palynological evidence for late miocene, pliocene and early pleistocene climate changes in the...

Quaternary Science Reviews, Vol. 10, pp. 147-162, 1991. 0277-3791/91 $0.00 + .50 Printed in Great Britain. All rights reserved. (~) 1991 Pergamon Press plc

P A L Y N O L O G I C A L E V I D E N C E F O R L A T E M I O C E N E , P L I O C E N E A N D E A R L Y P L E I S T O C E N E C L I M A T E C H A N G E S IN T H E M I D D L E U.S. A T L A N T I C C O A S T A L

P L A I N

Johan J. Groot Delaware Geological Survey, University of Delaware, Newark, DE 19716, U.S.A.

Palynomorph assemblages from eight geologic formations in the Middle Atlantic Coastal Plain, ranging in age from latest Miocene to Early Pleistocene, have been interpreted in terms of terrestrial paleoclimates. The data suggest that a warm- temperate to subtropical climate, warmer than at present, prevailed at the close of the Miocene and the beginning of the Pliocene. At that time, there was little or no temperature gradient within the study area (36030 ' to 39°N). This warm period was followed by a warm-temperate interval in Virginia and North Carolina, with temperatures probably not very different from those of today, although a slight warming trend probably occurred during the deposition of the Colerain Beach Member of the Chowan River Formation. A definite cool interval is indicated by the presence of spruce pollen in the Bacons Castle Formation of Virginia. This interval is interpreted to have begun about 2.3-2.4 Ma, or possibly slightly later, simultaneous with the cooling that has been recorded in deep-sea cores of the North Atlantic Ocean, and in the pre-Tiglian of western Europe. This was followed by a warmer-than-present period that may be correlated with the Tiglian of the latest Pliocene. Finally, palynologicai data from the Cape May Formation of New Jersey suggest that a warm-temperate (warmer than at present) climate prevailed during the Early Pleistocene.

INTRODUCTION

Early palynological investigations were concerned with the analysis of pollen assemblages in Quaternary sediments, with the primary objective of describing vegetational changes that could be interpreted in terms of variations in climate. Thus, pollen analysis was originally used to study the paleoclimatology of the last few hundred thousand years, and, in considerable detail, the climate variations of postglacial western Europe. Most of these studies were done by botanists. Only in the last sixty years has there been more emphasis on paleopalynology, or the stratigraphic distribution of palynomorphs.

The interpretation of palynomorph assemblages in terms of climate depends, in part, on the occurrence of pollen of extant taxa with known climatic requirements, and on the assumption that these requirements have not significantly changed during the last few million years. Traverse (1988) opined that the presently existing taxa are not very different, at least on the generic level, from those up to ten million years ago; therefore it should be possible to interpret late Neogene palynomorph assemblages in terms of terrestrial climate changes. And as these assemblages comprise some exotic taxa of approximately known stratigraphic ranges, they are also useful in interpreting the age of the formations in which they occur.

In the middle Atlantic Coastal Plain, clastic sediments of late Miocene, Pliocene and Quaternary age form the surficial deposits covering most of the study area (Fig. 1). Among the geologic formations that have been recognized in that area are the Eastover, York- town, Chowan River, Bacons Castle and Windsor formations of Virginia and North Carolina, the Brandy-

wine Formation of Maryland, the Bethany, Beaver- dam, and Omar formations of Delaware, and the Cape May Formation of New Jersey (Ward and Blackwelder, 1980; Blackwelder, 1981; Ramsey, 1988; Groot et al., 1990).

Some palynological investigations of Neogene sediments in the middle Atlantic Coastal Plain have been done in the past, but several of these studies were concerned with Miocene formations (Leopold, 1969; Goldstein, 1974; Rachele, 1976; Brush, cited in Han- son, 1981). Owens and Denny (1979), however, studied the Pliocene Beaverdam Formation of Delaware and reported the presence of Pterocarya. Groot et al. (1990) described pollen assemblages from the late Miocene and Pliocene formations of Virginia, and reviewed the palynostratigraphy of Miocene, Pliocene, and Early Pleistocene deposits in the middle Atlantic region. These studies provided the palynostratigraphic basis for the paleoclimatic investigation that is the subject of this paper.

Sample Numbering System The sample numbering system is that used by the

Delaware Geological Survey. Numbers in the 20 thousand series indicate core samples from Delaware, 40 thousand and 80 thousand series are, respectively, outcrop and auger samples from the State, and 50 thousand series are samples obtained from outside Delaware. Sample localities are shown in Fig. 1.

