Age and correlation of emerged pliocene and pleistocene deposits, U.S. Atlantic Coastal Plain

Palaeogeography, Palaeoclimatology, Palaeoecology, 47 (1984) : 21--51

21

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

AGE A N D CORRELATION OF EMERGED PLIOCENE A N D P L E I S T O C E N E D E P O S I T S , U . S . A T L A N T I C C O A S T A L PLAIN THOMAS M. CRONIN ~, L A U R E L M. BYBELL 1, RICHARD Z. POORE ~, BLAKE W. BLACKWELDER 2, JOSEPH C. LIDDICOAT ~ and JOSEPH E. HAZEL ~

1U.S. Geological Survey, Reston, VA 22092 (U.S.A.) 2Tenneco Oil, Exploration and Production, Houston, T X 77001 (U.S.A.) 3Lamont-Doherty Geological Observatory, Palisades, N Y 10964 (U.S.A.) 4 A m o c o Production Co., Research Center, Tulsa, OK 74102 (U.S.A.) (Received April 13, 1984)

ABSTRACT Cronin, T. M., Bybell, L. M., Poore, R. Z., Blackwelder, B. W., Liddicoat, J. C. and Hazel, J. E., 1984. Age and correlation of emerged Pliocene and Pleistocene deposits, U.S. Atlantic Coastal Plain. Palaeogeogr., Palaeoclimatol., Palaeoecol., 47 : 21--51. Paleontologic and paleomagnetic investigations were conducted on several hundred Pliocene and Pleistocene marine samples from five regions of the emerged Atlantic Coastal Plain: (1) the Delmarva Peninsula, (2) eastern Virginia, (3) central and northern North Carolina, (4) southern North Carolina and northeastern South Carolina, and (5) the Charleston area, South Carolina. Molluscan and ostracode interval and assemblage zonations, which are the primary means of regional correlation, have been calibrated using pl. nktic biochronologic, paleomagnetic, radiometric and amino-acid racemization data These multiple dating criteria were used to determine the age and, where possible, the duration of marine transgressive/regressive sequences. A correlation chart illustrates the age relationships of 27 formations from five regions. One important conclusion is some of the Yorktown Formation of Virginia and North Carolina (including the " D u p l i n " Formation), and some of the Raysor of South Carolina are late Pliocene in age. The late Pliocene Chowan River F o r m a t io n of North Carolina is older than the early Pleistocene Waccamaw Formation o f South Carolina, which in turn may be older than the James City Formation of North Carolina. During the last 1.0 million years, multiple marine transgressions occurred in each region, but the age of these middle and late Pleistocene formations often may differ from one area to the next. A significant result of the study is the evidence for the lack of time equivalence of formations in the five different regions; that is, the sequence of marine transgressions in one region does not necessarily correspond to that in another. This appears to be the result of differing subsidence and uplift histories, the patchiness of the depositional record, and the limitations of the dating techniques in light of the rapidity and frequency of sea-level fluctuations. INTRODUCTION The emerged and submerged Atlantic Coastal Plain includes the region lying between the Fall Line and the continental shelf/slope break and forms the inner feather edge of a mature, passive continental margin. The goals of

22 this paper are to: (1) synthesize the knowledge of the age of Pliocene and Pleistocene marine deposits of the Coastal Plain; (2) correlate these deposits with more continuous deep-sea marine sections; and (3) outline major oceanographic events of the North Atlantic Ocean during the last 5 million years (m.y.) reflected in the Coastal Plain record. Some specific problems we address include the evidence for a major mid-Pliocene oceanographic climatic event along eastern North America, the nature of the Pliocene/Pleistocene boundary in this region, and the correspondence of Coastal Plain marine history with deep-sea climatic data. Our study is concerned primarily with marine deposits in Virginia, North and South Carolina, and, to a lesser extent, Maryland and Delaware. However, we draw heavily on published and unpublished data from the adjacent continental shelf, correlative deposits from Florida and Georgia, and from deep-sea cores. We limit our discussion to the interval from the base of the Pliocene to the end of the last interglacial period, about 70,000 years before present (yr. B.P.), after which sea level dropped and did n o t return to its present position until the final post-glacial Holocene transgression. The Miocene of the Coastal Plain is treated elsewhere (Ernissee et al., 1977; A b b o t t , 1978, 1984, this issue). WESTERN NORTH ATLANTIC OCEANOGRAPHY The post-rifting Mesozoic and Cenozoic history of the U.S. Atlantic margin contains a detailed, but as y e t poorly studied, record of eustatic sea-level fluctuations and the emerged Coastal Plain contains important parts of this record for the western North Atlantic. Within this regional framework, emerged fossiliferous marine deposits assume a dual significance. In one sense, Coastal Plain deposits should be viewed in an oceanographic context because these marine sediments yield a local paleoclimatic and sea-level record. Valuable marine paleotemperature data can be obtained by comparing modern and fossil zoogeographic distributions of temperature-sensitive benthic species in a manner similar to that used for deep-sea planktic faunas and floras. Emerged marine deposits also yield data on the timing and extent of transgressive/regressive events which are related to rates of sea-floor spreading, global sea-level cycles and glacial/deglacial events. Understanding the factors that affect global sea-level fluctuations depends on the study of such passive margin geologic records. In the case of the Pliocene and Pleistocene of the Atlantic Coastal Plain, glacio-eustatic influences predominate over tectonic factors. Conversely, although the studied material is marine in origin, the Coastal Plain lies on the eastern edge of the North American continent and is underlain by continental basement and crust. By dating and correlating marine deposits and shorelines, information on the timing and location of neotectonic vertical crustal movements can be related to offshore continental margin depositional troughs and platforms to provide a better understanding of passive margin tectonics.

23 PREVIOUS C O R R E L A T I O N STUDIES

To adequately synthesize Coastal Plain correlations, it is necessary to briefly review the highlights of previous work. Three main periods can be recognized. The first, from about 1840 to 1950, was predominantly a time of discovery during which an early split occurred between paleontologists and physiographers whose respective approaches to correlation differed radically. By comparing the diverse molluscan faunas from the Duplin and Yorktown Formations of North Carolina and Virginia to contemporaneous European faunas, particularly the Suffolk Crag, Charles Lyell (1845) established paleontology as a primary means of correlation in the region; he proposed a Miocene age assignment for beds now placed in the Yorktown and Duplin Formations (an age assignment that persisted well over a century until micropaleontologic data revealed a Pliocene age). Lyell's summary of Coastal Plain geology included a discussion of Miocene faunas and climates and set a precedent for his successors to define and recognize formations on the basis of fossils, not on lithology. Subsequent monographic study of Coastal Plain mollusks by T. A. Conrad, W. H. Dall, J. A. Gardner, and others provided the framework for correlations carried out by numerous Coastal Plain geologists. At this same time, a school of geologists emerged who viewed the Coastal Plain as a physiographic province because the region consists primarily of a complex sequence of gently inclined "terraces" bounded landward by marine cut scarps or other shoreline geomorphic features and often underlain by fossiliferous marine and marginal marine deposits. While paleontologists studied Cretaceous through Pleistocene stratigraphy, geomorphologists emphasized what they considered Pleistocene features. Beginning with the terrace studies of McGee (1888a, b) and Shattuck (1901, 1906) and culminating in the "terrace-formation" concept of C. W. Cooke (1941 and earlier), members of the morphostratigraphic school of correlation contended that the seaward-dipping planes represented former sea b o t t o m s and the landward border of each was a scarp, a beach or other paleo-shoreline feature. The higher the elevation of a surface, the older it was, although all were believed to be Pleistocene. A major corollary of this hypothesis, explicitly stated by Cooke (1930), was that marine transgression could be correlated solely on the basis of their elevation -- i.e., they could be traced over long distances along the Atlantic and Gulf Coasts and even to other presumed stable coasts. During the last fifty years, there has been much controversy over the validity of Cooke's approach, the terrace formation nomenclature introduced by Cooke and the distance a single terrace could be reliably traced. Criticism has centered on four points: (1) n o t all terraces are Pleistocene (Colquhoun et al., 1968); (2) the assumption of tectonic stability is not justified (Winker and Howard, 1977); (3) terrace formations are not lithologic units and their definition does not conform to standard stratigraphic

