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 along the basin and corresponds with a major change in sedimenta- axis beneath the present continental shelf (Fig. 1A). tion. Northward drift of the U.S. Atlantic Margin The Paleogene shelf shows a distinctive evolving during the Paleogene positioned the North Carolina profile on seismic profiles, consisting of an inner- shelf between 30j and 36j north latitude, placing it shelf and inner-shelf break, as well as a deeper outer north of the tropical latitudes (Scotese, 1995; Smith et shelf, which breaks seaward onto the continental al., 1994). slope. Paleogene sediments are erosionally terminated Low Cenozoic subsidence rates on this passive at or beneath the modern continental shelf break margin (minimum 1.0 cm/ky uncompacted; 1.4 cm/ (Popenoe, 1985). A thick basin-fan complex lies at ky with 50% decompaction of muds) postdate the the foot of the continental slope and is composed of B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 25 deep water sediments with a major component of present coastline (Fig. 4B; Brown et al., 1972; Zarra, resedimented shelf material (Poag, 1992). 1989; Baum et al., 1978; Ward et al., 1978). Offshore, the combined Lower and Middle Eocene strata are over 300 m thick beneath the modern shelf, thickening 3. Stratigraphic framework to 400 m beneath the continental margin (Fig. 4B; Popenoe, 1985). Upper Eocene units generally are 0 Previous studies of the North Carolina Paleogene to 10 m thick onshore, and consist of sandy molluscan concentrated on thin exposures on the Cape Fear Arch limestone and quartz sand (New Bern Formation; Fig. and updip outliers (Figs. 1A,B and 3; Thayer and 3; Baum, 1977). Textoris, 1972; Baum et al., 1978; Ward et al., 1978; One uppermost Paleocene to Lower Eocene se- Otte, 1981; Zullo and Harris, 1987), on offshore quence was recognized by Zarra (1989) and Harris et seismic data (Popenoe, 1985; Snyder et al., 1994) or al. (1993). A single Middle Eocene subsurface se- on the biostratigraphic dating of depositional sequen- quence was recognized by Zarra (1989), but Harris et ces (Brown et al., 1972; Zarra, 1989; Harris et al., al. (1993) recognized four Middle Eocene sequences, 1993). Subsurface facies were only broadly identified plus one additional Upper Eocene sequence (Fig. 3). and have not been integrated with seismic data across much of the basin; however, the areal extent and 3.3. Oligocene updip expression from each time stage has been mapped (Harris and Laws, 1997). Oligocene strata range from 0 to over 100 m thick onshore, thickening basinward to over 400 m beneath 3.1. Paleocene the modern continental shelf (Fig. 4C,D). They in- clude the Lower Oligocene Trent Formation (marl, Paleocene sediments are up to 100 m thick in the fine sand, and sandy molluscan limestone), and the onshore subsurface, increasing to more than 200 m Upper Oligocene Belgrade Formation of silty–sandy offshore (Fig. 4A; Spangler, 1950; Brown et al., molluscan limestones (Fig. 2; Baum et al., 1978; Zullo 1972; Zarra, 1989; Harris and Laws, 1994). They and Harris, 1987). One Lower Oligocene sequence unconformably overlie Cretaceous sediments along and three Upper Oligocene sequences were recog- the basin margin, but probably are conformable nized by Zarra (1989) and Harris et al. (1993) (Fig. 3). downdip. They are mapped as the Beaufort Forma- The Oligocene units are unconformably overlain by tion, which includes Lower Paleocene updip sand Lower Miocene–Pliocene strata along the basin mar- (Yaupon Beach Member; Harris and Laws, 1994) gin (Baum et al., 1978; Zullo and Harris, 1987). and downdip siliceous mudstone (Jericho Run Mem- ber), and an Upper Paleocene sandy molluscan limestone (Mosely Creek Member; Fig. 3). These 4. Lithofacies and depositional environments correspond with the Lower and Upper Paleocene sequences identified in the subsurface by Zarra Seismic dip lines show that the Paleogene shelf had (1989) and Harris et al. (1993). a distinctive profile consisting of a shallow inner shelf, inner shelf break, deep shelf, and continental 3.2. Eocene slope (Figs. 5 and 6). This dual-break geometry has been noted elsewhere on the western Atlantic margin Lower Eocene sediments are 0 to 20 m thick, and (cf. Miller et al., 1998) and has a major influence on are confined to the subsurface (Brown et al., 1972; the distribution of facies across the basin. The wells Zarra, 1989). They were included in a single deposi- studied penetrate only the inner shelf portion of this tional sequence by Zarra (1989). Middle Eocene strata shelf profile, so the facies making up the seismic units are mapped as Castle Hayne Limestone, which is offshore are inferred from their deep water counter- dominated by bryozoan–echinoderm limestones. Eo- parts, which developed updip during times of regional cene stratal thicknesses range from less than 15 m flooding across the inner shelf. Lithofacies were updip to over 200 m in the subsurface beneath the defined in the subsurface by examining 1600 thin 26 ..Cfe,J rdRa eietr elg 6 20)21–57 (2004) 166 Geology Sedimentary / Read Fred J. Coffey, B.P.

Fig. 3. Regional stratigraphic framework for the Paleogene of the North Carolina coastal plain. Global and regional eustatic curves of Haq et al. (1988) and Kominz and Pekar (2001) are included for comparisons later in the text. Biostratigraphic zonations and radiometric time scale are from Berggren et al. (1995). B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 27

Fig. 4. Isopach maps showing sediment thicknesses of the four time intervals studied in the Albemarle Basin. Offshore isopachs were modified from Popenoe (1985), and onshore data was modified from Brown et al. (1972) and Harris and Laws (1997). Seismically defined terminal inner shelf breaks are marked with dashed black line, roughly parallel to the orientation of the modern coastline. (A) Paleocene isopach map, showing gradual eastward thickening in the north, a major erosional, non-depositional area to the south, bordered further south by an east to west- trending lobe. (B) Eocene isopach map, showing southeasterly thickening in the north and a southwest- to northeast-trending belt of marine erosional incision, and non-deposition. (C) Lower Oligocene isopach map, showing southeasterly thickening onshore, local sediment lobes (in part lowstand deposits) near terminal inner shelf break, a north–northeast-trending belt of marine erosion/nondeposition, and a strike-parallel sediment lobe on the deep shelf. (D) Upper Oligocene isopach map, showing gradual eastward thickening onshore to offshore, with major sediment lobes (in part lowstand deposits) near the terminal inner shelf break; strike-parallel marine erosional incision/nondeposition to seaward, and a large elongate, lobate sediment body on deep shelf. The contour interval is 50 m on all diagrams. sections of well cuttings from 24 wells. Coffey and The major lithofacies and their inferred deposition- Read (2002) discuss the methodology for cutting al settings discussed below have been simplified from analysis used in this study. Coffey (1999), and are summarized in Table 1. The 28 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57

Fig. 5. Generalized mixed carbonate/siliciclastic facies distribution across the Paleogene shelf. Note distinctive depositional profile with a low- relief shoreface, passing out onto a wave-swept region on the inner shelf, passing out into a sediment accreting region on the slightly deeper inner shelf (10 to 50 m plus), an inner shelf break sloping gently (f 1j) to a boundary current-influenced deep shelf at depths greater than 100 m deep, which terminates against the continental slope. The relative abundance of lithofacies within the general facies assemblage is noted by bars at base (bold indicates greater abundance). following facies assemblages have been grossly ar- Quartz sands are almost exclusively quartz grains, ranged from shallow to deep (Fig. 5). while skeletal fragment quartz sands have greater than 50% quartz sand grains, which form the rock 4.1. Quartz sand and skeletal fragment quartz sand framework. These facies are shoreface to shallow shelf depos- These facies have 0- to 10-m-thick units, with its, based on their nearshore, mollusk-dominated bio- light gray and massive to crudely bedded sand in tas, abundant terrigenous detritus, their positions outcrop and core. They consist of variable amounts adjacent to bases or tops of upward-deepening and of fragmented, angular to rounded skeletal debris upward-shallowing successions (respectively), and (bivalves, oysters, barnacles, and minor echino- their similarity to the modern nearshore facies on derms) and medium to coarse-grained quartz sand. the Carolina continental shelf (cf. Milliman et al., B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 29

1968; Blackwelder et al., 1982). The lack of sedimen- 4.4. Fine to medium quartz sand/silt tary structures in the sands in outcrop probably was due to pervasive burrowing by bivalves, which com- This facies is confined to the subsurface, where monly are present. This homogenization could have it forms 3- to 15-m-thick units (Table 1). They are obliterated any eolian or nearshore mechanical sedi- dark yellow to pale brown units that are massive in mentary structures. core. They consist of subrounded to angular, fine quartz sand and silt with clay matrix, together with 4.2. Sandy mollusk fragment grainstone/packstone diatoms, planktonic and benthic foraminifera tests, and some small gastropod, bivalve, and echinoderm These facies form discontinuous units 1 to 5 m debris. thick, which are light gray and massive to heavily These muddy, fine quartz sands and silts formed burrowed (Table 1). They consist of a framework of in low to moderate energy settings on the inner shelf abundant bivalve, oyster, and barnacle fragments, less that favored deposition of fines, together with tests than 50% medium to coarse quartz sand, and minor of benthic and pelagic microorganisms. These facies lime mud. probably formed seaward of higher energy shoreface Sandy mollusk fragment grainstone/packstone fa- sands, as on the Queensland shelf of eastern Aus- cies probably were formed by physical and biological tralia (cf. Johnson and Searle, 1984). Fines probably fragmentation of mollusks and barnacles on the shore- were carried in from river systems during floods, face and nearshore shelf, the fragmented material then moved out onto the shelf as muddy sediment being winnowed and transported by waves and cur- plumes transported under the influence of longshore rents, to accumulate as localized fragmented skeletal currents. When lacking open marine faunas, these sands. Similar facies are common on the nearshore strata also may have formed as local estuarine/lagoon parts of warm temperate to subtropical shelves today fills. (Collins, 1988; James et al., 1994). 4.5. Bryozoan–echinoderm grainstone/packstone 4.3. Sandy, whole mollusk packstone/grainstone These facies occur in sheet-like units 2 to 15 m These facies occur as thin sheets, lenses, and small thick, which locally are stacked to form a large banks 0.25 to 5 m thick, which are interlayered with sediment buildup 150 m thick beneath Cape Hatteras quartz sand facies (Table 1). They are massive to (Fig. 1). Strata are white to light gray, and may have burrowed grainstones/packstones composed of gravel- meter-scale sand waves and cross bedding. They sized, whole bivalves (commonly leached), oysters, consist of a diverse medium sand and gravel-sized and variable amounts (less than 50%) of interstitial carbonate skeletal assemblage dominated by whole very fine to fine quartz sand and lime mud. and fragmented bryozoans, echinoderms, benthic and These shell beds were formed by mollusk-domi- planktic foraminifera, and bivalves, which sometimes nated inner shelf assemblages (cf. Collins, 1988). The has interstitial fine to medium quartz sand and glau- abundant whole shells suggest deposition in mid- to conite. Lime mud matrix ranges from sparse to lower shoreface or shelf settings of a wave-dominated abundant, with bladed and syntaxial marine cements shelf, where sedimentation rates and lack of intense in some mud-lean samples (Table 1). wave reworking inhibited biologic and physical frag- By comparison with similar facies on modern mentation of valves. Interstitial spaces between the open continental shelves, these likely formed in shells typically were filled by infiltrated terrigenous water depths from 30 to 100 m (Nelson et al., sand and silt probably carried in by longshore drift, 1988; Collins, 1988; James et al., 2001). The open along with lime mud produced by in situ skeletal shelf setting is supported by very diverse biotas breakdown. This mud may have been deposited (Fallaw and Wheeler, 1963; Zullo and Harris, during low energy periods, possibly under the influ- 1987; Baum, 1977) and the evidence for sweeping ence of local seagrass cover typical of inner shelf by storm or swell waves, which formed the sand areas today (Wanless et al., 1995). waves and cross-bedded units, some of which were 30 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57

