A chronology of Pliocene sea-level fluctuations: The U.S. Middle Atlantic Coastal Plain record

0277-3791/91$0.00 + so @ 1991 Pergamon Press plc

QuuterncrryScknce Reviews. Vol. 10. pp. 16%174. 1991 Printed in Great Britain. All rights reserved.

A CHRONOLOGY

OF PLIOCENE SEA-LEVEL FLUCTUATIONS: ATLANTIC COASTAL PLAIN RECORD

THE U.S. MIDDLE

David E. Krantz College of Marine Studies, University of Delaware, Lewes, DE 19958, U.S.A.

The sequence of marine and marginal-marine silts and sands of the U.S. Middle Atlantic Coastal Plain preserves a relatively complete record of Pliocene sea-level highstands. A general chronology for the deposition of these units has been developed previously from studies of the microfossil biostratigraphy complemented by a limited number of paleomagnetic and radiometric dates. Within this broad temporal framework it is currently difficult to assign ages to individual transgressive events with a resolution much better than 0.5 Ma. The age model proposed here attempts to refine the depositional history of the Atlantic Coastal Plain by correlation with the more continuous deep-ocean record. The model assigns probable ages to the Pliocene transgressions onto the Atlantic Coastal Plain based on evidence for sea-level highstands inferred from high-resolution deep-ocean 6”O records. At least five major transgressive-regressive events can be correlated with presumed eustatic fluctuations. The temporal resolution for the timing of the transgressive events is greatly improved over that of the biostratigraphy alone. Many aspects of the proposed chronology require verification by more detailed field study, but it is offered as a testable working hypothesis.

INTRODUCTION

Late Neogene marine sediments of the U.S. Atlantic Coastal Plain provide a detailed record of sea-level fluctuations. Major advances have been made during the past two decades in defining the regional stratigraphy and depositional history of these sediments. Along with the improved understanding of the physical stratigraphy, age estimates for the times of transgression have been greatly refined. Even so, the biostratigraphic control often does not allow age assignments with precision much better than 0.5-l Ma. This paper will develop a chronology of Pliocene transgressive and regressive events using the best biostratigraphic control available from Atlantic Coastal Plain strata in conjunction with high-resolution 6180 records from deep-ocean cores. The proposed chronology is intended as a testable model that can be used to further refine the late Neogene geologic history of the region and to better correlate the Atlantic Coastal Plain stratigraphic record with global events. By extension, this model may also provide a test for eustatic sea-level fluctuations.

PLIOCENE STRATIGRAPHY OF THE MIDDLE ATLANTIC COASTAL PLAIN

Marine and marginal-marine sediments preserved on the U.S. Middle Atlantic Coastal Plain record a minimum of five transgressive events during the Pliocene. These units are extensively exposed in the coastal plain of southeastern Virginia and northeastern North Carolina. The regional stratigraphy and biostratigraphy of these units is well documented by previous studies (Johnson, 1969; Oaks and Coch, 1973; Ward and Blackwelder, 1980; Johnson and Peebles, 1984;

Johnson et al., 1987; Newell, 1985; Mixon, 1985; Bailey, 1987; Mixon et al., 1989). Ward and Blackwelder (1980) defined the lectostratotype of the Yorktown Formation in eastern Virginia and identified four members which represent three early to middle Pliocene transgressions. In stratigraphic order from lowest to highest these are the Sunken Meadow, Rushmere, Morgarts Beach and Moore House Members (Fig. 1). The Rushmere and Morgarts Beach Members were deposited during the same transgression, but represent distinctly different lithologies representative of an open shelf and a protected lower-energy environment, respectively. Fluvial, estuarine and marine strata that are correlative with the Yorktown Formation in the type area can be identified in the subsurface of the Delmarva Peninsula (Mixon, 1985) and the northeastern Virginia coastal plain (Newell, 1985; Mixon et al., 1989a). The upper Pliocene is represented on the Middle Atlantic Coastal Plain by the Chowan River, Bacons Castle, Windsor and James City Formations (Oaks and Coch, 1973; Blackwelder, 1981~; Ramsey, 1988). The Chowan River Formation is exposed primarily in northeastern North Carolina, although scattered exposures occur in southeasternmost Virginia (Fig. 2d). The Bacons Castle Formation has been mapped in detail in Virginia south of the York and Pamunkey Rivers (Coch, 1968; Ramsey, 1988) and sediments that are probably correlative have been identified in northeastern Virginia and northeastern North Carolina (Mixon et al., 1989b) (Fig. 2e). The James City Formation in North Carolina and the Windsor Formation in Virginia both pinch out updip against the Surry Scarp (Fig. 2f) and are tentatively correlated. In a pattern of preservation typical of the Tertiary units of the Atlantic Coastal Plain, the marine sediments of each Pliocene transgression unconformably 163