24 practice; and (4) the belief that all terraces were marine (Hack, 1955). Yet Cooke's speculative attempt to correlate Coastal Plain high stands with interglacial intervals was partially correct because m o s t deposits do contain interglacial faunas and floras. In light of the revolutionary advances in our understanding of Pliocene and Pleistocene climatic history during the last decade, particularly by the CLIMAP Project (CLIMAP, 1976; Hays et al., 1976), and the potential for refinement of this record via the hydraulic piston core (Prell et al., 1980), attempts to correlate Coastal Plain marine events to North Atlantic and hemispheric climatic events now seem appropriate. The second phase in the study of Coastal Plain geology was a period of descriptive lithostratigraphic regional studies in which subsurface augering provided data on material n o t exposed in natural outcrops. A synthesis of m a n y studies conducted during this period can be found in Oaks and DuBar (1974) and references therein. Regional studies concentrated on lithostratigraphic descriptions, augmented by geomorphic study, and numerous new formation names were proposed which form the framework within which later correlation studies must proceed. A new period of Coastal Plain correlation began in the early 1970s with the application o f quantitative statistical techniques to micropaleontologic data (Hazel, 1971a) and the use of planktic organisms for correlating onshore marine deposits to a global planktic zonation (Akers, 1972; Akers and Koeppel, 1973). These studies revised the age of the Y o r k t o w n and Duplin Formations from Miocene to Pliocene and ushered in a period during which numerous new dating and correlation techniques were applied to Coastal Plain geology. The results of many of these studies will be incorporated into the summary below. Significant aspects to this present period of study are that the Coastal Plain marine record can be partially correlated to a global time scale and the sequence of marine events and climatic history can be interpreted in light of records from other regions. LIMITATIONS OF CORRELATION CRITERIA Coastal Plain correlations have remained obscure and imprecise for many reasons. The m o s t difficult obstacle to overcome is the extremely fragmentary depositional record of the past 5 m.y. Marine and marginal marine deposition -- that is, when sea level was relatively higher than that of the present day -- probably occurred only between 5% to 20% o f the Pliocene and Pleistocene, and with a few possible exceptions, occurred only during peak interglacial intervals. This problem is further c o m p o u n d e d by the erosion of marine deposits by younger transgressions. F o r instance, it has been demonstrated that most shelf deposits o f f Georgia and South Carolina were deposited during the last (Holocene) deglacial transgression (Pilkey et al., 1981), and that the record of most earlier interglacials has been obliterated from this part of the shelf. N o t only are the marine facies of these transgressions

25 scattered, but the high degree of fluvial dissection during low stands of sea level has destroyed much of the associated geomorphic expressions of paleoshorelines. Hence, tracing a particular beach, erosional scarp, or relict barrier island over a long distance is usually difficult and the net effect of these proceases is a very patchy onshore marine record. The second major limitation inhibiting accurate correlations is the broad range of depositional environments and the strong facies control over preserved faunal and floral assemblages. Depth, salinity, substrate and water temperatures are the environmental parameters of greatest significance to the presence or absence of benthic species (Hazel, 1975; Cronin, 1979). Pliocene and Pleistocene Coastal Plain paleoenvironments range from fluvio-estuarine to open shelf sublittoral and, within this framework, discrete biofacies of benthic organisms can often be recognized. For example, quantitatively defined late Pleistocene ostracode biofacies have been delineated which signify estuarine, lagoonal, oyster bank, open sound, and inner sublittoral environments (Cronin, 1979) and modern temperature-controlled biofacies (Hazel, 1971b, 1975; Valentine, 1971). Recognition of such facies variation is, of course, a prerequisite for using benthic organisms for correlation. Depositional environments deeper than continental shelf are not represented in onshore deposits and therefore are not considered here. Planktic organisms, especially foraminifers and calcareous nannoplankton, are relatively sparse in shallow-water deposits compared to open ocean environments and have only recently been examined from Coastal Plain deposits. Nevertheless, m a n y biostratigraphically diagnostic species are present making correlation to a global time scale possible and, in conjunction with benthic organisms, permitting more accurate regional correlations. The control of species distribution by water temperatures is very important for Coastal Plain correlations because the last 5 m.y. was a time of rapid and frequent climatic change, especially in the North Atlantic where complex interaction occurred between ice sheet growth and decay and oceanographic fluctuations. Also, our study area includes m o d e m temperate and subtropical marine climatic zones separated by Cape Hatteras, a region of steep thermal gradient and, hence, a zoogeographic boundary for many temperature-sensitive species (Valentine, 1971; Hazel, 1975). The disappearance or appearance of a particular species in Coastal Plain deposits is often, therefore, related to a changing environment, rather than an evolutionary event. This fact does not diminish their usefulness as time markers of major marine events, and in fact, this "ecostratigraphic" approach is essential in Coastal Plain geology where climatic events are ultimately the mechanism controlling marine deposition. Reworking of microfossils can be a problem where there is erosion and redeposition of older sedimentary units by subsequent transgressions. Lower Tertiary and Cretaceous foraminifers, ostracodes, and calcareous nannofossils are sometimes present in Pliocene and Pleistocene deposits and usually represent material reworked from the immediately underlying geologic for-

26 mation. Reworking is not considered a major problem for several reasons. First appearance events were usually weighed more heavily than last appearances. Also, several means are available to judge if ostracode valves or foraminifer tests were reworked. Some specimens are poorly preserved, abraded and blackened and, in the case of ostracodes, the overall population structure can be analysed for percentages of juveniles and adults of various species. Since fragile juvenile valves are less likely to be reworked, a complete o n t o g e n y of early instars through adult stages probably signifies an indigenous death assemblage. Finally, environmentally anomalous specimens m a y indicate reworking or significant transport. For example, the occurrence of a few specimens of the brackish-water ostracode genus Cyprideis, n o t known to live in shelf environments, in conjunction with an open marine, shelf assemblage, indicates t h e y might be transported from their original habitat. LITHOSTRATIGRAPHY Outcrop, subsurface power auger and core material from the Coastal Plain localities shown in Figs.l, 2 and 3 were studied. Because there is often disagreement about the validity of certain stratigraphic names, our approach was to examine as m a n y formations at the type localities for as m a n y geochronologic types of data as possible, regardless of the criteria on which the unit was originally defined. Original references and locality information for the formations studied are given in Appendix I. Table I lists, for each formation studied, the criteria on which it was defined and the geochronologic results obtained in our study or available from the literature. These results will be discussed below in detail. A note of caution should be mentioned about applying the results of Atlantic Coastal Plain correlations in one region to correlation of units in another area because: (1) the areal and subsurface extent of m a n y formations is known only for small areas and extrapolation of a formation name outside its immediate type area can raise problems; (2) there are differing philosophies on w h a t constitutes a formation in the Coastal Plain (i.e., litho-, bio- and morphostratigraphic definitions); (3) the sedimentary record is so patchy t h a t tracing of a single lithologically distinguishable unit is very difficult; {4) m a n y marine transgressions deposited several lithofacies ranging from dunes, beach sands, backbarrier muds to shelf sands so that distinguishing the same facies from two separate transgressions on lithologic grounds is difficult unless independent dating of the marine facies is available. One approach to unraveling Coastal Plain history is to establish the areal extent and the timing {especially the duration) o f each marine event and thus the duration of periods of nondeposition through detailed stratigraphic study. Consequently, in trying to place the Coastal Plain marine deposits in a chronostratigraphic framework, we prefer to assign marine transgressions to stratigraphic units defined in continuously deposited deep-sea cores rather

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Fig.3. Localities in South Carolina studied for biostratigraphy and/or magnetostratigraphy. Stars indicate type localities. BIOSTRATIGRAPHIC ZONATIONS Four paleontologic groups mollusks, ostracodes, planktic foraminifers and calcareous nannofossils -- were studied in detail and each has its o w n advantages and disadvantages for Coastal Plain work. The following are summaries of: (1) the zonation followed for each group; (2) the methodo l o g y o f zoning for the regional groups (ostracodes and mollusks); and (3) the more significant planktic datum events for foraminifers and nannofossils. Fig.4 shows the time relations among the zonations. Table I shows -

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30 TABLE I A t l a n t i c C o a s t a l Plain F o r m a t i o n s S t u d i e d a Formation a

Criteria c

Mollusk zones-

Ostracode zones e

Foram f Magnetic (N) Chron

Nanno f U-series and (NN) He/U (103 yr.)

Amino acid (103 yr.)