Table 1 Summary of lithofacies Facies Quartz/sand/skeletal Sandy mollusk- Sandy, whole mollusk Fine to medium fragment quartz sand fragment grainstone/ packstone/grainstone quartz sand/silt packstone (shell beds) Stratigraphic occurrence Occur with shell beds, Interlayered with shell Sheets, lenses, and Not present in and thickness especially in Upper beds and quartz sands; small banks outcrop; associated Eocene and Oligocene; common in Oligocene associated with with sands in 0.5 to 10 m thick, but interval; form stacked quartz sands and subsurface; 3 to rarely greater than 1 m units; 1 to 5 m thick skeletal quartz ands; 15 m thick; in outcrop 0.25 to 3 m thick; common in Upper more common in Eocene and Oligocene Oligocene strata strata in northeast

Color Light gray Light gray to light Light gray to light Dark yellowish yellowish gray yellowish gray to brown Bedding and Massive to crudely Massive, heavily Massive/bioturbated Massive in core sedimentary structures bedded burrowed; laterally discontinuous in outcrop

Constituents Highly fragmented Abundant leached, Abundant leached Common subrounded/ angular to rounded fragmented and abraded whole mollusks and angular fine quartz skeletal material and mollusks and abundant variable amounts of sand/silt; clay matrix; abundant rounded rounded medium to very fine to fine common fine medium to coarse coarse quartz sand; quartz sand and silt; skeletal fragments quartz sand minor lime mud lime mud matrix sparse to abundant

Biota Bivalves, oysters, Bivalves, oysters, some Abundant bivalves Diatoms, planktic and barnacles; minor barnacles; minor and oysters; some benthic foraminifera; echinoderms echinoderms gastropods some gastropods, bivalves, and echinoderms

Glauconite Minor, very fine to Minor, fine to medium Minor, very fine Minor, very fine fine sand size sand size to fine sand size sand size Environment of Barrier/shoreface/ Bay/shoreface/shallow Bay and shallow Low-moderate energy Deposition shallow shelf inner shelf inner shelf inner shelf

infiltrated by lime mud during quiet periods. A warm 4.6. Phosphatic sand and hardground subtropical, rather than cool water setting is sug- gested by the presence of large benthic foraminifera Phosphatic sediment accumulations are up to 15 m (cf. nummulitids and lepidocyclinids), aragonitic thick and range from regional pavements to localized bryozoans, and mollusk assemblages (Baum, 1977; pebble lags and sands. Single or coalesced hard- Otte, 1981; Powell, 1981; Kazmer et al., 2001; grounds form planar to irregular surfaces that are Stanley and Hardie, 1998). Seasonal temperature heavily bored and encrusted by foraminifera and variations may have been ameliorated by the ances- bryozoa. Phosphatic lags often overlie hardgrounds tral Gulf Stream. and contain rounded, medium to coarse quartz sand B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 31

Bryozoan–echinoderm- Phosphatic sand and Glauconitic sand Fine skeletal Carbonate mudstone marl grainstone/packstone hardground packstone/wackestone

Dominant Middle Hardgrounds form Associated with Thin (3–5 m) units; Thick sections (50 m) in Eocene facies; semi-regional planktic marls; commonly associated Paleocene; in Paleocene; 2 to 15 m thick surfaces; may be more abundant with marls in Eocene/Oligocene, (locally stacked overlain by oolitic in northern relatively thin (2–10 m) into 150 m thick phosphatic sands Albemarle basin in subsurface; thin to 3 m buildup); less up to 0.5 m thick, (3–10 m thick) in outcrop over the arches common in Upper except in thicker Paleocene and Upper Oligocene Oligocene phosphorite accumulations of the northern basin (15 m) White to very light Yellowish brown to Dark green Light gray to light Light olive gray gray grayish black olive gray Some meter-scale Planar to irregular Generally Massive/bioturbated Massive, or thin bedded to sand waves in outcrop, surfaces, with massive in core laminated commonly large-scale borings; common in outcrop cross-bedded lags Medium sand-gravel; Minor skeletal material, Medium to very coarse Fine sand to gravel- Planktic tests and spicules bryozoans, echinoderms, commonly phosphatized; sand-sized, spherical sized benthic skeletal variable amounts of angular clams, and forams; common rounded to ovoid glauconite debris; variable planktic quartz silt to very fine sand variable fine angular to medium to coarse pellets and rounded biotas and very fine to in a matrix of silt to clay- subrounded medium quartz sand very fine to medium fine subangular quartz sized carbonate and quartz sand; sparse to quartz sand; minor sand in argillaceous terrigenous silt/clay; finely abundant lime mud planktic and benthic lime mud matrix disseminated phosphate matrix foraminifera; siliceous and oxides; silt/clay present in stringers or as ovoid fecal pellets Abundant bryozoa, Boring mollusks, Planktic and benthic Delicate bryozoans, Common planktic echinoderms, encrusting organisms foraminifera, minor sponge echinoderms, and benthic foraminifera, sponge brachiopods, moderate common (benthic spicules, and pycnodontid foraminifera, some spicules, radiolaria, benthic and planktic foraminifera, thick- oysters planktic foraminifera calcareous nannoplankton, foraminifera; minor red walled bryozoans) minor benthic foraminifera algae, crab fragments, and ostracodes Variable, fine to Common, medium to Very abundant, medium to Variable, very fine to Abundant, very fine to medium sand size coarse sand size very coarse sand size fine sand size fine sand size Storm-influenced Sediment starved/ Shallow (distal deltaic) to Deep shelf Subwave base deep shelf deep inner shelf current-swept deep shelf mid-deep shelf and glauconite (Table 1). Phosphate sands are yellow- sweeping and abrasion by swell-waves (Emery, brown, consist of a mixture of oolitic phosphate and 1965; Milliman et al., 1968; Collins, 1988; James et black phosphatized skeletal grains and clasts, and are al., 1994). In more deeply submerged settings, bound- often associated with a matrix of fine argillaceous ary currents may have swept the sediment surface, quartz sand or fine skeletal packstone/wackestone. suppressing deposition, and allowing hardground Phosphatic units commonly have intergranular fine pavement formation and encrustation. Pavements were to coarse rhombic dolomite cement. eroded and redeposited onto the updip inner shelf Fine sandy facies formed on the high energy inner during major transgressive events. Hence, major eu- shelf, due to suppressed sediment deposition by static fluctuations may have reworked localized phos- 32 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 phate accumulations into regional, time-transgressive Modern analogs of these facies form in deep shelf pavements across the inner shelf. Gyres spun off from settings below storm and swell wave base, perhaps at the main boundary current likely localized upwelling depths of 100 m or more (cf. Collins, 1988; James et to form the thick oolitic phosphate sands and impreg- al., 1999), although they may have been deposited at nated hardgrounds (Prokopovich, 1955; Riggs, 1984). shallower depths in updip parts of the Albemarle Some of the hardgrounds may have been modified by Basin that were protected from swell waves. Abun- exposure during lowstands of sea level but many do dant lime mud, delicate neritic benthic organisms, and not show obvious evidence of emergence (Moran, abundant planktonic foraminifera support a deep, 1989). Exposed phosphatic surfaces have crystal sand- open shelf depositional setting. or silt-infiltrated shell molds and depleted stable iso- topes beneath the surface (Baum and Vail, 1988). 4.9. Carbonate mudstone/marl

4.7. Glauconitic sand These units range from a few meters to 50 m thick. They are light-blue gray, massive (locally thin bedded This facies occurs in units 3 to 10 m thick, especially to laminated), and composed of a diverse open marine in the more siliciclastic-dominated northern parts of the fauna (planktonic foraminifera, radiolarians, calcare- basin. They are dark green sediments composed of ous nannoplankton, sponge spicules, and sparse ben- abundant medium to very coarse sand-size glauconite thic foraminifera), together with variable amounts of pellets, along with planktonic and benthic foraminif- quartz silt and very fine sand in a matrix of argilla- era, minor very fine to medium quartz sand, and ceous carbonate silt and clay (Table 1). Marls are argillaceous stringers of terrigenous silt or clay. variably silicified and dolomitized, more commonly These facies developed in low-energy open shelf when radiolaria and spicules are abundant. settings with low sedimentation rates. Planktonic fo- The abundant fines suggest that the marls formed raminifera in some units suggest deeper shelf settings, below swell wave base, probably in water depths but thicker units that occur in updip positions contain- greater than 100 m on an open shelf, although some ing a coarse quartz sand component may have formed strata may have formed at shallower depths in pro- on the shallow shelf. Modern shelves show wide range tected updip portions of the Albemarle basin (cf. of water depths for glaucony formation (Cloud, 1955; Collins, 1988; James, 1997; Marshall et al., 1998). Gorsline, 1963; James et al., 1999). Sands likely They formed by accumulation of planktonic tests, formed beneath cool, normal salinity waters with skeletal debris winnowed from upslope, and variable elevated dissolved silica concentrations, in areas with amounts of fine terrigenous siliciclastics carried across abundant phyllosilicate clays and organic matter, and the shelf during major storms. Being largely below relatively reducing conditions, such as distal deltaic wave base, deep shelf facies were not affected by settings or areas of the shelf downdip from fine clastic surface wave energy, but offshore seismic suggests point sources (Harder, 1980; Cloud, 1955). they were subjected to periodic reworking and inci- sion by shore-parallel boundary currents. 4.8. Fine skeletal packstone/wackestone

These facies occur as 3- to 5-m units that are light 5. Sequence stratigraphy gray and massive to burrowed (Table 1). They consist of fine sand to gravel-sized benthic skeletal debris Regional cross-sections were constructed from the (delicate bryozoa, echinoderms, and foraminifera), onshore well data using available seismic data and planktonic foraminifera, and very fine to fine quartz published biostratigraphic data to constrain correla- sandandglauconiteinanargillaceouslimemud tions. Wells penetrate sections only on the inner shelf, matrix. This lithology resembles the matrix of some so offshore data for the deep shelf is limited to seismic of the previously mentioned, coarser bryozoan-rich profiles (Fig. 6A,B,C). Seismic stratigraphic surfaces skeletal limestones, and so may be overestimated in were interpreted using offshore data from Popenoe cuttings counts. (1985) and various offshore and onshore data curated B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 33

