Prograding Shelf Research Articles

Postglacial evolution of the Lake Constance shelf and its relation to a 10-km long chain of submerged Neolithic mounds

A recent bathymetric survey of Lake Constance revealed ~ 170 mounds composed of loosely deposited rocks aligned in a ~ 10-km long chain along the southern Swiss shoreline in a water depth of 3–5 m. The mounds are 10–30 m in diameter and up to 1.5 m high. Over their entire length of occurrence, the mounds are estimated to be composed of ~ 60 million individual boulders, with a total weight of ~ 78,000 t. A ground penetrating radar (GPR) survey showed that the mounds are not linked to the glacial substrate but were rather deposited artificially on the edge of a prograding shelf composed of Holocene lake sediments. Here, we present the results of a coring campaign with four piston cores along a GPR transect across one of the mounds. The cores recovered the full Holocene sedimentary succession all the way into the basal till that is overlain by lacustrine sediments dating back to ~ 14,400 cal. yrs BP. The four cores are merged into a ~ 12.4-m long composite section reflecting continuous sedimentation from the siliciclastic-dominated Late Glacial to the carbonate-rich Late Holocene. The stratigraphic horizon representing the mound’s construction was radiocarbon-dated to ~ 5600–5300 cal. yrs BP, placing them in the Neolithic period. This age was confirmed by radiocarbon dating of wood samples collected during underwater excavation of the mounds. Geochemical analysis of the Holocene sedimentary succession shows generally high carbonate contents (average of 69%). The interval from 5750 to 4950 cal. yrs BP, a part of the mound period, is characterized by a Holocene minimum in carbonate content (average of 57%) and by larger mean grain sizes. Comparing these values to those from a recent surface-sediment depth transect indicates that this was a period of rather low lake levels, which might have favoured mound construction. Correlations to nearby archaeological sites and to the general West-Central European lake-level record indicates that the mounds likely were built during a short phase of low lake levels during a general trend of climatic cooling followed by a lake-level transgression.

Deep‐water sand transfer by hyperpycnal flows, the Eocene of Spitsbergen, Arctic Norway

AbstractFlood‐generated hyperpycnal flows are dense, sediment‐laden, turbulent flows that can form long‐lived, bottom‐hugging turbidity currents, which undoubtedly transport large volumes of fine‐grained sediments into the ocean. However, their ability in transferring sand into deep‐water basins is debated. This study presents sedimentological evidence of sandy hyperpycnal flow deposits (hyperpycnites) in a series of basin floor lobe complexes associated with a progradational shelf margin in the Eocene of Spitsbergen, Arctic Norway. Four coexisting types of sediment gravity flow deposits are recognized: (i) sandy hyperpycnites deposited by quasi‐steady hyperpycnal flows; (ii) turbidites deposited by waning, surge‐type turbidity currents; (iii) hybrid event beds deposited by transitional flows; and (iv) mass transport deposits emplaced during rare slope failures. The abundance of thick‐bedded massive sandstones, frequent bed amalgamation, the distribution of hyperpycnites across the lobes and the abundance and systematic occurrence of plant‐rich hybrid event beds and associated climbing ripple cross‐laminated beds in the lobe fringes, suggest that hyperpycnal flow was the most important mechanism driving lobe progradation. Shelf‐edge positioned fluvial channels linked to the basin floor lobe complexes via deeply incised, sandstone‐filled slope channels, suggest that rivers fed directly onto the slopes where their dense, sand‐laden discharges readily generated quasi‐steady hyperpycnal flows that regularly reached the basin floor. The composite architecture and complex waxing–waning flow facies configuration of the hyperpycnites is consistent with sustained and concomitant suspension and traction deposition under fluctuating subcritical to supercritical conditions. Similar sandstone beds occur on the clinoform slopes, indicating that the hyperpycnal flows operated likewise on the slope. Plant‐rich hybrid event beds indicate transformation of initially turbulent flows by relative enrichment of clay and plant material via progressive sand deposition to such an extent that it suppressed turbulence. The multi‐faceted character of the hyperpycnites reported here, challenges traditional beliefs that hyperpycnites assumingly preserve the waxing–waning signal of single‐peaked floods.

