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Outcrop Expression of a Continental-Margin-Scale Shelf-Edge Delta from the Cretaceous Magallanes Basin, Chile

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.; present address: Chevron Energy Technology Company, San Ramon, California 94583, U.S.A.; jcovault{at}chevron.com
² Chevron Energy Technology Company, San Ramon, California 94583, U.S.A.
³ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.

Shelf-edge deltas are the primary agents of sediment delivery to deeper-water slope and basin-plain depositional environments, and they represent significant targets for hydrocarbon exploration. Subsurface shelf-edge deltas from passive margins have been extensively studied with seismic-reflection data, and only recently have outcrop analogs been documented (e.g., smaller-scale shelf-and-slope systems in the Eocene Central Basin, Spitsbergen, and the Cretaceous Western Interior Seaway, North America). This study characterizes stratigraphic architecture and interprets depositional processes of outcropping deep-water upper-slope and deltaic strata of the Tres Pasos and Dorotea formations of the Late Cretaceous Magallanes foreland basin, southern Chile. The Dorotea delta system at Cerro Escondido is the topset element of an unstable, continental-margin-scale clinoform. Topset-to-basin-floor relief was on the order of two kilometers as a result of inherited tectonic relief from a precursor extensional-basin phase combined with the effects of thrust loading and foreland flexure. The superbly exposed Cerro Escondido outcrop exhibits a depositional-strike perspective of ~ 300 m of shelf-edge delta deposits, including two generally upward-coarsening lithofacies successions (each succession up to ~ 200 m of measured thickness). Lithofacies successions are composed of upward-shoaling lithofacies associations, including prodelta turbidites overlain by thick wave-reworked delta-front, or shoreface, sandstones and subaqueous delta-plain distributary-channel and interdistributary deposits. Successions at Cerro Escondido are distinctively different from upward-shoaling deposits documented in other outcrop-based studies: they include thicker, coarser-grained delta-plain and delta-front strata and relatively coarse-grained prodelta turbidites in pockets of shelf-edge accommodation created as a result of mass wasting. Conditions inherent to the relatively unstable, continental-margin-scale, linked Dorotea shelf and Tres Pasos slope facilitated the development of successions at Cerro Escondido. Therefore, outcrops at Cerro Escondido provide unique insights into shelf-edge architecture and development, which can be applied to models of continental-margin evolution.