Downsystem grain-size trends and mass balance of an ancient wave-influenced sediment routing system: Middle Jurassic Brent Delta, northern North Sea, offshore UK and Norway

Authors

  • Ikenna C. Okwara Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK and Department of Geology, University of Nigeria, Nsukka, Nigeria
  • Gary J. Hampson Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK https://orcid.org/0000-0003-2047-8469
  • Alexander C. Whittaker Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
  • Gareth G. Roberts Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK https://orcid.org/0000-0002-6487-8117

DOI:

https://doi.org/10.57035/journals/sdk.2025.e31.1336

Keywords:

Sediment grain size, Sediment mass-balance, Sediment routing system, Source-to-sink, Brent Group

Abstract

We reconstruct spatial variations in grain size in the sediment routing system of the data-rich Middle Jurassic Brent Group of the northern North Sea, using published stratigraphic, thickness, palaeogeographic, provenance and age constraints combined with representative core and wireline-log data. Facies associations provide a textural proxy for gravel, sand and mud grain-size fractions, and their distributions define spatio-temporal variations in grain size within four stratigraphic intervals (J22, J24, J26, J32 genetic sequences). Sediment was sourced from the west (Shetland Platform), east (Norwegian Landmass) and south (Mid-North Sea High). The associated sediment routing systems were geographically distinct in the oldest (J22) and youngest (J32) genetic sequences, but combined to feed a large wave-dominated delta (‘Brent Delta’) in genetic sequences J24 and J26. Few of the Brent Group sediment routing systems exhibit the downsystem-fining grain-size trend predicted by sediment mass balance theory. Deviations from this reference trend reflect: (1) sparse sampling of channelised fluvial and fluvio-tidal sand bodies in upsystem locations; (2) preferential trapping of sand in underfilled antecedent and syn-depositional, half-graben depocentres in genetic sequences J22 and J32; and (3) nearshore retention of sand by shoaling waves in wave-dominated shoreface and barrier-strandplain systems. This third type of deviation reveals that spatial facies partitioning due to shallow-marine process regime distorts the simple downsystem-fining reference trend, and supports the interpretation that large volumes of predominantly muddy sediment were bypassed beyond the ‘Brent Delta’ into neighbouring basins. Our analysis demonstrates a practical approach to interpret sediment supply and sediment dispersal in the stratigraphic record.

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Lay summary

Patterns of sediment accumulation at the Earth’s surface, and its archive in the stratigraphic record, reflect the interaction of space in which sediment can accumulate with the supply of sediment. Sediment supply is hard to constrain in the stratigraphic record, and its role is therefore under-appreciated. We use a compilation of published data that describe deposits of the Middle Jurassic ‘Brent Delta’ in the northern North Sea to test a method to constrain sediment supply that is based on theory and laboratory experiments. We reconstruct lateral trends in sediment grain size in the ‘Brent Delta’ deposits, and then analyse them within the context of the mass of the preserved deltaic deposits and independently derived estimates of sediment supplied to the delta from its mountainous hinterlands. The ‘Brent Delta’ deposits rarely show the simple downsystem-fining trends in grain size predicted by theory. Instead these trends are distorted by sparse data, by preferential trapping of sand in areas of thickened deltaic-shoreline deposits bounded by faults, and by retention of sand at the deltaic shoreline due to waves and wave-generated alongshore currents. The observed grain-size patterns imply that large volumes of mud were bypassed beyond the ‘Brent Delta’ shoreline. This mud volume is required to balance the mass of ‘Brent Delta’ deposits with the mass of sediment supplied from its hinterlands.

1. Introduction

Sediment supply and accommodation space are widely considered to be the two principal controls on stratigraphic architecture (e.g., Posamentier & Vail, 1988; Catuneanu et al., 2009; Allen et al., 2013; Romans et al., 2016). For instance, coupled landscape-basin models (e.g., Armitage et al., 2011) clearly show how changing sediment supply as opposed to changing accommodation-space generation should give rise to distinct stratigraphic signatures. Sediment supply is often more difficult to constrain than accommodation space, with the result that sediment supply is rarely explicitly interpreted in studies of stratigraphic architecture (e.g., Heller et al., 1993; Galloway, 2001; Brommer et al., 2009; Hampson, 2016; Burgess & Steel, 2017; Zhang et al., 2019). In contrast, source-to-sink studies have developed several approaches over the last two decades to estimate sediment flux from geologic and geomorphic evidence. These include, for instance, the empirical BQART model of suspended sediment load applied to modern or palaeo-catchments (Syvitski & Milliman, 2007; Zhang et al., 2018; Watkins et al., 2019; Lyster et al., 2020; Ravidà et al., 2021), empirical geomorphological scaling relationships (Sømme et al., 2009; Nyberg et al., 2018; Snedden et al., 2018), and palaeohydrological scaling models applied to stratigraphic observations (Holbrook & Wanas, 2014; Bhattacharya et al., 2016; Garefalakis & Schlunegger, 2018). These approaches to estimating sediment flux have been argued to show broad consistency with each other, and with mapped sediment volumes and ages in rich datasets from the stratigraphic record (Brewer at al., 2020; Lyster et al., 2020; Heins, 2023). However, facies-based grain-size data are rarely incorporated into these approaches (e.g., Reynolds, 2019).

Sediment routing systems record the cascade of sediment from its generation by erosion in source regions to deposition in sinks, via a region of sediment transfer (e.g., Allen, 2017). Preferential deposition of coarser grains during sediment transport gives rise to downsystem-fining grain-size trends that can provide a reference for comparison between sediment routing systems when normalised with respect to accommodation space (i.e., ‘sediment mass balance’; Figure 1; Strong et al., 2005; Whittaker et al., 2011; Paola & Martin, 2012; Michael et al., 2013, 2014; Hampson et al., 2014). Thus, downsystem-fining trends in grain size can in principle be used in combination with sediment flux estimates and, potentially, provenance constraints to infer spatio-temporal variations in sediment supply. The extent to which facies partitioning may complicate spatial interpretations of grain size trends remains contentious (c.f. Michael et al., 2013, 2014), but its use offers the means to apply facies-based sequence stratigraphic analysis to constrain the role of sediment supply on stratigraphic architecture and sediment routing and dispersal (Figure 1).

Figure 1. (A) Schematic illustration of a sediment routing system, which links sediment source areas to depositional sinks, and controls on its development (after Allen & Heller, 2011). These controls force the moving boundaries of the system (e.g., gravel front, sand front, shoreline) to migrate. Stratigraphy results from mass extraction (i.e., to deposition) from the surface sediment flux, and the depositional flux depends on (B) the spatial and temporal distribution of accommodation. (C) Sediment distribution in a mass balance framework, in which downsystem distance is transformed to the mass extraction domain; the downsystem coordinate χ shows the depositional mass extracted upsystem, normalised to the total sediment mass deposited in the sediment routing system from source to sink (e.g., Paola & Martin, 2012). The mass balance framework allows sediment routing systems of different scales and spatio-temporal accommodation-space distributions to be compared

In this paper, we reconstruct downsystem variations in grain size within a sediment mass balance context for the well-constrained, data-rich sediment routing systems of the Middle Jurassic Brent Group and coeval strata, northern North Sea, offshore UK and Norway. These strata include some of the most prolific hydrocarbon reservoirs in the North Sea basin (e.g., Husmo et al., 2003), which are now being evaluated for geological CO2 sequestration (e.g., North Sea Transition Authority, 2023). There is an enormous amount of legacy seismic, well, core, and biostratigraphic data and previous work on the Brent Group reservoirs and play in the public domain. In a recent synthesis, Okwara et al. (2023) used these data to estimate the net-depositional sediment mass of the ‘Brent Delta’ sediment routing system(s) to be 18.9 x 106 Mt (median P50 value, with a range from 10th to 90th percentile probability values, P10-P90 range, of 14.8 x 106 Mt to 23 x 106 Mt) deposited in 8.1 Myr, resulting in a net-depositional sediment budget of 2.33 Mt/yr (P50 value, with a P10-P90 range of 1.96–2.78 Mt/yr). Okwara et al. (2023) also used the BQART sediment load model of Syvitski & Milliman (2007) with inputs derived from published data to estimate the source-area sediment budget of 17.4 Mt/yr (P50 value, with a P10–P90 range of 13.9–23.0 Mt/yr). Thus, the estimated source-area sediment budget is almost an order of magnitude greater than the estimated net-depositional sediment budget. This discrepancy implies that the ‘Brent Delta’ sediment routing system(s) was oversupplied with sediment, and sediment bypass beyond the ‘Brent Delta’ is required to achieve sediment mass balance (Okwara et al., 2023). We extend the sediment mass balance analysis of Okwara et al. (2023) by reconstructing grain-size trends within their estimated net-depositional sediment mass.

We aim to constrain and assess sediment supply and dispersal within the wave-dominated ‘Brent Delta’ using downsystem variations in grain size, and thereby to evaluate potential sediment bypass to more basinward locations. This aim is important because it addresses the extent to which downsystem variations in grain size together with sediment mass balance can provide a simple, pragmatic tool to interpret the role of sediment supply on stratigraphic architecture. The specific objectives of the paper are threefold: (1) to document upsystem-to-downsystem variations in grain size in the Middle Jurassic Brent Group and coeval strata in the northern North Sea; (2) to use these grain-size variations to constrain sediment supply to, dispersal in, and bypass beyond the ‘Brent Delta’ sediment routing systems; and (3) to compare these results to estimates of sediment supply derived from the BQART sediment-load model (Okwara et al., 2023) and to qualitative constraints from facies models and sequence stratigraphic interpretations of the ‘Brent Delta’ (e.g., Budding & Inglin, 1981; Scott, 1992; Johannessen et al., 1995; Olsen & Steel, 1995; Løseth & Ryseth, 2003; Went et al., 2013).

2. Geological setting

2.1. Tectono-stratigraphic and climatic context

The Middle Jurassic Brent Group and associated strata were deposited in the northern part of the North Sea, within a North–South-trending depocentre that subsequently developed into the Viking Graben and Horda Platform (Figure 2A, B; e.g., Husmo et al., 2003). The proto-Viking Graben depocentre is bounded by the Shetland Platform to the west, Norwegian Landmass to the east, and Mid-North Sea High to the south (Figure 2B).