Fig. 6. Line drawings of 2D seismic lines from the North Caroline coastal plain. (A) USGS dip line 31 (BV-BU; Hutchinson et al., 1992), showing Paleogene supersequence boundaries, lowstand wedges, flooding surfaces, and terminal inner shelf breaks (onlaps define supersequence boundaries; regional downlap surfaces define maximum flooding surfaces). (B) Onshore strike line (Cities Service, Citgo, courtesy N.C. Geol. Survey), showing zones of clinoform development in Eocene strata (Hatteras buildup). (C) Offshore shelf strike seismic line Gyre 81 (USGS, Popenoe, 1985), showing locally developed clinoforms, seismic-scale erosion, and lobate geometries of units on the deep shelf. Line locations are shown in Fig. 1B. by the North Carolina Geologic Survey. Regional nannofossil control provided by Laws and Bralower subsurface isopach maps that document the updip (unpublished). Calcareous nannofossils were of limit- expression of sediment packages were generated by ed use in constraining ages, due to considerable Harris and Laws (1997), using onshore well data vertical mixing of fines with muds during drilling (Table 2). The Paleocene–Eocene cross sections were (Laws, personal communication, 1999). In addition, datumed at the top Eocene surface. Oligocene cross most of the grain-rich, updip lithologies encountered sections were hung from the top of the Oligocene. were not conducive to the preservation of these Age correlations between wells were based on bio- microfossils. stratigraphic control published by Brown et al. (1972) Biostratigraphic age picks were plotted onto inter- and Zarra (1989), who subdivided the Paleogene val transit time logs (inverse of sonic velocity) from subsurface into Lower and Upper Paleocene, Lower, five wells and then transposed onto regional seismic Middle, and Upper Eocene, and Lower and Upper lines to tie correlations between wells. This was Oligocene intervals (Table 2). Greater weight was essential in areas showing broad, low angle clino- placed on the more recent planktonic foraminiferal forms, which had not been previously recognized, picks of Zarra (1989), with additional calcareous because these subtle features were below the resolu- 34 ..Cfe,J rdRa eietr elg 6 20)21–57 (2004) 166 Geology Sedimentary / Read Fred J. Coffey, B.P.

Fig. 6 (continued). B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 35 tion of the biostratigraphic data. Isopach maps of the fluctuations recorded by the strata. Regional correla- units onshore (Fig. 4) were constructed from outcrop tions then were incorporated into a sequence strati- and well data and were used to modify the maps of graphic framework (Vail and Mitchum, 1977; Van Brown et al. (1972) and Harris and Laws (1997). Wagoner et al., 1990; Sarg, 1988). Offshore isopachs generated from seismic by Popenoe Sequence boundaries were recognized on the cut- (1985) were integrated with onshore data to construct tings logs by upward-shallowing trends of shelf car- basinwide isopach maps. bonate facies into skeletal quartz sands, with the Given the problems inherent in well cuttings data sequence boundary being placed at the base of the (Coffey and Read, 2002), lithologic types present in interval showing a major increase in percentage of the each well sample interval were grouped into the shallowest-water lithofacies (Fig. 7A,B). The percent- following facies associations prior to correlation be- age of quartz sand generally increases gradually tween wells: upward to the sequence boundary, then reaches a maximum just above the boundary. Quartz sand-based 1. Shoreface-nearshore inner shelf association: quartz sequences are Type 2 boundaries and are concentrated sandstone, mollusk-fragment quartz sandstone, and in the central basin, where accommodation rates are sandy whole- and fragmented-mollusk rudstone, greater. In updip areas and areas with reduced accom- grainstone and packstone. modation space, phosphatized hardgrounds often 2. Offshore, inner shelf association: bryozoan–echi- mark the sequence boundaries (LaGesse, 2003). There noderm grainstone/packstone (mainly Eocene) and is little evidence of subaerial exposure associated with sandy barnacle echinoderm grainstone-packstone sequence boundaries in this basin, which makes these (Oligocene). surfaces difficult to recognize in outcrops, cores, and 3. Wave- and current-swept shallow to deep shelf logs. Instead, they were defined by major seaward association: phosphatic sands/wackestone, and shifts in facies associations. In downdip wells lacking carbonate hardgrounds. These may occur on sandy intervals, sequence boundaries were placed near wave-abraded nearshore shelf (hardgrounds), and the tops of upward-shallowing trends, expressed by on deeply submerged inner shelf due to boundary increasing percentages of inner shelf units above deep currents and upwelling (hardgrounds and phos- shelf facies. Phosphatic hardgrounds occur at many phatic units). sequence boundaries in outcrop; however, downdip, 4. Deeper water shelf association: fine skeletal pack- they also occur on transgressive and maximum flood- stone/wackestone or very fine to fine silty sands. ing surfaces. As these thin features make up only a These units are dominated by delicate benthonic small percentage of the well cuttings within a sample and pelagic biotas, relative to the current-swept interval, they were not used as the primary criteria for assemblage. differentiating bounding surfaces or systems tracts. In 5. Sub-wave base, very deep shelf association: lime general, Paleocene and Eocene sequences consist of mudstone and silty marl. greater amounts of muddy to skeletal, open shelf carbonate material (Fig. 7A), while Oligocene sequen- All of the facies observed in the wells were ces have significantly greater amounts of siliciclastic deposited on the geomorphic inner shelf, but in a material and mollusk-dominated carbonate skeletal wide range of water depths, depending on the position material (Fig. 7B). of the shelf surface with respect to Paleogene relative Transgressive systems tracts were defined where sea level. The dominant facies association recognized units showed an upward increase in proportion of from each sample interval in the wells were used to deeper water facies (Fig. 7). The accompanying draw the facies cross sections; correlations were con- upsection decrease in the abundance of shallow water strained by seismic and biostratigraphic data. The facies in the cuttings reflects progressive landward open, wave-dominated configuration of this passive migration of facies during transgression. margin resulted in laterally continuous, shore-parallel Maximum flooding surfaces were placed at the facies assemblage distribution, which facilitates cor- bases of intervals in the wells characterized by the relation of strata and recognition of major eustatic highest percentage of deepest water facies. They 36 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57

Table 2 Regional stratigraphic and seismic expression of supersequences Supersequence Age/outcrop expression Onshore thickness Updip extent/expression (Harris and Laws, 1997; (Harris and Laws, 1997) Brown et al., 1972) Upper Oligocene P21–P22+; quartz sandy 0–50 m; thickest near Limited to eastern basin; (Supersequence 5) mollusk carbonates record siliciclastic point sources localized thick onshore prograding supersequence northwestern Onslow Bay; highstand downdip of previous (Supersequence 4) limit, depositional Lower Oligocene P19–P21; basal silty marl 0–70 m; generally less Widespread thin unit across (Supersequence 4) marks supersequence MFS; than 15 m eastern basin; 150 to 250 sandy, mollusk-rich carbonates km seaward of updip record supersequence highstand Supersequence 3, depositional Middle–Upper Eocene P11–P16; bryozoal carbonates 0–200 m; thickest beneath Widespread across basin; (Supersequence 3) record prograding Cape Hatteras (Fig. 1A) thickest beneath Pamlico supersequence highstand Sound, erosional; likely sequences Mollusk-rich Upper extends updip to near Fall Line Eocene carbonates mark short- lived flooding event during supersequence highstand Lower Eocene P8–P9; not recognized in 0–40 m; thickest beneath Confined to subsurface in (Supersequence 2) outcrop; possibly preserved as Cape Hatteras central basin, depositional; thin bryozoal carbonates updip Highly thinned to absent across Onslow Black Uppermost Cretaceous– P1–P4; glauconitic sands 0–100 m; Thickest in east- Limited across Onslow Paleocene (Supersequence 1) mark late supersequence central basin Block, erosional (local lobes transgression to early highstand offshore); widespread across Albemarle Block, Thanetian up to 100 km further updip than Danian, depositional typically underlie muddy carbonates and silty marl well cuttings, because of their limited thickness, the downdip, and skeletal carbonate updip (Figs. 7–9). poor resolution of the seismic and biostratigraphic Maximum flooding surfaces could not be identified in framework, and problems inherent in the cuttings thin (less than 10 m) sequences, because they were analysis (Coffey and Read, 2002). As such, they will beyond the resolution of the cutting data. be discussed only generally in this paper. Dramatic Highstand systems tracts were recognized by up- thinning onto structural highs and in updip areas section increase in shallow water, quartz-rich facies. further complicated correlation, because cuttings sam- They could be recognized only where a maximum ple intervals in these areas became too large to resolve flooding surface could be defined; otherwise, trans- heavily eroded and thinned stratigraphy between out- gressive and highstand systems tracts were not sub- crop exposures (Fig. 10). divided (Fig. 7A,B). Five unconformity-bounded supersequences are 5.1. Supersequence 1 (Paleocene) recognized, most containing a seismic-scale lowstand wedge along the inner shelf margin (Figs. 3 and 6A). This supersequence is largely Paleocene in age but Each supersequence is expressed on the inner shelf as includes uppermost Cretaceous units near its base. It a grossly upward-deepening to upward-shallowing includes the Beaufort Group, comprised of the Danian succession of component third-order depositional Jericho Run/Yaupon Beach formation and the Thane- sequences (Figs. 8–10). Component sequences were tian Mosely Creek/Bald Head Shoals formations (Fig. more difficult to identify and map regionally using the 3), and spans Plankton Zones P1, P2 and P4 (Zarra, B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 37

Onshore seismic Onshore third-order Inner-shelf seismic Deep shelf Terminal high expression sequences (this study) expression seismic expression shelf position

Thin parallel reflectors 3 + Series of lobate, Low angle, strike- 20 km seaward of seaward-dipping parallel clinoforms modern shoreline; clinoform wedges in north; broad, parallels coastline in (0.5–1j slopes, triangle-shaped lobe north; up to 100 km 200–300-m relief) that terminates at offshore in south continental shelf break Thin parallel reflectors ; 3 Thin with localized Strike-parallel, Highly variable (eroded); localized progradation steep clinoforms discontinuous mounded broadly coastline parallel in north (1j slope, 250 m relief) geometries with clinoforms

Variable imaging; regional 5 + Early: Regional Broadly mounded lobes 30 km seaward of modern downlap surface overlain downlap surface, then along strike; seaward- shoreline; parallels coastline by multidirectional common clinoform thickening wedge in north; up to 60 km clinoforms in central development offshore in south basin (Hatteras Buildup)

Poorly imaged; regional 1–2 Basal lowstand wedge; Poorly imaged N/A downlap surface regional downlap surface

Thin, parallel reflectors 2–3 Broad, low angle Poorly imaged 15 km east of modern clinoforms and local shoreline at Cape Lookout wedges at terminal (Fig. 1A) shelf margin