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FIG. I C‘orrclaliou 01 Pliocene mxine xtlimcntarq units of the U.S. ,Zllantic (‘oatal Plain. hllnlmum and maximum ag,es lotcoch unit xc‘ hased on foraminiferal and calcarcous nannoplankton biostratigraphy nnd B limited amount of paleomagnettc and radiomctrlc data. l‘hc boundaries dehmit the possible ranec of time for each unit and do not Indicate continuous depoGtion. Similarly. overlapping age ranges for units in different embaymenta do not neccsaar~ly imply deposition during the xmic transgressivc cvcnt. Sourccs of age cstimata arc‘ discussed in the text. Placement of the Bacon\ (‘;tdle Formation is hascd solclv on stratigraphic position; age data are not prcscntly availahlc for the unit. I.hc I .0-I 7 Ma date\ art‘ for the lower Formation

overlie marine sediments of preceding transgressions (Ward and Strickland. 1985). The unconformities separating two superposed transgressive packets are traceable regionally, and those representing the lowest stands of sea level can be identified in many continental-shelf sequences (Poag, 1984. lYX7; Poag and Ward. 1987). These regional unconformities were created by tluvial erosion and dissection during a lowstand, followed by shoreface erosion during the ensuing transgression. Consequently, regressive facics are rarely preserved and no known section contains a complete record of continuous deposition. Because these coastal plain embayments represent the extreme landward fringe of marine deposition. only the highest stands of sea level are preserved in the sedimentary record. Lower-amplitude intervening highstands may have been completely removed by erosion or reworked and mixed into transgressive lag deposits. Most of the units are bounded to the west by an escarpment which marks the landward extent of the transgression. Littoral and estuarine facies occur in some updip areas, but these units arc commonly dissected extensively and are preserved primarily on interfluves (Johnson e/ (II., 1987; Mixon L’Iul., 1989b). There appears to be a significant tectonic component to the initial deposition (i.e. producing accommodation

space) and preservation of the Pliocene marine sediments of the Atlantic Coastal Plain (Cronin, IYXl: Dowsett and Cronin, IYYO). Continual regional uplift (Hack. lY82; Markewich, 19X5) has raised these units above subsequent sea-level Posthighstands. depositional warping and uplift of the Atlantic Coastal Plain is evidenced by the pronounced rise of the Orangeburg Scarp from ;tn elevation of -35 m above sea level in the coastal plain of Georgia (which is believed to be tectonically stable; Huddlestun, 1YXX)to - 100 m along the crest of the Cape Fear Arch in North Carolina (Winker and Howard. 1977; Cronin. 19XI : Colquhoun, 19%; Dowsett and (‘ronin, lY90). The Orangeburg Scarp was cut and probably reoccupied by one or more highstands between -3.5-3.0 Ma. The equivalent geomorphic feature in Virginia is the Chippenham-Thornburg Scarp (Johnson c’t (11.. lY87: Mixon ~‘t al.. 19XYb). Biostrutigruphic

Control ott ‘Times of‘ Trunsgression

A well-developed regional biostratigraphy based on ostracodes and mollusks (Hazel. 1971. 1983; Cronin and Hazel, 1980; Blackwelder, 19Xlb; Ward and Blackwelder, lY80, 19X7) allows the correlation of late Ncogene units within the Atlantic Coastal Plain. However, most of the taxa are shelf-dwelling organisms

A Chronology

of Pliocene

Sea-Level

Fluctuations

165

FIG. 2. Depositional basins for the Pliocene marine sediments of the Middle Atlantic Coastal Plain. Modified from Oaks and Coch (1073). Ward and Blackwelder (IYSO), Blackwelder (1981~). Ward and Strickland (IYSS), Bailey (1987) and Mixon et al. (1989). (A) Areal extent of the lower Pliocene marine sediments of the Sunken Meadow Member of the Yorktown Formation. (B) The middle Pliocene Rushmere and Morgarts Beach Members of the Yorktown Formation. Fluvial and nearshore sands and gravels in updip regions that are believed to be associated with the Rushmere-Morgarts Beach transgressive event are indicated by the stippled pattern. (C) The upper middle Pliocene Moore House Member of the Yorktown Formation. (D) The upper Pliocene Chowan River Formation. (E) The upper Pliocene Bacons Castle Formation. The fluvial sands and gravels of the Varina Grove Member are indicated by the stippled pattern; the estuarine sediments of the Barhamsville Member by the dashed pattern. (F) The uppermost Pliocene or lower Pleistocene James City and Windsor Formations. The nearshore coarse sands of the Windsor Formation are indicated by the stippled pattern; the marine shallow-shelf sediments of the James City Formation by the dashed pattern. Correlation of these two units is speculative.

