Mixed Carbonate–Siliciclastic Sequence Stratigraphy of a Paleogene Transition Zone Continental Shelf, Southeastern USA

Sedimentary Geology 166 (2004) 21–57 www.elsevier.com/locate/sedgeo Mixed carbonate–siliciclastic sequence stratigraphy of a Paleogene transition zone continental shelf, southeastern USA Brian P. Coffey*, J. Fred Read Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061-0420, USA Received 4 April 2003; received in revised form 5 November 2003; accepted 25 November 2003 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 * Corresponding author. Current address: Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6. E-mail addresses: [email protected] (B.P. Coffey), [email protected] (J. Fred Read). 0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2003.11.018 22 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 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. D 2004 Elsevier B.V. All rights reserved. Keywords: Sequence stratigraphy; Atlantic; Paleogene; Carbonate; Siliciclastic; Transition zone 1. Introduction 2. Regional setting Little is known about the facies relationships and The Paleogene sediments of the study area lie response to changes in relative sea level in transition within the Albemarle Basin, North Carolina. This zones between subtropical carbonate-dominated and basin is bounded on the south by the Cape Fear Arch temperate siliciclastic-dominated strata. This lack of and on the north by the Norfolk Arch (Fig. 1A; Bonini understanding is because many areas recognized as and Woollard, 1960; Harris, 1975). Two main struc- transitional settings are modern to very young geo- tural blocks separated by the southeast-trending Neuse logically and cover broad areas with sparse outcrop hinge underlie the basin and influenced depositional coverage. This study utilizes a broad range of subsur- patterns (Harris and Laws, 1997). These blocks in- face data, notably including plastic-impregnated thin- clude the structurally high Onslow Block to the south, sections of well-cuttings and seismic, supplemented which passes southwestward into the Cape Fear Arch, with outcrop data to document the detailed vertical and the generally low-lying Albemarle Block to the and lateral facies distributions across a basin within a northeast, which passes northward into the Norfolk sequence stratigraphic framework. The integrated Arch (Fig. 1A). In addition, the basement is cut by methodology provides the lithologic expression of a east–west trending faults that were active in the Paleogene mixed carbonate–siliciclastic succession to Paleocene, which are overprinted by more recently be inserted into the gross geometric framework iden- active northeast trending faults (Graingers Wrench tified through seismic stratigraphic mapping. System; McLauren and Harris, 2001). These formed The Paleogene Albemarle basin of eastern North a series of small horsts and grabens that influenced Carolina (U.S. Atlantic coastal plain) provides an local thickness patterns in the Paleogene. excellent study area to document transitional deposi- The Paleogene succession of the North Carolina tion, because it straddles the carbonate–siliciclastic continental shelf developed on a thick succession (0 to transition on the Atlantic passive margin. It formed as 12 km) of Upper Triassic to Lower Jurassic rift-related a swell-wave swept shelf fronting a major ocean body, siliciclastic strata and largely marine Middle Jurassic and was heavily influenced by shelf boundary currents to Upper Cretaceous shelf carbonates and siliciclastics through much of the Paleogene. These currents had a (Klitgord et al., 1988). By the onset of the Cenozoic, major impact on both lateral and temporal shelf facies the Atlantic Ocean bordering North Carolina had distribution, with significant changes in sedimentation widened to a major ocean basin, with open ocean to during major eustatic variations. Incorporation of the south, and a sill to the north undergoing continued changes in current behavior into a sequence strati- rifting (Fig. 2; Scotese, 1995). In the early Paleogene, graphic model provides insight into the processes that the Mid-Atlantic and Mid-Pacific water masses were controlled deposition in this basin. The methodology connected; this passage became increasing restricted of integrating well cuttings data with seismic outlined as the Panama Isthmus developed (Fig. 2). During in this paper provides a means of capturing valuable mid-late Paleogene sea level highs, the ancestral Gulf lithologic information necessary to reconstruct a ba- Stream cut became active, cutting across Florida via sin-scale depositional history in areas with limited the Suwannee Straits during sea-level highstands, and outcrop and core data, and is widely applicable to then flowing northeastward along the southeastern Tertiary carbonate-prone basins worldwide. U.S. margin (Fig. 2; Pinet and Popenoe, 1985; Hud- B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 23 Fig. 1. Location of Albemarle Basin, eastern USA (inset map) study area, showing: (A) major structural features, and isopachs (in meters) of the Paleogene interval (Modified from Popenoe, 1985; Brown et al., 1972). Locations of stratigraphic cross-sections (A-AV, B-BV, and C-CV) are shown with dashed bold line. (B) Locations of wells, outcrops, cores, and seismic data used in this study. Bold seismic lines are referred to in Fig. 6. 24 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 Fig. 2. Eocene paleogeographic reconstruction of the northern Atlantic generated by Scotese (1995). Note study area position is north of 30j latitude at time of deposition (black box). Also note elevated relative sea levels and the open connection between the Atlantic and Pacific Oceans prior to closing of the Panama isthmus. The position of the Suwannee Straits, utilized by the ancestral Gulf Stream, is indicated. Presence of highlands in the southern Appalachians (just west of the study area) during this time is questionable. dleston, 1993). The major continental margin prom- bulk of thermotectonic subsidence related to Mesozo- ontory at Cape Hatteras (Fig. 1A) marks the location ic rifting. Paleogene strata form a seaward-thickening where the modern (and likely ancestral) Gulf Stream wedge, with erosional remnants near the present fall detaches from the North American continental shelf, line. This wedge thickens to 750 m

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