Contrasting Neogene–Quaternary continental margin evolution offshore mid-north Norway: Implications for source-to-sink systems

The Neogene–Quaternary development of the ∼700 km long mid-Norwegian and Lofoten–Vesterålen continental margin is reconstructed using a dense grid of 2D seismic data and exploration wellbores. Overall, widespread ocean current-controlled contourite drifts built up along the whole margin segment from the mid-Miocene onwards (c. 11 Ma, Kai Formation). The onset (c. 8.8 Ma) of a large inner shelf progradation (Molo Formation) was, however, restricted to the southern part of the study area, the inner mid-Norwegian shelf. In the Quaternary (c. 2.7 Ma), grounded ice sheets repeatedly brought large sediment volumes (Naust Formation) to the shelf beyond the Molo Formation. A similar build-out is less pronounced further north, where contourite drift growth instead continued and resulted in build-up of the Lofoten and Vesterålen drifts. In contrast, the drifts of the southern part of the study area occur for the most part stratigraphically below, interbedded with and distal to the progradational Molo and Naust deposits.The study area exemplifies pronounced variability in Neogene–Quaternary continental margin growth. The wide and gently dipping mid-Norwegian margin facilitated coastal and shelf progradation related to fluvial and glacial processes, while the narrow and steep Lofoten–Vesterålen margin received little input from these sources although exposed to the same paleoclimate. Instead, erosion of canyons promoted downslope reworking across the slope and into the deep basins. This low sediment input is interpreted to be controlled by the alpine relief in the north resulting in a small source area and thus low fluvial and glacial sediment input. To the south, hinterland relief allowed for a much larger fluvial and later, glacial source area. Both margin segments were also influenced by contour currents throughout the studied period. We emphasize their importance for understanding the role of erosion and deposition in source-to-sink systems, and thus the need for these processes to be integrated within source-to-sink models.

Peculiarities of Tertiary petroleum systems evolution under prograding shelf environment on the continental margin of the East Siberian Sea

The upper part of the sedimentary cover within the East Siberian Sea shelf comprises Cenozoic clinoform deposits, which accumulated in passive continental margin settings. In the Eastern Arctic, the productivity of clinoform deposits has been proved on the Alaska North Slope and in the Beaufort–Mackenzie Basin. Considering that Cenozoic clinoform deposits are widely represented in the Russian part of the Eastern Arctic, they undoubtedly attract considerable interest from the standpoint of hydrocarbons prospecting. However, despite increasingly closer attention to this interval of the sedimentary section, it is still poorly understood due to its complicated geology. The lack of drilled wells in the region imposes a considerable limitation on an understanding of sedimentary basins development. In this situation, geophysical data become the primary source of information for building geologic models in the Russian sector of the Eastern Arctic. An assessment of hypothetical Cenozoic petroleum systems of the East Siberian Sea is the main objective of this paper. It is to be said research performed under high uncertainty of input data. The results obtained from basin analysis and numerical modeling indicate the possibility that an active petroleum system may exist in the Cenozoic sedimentary wedge of the East Siberian Sea. The outlook for the clinoform complex largely depends on the source rock maturity, i.e., higher prospects should be expected in areas where the prograding wedge has maximum thickness. Considering all factors (reservoir quality prediction, proximity to a hydrocarbon kitchen, timing), the Eocene–Oligocene part of the sedimentary section appears to offer the greatest promise within the study area. Here, predominantly oil accumulations may be expected at a depth of 2.5–3.5 km below sea bottom.

Seismic geomorphology and evolution of the Melville Bugt trough mouth fan, northwest Greenland

