ALBERT

Description: Lower Eocene shelf-slope clinoforms are exposed in 1 x 10 km mountainside outcrops in the Central Tertiary Basin, Spitsbergen. Where clinoforms are sand-prone they include a deepwater sand complex. Submarine fans represent an early, basin-floor aggradational phase of clinoform growth, whereas later growth of the same clinoform involves a phase of shelf-margin accretion. Individual fans, within stacked series, can be distinguished when traced towards the slope, where a thickening wedge of mudstones separates successive fan bodies. The sand-prone parts of basin-floor fans are some 15-60 m thick and extend into the basin by up to 10-12 km. The lower levels of any fan consist of ripple- to parallel-laminated thin-bedded turbidites interbedded with some thick-bedded turbidites. This association changes irregularly upwards to a succession dominated by thick beds that are structureless and parallel-laminated. The thin-bedded facies are interpreted as turbidite sheets that formed as channel-mouth sandy lobes, sandy levees and crevasse splays. The erosively based, thick-bedded facies are interpreted as constructional channel-fill sandstones. The shallow channels fed sheet-complexes both laterally and distally. The apparent short basinward extent and longitudinal palaeocurrents for the youngest fans suggest that downslope sediment transport became longitudinally deflected by anticlinal topography once sediment reached the basin floor.

Description: Many modern deltas exhibit a compound geometry that consists of a shoreline clinoform and a larger subaqueous clinoform connected through a subaqueous platform. Despite the ubiquity of compound clinoforms in modern deltas, very few examples have been documented from the ancient sedimentary record. We present recognition criteria for shelf compound-clinoform systems in both tide- and wave-dominated deltas by integration of ancient and modern examples from multiple types of data. The compound clinothem can be identified by using a combination of: (1) the three-dimensional (3-D) configuration identified in bathymetric or seismic data, (2) bipartite stacked regressive units, consisting of a lower muddy coarsening-to-fining-upward (CUFU) or coarsening-upward (CU) unit (30–100 m thick) and an overlying sandier CU unit (5–30 m thick) (together they represent the subaqueous and shoreline clinoform pair), and (3) distinct facies described herein, though both types of delta have highly bioturbated mudstone and siltstone bottomsets. Tide-dominated deltas have muddy foresets with tidal scours containing tidal rhythmites or inclined heterolithic strata in the subaqueous clinothem overlain by river and tidal deposits of the shoreline clinothem. Wave-dominated deltas show mainly wave-enhanced sediment-gravity-flow (WSGF) beds and some thin hummocky/swaley cross-stratified (HCS/SCS) sandstones toward the top in the subaqueous muddy foreset, and upward-thickening HCS/SCS and trough/planar cross-bedded sandstones interbedded with siltstones in the shoreline clinothem. The subaqueous platform, which links the clinoform couplet, shows evidence of frequent tidal or wave reworking and redeposition. The platform in tide-dominated deltas is characterized by tide-generated heterolithic strata (e.g., bidirectional current-rippled and cross-stratified sandstones, spring and neap tidal bundles, tidal rhythmites) with occasional storm-wave–influenced strata. In contrast, the wave-dominated platform comprises small-scale swales with scours and mud clasts and some WSGF deposits. The proposed criteria can aid in the recognition of compound deltaic clinothems in other basins, particularly those with limited amounts and/or types of data.