Figure 2. (A) Unrestored Middle Jurassic palaeogeographic reconstruction of the North Sea (Ziegler, 1990; Torsvik et al., 2002). (B) Restored Middle Jurassic palaeogeographic reconstruction of the Northern North Sea showing palaeo-landmasses and basins (Ziegler, 1990). Note the extent of the proto-Viking Graben (VG) and sediment input into the basin from the Shetland Platform (SP), Norwegian Landmass (NL), and Mid-North Sea High (MNSH). Additional tectonic elements include the Central North Sea (CNS), Egersund Basin (EB), Faroes-Shetland Basin (FSB), Horda Platform (HP), London-Brabant Massif (LBM), Moray Firth Basin (MFB), Møre Basin (MB), Rhenish Massif (RM), Rockall Basin (RB), South Permian Basin (SPB), Unst Basin (UB), West Hebrides Basin (WHP). The mapped extent of the subcrop beneath the ‘Mid-Cimmerian Unconformity’, which formed due to initiation of the MNSH uplift (Underhill & Partington, 1993) is shown. Depocentres supplied by abundant clastic sediment occur in the Faroes-Shetland Basin (FSB), South Permian Basin (SPB), Egersund Basin (EB) and in northern Germany, in addition to the ‘Brent Delta’ depocentre in the Viking Graben (VG) and Horda Platform (HP). (C) Simplified lithostratigraphic column for the proto-Viking Graben (Figure 2B) highlighting the main phases of structural evolution in relation to deposition of the Brent Delta sediment routing system(s) (Figure 3).

The proto-Viking Graben was initiated during Late Permian to Early Triassic rifting (Barton & Wood, 1984; Steel & Ryseth, 1990; Færseth, 1996; Zanella & Coward, 2003; Duffy et al., 2015), and underwent post-rift thermal subsidence during Late Triassic to Middle Jurassic times (Figure 2C). Middle Jurassic thermal doming and uplift of the Mid-North Sea, south of the proto-Viking Graben, resulted in erosion of Triassic to Lower Jurassic strata here, and the development of an intra-Aalenian unconformity, widely referred to as the ‘mid-Cimmerian Unconformity’ . Middle to Late Jurassic strata onlap this unconformity, recording later collapse of the dome (Underhill & Partington, 1993, 1994; Husmo et al., 2003). Subsequent Late Jurassic to Early Cretaceous rifting resulted in development of the trilete North Sea rift, and established the present-day configuration of structural elements, including the Viking Graben and Horda Platform (e.g., Underhill & Partington, 1993; Zanella & Coward, 2003). In the developing Viking Graben, Late Jurassic to Early Cretaceous rifting reactivated some North–South-trending, Permo-Triassic rift faults and generated additional northeast-southwest-trending faults (Færseth, 1996; Zanella & Coward, 2003; Duffy et al., 2015; Phillips et al., 2019).

The proto-Viking Graben depocentre occupied a palaeo-latitude of c. 44–48°N during the Middle Jurassic (Figure 2A; Ziegler, 1990; Torsvik et al., 2002). Coal petrology, palynological data, palaeo-latitude reconstructions, strontium isotope data, oxygen isotope data, and ocean-atmosphere numerical models indicate that the depocentre was subject to a sub-tropical and humid climate throughout the Middle Jurassic, with mean annual temperature estimated to have been ca. 20°C (Abbink et al., 2001; Sellwood & Valdes, 2006, 2008; Prokoph et al., 2008).

2.2. Stratigraphic and palaeogeographic framework of the ‘Brent Delta’

The stratigraphic and palaeogeographic framework of the Brent Group and related strata have been documented in many regional, basin-scale studies are broadly consistent with each other.

In this study, we use the facies-association classification scheme (Table 1) and stratigraphic framework (Figure 3) synthesised by Okwara et al. (2023), which is based on the “J sequences” (J22, J24, J26, J32) proposed by Mitchener et al. (1992) and applied to a dataset of wells used in previous stratigraphic studies (Figure 4, Table S1; Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Hampson et al., 2004; Kieft et al., 2011). Facies associations are based on sedimentological characteristics in core, and are calibrated to wireline-log characteristics in uncored wells and intervals (Table 1). The basic, and widely used, facies model of the Brent Group is of a wave-dominated delta, in which a wave-dominated shoreface and barrier bars were developed in front of a coal-bearing coastal plain containing lagoons and fluvio-estuarine channels (Budding & Inglin, 1981). Weakly wave-influenced (bioturbated) shorefaces and fan deltas were developed locally in the J22 and J32 genetic sequences (e.g., Graue et al., 1987; Helland-Hansen et al., 1992; Johannessen et al., 1995; Fjellanger et al., 1996; Muto & Steel, 1997; Morris et al., 2003; Hampson et al., 2004). Each “J sequence” is a genetic sequence (sensu Galloway, 1989) bounded by biostratigraphically calibrated maximum flooding surfaces of basin-wide extent (Partington et al., 1993). Each genetic sequence is outlined below, and illustrated via isopach and palaeogeographic maps (Figures 5-8), core and wireline logs from representative wells (Figure S1), well correlation panels (Figure S2), and a net-depositional sediment budget (Figure 9A; Okwara et al., 2023). We use the revised biostratigraphic picks of Kieft et al. (2011) to refine local definition of the genetic sequences in Norwegian quadrant 15; in particular, these data place the J32 genetic sequence within the coastal-plain and lagoonal deposits of the Sleipner Formation (e.g., well 15/3-3 in Figure S2C).

Facies association Lithostratigraphic distribution Sedimentological description Proportion of grain-size class Wireline log response
1. Coastal plain
1.1. Fluvial channel Ness Fm.; Sleipner Fm.; Bruce ‘C’ Sands Poorly to moderately sorted, fining-upward, fine- to coarse-grained sandstone (2-8 m thick). Pebble lag above erosional base. Cross-beds and asymmetrical ripples. Bioturbation absent to low in intensity. g - 5%; s - 90%; m - 5% Low, upward-increasing gamma ray (20–40 API), moderate sonic (60–80 µs/ft), moderate to high density (2.4–2.6 g/cm3), low neutron (2–25 p.u).
1.2. Floodplain Ness Fm.; Sleipner Fm. Mudstone (2-10 m thick) interbedded with very fine- to medium-grained sandstone. Plant root traces common. Bioturbation absent to moderate in intensity. g - 0%; s - 20%; m - 80% Variable, high gamma ray (60–90 API), high sonic (80–100 µs/ft), moderate to high density (2.35–2.7 g/cm3), moderate to high neutron (15–50p.u.).
1.3. Swamp Ness Fm.; Sleipner Fm.; Bruce B-C Coal Coal and carbonaceous shale (<1 m thick). Bioturbation absent. In situ biogenic accumulation Low gamma ray (20–35 API), very high sonic (110–130 µs/ft), low density (1.6–1.8 g/cm3), high neutron (50–60 p.u.).
2. Marginal marine
2.1. Lagoon Ness Fm.; Sleipner Fm. Mudstone (1-4 m thick) with rare beds of very fine- to fine-grained sandstone. Parallel lamination, symmetrical and asymmetrical ripples. Rare hummocky cross-stratification. Sparse to moderate bioturbation intensity. g - 0%; s - 20%; m - 80% High gamma ray (70–80 API), moderate sonic (80–90 µs/ft), high density (2.6–2.65 g/cm3), moderate to high neutron (30–45 p.u.).
2.2. Fluvio-tidal / estuarine channel and fan delta Broom Fm.; Oseberg Fm.; Etive Fm.; Ness Fm.; Tarbert Fm.; Bruce ‘B’ Sands Moderately to well-sorted, medium- to very coarse-grained sandstone (10-25 m thick) with sharp or erosional base. Cross-beds, asymmetrical ripples, mud drapes. Bioturbation is generally low to moderate in intensity. g - 5%; s - 90%; m - 5% Low to moderate, uniform to upward-increasing gamma ray (15–45 API), moderate to high sonic (60–95 µs/ft), moderate to high density (2.2–2.65 g/cm3), low to moderate neutron (0.5–25 p.u.).
2.3. Bay-head and lagoonal delta Ness Fm.; Sleipner Fm. Very fine- to medium-grained, well to moderately sorted sandstone (3-7 m thick) usually interbedded with lagoon mudstone (FA 2.1). Symmetrical and asymmetrical ripples, low-angle cross-lamination, climbing asymmetrical ripples. Generally sparse to moderate bioturbation intensity. g - 0%; s - 80%; m - 20% Typically upward-decreasing gamma ray (20–70 API), moderate to high sonic (70–90 µs/ft), moderate density (2.3–2.55 g/cm3), low to moderate neutron (6–28 p.u.).
3. Shallow marine to shelf
3.1. Weakly wave-influenced (bioturbated) shoreface Broom Fm.; Oseberg Fm.; Tarbert Fm. Moderately to well-sorted, coarsening-upward, fine- to coarse-grained sandstone (10-20 m thick). Planar-parallel lamination and low-angle cross-lamination. Moderate to high bioturbation intensity in lower parts and low bioturbation intensity in upper parts of coarsening-upward units. g - 0%; s - 100%; m - 0% Moderate, upward-decreasing gamma ray (40–70 API), moderate sonic (75–85 µs/ft), moderate to high density (2.35–2.62 g/cm3), moderate neutron (20–30 p.u.).
3.2. Wave-dominated upper shoreface and barrier bar Etive Fm.; Hugin Fm.; Tarbert Fm. Well-sorted, coarsening-upward, fine- to medium-grained sandstone (8-35 m thick). May appear structureless, but planar and trough cross-beds, asymmetrical ripples, planar-parallel lamination, low angle cross-lamination and dewatering structures occur. Absent to low bioturbation intensity. g - 0%; s - 100%; m - 0% Low, upward-decreasing gamma ray (10–35 API), moderate to high sonic (70–90 µs/ft), moderate density (2.2–2.6 g/cm3), low to moderate neutron (3–30 p.u.).
3.3. Proximal lower shoreface Rannoch Fm.; Hugin Fm.; Bruce ‘A’ Sands Coarsening-upward succession of micaceous, well-sorted, fine- to lower medium-grained sandstone (5-25 m thick). Low-angle and hummocky cross-stratification, minor symmetrical and asymmetrical ripple cross-lamination. Bioturbation intensity is sparse to low. g - 0%; s - 100%; m - 0% Moderate, upward-increasing gamma ray (40–70 API), moderate sonic (70–85 µs/ft), variable density (2.3–2.7 g/cm3), moderate neutron (20–36 p.u.).
3.4. Distal lower shoreface and offshore lower Rannoch Fm.; Heather Fm. Dominantly mudstone, may be interbedded with pinstripe laminae and thin beds of very fine- to fine-grained sandstone. Pyrite nodules and scattered shells occur. Bioturbation intensity is moderate to intense. g - 0%; s - 0%; m - 100% High gamma ray (55–120 API), moderate to high sonic (80–95 µs/ft), high density (2.5–2.65 g/cm3), variable neutron porosity (12–40 p.u.).
Table 1. Summary of interpreted facies associations, their sediment grain-size characteristics, and their wireline-log signatures (after Supplementary Material of Okwara et al., 2023). Abbreviations for grain-size classes: g – gravel, s – sand, m – mud.