Trough mouth fans on glaciated margins are a common sedimentary feature that contain a record of ice sheet advance and retreat across the continental shelf. This study uses 2D and 3D seismic reflection data across the Melville Bugt Trough Mouth Fan (MB-TMF), offshore northwest Greenland, to document its stratigraphic architecture, glacigenic landforms, and marine deposits. The MB-TMF stratigraphy is characterised by rapid progradation of the continental shelf (over 100 km since ∼2.7 Ma) and heterogeneous truncation or subsidence of topset strata. The variable topset character relates to the repeated growth and retreat of the Greenland Ice Sheet across the shelf since ∼2.7 Ma. Seismic geomorphology and facies analysis of the prograding clinoforms show repeated sequences of debris flow deposits and gullies that are taken to infer gravity-driven processes and the presence of meltwater-related hyperpycnal flows in areas proximal to the ice sheet on the outer shelf. Several sets of mega-scale glacial lineations confirm the presence and flow pathways of successive ice streams since ∼1.3 Ma. Evolution of the MB-TMF can be summarised into four stages that were controlled by variations in ice sheet erosion, topographic forcing of ice stream flow, and changes in accommodation related to glacigenic deposition and tectonic subsidence. These results show that during the Middle Pleistocene Transition there is an apparent switch from a meltwater-dominated depositional regime, to one with greater emplacement of materials by ice directly at the palaeo-shelf edge. These results provide significant new detail about the large-scale glacigenic and marine depositional processes during the repeated fluctuations of the northwest Greenland Ice Sheet into Melville Bugt since ∼2.7 Ma.

Eustatic, Climatic, and Oceanographic Influences on Geomorphology and Architecture of Isolated Carbonate Platforms: Miocene, Northwest Shelf, Australia

Abstract The Miocene represents an interval of marked global change, and this evolution is reflected in carbonate platforms from this epoch. Seismic stratigraphic characterization of high-resolution (ca 60 Hz) 3D seismic data from the Browse Basin, offshore Australia, reveals a middle to upper Miocene three-part seismic stratigraphic subdivision. Each unit consists of several seismic sequence sets and their component sequences. Seismic stratal geometries and seismic facies define a prograding shelf (Langhian and older), a barrier-reef complex with scattered platforms (upper Langhian–early Tortonian), and aggrading and prograding isolated platforms (early Tortonian–Messinian). The data permit description and interpretation of high-fidelity stratigraphic details of the initiation, expansion, termination, and geomorphology of over 100 platforms in this interval. The results reveal that the isolated platforms initiated following the Middle Miocene Climatic Optimum. The succession includes major seismic stratigraphic boundaries and overall patterns of platform growth and demise that correspond roughly with periods of pronounced eustatic change associated with initiation of eastern Antarctic ice sheets. Although invoking a eustatic control for coarse trends may be tempting, mismatch between the numbers and ages of sequences, as well as the variable stacking patterns among contemporaneous platforms regionally, precludes such an interpretation; conversely, some globally recognized eustatic changes do not have a pronounced manifestation in this area. Thus, it appears that the eustatic signal combined with dynamic physical regional processes such as waves, currents, and variable subsidence creates the complex architecture and geomorphology of platforms. These results illustrate how global changes can interact with local controls to create diverse patterns of birth, growth, and demise of carbonate platforms and drive local stratal heterogeneity.

Tectonostratigraphy and evolution of the West Greenland continental margin

Large structural highs and sedimentary basins are identified from mapping of the West Greenland continental margin from the Labrador Sea to the Baffin Bay. We present a new tectonic elements map and a map of thickness from the seabed to the basement of the entire West Greenland margin. In addition, a new stratigraphic scheme of the main lithologies and tectonostratigraphy based on ties to all offshore exploration wells is presented together with seven interpreted seismic sections. The work is based on interpretation of more than 135 000 km of 2D seismic reflection data supported by other geophysical data, including gravity- and magnetic data and selected 3D seismic data, and is constrained by correlation to wells and seabed samples. Eight seismic mega-units (A–H) from the seabed to the basement, related to distinct tectonostratigraphic phases, were mapped. The oldest units include pre-rift basins that contain Proterozoic and Palaeozoic successions. Cretaceous syn-rift phases are characterised by development of large extensional fault blocks and basins with wedge-shaped units. The basin strata include Cretaceous and Palaeogene claystones, sandstones and conglomerates. During the latest Cretaceous, Paleocene and Eocene, crustal extension followed by oceanic crust formation took place, causing separation of the continental margins of Greenland and Canada with north-east to northward movement of Greenland. From Paleocene to Eocene, volcanic rocks dominated the central West Greenland continental margin and covered the Cretaceous basins. Development of the oceanic crust is associated with compressional tectonics and the development of strike-slip and thrust faults, pull-apart basins and inversion structures, most pronounced in the Davis Strait and Baffin Bay regions. During the late Cenozoic, tectonism diminished, though some intra-plate vertical adjustments occurred. The latest basin development was characterised by formation of thick Neogene to Quaternary marine successions including contourite drifts and glacial related shelf progradation towards the west and south-west.