Description: Shelves have previously been classified according to a wide range of criteria, such as tectonic, morphological, climatic, and process-based classifications. Here, the formation of shelves is discussed in the context of conditioning from sedimentary and tectonic processes. A three-fold division of shelves is proposed: sedimentary shelves, combined structural–sedimentary shelves, and structural shelves. With a definition of a shelf as a shallow-marine surface of large areal extent located around the margin of a deeper basin (relief hundreds to thousands of meters), most cases of shelf formation can be explained by means of sedimentation, with the only contribution from tectonics being long-term accommodation provided by basinal subsidence. These sedimentary shelves are formed by virtue of differential sediment deposition basinwards in combination with nucleation and propagation of a break in slope around or close to the shoreline. Additional conditions for the formation of sedimentary shelves include (1) deep frontal waters; (2) a hinterland that can deliver a sediment budget large enough to prograde the margin; and (3) transgressions that periodically flood back across the low-gradient coastal and alluvial plains.A structural shelf is a shelf where the shelf edge is a subaqueous structural feature (e.g., a fault escarpment), not propagated by sedimentation, usually sediment starved. Commonly, sediment has draped and infilled smaller-scale topography. Combined structural–sedimentary shelves have a direct structural nucleation of the shelf–slope break. This initial break is then blanketed and propagated by sediments retaining a shelf–slope-break topography which is displaced relative to its structural heritage.

Description: Trinidad is an uplifted segment of the Neogene Orinoco shelf, created largely by the paleo–Orinoco River and delta. The Atlantic-facing shelf-margin sediment prism has an internal clinoform architecture, with both sandy marine and nonmarine topsets and muddy deepwater slope deposits that contain turbidite channels and collapsed shelf-edge blocks. The shelf prism is 〉 10 km thick and 〉 200 km wide at present, built from late Miocene to present. The shelf margin had an irregularly rising trajectory towards the Atlantic, with very thick topset aggradation and rapid progradation of the fronting deepwater slope. The 18–33 km/My progradation rate for the shelf margin with exceptionally high shelf-subsidence rates (up to 1000 m/My) documents a high sediment supply, despite very high shelf subsidence, and strongly suggests prominent sand bypass from shelf into deepwater areas for much of the Neogene interval. The early proximal, onshore south Trinidad part of this sediment prism, the late Miocene and early Pliocene Cruse Formation, irregularly outcrops for tens of kilometers along southern Trinidad in a disrupted but near-downdip (shelf to basin) direction from west to east. This distribution allows the late Miocene shelf-break position in the paleo–Columbus Channel to be identified in the outcrops, separating a western shelf and shelf-edge delta segment from an eastern highly deformed segment with very large (house size) blocks of shallow-water facies, collapsed from the shelf edge, that are disoriented and embedded in deformed slope mudstones. These eastward distorted outcrops are interpreted as within the headward reaches of a Columbus Canyon system, otherwise known from seismic interpretation offshore in the Columbus Channel. In this eastward outcrop area below the shelf edge, the facies are mainly large, shelf-edge collapse mass-transport blocks, as well as in situ turbidite-filled slope channels, thin-bedded turbidites, debrites, and abundant slope mudstones. Most of the sandstone blocks and associated chaotic beds contain highly deformed parallel-laminated and hummocky cross strata, betraying their former shelf location. Landward of the identified shelf-edge location, there are stacked parasequences (each 3–15 m thick) of undeformed, upward-coarsening shelf-edge delta deposits, in places sharply truncating (erosional truncation) the slope mudstones and mass-transport deposits. The significant downcutting of most of the topset Orinoco channels, their position so close to the shelf edge, as well as the generally erosional contact between base of topsets and underlying slope mudstones suggests that the topset deltas were forced regressive, probably driven across the shelf by falling relative sea level, despite high subsidence rates at this time. The outcropping facies architectures, both vertically and laterally (from shelf to deepwater slope) generate a hypothesis that the early paleo–Orinoco shelf margin grew by an alternation of upward- and basinward- growing clinoforms. Particularly marked is the earliest Pliocene period of marked forced regression of the Orinoco Delta, possibly when short periods of icehouse eustatic sea-level fall outpaced subsidence rate. This lesson from the outcrop area is used to further understand the flat–rise–flat–rise, shelf-edge trajectory pattern that persisted throughout the Neogene margin development, as seen on seismic lines across the Columbus Basin.