INTERPRETATION OF PALYNOLOGICAL DATA IN TERMS OF CLIMATE

Routine identification of palynomorphs is, in general, possible only on the generic level, and in some

147

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78 ° 40*

38 Q

36

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VIBGI

~ASTO~

76 ° 74 °

PENNSYLVANIA ~ /¢" / / ~ " " t

BEY \,,

' I) ', \ +I~)~cAPE ~A~ " ( , ~, "\ ~~AIRPORT BOREHOLE

y / \ •

LOWER " ~ t BEAVERDAM'-"S-_ ~/ / • F . ~d~-UPPER B~AV~ROA~ FM

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. . . . . . . . . . . . . . . . . . . . . . . . . . C)WAN RIVER FM, YADKIN PIT h~ GOMEZ PIT

,..QiN.,, ~_.^.~

CHOWAN RIVER F M ~ ~ ~ ~ , ~ ~ COLERAIN =EACH MBR,~ ~ ' ~ - ~ - ~ ~ 1\

, EDENHOUSE :MBR-.~-q .~, / ~,~, ,,

26 0 F I MILE8

KILOMETER8

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FIG. 1. Map of the study area showing sample localities.

25 4

cases, on the plant family level. Some taxa occur in a wide variety of climates, but may be indicative, if not of climate, of environment of deposition. For instance, high percentages of the pollen of Chenopodiaceae, Compositae, and Gramineae suggest, in the study area, an estuarine or lagoonal environment (Groot, 1966). Abundant pollen of Typha lati[olia, Sparganium, Myriophyllum, and Gramineae suggest a freshwater marsh, without much implication as to the temperature prevailing at the time of deposition.

Although this paper is primarily concerned with climate changes, some attention must be paid to environments of deposition, because they have a bearing on the provenance of palynomorphs. Whereas a small freshwater body receives pollen and spores produced essentially by the local vegetation only, fluviatile and estuarine sediments may contain palynomorphs from a large drainage basin, although the vegetation close to the site of deposition is likely to furnish the vast majority of the pollen and spores.

Palynological Evidence for Climate Change 149

Palynomorphs in marine sediments may have been transported by ocean currents as well as by rivers, and the possibility must be considered that changes in pollen assemblages from such sediments may be the result, at least in part, of changes in currents rather than in climate.

Taxa that are considered indicators of climate in the study area are listed in Table 1.

The vegetation of the northern part of the study area (the Coastal Plain of Delaware) lacks both cold and subtropical plant genera. The present climate of this area is called temperate. Trewartha and Horn (1980) include this area in the temperate-continental zone. The southern part of the study area (Virginia, northern North Carolina) has a vegetation that comprises temperate and subtropical taxa, including Cyrilla, Symplocos, and Taxodium. In this part of the study area the present climate may be called warm- temperate, although Trewartha and Horn (1980) classi- fied it as subtropical-humid. Clearly, some latitudinal differences in the present vegetation occur.

Davis and Webb (1975) made a study of the contemporary distribution of pollen in eastern North America. In the Coastal Plain of Delaware, Maryland, Virginia and northern North Carolina Pinus and Quer- cus pollen together comprised 60% or more of the

assemblages and Carya about 5%. Pollen of southeastern trees, Taxodium, Liquidambar and Nyssa were numerous in the Cape Hatteras area, but amounted to only 5% of total tree pollen in the Delmarva peninsula and the western part of the Coastal Plain of Virginia and North Carolina. No Picea and Tsuga were found. Davis and Webb (1975, p. 426) concluded that there is a "close geographical correlation between pollen assemblages and vegetation."

A study of modern pollen-vegetation relationship in the south-eastern United States was made by Delcourt et al. (1983), using mainly pollen obtained from lakes, ponds, and swamps. That study presented a series of maps showing various genera as percentages of the forest vegetation, and as percentages of arboreal pollen. Not surprisingly, Pinus is somewhat over- represented in the pollen assemblages, and some genera, e.g. Liquidambar, Abies, and Larix are under- represented. Under-representation is also true for trees that are mainly insect pollinated, e.g. Tilia and Acer.