and do not allow reliable correlation to a global chronology. Open-ocean microfossils are generally sparse in these shelf sediments, but several studies (Akers, 1972; Akers and Koeppel, 1973; Snyder et al., 1983; Cronin et al., 1984; Ward and Huddlestun, 1988; Huddlestun, 1988; Dowsett and Cronin, 1990; Dowsett and Wiggs, 1991) have identified key species of planktonic foraminifera and calcareous nannoplankton that allow correlation of these units to standard biostratigraphic zonation. Cronin et al. (1984) compiled the available biostratigraphic data, and a limited amount of paleomagnetic and radiometric data. Figure 1 summarizes the currently available age constraints for the deposition of each transgressive unit. As demonstrated by Fig. 1, the uncertainty of the age estimates for most of the units ranges from 300 ka to more than 1 Ma. The Sunken Meadow Member of the Yorktown Formation and the lower Duplin Formation are the most poorly constrained units, being placed

generally within early zone N19 (4.8 to -3.5 Ma; planktonic foraminiferal biozones of Blow, 1969; as updated by Berggren et al., 1985) (Cronin et al., 1984; Cronin, 1988). An equivalent unit, the Wabasso Formation, in the subsurface of eastern Georgia falls within zone PLl (of Berggren, 1973), which spans the period 4.9-3.7 Ma (Huddlestun, 1988). The Cobham Bay Member of the Eastover Formation, which underlies the Yorktown Formation in Maryland, Virginia and North Carolina, is confined to the uppermost Miocene by the presence of Globorotalia conoidea (P.F. Huddleston, pen. commun., 1990). The last appearance of G. conoidea in late zone N17 or earliest N18 allows a youngest possible age of -4.9 Ma for the Eastover Formation (Kennett and Srinivasan, 1983). The middle Pliocene transgressions represented by the Rushmere-Morgarts Beach and Moore House Members of the Yorktown Formation are bracketed between 4.0 and 3.0 Ma, late in zone N19-20 or late

PLl to PL3 (Akers, 1972; Akers and Koeppel, 1973; Snyder ef al., 1983; Dowsett and Wiggs, 1991). This age assignment is consistent with that of the Duplin/ Yorktown and Raysor Formations in South Carolina and North Carolina (Cronin et al., 1984; Ward and Huddlestun, 1988; Dowsett and Cronin, 1990), and the Jackson Bluff Formation of western Florida (Huddlestun, 1988). The middle Pliocene transgressive events were the highest stands of sea level on the Atlantic Coastal Plain during the late Neogene. Extensive deposition of marine sediments stretched from Virginia into South Carolina and Georgia (Fig. 2b), overlapped the Cape Fear Arch, and lapped onto the crystalline rocks of the Piedmont (Ward and Blackwelder, 1980; Ward and Strickland. 1985; Bailey, 1987). Middle Pliocene marine silty sands can be found at 3(&40 m and nearshore sand and gravel deposits at 7(r75 m elevation in the western coastal plain of Virginia (Ward and Blackwelder, 1980; Newell, 1985; Johnson et al.. 1987: Mixon et ul.. 198913). A relatively brief lowstand, which probably dropped below modern sea level, produced an unconformity that separates the Moore House Member from the underlying Rushmere-Morgarts Beach strata (Ward and Blackwelder, 1980). In contrast to the extensive flooding of the continental margin by the RushmereMorgarts Beach Sea, marine deposition during the Moore House transgression was limited to the eastern third of the Virginia coastal plain and a restricted embayment in eastern North Carolina (Fig. 2~). This transgression has not been unequivocally identified in South Carolina or Georgia. The regression that followed the deposition of the Moore House Member is marked by extensive erosion on the Atlantic Coastal Plain and the adjacent shelf as indicated by a prominent regional unconformity (Poag, 1984, 1987; Poag and Ward, 1987). This regressive phase also coincides with a major extinction of the Atlantic Coast molluscan fauna (Stanley and Campbell. 1981; Stanley, 1986). The Chowan River Formation was deposited during the late Pliocene in an embayment centered in northeastern North Carolina and extending into southeasternmost Virginia (Fig. 2d). Marine strata deposited in South Carolina (the Bear Bluff Formation) and Florida (the lower Caloosahatchee Formation) have been tentatively correlated with the Chowan River Formation (Blackwelder, 1981a,b; Ward and Blackwelder, 1987). Lithofacies and biofacies changes evident in exposures of the Chowan River Formation in North Carolina imply a relatively rapid transgression, which produced a sediment-starved shelf, followed by a gradual regression (Bailey, 1977; Blackwelder, 1981~; Bailey and Tedesco, 1986). The sequence is capped in some localities by estuarine and/or lagoonal crossbedded sands and clayey silts. The sediments of the Chowan River Formation are magnetically reversed (Liddicoat et al., 1979). Deposition most likely occurred within the Matuyama Chron