Description: The Iles Clastic Wedge is a 500 m thick, 3 My duration, third-order sequence that built out eastward in the Cretaceous Western Interior Seaway. The wedge also contains high-frequency regressive-to-transgressive sequences that are irregularly stacked in a basinward-stepping pattern (lower limb) and in a landward-stepping pattern (upper limb). The entire wedge and the component cycles were analyzed in terms of vertically monitored sandstone-mudstone proportion, thickness, and facies distribution. The measured profiles through the Iles Clastic Wedge form a 300 km long, source-to-sink transect from southeast Rock Springs uplift, Wyoming to Kremmling, Colorado. The sandstone proportion in the entire wedge (and also in the basinward-stepping half of the wedge) attains a maximum in the proximal reaches (fluvial and tidal-fluvial/estuarine channels) of the study transect and decreases unsteadily toward the medial and distal zone. A slight secondary increase in sand proportion also appears irregularly in the medial shoreline zone. On the other hand, the sandstone proportion in the landward-stepping half of the wedge reaches a maximum in the medial (tidal-fluvial and estuarine channels and delta-front) to distal zone (basinal regressive delta) of the wedge and decreases slightly sourceward. Along individual fourth-order sequences, the sandstones and mudstones indicate a more nuanced partitioning, with three marked sandstone maxima (proximal, medial, and distal zones), separated by zones with abundant mudstone. These sandstone peaks are produced by the presence of fluvial and tidal-fluvial/estuarine channel sandstones in the most proximal zone, delta front/shoreface in the medial reaches, and basinal regressive delta front in the distal zone. The mudstone peaks represent the muddy, coal-bearing coastal plain and the prodelta area. This accentuated sandstone and mudstone partitioning at shorter time scales (few 100 Ky) becomes blurred at the longer time scale (3 My) because of the progressive basinward, then landward, offset of successive high-frequency sequences that form the larger clastic wedge.

Description: The linkage between relative sea-level change, shelf-edge architecture, and evolution of Maastrichtian basin-floor fans in the Washakie Basin, Wyoming, has been investigated at the scale of lobes, lobe complexes, and submarine fans using 630 wireline logs. The basin-floor fan deposits of two adjacent clinothems form lobate shapes on the toe of slope and basin floor. The earlier lobe complexes of the two clinothems are only weakly developed (from no deposition to up to 3.9 km3 respectively in Clinothems 9 and 10), indicating small volumes of sandy sediment delivered to deep water. The lobe complexes (up to 6.4 km3 of each lobe complex) of Clinothem 9 aggraded with fixed slope channels and without strong basinward or lateral migration (40–170 m aggradation, 4–8 km progradation with 4 km lateral shift) and did so in concert with a highly aggradational shelf edge (50 m/100 ky with 5.5 km progradation) during a period of interpreted relative sea-level rise. In contrast, the deep-water lobe complexes (up to 11.5 km3 of each lobe complex) of Clinothem 10 prograded continuously for 15–18 km on the basin floor (with 60–210 m aggradation) coeval with a flattish shelf-edge progradation (25 km/100 ky with 25 m aggradation) and an interpreted minimal sea-level rise or stillstand. The depocenters of lobe complexes in Clinothem 10 switched laterally (7–14 km) by compensational stacking and slope-channel avulsions. During the late development of both clinothems, the deep-water lobe complexes became smaller (up to 1.9 and 6.1 km3 respectively in Clinothems 9 and 10) or retreated concurrently with shelf flooding. Washakie Basin deep-water fans thus evolved through stages of initiation, aggradation or progradation, and retreat of lobe complexes. The submarine-fan growth stages of these deep-water depocenters were surprisingly well linked to coeval changes in shelf-edge trajectory between successive, ca. 100 ky maximum flooding events on the shelf. We suggest that the close linkage of lobe-complex stacking pattern with shelf-edge behavior was because the Washakie Basin formed under greenhouse conditions with a continuously high, Laramide sediment discharge to the deep-water fans while the feeder deltas were at the shelf edge, despite significant sediment reworking of shelf-edge deltas by waves and tides.