1989). The regional distribution and seismic expres- Supersequence 1 lowstand systems tract consists of sion is summarized in Table 2. uppermost Cretaceous quartz sands, which grade Supersequence 1 forms a broad, easterly thicken- downdip into phosphatized hardgrounds (Well 15, ing wedge, but extends only a limited distance updip Fig. 9A), and then into argillaceous lime mud in the from the present coastline in the southern study area basin center, where they appear to conformably over- (Harris and Laws, 1997; Fig. 4A). Near its updip limit lie Upper Cretaceous marl (Figs. 8A and 9A). Phos- on the Albemarle Block, the unit thickens locally into phatized hardgrounds mark the Supersequence 1 small grabens (McLauren and Harris, 2001). In the transgressive surface in several wells along the flanks south, just east of the Cape Fear Arch, the unit of the central basin. Updip, the transgressive systems narrows to form localized west–east trending lobes tract consists of condensed sections of glauconitic and (0 to 200 m thick; Fig. 4A). These lobes lie near or phosphatic sands, whereas silty marls dominate down- seaward of the terminal Supersequence 1 inner shelf dip sections. The supersequence TST is below reso- break, as recognized on shelf seismic data, and are lution on the available offshore seismic data. separated by a northeast-trending erosional/non-depo- The Supersequence 1 maximum flooding surface is sitional re-entrant. placed at the base of a thick, regional silty marl that Updip, the basal Paleocene supersequence bound- overlies shallower water facies. This maximum flood- ary unconformably overlies Upper Cretaceous quartz ing surface is well expressed on offshore seismic mollusk sands and shell beds that generally are well profiles as a regional downlap surface near the base cemented (Zullo and Harris, 1987; Fig. 9A). The of Supersequence 1 (Fig. 6A). 38 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57

Updip on the Onslow Block, the highstand sys- upward into shelfal quartz-mollusk sands and bryo- tems tract is highly thinned and dominated by zoan carbonates. These shelfal facies extend seaward nearshore quartz-mollusk sands. Over much of the as a broad incipient bank beneath the Hatteras area, more low lying Albemarle Block, the highstand interfingering along strike to the north and south systems tract consists of silty marls, which grade with deeper water marls. In the northern study area, B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 39 the highstand is dominated by marls and thin quartz- east–southeast, reaching 250 m beneath Cape Hatte- mollusk sands (Fig. 8A). The highstand systems tract ras, where seismic data define a major carbonate bank on onshore seismic data has relatively flat-lying, (100 m thick), marked by clinoforms that dip to the parallel reflectors indicative of aggradation (Fig. north, south, and east (Fig. 6b). Further downdip, 6B). On offshore seismic, the highstand systems tract offshore low angle clinoforms with up to 150-m relief has low-angle, parallel clinoforms with up to 100-m and 0.5j slopes downlap onto the top-Paleocene relief and 0.5j slopes that downlap seaward onto the reflector at the terminal Eocene inner-shelf break top-Cretaceous (?) reflector (Fig. 6A). The terminal (Fig. 6A). In the southern study area, offshore iso- highstand inner-shelf break can be recognized on pachs suggest the Eocene succession has been eroded seismic profiles by a change in slope on the top of in a southwest to northeast-trending, 50-km-wide belt, the Paleocene shelf (Fig. 6A). Where mappable, this as well as in a smaller erosional belt to the southeast feature roughly parallels ( f 20 to 30 km offshore) (Popenoe, 1985; Fig. 4B). These eroded sediments the modern coastline, except where it bends seaward form a complex lobate isopach pattern to the northeast (up to 100 km) of the coastline marginal to the Cape (Fig. 4B); deep shelf seismic reflectors in this area are Fear Arch (Fig. 4A). The discontinuous top-Paleo- wavy and irregular (Fig. 6C). cene reflector appears to correspond with a contact between skeletal carbonate and overlying quartz 5.3. Supersequence 2 sandy facies in wells, which marks the top Paleocene supersequence boundary. Supersequence 2 is Lower Eocene and ranges from Previous work (Zarra, 1989; Harris and Laws, pre-NP Zone 12/13 (Zone P8; Bralower, personal 1997) and the well data from this study suggest that communication, 2000) through Zone P9 (Zarra, there are two to three subseismic depositional sequen- 1989), spanning the upper two-thirds of the Ypresian ces within Supersequence 1. The bases of these (Fig. 3). It is up to 40 m thick and is confined to the sequences are defined by areally extensive quartz subsurface of the central basin. Offshore, Superse- sand-prone units, which punctuate the carbonate-dom- quence 2 is underlain by a Type 1 sequence boundary inated succession (Figs. 8A and 9A). (cf. Van Wagoner et al., 1990) with a well-developed, seismically defined lowstand wedge seaward of the 5.2. Supersequences 2 and 3 (Eocene) Supersequence 1 terminal inner-shelf edge (Fig. 6A). No wells penetrate the lowstand strata. Two supersequences have been mapped in the The transgressive systems tract consists of a sev- Eocene interval from the Albemarle Basin . However, eral meter thick, regional quartz sand, which grades the two packages are difficult to differentiate on shelf up into quartz mollusk sands, then into a thin bryo- seismic data; a single lowstand wedge is evident at the zoan limestone. It is below detection on offshore base of the Lower Eocene (Fig. 6A).Hence,the seismic data. Onshore, the supersequence maximum Eocene succession is described as a single unit off- flooding surface is picked at the base of a marl unit shore. The Eocene section thickens gradually to the that overlies transgressive strata and extends up to

Fig. 7. Example of raw data (right) and interpreted data (left) from analysis of thin-sectioned well cuttings. (A) Eocene, carbonate-dominated sequence from Baylands #1 well (depths shown in feet alongside column). High percentages of quartz sand occur in the lowstand. Transgression is indicated by dramatic increase in percentage of open shelf skeletal carbonate cuttings fragments, coupled with an upsection decrease in shallow shelf facies. The Highstand Systems Tracts shows upsection increase in shallow shelf, quartz-mollusk-dominated facies. The MFS is arbitrarily placed beneath the interval with minimum nearshore (quartz-mollusk) facies, ideally coupled with the maximum abundance of deep water facies fragments in cuttings. (B) Oligocene, siliciclastic-dominated sequences from the Mobil #1 well, showing variations in sequence expression in these facies associations. Sequence boundaries were picked at the bases of thick quartz sand intervals. Transgressions were noted by dramatic decrease in clean quartz sand, as increased shallow shelf mollusk-fragments are admixed with quartz sand. Maximum floods may be expressed as thin intervals of argillaceous carbonate muds and wackestones with abundant pelagic biotas. Highstand strata are marked by abundant mollusk-fragment packstones/grainstones, with gradually increasing quartz sand content. Black circles to the left of the raw cuttings data indicate thin-sectioned cuttings sample intervals. Note additional stratigraphic resolution provided by the cuttings data over the available vintage wireline log data. 40 ..Cfe,J rdRa eietr elg 6 20)21–57 (2004) 166 Geology Sedimentary / Read Fred J. Coffey, B.P.

Fig. 8. Interpretive ‘‘strike’’ cross-section, Albemarle Basin, showing inferred dominant lithologic units, supersequence boundaries, and supersequence maximum flooding surfaces, based on the cuttings data. Correlations are constrained by regional biostratigraphic age control and seismic data. Location of the cross section is shown in Fig. 1A. (A) Paleocene and Eocene cross section, showing multiples orders of sequence stratigraphic information revealed by cuttings data (Top Eocene datum). (B) Oligocene strike cross section, showing dominance of shallower water facies assemblages (Top Oligocene datum). ..Cfe,J rdRa eietr elg 6 20)21–57 (2004) 166 Geology Sedimentary / Read Fred J. Coffey, B.P.

Fig. 9. Interpretive dip cross-section B-BV, Albemarle Basin and offshore shelf, showing inferred dominant lithologic units, supersequence boundaries, and supersequence maximum flooding surfaces, based on the cuttings data. Interpretation constrained by regional biostratigraphic age control and seismic data. Schematic offshore projection is based on lowstand wedges and terminal shelf edges identified from shelf seismic data. Location of cross section is shown in Fig. 1. (A) Paleocene and Eocene dip section, revealing large-scale supersequence set and component supersequence stacking patterns (Top Eocene datum). (B) Oligocene dip section, showing extensively progradational stacking patterns in predominantly shallow shelf facies assemblages (Top Oligocene datum). 41 42 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57

Fig. 10. Highly thinned, updip dip cross-section C-CV, showing general lithofacies trends and sequence stratigraphy of the prograding Supersequence 3 highstand from the Middle Eocene Castle Hayne Formation (limestone) on the Cape Fear arch (Onslow Block of Harris and Laws, 1997). Lithologic data and age picks from updip outcrops are compiled from Worsley and Laws (1986) and Zullo and Harris (1987). Note highly variable package thicknesses and irregular erosional removal of section in updip areas of the basin. Location of cross section is shown in Fig. 1A.

160-km updip of the modern shoreline. Offshore, sequence 2 sands on the dip cross-section (Fig. 9A) seismic lines indicate the maximum flooding surface suggests that two sequences may be developed. How- as the base of a regional downlap surface that merges ever, the middle sand is present only in Well 15 and with the top Paleocene reflector on the deep shelf thus may be a local feature, rather than a sequence (Fig. 6A). base. The Supersequence 2 highstand systems tract is an upward-shallowing succession of deep shelf marl and 5.4. Supersequence 3 wackestone/mudstone, overlain by bryozoan skeletal limestone and minor quartz sandstone (Fig. 9A). Supersequence 3 includes the Castle Hayne Lime- Downdip in the Cape Hatteras region, the site of a stone and the overlying New Bern Formation (Fig. 3; subsequent Middle Eocene carbonate buildup, a subtle Harris and Laws, 1997). This supersequence is not thickening of bryozoan limestone occurs in highstand only largely Middle Eocene (Lutetian–Bartonian) in strata, thinning both along strike to the northwest and age, ranging from NP Zone 15 to NP 17 (Zullo and southeast and downdip to the east (Figs. 8A and 9A). Harris, 1987), but also includes the overlying Upper In offshore seismic profiles, gently sloping clinoforms Eocene strata (Priabonian) spanning Zones P15–17 of the highstand systems tract downlap seaward onto (Zarra, 1989). Downdip, only Zones P15 and 16 have the top-Paleocene reflector on the deep shelf (Fig. 6A). been recognized in the subsurface (Zarra, 1989). Harris and Laws (1997) recognized a single third- Supersequence 3 is up to 200 m thick and has a order sequence within the Lower Eocene succession. highly erosional updip limit. Based on the distribution In this study, the stratigraphic distribution of Super- of Middle Eocene outliers, the depositional limit of B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 43