Interpretation of pollen assemblages in terms of vegetation and climate is, when lake or pond sediments are used, possible in broad terms. When dealing with fluvial, estuarine, and marine sediments, long distance transport by water is a possibility. Because 95% of all pollen settle down (in the atmosphere) within one

TABLE 1. Climate and taxon distribution in eastern North America

Climate Taxon Present distribution in eastern North America (Elias, 1987)

Cold Picea (spruce)

Abies (fir)

Cool Tsuga (hemlock)

Larix (tamarack)

Cool-temperate to Betula (birch) warm-temperate

Temperate to sub-tropical

Warm-temperate to tropical

Alnus (alder)

Fagus (beech)

Fraxinus (ash)

Carya (hickory)

Castanea (chestnut; chincapin)

Celtis (hackberry)

llex (holly)

Liquidambar (sweet gum)

Nyssa (tupelo)

Quercus (oak)

Tilia (basswood)

Ulmus (elm)

Pterocarya (wingnut)

Sciadopitys (umbrella pine)

Cyrilla (leather wood)

Engelhardia

Symplocos (sweet leaf)

Taxodium (bald cypress)

Northern New England, Canada, high mountains of North Carolina

Northern New England, Canada, high mountains of North Carolina

Northeastern North America, Appalachian Mountains

Northeastern North America

New England to northern Florida, depending on species, but mostly in cool temperate regions; dwarf birch in Arctic

New England to northern Florida, depending on species, but mostly in cool temperate regions

New England to northern Florida

New England to Florida, depending on species

Quebec to northern Florida

New England to northern Florida

Southern New England to Florida, depending on species

Southern New England to central Florida, depending on species

New Jersey to central Florida

New England to Florida

Quebec to northern Florida

New England to Florida, depending on species

Northern New England to central Florida

Extinct in North America. Extant in Caucasus Mountains, and southeast Asia

Extinct in North America. Extant in southern Japan

Virginia to south central Florida

Extinct in the eastern United States. Extant in central America

Southern Delmarva peninsula to central Florida

Southern Delmarva peninsula to southernmost Florida. Common from Virginia southward in the Coastal Plain

150 J.J. Groot

kilometer of a tree (Traverse, 1988), river and other water currents are likely to transport primarily pollen of plants that grow near stream banks and in coastal areas. Vesiculate pollen can float for a considerable time and distance (Traverse, 1988) and some fern spores are also apparently buoyant (Koreneva, 1971). In interpreting pollen assemblages in terms of vegetation and climate, it is therefore advisable to consider the provenance of the pollen assemblages and the settling properties of the palynomorphs as much as possible.

The late Neogene sediments of the study area are mainly near-shore marine or estuarine, but fluvial, deltaic and freshwater marsh deposits also occur. These different environments can produce different contemporaneous pollen assemblages within one climate region. It would therefore be unwise to depend primarily on quantitative data (percentages of Pinus, Quercus, Carya, etc.) to interpret climate changes. In order to do so, it is preferable to determine the presence and/or absence of pollen of those taxa that are good temperature (or moisture) indicators, e.g. Tax- odium, Cyrilla, Symplocos, Engelhardia, Sciadopitys and Picea, even if the frequency of their occurrence is low.

PALYNOMORPH ASSEMBLAGES OF THE EASTOVER, BRANDYWINE AND BETHANY-

LOWER BEAVERDAM FORMATIONS

The palynomorph assemblages of these formations are shown in Table 2. They have been grouped together because they have much in common. The Eastover Formation (Ward and Blackwelder, 1980), is a transgressive unit of late Miocene age, but the formation may be of early Pliocene age near the Fall Line in Virginia (Newell and Rader, 1982). A sample (58092) obtained in the Jack Quarry (near the Fall Line), is exceptional in that the pollen assemblage indicates deposition in a freshwater marsh environment; it also includes Sciadopitys, usually found in Pliocene sediments of the middle Atlantic Coastal Plain.

The Brandywine Formation at the Brandywine site, Maryland (Fig. 1) has recently been described by McCartan et al. (1990). The formation at this site is of fluviatile origin and contains many plant fossils. It is reportedly of late Miocene age.

The Bethany formation in southern Delaware is essentially a gray silt and clay with interbedded sands. The environment of deposition has been interpreted as being deltaic (Groot et al., 1990). The overlying Beaverdam Formation consists mainly of medium sands with scattered beds of gravelly sand and greenish gray silty clay. The lower part of the Beaverdam is of fluvial-deltaic origin. The upper part of the formation contains pollen assemblages rather different from those in the Bethany and lower Beaverdam, and will be discussed in conjunction with the Pliocene formations of Virginia and North Carolina.