(2.47-0.73 Ma) prior to the Olduvai Event at 1.88 Ma. Biostratigraphy provides poor age control for the Chowan River Formation, but the unit is placed generally within zone N21 (Cronin et al., 1984). The Bacons Castle Formation unconformably overlies the Yorktown and Chowan River Formations in eastern Virginia (Coch. 1968; Oaks and Coch, 1973; Ramsey, 1988). Channel incision which produced approximately 30 m of relief on the unconformity surface indicates significant fluvial downcutting during a preceding lowstand of sea level. Two facies associations within the Bacons Castle Formation represent fluvial deposition of sands and gravels (the Varina Grove Member; Ramsey, 1988) and estuarine deposition of silty sands and muds (the Barhamsville Member) (Fig. 2e). The transition between the fluvial and estuarine facies occurs consistently at 40 m elevation, marking the maximum incursion of the highstand. The Bacons Castle Formation does not contain calcareous fossils; consequently. age assignment is by/ relative stratigraphic position alone. Although equivalent and presumably correlative sediments exist in northeastern North Carolina, the unit has not been mapped in that area. Because the transgression which deposited the Bacons Castle Formation beveled the Chowan River Formation in its updip areas. the relative height of sea level during the former event is uncertain. The James City Formation is restricted to a relatively small embayment in eastern North Carolina (Fig. 2f) that is bounded by the Surry Scarp to the west and the northern flank of the Cape Fear Arch to the south (Blackwelder, 1981~). The nearshore coarse sands of the Windsor Formation in southeastern Virginia (Oaks and Coch, 1973; Oaks and DuBar, 1974; Mixon ef al.. 1989b) which also abut the Surry Scarp are probably correlative, but detailed stratigraphic surveys that link the two units are not presently available. The James City Formation unconformably overlies the Chowan River Formation in North Carolina (Hazel. 1983; Ward and Blackwelder, 1987). Channel cutting and subsequent infill seen in exposures at the Lee Creek Mine near Aurora, North Carolina, imply that the sediments of the James City Formation may have been deposited during two or more transgressions. The Waccamaw Formation, deposited south of the Cape Fear Arch. is considered to be in part a temporal equivalent of the James City Formation, although the lower Waccamaw is probably older (Cronin and Hazel, 1980; Hazel. 1983; Ward and Blackwelder, 1987). Age assignments for the James City-Windsor transgression(s) are equivocal. Other than ostracodes. practically no diagnostic microfossils have been found in the James City Formation. and the Windsor Formation is non-fossiliferous. Amino acid racemization data require the James City to he at least 1 .O Ma old, with an age of -1 .S Ma more likely (Wehmiller et al.. 1988). Limited paleomagnetic evidence places the James City Formation and the upper Waccamaw Formation in reversed intervals within the Matuyama Chron (Liddi-

A Chronology

of Pliocene

Sea-Level

coat et al., 1979; Cronin er al., 1984). If the deposition of the James City Formation occurred after the Olduvai Event (a period of normal polarity), its age is bracketed between 1.66 and 0.73 Ma. The foraminiferal and nannofossil assemblage of the lower Waccamaw Formation confines that unit to -1.9-1.7 Ma (late N21 to early N22), whereas the upper Waccamaw is younger than 1.6 Ma (Akers, 1972). DEEP-OCEAN

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Previous authors have correlated the Pliocene stratigraphy of the Atlantic Coastal Plain with global transgressive and regressive events inferred from deepocean records (cf. Blackwelder, 1981a; Cronin, 1981). Because of the relatively coarse resolution of the deepocean records available at the time of these publications, only a very general picture emerged. Since the advent of the hydraulic piston corer, the Deep Sea Drilling Project and the successor Ocean Drilling Program have recovered a number of cores with very complete late Neogene sections. Close-interval sampling of these cores for stable isotope analyses, microfossil assemblages and sediment composition (i.e. percent CaCOs) has produced extremely detailed highresolution records of regional and global paleoceanographic and paleoclimatic variability. With these records, glacial-interglacial fluctuations with periods less than 100 ka can be documented back through the Pliocene. Several deep-ocean olsO records were used to develop a model for correlation of the Atlantic Coastal Plain sequences with global events. The o’s0 records provide a proxy for global ice volume and hence a measure of eustatic fluctuations in sea level. Considerations that went into the choice of records include the temporal resolution and completeness of the record, the availability of good chronological control, and the absence of significant local effects such as salinity changes or exaggerated temperature changes. Prentice and Matthews (1988) developed a similar composite ice-volume history for the Cenozoic relying primarily on low-latitude planktonic o’s0 records. Their contention that benthic records show a significant component of bottom-water cooling is certainly valid for the entire Cenozoic. However, during the Pliocene, the variability caused by temperature has approximately the same effect on the planktonic 6lsO records as on both planktonic and the benthic. Consequently, benthic records were selected and the covariance between the two is used to identify ice-volume fluctuaions during the Pliocene. Trends which appear in both the planktonic (i.e. surface water) and benthic (i.e. bottom water) records are taken to indicate a change in global ice volume and not a change of water temperature (Prell, 1984). The olsO records are presented in blocks of time representative of the general climatic regimes of the Pliocene. The latest Miocene and early Pliocene (6.04.0 Ma; Fig. 3) are marked by a series of pronounced

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glaciations near the Miocene-Pliocene boundary, followed by a gradual and general warming into the middle Pliocene. The middle Pliocene (from -4.0-2.5 Ma) is not a formal stratigraphic subdivision but a distinct climatic regime characterized by extreme global warmth (cf. Budyko, 1982). The latter part of the middle Pliocene was marked by a stair-step transition into a cooler climate and the development of significant northern hemisphere ice (Shackleton et al., 1984; Keigwin, 1987b). Following a major glacial phase from 2.4-2.3 Ma, the late Pliocene and the early Pleistocene climate entered a period of low-amplitude fluctuations dominated by the 41-ka periodicity of obliquity (tilt) (Raymo et al., 1989). All of the records presented have been smoothed with a three-point running average to reduce the highfrequency variability and highlight the major events.