Supersequence 3 extended far updip of its present day onshore seismic strike and offshore dip lines. This erosional zero isopach, and significantly landward of downlap surface appears to correspond updip with the the underlying supersequences (Fig. 4B).Itisthe previously mentioned marl and phosphatic conglom- most regionally extensive Paleogene unit of the North erate of the New Hanover member (Fig. 10). Carolina coastal plain. The Supersequence 3 highstand systems tract con- The base of Supersequence 3 appears to be a Type sists of progradational upward-shallowing sequences II sequence boundary (cf. Van Wagoner et al., 1990), grading from skeletal packstone/wackestone, then based on the absence of a lowstand wedge seaward of bryozoal packstone/grainstone, and quartz-mollusk the preceding Supersequence 2 terminal inner shelf sand. Highstand strata cap the Hatteras buildup and break on offshore seismic (Fig. 6A). Updip, the subsequently fill in the low areas along strike north sequence boundary is a major erosional unconformity and southeast of the buildup as a series of seismically between Middle Eocene units of Supersequence 3 and defined clinoforms, which flatten away from the the underlying Cretaceous sediments; elsewhere, it buildup (Fig. 8A). A thick quartz sand caps the unconformably overlies Paleocene sediments of buildup and drapes its flanks, extending downdip onto Supersequence 1 (Harris et al., 1993; Harris and the floor of the inner shelf, where it underlies the post- Laws, 1997). Contacts between Middle Eocene rocks buildup strata that fill topographic lows adjacent to the of Supersequence 3 and underlying Supersequence 2 buildup. This sand extends along strike onto shallow (Lower Eocene) sediments have been observed only shelf areas of both the Onslow Block to the southwest in well cuttings from the deeper parts of the Albe- and the northern part of the Albemarle Block (Fig. marle Basin, which suggest the units are generally 8A). Landward (northwest) of the buildup, the sandy disconformable. facies merge into a single thick prograding sandy unit A large sediment buildup makes up the bulk of the (Fig. 9A). Sand-prone units within the post-buildup supersequence transgressive systems tract beneath the infill succession are focused on the flanks of the Cape Hatteras area (Figs. 1 and 6B). This buildup, sediment buildup, dying out along strike away from informally named the Hatteras buildup, is roughly 100 the buildup. North (along strike) of the buildup, the m thick by 50 km wide and consists of bryozoan– infill succession has several thick skeletal wackestone echinoderm grainstone/packstone (Fig. 8A). Along units that are notably absent from the southern side of strike (to the north and southwest) away from the the buildup (Fig. 8A). On the southern basin margin buildup, the transgressive systems tract is condensed. (adjacent to the Cape Fear arch), the highstand con- Updip on the Onslow Block and Cape Fear Arch, the sists of a series of prograding skeletal carbonate- transgressive systems tract and maximum flooding dominated sequences (Fig. 10). surface are recorded by a thin, condensed marl (Zone The most updip highstand (?) units of Superse- NP15) and reworked phosphatic conglomerate named quence 3 occur as erosional outliers composed of the New Hanover member (Ward et al., 1978; Zullo quartz sands with silicified mollusks (Cabe, 1984; and Harris, 1987; Baum and Vail, 1988). Immediately Harris and Laws, 1997); however, the age of these offshore, the transgressive systems tract can be iden- strata are poorly constrained. The updip edge of the tified as a thin unit between the toes of overlying Upper Eocene portion of the highstand of Superse- downlapping clinoforms and the underlying top-Low- quence 3 is largely erosional and shifted significantly er Eocene highstand reflectors. Further offshore, the seaward of the Middle Eocene updip limit. These seismic data suggests that the transgressive systems Upper Eocene units are dominated by sandy mollusk tract reappears as a broadly mounded wedge of deep grainstone/packstone and skeletal wackestone–mud- water sediment that thickens toward the continental stone; they are thickest north of the Hatteras buildup, shelf edge (Fig. 6C). where they fill the remaining low topography (Figs. The Middle Eocene maximum flooding surface is 8A and 9A). placed at the base of a regional wackestone/mudstone Offshore, the Supersequence 3 highstand systems unit in the middle of the buildup section. This surface tract has gently clinoformed reflectors near the slopes away from the buildup, merging with the floor terminal inner shelf break. Offshore seismic strike- of the shelf to form a regional downlap surface on the lines reveal complex mounded geometries, lateral 44 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 pinchout, and complex downlap relations on the The seismically defined Supersequence 4 lowstand deep shelf (Fig. 6C). forms a thin (f 35 m), elongate wedge extending 3 Five depositional sequences were previously rec- km seaward of the Supersequence 3 terminal inner- ognized in the Middle to Upper Eocene interval; three shelf break. Supersequence 4 reflectors (lowstand and occur in the Lutetian–Bartonian, one at the Barto- possibly early transgressive systems tracts) onlap the nian–Priabonian boundary, and one within the Pria- top of the Supersequence 3 boundary updip and bonian (Harris and Laws, 1997). In updip outcrops, gently downlap onto the deep shelf (Fig. 6A). The the third-order sequences are highly thinned and often thin (15 m) supersequence transgressive systems tract lack the basal quartz sandy lowstand facies observed is subseismic, but consists of regional phosphatic in well downdip. Instead, they contain well-developed sands and hardgrounds and mollusk-rich skeletal basal phosphatic hardgrounds at sequence boundaries carbonates in well cuttings. The Supersequence 4 (cf. Baum and Vail, 1988). In the well data, at least maximum flooding surface is picked onshore at the five sequences can be recognized by the presence of base of a thin (5 m) regional marl, which passes updip sand-prone intervals at their bases, but the correlation and along strike to the north into phosphatic sands of each subsurface sequence to those mapped in and hardgrounds (Fig. 9B). Offshore, the maximum outcrop is not clear due to limited age control and flooding surface is a regional seismic downlap surface pervasive erosion in updip areas. that corresponds with the top of Supersequence 3 (Fig. 6A). 5.5. Supersequence 4 (Lower Oligocene) The Supersequence 4 highstand systems tract con- sists of silty foraminifer sands and local sandy, phos- Supersequence 4 includes the Trent or lower River phatic oolite packstone/wackestone, which grade Bend formations (Ward et al., 1978; Zullo and Harris, upward into sandy mollusk wackestone/packstone 1987; Harris et al., 2000). It is Lower Oligocene and sandy echinoid-barnacle-mollusk grainstone with (Rupelian) in age, spanning Zones P19/20 (Zarra, minor interbedded clean quartz sands. Overall, the 1989), which is supported by 87Sr/86Sr dating (Harris units fine along strike away from the Hatteras buildup, et al., 2000; Fig. 3). However, Harris and Laws (1997) as well as downdip (Figs. 8B and 9B). Onshore, the point out that some units included in the Rupelian top Supersequence 4 highstand unit corresponds with based on foraminiferal evidence, can be assigned to a phosphatic hardground (overlain by quartz skeletal the earliest Chattian (NP zone 24) based on calcareous sands) that corresponds with a significant regional nannofossils assemblages. seismic peak. The updip limit of Lower Oligocene Superse- In offshore areas lacking Lower Oligocene low- quence 4 is depositional, but has shifted 150 to 250 stand deposits, the highstand has prograded out a km basinward relative to the inferred updip deposi- short distance seaward of the Supersequence 3 termi- tional limit of Supersequence 3, (Harris and Laws, nal inner shelf margin, with toes of clinoforms that 1997). However, its updip limit lies 20–30 km drape the underlying Eocene margin and pinch out landward of the erosional edge of the Upper Eocene onto the deep shelf floor (Fig. 6A). In areas where the (Priabonian) highstand portion of Supersequence 3 Lower Oligocene lowstand deposits are present, the (Harris and Laws, 1997). Supersequence 4 is 0 to 15 highstand units form a broadly mounded aggrada- m thick onshore, but thickens offshore to greater than tional unit that fills in the accommodation space above 50 m. It locally exceeds 100 m in thickness within the lowstand deposits. Here, the clinoform reflectors two small sediment lobes near the inner-shelf margin onlap the underlying Eocene inner shelf margin and (Fig. 4C). downlap onto the deep shelf just seaward of the Offshore, just seaward of the Supersequence 3 prograded edge of the lowstand (?) unit. The terminal terminal inner shelf break, Supersequence 4 pinches inner shelf break of Supersequence 4 is irregular in out both along strike (to the northeast and southwest) strike but trends broadly southwest (Fig. 4C). Over and downdip (to the southeast), but thickens into 50 to much of the deeper shelf, seismically defined isopachs 200 m thick circular to elongate sediment lobes near show that the Lower Oligocene sediments are absent the continental shelf edge (Fig. 4C). along a strike-parallel band just seaward of the inner B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 45 shelf margin but reappear downdip as broadly the inner shelf slope and deep shelf as a series of low mounded units on the deep shelf (Popenoe, 1985; angle clinoforms. They form a thin, strike-parallel Fig. 4C). wedge that thickens and thins along strike, locally Wells containing dominantly deeper water marl forming lobes just seaward of the Supersequence 4 and fine foraminifer sands typically consist of a single terminal shelf edge (Fig. 4D). upward shallowing succession that lacks recognizable The transgressive systems tract of Supersequence 5 third-order sequences (Harris et al., 2000). However, in onshore wells is a discontinuous unit 10 to 15 m up to three sequences can be recognized in offshore thick, composed of sands and sandy-mollusk facies seismic profiles (Snyder et al., 1994) and in cuttings that cannot be resolved on seismic (Figs. 8B and 9B). from a few onshore wells dominated by shallow water The maximum flooding surface of the Upper Oligo- facies (Fig. 8B, wells 6, 7, 8). The sequences appear to cene supersequence is difficult to trace regionally but be upward deepening, followed by upward shallowing is marked by a major decrease in sand and mollusk- units bounded by thin sands or sandy mollusk units, fragment sand cuttings, coupled with an increase in which sandwich slightly deeper water phosphatic delicate skeletal carbonates and carbonate mud, in the sands and muddy carbonates. wells. The maximum flooding surface cannot be defined on the onshore seismic lines, because the 5.6. Supersequence 5 (Upper Oligocene–Lower reflectors are relatively flat lying. Offshore, it is a Miocene) regional downlap surface onto the Supersequence 5 lowstand wedge on seismic data. Further seaward, Supersequence 5 is mainly Upper Oligocene but where the wedge is absent, the maximum flooding may extend into the basal Miocene. The Lower surface is a downlap surface on top of the underlying Miocene units in the onshore wells were above the supersequence (Fig. 6A). designated study interval, so cuttings from this inter- The Supersequence 5 highstand systems tract in val were not analyzed. Offshore isopach maps of onshore wells is dominated by an aggradational Supersequence 5 (Fig. 4D) and seismic line drawings succession of quartz, quartz-mollusk, and mollusk- adapted from Popenoe (1985) (Fig. 6C) may include fragment sands, grading northward into phosphatic Lower Miocene with the Upper Oligocene units. sands and silty sands. The highstand units form a Onshore, the base of the Upper Oligocene superse- grossly upward-shallowing succession that becomes quence is in NP Zone 24 (Parker, 1992) and extends more mollusk-rich and then sandy upsection. Off- up into the Lower Miocene (Aquitanian Zone N4; shore, the inner shelf margin has high relief (over 200 Zarra, 1989). Thus, the data suggest a significant m) above the deep shelf; a small late highstand break between the Upper Oligocene and the underly- carbonate buildup may be developed near the inner ing Lower Oligocene supersequence (Fig. 3). shelf break (Fig. 6A). The highstand deposits along Supersequence 5 is 0 to 50 m thick beneath the the inner shelf margin occur as gently seaward-dip- present coastal plain (Fig. 4D). Supersequence 5 ping clinoforms; further seaward, gentle clinoforms thickens offshore into a series of elongate to lobate exist within strike-parallel lobes on the deep shelf wedges (roughly 50 km across and 50 to 350 m thick) (Fig. 4D). just seaward of the underlying Supersequence 4 At least three sequences can be recognized within terminal inner-shelf margin that thin downdip into a the inner shelf succession of Upper Oligocene Super- northeast-trending zone of truncation (Fig. 4D). sequence 5 from offshore seismic data (Snyder, 1982). Possible lowstand systems tract strata of Super- These sequences are also evident in some wells but sequence 5 are expressed on the offshore seismic as cannot be traced with confidence between wells, due low relief, mounded units above an irregular surface to limitations in biostratigraphic data, onshore seismic on the upper slope of the Supersequence 4 terminal data, and localized sediment accumulation/preserva- inner shelf margin (Fig. 6A). Elsewhere, strata form tion. Where evident, the cuttings suggest that sequen- localized wedges at the foot of the Supersequence 4 ces consist of transgressive basal quartz sands and inner shelf margin. These lowstand units onlap the mollusk-fragment sands, overlain by upward shallow- terminal inner shelf margin updip and downlap onto ing highstand successions of phosphatic sand, fine 46 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 skeletal packstone/wackestone, or silty fine sand that 1997; Miller et al., 1998). The overall deepening to transition upward into quartz-mollusk sand facies. shallowing trend observed in this Paleogene succes- sion defines a supersequence set that reflects long- term global eustatic response to major tectonism. 