The age of the Bethany-lower Beaverdam is believed

to be latest Miocene or earliest Pliocene in view of the similarity of their pollen assemblages and those of the Brandywine and Eastover formations (Groot et al., 1990).

The pollen assemblages shown in Table 2 are characterized by generally high, although variable, percentages of Quercus and Carya, the consistent presence of pollen of the Taxodiaceae-Cupressaceae- Taxaceae (TCT), and low Pinus frequencies. Repre- sentatives of a cold climate (e.g. Picea) are absent in most samples, and the few pollen of that genus may have been transported some considerable distances, either by rivers or by winds. On the other hand, representatives of temperate to subtropical climates are dominant, particularly Quercus and Carya. Taxodium, Pterocarya, Sciadopitys and Liquidambar are present. Rare occurrences of Symplocos, Cyrilla and Engelhar- dia suggest warmth also. Consequently, the palynomorph assemblages indicate a warm-temperate to subtropical, moist climate, similar to that of the present southern Atlantic Coastal Plain. Some temperature difference with latitutde is suggested by the occasional presence of Picea and Tsuga in Delaware and their near absence farther south. On the other hand, it is also possible that these two taxa reached Delaware from the north by way of the Delaware River, as did a few reworked Cretaceous palynomorphs that were eroded from outcrops adjacent to the Fall Line. However, the similarity of the late Miocene-early Pliocene pollen assemblages spanning two degrees of latitude is striking and if there was a latitudinal temperature gradient, it must have been small, probably smaller than at present.

PALYNOMORPH ASSEMBLAGES OF THE YORKTOWN, CHOWAN RIVER, THE UPPER

BEAVERDAM AND BACONS CASTLE FORMATIONS

The Yorktown Formation consists mostly of shelly sand with abundant pelecypod shells, ostracodes, and foraminifera. The marine character of the formation is also indicated by the common occurrence of dinoflagellate cysts and 'microforms'. Although palynomorphs are fairly abundant in samples from the Yorktown, preservation is generally poor, with many of the large pollen grains (e.g. Pinus and Carya) partially de- stroyed, presumably by bottom-dwelling organisms.

The Chowan River Formation consists of fine to coarse silty sand with scattered pebbles at its base. Dinoflagellate cysts have been found in all five samples collected from the Chowan River.

Ramsey (1988) found evidence for the unconform- able contact between the Bacons Castle Formation and the Yorktown and older formations. The Bacons Castle also lies unconformably on the Chowan River Forma- tion in northeastern North Carolina (Johnson et al., 1987). Certainly, the lithology of the Bacons Castle differs from that of the other Pliocene formations of Virginia in that it is coarser, with, in some places, basal

P a l y n o l o g i c a l E v i d e n c e fo r C l i m a t e C h a n g e 151

T A B L E 2. P a l y n o m o r p h a s s e m b l a g e s o f t h e E a s t o v e r , B r a n d y w i n e , a n d B e t h a n y - l o w e r B e a v e r d a m f o r m a t i o n s , e x p r e s s e d in p e r c e n t a g e s o f t he p o l l e n s u m e x c l u d i n g r e w o r k e d p a l y n o m o r p h s

T a x o n S a m p l e n u m b e r s

58038 58056 58092 58035 83039 83042 83094 20959 83217

A c e r - - .9 . . . .

A l n u s 2 10 2 3 8 1 B e t u l a 1 4 2 - - 2 4

C a r p i n u s - O s t r v a - - P * - - P 2 5

C a r y a 2 10 2 29 11 14 C a s t a n e a . . . . 16 7 C o r y l u s . . . . . .

F a g u s - - 5 . . . .

F r a x i n u s 2 - - ? 4 - - - -

l l e x - - P - - 2 1 - -

J u g l a n s - - P - - p - - _ _

L i q u i d a m b a r - - 4 1 P 2 1

M y r i c a P - - - - p - - _ _

N y s s a 1 3 - - P - - - -

P o p u l u s . . . . . .

Q u e r c u s 52 31 28 24 23 25 S a l i x . . . . . .

T i l i a - - - - - - p - - - -

U l m u s 2 - - 1 P 1 - -

C e l t i s . . . . . .

E n g e l h a r d i a - - 1 . . . .

P t e r o c a r y a 2 1 P 6 1 3

C y r i l l a - - - - 1 P - - - -

S y m p l o c o s - - P . . . .

