Anatomy of shelf deltas at the edge of a prograding Eocene shelf margin, Spitsbergen

Abstract Although shelf‐edge deltas are well‐imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand‐prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf‐edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand‐prone, shelf‐margin clinoforms. The shelf‐edge deltas of the Battfjellet Formation on Litledalsfjellet and Høgsnyta produced a 3–5 × 15 km, shelf edge‐attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth‐bar deposits in its shelf‐edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short‐headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf‐edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane‐parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flat‐laminated intervals. The examined shelf‐edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity‐flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward‐directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive‐to‐transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf‐margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re‐established above the shelf edge.     

Clinoform geometry,geomorphology, facies character and stratigraphic architecture of a sand‐rich subaqueous delta: Jurassic Sognefjord Formation,offshore Norway

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta‐scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud‐prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta‐scale subaqueous clinoforms are common in recent tide‐influenced, wave‐influenced and current‐influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient delta‐scale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super‐giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening‐upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave‐dominated or current‐dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic ‘envelope’ of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform ‘envelope’ rolls over to define a topset–foreset–toeset geometry. All clinoforms are consistently oriented sub‐parallel to the edge of the Horda Platform (N005–N030). In the eastern half of the field, individual foresets are relatively gently dipping (1° to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine‐grained, hummocky cross‐stratified sandstones. Towards the west, clinoforms gradually become steeper (5° to 14°) and bound thicker (15 to 60 m) clinothems that comprise medium‐grained, cross‐bedded sandstones. Topsets are consistently well‐developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near‐linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north‐east and sculpted by the action of currents sub‐parallel to the clinoform strike.     

Trajectory analysis and inferences on geometric relationships of an Early Triassic prograding clinoform succession on the northern Barents Shelf

A sedimentary succession studied along three parallel seismic lines details a platform-edge progradation of 21–36 km in a northwesterly direction across the northwestern Barents Shelf. The intra-shelf clinoform succession is bounded at bottom and top by Base Olenekian and Early Ladinian seismic reflectors. The ca 800 m thick succession can be resolved into seven distinct clinothems. The system is characterized by an early sub-horizontal platform-edge trajectory with extensive progradation, limited relative sea level rise and restricted accommodation. Thereafter the system outlines a largely ascending trajectory, marking a major rise in relative sea level and creation of significant accommodation. The platform-edge appears to back-step along one line suggesting that relative sea level rise out-paced sediment influx and preserved a clinothem with a trajectory characterized by accretionary transgression. Thereafter the trajectory is overall ascending regressive, with some variation of the trajectory angle, culminating in a flat and finally descending trajectory with oblique clinoforms outlining extensive progradation and another period of limited accommodation. The clinoforms downlap onto a succession of basin-floor deposits which appear to comprise at least two separate periods of deposition, forming two separate units. The first five clinothems downlap onto the first basin-floor unit. The shift to downlap onto the second unit occurs around the second period of extensive platform-edge advance, suggesting limited accommodation promoted bypass of significant amounts of sediment to the basin floor.The Gardarbanken High has been considered an obstacle to Early Triassic sediment progradation in this part of the basin. This inference can be corroborated based on the seismic attributes, which show sediment infill and onlap near the High. The influence is also noticeable in the reduced slope relief near the High, indicating that the basin floor was topographically higher. However, other geometric attributes cannot provide any definitive measures of structural influence.The thickness of preserved topsets and the distance from the platform-edge to the toe pinch-out point of each clinothem is found to be inversely proportional. This relationship is most marked in the fully developed sigmoidal clinoforms, whereas the link appears weaker in the oblique clinoforms. A near-perfect correlation between clinothem average vertical thickness (the average sedimentary rock accumulation within the clinothem) and advance of the toe is found, with only a relatively close relationship between clinothem average vertical thickness and advance of the platform-edge. In the studied system it therefore appears the advance of the toe is governed solely by sediment influx while the advance of the platform-edge is also influenced by relative sea level.     

Synthetic seismic illumination of small-scale growth faults,paralic deposits and low-angle clinoforms: A case study of the Triassic successions on Edgeøya,NW Barents Shelf