Bear Bluff Canepatch Chowan River

litho. litho. litho. paleo. p aleo. paleo. litho. litho. litho. litho. litho. litho. morp hol.

4 2 4

P. rnucra B. sapeloensis P. m e s a c o s t a l i s

19--21 23

16--18 20--21 16--18

550--350

4 5

P. rnesacostalis P. m u c r a

morphol,, lltho. morphol. litho. litho. morphol., litho. morphol., litho. pale o. litho. litho.

1

B. sapeloensis

5 1 3

B. sapeloensis M. barclayi B. sapeloertsis P. c o n v o l u t a

"Croatan" "D uplin" Flanner Beach "Goose Creek" James City "Neuse" Norfolk Omar Penholoway] Wicomico "Princes~ Anne" "Silver Bluff" Raysor Socastee Talbot Pamlico Waccamaw Y o r k t o w n 1b Yorktown 2 b

1

Gauss Brunhes Matuyama

16--18 16--18

21

Brunhes

Zone C P. m u c r a

3 1 1

Zone Zone Zone Zone

A C C C&n

440 2400--1900

22--23

Matuyama

23

Brunhes Brunhes

20--21 16--18 19 20--21

200

72

350--70

95

250--200

P. c o n v o l u t a

16--18

19--21 Brunhes

240 19 120

B. sapeloensis 3 6

5

P. c o n v o l u t a P. i n e x p e e t a t a O. vaughani

22 19 21

Matuy area

19 13--15 16--18

1600--1000 2800--2400

a I n t h e case o f m o r p h o s t r a t i g r a p h i c u n i t s , m a r i n e d e p o s i t s a s s o c i a t e d w i t h t h e t e r r a c e w e r e s a m p l e d f r o m t h e g e n e r a l r e g i o n o f t h e t y p e a r e a f o r t h i s area. b y o r k t o w n 1 is e q u i v a l e n t t o t h e S u n k e n M e a d o w s M e m b e r o f B l a c k w e l d e r a n d W a r d ( 1 9 7 6 ) ; Y o r k t o w n 2 is equivalent to the Rushrnere, Morgarts Beach, and Moor e Hous e Me m be rs of Blackwelder and Ward (1976). CReferences identify original descriptions o f f o r m a t i o n s , d M o U u s k z o n e s f r o m B l a c k w e l d e r ( 1 9 8 1 a ) ; see Fig.4. e O s t r a c o d e z o n e s f r o m H a z e l ( 1 9 7 l a ) , C r o n i n a n d H a z e l ( 1 9 8 0 ) , C r o n i n ( 1 9 8 1 ) ; see F i g . 4 f o r c o m p l e t e n a m e s o f o s t r a c o d e z o n e s , f F o r a m i n i f e r a a n d n a n n o f o s s i l z o n a t i o n s are m o d i f i e d f r o m t h e s t a n d a r d z o n a t i o n s o f B l o w (1969) and Martini (1971) (see text for discussion).

the kinds of geochronologic data available for each stratigraphic unit. In the following sections, the stratigraphic ranges o f some important taxa are briefly discussed. Certain anomalous or as y e t equivocal occurrence data t h a t have arisen in the study are also summarized. Mollusks

Despite the extensive use of mollusks for field recognition of Coastal Plain formations, formal molluscan zones have been lacking until recently for the Pliocene and Pleistocene of the region. Six molluscan range and interval zones, abbreviated M 1 - M 6 in Fig.4, have been proposed which are based on several hundred samples from the region (Blackwelder, 1981a). Interval

31

>-

Magnetics

/ S t a n d a r d Planktic Ostracode L Zones J Molluscan Assemblage Zones o~ I ~ ~ Biostratigraphic :.'~ ~ S.C. & / Va. & ~ I ~= Zonat,on

SOU ~outhern N.C. northern

Bensonocythete sapeloensis Neocaudites

N.C.

Mo,~

No Formal

....

......

Zonation

atlantica

~

q

Anadara brasiliana i Range Zone ~ ovalis ~ A . brasi~lia-na~ M 2 !Range Z o n e | A. o v a l i s ' ~ ~ l n t er val ZoEe

M1

Anadara

/

NN19

Puriana convoluta

N22

M3

Glycyrneris subovata-

Noetia limula-

Puriana

Anadara ovalis

mesacostalis

Interval

Zone

NN18

Paracytheridea

Anadara

ovalis

Interval

Zone

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lirnula-

=

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ii N 4

Interval Zone

mucra

- NN16

N21

! Chesapecten

Orionina vaughani

M5

Noetia

madisoniuslimula

Interval Zone Murrayina barclayi

14.0 ¸

NN15 r 'Pterygo" cythereis ~ - inexpectata NN 14

L/Chesapecten jeffersonius| Chesapecten madisonius _MS L Int .... I Zone

,

i

i N19

i NN13 i

i !i l

Fig.4. Correlation o f planktic foraminifer, calcareous nannofossil, ostracode and mollusk zones with absolute and magnetic time scales for Atlantic Coastal Plain m a r i n e deposits.

z o n e s are d e f i n e d as the t i m e interval b e t w e e n t w o distinctive biostratigraphic h o r i z o n s (Hedberg, 1 9 7 6 ) , u s u a l l y the first or last a p p e a r a n c e o f a t a x o n . F o r e x a m p l e , the Glycymeris subovata--Anadara ovalis Interval Z o n e spans the time b e t w e e n t h e last a p p e a r a n c e o f G. subovata and the first a p p e a r a n c e o f A. ovalis. T h e s e z o n e s are especially useful in the field and in cases w h e r e m i c r o f o s s i l s are rare or p o o r l y preserved. In addition, to insure m a x ~ n u m utility, these z o n e s w e r e d e f i n e d using m o l l u s c a n species c o m m o n l y e n c o u n t e r e d in C o a s t a l Plain d e p o s i t s rather than o n relatively rare species w h i c h require e x t e n s i v e c o l l e c t i n g and e x p e r i e n c e w i t h m o l l u s c a n t a x o n o m y . T h e ranges o f s e l e c t e d t a x a are given in B l a c k w e l d e r ( 1 9 8 1 a ) .

32

Ostracodes Ostracode assemblage zones have been defined by cluster analysis of Yorktown Formation assemblages (Hazel, 1971a) and Principal Coordinate (PCOORD) analysis for numerous formations in South Carolina and southeastern North Carolina (Cronin, 1981). Separate zonations were erected for regions north and south of about 35°N because during m u c h of the Pleistocene and Pliocene, as is the case today, the central North Carolina area was a major zoogeographic b o u n d a r y between relatively warm subtropical regions to the south and temperate regions to the north. Although ostracode assemblages have some species in c o m m o n to both areas, there is a high degree of provinciality and the separate zonations minimize temperature effects over the species distributions. The PCOORD analysis allows for data reduction of large numbers of samples and species and graphically illustrates the relationship am ong samples from different formations based on the contained species assemblage. Often, this is the best way to compare two sections because of the difficulty in using lithologic criteria for correlation in the Coastal Plain and the lack of continuous sections. In the South Carolina analysis (Cronin, 1981) samples representing the marine facies of each studied section were selected wherever possible to minimize the influence of salinity on the biofacies. Fig.5 is taken from Cronin (1981) and shows a plot of the first and second coordinate axes showing the gradational nature of sample clusters. Samples taken from type localities and planktic foraminifer and nannofossil data are indicated. Boundaries between assemblage zones were selected on the PCOORD results and also on the field relationships of the formations and the planktic age control for selected samples. The name given to each zone is that of a characteristic ostracode species but one not necessarily restricted to the zone. The age of the tops and bases of the zones have been estimated from the best available age data from planktic datum events, magnetostratigraphy and radiometric ages and can be changed with additional data. The taxon and concurrent range zones of fifty ostracode species used to recognize these zones are given in Fig.6 o f Cronin (1981); scanning electron photomicrographs of m a n y of these species are given in Cronin and Hazel (1980) and Cronin (1979). The zonal scheme for south of 35°N should not be extended outside the study region either to the north, where the zones o f Hazel (1971a) apply, or south into Florida, where the Plio/Pleistocene ostracodes require study (see Hazel, 1977).

Planktic biochronology One major goal in was to determine the water, temperate and ating planktic groups

investigating planktic groups in Coastal Plain deposits preservation and frequency of these groups in shallowsubtropical climatic zones. Our approach to incorporinto regional biostratigraphy has been to review the

•

' •[BZ4B

•

\\

\

/

~'"

•

[B

N

/

/

•~-! ~ 4 /

i/

-e" z)~

9.-

\

NN19

e'~, .