T r i c o l p o r o p o l l e n i t e s

e d m u n d i i - - - - - - p - - - -

C u p u l i f e r o i d a e p o l l e n i t e s

f a l l a x a n d l i b l a r e n s i s - - P 1 - - 1 1

P i c e a a n d A b i e s . . . . 3 1? P i n u s 7 6 2 2 6 1

T s u g a - - 2 - - - - 5 - -

T . d i v e r s i f o l i a t y p e - - P - - - - 1 - - S c i a d o p i t y s - - 1 P - - 6 4

T a x o d i a c e a e - C u p r e s s a c e a - T a x a c e a e 6 4 5 5 8 21

T a x o d i u m t y p e 2 2 2 4 2 4 S e q u o i a - t y p e - - - - - - 2 - - - -

A r t e m i s i a - - - - p - - _ _ _ _

C a r y o p h y l l a c e a e a n d C h e n o p o d i a c e a e 3 - - 1 - - - - - -

C o m p o s i t a e 9 5 9 6 1 3 G r a m i n e a e 11 - - 40 3 1 1 C y p e r a c e a e - - - - p - - - - - - E r i c a c e a e - - 2 . . . . H y d r o c h a r i t a c e a e . . . . . . T y p h a / S p a r g a n i u m - - - - p - - - - - -

M y r i o p h y l l u m - - - - p - - - - - -

N y m p h a e a c e a e - - - - p - - - - __ O t h e r h e r b s - - P 2 - - - - 2 L y c o p o d i u m . . . . . 1

O s m u n d a . . . . . .

P o l y p o d i a c e a e - - 3 1 - - 1 2 S p h a g n u m t ype - - 2 - - - - 1 - - D i n o c y s t s , m i c r o f o r a m s P . . . . . R e w o r k e d t - - 1 . . . . N A P 23 12 53 9 4 9

E a s t o v e r E a s t o v e r E a s t o v e r B r a n d y w i n e B e t h a n y B e t h a n y F o r m a t i o n F o r m a t i o n F o r m a t i o n F o r m a t i o n

( n e a r Fa l l L i n e )

- - 1 - -

4 2 5 3 - - 1

- - e - -

3 1 36 19

- - 1 1

- - _ _ . 9

- - 1 - -

- - p - -

P 5 1

P - - 1

41 20 42

P P - -

- - 1 1

P ? 1 P P 2 p - - ?

- - p - -

P

- - p - -

p - - _ _

6 3 2 4 1 P

P P - - P P 1

4 15 11 - - 1 3

. 9 - - _ _

p - - _ _

P P 3 - - 9 _ _

- - - - 1

- - 3 P

P - - 5

P P P 2 5 11

B e t h a n y B e a v e r d a m B e t h a n y F o r m a t i o n o r l o w e r

B e a v e r d a m

F o r m a t i o n F o r m a t i o n F o r m a t i o n F o r m a t i o n ( u p p e r ( l ower ) p a r t )

* P = < 1 % . t A s a p e r c e n t a g e o f the n o n r e w o r k e d p o l l e n s u m .

152 J.J. (}rool

cobble gravel (Ramsey, 1988), and a distinct change in the heavy mineral suites from hornblende-dominated in the Yorktown Formation to zircon-dominated in the Bacons Castle (Sinnott, 1955; Lawrence, 1974). The Bacons Castle is considered to be of tidal-fat and fluvial-estuarine origin (Ramsey, 1988). Dinoflagellate cysts are rare in the sediments sampled for palynological analysis.

The age of the Yorktown Formation has been the subject of considerable discussion. However, micropaleontological data and U-series dates reported by Cronin et al. (1984) indicate that the basal part of the formation is not older than 4.8 Ma, and that the upper part is not younger than 2.4 Ma. It is probable that the age of the formation ranges from about 4 to 3 Ma (H. Dowsett, oral commun.). During that time interval deposition was not continuous; at least three transgressive events have been recorded (Ward and Blackwel- der, 1980).

The age of the Chowan River Formation is not entirely clear. He/U data on corals gave dates of 1.91 and 2.4 Ma (Blackwelder, 1981). Cronin et al. (1984), stated that the formation contained calcareous nanno- plankton zones NN 16-18, suggesting a possible age ranging from 3.4 to 2 Ma. Liddicoat et al. (1983) found that Chowan River deposits at the type locality and in the Yadkin pit had reversed polarity. Thus, the combination of paleontological and magnetic data suggest that the age of this formation is slightly younger than 2.48 Ma (the Matayama-Gauss boundary).