16X

D.E.

The age assignments used for each record are derived from the age model of the original author or from a revised model developed from additional data. The age estimates and correlation proposed by Curry and Miller (1989) for the 3.0-2.0 Ma interval are used to revise the Hole 552A record (Shackleton and Hall, 1985). Likewise, the chronology for the planktonic h’“0 record of iHole 572C (Prell, 1985) is revised following the correlation of the carbonate stratigraphy and orbitalparameter tuning of Holes 572A and 572C proposed by Hagelberg and Pisias ( 1990). Mujor Glohul Events During the Pliocene The late Miocene and early Pliocene are represented here by the planktonic hlXO records from Hole 572C (equatorial eastern Pacific; Prell. 1985) and Hole 58X (subtropical southwestern Pacific: Hodell and Kennett. 19X6). and benthic b’“0 records from Hole 58X and Hole 552A (northeastern North Atlantic; Keigwin, 1987) (Fig. 3). The latest Miocene was the culmination of a general global cooling and ice-volume increase which began in the middle Miocene (Savin ef rrl.. 1985; Miller et ul., lY87). Sea level had fallen close to the modern level by the late Miocene. A series of glaciations at 5.2. 5.0 and 4.8 Ma each dropped sea level by as much as 6&70 m (Fig. 3d; Keigwin, 1987~). Global climate gradually moderated. leading into a relatively brief but intense warm event at approximately 4.5-4.4 Ma. This event in the a’“0 records appears to coincide with evidence from Antarctica for period of much-reduced ice cover and marine transgression which began at -4.5 Ma (Webb et al.. 1984; Pickard et rd.. 1988). The climate cooled again, but did not enter a full glacial phase, from 4.4-4.1 Ma. For the interval between 4.0 and 2.0 Ma (Fig. 4). the planktonic hlxO record from Hole S72C is used in conjunction with benthic records from Holes 606 (central North Atlantic; Keigwin, 1987b), 665A (eastern equatorial Atlantic; Curry and Miller, 1989) and SS2A (northeastern North Atlantic; Shackleton and Hall. 1985). Global climate warmed to a maximum for the late Neogene from approximately 4.&3.0 Ma. Evidence from Antarctica again suggests warming, a significant retreat of the ice sheet, and a marine transgression during this interval (Webb et al.. 1984; Harwood, 1985). This period of global warmth was punctuated by moderate cooling events at 3.65-3.45 Ma and 3.2-3.1 Ma. The 3.2-Ma event was the more extreme, and has been widely recognized in deep-ocean cores (Prell. 1984. 1085; Shackleton and Hall. 1985; Weissert et al.. lY84; Keigwin, 1987a; among many others). The 3.6-Ma event appears to coincide with a glacial advance in Patagonia (Mercer, 1983). A brief rebound from the 3.2-Ma cooling event is centered at 3.0 Ma (Fig. 4). This was the last major warm episode during the middle Pliocene. The period from 2.9 to 2.5 Ma is characterized by progressive cooling and increasing ice volume which led into a major glacial phase at 2.4-2.3 Ma (Keigwin, 1987a; Curry and Miller. 1989). The 2.4-Ma event is widely

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FIG. -1. IHIgh-resolution deep-ocean b’“O records for the Middle Pliocene. Planktonic h”O record is from DSDP Site 571 (Prcll, 19X5). and henthic records arc from Site 606 in the central North Atlantic (Keigwin, 19874, Site 665, eastern equatorial Atlantic (Curry and Miller. 19X9) and Site 552 (Shackleton and Hall. 19x5).

regarded as marking the onset of significant northern hemisphere glaciation. During the 2.4-Ma glacial. global ice volume increased to at least two-thirds that of the late Quaternary glaciations (Curry and Miller. 1989). implying a sea-level drop of 80--90 m. The most complete and detailed record of late Pliocene and Pleistocene ice-volume fluctuations currently available is that of Site 607 (Ruddiman er ul., 1989: Raymo ef ul., I98Y). The age model for Site 607 was developed by splicing several cores from Sites 607 and 609 to produce a continuous composite section. The correlation between cores was based on highresolution (3.5ka average sample interval) records of percent CaCOI, benthic foraminiferal a”0 and 6%. and sea-surface temperature (derived from microfossil assemblages). The 6’“O record of the composite section was then tuned to the obliquity (41ka) and eccentricity (96-ka) components of orbital cyclicity. The Pliocene portion of the Site 607 record (Raymo et al.. 1989) is presented in Fig. 5.