Inherited Depositional Topography Control on Shelf-Margin Oversteepening, Readjustment, and Coarse-Grained Sediment Delivery to Deep Water, Magallanes Basin, Chile

A basin-margin depositional system is the stratigraphic product of terrigenous sediment delivery to the ocean, comprising a flat to low-gradient shelf, or topset, which transitions to a steeper deep-water slope, and, ultimately, a relatively flat basin floor. Erosional and depositional processes across these physiographic domains approximate a clinoform in the stratigraphic record. The shelf margin is a critical environment for terrigenous sediment dispersal because it links the shelf to deep water, and the coarse-grained deposits of associated strata are important subsurface reservoirs. Here, we characterize the shelf-margin and upper slope stratigraphy of the outcropping Upper Cretaceous Tres Pasos and Dorotea formations, Magallanes Basin, southern Chile. The Late Cretaceous Magallanes retroarc foreland basin was an elongate trough oriented parallel to the southern Andean arc and fold-and-thrust belt. The Tres Pasos and Dorotea formations record southward (basin axial) progradation of a high-relief shelf and slope system (>1000 m paleo-water depth) represented by a stratigraphic succession up to 3 km thick that is exposed for tens of kilometers along depositional dip. The character and distribution of deposits that define shelf margins contain evidence for a variety of processes related to deposition, erosion, sediment bypass, and mass wasting. The overall architecture of the Magallanes Basin strata is indicative of a graded basin-margin system interrupted by periods of slope oversteepening and development of out-of-grade conditions. These punctuated periods are recognized by sedimentological evidence for enhanced sediment bypass of the upper slope, and thick submarine fan successions in more distal segments. Development of oversteepened depositional topography is particularly significant as it instigated the only two major periods of coarse-grained sediment delivery to deep water over ~8 Myr during the Campanian. The controls on sediment dispersal beyond the shelf margin are commonly discussed in terms of allogenic forcings, such as tectonics, climate, and receiving-basin geometry, as well as autogenic behavior, such as delta-lobe switching. However, inherited depositional topography does not clearly fit within an allogenic/autogenic dichotomy. Depositional topography inherited from basin-margin evolution influences the position of subsequent shelf margins, which promote coarse-grained sediment delivery to deep water.

Cenozoic Clinoform Complexes and the Geological History of the North Chukchi Basin (Chuckchi Sea, Arctic)

A new seismostratigraphic model was developed for the sedimentary cover of the North-Chukchi Basin and the main paleogeographic events that occurred within this territory were interpreted. The post-rift sedimentary history of the North-Chukchi basin can be divided into three stages: the late Cretaceous–Paleocene with strong progradation of shelf sedimentary packages to the north, the middle Eocene–Miocene with gradual differently oriented filling of the basin from different sides, and the late Miocene until the present. At the last stage, the shelf outlines were formed along with slow aggradation of the clinoform complex with an insignificant northern deviation.

Seaward growth of Antarctic continental shelves since establishment of a continent-wide ice sheet: Patterns and mechanisms

The location and setting of the continental shelf break along the Antarctic continental shelves influences the paleoceanographic circulation patterns as well as sedimentation and ice sheet dynamics. By evaluating the available multichannel seismic reflection data and drill core information, we mapped the base of progradational features associated with the first advances of grounded ice sheet to the outer shelf. The distance between the modern shelf edge and the pre-glacial shelf edge varies from >200 km (e.g. Ross Sea) to <6 km (e.g. Thurston Island, West Antarctica). We identified multiple processes and mechanisms which influence the amount of shelf progradation during the establishment of a grounded ice sheet on the outer Antarctic shelves. The tectonic structure of the shelves proves to be a key factor by establishing the width of the shelf as well as local tectonics such as horst/graben and fault systems, which guide the ice stream flow and therefore the resulting sedimentation. Unlike expected, the erosional potential of the hinterland seems to play a minor role in building progradational sequences, whereas fewer long-term grounding events on the shelf edge result in more continental shelf progradation than presumed from a more dynamic ice sheet. By using these observed relationships to extrapolate the former shelf edge to unmapped regions around the Antarctic continent, we estimate an overall seaward growth of the continental shelves by 7% in area and 1.28 ∗ 106 km3 of approximated sedimentary volume deposited along the shelf edge and upper slope.