The Bacons Castle Formation is the youngest known Piiocene deposit of Virginia, and is assumed to be younger than 2 or 2.3 Ma. The ages of the Pliocene formations are further discussed in relation to their pollen assemblages.

The pollen assemblages of the Yorktown Formation (Table 3) are dominated by Quercus and Pinus, and Carya is a common constituent. Exotic taxa present are Sciadopitys and Pterocarya. Thus, temperate to subtropical taxa are dominant. On the other hand, pollen of warm-temperate to tropical taxa, e.g. Taxodium, Sym- plocos, and Cyrilla, are rare or absent. Some cool climate taxa are represented by Tsuga, and, surprisingly, by Larix in a sample obtained in the Yadkin pit. Under present climatic conditions, Tsuga and Larix pollen have not been found in lake sediments in Virginia and North Carolina. Their presence in the marine Pliocene can perhaps be attributed to long- distance transportation.

The nonarboreal palynomorphs are mainly dino- cysts, microforms and spores of Sphagnum and the Polypodiaceae. The pollen assemblages suggest a terrestrial climate not very different from that of the present, and a nearshore marine environment of deposition.

The assemblage of the Edenhouse Member of the Chowan River Formation (Blackwelder, 1981), (samples 58100 and 58101, Table 3) is dominated by Pinus. Temperate to subtropical taxa comprise, respectively, 17 and 21% of the arboreal pollen sums of these samples. Pollen of cool climate taxa, Picea, Larix and Tsuga, are

present but rare. Warm-temperate to subtropical taxa, e.g. Taxodium, are even rarer. Pterocarya is the only exotic taxon identified. The most frequently occurring nonarboreal palynomorphs are those of the Grami- neae, Compositae, and Sphagnum, and dinoflagellate cysts are common. The Edenhouse assemblage suggests a terrestrial climate similar to that of today, and a near- shore marine environment of deposition.

Quercus and Pinus are the most frequent taxa in the Colerain Beach Member (overlying the Edenhouse) of the Chowan River Formation. Temperate to subtropical taxa dominate the arboreal assemblage. Exotic pollen are those of Pterocarya and of Araucaria(?), or possibly Araucariacites, reworked from Mesozoic (Triassic-Jurassic?) deposits. Warm-temperate to tropical taxa as well as cool-climate taxa are rare. The nonarboreal pollen assemblage suggests an estuarine or near-shore marine environment. Dinoflagellate cysts are present, but not common. The terrestrial climate was perhaps slightly warmer than that prevailing during the deposition of the Edenhouse Member, and perhaps somewhat warmer than at present.

The preservation of the palynomorphs in other samples of the Chowan River Formation (58011 and 58107) is poor. The main constituents are Pinus, Quercus, and Carya. No Pterocarya was identified, and the presence of Sciadopitys is questionable. Warmth is suggested by rare occurrence of Taxodium and CyriUa, and perhaps Engelhardia. On the other hand, Tsuga suggests a relatively cool and moist climate.

In general, the palynological evidence suggests a warming trend during the deposition of the Chowan River Formation, without reaching the temperature that prevailed during the deposition of the Eastover and Brandywine formations at the end of the Miocene and the beginning of the Pliocene.

The palynomorph assemblage of the Bacons Castle Formation of Virginia is dominated by Quercus and Carya. The Pinus percentages in the Bacons Castle are much lower than those of the Yorktown and Chowan River formations. Exotics are Pterocarya, and, in one sample, Tricolporopollenites edmundii. The most re- markable feature of the assemblage is that Picea pollen are present in significant quantity, 6% in sample 58003, and 4% in sample 58040. It is very unlikely that this is due to long distance transport of Picea pollen, and the conclusion is therefore that a relatively cool climate prevailed at the time and near the site of deposition.

As no fossils have been found in the Bacons Castle of Virginia, except pollen and spores, the precise age of this formation is difficult to determine. However, two types of evidence suggest that it is younger than 2.4 or 2.3 Ma: the formation is younger than the Chowan River Formation, and its pollen assemblage records the first rather sudden cooling in the late Pliocene. This cooling is clearly recorded in the deep-sea record of the North Atlantic (Zimmerman et al., 1984), and is also described by Braun (1989), who stated that the first major glaciation in North America reached its maxima at about 2.37-2.4 Ma, and again at 2.33 Ma. Although


the coincidence of climatic deterioration in the Coastal Plain of Virginia, in the North Atlantic and the northern part of North America is no proof of them being contemporaneous, it appears very likely, and therefore the age of the Bacons Castle is considered to be younger than about 2.4 or 2.3 Ma.

