Updip sequence development on a wave- and current-dominated, mixed carbonate-siliciclastic continental shelf: Paleogene, North Carolina, eastern U.S.A.

Cores, quarry exposures, and exploratory wells in the Paleogene sediments of the Albemarle Basin, North Carolina, span the transition between the southern carbonate and northern siliciclastic provinces of the eastern U.S. continental shelf succession. This swell-wave- and current-dominated shelf succession differs from the standard carbonate sequence-stratigraphic model because available accommodation is limited by the depth of wave abrasion and scour. The study area is on the thin updip part of the shelf, which includes the relatively positive Cape Fear Arch (Onslow Block) and the slowly subsiding (1.5 cm/ky) Albemarle Block to the north.The Paleogene supersequence set has a basal boundary that is a hardground on Cretaceous shoreface/shallow-shelf mollusc facies. It is overlain by a thin Paleocene sequence of transgressive, deeper offshore, glauconitic fine sands to deep marine silt–shale. Five regionally mappable, vertically stacked Eocene sequences (0–30 m thick), contain coastal sands, shoreface sandy-mollusc rudstone, offshore bryozoan grainstone–packstone and subwave base fine wackestone–packstone and marl. Eocene sequences commonly are bounded by hardgrounds overlain by thin local lowstand shallow-marine sands; they consist of a thin transgressive, shallow-marine unit (commonly absent), overlain by an upward-shallowing highstand marine succession. On the arch, lowstand and transgressive units may be condensed into lags. The lower Oligocene succession on the arch has a single marl to fine foram sand-dominated sequence whereas downdip, two to three siliciclastic-rich sequences are developed, capped by nearshore sandy molluscan facies. The upper Oligocene section is dominated by possibly three sequences composed of basal, thin sands grading up into variably sandy mollusc rudstone.Sequence development was influenced by differential movement of basement blocks (which influenced localization of fines in lows), coupled with increasing third-order eustatic sea-level changes during later Paleogene global cooling. This was coupled with swell-wave and current sweeping of the shelf that effectively decreased available accommodation by 20–30 m and generated the distinctive hardgrounds at sequence boundaries, and variable development of lowstand and transgressive system tracts. The well-developed highstand deposits reflect maximum accommodation allowing deposition of an upward-shallowing succession that terminated at the depth of wave abrasion and current scour on the open shelf. The sequence-stratigraphic development contrasts markedly with that from tropical shelves, which tend to be aggraded to sea level.