" ~

'o~qVo{{\

•

~ ;,

'd!!

/ ~ - .....

:

\j

N22

N"Duplin"

:"F 22~ Wa,:~amaw__

Bear Bluff

OZ>4

NNI6 I£~ :.B• 17qB

i-%

LB 1,H,&• N21 23

%( ~

•l B

A Zone

MurrcJyino barclag, i

23

J'"~

"~.(~

~,~ •~ .

.'aner X ~NN2() 21 wand•

IRST & SECOND COORDINATES

~nholoway

/ / ./ . . . . .

--....~%j ........

m 'e

4

Aid-Late Pleistocene

~ig.5. Plot of first and second Principle Coordinate axes for ostracode samples from South Carolina and southern North Carolina ;howing type localities, planktic data for key samples, and assemblage zone clusters of samples. Adapted from Cronin (1981).

Puriana convoluta /

L,e,,:;~,4,,

i.5 z,

Raysor

~

•

- - ~ "Goose s ~ l ~ . . Creek"

"~~

mucra

A Zone

Paracytheridea

Pliocene

NNI5 1 N19

0 0

34

[Paleomagnetics St,andard Datum Events Primary luxleyi ruxleyi

Secondary

acme

acunosa

H. sellii )ceanica "aribbeanica n a c i n t y r e i , G. obliquus, b r o u w e r i , C. rugosu~; ~runcatulinoides ~N.

pentaradiatus tamalis ~ltispira • .~. . otomellopsls s p p . nargaritae • tamalis tricorniculatus nepenthes

dutertrei

~2P~D. surculus, G. w o o d i ~ - ~ N. a c o s t a e n s i s . flata ~ IG." m "~'Sphenolithus s p p . < ~ P. lacunosa

]symmetricus

•ugosus ~.hiscens crassaformis tumida ;uinqueramus margarita e ~ricorniculatus

,~JG. p u n c t i c u l a t a

Fig.6. Ranges of diagnostic calcareous nannofossils and planktie foraminifers used in Coastal Plain correlations.

literature on t a x o n ranges, primarily from deep-sea cores, to establish which planktic d atu m events can be calibrated to the magnetic or oxygen isotope time scales and to use these taxa as time markers. F o r t u n a t e l y , Berggren et al. (1980) have recently synthesized an extensive a m o u n t of Q u a t e r n a r y correlation data and recognized 45 datum events for the Quaternary, including m a n y foraminifer and nannofossil events. Although m a n y of the absolute ages attached to these events are interpolations and m ay eventually be revised, t h e y still provide u n p r e c e d e n t e d accuracy for the localities yielding diagnostic taxa. Thus, we use them as absolute age devices to place maxi-

35 mum and minimum ages on benthic group zonal boundaries, to date climatic and sea-level events, and to put age limits on lithostratigraphic units. Conversely, currently used LADs and FADs are bound to require age revision as more radiometric data become available and these events are integrated with magnetic events. The uranium series and K-At age obtained on Coastal Plain material can also provide additional absolute age data which augments magnetostratigraphy in dating biochronologic datums. The following synthesis comes from data contained in the following primary references: Hays et al. (1969), Martini (1971), R y a n et al. (1975), Saito et al. (1975), Gartner and Emiliani (1976), Gartner (1977), Haq et al. (1977), Haq and Berggren (1978), Poore (1979), Poore et al. (1984) and others. Planktic data were calibrated to the recently published magnetic time scale o f Mankinen and Dalrymple (1979).

Calcareous nanno[ossils Calcareous nannofossils are present in varying abundances and in low diversities (usually 2--10 species/sample) in Pliocene and Pleistocene marine deposits in the study area. Although discoasters are extremely rare, the following diagnostic nannofossil taxa could be recognized with a high degree of certainty (see Fig.6):

1. Emiliania huxleyi (Lohman) (FAD = 0.27 m.y.) 2. Pseudoemiliania lacunosa (Kamptner) (LAD = 0.45 m.y.). 3. Helicosphaera sellii (Bukry and Bramlette) (LAD = 1.1 m.y.). 4. Cyclococcolithus macintyrei (Bukry and Bramlette) (LAD = 1.6 m.y.). 5. Gephyrocapsa small species (FAD = 1.6--1.8 m.y.). 6. Pseudoemiliania lacunosa (Kamptner) (FAD = 3.4 m.y.). It should be noted that due to the discontinuous nature of Coastal Plain marine sedimentation, the occurrences of these diagnostic planktic taxa may not represent actual first and last appearences that are synchronous with appearences in the deep-sea record. Rather they are more appropriately referred to as the highest and lowest recorded occurrences in the Pliocene and Pleistocene in the region. Gephyrocapsa small species includes species with obvious cross bars when observed with a light microscope. This datum is used as a close approximation of the Pliocene/Pleistocene boundary (Haq et al., 1977), due to the absence of Discoaster brouweri Tan Sin Hok in Coastal Plain sediments. Low specimen abundances and poor preservation made scanning electron microscopic study of Gephyrocapsa impractical at present and in this paper G. oceanica and Gephyrocapsa small species are the only categories recognized for the genus. The age of the P. lacunosa FAD appears to be about 3.4 m.y. but in some regions it may be slightly older (Mazzei et al., 1979). The occurrence of this species with certain planktic foraminifers poses problems for dating some

36 deposits of the " D u p l i n " and Yorktown Formations. Akers and Koeppel (1973) f o u n d P. lacunosa in the Yorktown Formation at the Lee Creek Mine, Aurora, N.C. and in Hampton, Virginia at Rice's pit. We have found this species in numerous samples at Lee Creek mine in the four members of the Yorktown Formation as defined by Blackwelder and Ward (1976). It also occurs in deposits of the upper Raysor Formation near Florence, North Carolina t h a t are correlative with the type locality of the " D u p l i n " Formation. However, this species was not found in deposits of the Raysor Formation assigned to the Murrayina barclayi assemblage zone in South Carolina nor in deposits of the Sunken Meadows Member of the Yorktown Formation that outcrop on the James River in southeastern Virginia. It is not clear if its absence is due to environmental conditions or because these sediments were deposited before its first appearence. Because P. lacunosa occurs with foraminifer species indicating Zone N19 of Blow (1969) at Lee Creek Mine, the latter alternative seems more probable. The overlap ofP. lacunosa with Reticulofenestrapseudoumbilica (Gartner), a species absent in most Pliocene sediments examined, is also a problem yet to be resolved. F o r example, R. pseudoumbilica has been found in the Eastover Formation, a unit considered late Miocene to early Pliocene in age, but it is absent in the overlying Yorktown Formation in Virginia and at Lee Creek mine in North Carolina. In summary, the age of the base of the Yorktown is therefore at most 4.8 m.y. based on the occurrence of Globorotaliapuncticulata. The occurrence of P. lacunosa in the lowermost part of the Yorktown Formation at Lee Creek Mine and its absence in correlative deposits to the north and south suggests that the first appearance of this species may be somewhat older than 3.4 m.y. in this region, but also that much o f the Yorktown Formation may be significantly younger than 4.8 m.y. We tentatively place the base of the Y o r k t o w n at 4.0 m.y. in Fig.8 pending additional data on the exact age of the P. lacunosa FAD. The base of the Raysor Formation appears to be slightly younger than the base of the Yorktown (Fig.8) but this formation contains Globigerina nepenthes Todd in its lowermost part. If this species is not reworked, it is an important occurrence for dating the Raysor Formation and it should be searched for in future study of the Pliocene of the region. At present, the base of the Raysor is placed at 3.8--3.6 m.y. The " D u p l i n " Formation at its type locality in North Carolina was considered by Blackwelder and Ward (1979) to be equivalent to the middle and upper parts of the Yorktown F o r m a t i o n (Rushmere, Morgarts Beach, and Moore House Members). We approximate the age of the Moore House Member and the " D u p l i n " Formation to be about 3.2--2.8 m.y. Emiliania huxleyi (Lohman) has not been found in any sample examined. Because it first appeared at 275,000 yr. B.P. and its acme zone began 85,000--75,000 yr. B.P., it is not surprising t h a t it does not occur in emerged Coastal Plain deposits for several reasons. Between 275,000 and 85,000 yr. Gephyrocapsa would still be expected to dominate over E. huxleyi, and

37

E. huxIeyi m a y n o t have been as a b u n d a n t or well preserved in shallowwater, marginal marine environments characteristic of the mid- and late Pleistocene of the region. Finally, the marine record of the last 275,000 yr. is very incomplete in the Coastal Plain and marine deposition occurred only briefly during interglacial high stands of sea level. The regression near the isotope stage 5/4 transition, the beginning of the Wisconsin glaciation, is dated at about 75,000 yr. B.P. and caused sea level to drop below present sea level until the final Holocene sea-level rise during the last 15,000 yr. Therefore, a period o f emergence existed that corresponds almost exactly with the acme zone o f E . huxleyi. At depths below Wisconsin glacial sea level on the co n tin en ta l slope, Valentine et al. (1980) have found this species in Pleistocene deposits and it is likely a m or e refined calcareous nannofossil biostratigraphy will come from these regions.