McCartan et al. (1990, p. 313) reported that the Bacons Castle of Maryland contains subtropical benthic mollusks. However, the Bacons Castle of Virginia has no mollusks at all (Ramsey, 1988) and the two units are probably not of the same age.

The environment of deposition of the Bacons Castle Formation, where sampled, is probably lagoonal or estuarine, as is suggested by the presence of pollen of the Chenopodiaceae, Compositae and Gramineae.

Quercus-Carya assemblages with relatively low Pinus percentages also occur in the upper part of the Beaverdam Formation of Delaware. The only exotic is Pterocarya. Picea is present in all four samples exami- ned, but it does not exceed one percent of the pollen sum. The environment of deposition of the upper Beaverdam is probably estuarine.

than in the higher samples, scattered exotic pollen are present, including Engelhardia, Pterocarya, Symplo- cos, Tricolporopollenites edmundii, and Sciadopitys. Also Taxodium pollen are present in all but one sample. These assemblages suggest a warm-temperate, moist climate and, if the exotic pollen are not reworked from Tertiary deposits, a Pliocene age.

Amino acid age estimates of the upper part of the Omar Formation include one of 500 + 100 ka on shell material obtained in borehole Qi51-04 at a depth of 12.8--13.2 m (-6 .4 to -6 .8 m msl) (Wehmiller, in Groot et al., 1990). No exotic pollen grains were found in this borehole above a depth of 25.9-26.4 m (-19.5 to -20 m msl). These data suggest that no exotic taxa were present in southern Delaware in Middle Pleisto- cene or even Early Pleistocene time. Data from other areas (see the following section of this report) suggest that the presence of exotics indicates a pre-Quaternary age. The lower part of the Omar, filling a paleovalley in the Pliocene Beaverdam Formation, is therefore considered to be of late Pliocene age; it was deposited at a time of a warm-temperate climate.

PALYNOMORPHASSEMBLAGES OF THE OMAR FORMATION

The Omar Formation is a heterogeneous unit consist- ing of interbedded sands, silty sands, clayey silts and silty clays. Lithologic changes occur over short distances, laterally and vertically. Shell beds occur in the formation at several localities, commonly of Crassos- trea virginica. Ramsey (in Groot et al., 1990, p. 4) described the lithostratigraphy of the Omar as follows: "The Omar Formation unconformably overlies the B e a v e r d a m . . . It fills a paleovalley that cuts into the Beaverdam . . . The interbedded fine sand, silt, and clay that fills the paleovalley is the oldest part of the Omar. It is either gradational with or unconformably overlain by . . . other lithofacies".

The age of the Omar Formation has been a controv- ersial matter but generally was considered to be Quaternary. Groot et al. (1990) divided the formation into a lower part of late Pliocene age, and an upper part of Quaternary age.

The palynomorph assemblages of the Omar Forma- tion in borehole Qh44-01, drilled and cored at the type locality of the formation, are shown in Table 4. Sample 20769 at +0.3 m msl (mean sea level) is exceptional because its arboreal pollen content is nearly exclusively composed of Pinus and Picea; the nonarboreal compo- nent consists mainly of spores of Sphagnum and the Polypodiaceae. A boreal forest, a cold climate, and a Quaternary age are implied. Above sample 20769, the pollen assemblages suggest a temperate, moist climate and environments of deposition ranging from Sphag- num bog (sample 20763) to bodies of fresh or brackish water. No exotic pollen were found except one grain of Ulmus serotina type.

Below +0.3 m msl the pollen assemblages differ in that Quercus generally occurs in higher frequencies

PALYNOMORPH ASSEMBLAGES OF QUATERNARY AND UPPER TERTIARY SEDIMENTS, CAPE MAY, NEW JERSEY

In 1989, a 92 m continuous core was obtained at the Cape May County airport by the U.S. Geological Survey in cooperation with the New Jersey Geological Survey. Seventeen samples of fine-grained clastic sediments were processed for palynological analyses; one of these was barren, and three samples contained an insufficient number (<100 specimens) or such poorly-preserved palynomorphs to make a reliable count possible. The results of the analyses are shown in Table 5.