Fault activity and sandstone-body geometries and spread, all bear significant weight to understanding the potential hydrocarbon systems on the NW Barents Shelf. Synthetic seismic modelling of onshore sedimentary successions provides insight into the seismic resolution and expression of various sedimentological features on Edgeøya: (i) sandy growth basins in pro-delta mudstones (ii) paralic sedimentary deposits and (iii) low-angle tidally influenced progradational successions.Synthetic modelling suggests that the lithological contrast associated with sand-infilled growth basins in pro-deltaic shales, even with offsets as small as 50–75 m, will create distinct geometries in seismic data. Modelling suggests that while optimal 90° illumination, low frequency bands and more typical sedimentary velocities around 2000 m/s will generate very clear discernible growth faults, the angular lithological contrast should generate discernible features even with the high velocities and typical 20–30 Hz frequency band of seismic sections on the NW Barents Shelf. Comparing to actual seismic data in which multiples, noise and in places overburden are influential, it is possible to identify growth-fault geometries more confidently, and to link them to larger planar fault activity.Modelling other features identified in the paralic sedimentary system it is apparent that many of the massive channel or lenticular shaped sandstone bodies should be identifiable in the actual seismic, although their expression is less distinct and more easily misinterpreted than that of growth faults. It is apparent that features such as igneous intrusions, unless imaged in a “perfect” survey, can be difficult to properly identify, particularly near-vertical connections. The velocity contrast creates strong impedance along horizontal sections, but heavily fractured igneous intrusions with lower velocities could easily be assumed to be isolated sandstone bodies. While the modelling appreciates the overall wedging nature of the successions, the simplified lithological observations onshore cannot predict the probable erosion/condensation contrast associated with low-angle clinoforms which are visible in the seismic data, and hence not reproduce them easily in the models.     

The Lower Cretaceous succession of the northwestern Barents Shelf: Onshore and offshore correlations

The Lower Cretaceous succession in the Barents Sea is listed as a potential play model by the Norwegian Petroleum Directorate. Reservoirs may occur in deep to shallow marine clastic wedges located in proximity to palaeo-highs and along basin margins. In addition, shelf-prism-scale clinoforms with high amplitude anomalies in their top- and bottomsets have been reported from reflection seismic but they have never been drilled. In Svalbard, the exposed northwestern corner of the Barents Shelf, Lower Cretaceous strata of shelfal to paralic origin occur, and includes the Rurikfjellet (Valanginian–Hauterivian/lowermost Barremian), Helvetiafjellet (lower Barremian–lower Aptian) and Carolinefjellet formations (lower Aptian–middle Albian). By combining sedimentological outcrop studies and dinocyst analyses with offshore seismic and well ties, this study investigate the link between the onshore strata and the offshore clinoforms. Age-vise, only three (S1–S3) of the seismic sequences defined in the offshore areas correlate to the onshore strata; S1 correspond to the Rurikfjellet Formation, S2 to the Helvetiafjellet Formation and the lower Carolinefjellet Formation, and S3 to the upper Carolinefjellet Formation. Offshore, all three sequences contain generally southward prograding shelf-prism-scale clinoforms. A lower Barremian subaerial unconformity defines the base of the Helvetiafjellet Formation, and its extent indicates that most of the Svalbard platform was exposed and acted as a bypass zone in the early Barremian. Onshore palaeo-current directions is generally towards the SE, roughly consistent with the clinoform accretion-direction towards the S. The local occurrence of a 150 m thick succession of gravity flow deposits transitionally overlain by prodelta slope to delta front deposits in the Rurikfjellet Formation, may indicate that shelf-edges also developed in Svalbard. The late Hauterivian age of theses deposits potentially highlights the inferred offlapping nature of the Lower Cretaceous strata as they predate the lower Barremian unconformity, and thus record a hitherto unknown regression in Svalbard. The presence of the lower Barremian subaerial unconformity in Svalbard, the general southeastward palaeo-current directions, and the age-equivalent clinoform-packages south of Svalbard, suggests that the onshore and offshore strata is genetically linked and was part of the same palaeo-drainage system.     

Defining the shelf edge and the three‐dimensional shelf edge to slope facies variability in shelf‐edge deltas

Shelf‐edge deltas play a critical role in shelf‐margin accretion and deepwater sediment delivery, yet much remains to be understood about the detailed linkage between shelf edge and slope sedimentation. The shelf edge separates the flat‐lying shelf from steeper slope regions, and is observable in seismic data and continuous outcrops; however, it is commonly obscured in non‐continuous outcrops. Defining this zone is essential because it segregates areas dominated by shelf currents from those governed by gravity‐driven processes. Understanding this linkage is paramount for predicting and characterizing associated deepwater reservoirs. In the Tanqua Karoo Basin, the Permian Kookfontein Formation shelf‐slope clinothems are well‐exposed for 21 km along depositional strike and dip. Two independent methods identified the shelf‐edge position, indicating that it is defined by: (i) a transition from predominantly shelf‐current to gravitational deposits; (ii) an increase in soft‐sediment deformation; (iii) a significant gradient increase; and (iv) clinothem thickening. A quantitative approach was used to assess the impact of process‐regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and photographic panels, and integrated with mapped key surfaces to construct a stratigraphic grid. Spatial variability in facies proportions highlights two types of shelf‐edge depositional zones within the same shelf‐edge delta. Where deposition occurred in fluvial‐dominated zones, the slope is sand rich, channelized with channels widening downslope, and rich in collapse features. Where deltaic deposits indicate considerable tidal reworking, the deposits are thin and pinch‐out close to the shelf edge, and the slope is sand poor and lacks channelization. Amplification of tidal energy, and decrease in fluvial drive on the shelf, coincides with a decrease in mouth bar and shelf‐edge collapse, and a lack of channelization on the slope. This analysis suggests that process‐regime variability along the shelf edge exercised significant control on shelf‐edge progradation, slope channelization and deepwater sediment delivery.     