Planktic foraminifers A p p r o x i m a t e l y 37 species and forms of planktic foraminifera were found, o f which 13 served as biochronologic datum events (Fig.6). S o m e of the more i m p o r t a n t taxa and the deposits they date are as follows: (1) Globigerina woodi and Neogloboquadrina acostaensis. The occurrence o f these species in deposits of the Bear Bluff, " D u p l i n " and Raysor Formation places an u p per age limit of about 2.8 m.y for these deposits. (2) Globoquadrina altispira/Globorotalia crassaformis places an age range o f 5.0--3.0 m.y. on deposits of the R ays or F o r m a t i o n in Sout h Carolina. (3) Globorotalia puncticulata in samples of the Y o r k t o w n and Raysor Formations give a m a x i m u m age of 4.8 m.y. (4) Globigerina nepenthes in the lower part of the Raysor F o r m a t i o n indicates a min imu m age o f 3.7 m.y. The concurrence of Neogloboquadrina acostaensis and Neogloboquadrina pachyderma (s.s.) in a sample of the " D u p l i n " F o r m a t i o n is som ew hat anomalous and may signify an age as y o u n g as 2.8--2.5 m.y., however, the few specimens of N. acostaensis m a y be reworked. T he occurrence of Globigerinoides obliquus in the Waccamaw F o r m a t i o n at Calabash, Walkers Bluff and Old Dock, N.C. appears to represent the u p p e r m o s t part of that species range in the oldest Pleistocene because it occurs with Globorotalia truncatulinoides and the nannofossil Gephyrocapsa. These sediments appear to straddle the Pliocene/Pleistocene b o u n d a r y and range in age from 1.9 to 1.6 m.y. Deposits o f the Waccamaw F o r m a t i o n at its t ype locality at Tilly's Lake on the Waccamaw River (Blackwelder, 1979) and along the Intercoastal Waterway near Myrtle Beach, S.C. lack Globigeriuoides obliquus, have reversed magnetic polarity and contain the nannofossils Gephyrocapsa and Helicosphaera sellii and are slightly y o u n g e r than the N o r t h Carolina Waccamaw Form at i on samples.

38

Magne tostra tigraphy M i n i m u m and m a x i m u m ages o f m a n y s e d i m e n t a r y u n i t s and s t r a t i g r a p h i c sections in the C o a s t a l Plain can be d e t e r m i n e d b y i n t e g r a t i o n of biostratigraphic and p a l e o m a g n e t i c data. P a l e o m a g n e t i c d a t a also helps to p l a c e ages on the t o p s a n d b o t t o m s o f regional b i o s t r a t i g r a p h i c z o n e s and f a u n a l d a t u m events. T a b l e I I is a c o m p i l a t i o n of p a l e o m a g n e t i c results for C o a s t a l Plain f o r m a t i o n s . It includes t h e t y p e o f s a m p l e a n a l y s e d ( o u t c r o p or s u b s u r f a c e ) , t h e d e m a g n e t i z i n g l a b o r a t o r y t r e a t m e n t u s e d ( a l t e r n a t i n g field a n d / o r thermal), and the polarity assignment. In S o u t h Carolina, t h e M a t u y a m a / B r u n h e s o c c u r s in the t i m e interval bet w e e n t h e d e p o s i t i o n o f t h e C a n e p a t c h F o r m a t i o n ( n o r m a l p o l a r i t y ) a n d the W a c c a m a w F o r m a t i o n (reversed p o l a r i t y ) . T h e p a l e o m a g n e t i c d a t a in Fig.7 c o m e f r o m t h e t y p e localities l o c a t e d a l o n g t h e I n t e r c o a s t a l W a t e r w a y (Canepatch Formation) and the Waccamaw River (Waccamaw Formation). Similar results h a v e been o b t a i n e d at o t h e r sites in the M y r t l e Beach, S.C. area and give a m a x i m u m age of for the C a n e p a t c h and a m i n i m u m age for t h e W a c c a m a w F o r m a t i o n s of 0.73 m . y . O n the D e l m a r v a Peninsula, d e p o s i t s n e a r C h i n c o t e a g u e , Virginia t h a t are assigned t o t h e O m a r F o r m a t i o n ( O w e n s and D e n n y , 1 9 7 9 ) r e c o r d n o r m a l m a g n e t i c p o l a r i t y a n d s u b s t a n t i a t e u r a n i u m series ages for a B r u n h e s age ( M i x o n et al., 1 9 8 2 ) . O u t c r o p s o f d e p o s i t s c o n s i d e r e d t o be n o n m a r i n e facies o f the Y o r k t o w n F o r m a t i o n y i e l d e d reversed m a g n e t i c p o l a r i t y ( L i d d i c o a t and Newell, 1 9 8 1 ) ; h o w e v e r w i t h o u t p a l e o n t o l o g i c d a t a f r o m this site, it r e m a i n s u n c l e a r w h e t h e r these d a t a i n d i c a t e the G i l b e r t C h r o n or a reversed e v e n t w i t h i n the Gauss. T h e s t r a t i g r a p h i c r e l a t i o n s h i p s o f the d e p o s i t s to t h e type Yorktown have not been adequately demonstrated. TABLE II Summary of Atlantic Coastal Plain paleomagnetic data Formation

Outcrop Core A.F. Thermal Polarity Chron demagnetization demagnetization

Norfolk Socastee Flanner Beach Omar Canepatch James City "Croatan" Waccamaw Bear Bluff "Goose Creek" Yorktown

x x x x x x x x x x x

"Duplin"

x

x

x x x x x x x x x x x x

x

N N N N N R? N? R N? N? R ?

Brunhes Brunhes Brunhes Brunhes Brunhes Matuyama Gauss? Matuyama Gauss? Gauss? Gilbert (Gauss?) ?

39 N 1

/

~k WaccamawFm \

\

\

~ ~52~ 5 0 1sO / oo/

Fig.7. Brunhes/Matuyama boundary in the Atlantic Coastal Plain illustrated by equal area plot of paleomagnetic directions for the Canepatch Formation (Normal Polarity) and Waccamaw Formation (Reversed Polarity) from their type localities in South Carolina. The data are for A.F. demagnetization and the numbers adjacent to the symbols indicate the level of demagnetization in oersteds. Solid symbols are plotted on the lower hemisphere, open symbols on the upper hemisphere.