6. Controls on sequence development Similarly, supersequences formed in response to sig- nificant tectonism elsewhere in the world, which 6.1. Duration and subsidence rates directly influenced global circulation, climate, and eustasy. Based on published chronostratigraphic charts, the Paleogene supersequences in North Carolina range in 6.1.1. Controls on Supersequence 1 deposition duration from about 5 to 14 my (Berggren et al., Upper Cretaceous sediments in updip areas were 1995). Each supersequence consists of two to five exposed during a latest Cretaceous lowstand that may third-order depositional sequences 0.5 to 5 my in have formed in response to global cooling, as sug- duration (cf. Weber et al., 1995). Of the 41 my gested by mollusk-dominated carbonate assemblages duration of the Paleogene, only about 75% of the in the region and global isotope excursions (cf. time has a recognized depositional record on the inner Barrera et al., 1987). Deposition of Supersequence shelf in North Carolina (onshore); the remainder is 1, which extends from the latest Cretaceous to the represented by unconformities. latest Paleocene (Plankton Zones P1, P2, and P4; Total subsidence rates for the onshore Paleogene Zarra, 1989), probably was initiated during latest sections average 1 cm/ky (minimum, compacted), Maastrichtian sea level rise (cf. Haq et al., 1988; roughly half of the Cretaceous subsidence rates, based Frakes et al., 1994) associated with transition into on gross thickness variations between the two inter- global greenhouse climate. This total relative sea level vals in deep onshore wells. Overall, the accumulation rise would have needed to be at least 100 m, because rates for the Albemarle Basin are relatively low it caused Upper Cretaceous near-shore facies to be compared to accumulation rates calculated from much overlain by deep water marls (deposited in water thicker sections in offshore wells elsewhere on the depths probably in excess of 100 m; cf. Collins, western Atlantic margin, where accommodation was 1988) on the Albemarle Block (cf. wells 2, 9 and not the limiting factor and subsidence was slightly 16; Fig. 8A). This eustatic rise probably was assisted higher (Steckler and Watts, 1982; Heller et al., 1982). by differential subsidence and consequent water load- Sediment accumulation rates were even lower on the ing, especially on the Albemarle Block. In the central, arches, which may be sites of Cenozoic faulting and deeper part of the Albemarle Basin, Paleocene marls relative uplift, locally associated with compression appear to form a correlative conformity on Cretaceous (Brown et al., 1972, 1982; Bramlett et al., 1982; marls. The Haq et al. (1988) chart suggests that the Reinhardt et al., 1984; Owens and Gohn, 1985; Soller, Lower Paleocene (Danian) interval should show 1988; Prowell, 1989; Berquist and Bailey, 1998). greater onlap than the Upper Paleocene (Thanetian) Abrupt changes in sediment thickness onshore are interval (Fig. 3). Yet in North Carolina, the Thanetian associated with numerous, small-offset faults on seis- shows much greater onlap, suggesting increased sub- mic data (Fig. 8; Baum, 1977) and local thickenings sidence of the Albemarle Block in the Upper Paleo- into small graben-like depressions (McLauren and cene (Harris and Laws, 1997). In contrast, because the Harris, 2001). Variation in subsidence rates across Onslow Block underwent differential uplift (or only the basin strongly controlled the thickness and to a limited subsidence), Thanetian strata only slightly lesser extent, the facies distribution of the Paleogene overstep Danian units in the southern study area units. Thus, local subsidence had a major influence on (Harris and Laws, 1997). Because the Onslow Block the overall thicknesses of the supersequences. How- remained elevated, shallow shelf facies dominate the ever, eustatic sea level changes were the major influ- Paleocene section in the southern study area. ence on the timing of supersequence and sequence Regional climatic warming throughout the Paleo- development in North Carolina and elsewhere on the cene resulted in transition from wet temperate to moist Atlantic margin (Harris et al., 1993; Harris and Laws, subtropical climates on the Atlantic margin, in step B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 47 with global warming trends (Nystrom et al., 1991; quence lowstand quartz sands were localized downdip Frakes et al., 1994). The resultant increased forest in the depocenter, and may have formed a subtle cover trapped coarser detritus, allowing only silt and topographic high that stimulated later bryozoal car- clay to be carried out to the shelf, favoring widespread bonate deposition when the warm Upper Paleocene deposition of silty marls on the shelf. Absence of the waters flooded the basin (Fig. 8A). This precursor to ancestral Gulf Stream also favored uniform hemi- the Hatteras buildup may have been initiated from pelagic deposition on the deep shelf. Widespread interaction of the recently activated ancestral Gulf accumulation of sponge spicules in the marls could Stream (which became active on the shelf during the have been promoted by cool stratified bottom water early Late Paleocene; Huddleston, 1993; Pinet and (Collins, 1988) or these could have been a response to Popenoe, 1985) with a seaward bend in the continen- input of dissolved silica from tropical weathering of tal margin, which localized deposition of sediment the continent. undergoing along-shelf transport and provided The appearance of large benthic foraminifera in nutrients via local upwelling. Upper Paleocene deep inner shelf facies reflect relatively warm waters on the shelf, compatible with 6.1.2. Supersequence 2 controls Paleocene warming (Frakes et al., 1994).This Supersequence 2 is bracketed between Lower Eo- warming culminated in the short-lived (2 my) Pa- cene transgressive strata dated as Nannofossil Zone leocene–Eocene thermal maximum. This event is NP12 and the previously mentioned underlying associated with a widespread global extinction in Supersequence 1 highstand, which corresponds with both the marine and terrestrial realms, turnover from uppermost Paleocene Plankton Zone P4 (Zarra, 1989; thermohaline to stenohaline ocean circulation, and a Bralower, personal communication, 1999). As such, global negative 13C isotope shift at the Paleocene/ the base of this event may correspond with the global Eocene boundary, perhaps triggered by dissociation cycle lowstand at the base of TA2.2 of Haq et al. of methane hydrates or increased tropical weathering (1988) (Fig. 3). This sea level lowstand is marked by (Zachos et al., 1994; Berggren et al., 1998; Bralower an abrupt, short lived increase in oxygen isotope et al., 2002; Gammon et al., 2000). By the end of values (cooling and/or ice buildup) after the Paleo- the Paleocene, the Supersequence 1 inner-shelf cene–Eocene thermal maximum in the deep sea break had prograded up to 15 km seaward of the (Bralower et al., 2002) and could have involved a preceding uppermost Cretaceous terminal inner-shelf relative sea-level fall of over 100 m (cf. Haq et al., break, indicating sedimentation well in excess of 1988). Such a fall would have exposed the shelf for up accommodation on the shelf. In addition, increased to 3 my, allowing lowstand siliciclastics to form the coarse siliciclastic input into downdip component observed sediment wedge seaward of the terminal sequences suggests decreased accommodation in Supersequence 1 inner shelf margin (Fig. 11A). updip areas. Haq et al. (1988) show a major landward shift in Ages of the Paleocene sequences led Harris and deposition in the Ypresian (Lower Eocene), relative to Laws (1997) to invoke a eustatic origin for the the Thanetian (Upper Paleocene); this flooding event Paleocene sequences in North Carolina, as did Miller corresponds with the Paleogene maximum global sea et al. (1998) in New Jersey. At least two relative sea level. However, Harris and Laws (1997) point out that level falls near the Danian–Thanetian boundary punc- the opposite occurs in North Carolina, with the tuated deposition of Supersequence 1 to form the two Ypresian depositional edge shifted some 90 km basin- sandy units within the depocenter (Figs. 8A and 9A), ward relative to the Thanetian. This suggests that both which have also been recognized in offshore seismic the Onslow and Albemarle blocks underwent uplift in (Popenoe, personal communication, 1999). These the Lower Eocene (Harris and Laws, 1997), limiting relative falls could have been associated with the Supersequence 2 deposition to the downdip axis of the eustatic sea level falls at the top of the Danian and Albemarle basin. This eustatic sea-level rise was in the early Thanetian (the Haq et al., 1988 sea level initiated just prior to NP12/13 (Bralower, personal lowstands 1.4 and 2.1; Fig. 3), but biostratigraphic communication, 1999) and perhaps involved a relative control is insufficient to verify this similarity. Se- sea level rise in excess of 100 m. The rise resulted in 48 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 49 deposition of thin, shallow water transgressive units Laws, 1986; Zullo and Harris, 1987), moving the on the inner shelf, followed by thin, but widespread coastal depositional edge some 175 to 200 km updip deep shelf marls (Figs. 8A, 9A and 11B). Highstand of the Ypresian (Lower Eocene) updip limit, as deposition continued through NP13 and formed an indicated by the presence of Middle Eocene erosion- upward shallowing succession with renewed deposi- al remnants far updip of their present day erosional tion of bryozoal sediments on the low relief Hatteras edge (Haq et al., 1988; Harris and Laws, 1997; Fig. buildup near the inner shelf margin. Global isotopic, 4B). Given this age, this flooding event spans the faunal, and floral data indicate that the Cenozoic sea level highstands of cycle 3.2–3.3 of Haq et al. thermal maximum occurred in the Early Eocene (1988). Harris and Laws (1997) point out that this (Miller et al., 1987, 1998; Prothero, 1994; Berggren large scale Middle Eocene (Lutetian–Bartonian) et al., 1998), supported in this study area by the coastal onlap is not predicted by the Haq et al. presence of large benthic foraminifera (cf. nummuli- (1988) chart, which indicates that highstand sea tids and lepidocyclinids) in these bryozoal shelf sedi- levels should be lower than those in the Lower ments. Highstand deposition terminated at the end of Eocene. This discrepancy suggests that the Onslow NP13 (Bralower, personal communication, 1999) near and Albemarle blocks may have undergone increased the top of the Lower Eocene, when thin, prograding subsidence at this time, resulting in a significant sequences with siliciclastic caps were abruptly relative sea level rise in the region (Harris and Laws, flooded by the thick shelf carbonate package of 1997). Supersequence 3. Thin phosphatic conglomerates on the southern basin margin (Onslow Block) that were deposited 6.1.3. Supersequence 3 controls following deep submergence of the inner shelf reflect The Supersequence 3 boundary spans NP14/part of active Ancestral Gulf Stream scouring and sediment- P10 (Zarra, 1989; Harris and Laws, 1997), and may bypass, with the lower sequences being highly thinned include the sea level lowstand of Haq et al. (1988) cycle by erosion and lack of accommodation space (Figs. TA3 (Fig. 2). Although Haq et al. (1988) suggest sea 8A, 10 and 11B). To the north (Albemarle Block), level fall of over 150 m occurred at the Early to Middle much of the accommodation space on the inner shelf Eocene boundary, this is not evident in the Super- during transgression was filled by the large Hatteras sequence 3 sequence boundary in North Carolina, buildup (Fig. 8A). These Eocene shelf waters were which is a Type II boundary that appears to lack a marginally subtropical and well oxygenated, favoring lowstand wedge. This sea level fall resulted from latest widespread development of echinoderm–bryozoan Early Eocene global ocean cooling that continued into facies with scattered large benthic foraminifera (num- the Middle Eocene, a period of some 3 my (McGowran mulitids and lepidocyclinids). Buildup deposition et al., 1997). It is represented by deposition of a ceased during NP16/17 time (late Middle Eocene) widespread quartz sand unit in the Albemarle Basin when onset of global cooling, aridification, and rela- center (Fig. 8A) and a major hiatus across the updip tively prolonged sea level fall (Miller et al., 1987; Haq inner shelf (Harris and Laws, 1997; Fig. 11A). et al., 1988; McGowran et al., 1997) increased fluvial In North Carolina, relative sea-level rise peaked siliciclastic input (cf. Cecil, 1990) to form the wide- during Middle Eocene zone NP15 (Worsley and spread, relatively thick quartz sand blanket extending