Figure 3. Sequence stratigraphic framework for the ‘Brent Delta’ synthesised from various published references (Deegan & Scull, 1977; Mitchener et al., 1992; Partington et al., 1993; Sneider et al., 1995; Johannessen et al., 1995; Fjellanger et al., 1996; Hampson et al., 2004). The Aalenian to Bathonian genetic sequences (J22, J24, J26 and J32) of Mitchener et al. (1992) are used in this study. Possible tectonic drivers in the Shetland Platform, Norwegian Landmass, and Mid-North Sea High source regions and depositional sink, climate (Prokoph et al., 2008), eustatic sea-level change (Haq, 2018), and estimated relative contributions from the source regions to the sediment budget of the ‘Brent Delta’ sediment routing system(s), based on the relative proportion of detrital garnet compositional suites (Morton, 1992), are shown (after Okwara et al., 2023). Facies Association (FAs) in each Gross Depositional Environment are summarised in Table 1.

Figure 4. Maps locating: (A) the study area (Figure 4B); and (B) wells in the study dataset (numbered according to Table S1), and regional stratigraphic correlation panels (Figure S2). To aid clarify, two west-east-oriented (transverse) correlation panels (Figure S2A, B) are shown in blue and one north-south-oriented (axial) correlation panel (Figure S2C) is shown in red. Wells in the study dataset and previously published interpretations of these wells are summarised in Table S1.

The J22 genetic sequence is characterised by regression and subsequent transgression of fluvio-estuarine channels (facies association 2.2, Table 1) and weakly wave-influenced (bioturbated) shorefaces (facies association 3.1, Table 1), comprising the Broom Formation, from the western basin margin (Figures 5B, S1A, C, S2A; e.g., Mitchener et al., 1992; Hampson et al., 2004). At the same time, fluvio-estuarine channels and fan deltas (facies association 2.2, Table 1), comprising the Oseberg Formation, underwent regression and subsequent transgression from the eastern basin margin (Figures 5B, S1B, S2B; e.g., Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996). Coeval coastal plain strata (facies associations 1.1–1.3, Table 1), including the Bruce ‘C’ Sands, accumulated during minor northward shoreline regression and subsequent transgression in the southernmost basin axis (Figures 5B, S2C; e.g., Mitchener et al., 1992). Thickness changes in the J22 genetic sequence (Figure 5A) are relatively small. They are either confined to the hangingwalls of a few extensional faults or occur above deeply buried, fault-bounded depocentres developed during Lower Triassic rifting (Figure 2C). The former are aligned sub-parallel to palaeoshoreline trends (Figure 5B), suggesting that the hangingwall depocentres of these extensional faults were underfilled prior to, and thus influenced deposition of, the J22 genetic sequence .

Figure 5. (A) Isopach map and (B) palaeogeographic reconstruction for genetic sequence J22 (Figure 3), during maximum progradation of the eastward prograding ‘Broom Delta’ and westward prograding ‘Oseberg Delta’ (after Okwara et al., 2023 and references therein). Maps show the extent of deposition prior to Late Jurassic erosion (dotted lines; Husmo et al., 2003) and the location of Shetland Platform, Norwegian Landmass and Mid-North Sea High source areas (Ziegler 1990; Underhill & Partington, 1993). Facies Association (FAs) in each Gross Depositional Environment (Figure 5B) are summarised in Table 1 and illustrated in Supplementary Figures S1 and S2. Cored and uncored wells in the study dataset are shown as green and red circles, respectively. Red rectangles indicate the areas for which sediment masses have been calculated for three distinct sediment routing systems, with wells in each area projected into a transverse (Figure 10A, B) or axial transect (Figure 10C) through the sediment routing system.

The J24 genetic sequence is characterised by northward regression along the basin axis of a wave-dominated shoreface and barrier bar (facies associations 3.2–3.4, Table 1; Rannoch and Etive formations) and associated coastal plain, lagoon and fluvio-estuarine channels (facies associations 1.1–1.3 and 2.1–2.3, Table 1; lower Ness Formation) (Figures 6B, S1, S2; e.g., Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Husmo et al., 2003; Hampson et al., 2004). In the J24 genetic sequence (Figure 6A), thickness changes are confined to the hanging walls of a few extensional faults and to locations above deeply buried Lower Triassic depocentres (e.g., Folkestad et al., 2014). Palaeoshorelines were oriented approximately west-east (Figure 6B). Their orientation is nearly perpendicular to, and continues across, syn-depositional extensional faults (Figure 6B), suggesting that the associated depocentres were filled and/or overfilled (e.g., Hampson et al., 2004). The J24 genetic sequence thins northward of the maximum-regressive palaeoshoreline position (Figure 6), reflecting underfilling of relatively deep bathymetry offshore of this palaeoshoreline.

Figure 6. (A) Isopach map and (B) palaeogeographic reconstruction for genetic sequence J24 (Figure 3), during maximum progradation of the northward-prograding ‘Brent Delta’ (after Okwara et al., 2023 and references therein). Maps show the extent of deposition prior to Late Jurassic erosion (dotted lines; Husmo et al., 2003) and the location of Shetland Platform, Norwegian Landmass and Mid-North Sea High source areas (Ziegler 1990; Underhill & Partington, 1993). Facies Association (FAs) in each Gross Depositional Environment (Figure 6B) are summarised in Table 1 and illustrated in Supplementary Figures S1 and S2. Cored and uncored wells in the study dataset are shown as green and red circles, respectively. The red rectangle indicates the area for which sediment mass has been calculated for a combined sediment routing system, with wells in the area projected into an axial transect (Figure 11) through the sediment routing system.

The J26 genetic sequence is characterised by continued, minor northward regression followed by aggradation and subsequent transgression of wave-dominated shorefaces and barrier bars (facies associations 3.2–3.4, Table 1; Etive and, locally, Tarbert formations) in the northern part of the basin, and aggradation of coastal plains, lagoons and fluvio-estuarine channels (facies associations 1.1–1.3 and 2.1–2.3, Table 1; upper Ness Formation, Bruce ‘B’ Sands) further south (Figures 7B, S1, S2; e.g., Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Husmo et al., 2003; Hampson et al., 2004). The thickness of the J26 genetic sequence (Figure 7A) indicates that the few active fault-bounded depocentres were filled and/or overfilled (e.g., Hampson et al., 2004), and that relatively deep, underfilled bathymetry lay offshore of the west-east oriented, maximum-regressive palaeoshoreline position (Figure 7).

Figure 7. (A) Isopach map and (B) palaeogeographic reconstruction for genetic sequence J26 (Figure 3), during aggradation of the ‘Brent Delta’ (after Okwara et al., 2023 and references therein). Maps show the extent of deposition prior to Late Jurassic erosion (dotted lines; Husmo et al., 2002) and the location of Shetland Platform, Norwegian Landmass and Mid-North Sea High source areas (Ziegler 1990; Underhill & Partington, 1993). Facies Association (FAs) in each Gross Depositional Environment (Figure 7B) are summarised in Table 1 and illustrated in Supplementary Figures S1 and S2. Cored and uncored wells in the study dataset are shown as green and red circles, respectively. The red rectangle indicates the area for which sediment mass has been calculated for a combined sediment routing system, with wells in the area projected into an axial transect (Figure 12) through the sediment routing system.

The J32 genetic sequence records the initiation of Upper Jurassic rifting (Figures 2C, 3), which is expressed as pronounced thickening into the hanging walls of extensional faults as they propagated and linked (Figure S2A; e.g., Davies et al., 2000; McLeod et al., 2000), significant time gaps across unconformities confined to footwall crests and rift shoulders, onlap on to rift-generated topography, and the extrusion of volcanic rocks on the Mid-North Sea High (e.g., Mitchener et al., 1992; Quirie et al., 2019). The genetic sequence is characterised by net-transgressive stacking of a range of shallow- and marginal-marine deposits, including fluvio-estuarine channels and fan deltas (facies association 2.2, Table 1), weakly wave-influenced (bioturbated) shorefaces (facies association 3.1, Table 1), wave-dominated shorefaces (facies associations 3.2-3.4, Table 1) (Tarbert Formation, Bruce ‘A’ Sands and Hugin Formation) and offshore mudstones (facies association 3.4, Table 1; Heather Formation), which overlie aggradational coastal-plain and lagoonal deposits (facies associations 1.1–1.3 and 2.1–2.3, Table 1; upper Ness Formation and Sleipner Formation) (Figures 8B, S1, S2; e.g., Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Husmo et al., 2003; Hampson et al., 2004). Palaeoshorelines near the base of the J32 genetic sequence were oriented west-east (cf. Figure 7B), but evolved to a north-south orientation, parallel to major fault trends in the basin and consistent with the development of underfilled hanging wall depocentres, as they retreated towards the basin margins (Figure 8B).

Figure 8. (A) Isopach map and (B) palaeogeographic reconstruction for genetic sequence J32 (Figure 3), during transgression of the ‘Brent Delta’ and shoreline retreat to the western, eastern and southern basin margins (after Okwara et al., 2023 and references therein). Maps show the extent of deposition prior to Late Jurassic erosion (dotted lines; Husmo et al., 2003) and the location of Shetland Platform, Norwegian Landmass and Mid-North Sea High source areas (Ziegler 1990; Underhill & Partington, 1993). Facies Association (FAs) in each Gross Depositional Environment (Figure 8B) are summarised in Table 1 and illustrated in Supplementary Figures S1 and S2. Cored and uncored wells in the study dataset are shown as green and red circles, respectively. Red rectangles indicate the areas for which sediment masses have been calculated for three distinct sediment routing systems, with wells in each area projected into a transverse (Figure 13A, B) or axial transect (Figure 13C) through the sediment routing system.