River‐dominated,shelf‐edge deltas: delivery of sand across the shelf break in the absence of slope incision

Shelf‐edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un‐incised, represent the main increment of shelf‐margin growth into the basin, for that period. The three‐dimensional complexity of shelf‐edge delta systems and along‐strike variability at the shelf edge in particular, remains understudied. The Permian–Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three‐dimensional exposure (>100 km²) and therefore a unique opportunity to evaluate shelf‐edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer‐shelf areas, parasequences are characterized by undeformed, river‐dominated, storm‐wave influenced delta mouth‐bar sandstones interbedded with packages showing evidence of syn‐depositional deformation. The amount and intensity of soft‐sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth‐bar packages transport material down slope. (ii) On the upper slope, mouth‐bar and delta‐front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer‐grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river‐dominated, shelf‐edge deltas transport large volumes of sand to the upper slope, even when major shelf‐edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un‐channellized low density turbidites is distributed evenly along strike.     

Upper Oligocene–Miocene clinoforms of the foreland Austral Basin of Tierra del Fuego, Argentina: Stratigraphy, depositional sequences and architecture of the foredeep deposits

The Upper Oligocene–Miocene deposits of the foreland Austral Basin of Tierra del Fuego represent the youngest foredeep fill, developed in front of the adjacent fold and thrust belt. They consist of superbly exposed, sub-horizontal clastic successions of more than 600 m of sedimentary thickness. The study of 11 sections by means of facies analysis and sequence stratigraphic criteria enabled the identification of five depositional sequences (SI–SV), bounded by unconformities (dI-dV) involving hiatuses of different magnitudes. The basal sequence (SI) includes two members: A, mudstone dominated, deposited by cohesive flows; and B, glauconite-rich, sandstone dominated, deposited by episodic turbidity currents. The remaining sequences (SII–SV) are composed of complex arrangements of fine conglomerates, coarse- to fine-grained sandstones, and mudstones that were deposited mainly by hyperpycnal flows. The basal unconformities of the SI to SIV involve minor hiatuses, while that of the SV is a major order unconformity. Two types of clinofom geometries are recognized in the foredeep sequences. Type a clinoforms present a wedge shaped geometry and characterize the foredeep infill during the compressional tectonic regime. Regarding this clinoform type, SI is situated closer to the orogen and shows variations in the bedding dip with development of internal unconformities. SII to SIV are situated towards the foreland and are characterized by subhorizontal conformable beds of large lateral extension. Type b clinoforms, with sigmoidal geometry, show a clear northeast progradation related to a progressive foredeep fill under tectonic quiescence. This clinoform type characterizes the deposits in SV. The recognition of hyperpycnites and different types of clinoform geometries in these sequences incorporates new concepts in reservoir prospects, which are critical for the evaluation of the petroleum system in the Austral Basin.     

Growth faults above shale – Seismic-scale outcrop analogues from the Makran foreland,SW Pakistan

The southern Makran fold-thrust belt, Pakistan, displays unique outcrop examples of well-exposed, kilometre-scale, listric growth faults that displace Miocene-age deltaic growth strata by several hundreds of metres to kilometers. The largest growth faults are counter-regional (landward-dipping), bounding major clastic depocentres exposed over areas > 1000 km². Stratal offset along these faults can exceed 1.5 km. Fault-zone thicknesses range between ca. 100 and 400 m, and average fault thickness-displacement ratios are around 1:10. High-resolution satellite data show in unprecedented detail the faults and the stratigraphic architecture of associated growth sequences, which comprise kilometre-scale progradational clinoforms, thick mudstone units and basinwards wedging sandstone-shale deposits. The true vertical thickness of the syn-kinematic record is, in places, up to 8 km, making the outcrop examples equivalent to major growth faulted successions known from seismic data of large deltas, and at least an order of magnitude larger than other outcrop examples. A comparison of the Makran outcrops with seismic-reflection examples offshore NW Borneo reveals distinct similarities in the gross depocentre geometries and internal architecture. The key control for growth faulting is interpreted to result from sedimentary loading, with rapid sedimentary progradation causing the development of rollover synclines by differential compaction and fluid expulsion, and counter-regional growth faults preferentially forming on the basinward side of these synclines. The data and interpretations presented can be used to assess the key parameters that contribute to the development of growth faults and growth successions above shale, reinforcing structural and stratigraphic observations from seismic interpretation and modelling studies in demonstrating their occurrence in exposure.