Late Jurassic evolution of the Jameson Land Basin, East Greenland – implications of the Blokelv-1 borehole

Data from the recently drilled, fully cored Blokelv-1 borehole and previous cored boreholes in the Upper Jurassic of Jameson Land, central East Greenland, are integrated with published field studies to address the depositional evolution of the Jameson Land Basin in the Oxfordian–Volgian. In Jameson Land, the succession represents a marine shelf-to-basin transect in a W–SW-dipping half-graben. Laminated organic-rich mudstones were deposited in the central deep parts of the basin and grade up-slope into bioturbated sandy mudstones. Extensive shallow marine – deltaic sand prograded from the western and northern basin margins and formed prominent sandy shelf-edge wedges. Sand-rich density flows initiated by periodic collapse of the shelf edge deposited massive sand bodies on the slope and basin floor; these sands were prone to post-burial remobilisation to form injectite bodies. Basin evolution was controlled both by relative sea-level changes, typically correlatable with regional and global sea-level curves, and by rift tectonics. During periods with high relative sea level, the organicrich muddy facies onlapped the sandy shelf environments; such periods of basinal expansion and onlap are recorded in the lower Oxfordian (Q. mariae Chronozone), the middle–upper Oxfordian (C. tenuiserratum – A. glosense Chronozones) and uppermost Oxfordian – upper Kimmeridgian (A. regulare – A. autissiodorensis Chronozones); the deepening, transgressive trend culminated in the mid-Kimmeridgian (A. eudoxus Chron). Marked progradation of the sandy shelf and associated deposition of gravity-flow sands on the slope and basin floor occurred in the early Oxfordian (C. cordatum Chron), the middle Oxfordian (C. densiplicatum Chron), the late Oxfordian (A. serratum Chron) and the early Volgian (P. elegans Chron). The basin architecture reflects periodic differential subsidence on the W- to SW-dipping fault block. The lower to middle Oxfordian is highly condensed in the east (&lt;10 m) and thickens markedly towards the west (&gt;300 m), reflecting accumulation during rift/fault-controlled block rotation. The upper Oxfordian – Kimmeridgian, in contrast, shows a broadly symmetrical distribution and records uniform regional subsidence.

Laurel Gas Play, Canning Basin - Recent Stratigraphic Learnings

Well, core and seismic data from the Lennard Shelf and Fitzroy Trough were integrated to produce a predictive sequence stratigraphic framework of the Laurel Formation consisting of Lower, Middle and Upper depositional sequences. Significant encouragement for potentially commercial flow rates was achieved by the hydraulic stimulation of the Middle Laurel in the Eastern Gas Province (Valhalla North 1 and Asgard 1 wells) in late 2015 and in the Western Gas Province (Yulleroo) in 2010. The Laurel tight gas play extends over a 20,000km2 area developed within a 2000m thick succession of marine clastics and carbonates. Condensate rich wet gas associated with overpressure is encountered regionally within low porosity and permeability sands at depths below 2000m. This was correlated across the basin from Yulleroo to the Meda Embayment and Northern Gas Province where a number of prospective plays were identified. Upper Laurel shallow marine sandstones commonly possess good oil and gas shows and have potential for tight gas where overpressured. A prominent Middle Laurel lowstand prograding shelf slope wedge supported by the presence of conglomerates in updip wells suggest better clastic sediment supply and the potential for the development of conventional lowstand topset sandstone reservoirs in the Northern Province. The Middle Laurel interval in the Northern Gas Province likely consists of a similar and potentially better tight gas reservoir than the interval stimulated in the Eastern Gas Province. Further drilling is required to confirm this and also to prove the viability of the tight gas play in the Northern Gas Province