2.3. Provenance and sediment source areas

Petrographic data (e.g., Morton, 1985, 1992; Hamilton et al., 1987; Hurst & Morton, 1988; Mearns, 1992) and palaeogeographic reconstructions (e.g., Mitchener et al., 1992; Fjellanger et al., 1996; Husmo et al., 2003) indicate that sediment was routed consistently to the ‘Brent Delta’ from three main source areas at the margins of the proto-Viking Graben: the Shetland Platform, Norwegian Landmass, and Mid-North Sea High (Figure 2B; e.g., Okwara et al., 2023). The Norwegian Landmass lay to the east (Figure 2B), was composed of Precambrian gneisses and granites and Caledonian metasediments and granitic rocks ), and had an estimated relief of 1–1.2 km during the Middle Jurassic (Gabrielsen et al., 2010; Medvedev & Hartz, 2015; Okwara et al., 2023). The Shetland Platform lay to the west (Figure 2B), and was composed mainly of Devonian, Carboniferous and Permo-Triassic sedimentary rocks reworked from Precambrian metasediments of greenschist to upper amphibolite grade, Caledonian granites, and quartzo-feldspathic gneisses (Zanella & Coward, 2003; Morton et al., 2004). The palaeotopography of Shetland Platform catchments is poorly constrained (e.g., Okwara et al., 2023 used a value of 1.0 km in their application of the BQART sediment load model, assuming a similar uplift history to the Norwegian Landmass). The Mid-North Sea High lay to the south (Figure 2B), was composed of sandstone-dominated Triassic and mudstone-dominated Lower Jurassic sedimentary rocks eroded at the ‘mid-Cimmerian Unconformity’ ), and had a maximum relief of 0.3-0.5 km (Underhill & Partington, 1993; Okwara et al., 2023).

2.4. Source-area and depositional-sink sediment budgets

Okwara et al. (2023) used a synthesis of previous work on the sedimentology, stratigraphy, provenance and source areas of the ‘Brent Delta’ sediment routing system(s) to estimate the sediment mass supplied from the source areas, the mass of sediment deposited in the deltaic strata, and related sediment budgets. The source-area sediment budget was estimated using the BQART sediment load model of Syvitski & Milliman (2007) and independent geometrical reconstruction of eroded volumes from the Mid-North Sea High source area. The sediment mass in the ‘Brent Delta’ depositional sink was reconstructed using isopach maps, palaeogeographic reconstructions and facies analysis of core and well-log data, and its mean net-depositional sediment budget was estimated as 2.0-2.8 Mt/yr. In detail, the net-depositional sediment budget increased from 0.24-0.69 Mt/yr for the J22 genetic sequence to 2.54-8.09 Mt/yr for the J32 genetic sequence. Okwara et al. (2023) attributed this increase to thermal doming and uplift of the Mid-North Sea High source area (cf. Underhill & Partington, 1993, 1994), uplift of the Norwegian Landmass source area (cf. Gabrielsen et al., 2010; Ksienzyk et al., 2014; Medvedev & Hartz, 2015), and potential catchment area expansion, and rift-related uplift of the Shetland Platform and Norwegian Landmass during deposition of the J32 genetic sequence (cf. Helland-Hansen et al., 1992; Davies et al., 2000; Folkestad et al., 2014). The total sediment budget supplied by the three source areas was estimated as 13.9–23 Mt/yr, an order of magnitude higher than the net-depositional sediment budget, which implies significant bypass of sediment beyond the mapped limits of the ‘Brent Delta’ depocentre (Okwara et al., 2023).

3. Dataset and method

3.1. Dataset

We use data from 84 representative exploration wells (Figure 4, Table S1), each of which has been interpreted in previous studies within one of four regional genetic stratigraphic and sequence stratigraphic frameworks (Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Hampson et al., 2004; Figure 3). Wells were selected based on their geographical distribution (Figure 4B) and their use in existing regional genetic stratigraphic and sequence stratigraphic frameworks (Mitchener et al., 1992; Sneider et al., 1995; Fjellanger et al., 1996; Hampson et al., 2004; Figure 3, Table S1). These regional frameworks are all based on lithofacies interpretations that are consistent with each other and that capture the full range of facies associations in the ‘Brent Delta’ sediment routing system(s), as summarised in Table 1 (after Supplementary Material of Okwara et al., 2023). In parts of the study area that contain many closely spaced fields with Brent Group reservoirs (e.g., the southern part of UK quadrant 211 and northern part of UK quadrant 3; Figure 4B), we were able to select wells from a large number of candidates. Other parts of the study area contain only few wells that penetrate Middle Jurassic strata (e.g., UK quadrant 9 and Norwegian quadrants 24 and 25; Figure 4B). A total of 1360 m of core from eight of the 84 studied wells (Table S2) was described using sedimentological facies analysis, in order to define the overall lithological and grain-size characteristics of facies associations, and to calibrate the lithological composition of uncored intervals and wells (see Supplementary Material of Okwara et al., 2023 for details). The isopach and palaeogeographic maps of Okwara et al. (2023) were used to constrain the thickness and gross facies-association distributions of the J22, J24, J26 and J32 genetic sequences between the studied wells (Figures 5-8).

3.2. Method

In our analysis, we first use steps i-iii of the method of Okwara et al. (2023) to estimate the net-depositional sediment mass of the ‘Brent Delta’ deposits: (1) definition of age-constrained stratigraphic intervals (J22, J24, J26 and J32 genetic sequences in Figure 3); (2) facies analysis to constrain the distribution of coastal-plain, marginal-marine, and shallow-marine-to-shelf deposits in each stratigraphic interval (Table 1); and (3) generation of isopach maps for each stratigraphic interval (Figures 5A, 6A, 7A, 8A).

The fourth step in our method is delineation of sediment routing system(s) in each stratigraphic interval using palaeogeographic maps (Figures 5B, 6B, 7B, 8B). In the J22 and J32 genetic sequences, palaeoshorelines adjacent to the western, eastern and southern basin margins are palaeogeographically distinct (Figures 5B, 8B), such that we consider each of them to belong to a separate sediment routing system (or a combined group of closely spaced sediment routing systems) lying directly downsystem of, and fed by, each source area. In the J24 and J26 genetic sequences, these three sediment routing systems are considered to combine, such that they fed a single west-east-trending palaeoshoreline (Figures 6B, 7B). For these genetic sequences, we can only analyse the combined sediment routing system. The resulting delineation of sediment routing system(s) is consistent with that of Okwara et al. (2023, their figure 12), which was based on the detrital garnet compositional data of Morton (1992). However, a smaller proportion of the J22 and J32 deposits is sampled (i.e., cumulative area of three red boxes in Figures 5, 8) than of the J24 and J26 deposits (i.e., area of one red box in Figures 6, 7), due to the absence of clear palaeogeographic boundaries between sediment routing systems in the J22 and J32 genetic sequences.

The fifth step is calculation in each sediment routing system in each stratigraphic interval of: sediment volumes; volumetric proportions of facies associations; volumetric proportions of broad grain-size classes (gravel, sand, mud); and proportions of the broad grain-size classes by sediment mass. Our fifth step is equivalent to step iv in the method of Okwara et al. (2023), but is applied to the sediment routing systems in each stratigraphic interval, as identified in our fourth step, rather than to the entire stratigraphic interval. This fifth step requires the assumption that each facies association has a constant grain-size composition that is independent of upsystem-to-downsystem position (cf. Hampson et al., 2014). This assumption is consistent with previous facies analyses of the ‘Brent Delta’ deposits (Table 1; see Supplementary Material of Okwara et al., 2023 for details), which we consider to reflect deposition on a broad, low-gradient coastal plain and shelf that was subject to little direct influence from upstream variations in water discharge and sediment transport capacity. The assumption also does not account for grain-size variations within the proportions of gravel, sand and mud in each facies association. Use of the assumption allows variations in the proportion of facies associations from upsystem to downsystem to be treated as a first-order descriptor of trends in grain size at the scale of the sediment routing system (i.e., “architectural fining” of Hampson et al., 2014), but is not appropriate to analyse more subtle grain-size variations.

To implement the fifth step in our method, the following proximal-to-distal transects were constructed for each sediment routing system in the J22 and J32 genetic sequences: (1) west to east transect for systems fed from the Shetland Platform (Figures 5, 8); (2) east to west transect for systems fed from the Norwegian Landmass (Figures 5, 8); and (3) south to north transect for systems fed from the Mid-North Sea High (Figures 5, 8) or, in the J24 and J26 genetic sequences, the combination of all three sediment source areas (Figures 6, 7). Wells and groups of 2-14 wells that are closely spaced (<2 km) along depositional strike (i.e., perpendicular to the proximal-to-distal transect) were projected into the transect lines (in red boxes in Figures 5–8). The proportions of each facies association were calculated in each projected well, using wireline-log data calibrated to cores (e.g., Figures S1, S2), and mean facies-association proportions were calculated in each projected group of closely spaced wells. The proportions of gravel, sand and mud grain size classes in each projected well and projected group of wells were calculated from these facies-association proportions and the proportions of gravel, sand and mud assigned to each facies association (Table 1). The wells and groups of wells therefore provide estimates of facies-association proportions and grain-size proportions at specific downsystem distances in each sediment routing system transect. Facies-association proportions and associated grain-size proportions at other points in each transect were estimated by simple linear interpolation between neighbouring pairs of wells and groups of wells in the transect. Facies-association and grain-size volumes between wells were estimated by multiplying the along-strike (i.e., transect-perpendicular) volumetric increments between wells by the mean facies-association and grain-size proportions of the pair of wells that bound each volumetric increment. The sum of these along-strike (i.e., transect-perpendicular) facies-association and grain-size volumetric increments for a particular transect constitutes the facies-association and grain-size volumes in the corresponding sediment routing system. Compacted grain-size volumes along each proximal-to-distal transect are converted to sediment masses using bulk-density values derived from density logs in the studied wells. Gravel conglomerates and sandstones are assigned a bulk density of 2400 kgm-3, and mudstones a bulk density of 2500 kgm-3 (cf. Okwara et al., 2023). These values include the effects of compaction and cementation. Coal, which is generated biogenically in situ (e.g., Budding & Inglin, 1981; Livera, 1989), is excluded from our calculations of sediment mass.