Geochemical techniques A detailed discussion of radiometric and amino-acid dating of Coastal Plain deposits is beyond the scope of this paper, however, the available data are important for correlation. Geochronologic data are listed in Table I and have been incorporated into Fig.8. The following sources have been used: --Amino-acid racemization: Belknap and Wehmiller (1980), Wehmiller and Belknap (1982). - - 2 3 ° T h / 2 3 4 U : Cronin et al. (1981), Mixon et al. (1982). - - He/U: Bender {1973), Blackwelder (1981b). References to the geochemistry and the dating limitations of each technique c an be f o u n d in these papers. INCEPTION OF N O R T H E R N HEMISPHERIC GLACIATION

In addition to these age assignments using planktic age control, we have reconstructed Pliocene marine paleoclimates using ostracode zoogeographic data from the formations studied. Fig.9 shows the general trend in marine climates during the Pliocene and Pleistocene for the study area and a corn-

40

Standard Ptanktic

Paleomagnetics

Southern

Zones

i I Jg~]

~ ~

<~ E,

Southeastern

Central &

North Carolina

Charleston

Virginia

Northern North Carolina

& Northeastern

Area South Carolina

Delmarva Peninsula

Soutl ~ Carolina

Slnepux(ant Fm

~ Norf elk Forma ions~ ~Core~ Creek"~ \Band ~ "Great Bridge" F m ~ "Omar" Formation

Omar Formation ~

FlannerBeach Fo ma on - "=

(at RaDDahanneck River Va )

\\

\

~-'"S u

_ ver 3 1 f l "

Socastee Formation

~ ~]Prmcess Anne. ~; ~) Pare ¢c

CaneDatch . Formation_

Talbot For~ ~t~on ~(near Beaufort S C )

-omar-F.... f,oi

(al D=rIcksen Cree O

James City Formation

.$ t.o

E

o~

Penholoway Formation Waccamaw Formation

W

(at Summervllle SC)

(at rllly's LaKe Intraeoastal Waterway S C )

J

~

2.0~

C h o w a n River Formation

~E o~ LL L)

Waccamaw : ~ (at Walkers B uN Old Dock & Calabash N.C.) Celer am B e a c h - \ Member

Wicomloo Formation 9

Edenbouse Member

(al Bowers n,, h V a )

Bear Bluff

L

~

i F ..... , o °

Moore House~ ember

~

~'DL }lin" F o r m a t i o n

-~n,o~rts--.,._ =

Morgarts Beach

Beach

Member

"Goose Creek Marl" &

tat Natora~ We,

NC ~ Florence S C ) =

_

Raysor Formation Rushmere Member

o

E _ = o ~ NO

~

{at Edlsto River Member ~aysor F o r m a t i o n 1 (at Pee Dee River, SC.)

4.0

:

O~

SC - " ~~ - -

41 m

Observed Paleomagnetics Standard Paleoclimates Sea Level (M), ' Planktic >:~ Zones South Carolina ~

.=

"~

o

IA_

._

"~

09

I'-

mr./) I'-"

n

I'--

+25

~50

+75

I

b 1.o

NN 191 N22 E

b

! I

3

!

N21

O

E_

."

(D

("

"-..,..

3.0

"%..,

NN15

,..,, :l

N19

!4.o- i

N

8

L

UJ

S

Ii

NN i

i

i

T

o

Fig.9. P a l e o c l i m a t i c a n d relative sea-level h i s t o r y , A t l a n t i c Coastal Plain. P a l e o c l i m a t e curves r e p r e s e n t m a r i n e c l i m a t e s only d u r i n g m a r i n e d e p o s i t i o n o n the e m e r g e d Coastal Plain. N o t e increased p r o v i n c i a l i t y r e f l e c t e d in t h e b e n t h i c m a r i n e o r g a n i s m s a b o u t 3.0 m.y. b e t w e e n n o r t h e r n a n d s o u t h e r n areas. R e l a t i v e sea-level c u r v e based o n p r e s e n t elevation o f p a l e o - s h o r e l i n e s in S o u t h C a r o l i n a a n d is not c o r r e c t e d for vertical crustal movem e n t s . See C r o n i n ( 1 9 8 1 ) f o r d e t a i l e d discussion o f c o n t r i b u t i o n s o f glacio-eustasy a n d n e o t e c t o n i c s t o t h e local sea-level record.

Fig.8. C o r r e l a t i o n c h a r t o f A t l a n t i c Coastal Plain f o r m a t i o n s . Solid lines d e l i n e a t i n g form a t i o n b o u n d a r i e s signify relatively c o n f i d e n t m i n i m u m o r m a x i m u m age f o r t h e u p p e r m o s t o r l o w e r m o s t p a r t o f the f o r m a t i o n respectively b a s e d o n p l a n k t i c b i o c h r o n o l o g y , m a g n e t o s t r a t i g r a p h y , a n d / o r r a d i o m e t r y . D a s h e d lines i n d i c a t e u n c e r t a i n t y as t o e x a c t age of t o p o r b o t t o m o f f o r m a t i o n . T h e d u r a t i o n of t i m e i n d i c a t e d for a f o r m a t i o n does n o t i m p l y c o n t i n u o u s d e p o s i t i o n in the region d u r i n g t h a t i n t e r v a l b u t o n l y t h e possible chron o s t r a t i g r a p h i c age b a s e d o n various criteria~ A p p e n d i x I gives i n f o r m a t i o n o n t h e lithostratigraphic units studied.

42 posite relative sea-level curve for South Carolina. It must be emphasized that the paleoclimate conditions plotted only represent times of marine deposition on the emerged Coastal Plain. Further, because such a large region was studied, a single sea-level curve does not adequately reflect the details or complexities of the sea-level histories in particular regions which are expected due to different subsidence and uplift histories (Cronin, 1981). Local neotectonic movements and post-depositional erosion effects appear to be reflected in the lack of age equivalence between many formations shown in Fig.8 that are sometimes thought to be correlative. Because deposition was interrupted by long and frequent intervals of nondeposition caused by global eustatic sea-level low stands, most of the paleoclimatic and sea-level record is missing. F o r the following discussion, the study area is divided into t w o regions: one north and the other south of the central North Carolina/Cape Hatteras area. Climatic data from the northern region were taken from Hazel (1971b) who used modern temperature data on 59 extant species to estimate Pliocene and early Pleistocene annual temperature ranges. In Fig.9, only the climatic zone is given; Hazel (1971b) gives specific temperature data. Ostracode data from the southern region do not permit specific annual temperature ranges because several key species no longer inhabit the Atlantic Coast and precise temperature data are n o t available (i.e., Cativella, Acuticythereis, Radimella). Other taxa are extinct, and their general paleoclimatic potential is derived from their association with diagnostic species indicating temperate climates (i.e., Cytheridea, Murrayina). The paleoclimatic trends north and south of central North Carolina were generally in phase with each other and, as expected, climates to the south were warmer. Roughly between 4.0 and 3.0 m.y., relatively cool climates prevailed relative to today. During the middle Murrayina barclayi Zone, the cryophilic species Pterygocythereis inexpectata, Actinocythereis dawsoni, Cytheridea virginiensis, and Echinocythereis planibasalis disappeared and the thermophilic species Acuticythereis laevissima, Cativella naevis and Radimella confragosa appear in low numbers. In South Carolina near the Murrayina barclayi/Paracytheridea mucra assemblage zonal boundary, several local biostratigraphic events occur which reflect a major change in the western Atlantic oceanography. Cryophilic species restricted to regions with temperatures cooler than 20°C disappear from Virginia and North Carolina (Hazel, 1971b), and simultaneously to the south, a fauna representing a transitional subtropical/tropical climate appears in S o u t h Carolina. Particularly indicative of this change is the dominance of Neocaudites spp., Acuticy-

thereis laevissima, Radimella confragosa, Pellucistoma magniventra, Palaciosa minuta, and Orionina vaughani, which indicate winter temperatures above 12--15°C during the lower to middle P. mucra Zone. Further, the absence of temperate species indicates summer temperatures above 27°C. The precise age of this warming event is difficult to assess, but all evidence suggests a 3.2--2.8 m.y. age as discussed above.

