Deltas on Falling-Stage and Lowstand Shelf Margins, the Eocene Central Basin of Spitsbergen: Importance of Sediment Supply
Piret Plink-Björklund, Ron Steel, 2005. "Deltas on Falling-Stage and Lowstand Shelf Margins, the Eocene Central Basin of Spitsbergen: Importance of Sediment Supply", River Deltas–Concepts, Models, and Examples, Liviu Giosan, Janok P. Bhattacharya
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Shelf-margin transects across the Eocene Central Basin of Spitsbergen provide seismic-scale (1 km x 15 km) outcrops of shelf- margin clinothem complexes with and without basin-floor fans (Type I and Type II respectively). Type I and Type II shelf margins reflect broadly similar timing of deposition in a sea-level cycle: (1) shelf-margin progradation or basin-floor aggradation during the falling stage, (2) lower-slope or basin-floor aggradation during the early lowstand, (3) intra-lowstand flooding back onto the shelf edge, and (4) shelf- margin progradation during the late lowstand. The Spitsbergen database strongly suggests that the sediment budget was partitioned largely onto the shelf and coastal plain during the development of transgressive and highstand systems tracts, whereas sands were distributed beyond the shelf edge during falling stage and lowstand. The conventional explanation for the differences in architectural and sediment-volume partitioning between Type I and Type II margins is that the magnitude or duration of sea-level fall was greater in the case of Type I. We argue here for a possible alternative explanation, that higher rates of sediment fallout at the shelf edge and upper slope during the falling stage can damp incision and prevent deep channeling at the shelf margin.
Type I shelf-margin complexes show severe erosion of the falling-stage shelf-edge deltas by the delta’s own distributary channels. Time- equivalent basin-floor fans can be “walked out” and linked to the eroded falling-stage deltas in these clinothem sets. The late-lowstand part of this type of shelf margin consists of prograding shelf-edge deltas, causing a prominent late episode of shelf-margin accretion. These late- lowstand deltas are generally more muddy than those of the Type II margins. Heterolithic, thin-bedded turbidites dominate the delta-front succession, along with slumped units.
In contrast, Type II shelf margins accreted with an amalgamated succession of falling-stage, early lowstand, and late lowstand deltas, and have no basin-floor fans. The falling-stage deltas are highly progradational, fluvially dominated, and have a sandy turbidite-prone delta front that reaches the base of slope. The tops of these deltas are severely eroded by a subaerial to subaqueous unconformity that is overlain by amalgamated channels. This unconformity is the sequence boundary that formed during the entire fall to the lowest sea-level position. Despite the documented sea-level fall below the shelf edge, no sand was partitioned onto the coeval basin floor. All the sandy sediment was trapped in the shelf-margin deltas, causing significant shelf-margin progradation. The falling-stage deltas are succeeded by an aggradational and landward-onlapping succession of turbidites, covered by a widespread intra-lowstand flooding surface. No coeval deltas at the shelf edge, or coeval basin-floor fans, were documented. The flooding surface is overlain by deltas that formed late in the lowstand. These late lowstand deltas prograded onto the slope, but they have a strong aggradational component; their distributaries and mouth bars are wave-reworked, and delta-front turbidites become muddy as high as on the middle slope. These deltas represent an important second episode of shelf-margin progradation. Although the latter deltas were deposited during the rising limb of the sea-level cycle, they are still overlain by a transgressive systems tract and maximum flooding surface.
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Deltas are amongst the most environmentally and economically important coastal sedimentary environments. Studies of deltas lag behind research in both fluvial and deep-water depositional systems, as well as more geomorphologically oriented land studies. This knowledge lag reflects both a reorientation of the petroleum industry in the last two decades toward deep-water systems, as well as the difficulty of working across the shoreline with the traditional tools used for oceanographic or land-based work. However, deltaic studies are experiencing a renewed focus, because of their global importance in environmental and other societal concerns. This volume stems from a special session: Deltas: Old and New, held at the Annual Geological Society of America conference in October 2002, that was convened to highlight these new directions in deltaic research.