A Chronology

FIG. 5. The Pliocene

portion

of Pliocene

Sea-Level

of the Site 607 benthic b’“0 record from the central stage designations are those of the original

During the late Pliocene, global climate settled into a phase of low-amplitude glacial-interglacial fluctuations with a 41-ka period forced by the obliquity cycle. This pattern persisted until the middle Pleistocene transition to a dominant lOO-ka cycle beginning at -0.7 Ma. Coming out of the 2.4-Ma event, peak interglacials slowly increased to a maximum between 2.2-2.1 Ma (stage 87). By the latest Pliocene, peak interglacials were noticeably cooler and glacials were significantly more severe. CHRONOLOGY OF ATLANTIC COAST TRANSGRESSIVE AND REGRESSIVE EVENTS

The correlation of Pliocene marine sedimentary units and erosional unconformities along the length of the Atlantic Coastal Plain (from Virginia through Georgia) implies strongly that eustatic sea level was the dominant control on the deposition of marine sediments on this continental margin. Although tectonic movements may have played a role in providing space for deposition and the post-depositional preservation of those sediments, tectonism alone cannot explain the timing of the transgressions and regressions. On the other hand, the uplift of the Atlantic Coastal Plain since the Pliocene complicates the assignment of absolute heights to the sea-level highstands (Cronin, 1981; Dowsett and Cronin, 1990). The maximum elevations of Pliocene marine sediments quoted in this paper have not been corrected for uplift. A model of sea-level fluctuations was developed by converting the 6”O values from the combined deepocean records (Figs 3-5) using the relationship of 10 m of sea-level change per 0.11%0 o’s0 (Fairbanks and Matthews, 1978; Fairbanks, 1989). Average minimum 6”O values for the Holocene or stage 5e for the Site (or nearby sites when not available) were used as a reference for ice-volume offsets. To account for bottom-water cooling, late Pliocene glacial lowstands were scaled to the late Pleistocene full glacials using as references the Pleistocene portion of the Site 607 record (Ruddiman et al., 1989) and a minimum sea level of -120 m (Fairbanks, 1989). An envelope

169

Fluctuations

North Atlantic authors.

(Raymo

et al., 1989). Isotope

around the range of sea-level variations was constructed by connecting the maximum highstands and minimum lowstands calculated from each 6”O record. In general there is extremely good correspondence between the individual records. Even so, differences in sampling interval and age models create some minor discrepancies. The composite envelope (Fig. 6) represents an averaging of the individual records for each time interval of -0.1 Ma. Sea levels calculated from the benthic and planktonic records agree fairly closely, although some extremely high peaks are suggestive of a temperature effect (i.e. warming of up to 1’C). The calculated middle Pliocene highstands closely approximate sea levels estimated for the Atlantic Coastal Plain transgressions. In the proposed model, the Sunken Meadow Member of the Yorktown Formation, the lower Duplin Formation, and the Wabasso Formation correspond to the brief warming from 4.5-4.4 Ma. Further work on the biostratigraphy of the Sunken Meadow Member will be required to verify this age assignment. Maximum sea level for the Sunken Meadow Member is estimated at 25-35 m from the elevation of marine sediments preserved on the Virginia coastal plain. This is substantially lower than the 40-50 m sea levels calculated from the planktonic o’s0 records, implying a significant surface-water temperature effect during the 4.5-Ma event. The unconformity between the Sunken Meadow and Rushmere Members of the Yorktown Formation represents the moderate drop to slightly below modern sea level from 4.4 to 4.1 Ma. The maximum transgression of the Duplin and Rushmere-Morgarts Beach Sea occurred during the middle Pliocene period of global warmth, 4.0-3.2 Ma. Dowsett and Cronin (1990) evaluated the extreme cases of regional uplift to estimate a sea level of 35 + 18 m for this middle Pliocene transgression; a range between 30-40 m is probably realistic. The 6”O records indicate that sea level remained high throughout this period with some moderate oscillations, notably a pair of drops between 3.65 and 3.45 Ma. Repetitive lithologic changes within the Rushmere and

170

I1.E

Sea c

Fdh~

Level

(m) Risinn

50

Middle Atlantic Coastal Plain Formations

i

i

1 .t

2.0

25

25

4.5

5.0

5.5

Upper

Eastover

Fm?

5.5

FIG. 6. Model of Plioccnc sea-level fluctuations derived from the deep-ocean h’“O records presented in Figs 3-S. and a proposed correlation with the marine sedimentary units of the Middle Atlantic Coastal Plain. The upper and lower curves define the average range of the highstands and lowstands for each period of -0.1 Ma. Some 01 the more extreme peaks were scaled down in cases of discrepancica between the henthic and planktonic records (for example the extreme excursion of the planktonic records during the 4.5-Ma event suggests a significant temperature effect). Major regressions are marked with arrows. For comparison, the maximum sea levels estimated for the Pliocene transgressions onto the Atlantic Coastal Plain arc 25-Z m for the Sunken Meadow Member of the Yorktown Formation. 3(b_JO m for the Rushmere and Morgarts Beach Members. IS-20 m for the Moore House Member. -15 m for the Chowan River Formation and 5-10 m for the James City Formation.