Fig. 11. Schematic depositional reconstructions of stages of supersequence development in this mixed carbonate–siliciclastic open shelf setting. (A) Supersequence lowstand, with localized lowstand wedge development, absence of a broad, wave-dominated shallow shelf, and slope incision by boundary currents. (B) Supersequence late transgression/maximum flood, marked by extensive shelf flooding and development of a broad, wave-dominated inner shelf composed largely of detrital carbonate skeletal material. Landward migration of boundary currents onto the shelf resulted from major eustatic rise, and is expressed as broad, shore-parallel erosional features and local accumulations of phosphatic material. Also note limited siliclastic input onto the shelf, likely in response to development of bays and estuaries in updip settings. (C) Supersequence late highstand characterized by reduced accommodation space and resultant progradation of shallow shelf facies. Increased siliciclastic material is delivered to the shallow shelf. Open shelf skeletal carbonate assemblage belts are pushed seaward by the combined effects of limited accommodation space and approaching siliciclastic material on the inner shelf. Also note the entrenchment of boundary currents on the deep shelf, in response to slowly falling relative sea levels during late stages of the highstand. 50 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 across the Albemarle Block and mantling the Hatteras through the Suwannee Straits of northern Florida/ buildup (Fig. 8A). southern Georgia (Huddleston, 1993) and onto the With renewed flooding in NP17 time, bryozoal North Carolina shelf. This caused erosional incision carbonates were widely deposited throughout the and local wholesale removal and remobilization of region, filling in remaining topographic lows north Eocene deep shelf sediment into elongate lobes (Fig. and south of the Hatteras buildup, to form a flat- 4B; Popenoe, 1985). It also may have elevated shelf topped shelf. Initial relatively cool water conditions water temperatures, facilitating sediment production are suggested by the paucity of larger foraminifera by more prolific subtropical faunas. Overall, Eocene from much of the shelf section. However, these larger shelf sediment production and accumulation outpaced foraminifera become more abundant up-section, as accommodation rate, causing the terminal Middle waters warmed during the latest Middle Eocene Kir- Eocene inner-shelf break to prograde up to 13 km thar Restoration, the Cenozoic thermal maximum seaward of the position of the uppermost Paleocene recognized in the Southern Ocean (McGowran et al., shelf break. 1997). This was followed by a relative sea-level fall Based on their ages, Harris et al. (1993) equated (associated with cooling) at the end of the Middle the Middle to Late Eocene sequences onshore to the Eocene estimated to be over 100 m by Haq et al. Haq et al. (1988) global cycles TA3.3, 3.4, 3.5/3.6, 4.1 (1988), or as little as 20 to 30 m (based on isotopic and 4.2/3. Miller et al. (1998) used isotopes, magneto, evidence; Miller et al., 1998). This sea level fall and biostratigraphy to correlate late Middle to Late effectively terminated widespread bryozoal limestone Eocene cycles in New Jersey to global sea level deposition over much of the North Carolina. changes, implicating an ice volume control, whereby The Late Eocene (Priabonian) relative sea levels the growth and decay of small ice sheets prior to rose up to 50 m above the previous Middle Eocene continental glaciation caused sea level fluctuations of lowstand position (Haq et al., 1988), but remained far less than 20 m. Although we lack sufficient age below the Middle Eocene highstand levels (Fig. 3). control from the North Carolina subsurface sequences This is manifested in a large-scale (150 km) seaward to evaluate timing of sea level changes, there is a shift of the Upper Eocene (Priabonian) updip deposi- strong possibility that they were driven by global tional limit, relative to that of the preceding Middle eustasy, albeit with smaller amplitude sea level Eocene updip limit (Harris and Laws, 1997). Middle changes (10 to 20 m) more akin to those suggested Eocene stratal filling of accommodation space by Miller et al. (1998) than those indicated by Haq et resulted in a much narrower, shallow shelf profile al. (1988). during Upper Eocene flooding (Fig. 11C). As a result, much of the shelf remained emergent or very shallow 6.1.4. Supersequence 4 controls in the south. Only north of the Hatteras buildup on the Isotopic data and Antarctic dropstones indicate that Albemarle Block, where downwarping and compac- an abrupt major global cooling in the Late Eocene tion of fine sediments generated accommodation marked the transition from global greenhouse to space, were water depths sufficient to allow thin units icehouse climates and culminated in the basal Oligo- of deeper shelf muddy carbonates to be deposited cene sea level lowstand (Miller et al., 1987; Denison during the Upper Eocene third-order highstand (Fig. et al., 1993; Prothero, 1994; Zachos et al., 1994; 8A). Intercalated cooler-water mollusk shell beds and McGowran et al., 1997). The cooling increased aridity fine terrigenous silts and sands formed on the much and decreased forest cover, resulting in increased narrowed shelf during Late Eocene lowered sea levels, siliciclastic input onto the Oligocene Atlantic shelf likely in response to arid, cooler climates (Harris and and ocean-basin floor (Fig. 8B; Poag, 1992). The Laws, 1997; Lees and Buller, 1972).Thisclastic lowered sea levels and increased clastic influx also influx and change in shelf geometry may have caused more pronounced development of Lower Ol- inhibited bryozoan-rich carbonate assemblages com- igocene lowstand deposits seaward of the terminal mon in Middle Eocene strata. Eocene inner-shelf margin in North Carolina. On the The Eocene highstands of sea level allowed the inner shelf, patchy quartz sands were deposited during ancestral Gulf Stream to flow in a northeastward path third-order lowstands and early transgressions. Low- B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 51 ered Early Oligocene sea levels also caused the Decrease in abundance of bryozoans may reflect ancestral Gulf Stream to migrate south to flow increased nutrients via upwelling on the shelf, which through the Florida Straits (Huddleston, 1993),as may have favored mollusks over oligotrophic bryo- well as more consistently further downdip, from the zoans (cf. Collins, 1988; James et al., 2001). deep outer shelf to seaward of the continental shelf Although up to three third-order sequences are break. This resulted in widespread erosion of the deep evident on offshore seismic (Snyder et al., 1994) shelf and continental slope as it flowed northward and in a few of the onshore wells, there is not along the North Carolina margin (Fig. 11C). sufficient biostratigraphic or radiometric control to Return to warmer climates caused a large Early evaluate whether they are synchronous with the three Oligocene sea-level rise (estimated to be at least 50 m Lower Oligocene events from New Jersey described by Kominz et al., 1998, and 75 to 100 m by Haq et al., by Kominz and Pekar (2001) (Fig. 3). However, the 1988), which drowned the North Carolina inner shelf New Jersey data suggest that the western Atlantic and resulted in widespread deposition of marl (Trent shelf was subjected to at least three Early Oligocene Marl) across the inner shelf. However, this Early eustatic events. The early events involved at least 50 Oligocene flooding event only moved the most updip m of eustatic sea level change (and probably more, shoreline position to roughly the location of the given that sea level probably fell further during preceding Upper Eocene shoreline limit, far downdip depositional hiatuses), requiring almost complete gla- of the most landward limit of the Middle Eocene ciation of Antarctica during lowstands (Kominz and shoreline. The actual updip limit of the Lower Oligo- Pekar, 2001). Subsequent sequences formed under cene is not well defined, due to Neogene erosion smaller sea level changes of 40 m or less (Kominz (Harris and Laws, 1997). High sea levels caused the and Pekar, 2001). Sediment input exceeded accom- ancestral Gulf Stream to migrate landward onto the modation as a result of falling sea level toward the end deep shelf. These highstand currents then incised a of the Lower Oligocene. Thus, the terminal Lower swath across the deep shelf and reworked hemipelagic Oligocene inner-shelf break prograded up to 13 km sediments into broad lobes (Figs. 4C and 8B; Popenoe seaward of the previous (Eocene) supersequence shelf et al., 1987). Boundary currents also may have spalled break during the Supersequence 4 highstand. gyres onto the deep shelf north of the bend in the margin at Cape Hatteras, which initiated localized 6.1.5. Supersequence 5 controls upwelling and deposition of the phosphatic units (cf. Major global cooling in the mid-Oligocene (Rupe- Riggs, 1984; Fig. 8B). lian–Chattian boundary) is evident in the deep sea The cooler climate and lowered sea-levels through isotope record (Miller et al., 1987). This resulted in a the Lower Oligocene, relative to the Eocene, in- global sea-level fall of approximately 100 m (Haq et creased siliciclastic deposition on the shelf, resulting al., 1988). This is erroneously shown as occurring at in an overall shallowing upward succession on the 30 my on the Haq et al. (1988) chart, but actually inner shelf from marls into fine silty sands or oolitic occurs at 28.5 my (Berggren et al., 1995), which phosphates, then into molluscan/barnacle/echinoderm coincides with a major sea level fall evident in New sandy carbonates and quartz sands. Dominantly shal- Jersey (requiring almost complete Antarctic glacia- low water inner shelf facies over the Hatteras buildup tion; Kominz and Pekar, 2001; Fig. 3). In North area indicates that it was a positive area, while deeper Carolina, this large eustatic fall caused localized water sediments formed along strike and downdip in deposition of the Supersequence 5 lowstand sediment areas of greater accommodation. Climates during the wedges on and at the seaward foot of the Lower highstand(s) remained relatively warm, based on the Oligocene (Supersequence 4) terminal inner-shelf nearshore subtropical Rupelian mollusk assemblages break (Fig. 6A). (Baum, 1977; Rossbach and Carter, 1991), perhaps in Haq et al. (1988) show that the subsequent sea part reflecting warming by ancestral Gulf Stream level rise was only 50 m above the earlier medial waters. However, large benthic foraminifera, scattered Oligocene lowstand position, far below the previous coral, and aragonitic bryozoans characteristic of Eo- highstand position. However, Kominz and Pekar cene warm water carbonates are notably absent. (2001) show that New Jersey data suggest that 52 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 during the Late Oligocene highstand, global sea Three to four third-order sequences evident in level possibly flooded shelves to Early Oligocene seismic (Snyder et al., 1994) and locally in well highstand levels (Fig. 3). In North Carolina, this sea cutting logs were deposited on the inner shelf during level rise returned the updip depositional limit of the the Upper Oligocene. Scattered distribution of these Upper Oligocene on the Onslow Block to roughly strata, coupled with lack of biostratigraphic and ra- the same updip limit as observed in Lower Oligo- diometric control on ages of individual sequences cene Supersequence 4. However, on the Albemarle prevents direct correlation of these sequences with Block, its updip limit was up to 30 km downdip of the global eustatic curves of Haq et al. (1988) or the the updip limit of Supersequence 4, suggesting uplift regional eustatic curves of Kominz and Pekar (2001) on the Albemarle Block (Harris and Laws, 1997; for the New Jersey shelf, which show three Late Fig. 4D). Oligocene global sea level cycles with magnitudes Basal Late Oligocene sea-level rise flooded the of 50 to 60 m. In general, the relatively shallow water North Carolina inner shelf to shallow depths, initiating facies of the inner shelf sequences over much of the widespread deposition of inner shelf sands and