Overview of geologic evolution and hydrocarbon generation of the Pannonian Basin

The Pannonian Basin is an intraorogenic extensional region floored by a complex system of Alpine orogenic terranes and oceanic suture zones. Its formation dates back to the beginning of the Miocene, and initial fluvial-lacustrine deposits pass into shallow to open marine strata, including a large amount of calc-alkaline volcanic materials erupted during the culmination of the synrift phase. The onset of the postrift phase occurred during the Late Miocene, when the basin became isolated and a large Pannonian lake developed. Early lacustrine marls are overlain by turbiditic sandstones and silts related to a progradational shelf slope and a delta plain sequence passing upward into alluvial plain deposits and eolian sands. A remarkable nonconformity at the top of lacustrine strata associated with a significant (4–7 my) time gap at large parts of the basin documents a neotectonic phase of activity, manifested by regional strike-slip faulting and kilometer-scale differential vertical movements, with erosion and redeposition. Subsidence and burial history modeling indicate that Middle and Late Miocene, fairly organic-rich marine and lacustrine (respectively) shales entered into the oil-generation window at about the beginning of the Pliocene in depocenters deeper than 2.5–3 km, and even reached the wet to dry gas-generation zone at depths exceeding 4–4.5 km. Migration out of these kitchens has been going on since the latest Miocene toward basement highs, where anticlines and flower structures offered adequate trapping conditions for hydrocarbons. We argue that compaction of thick sedimentary piles, in addition to neotectonic structures, has also been important in trap formation within the Pannonian Basin.

Petroleum systems framework of significant new oil discoveries in a giant Cretaceous (Aptian–Cenomanian) clinothem in Arctic Alaska

Recent oil discoveries in an Aptian–Cenomanian clinothem in Arctic Alaska demonstrate the potential for hundred-million- to billion-barrel oil accumulations in Nanushuk Formation topsets and Torok Formation foresets–bottomsets. Oil-prone source rocks and the clinothem are draped across the Barrow arch, a structural hinge between the Colville foreland basin and Beaufort Sea rifted margin. Stratigraphic traps lie in a favorable thermal maturity domain along multiple migration pathways across more than 30,000 km2 (10,000 mi2). Sediment from the Chukotkan orogen (Russia) filled the western Colville basin and spilled over the Beaufort rift shoulder, forming east- and north-facing shelf margins. Progradational shelf margin trajectories change abruptly to “sawtooth” trajectories at midclinothem, the result of reduction in sediment influx. Two stratigraphic trap types are inferred in Nanushuk basal topsets in the eastern part of the clinothem: (1) lowstand systems tracts, inferred to reflect forced regression, include a narrow, thick progradational stacking pattern perched on a sequence boundary on the upper slope; and (2) highstand-progradational systems tracts include a broad, thin wedge of shingled parasequences above a toplap surface. Both include stratigraphically isolated sandstone sealed by mudstone. Trap geometries in Torok foreset and bottomset facies in the same area include basin-floor fan, slope-apron, and slope-channel deposits that pinch out upslope and are sealed by mudstone. Significant potential exists for the discovery of additional oil accumulations in these stratigraphic trap types in the eastern part of the clinothem. Less potential may exist in the western part because reservoir–seal pairs may not be well developed.

A Late Cretaceous mixed (turbidite-contourite) system along the Uruguayan Margin: Sedimentary and palaeoceanographic implications