43 This relatively warm marine event was probably of very short duration because a major regression is signaled by the influx in the P. mucra Assemblage Zone of the brackish-water ostracodes Cyprideis and Cytheromorpha curta as the d o m i n a n t ostracode taxa in the uppermost Raysor Formation (= the type "Duplin"). Oaks and DuBar (1974) and Blackwelder (1981c) discussed the abundant stratigraphic and geomorphic evidence for this regression. The sequence of events, therefore, consists of an overall, although not necessarily continuous, warming from about 4.0 to 3.2 m.y., which may have been interrupted by cool periods of nondeposition, followed by an intensified pulse of warmer water along the coast between 3.2 and 2.8 m.y., followed by a major regression roughly 2.8--2.4 m.y. The near-synchroneity of the final warming with a drop in relative sea level appears anomalous if the regression was caused by a climatically induced eustatic sea-level drop stemming from a global or hemispheric cooling. Yet, evidence for a major glacial event, believed to be the initial Cenozoic Northern Hemispheric glaciation, comes from several sources. In deep-sea cores, North Atlantic faunal data (Berggren, 1972), stable isotopic fluctuations (Shackleton and Opdyke, 1977), and lithologic data on ice-rafted debris (Shor and Poore, 1979) all signify the beginning of a major cooling event at about 3.0 m.y., perhaps reaching peak intensity cold by 2.5 m.y. There is much additional oceanic and continental evidence for cooling in the mid-Pliocene (see Berggren and Van Couvering, 1974), but correlation with distant regions are at present unwarranted because of ambiguous age control and the distinct possibility of diachronous climatic events in different areas. The most reasonable explanation for the data presented above involves a diversion of warm Gulf Stream water from the Gulf of Mexico approximately 3.6 (Saito, 1976) to 3.1 (Keigwin, 1978) m.y. through the Straits of Florida, along eastern North America. Our data favor the younger age for this event. The cause for a sudden shift in Gulf circulation probably resulted from the closing of the Isthmus of Panama at roughly the same time (see Keigwin, 1978); however, it is unclear whether the Isthmus closed because o f a glacio-eustatic sea-level drop, tectonic movements in Central America, or a combination of both. Nevertheless, the concordance of planktic events in the Panama and Columbian basins (Keigwin, 1978), in the Straits of Florida (Brunner, 1978), along eastern North America (Hazel, 1983, this paper) and in the north-central North Atlantic (Berggren and Van Couvering, 1974; Shor and Poore, 1979) does not seem to be spurious. This series of events seems to have brought about the western North Atiantic oceanographic conditions that, with slight modification, seemed to predominate during the interglacial periods of the past 3.0 m.y. NEOGENE/QUATERNARY BOUNDARY The precise location and age of the Neogene/Quaternary (N/Q) boundary awaits the selection of a boundary stratotype, and thus the identification of

44 bio-, magneto- and ecostratigraphic criteria through which other sections can be correlated to it. In southern Italy, the La Castella and the Santa Maria di Catanzaro stratotype Calabrian sections are not suitable for determining the boundary because both have large hiatuses in the depositional record (Nikiforova, 1978). The most promising candidate appears to be the Vrica section in southern Italy that includes a continuous fossiliferous marine sequence but which does not yet have firm magnetostratigraphic and radiometric age control. Pending selection of a stratotype, we follow Haq et al. (1977) who placed the boundary at the top of the Olduvai paleomagnetic event, about 1.7 m.y., based on nannofossil and planktic foraminifer study of the La CasteUa and Santa Maria di Catanzaro sections and six magnetically dated continuous deep-sea cores. Several of the planktic LAD and FAD datums identified by Haq et al. (1977) and further refined by Berggren et al. (1980) are useful criteria for locating the Neogene/Quaternary boundary in the Coastal Plain. For example, the occurrence of Gephyrocapsa oceanica and Gephyrocapsa small species are useful for recognizing the Quaternary. Gephyrocapsa aperta (which is known from Pliocene deposits) has not yet been found in the Coastal Plain. Discoaster brouweri and the foraminifera Globigerinoides obliquus occur rarely in Coastal Plain deposits but when present indicate a Neogene age. In terms of ostracode assemblage zones, in South Carolina the boundary is equivalent to the boundary between the Paracytheridea mucra and the Puriana convoluta Assemblage Zones. Deposits along Walkers Bluff, Calabash, and in Old Dock, North Carolina appear to be slightly older than P. convoluta Zone assemblages in South Carolina. The early Pleistocene was generally one of warm climates as indicated by the occurrence in the Waccamaw Formation of tropical ostracodes such as Palaciosa and Caudites, and abundant Radimella and Neocaudites. Hazel (1971b) found a similar climatic situation in North Carolina during the upper Puriana mesacostalis Assemblage Zone. The abundance of Globigerinoides spp. in the Waccamaw also signifies relatively warm surface waters between 1.9 and 1.3 m.y. A significant turnover in the ostracode fauna occurred at this time with the extinction of at least fourteen species and the appearance of at least ten species (Cronin, 1981, fig.6). This faunal change appears exaggerated because in rock stratigraphic terms, the actual boundary at 1.7 m.y. has not been recognized by a continuous section in the eastern U.S. and is probably represented by unconformities between the Bear Bluff and Waccamaw Formations in South Carolina, and the lower and upper Croatan Formation in North Carolina. The paleomagnetic data for the Bear Bluff, Waccamaw and middle Pleistocene Canepatch Formations (Liddicoat et al., 1979) generally support the biostratigraphic data. Bear Bluff samples yielded magnetic data suggesting a probable Gauss Chron; Waccamaw samples are reversed and are placed in the Matuyama. Following a depositional hiatus of perhaps several hundred thousand years, normal magnetism is found in the Canepatch Formation in South Carolina and also in younger marine deposits of the Coastal Plain, all

45 considered to have formed during the Brunhes Chron. In North Carolina, the Pliocene/Pleistocene boundary is located between the lower and upper parts of the Puriana mesacostalis Assemblage Zone. Together, the data indicate there was marine deposition at the Neogene/ Quaternary boundary and a major marine transgression during the early Pleistocene beginning roughly 1.6 m.y. ago. Continuous marine deposition in which the Olduvai magnetic event might be recognized and the Neogene/ Quaternary boundary identified may yet be found, particularly in thicker Neogene and Quaternary sections in the Albemarle Sound or more likely on the continental slope. Work is underway towards this goal. Additional evidence for a warm early--middle Pleistocene comes from the northern Delmarva Peninsula, represented by samples labeled Omar Formation at "Diricksen Creek" in Fig. 8. These samples, provided by James Demarest (University of Delaware), have yielded the following thermophilic ostracodes: Reticulocythereis floridana Puri, 1960, Megacythere repexa Garbett and Maddocks, 1979, Loxoconcha sarasotana Benson and Coleman, 1963, Paracytheridea altila Edwards, 1944, and some unnamed species. Although a diverse marine facies has not y e t been found for this unit, these preliminary ostracode data suggest a strong warm-water pulse and possibly correlation of these deposits with the Waccamaw or, if the amino-acid age estimates are correct (Wehmiller and Belknap, 1982), the Canepatch Formation (Fig. 8). THEBRUNHESCHRON The past 730,000 yr. were characterized by relatively brief high stands of sea level, terminated by abrupt regressions which reflect glacio-eustatic sealevel lowering. Although radiometric data are scarce and the middle and late Pleistocene onshore marine chronologies differ from region to region, the composite data for the entire coast for the timing of high stands generally corresponds to climatically warm periods in the open ocean North Atlantic and appear to correspond to expected warm peaks as predicted from the Milankovitch astronomical theory of global climates (Cronin et al., 1981). Biostratigraphically, the two ostracode assemblage zones were defined on the basis of ostracode species ranges, detailed zoogeographic study of the late Pleistocene appearence of the modern ostracode shelf fauna, and radiometric dating of key localities (Zones B and C o f Cronin and Hazel, 1980). Calcareous nannofossil LADs and FADs are also useful in correlating these young deposits. The occurrence of Gephyrocapsa supports a uranium series age of 440,000 yr. on the Canepatch Formation (Cronin et al., 1981), a midPleistocene deposit with a very diagnostic ostracode assemblage (Cronin and Hazel, 1980). A n o t h e r problematical unit, the James City Formation of North Carolina contains Pseudoemiliania lacunosa and Gephyrocapsa indicating an age of about 1.7--0.5 m.y. The absence of Helicosphaera sellii, provides evidence for an age younger than 1.3 m.y. The ostracode fauna

46

suggests an age near the younger part of this range, roughly between 1.3 and 0.7 m.y. This formation has yielded equivocal paleomagnetic data indicating reversed polarity. In marine deposits dated by uranium series ages as younger than 250,000 yr. B.P., Gephyrocapsa occurs frequently. Although Emiliania huxleyi (FAD about 275,000 yr. B.P.) has n o t y e t been found, this is n o t surprising because Emiliania huxleyi was rare relative to Gephyrocapsa during the interval 275,000 until 85,000 to 75,000 yr. B.P. when it reached its acme zone (Gartner, 1977). ACKNOWLEDGEMENTS