Morgarts Beach Members might record some of these minor sea-level fluctuations. The global cooling event at 3.2-3.1 Ma, which is expressed most dramatically in the Hole 606 record (Fig. 4b), could represent a rapid drop of 75-100 m from the peak middle Pliocene highstand. This regression coincides with the unconformity that separates the Rushmere-Morgarts Beach strata from the Moore House Member of the Yorktown Formation. The Moore House Member is the last highstand of the Yorktown era known to be preserved above modern sea level. Maximum elevations of Moore House sediments in the lower Virginia coastal plain are i5-20 m. The brief warming event of 3.1-3.0 Ma immediately precedes a long period of progressively increasing ice volume and is taken to represent the Moore House highstand. Because of the high-frequency low-amplitude character of the late Pliocene h’s0 record, assigning the Chowan River, Bacons Castle, Windsor and James City

Krant7

Formations to particular events is very tenuous. Further complicating the correlation is the fact that few highresolution records exist for this period which can be used to compare with the Site 607 record. The Hole 552A record was sampled on a coarser scale and includes several gaps from core breaks (Fig. 11 of Raymo er al., 1989). The deposition of the Chowan River Formation is placed loosely between 1.9 Ma and the glaciations at 2.4-2~3 Ma. Within this window of time, global climate recovers quickly from the 2.4-Ma event but the interglacials remain fairly moderate until stage X7 at 2.14 Ma. The peak interglacial of stage X7 is a relatively broad feature with b”O values as much as 0.10~0.15% lighter than those of stages 99 through X9. These b’“0 values could indicate that sea level during stage X7 was 9-14 m higher than the other highstands of this period. For comparison, the maximum elevation of Chowan River marine sediments is approximately 15 m. The lower Waccamaw Formation is constrained within 1.9-1.7 Ma by biostratigraphic control. During this period, interglacial stages 75, 73 and 71 all have very similar a’“0 values and are potential candidates. The St80 values suggest that sea level during stage 75 came within +5 m of modern. If the biostratigraphic data would allow an age as old as 2.0 or 2.1 Ma for the lower Waccamaw Formation. stage X1 (2.01 Ma) is an even better possibility. The sea levels calculated for stage 81 are between +5 and Cl0 m. A prominent highstand during stage 81 may have also deposited the Bacons Castle Formation. The deeply incised unconformity at the base of the Bacons Castle Formation would have been created by lowstands during stages 86 and 82. The records from Sites 665 (Curry and Miller, 1989) and 709 (unpublished data; K.G. Miller, pus. commurz., 1991) show one or more glacials around 2.1 Ma which have 6”O values heavier than those during the 2.4-2.3 Ma glaciations. Sea levels during these glacial episodes probably approached - 100 m. The full glacial 6’“O values for these events may have been aliased in the Site 607 record. If the James City and the upper Waccamaw Formations are retained in the Pliocene and considered younger than the Olduvai Event (i.e. younger than 1.66 Ma), they would be restricted to stages 65 and 63, neither of which was an exceptionally high stand of sea level. The early Pleistocene stages may be a more likely possibility. The next period of interglacials with significantly reduced ice volume occurs from -1.3 to 1.1 Ma when stages47 (1.31 Ma), 43 (1.23 Ma) and 37 (1.11 Ma) equal or exceed the late Pleistocene full interglacials of stages 1 and 5 (Ruddiman et al., 1989). Without further information, it is not possible to assign the James City, Windsor and upper Waccamaw Formations to any specific interglacial stage. The strong possibility exists that these formations actually reprcsent sediments deposited during several glacialinterglacial cycles.

A Chronology

of Pliocene

An Alternative Correlation of Late Pliocene Events

The sea-level history derived from the h’s0 records suggests an alternative chronology for the upper Pliocene marine units of the Atlantic Coastal Plain. A persistent feature in all of the deep-ocean a’s0 records is a series of highstands immediately preceding the 2.4-Ma glaciation (stages 103, 105 and 107 in the Site 607 record; Fig. 5). The a’s0 values for these interglacials suggest that sea level rose to 15-20 m. In the current interpretation of the Atlantic Coastal Plain stratigraphy, these transgressions do not correspond with any known sedimentary units. The most finite age constraint for the Chowan River Formation is the paleomagnetic data which places it younger than 2.47 Ma. However, this age allows deposition during a stage 103 highstand at 2.45 Ma and, depending upon the age model, possibly as early as stages 105-107 (2.5-2.54 Ma; Raymo et al., 1989). In this scenario, the unconformity separating the Chowan River Formation from the Yorktown Formation would correspond to the major regressions at 2.9 and 2.6 Ma. The lower Waccamaw and Bacons Castle Formations could represent deposition during stage 81 as discussed above, or some combination of stages 91 through 87 (2.2-2.1 Ma). Comparison of the Atlantic Coastal Plain Record with Other Eustatic Curves

The sea-level record for the Atlantic Coastal Plain compares well in general with the third-order cycles in the eustatic curve of Haq et al. (1987), but there are also some major discrepancies (Fig. 7). The sequence

Eustatic

Sea Level (m)