sandy Albemarle Block indicate that accommodation was mollusk carbonates of the Belgrade Formation (Figs. limited, so that tongues of thin, deeper water phos- 2 and 8B). Long-term eustatic sea level gradually fell phatic sands and muddy carbonates encroached part (over 30 m) throughout the Late Oligocene (Kominz way onto the shallow inner shelf only during peak and Pekar, 2001), thus maintaining shallow water, highstands. agitated settings during highstand progradation of During the Late Oligocene, the ancestral Gulf the inner shelf. Low diversity mollusk faunas with Stream shifted southward from the Suwannee Straits few warm water species indicate that these inner shelf to the Florida Straits, where it has remained through- waters were relatively cool (Rossbach and Carter, out the Neogene (Huddleston, 1993). From Florida, it 1991). Along strike to the north and downdip, greater flowed northeastward across the Carolina deep shelf, water depths were sufficient to allow muddy oolitic where its contour currents eroded sediments along a phosphates and silty-fine sands to accumulate; these southwest- to northeast-trending swath across the deeper water areas likely retained unfilled accommo- middle portion of the deep shelf (Fig. 4D). Localized dation from the earlier Oligocene shelf. Upper Oligo- accumulations of phosphatic sediments north of Cape cene oolitic phosphate sands, like their predecessors, Hatteras may have formed in response to gyre-in- probably were associated with gyres spun off the duced upwelling currents on the shelf from the ances- ancestral Gulf Stream north of the shelf promontory tral Gulf Stream, which developed during the greatest at Cape Hatteras, while the silty sands in the northern shelf flooding during late transgressive to early high- study area were distal to the extensive deltas devel- stand conditions. oped to the north in Virginia (Mixon et al., 1989; Poag, 1992). The limited accommodation on the inner shelf was 7. Discussion accentuated by the 30 m of long-term Late Oligocene sea level fall (Kominz and Pekar, 2001). Thus, accom- Results of this study yield several observations that modation space was easily exceeded by siliciclastic have broader implications in transition zone sedimen- sediment influx, combined with biogenic cool-water tary systems. Incorporation of well-cuttings generated carbonate production. This resulted in progradation of lithologic data into more coarse seismic and log-based the inner shelf break up to 7 km seaward of the stratigraphic frameworks resulted in significant im- Supersequence 4 (Lower Oligocene) terminal shelf provement in reconstructing depositional setting and margin, and the formation of broad clinoforms onto sequence stratigraphy. Vertical stratigraphic variations antecedent inner-shelf margins. Local high relief inner documented in this study demonstrate the importance shelf breaks and the ensuing sea level fall at the end of of considering modern analogs as a single time slice the Oligocene may have triggered local failure of the that cannot explain the temporal variations of ancient margin during highstand progradation, as suggested by depositional systems. For example, the modern At- possible slump scars on offshore seismic. lantic shelf is an excellent example of a Holocene B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 53 transitional mixed carbonate–siliciclastic system, but The broad, low-angle shelf geometry of this pas- it does not preserve evidence of Holocene shelf flood- sive margin resulted from major Late Cretaceous shelf ing events, because the modern system only records flooding. Reduced Paleogene sedimentation rates as- the expression of the long-term Cenozoic sea-level fall sociated with lower productivity non-tropical carbon- trend, as indicated by significant amounts of reworked ate systems were unable to fill the significant sediment on the inner-shelf and the absence of marl accommodation space created by this flooding, result- deposition in water depths less than several hundred ing in the development of the dual-break shelf profile meters. As such, the supersequence highstand depo- that slowly prograded seaward as sediment filled sitional model (Fig. 11C) only would be proposed if updip accommodation space. This dual-break geome- interpretation were limited to modern systems. try is present across the U.S. Atlantic margin and is Data from this study area suggest that bryozoan- common elsewhere on under-filled passive margins. dominated facies commonly associated with cool The low-relief inner-shelf is subjected to storm and water carbonates can extend into warm subtropical swell wave sediment winnowing and abrasion to waters, covering broad shelf areas. Association with water depths in the tens of meters. Wave agitation is large benthic foraminifera and subtropical mollusk sufficient to destabilize the seafloor, preventing estab- assemblages on the North Carolina shelf indicate lishment of non-encrusting benthic carbonate commu- warm water conditions across the inner-shelf, which nities. Limited carbonate sedimentation, coupled with we attribute in part to local climate amelioration by isolation of siliciclastic sedimentation in shallow set- the ancestral Gulf Stream. Warm currents act as a tings via wave-dominated deltas and longshore cur- buffer from seasonal temperature variations outside rents perpetuates the inner-shelf low sediment the tropical realm, favoring proliferation of the warm- accumulation zone through much of the superse- er affinity carbonate organisms on the deeper inner quence transgression, with decreasing areal extent shelf, while age equivalent updip sediments take on a during gradual highstand progradation. Strong wave more temperate appearance. The influence of bound- influence results in laterally extensive mud-lean facies ary currents on shelf deposition extends beyond belts (both carbonate and siliciclastic), which have favoring the dominant carbonate grain producer. excellent potential as hydrocarbon reservoirs. In this These currents are capable of significant shelf incision area, they form a major aquifer system with perme- during long-term sea-level highstands, as the positions abilities commonly exceeding several Darcies. of the currents migrate landwards, and then stabilize on the shelf during eustatic rise to stillstand. As these strike-parallel currents become pinned on the deep 8. Conclusions shelf, major erosion and reworking of sediment may occur over broad areas well below the influence of Data from thin-sectioned well cuttings, in conjunc- surface wave energy. Landward and seaward migra- tion with wireline logs, biostratigraphic, and seismic tion of these currents, in response to changing sea data were used to construct a sequence stratigraphic levels, also enables the development of regionally framework for the 0–500-m-thick subsurface Paleo- extensive, time-transgressive phosphatic hardground gene shelf succession in the Albemarle Basin of North pavements on the inner-shelf during highstands (Fig. Carolina. Paleogene mixed carbonate–siliciclastic 11). These well-cemented features are extremely re- open shelves typified by the North Carolina Paleogene sistant to erosion, favoring preservation of condensed are distally steepened ramps (cf. Read, 1985) that sections over the more prolific, but easily erodable developed on drowned Cretaceous shallow water highstand siliciclastic sands and grainy carbonates. shelf. The inability of non-reefal carbonate sediment Hence, the presence of well-cemented hardgrounds do producers and passive margin siliciclastic sediments not necessarily equate to a single depositional phe- to fill accommodation space created by Cretaceous nomena or sequence stratigraphic surface, as they may drowning gave rise to this distinctive dual-break shelf have been subjected to condensed sedimentation, geometry, which is typical of the southeastern U.S. erosion, exposure, or a combination of all three (often Atlantic margin even today. The inner shelf was the case in updip outcrops). characterized by quartz sand and sandy mollusk facies 54 B.P. Coffey, J. Fred Read / Sedimentary Geology 166 (2004) 21–57 inshore, passing seaward into a broad, wave-swept, Eocene, medial Oligocene, and end Oligocene gener- sediment-starved abraded shelf, and then into storm- ated supersequence boundaries and transitions to influenced bryozoan–echinoderm limestones to depths cooler water, quartz sand prone facies throughout of several tens of meters. Deeper water, fine-grained the region. Major warming events and associated sea carbonates and marls characterized deposition on the level highstands were associated with large-scale shelf inner-shelf only during major highstands. Marl depo- flooding and widespread warm water subtropical sition probably was widespread on the deep shelf carbonate deposition with conspicuous bryozoal car- throughout most of the sequence development, with bonates and a large Eocene carbonate buildup. The erosion and reworking of sediment bodies by deep ancestral Gulf Stream eroded and remobilized sedi- shelf boundary currents during highstands. ment on the deep shelf during highstands, and even Long-term shelf subsidence rates were low, but extended onto the inner shelf during the Middle major crustal blocks beneath the basin appeared to Eocene supersequence highstand; it scoured the upper have undergone a complex history of differential continental slope during lowstands. Complex interac- subsidence that modified the effects of eustatic sea tions of surface wave energy, deeper water boundary level changes across the coastal plain. Arches or currents, and significant Paleogene climatic variations structural blocks bordering the basin acted as positive documented in this study are common elements af- elements, localizing shallow shelf deposition during fecting deposition in transition zone mixed carbon- low magnitude sea level highstands, while the gener- ate–siliciclastic settings. The influence of these ally low lying Albemarle Block developed a thick, processes on deposition differs significantly from more open marine section. General agreement in tropical carbonate systems, where accommodation timing and magnitude of eustatic events observed in space is limited by high rates of sediment production. North Carolina with those documented elsewhere on Hence, revised depositional models are necessary to the U.S. Atlantic margin suggest that eustasy was the understand and predict complex facies variations primary control on sequence development, although through time in these non-tropical settings. biostratigraphic limitations make it difficult to tie individual sequences directly with dated eustatic events. Acknowledgements Widespread deep water conditions followed drown- ing of the inner shelf during the Paleocene, resulting We thank Bill Hoffman and John Nickerson of the in deposition of updip glauconitic sands and thick, North Carolina Geologic Survey, Greg Gohn and downdipshaleandmarl.Followingtheterminal- Chris Polloni of the U.S. Geological Survey, and Paleocene lowstand, Eocene flooding of the inner Daniel Textoris and Roy Ingram of the University of shelf initiated shelf deposition dominated by bryozoal North Carolina-Chapel Hill for access to data. W. limestones that locally developed into large inner- Burleigh Harris and Gerald Baum provided valuable shelf skeletal sand buildup. Widespread quartz sands, insights on regional stratigraphy. Tim Bralower and which were deposited during third-order lowstands, Richard Laws identified calcareous nannofossils in punctuate the carbonate-dominated succession. Fol- key intervals. Javier Martin-Chivelet, Noel James, and lowing Late Eocene cooling and onset of global Bruce Sellwood are thanked for providing helpful icehouse climates, Oligocene accommodation space reviews. This research was supported by grants from was greatly decreased, except for during initial deep AAPG, SPWLA, GSA, and Mobil. shelf flooding. Reduced accommodation, aided by cooler global climate and low relative sea level, favored widespread deposition of progradational References quartz sand and sandy molluscan facies in the two Oligocene supersequences. Barrera, E., Huber, B.T., Harwood, D.M., Webb, P.N., 1987. 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