The sixth, and final, step in our method iscalculation of downsystem extraction of sediment mass in each sediment routing system in each stratigraphic interval, using the proximal-to-distal transects described above in the fifth step (i.e., in the red boxes shown in Figures 5–8). The resulting downsystem extraction of sediment mass is normalised against the cumulative sediment mass in the sediment routing system volume (i.e., the parameter Chi, χ, of Strong et al. 2005), in order to provide a framework for comparison between the sediment routing systems (Strong et al. 2005; Paola & Martin, 2012). χ(x) is defined as:

(1) χ(x) = ∫0x r∆T (x)dx / 0L r∆T (x)dx

where r(x) is the rate of deposition (in dimensions LT-1) at a given downstream distance x, measured over a time interval ∆T, and L is the total length of the depositional system (Strong et al. 2005). For each sediment routing system, the value assigned to ∆T is the duration of genetic sequence J22, J24, J26 or J32 (Figure 3), as appropriate, and the value assigned to L is the length of the proximal-to-distal transect for the relevant sediment routing system (Figures 5-8).

For each of the sediment routing systems, downsystem changes in the fractions of gravel, sand and mud grain-size classes are described in terms of χ (Equation 1), with the total sediment mass for each sediment routing system defined for deposits in the area enclosing the proximal-to-distal transect of that system (i.e., by the red boxes in Figures 5-8). We also evaluate downsystem changes in the fraction of sand deposited in coastal-plain, marginal-marine and shallow-marine-to-shelf environments (Table 1), normalized to the sediment mass extracted by deposition (i.e., χ; Equation 1), along each proximal-to-distal transect (Figures 5-8).

3.3. Uncertainties

Okwara et al. (2023) used a Monte Carlo approach to characterise error and uncertainty in their net-depositional sediment budget (their figure 9), and their results are directly relevant to steps 1, 3 and 5 in our method. Uncertainties related to stratigraphy, age models, sediment volume calculations, and sediment volume-to-mass conversion are discussed and quantified in Okwara et al. (2023).

Uncertainty in the facies-association scheme that we use as a proxy for grain-size distributions (step 2) arises from the quantity, quality and representativeness of core data that constrain the definition of facies associations and their constituent grain-size-class proportions (Table 1), and in the calibration of wireline logs with core data to interpret facies associations in uncored wells and intervals (e.g., Figures S1, S2). Although we use a relatively small length of core (Table S2) and small number of wells (Figure 4, Table S1) to construct the facies-association scheme, it is similar to that used in many previous sedimentological studies of the Brent Group at local, sub-regional and regional scales (Budding & Inglin, 1981; Richards & Brown, 1986; Graue et al., 1987; Livera, 1989; Ryseth, 1989; Livera & Caline, 1990; Mitchener et al., 1992; Scott, 1992; Johannessen et al., 1995; Fjellanger et al., 1996; Muto & Steel, 1997; Løseth & Ryseth, 2003; Morris et al., 2003; Hampson et al., 2004). We also use a simple grain-size characterisation scheme with only three major grain-size classes (gravel, sand, mud), which helps to promote standardisation between different sedimentological studies (cf. “grain size bookkeeping” of Reynolds, 2019) and thereby mitigate some uncertainty.

Delineation of the sediment routing system(s) in each stratigraphic interval (step 4) depends on the type, distribution, and quality of data that underpin reconstructions of palaeogeography (Figures 5–8; e.g., Mitchener et al., 1992; Fjellanger et al., 1996; Husmo et al., 2003) and provenance (e.g., Morton, 1985, 1992; Hamilton et al., 1987; Hurst & Morton, 1988; Mearns, 1992). Uncertainty is defined by different palaeogeographic scenarios, and is mitigated in our analysis by using a simple model of three sediment source areas in the J22 and J32 genetic sequences that are amalgamated in the J24 and J26 genetic sequences (Figures 5-8).

Uncertainty in calculating facies-association and grain-size volumes in sediment routing systems (step 5) reflects the quantity, quality and distribution of well data that underpin characterisation of facies-association volumes in the proximal-to-distal transect for each sediment routing system (e.g., Figures 5–8, S1, S2). Uncertainty may arise because the wells and groups of wells in each sediment routing system transect are not representative of along-strike facies-association distributions. Uncertainty is greater where facies associations pinch out laterally along-strike, for example because the facies associations occur in channelised bodies. Such uncertainty is qualitatively assessed in our analysis by comparing the mean facies-association and grain-size proportions for groups of wells projected into the transect with the facies-association and grain-size proportions of individual wells in the group. It could be reduced by incorporating additional well data that constrain the distribution and pinch-out relationships of facies associations, particularly those that have channelised and lenticular geometries, but only at the expense of the time required for analysis. Given the large uncertainties in other steps in our method and absence of wells in large parts of the study area (Figure 4), we consider that the well database is sufficient for indicative, first-order approximations.

Uncertainty in the calculation of downsystem extraction of sediment mass (step 6) is compounded from sources of uncertainty in steps 1-5 of the methodology. In similar studies of ancient sediment routing systems, such compounded errors can result in halving or doubling of net-depositional mass flux estimates (e.g., Galloway, 2001; Hampson et al., 2014).

3.4. Outputs

Within the normalised sediment mass framework that is produced by the method outlined above, and associated uncertainty, we compare two parameters from upsystem to downsystem.

The first parameter is the proportion of gravel, sand, and mud grain-size classes in the sediment volume deposited by the sediment routing system in the proximal-to-distal transect. These grain-size proportions have been previously demonstrated to be independent of spatial variations in χ (Whittaker et al., 2011; Michael et al., 2013). Consequently, differences in grain-size proportions between sediment routing systems and/or between stratigraphic intervals can be attributed to: (1) the grain-size distribution input to the system; (2) along-strike influx to or efflux from the system; and/or (3) the grain-size distribution of bypassed sediment output from the system (cf. Hampson et al., 2014). The first of these three mechanisms incorporates any input of sediment via reworking at regional or sub-regional unconformities (e.g., locally coinciding with the base of the J22 and J32 genetic sequences; Figure 3), as well as fluvial sediment input from source-area catchments. The impact of sediment reworking, temporary sediment storage and autogenic sedimentation processes is more pronounced over shorter time intervals (i.e., small values of ∆T in Equation 1; e.g., Strong et al. 2005), and is minimised in our analysis by considering genetic sequences of long duration (0.9–3.9 Myr) that are bounded regionally by maximum flooding surfaces (Figure 3). The second mechanism suggests that the sediment routing system is not closed, consistent with deposition in a wave-dominated delta(s) or other shorelines with significant along-shore sediment transport by waves, tides and marine currents (e.g., Hampson et al., 2014). The third mechanism would suggest that the full downsystem extent of the system has not been mapped, as has recently been implied for the ‘Brent Delta’ sediment routing system(s) by the sediment mass balance analysis of Okwara et al. (2023).

The second parameter is the downsystem change in percentage-thickness, as a function of sediment mass extracted, of coastal-plain, marginal-marine, and shallow-marine-to-shelf sandstones. This parameter describes the upsystem-to-downsystem position of the palaeoshoreline, relative to the mass fraction of sandstone, in each sediment routing system, and may thus provide insights into the role of along-shore sand transport by waves, tides and marine currents.

4. Results

4.1. Grain-size fractions of the total sediment mass

4.1.1. Description

Similar grain-size fractions occur in the J22, J24, J26 and J32 genetic sequences, with gravel, sand and mud constituting 0.7–2.1%, 45–62% and 38–54% of the total sediment mass, respectively (Figure 9B). Given the large uncertainty in net-depositional sediment budget (Figure 9A; Okwara et al., 2023), it appears that the grain-size fractions supplied to the ‘Brent Delta’ sediment routing system(s) remained approximately uniform throughout its deposition. The mean grain-size fractions of the total sediment mass for the J22, J24, J26 and J32 genetic sequences in combination are 1.0% gravel, 55% sand and 44% mud.

Figure 9. (A) Net-depositional sediment budget for the four genetic sequences of the ‘Brent Delta’ sediment routing system(s) (Figure 3; after Okwara et al., 2023). Genetic sequences are shown from oldest (J22, left) to youngest (J32, right). Black line shows the median (P50) value and grey shading shows the 10th to 90th percentile range (P10-P90) for each genetic sequence, accounting for uncertainties in sediment volumes, volume-to-mass conversions and genetic sequence durations (Okwara et al., 2023). (B) Proportion of grain-size classes (gravel, sandstone and mudstone) by mass in genetic sequences J22, J24, J26 and J32, based on mapped sediment volumes (Figures 5-8) that are subdivided into facies proportions in 84 representative wells (Figures 4, S1; see Supplementary Material for details). Each facies is assigned a specific grain size composition (Table 1), and volume-to-mass conversions based on density log data are applied to each grain-size class (2400 kg m-3 = gravel conglomerate and sandstone bulk density, 2500 kg m-3 = mudstone bulk density; Okwara et al., 2023).

4.1.2. Interpretation

Given the similarity in the gravel, sand and mud grain-size fractions of the net-depositional sediment mass in the J22, J24, J26 and J32 genetic sequences, combined with the apparent regional consistency in heavy mineral provenance characteristics between these genetic sequences (Mearns, 1992; Morton, 1992), we speculate that sediment of a relatively uniform grain-size composition may have been supplied via sediment routing systems from the Shetland Platform, Norwegian Landmass and Mid North Sea High source areas. However, we lack detailed, quantitative petrographic and geochemical data to rigorously test this speculation (cf. Caracciolo, 2020). The similarity in grain-size fractions of the net-depositional sediment mass in each genetic sequence (Figure 9B) implies either that: (1) the relative proportions of sediment supplied are insensitive to increases in catchment erosion rates, as required to account for the increase in sediment flux from J22 to J32 (Figure 9A); or (2) the excess of a particular grain-size fraction is preferentially retained in source-area catchments (e.g., gravel) or bypassed downsystem beyond the depositional limit of the ‘Brent Delta’ (e.g., mud; Okwara et al., 2023).

4.2. Downsystem changes in gravel, sand and mud fractions

4.2.1. Genetic sequence J22

4.2.1.1. Description

In genetic sequence J22, there are clear downsystem decreases in gravel and sand fractions in the deposits of sediment routing systems sourced from the Shetland Platform to the west (Figures 5, 10A; Table S3) and from the Mid North Sea High to the south (Figures 5, 10C; Table S3). Deposits of the sediment routing system sourced from the Norwegian Landmass to the east exhibit more variable upsystem-to-downsystem changes in gravel and sand fractions, including localised downsystem increases and downsystem decreases, such that there is no overall trend (Figures 5, 10B). These latter changes in sand fraction occur within shallow-marine-to-shelf environments, rather than in coastal-plain and marginal-marine environments that contain channelised sand bodies (Figure 10E).