The Late Cretaceous is a critical period in the evolution of the South Atlantic. Significant changes in the tectonic configuration of continental landmasses and marine basins during this time lead to major shifts in the sedimentary and oceanographic regimes. Using a combination of 2D and 3D seismic data with well data from the Uruguayan Margin, this work provides a new, highly detailed description of the Late Cretaceous sedimentary evolution of the margin and its regional and global palaeoceanographic implications.The Polonio Drift System (PDS), occupies the middle to lower-slope of the Uruguayan Margin. The PDS is characterised by four channel-drifts, which propagate obliquely (120–170°) from the palaeoslope and exhibit over 800m of local relief relative to the surrounding seafloor. Seismic interpretation identified four seismic units (SU.1–SU.4), separated by five major discontinuities (R.1–R.5). These record the onset (Albian–upper Turonian, 113–89.8Ma), growth (Coniacian–upper Maastrichtian, 89.8–66Ma), and burial (Paleocene, 66–56Ma) phases for the PDS. During the onset stage, down-slope processes dominate but, three major shifts in the sedimentary stacking pattern occur. These coincide with palaeoceanographic changes in the South Atlantic during the Cenomanian–Turonian (100.5–89.9Ma), Campanian-Maastrichtian (83.6–66Ma) and Paleocene (66–56Ma), following the opening of the Falkland/Malvinas-Agulhas Seaway (FAS), the subsidence of the Walvis-Ridge and the opening of the Vema channel. Bottom current influence first occurs during the growth stage (Coniacian–Campanian) when mounded deposits shift from aggradational and slightly progradational towards the N (SU.2), to clear lateral migration towards the S (SU.3). This sedimentary pattern indicates a regional palaeoceanographic shift in bottom current flow from northerly, to a subsequent southerly direction. Bottom currents do not appear to have actively eroded sediment, although they apparently had the potential to pirate and entrain fine-grained material from turbidity plumes, which build on underlying bathymetric features such as basement horsts and channel-levees. These produce large mounded morphologies adjacent to channels. Drift growth ceases following the termination of shelf progradation (R.4), enabling sluggish bottom currents contributed to a homogeneous sedimentary drape. Bottom currents intensify in the beginning of the Paleocene, forming contourite depositional systems with well-developed contourite terrace along the middle slope. Further research can elucidate the nature and mechanics of turbidite–contourites interactions along continental margins. This work offers new insights for the implications and consequences of the interaction of turbidites and contourites, which can improve the characterisation of deep-water deposits and understanding palaeoceanographic interpretations.

Impact of Weddell Sea shelf progradation on Antarctic bottom water formation during the Miocene

The Weddell Sea is a main location of bottom water formation and, thus, an important component of global ocean circulation. In this study we examine the ocean and climatic responses to a shelf progradation induced by ice sheet advance and glacially transported sediments during the Miocene, using a general circulation model. Our investigations show that relative to a Miocene standard bathymetry, a farther southerly placed shelf break, as reconstructed in a state-of-the-art bathymetry for the Weddell Sea, enables enhanced Antarctic Bottom Water (AABW) formation and gyre transport during the middle Miocene for both relatively high and low atmospheric CO2 concentrations. Furthermore, CO2 sensitivity experiments show that an atmospheric CO2 decline for a setup with the southerly placed shelf break of a new bathymetry has only a minor impact on AABW formation, while the standard setup shows an increase. In combination, these impacts may explain the pronounced deep water formation in the southern high latitudes from the middle Miocene to the late Miocene.

Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil

ABSTRACT: Seismic facies analysis and seismic geomorphology are important tools for the analysis of depositional elements in subsurface. This paper aimed to investigate the character and genesis of depositional elements and erosive features associated with an Eocene progradational shelf margin in northern Santos Basin. Identified seismic facies are interpreted as shelf-margin deltas/shoreface deposits, tangential (oblique) clinoforms, sigmoidal clinoforms, topset reflectors, mass-transport deposits and turbidites. These facies are grouped into four associations representing periods of relatively constant environmental conditions. Association 1 is composed of shelf-margin deltas/shoreface deposits, tangential clinoforms and extensive sand-rich turbidites disposed as submarine channels and frontal splays. A progressive increase in clinoform angle within this association has been identified, culminating in high-relief sigmoidal clinoforms with less voluminous turbidites of facies association 2. Association 3 is composed by subparallel to divergent topset reflectors, interpreted as continental to shelfal deposits placed during base-level rises. These are always truncated basinward by slump scars, formed as a consequence of sediment overload at the shelf margin during aggradations. Association 4 is composed of sigmoidal clinoforms, mass-transport deposits and turbidites. Early clinoforms are steeper as a consequence of the topography of the slump scars. Subsequently, dip angles become progressively gentler as the system approach to the equilibrium profile. The steep physiography was favorable for canyon incision, which played an important role in turbidite deposition. Mass-transport deposits, formed subsequent to slope collapse, are composed of mud-rich diamictites, and show strong internal deformation.