Thanks go to Dr. L. W. Ward (USGS) for his helpful suggestions, to Dennis Darby and Randell Spencer (Old Dominion University) for their assistance in the field, Dr. B. J. Szabo (USGS) for his discussions on uranium dating, to Dr. John Wehmiller (University of Delaware) and Dr. Daniel Belknap (University of South Florida), for their help with amino-acid racemization, to J. P. Owens, L. M. McCarten, R o b e r t Weems and R. B. Mixon for providing samples. Special thanks go to Ellen C o m p t o n for planning and drafting the figures. APPENDIX I: Stratigraphic units Original reference and location of type locality (preceded by an asterisk) are given for each unit studied. Only the general region is given for units that were defined on the basis of morphologic criteria and for which no formal type locality was designated. Bear Bluff -- DuBar, J. R., Johnson, H. S., Thom, B. and Hatehell, W. O., 1974. Neogene Stratigraphy and Morphology, South Flank of the Cape Fear Arch, North and South Carolina. In: R. Q. Oaks and J. R. DuBar (Editors), Post Miocene Stratigraphy, Central and Southern Atlantic Coastal Plain. Utah State University Press, Logan, UT, p. 156. *East bank of Waccamaw River, 16.1 km east of Conway at Bear Bluff, Horry Co., S.C., Nixonville 7 1/2'. Canepatch -- DuBar, J. R., Johnson, H. S., Thorn, B. and Hatehell, W. O., 1974. Neogene stratigraphy and morphology, south flank of the Cape Fear Arch, North and South Carolina. In: R. Q. Oaks and J. R. DuBar (Editors), Post Miocene Stratigraphy, Central and Southern Atlantic Coastal Plain. Utah State University Press, Logan, UT, p. 164. *On south bank of Intercoastal Waterway about 10.5 km north of Myrtle Beach near Canepateh Swamp, Horry Co., S.C., Nixonville 7 112'. Chowan River -- Blackwelder, B. W., 1981. Stratigraphy of the upper Pliocene and lower Pleistocene marine and estuarine deposits of northeastern North Carolina and southeastern Virginia. U.S. Geol. Surv. Bull., 1502-B: 1--16. *Right bank of Chowan River, 1.9 km southeast of Colerain, Bertie Co., N.C. " C r o a t a n " -- Dall, W. H., 1892. Tertiary Fauna of Florida. Wagner Free Inst. Sci. Trans., 3 (Pt. 2): 213--216. *About 3.2 km below James City at Flanner Beach, south side of Neuse River, Craven Co., N.C., Havelock 7 1/2'. "DupUn" -- Dall, W. H., 1898. U.S. Geol. Surv. 18th Annu. Rep. Pt. 2, p. 388; published in 1897 as 55th Congr., 2nd Session H. Doc. 5. *Natural Well, Matthews family farm, 1.6 km SW of Magnolia, Duplin Co., N.C., Rose Hill 15'. Flanner Beach -- DuBar, J. R. and Solliday, J. R., 1963. Stratigraphy of the Neogene deposits, lower Neuse estuary, North Carolina. Southeast. Geol., 4(4): 213--233.

47 * A b o u t 3.2 km below James City at F l a n n e r Beach, south side of Neuse River, Craven Co., N.C., Havelock, 7 1/2' " G o o s e C r e e k " -- Sloan, E., 1907. S u m m a r y of Mineral Resources of S o u t h Carolina, pp. 12, 18, 19. * Y e a m a a s Hall, on Goose Creek, Berkeley Co., S.C., N o r t h Charleston 7 1/2' James City -- DuBar, J. R. and Solliday, J. R., 1963. Stratigraphy of the Neogene deposits, l o w e r Neuse estuary, N o r t h Carolina. Southeast. Geol., 4(4): 213--233. *West bank of Neuse River, 1.0 km d o w n from F o r t P o i n t light, Craven Co., N.C., New Bern 7 1/2' Neuse -- Fallaw, W. C. and Wheeler, W. H., 1969. Marine fossiliferous Pleistocene deposits in southeastern N o r t h Carolina. Southeast. Geol., 10(1): 35--54. * N o r t h bank of Neuse River, 17.7 km SE of E edge o f New Bern, N.C. 7 1/2' quad.; i m m e d i a t e l y west of m o u t h o f Beard Creek N o r f o l k -- Clarke, W. B. and Miller, B.L., 1906. G e o l o g y of the Virginia Coastal Plain. Va. Geol. Surv. Bull., 2 (pt. 1) : 20. *Dismal S w a m p Canal . . . . . Va. O m a r -- Jorden, R. R., 1962. Stratigraphy of the s e d i m e n t a r y rocks of Delaware. Del. Geol. Surv. Bull., 9 : 5 1 pp. *Omar, Del. Pamlico -- Clark, W. B., 1909. Geol. Soc. Am. Bull., 20: 657. *Pamlico Sound, Pamlico Co., N.C. P e n h o l o w a y -- Cooke, C. W., 1925. The Coastal Plain. Ga. Geol. Surv. Bull., 32: 81, f i g . l l . *Hortense, Brantly Co., Ga. Princess A n n e -- W e n t w o r t h , C. K., 1930. Sand and Gravel Resources of the Coastal Plain o f Virginia. Va. Geol. Surv. Bull., 32: 81. *Princess A n n e Co., Va. Raysor -- Cooke, C. W., 1936. G e o l o g y of the Coastal Plain of S o u t h Carolina. U.S. Geol. Surv. Bull., 8 6 7 : 1 8 9 pp.; ( n e o s t r a t o t y p e ) Blackwelder, B. W. and Ward, L. W., 1979. Stratigraphic revision of the Pliocene deposits of N o r t h and S o u t h Carolina. Geol. Notes, 23(1): 38. *Givhans F e r r y State Park on Edisto River, left bank, 1.8 km above Rt. 61 bridge, C o l l e t o n Co., S.C., Maple Cane S w a m p 7 1/2' Silver bluff -- Cooke, C. W., 1945. G e o l o g y of Florida. Fla. Geol. Surv. Bull., 941 : 248. *Dade Co., Fla. Socastee -- DuBar, J. R. Johnson, H. S., Thorn, B. and Hatchell, W. O., 1974. Neogene stratigraphy and m o r p h o l o g y , south flank of the Cape F e a r Arch, N o r t h and South Carolina. In: R. Q. Oaks and J. R. DuBar (Editors), Post Miocene Stratigraphy, Central and S o u t h e r n Atlantic Coastal Plain, U t a h State University Press, Logan, UT, p. 168. *West bank of Intercoastal Waterway near Socastee Swamp, a b o u t 0.3 km SW of U.S. Highway 501, Bucksville 7 1/2', Socastee Co., S.C. T a l b o t -- Shattuck, G. B., 1901. T h e Pleistocene p r o b l e m of the North Atlantic Coastal Plain. J o h n s H o p k i n s Univ. Circ., 20: 74. * T a l b o t Co., Md. Waccamaw -- Dall, W. H., 1892. Tertiary F a u n a of Florida. Wagner F r e e Inst. Sci. Trans., 3 (pt. 2): 209--213. *East bank o f Waccamaw River at Tilly's Lake, Nixonville 7 1/2', Horry Co., S.C. Wando - - McCarten, L., Weems, R. E. and L e m o n , E. M., Jr., 1980. The Wando F o r m a tion ( U p p e r Pleistocene) in the Charleston, S o u t h Carolina, arem In: N. F. Sohl and W. B. Wright (Editors), Changes in Stratigraphic N o m e n c l a t u r e by the U.S. Geological Survey, 1979. U.S. Geol. Surv. Bull., 1502-A: 110--116. *Banks C o n s t r u c t i o n Co. Pit, 10 km east of Charleston, S.C., north of Venning Rd., east of R o u t e 17A W i c o m i c o -- Shattuck, G. B., 1901. The Pleistocene p r o b l e m of the North Atlantic Coastal Plain. J o h n s H o p k i n s Univ. Circ., 20 : 74. *St. Mary's Co., Md. Y o r k t o w n -- Clarke, W. B. and Miller, B. L., 1906. G e o l o g y of the Virginia Coastal Plain. Va. Geol. Surv. Bull., 2 (pt. 1): 19. *Rushrnere landing on J a m e s River, 1.0 km from R u s h m e r e , Isle o f Wight Co., Va., Bacon's Castle 7 1/2'

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NOTE A D D E D I N P R O O F Since the original submission of this paper, an important volume on the Neogene/ Quaternary deposits of the Lee Creek Mine, North Carolina, has been published (C.E. Ray (Editor), 1983, Geology and Paleontology of the Lee Creek Mine, I. Smithson. Contrib. Paleobiol., No. 53) containing papers on ostracodes (by J.E. Hazel) and planktic foraminifers (by S.W. Snyder, L.L. Manger, and W.H. Akers). The foraminifer data generally support our conclusions that the Yorktown Formation represents an age of late early to early late Pliocene.