Mixed carbonate–siliciclastic sequence stratigraphy of a Paleogene transition zone continental shelf, southeastern USA

Abstract

The sequence stratigraphy and facies of the Paleogene in the subsurface of the Albemarle Basin, North Carolina was defined using 1600 thin sections of plastic impregnated well cuttings from 24 wells, wireline logs, biostratigraphic data, and seismic data. The facies formed in the transition zone between warm subtropical and temperate conditions on a swell-wave dominated, open shelf exposed to major boundary current activity. The shelf has a distinctive seismic profile consisting of a shallow inner shelf, inner-shelf break, deep shelf (depths in excess of 200 m), and the continental slope. The inner shelf was characterized by distinctive quartz sand and sandy mollusk facies inshore, passing seaward into a broad, wave-swept abrasional shelf, and then into storm-influenced bryozoan–echinoderm limestones to depths of several tens of meters. Argillaceous lime mud (marl) deposition was widespread across the deep shelf, extending onto the inner-shelf during major highstands. Sediment thickness trends were controlled by greater differential subsidence of crustal blocks within the Albemarle Basin, which considerably modified but did not obliterate the effects of eustatic sea level changes in this passive margin setting. Five supersequences were identified on seismic and in wells, each consisting of multiple regionally identifiable sequences. The Paleocene supersequence is dominated by widespread marl deposition, reflecting shelf flooding into the Late Paleocene thermal maximum. This warming corresponds with widespread inner-shelf skeletal carbonate deposition from the Late Paleocene through the Middle Eocene. The two Eocene supersequences identified are dominated by bryozoan–echinoderm-rich carbonates that formed a seismically definable sediment buildup 50 km wide by 100 m thick across the deepest inner-shelf during the Lower to early Middle Eocene. Middle to Upper Eocene supersequence highstand sequences indicate increased progradation and greater mixing of shelf carbonates with nearshore siliciclastics, likely in response to lowering sea-levels and cooling climate. The two Oligocene supersequences identified are dominated by coarse siliciclastic sand that is heavily admixed with mollusk-foraminifer-dominated carbonates. Rapid flooding, followed by extensive progradation of shallow shelf sediments in Oligocene sequences reflects continued eustatic lowering driven by the onset of icehouse climatic conditions. Increased incision and reworking of deep shelf sediments during Oligocene supersequence highstands resulted from increased ancestral Gulf Stream boundary current activity during icehouse times. Key elements controlling facies distribution through time include significant wave energy, which controls the width and distribution of inner-shelf facies belts through the degree of water bottom abrasion and winnowing. Boundary currents also are capable of large-scale reworking of deep shelf sediment; they also provide a mechanism for widespread inner-shelf hardground development during major transgressions. In addition, boundary currents play a major role in stabilizing local climate by buffering seasonal temperature variations, directly influencing which carbonate grain producers can thrive in this transition zone setting.