Figure 10. Downsystem variations in (A-C) percentage-thickness of gravel, sandstone and mudstone, and (D-F) percentage-thickness of coastal-plain, marginal-marine and shallow-marine sandstones for genetic sequence J22 (cf. Figures 5, S2A-C) in the study well database (Figure 4), as a function of sediment mass extracted (χ) (Equation 1, Figure 1). Downsystem variations are shown for: (A, D) transverse transect for western basin margin, sourced from Shetland Platform; (B, E) transverse transect for eastern basin margin, sourced from Norwegian Landmass; and (C, F) axial transect for southern basin margin, sourced from Mid-North Sea High (Figure 5). Each point in the plots represents the mean value for a group of wells located at a specific downsystem distance, with the number of wells in the group indicated at the top of the plot (e.g., “n=3”). The vertical error bar for each point shows the variation in percentage-thickness values between wells in the group.

4.2.1.2. Interpretation

Downsystem decreases in gravel and sand fractions in deposits of the sediment routing systems sourced from the Shetland Platform and Mid North Sea High occur at χ = 0.5 and χ = 0.6, respectively (Figures 10A, C), and are consistent with downsystem-fining due to sediment mass extraction by deposition (e.g., simple eastward pinchout of facies association 2.2, 3.1 and 3.2 sandstones in genetic sequence J22, Figure S2A) (Strong et al., 2005; Paola & Martin, 2012). The more complex changes in sand fraction in deposits of the sediment routing system sourced from the Norwegian Landmass (Figure 10B) are attributed to preferential accumulation of sand in fan deltas (facies association 2.2, Table 1) located in antecedent half-graben depocentres (Figure 5A; Graue et al., 1987; Helland-Hansen et al., 1992; Muto & Steel, 1997). Sediment routing from the Norwegian Landmass to these fan deltas was likely controlled by the three-dimensional topography associated with the antecedent half-graben, and is poorly approximated by the two-dimensional, proximal-to-distal transect used to characterise grain-size distribution (e.g., complex westward and eastward pinch-outs of facies association 2.2 and 3.1 sandstones in genetic sequence J22, Figure S2B).

4.2.2. Genetic sequence J24

4.2.2.1. Description

In genetic sequence J24, downsystem changes in grain-size fractions are considered in the context of a single, composite sediment routing system sourced from the Shetland Platform, Mid North Sea High and Norwegian Landmass (Figures 6, 11A; Table S4). The deposits of the composite sediment routing system exhibit a downsystem increase in sand fraction (from χ = 0 to χ = 0.3, Figure 11A), followed by a relatively uniform sand fraction (from χ = 0.3 to χ = 0.9, Figure 11A) and a rapid downsystem decrease in sand fraction (from χ = 0.9 to χ = 1.0, Figure 11A). The changes in sand fraction reflect an overall downsystem decrease in the proportion of coastal-plain and marginal-marine sand (from χ = 0 to χ = 0.9, Figure 11B) and a corresponding overall downsystem increase in the proportion of shallow-marine-to-shelf sand (from χ = 0.1 to χ = 0.9, Figure 11B), followed by a rapid downsystem decrease in the proportion of shallow-marine-to-shelf sand (from χ = 0.9 to χ = 1.0, Figure 11B).

Figure 11. Downsystem variations in (A) percentage-thickness of gravel, sandstone and mudstone, and (B) percentage-thickness of coastal-plain, marginal-marine and shallow-marine sandstones for genetic sequence J24 (cf. Figures 6, S2C) in the study well database (Figure 4), as a function of sediment mass extracted (χ) (Equation 1, Figure 1). Downsystem variations are shown for axial transect along basin centre, sourced from Shetland Platform, Norwegian Landmass and Mid-North Sea High (Figure 6). Each point in the plots represents the mean value for a group of wells located at a specific downsystem distance, with the number of wells in the group indicated at the top of the plot (e.g., “n=3”). The vertical error bar for each point shows the variation in percentage-thickness values between wells in the group. Key as for Figure 10.

4.2.2.2. Interpretation

These changes in sand fraction are interpreted to reflect sand accumulation predominantly in coastal-plain and marginal-marine channels in upsystem locations (from χ = 0 to χ = 0.3, Figure 11; facies association 1.1 and 2.2 sandstones in genetic sequence J24, Figure S2C), sand accumulation in a northward-prograding, wave-dominated shoreface and barrier-strandplain system (from χ = 0.3 to χ = 0.9, Figure 11; Figure S2), and palaeoseaward-decreasing sand content at the northward-progradational limit of the wave-dominated shoreface and barrier-strandplain system (from χ = 0.9 to χ = 1.0, Figure 11; facies association 3.2 and 3.3 sandstones in genetic sequence J24, Figures 6B, S2C) (Graue et al., 1987; Helland-Hansen et al., 1992; Mitchener et al., 1992; Fjellanger et al., 1996; Hampson et al., 2004).

4.2.3. Genetic sequence J26

4.2.3.1. Description

Downsystem changes in grain-size fractions in genetic sequence J26 are also considered in the context of a single, composite sediment routing system (Figures 7, 12A; Table S5). The associated deposits exhibit a relatively uniform sand fraction over much of their upsystem-to-downsystem extent (from χ = 0 to χ = 0.9, Figure 12A), but with two localised maxima (at χ = 0.1 and χ = 0.9, Figure 12A), followed by a rapid downsystem decrease in sand fraction (from χ = 0.9 to χ = 1.0, Figure 12A).

Figure 12. Downsystem variations in (A) percentage-thickness of gravel, sandstone and mudstone, and (B) percentage-thickness of coastal-plain, marginal-marine and shallow-marine sandstones for genetic sequence J26 (cf. Figures 7, S2C) in the study well database (Figure 4), as a function of sediment mass extracted (χ) (Equation 1, Figure 1). Downsystem variations are shown for axial transect along basin centre, sourced from Shetland Platform, Norwegian Landmass and Mid-North Sea High (Figure 7). Each point in the plots represents the mean value for a group of wells located at a specific downsystem distance, with the number of wells in the group indicated at the top of the plot (e.g., “n=3”). The vertical error bar for each point shows the variation in percentage-thickness values between wells in the group. Key as for Figure 10.

4.2.3.2. Interpretation

In most upsystem-to-downsystem locations (from χ = 0 to χ = 0.9, Figure 12B), the sand fraction is interpreted to reflect sampling of coastal-plain and marginal-marine channels (Figure S2). At downsystem distances for which multiple wells are grouped (e.g., for which n ≥ 4, Figure 12), there is sufficient sampling of channels and intervening interfluves that the mean value of sand fraction is constrained between 30 and 60% (Figure 12). At downsystem distances for which only one well is present (i.e., for which n = 1, Figure 12), unrepresentatively high or low values of sand fraction are recorded (e.g., 90% in the localised peak at χ = 0.1, Figure 12) depending on whether a channel or interfluve is sampled. The rapid downsystem decrease in sand fraction in downsystem locations (from χ = 0.9 to χ = 1.0, Figure 12) is interpreted to reflect palaeoseaward-decreasing sand content at the northward-progradational limit of the wave-dominated shoreface and barrier-strandplain system (facies association 3.2 and 3.3 sandstones in genetic sequence J26, Figures 7B, S2C; Graue et al., 1987; Helland-Hansen et al., 1992; Mitchener et al., 1992; Fjellanger et al., 1996; Hampson et al., 2004).

4.2.4. Genetic sequence J32

4.2.4.1. Description

Three distinct sediment routing systems are identified in genetic sequence J32 (Figures 8, 13A-C; Table S6). Deposits of the sediment routing system sourced from the Shetland Platform to the west (Figure 8) exhibit an overall downsystem decrease in sand fraction, but with a localised minimum between χ = 0.1 and χ = 0.3, in its deposits (Figure 13A). Deposits of the sediment routing systems sourced from the Mid North Sea High to the south and from the Norwegian Landmass to the east (Figure 8) both lack simple upsystem-to-downsystem trends in gravel and sand fractions (Figure 13B, C). For example, deposits of the sediment routing system sourced from the Norwegian Landmass High (Figure 8) exhibit a downsystem increase in sand fraction (from χ = 0 to χ = 0.4, Figure 13B) followed by an irregular decrease in sand fraction (from χ = 0.4 to χ = 1.0, Figure 13B). The changes in sand fraction occur within shallow-marine-to-shelf environments, rather than in coastal-plain and marginal-marine environments, which are less abundant in the deposits of this sediment routing system (Figure 13E). The sand fraction of deposits of the sediment routing system sourced from the Mid North Sea (Figure 8) increases downsystem overall, but this increase is subtle and masked by shorter-period changes (Figure 13C). This overall trend is composed of an irregular downstream decrease in the sand fraction within coastal-plain and marginal-marine channels (Figure 13F), combined with a downstream increase in the sand fraction within shallow-marine-to-shelf environments (from χ = 0.3 to χ = 1.0, Figure 13F).

Figure 13. Downsystem variations in (A-C) percentage-thickness of gravel, sandstone and mudstone, and (D-F) percentage-thickness of coastal-plain, marginal-marine and shallow-marine sandstones for genetic sequence J32 (cf. Figures 8, S2A-C) in the study well database (Figure 4), as a function of sediment mass extracted (χ) (Equation 1, Figure 1). Downsystem variations are shown for: (A, D) transverse transect for western basin margin, sourced from Shetland Platform; (B, E) transverse transect for eastern basin margin, sourced from Norwegian Landmass; and (C, F) axial transect for southern basin margin, sourced from Mid-North Sea High (Figure 8). Each point in the plots represents the mean value for a group of wells located at a specific downsystem distance, with the number of wells in the group indicated at the top of the plot (e.g., “n=3”). The vertical error bar for each point shows the variation in percentage-thickness values between wells in the group. Key as for Figure 10.

4.2.4.2. Interpretation

We attribute the upsystem-to-downsystem changes in sand fraction in deposits of the sediment routing systems sourced from the Shetland Platform and Norwegian Landmass (Figure 13) to the initiation and growth of syn-depositional faults that are interpreted to have generated complex, three-dimensional patterns in sediment routing and accumulation (Figure 8; Helland-Hansen et al., 1992; Mitchener et al., 1992; Fjellanger et al., 1996; Davies et al., 2000; McLeod et al., 2002; Hampson et al., 2004). These complex, three-dimensional patterns are poorly approximated by the two-dimensional, proximal-to-distal transects that we have used to characterise grain-size distribution (e.g., complex westward and eastward thickness variations and pinchouts of facies association 3.1-3.4 sandstones in genetic sequence J32, Figure S2A, B). In the deposits of the sediment routing system sourced from the Mid North Sea High, upsystem-to-downsystem changes in the sand fraction are attributed to sparse sampling of coastal-plain and marginal-marine channels in well data (n ≤ 3 at all downsystem distances, Figure 13C, F; facies association 1.1 and 2.2 sandstones in genetic sequence J32, Figure S2A) combined with sand accumulation in a series of retrogradationally stacked, southward-retreating, wave-dominated and weakly wave-influenced shoreface systems (from χ = 0.3 to χ = 1.0, Figure 13F; facies association 2.2 and 3.1-3.4 sandstones in genetic sequence J32, Figure S2A, B).