Haq et al. (1987)

3

Sea-Level

Fluctuations

boundaries from the base of TB 3.7 through TB 3.9 all correspond with prominent lowstands in the Atlantic Coastal Plain model. The timing and magnitude of the late Pliocene highstands also coincide reasonably well. However, the two models differ considerably in the timing, duration and elevation of the early and middle Pliocene transgressions. In particular, cycle TB 3.4 is completely out of phase with the Atlantic Coastal Plain record which shows a major regressive episode between 5.4 and 4.5 Ma. In contrast with the Haq et al. (1987) eustatic curve, the sea-level record developed for the late Neogene of Enewetak Atoll (Quinn and Matthews, 1990) compares extremely well, even in many details, with the Atlantic Coastal Plain model. The Enewetak record was derived by matching the results of a one-dimensional forward model with the sequence of unconformities, diagenesis, and carbonate deposition. The model was driven by the eustatic sea-level curve of Prentice and Matthews (1988) which was derived from low-latitude planktonic 6lsO records. SUMMARY: A WORKING MODEL The sea-level record developed here and the correlation with the marine sequences of the Atlantic Coastal Plain are intended as a working model. The model attempts to refine the geologic history of the region and to make certain predictions which can be tested by more detailed field work. One specific goal is to link the Neogene climatic record preserved in the coastal plain with that of the shelf and the deep ocean.

Sea Level (m) Atlantic Coastal Plain and Enewetak Models

2.5

z! P) 3.0 E F

171

3.5

FIG. 7. Comparison of the eustatic sea-level record from the standard ‘Vail Curve’ (Haq er al., 19X7) with the Atlantic Coastal Plain model (dashed lines in right diagram) and a sea-level model for the carbonate sequences of Enewetak Atoll (solid line in right diagram; Quinn and Matthews. 1990; see also Wardlaw and Quinn, 1991). Dashed lines in the left diagram are the sequence boundaries correlating with major lowstands.

Improvements in the age estimates for the Pliocene transgressions onto the Atlantic Coastal Plain that are predicted by the model include: (1) The unconformity separating the upper Miocene Eastover Formation from the lower Pliocene Sunken Meadow Member of the Yorktown Formation corresponds with a period of generally low sea level which stretched from 5.4 to 4.5 Ma. Two severe glacial events at 5.2 and 4.8 Ma, and a moderate event at 5.0 Ma. each dropped sea level by 4@60 m (Keigwin, lOX7b). (2) The Sunken Meadow Member of the Yorktown Formation was deposited during a brief episode of significantly reduced ice volume from 4.5-4.4 Ma. Following this highstand, sea level dropped to slightly below the modern level until -4.1-4.0 Ma. (3) The middle Pliocene maximum transgression that deposited the Rushmere and Morgarts Beach Members of the Yorktown Formation spanned the period from 4.0 to 3.2 Ma. During this time sea level rose above 3& 35 m on several occasions, cutting and re-occupying the Orangeburg, Chippenham and Thornburg Scarps. Even though sea level remained generally high throughout this period, lowstands during several oscillations probably approached modern sea level. (4) A major cooling event between 3.2-3.1 Ma dropped sea levels to 4&SO m and produced the unconformity separating the Moore House Member from the lower units of the Yorktown Formation. The Moore House transgression from 3.1 to -3.0 or 2.9 Ma immediately preceded a series of progressively more intense glaciations which culminated with a majol glacial phase at 2.4-2.3 Ma. If deposition on the coastal plain occurred during several low-ice-volume events between 2.9 and 2.5 Ma. the sediments were either removed by later transgressions or have not yet been identified. (5) Global climate gradually recovered from the 3.4Ma event and by 2.2-2.1 Ma sea levels during peak interglacials rose S-15 m. This is the most likely period for the deposition of the Chowan River Formation. An alternative which requires consideration is that the Chowan River was deposited during a highstand that immediately preceded the 2.4-Ma glacial episode. (6) The lower Waccamaw and Bacons Castle Formations most likely correlate with stage 81 at 2.01 Ma. although interglacial stages 75 through 71 are also reasonable candidates. (7) By the latest Pliocene, interglacials were significantly cooler and glacials were much more extreme. Although conditions during several late Pliocene interglacial stages approached modern, there are no obvious events which correlate with the upper Waccamaw. James City and Windsor Formations. Deposition during one or more early Pleistocene interglacial episodes might be a more likely possibility. ACKNOWLEDGEMENTS The ideas brought together in this paper wcrc several years during discussions with a number

generated over of colleagues.

Parttcular thanks lor their insIghts, suggestions and experience go to Jerrc Johnson, Buck Ward. Tom Cronin. Bob Thunell. Paul Huddlestun and Lyle Campbell. Reviews and constructive comments by Jerre Johnson. Kelvin Ramsey and Harry Dowsett improved the final manuscript and helped to refine many parts of the model. Kathy Witherell’s assistance in organizin g the data for the figures was

extremelv valu,lblc, . ‘

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A Chronology

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Paleontology

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the Lee Creek

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Curolina.