Sedimentological architecture, shelf-edge trajectories and evolution of an Oligocene reservoir, East Nile Delta

Abstract3D seismic data and wireline logs from 17 wells in the onshore East Nile Delta were utilized to study the stratigraphic architecture and evolution of Oligocene deposits. The Oligocene is readily recognized on seismic cross-sections by a series of northward prograding clinoforms. The Oligocene sequences show lateral facies changes from fluvial to deepwater via a shelf-edge delta. Such deltaic features include distributary channels, interdistributary bay fill, and a delta plain and front, gradually changing downslope into prodelta and slope deposits. The slope deposits include three main depositional elements: a deeply incised channel, a complex of laterally switched leveed channels and lobes, and crevasses and frontal splays. These depositional elements are formed in the main western slope basin and local eastern intraslope ponded sub-basin. The reactivated E–W- and NW-trending faults as well as NE–SW inverted structures are the main controls on slope physiography that formed the main corridors for Oligocene sediment distribution. Two main globally correlated Oligocene sequences are encountered being separated by the major drop in global eustatic sea level at 28.1Ma. The lower Oligocene sequence has two main stages: (1) the formation and filling of a basal canyon; and (2) the formation and progradation of shelf deltas to form shelf-edge deltas, and then progradation with an ascending shelf-edge trajectory (clinoforms 1). The upper Oligocene sequence is characterized by accretionary shelf-edge deltas with descending and then flat trajectories (clinoforms 2–4). The frontal splay and lobe complex exhibits potentially favourable reservoir continuity and areal coverage followed by the slope channels.

Wind-driven waterbodies: a new category of lake within an alternative sedimentologically-based lake classification

Lakes are common natural systems for which sedimentation is considered to be relatively simple, generally dominated by fluvial processes along the margin and prevailing low-energy settling in the central, deeper parts. However, for many lakes, higher-energy wind-driven processes dominate. As such, a new category of lakes is proposed, herein referred to as wind-driven waterbodies (WWB). WWB display a sedimentation largely dominated by wave related processes and wind-induced lake-scale water circulation evidenced by the construction of beach ridges, spits or cuspate spits along their shorelines, and by sediment drifts, sedimentary shelf progradation and erosional surfaces in their deeper, offshore domains. WWB are observed worldwide, they share a common physiography that favours wind-forced hydrodynamics and related sedimentation patterns. This physiography is expressed by the IWWB index, a ratio of the maximum representative fetch relative to mean basin depth. It is proposed that an index value greater than three favours the evolution of a lake as a WWB. The WWB concept represents a new end-member in an alternative, sedimentologically-based lake classification that is proposed in this paper.

Late Cenozoic submarine slope failures in the southern North Sea – Evolution and controlling factors

During the Late Miocene to early Pleistocene sedimentation in the southern North Sea Basin was dominated by a westward prograding depositional system. Progradation is evidenced by a series of large-scale, westward dipping clinoforms with amplitudes of up to 400 m. The clinoforms are related to a shelf-slope-basin physiography during deposition and their development and growth reflects the basinward migration of the Late Cenozoic shelf margin through time. Numerous submarine slope failures occurred on the shelf margin during this time, recognized as kilometer-scale mass-transport deposits (MTDs). Comparatively little is known about the earliest slope failures on this prograding shelf margin, yet their role is important in developing a coherent understanding of the origins of the instability of the margin as a whole. In this study we present detailed analyses of the first MTDs occurring on this Late Cenozoic shelf margin. Based on interpretation of 2D seismic reflection profiles, borehole data and integration of new chronostratigraphic datings the development and causes of slope instabilities are reconstructed. Three MTDs are distinguished within the German part of the southern North Sea, one (MTD1) that has been displaced in the Late Tortonian and two (MTD 2/3) in the Piacenzian. MTD 1 was triggered by salt-induced seismicity, as evident from salt-related faulting of the Late Cenozoic succession in its headwall domain. Pore pressure build up due to fluid migration from deeper levels in combination with loading imposed to the basin by the prograding shelf prism are the main factors controlling the initiation of MTDs 2 and 3.Subsequent slope failures occurring during shelf progradation within the Dutch North Sea are much more frequent compared to the earliest slope failures. The development from a relatively stable shelf margin towards a margin affected by repeated slope failures coincides approximately with the intensification of Northern Hemisphere Glaciations during Pleistocene times. The development and deposition of the MTDs in the Dutch North Sea is clearly linked to climate-driven environmental changes, whereas prior to the Pleistocene failure mechanisms are preferably limited to those independent of glaciations and associated sea level changes and therefore fewer failures have occurred.