5. Discussion

Paola & Martin (2012) argued that simple models of geomorphic relationships and downsystem grain-size fining in a mass-balance framework can provide a useful reference for interpretation of complex, real-world systems that deviate from the simple reference state. Models of closed sediment routing systems (i.e., with no sediment mass transfer to or from neighbouring systems) indicate that grain size fines downsystem, when normalised by the total sediment mass, due to sediment mass extraction by deposition (Strong et al., 2005; Paola & Martin, 2012). Most of the ‘Brent Delta’ sediment routing systems do not each exhibit a simple downsystem decrease in the proportion of facies associations of a given grain size (Figures 10A–C, 11A, 12A, 13A–C), and thus differ from reference models.

5.1. Why do the ‘Brent Delta’ deposits not exhibit a simple downsystem-fining trend in the facies-association proxy for grain size?

Although some of the sediment routing systems that we delineate in the ‘Brent Delta’ deposits exhibit a relatively simple downsystem-fining trend in the proportion of facies associations of a given grain size (Figure 10A, C), most show more complex upsystem-to-downsystem patterns in the facies-association proxy for grain size (Figures 10B, 11A, 12A, 13A–C). We identify three reasons for these complex patterns.

First, locally variable, irregular sand fractions in upsystem locations are attributed to sampling of laterally discontinuous, channelised sand bodies in coastal-plain and marginal-marine environments (facies associations 1.1 and 2.2, Table 1) by sparse, relatively widely spaced wells (from χ = 0 to χ = 0.3 in Figure 11; from χ = 0 to χ = 0.9 in Figure 12; from χ = 0 to χ = 1.0 in Figure 13C, F) (Figure 14). Such local variations may be mitigated by increasing the number of wells, where sufficient well data are available, and thus sampling more representatively the channelised sand bodies and their intervening interfluves.

Figure 14. Conceptual model synthesising sediment source regions, sediment influxes and effluxes, depositional environments, and downsystem variations in gravel (orange), sandstone (yellow) and mudstone (grey) in the axial ‘Brent Delta’ sediment routing system. See text for discussion.

Second, locally variable sand fractions in non-channelised, marginal-marine and shallow-marine-to-shelf environments (facies associations 2.2 and 3.1-3.4, Table 1) in genetic sequences J22 and J32 are attributed to infilling of rift-related, underfilled antecedent or syn-depositional topography with a complex spatial distribution (Figures 5A, 8A; from χ = 0 to χ = 1.0 in Figures 10B, E, 13A–B, D-E). Sediment routing and accumulation in the three-dimensionally complex topography is sparsely sampled by the two-dimensional, proximal-to-distal transect lines in Figures 5 and 8, which results in pronounced local minima and maxima in sand fraction. The resulting upsystem-to-downsystem patterns in gravel, sand and mud fractions are consistent with established interpretations of laterally stacked fan deltas (facies association 2.2, Table 1) in antecedent half-graben depocentres in genetic sequence J22 (Figure 5A; Graue et al., 1987; Helland-Hansen et al., 1992; Muto & Steel, 1997) and of shoreface sandstones (facies associations 3.1-3.3, Table 1) in growing, underfilled half-graben depocentres in genetic sequence J32 (Figure 8A; Helland-Hansen et al., 1992; Mitchener et al., 1992; Fjellanger et al., 1996; Davies et al., 2000; McLeod et al., 2002; Hampson et al., 2004). Where topographic steering of sediment routing systems is clearly defined (e.g., imaged in high-resolution seismic data) and well distribution allows, such under sampling may be mitigated by orienting transects along the axes of the sediment routing systems.

Third, downsystem-increasing or relatively consistent sand fractions in non-channelised, wave-dominated shallow-marine-to-shelf environments (facies associations 2.2 and 3.2-3.4, Table 1) are attributed to prograding or retrograding shoreface and barrier-strandplain systems that are not restricted to underfilled antecedent or syn-depositional depocentres (from χ = 0.3 to χ = 0.9 in Figure 11; from χ = 0.3 to χ = 1.0 in Figure 13C, F) (Figure 14). The progradational limit of such wave-dominated shoreface and barrier-strandplain systems is marked by an abrupt downsystem decrease in sand fraction (from χ = 0.9 to χ = 1.0 in Figure 11; from χ = 0.9 to χ = 1.0 in Figure 12) (Figure 14). These upsystem-to-downsystem patterns in the facies-association proxy for grain size are consistent with established interpretations of wave-dominated, shoreface and barrier-strandplain deposits in genetic sequences J24, J26 and J32 (Figures 6A, 7A, 8A; Graue et al., 1987; Helland-Hansen et al., 1992; Mitchener et al., 1992; Fjellanger et al., 1996; Hampson et al., 2004). Such wave-dominated shoreface and barrier-strandplain deposits are associated with nearshore retention of sand by shoaling fairweather waves (i.e., the ‘littoral energy fence’ of Swift & Thorne, 1991) and significant shoreline-parallel sediment transport, for example in response to oblique wave approach (e.g., Komar, 1976), as noted in previous interpretations of wave-dominated shoreface, strandplain and barrier island deposits in the ‘Brent Delta’ (Figure 14; e.g., Mearns, 1992; Morton, 1992; Scott, 1992; Løseth & Ryseth, 2003; Went et al., 2013).

For each of these three reasons, spatial facies partitioning obscures downsystem-fining trends, particularly in the point-based observational data provided by wells. Incorporating additional wells may mitigate these effects to an extent, provided that they sample representatively the underlying facies distributions. However, spatial facies partitioning produced by processes that systematically redistribute sediment (e.g., waves and tides in shoreface-shelf settings) are still likely to distort simple downsystem-fining trends. Accounting for these processes requires mapping potentially complex sediment routing and dispersal patterns, as preserved in stratigraphic architectures, and integrating these patterns across all grain-size fractions. In the ‘Brent Delta’ sediment routing systems, this approach would require adding the bypassed sediment mass to those characterised herein (Figures 10-13). If such careful analysis of sediment routing and grain-size fractionation were carried out, sediment mass balance theory predicts that downstream extraction of sediment in χ space should recover a simple downsystem-fining trend, as documented in physical experiments (e.g., Strong et al., 2005; Paola & Martin, 2012).

5.2. Can downsystem changes in the facies-association proxy for grain-size fractions constrain the budget of reworked or bypassed sediment mass?

Source-area sediment budgets, which describe sediment influx from source-area catchments to the ‘Brent Delta’, are almost one order-of-magnitude larger than the net-depositional sediment budgets for the ‘Brent Delta’ sediment routing systems (Okwara et al., 2023). This discrepancy can potentially be accounted for by two mechanisms. First, it may have arisen because a large sediment mass underwent net export (or bypass) from the ‘Brent Delta’ sediment routing systems northwestward to the Faroe-Shetland Basin and/or northward to the western Møre Basin. Second, erosion and sediment reworking at unconformities (e.g., at the base of the J22 genetic sequence, or between the J26 and J32 genetic sequences; Mitchener et al., 1992; Fjellanger et al., 1996; Hampson et al., 2004; Figure 3) may have reduced the net-depositional sediment budget. Below we consider the likely grain-size distributions released from the source-area catchments and by erosional reworking at unconformities, how these grain-size distributions of the sediment supply compare to downsystem changes in grain-size fractions in the ‘Brent Delta’ deposits, and the implications for the volume and grain-size characteristics of sediment that was potentially bypassed to the Faroe-Shetland Basin and/or western Møre Basin.

The proportion of gravel, sand and mud generated in, and exported from, catchments reflect their topography, exposed bedrock lithologies, climate, and sediment residence time (e.g., Palomares & Arribas, 1993; Arribas & Tortosa, 2003; Whittaker et al., 2010; Allen et al., 2015; Caracciolo, 2020; Watkins et al., 2020; Heins, 2023). In the source-area catchments that supplied sediment to the ‘Brent Delta’, the climate was sub-tropical and humid throughout the Middle Jurassic (Abbink et al., 2001; Sellwood & Valdes, 2006, 2008; Prokoph et al., 2008). Catchments on the Norwegian Landmass were composed of Precambrian gneisses and Caledonian metamorphic and granitic rocks (Morton et al., 2004), which probably generated grain-size distributions with a high proportion of medium-to-very-coarse sand and gravel (Palomares & Arribas, 1993; Allen et al., 2015; Figure 14). Shetland Platform catchments were composed of Devonian, Carboniferous and Permo-Triassic sedimentary rocks (Morton et al., 2004) that likely generated grain-size distributions with variable proportions of sand and gravel; high proportions of fine-to-coarse sand for reworked sandstones and relatively low proportions of sand for reworked mudstones and limestones (Arribas & Tortosa, 2003; Allen et al., 2015; Figure 14). Sandstone-dominated Triassic and mudstone-dominated Lower Jurassic sedimentary rocks were eroded from catchments on the Mid North Sea High (Underhill & Partington, 1993; Okwara et al., 2023), and probably generated grain-size distributions with high and low proportions of fine-to-coarse sand, respectively (Arribas & Tortosa, 2003; Allen et al., 2015; Figure 14). The grain-size distributions generated by fragmentation and weathering (Palomares & Arribas, 1993; Arribas & Tortosa, 2003; Allen et al., 2015) are modified by the duration and intensity of physical and chemical weathering during transport and storage (e.g., Heins & Kairo, 2007), although the resulting grain sizes retain a log-normal distribution (Allen et al., 2017).

Erosion at the base of fluvio-estuarine channels generated a sub-regional unconformity at the base of the J22 genetic sequence (Mitchener et al., 1992; SB177 of Fjellanger et al., 1996; SB100 of Hampson et al., 2004; Figure 3). The unconformity is cut into marine mudstones of the Dunlin Group, and the volume of eroded mudstones is less than that of marginal-marine sandstones in the J22 genetic sequence (i.e., <190 km3). This volume corresponds to a reworked-sediment mass of <480 Mt and reworked-sediment flux of <0.12 Mt/yr, less than one-third of the net-depositional sediment flux estimated for the J22 genetic sequence (0.43 Mt/yr; Okwara et al., 2023). The base of the J32 genetic sequence is marked by a sub-regional unconformity on the western margin of the Viking Graben, marked by erosion of up to 40 m (typically c. 20 m) of sandstones and mudstones over an area of approximately 40 km2 (SB1000 of Hampson et al., 2004; Figure 3). The resulting volume of reworked sediment is c. 0.8 km3, corresponding to a reworked-sediment mass of c. 2 Mt and reworked-sediment flux of c. 2.2 x 10-6 Mt/yr; this estimated flux is insignificant compared to the net-depositional sediment fluxes estimated for the J26 and J32 genetic sequences (3.1 Mt/yr and 4.1 Mt/yr, respectively; Okwara et al., 2023). In summary, erosional reworking at unconformities can account for very little of the discrepancy between source-area and net-depositional sediment budgets, and we instead invoke sediment bypass.

The downsystem-increasing or relatively consistent sand fractions in wave-dominated shoreface and barrier-strandplain deposits of genetic sequences J24 and J26 (from χ = 0.9 to χ = 1.0 in Figure 11; from χ = 0.9 to χ = 1.0 in Figure 12) support the nearshore retention of sand within the ‘littoral energy fence’ (Swift & Thorne, 1991) and corresponding preferential offshore transport of mud. Thus, the sedimentological characteristics of the high sand fraction of the net-depositional sediment budgets (45-62% in genetic sequences J22-J32; Figure 9B) are also consistent with significant mud bypass to the Faroe-Shetland Basin and/or western Møre Basin (Figure 14). In order to balance the sediment mass supplied from source-area catchments with the net-depositional sediment budgets of the ‘Brent Delta’ deposits solely by trapping gravel and sand within the ‘littoral energy fence’, a sediment supply of approximately 0.02 Mt/yr of gravel, 1.3 Mt/yr of sand and 18.7 Mt/yr of mud is required, with 17.7 Mt/yr of this mud being exported to the Faroe-Shetland Basin and/or western Møre Basin (i.e., only 1.0 Mt/yr of mud is accounted for in the net-depositional sediment mass; Figure 9B). This is an end-member estimate, because net-import of sand to wave-dominated shoreface and barrier-strandplain deposits via shoreline-parallel sediment transport, and/or net-export of mud by the same mechanism, can also contribute to balancing the source-area sediment budgets with the net-depositional sediment budgets. Estimates of sediment supply from the source areas may also be systematic underestimates, because the BQART model considers only suspended sediment load (see Okwara et al., 2023 for discussion). Notwithstanding the assumptions and uncertainties in our analysis, similarly large proportions of sediment bypass, associated with differential transport of sand and mud fractions, are noted in other source-to-sink studies (e.g., Angoche margin, Heins, 2023).

We consider that a combination of: (1) export of variably sandy sediment from the Shetland Platform, Norwegian Mainland and Mid North Sea High source-area catchments; (2) retention of sand in wave-dominated shoreface and barrier-strandplain deposits by the ‘littoral energy fence’; and (3) net-import of sand and/or next-export of mud by wave-generated longshore currents can account for much of the discrepancy between source-area sediment budgets and net-depositional sediment budgets of the ‘Brent Delta’ sediment routing systems (Figure 14; Okwara et al., 2023).

5.3. Implications for analysis of sediment routing systems in the stratigraphic record

The integration of downsystem variations in grain size (e.g., Reynolds, 2019), or a facies-association proxy for grain-size fractions with sediment flux estimates (e.g., Brewer et al., 2020) provides a means to interpret sediment supply as a control on stratigraphic architecture. Such interpretations are not straightforward, and need to be guided by a conceptual model of the sediment routing system(s) that explicitly accounts for spatial facies partitioning. For example, the facies model of a wave-dominated delta (‘Brent Delta’) is required to interpret retention of sand in shoreface and barrier-strandplain facies by the ‘littoral energy fence’ (Swift & Thorne, 1991); this nearshore sand retention significantly modifies the simple downsystem-fining trend identified as a reference by Paola & Martin (2012) (Figure 14). In addition, this facies model implies alongshore sediment transport that is consistent with heavy mineral provenance data (Morton, 1992). It also implies that the ‘Brent Delta’ sediment routing system(s) was not closed, and that the mapped sediment volumes do not account for the full sediment budget (cf. Angoche margin, Heins, 2023). Other deviations from the reference of a simple downsystem-fining grain-size trend are attributed to preferential trapping of sand and gravel by antecedent and growing, syn-depositional topography, and to sparse, unrepresentative sampling of channelised sand bodies and their coeval sand-poor interfluves. The former may be mitigated by organising and analysing well data along the axes of the sediment routing systems, where the spatial distribution of well data allows. The latter may be mitigated by increasing the number of wells, where sufficient well data are available.

Despite potential ambiguity in interpretations, we consider that the analysis of downsystem grain-size variations in a sediment mass balance context provides insights that constrain sediment supply as a control on stratigraphic architecture. In the context of the ‘Brent Delta’ sediment routing system(s), downsystem grain-size variations support the interpretation that large volumes of predominantly muddy sediment were bypassed beyond the down-dip limit of the wave-dominated delta front into the Faroes-Shetland Basin and Møre Basin (Figures 2, 14). This interpretation is consistent with estimates of sediment supply derived from the BQART sediment-load model (Okwara et al., 2023). In contrast, sequence stratigraphic interpretations are inconsistent, and have proposed either the presence (e.g., Johannessen et al., 1995; Olsen & Steel, 1995) or absence of down-dip sediment bypass from the ‘Brent Delta’ (e.g., Went et al., 2013).

6. Conclusions

We reconstruct upsystem-to-downsystem variations in grain size in the sediment routing systems of the Middle Jurassic (Aalenian – Bathonian) Brent Group and coeval strata of the Northern North Sea in a sediment mass balance framework, in order to constrain sediment supply to the ‘Brent Delta’. ‘Brent Delta’ deposits are subdivided into four previously defined, age-constrained stratigraphic intervals (J22, J24, J26 and J32 genetic sequences), and sediment routing systems delineated in each genetic sequence. Facies associations are used as a textural proxy for gravel, sand and mud grain-size fractions in the net-depositional sediment mass of each sediment routing system. Upsystem-to-downsystem variations in the facies-association proxy for grain-size fractions are normalised with respect to depositional sediment mass, so that they can be readily compared with each other.

A simple downsystem-fining trend in grain size, the reference case predicted by theory and physical experiments, is rare in the ‘Brent Delta’ sediment routing systems for three reasons. (1) Locally variable, irregular sand fractions in upsystem locations reflect sparse sampling of laterally discontinuous, channelised sand bodies in coastal-plain and marginal-marine environments. (2) Locally variable sand fractions in non-channelised, marginal-marine and shallow-marine-to-shelf environments in transverse sediment routing systems in the J22 and J32 genetic sequences are attributed to preferential trapping of sand in underfilled antecedent and syn-depositional, half-graben depocentres that are poorly represented in simple upsystem-to-downsystem transects. (3) Downsystem-increasing and relatively uniform sand fractions in non-channelised, wave-dominated shallow-marine-to-shelf environments in the J24 and J26 genetic sequences correspond to prograding or retrograding shoreface and barrier-strandplain systems, in which sand was retained near the shoreline by shoaling fairweather waves. By inference, wave-driven alongshore sediment transport was common in the ‘Brent Delta’ sediment routing system(s), which were not closed systems. This result demonstrates that spatial facies partitioning due to shallow-marine process regime is sufficient to distort the simple downsystem-fining trends predicted by sediment mass balance theory.

The ‘Brent Delta’ deposits in each genetic sequence have a similar composition by mass: 0.7-2.1% gravel, 45-62% sand and 38-54% mud. These compositions are consistent with net input of sand to and/or net export of mud from the ‘Brent Delta’ sediment routing system(s) by alongshore sediment transport. Our reconstructions of upsystem-to-downsystem variations in the facies-association proxy for grain size thus support the interpretation that large volumes of predominantly muddy sediment were bypassed beyond the down-dip limit of the ‘Brent Delta’, as implied by previous application of the BQART sediment-load model to the Brent Group sediment routing systems.

7. Acknowledgements

We thank Robert Duller, two anonymous reviewers, two anonymous Associate Editors and Editor Peter Burgess for their critical reviews and editorial assessments, which greatly improved the manuscript. We also thank Christopher Brewer, Christian Haug Eide, John Holbrook, Christopher Jackson, Howard Johnson and Oliver Jordan for constructive and encouraging discussions of this work, and the British Geological Survey (Nottingham, United Kingdom) and the Norwegian Petroleum Directorate (Stavanger, Norway) for permission to access subsurface core repositories and well data. Our research was supported by the Petroleum Technology Development Fund of Nigeria through a scholarship grant to ICO (project grant number: PTDF/ED/PHD/OIC/848/16).

8. Conflict of interest

The authors declare no conflict of interest.

9. Data availability statement

The data that support the findings of this study are publically available from the sources listed below: core, offshore UK can be viewed at the National Geological Repository, British Geological Survey (https://www.bgs.ac.uk/geologicaldata/nationalgeologicalrepository/); wireline logs, offshore UK from the UK National Data Repository, North Sea Transition Authority (https://ndr.nstauthority.co.uk); core, offshore Norway can be viewed at the Geobank, Norwegian Petroleum Directorate (https://www.npd.no/en/facts/geology/geobank/); wireline logs, offshore Norway from the Diskos Well Database, Norwegian Petroleum Directorate (https://www.npd.no/en/diskos/wells/); all other data are taken from the publications cited and listed in this paper. All calculations of upsystem-to-downsystem sediment mass are available in the supplementary files.

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Conceptual model synthesising sediment source regions, sediment influxes and effluxes, depositional environments, and downsystem variations in gravel (orange), sandstone (yellow) and mudstone (grey) in the axial ‘Brent Delta’ sediment routing system.

Published

2025-01-21

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How to Cite

Okwara, I. C., Hampson, G. J., Whittaker, A. C., & Roberts, G. G. (2025). Downsystem grain-size trends and mass balance of an ancient wave-influenced sediment routing system: Middle Jurassic Brent Delta, northern North Sea, offshore UK and Norway. Sedimentologika, 3(1). https://doi.org/10.57035/journals/sdk.2025.e31.1336