It may be noted that exotic pollen have been found in cores at a depth of 48.78 m and below. These pollen are few in number, but they are consistently present at those depths, and consistently absent above 48.78 m. Sciadopitys is usually found in Pliocene and upper Miocene sediments of the mid-Atlantic region, e.g. in the Bethany, Beaverdam, Yorktown, Chowan River and Bacons Castle formations. It was widespread in the Neogene of Europe and became extinct in northwest Europe at the end of the Pliocene (Traverse, 1988, Table 15.1). It is present in sample 58067 at 48.78 m, together with Engelhardia. The pollen assemblage at this depth is further characterized by a high percentage of Quercus, and a low frequency of Pinus, suggesting a late Neogene age, probably Pliocene, and a warm-temperate climate. The nonarboreal pollen content is dominated by Chenopo- diaceae, indicating an estuarine environment of deposition.

Sample 58111 at a depth of 51.83 m lacks Sciadopitys, but Engelhardia and Tricolporopollenites edmundii are present. The exact age of this sample is not clear; it could be Pliocene or late Miocene. The sediments

154 J.J. Groot

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T A B L E 4. P a l y n o m o r p h a s s e m b l a g e s o f t h e O m a r F o r m a t i o n , e x p r e s s e d in p e r c e n t a g e s o f t h e p o l l e n s u m e x c l u d i n g r e w o r k e d p a l y n o m o r p h ~

T a x o n S a m p l e n u m b e r s

20763 20766 20767 20768 20769 2077(I 20771 20772 20773 211774 20775

A c e r . . . . . . . . . . . . . . .

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l l e x - - - - P P I" P 2 P - - I '

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T r i c o l p o r o p o l l e n i t e s

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T a x o d i a c e a e - C u p r e s s a c e a - T a x a c e a e 2 2 5 2 - - 18 9 20 2 5 P

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C o m p o s i t a e 6 12 5 - - 1 P 2 - 6 2 t ' G r a m i n e a e I 5 25 4 - - 1 2 I 1 1 3

C y p e r a c e a e - - - - 3 I ' - - '~ - - ? 2 - - 1

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T y p h a / S p a r g a n i u m - - 2 1 ..... I . . . . . . [ . . . .

M y r i o p h y l l u m - - 1 . . . . . . . . .

N y m p h a e a c e a e . . . . . . . . . . . . . . . . . O t h e r h e r b s 3 4 ~ " 1 4 ~ P L y c o p o d i u m 1 P . . . . . P 2 - P --

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P o l y p o d i a c e a e - - 13 4 2 4 1 6 6 l 3 P , S p h a g n u m t y p e 44 5 - - - - 9 - - 2 - - P 2 - -

D i n o c y s t s , m i c r o f o r a m s . . . . . . . . . . . . R e w o r k e d ? . . . . . . . . . . . . . . N A P 60 42 42 11 20 4 16 16 15 9 8

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below this depth are probably of Miocene age, deposited in a marine environment during a period of warm- temperate and temperate climates.

Samples 58063-58066 and 58108-58110, at depths from 29.57-47.26 m have much in common. All have high percentages of Quercus and low Pinus frequencies. Pollen of the Taxodiaceae-Cupressaceae- Taxaceae have been found in all these samples, including Taxodium in some of them. In general, a warm-temperate climate and an estuarine environment of deposition is indicated. The consistent absence of exotics suggests a Quaternary age. The pollen assemblages are quite similar to those of zone S-6 of the Stetson Core, Dare County, North Carolina (York et al., 1989), the estimated age of which is 0.8-1.3 Ma, or Early Pleistocene. In the Cape May core at 29.9 m, Mulinia samples yielded mean A/1 values suggesting an earliest Pleistocene to late Tertiary age (K.J. Laco- vara, 1990, written commun.). Therefore, the combination of palynological and aminochronological data indicate that the sediments between 29.57 m and 48.78 m are of Early Pleistocene age.

The palynological record above 29.57 m is meager, but is quite different from that below this depth. Sample 58061, at 21.52 m, is dominated by Pinus. Tsuga, Carya, and some Quercus are also present. A temperate climate is suggested. Fresh-water algal remains and the absence of pollen of salt-tolerant plants like the Chenopodiaceae may indicate deposition in a fresh-water body.

The palynological record of the Cape May cores shows that the climates of the late Tertiary and that of the Early Pleistocene were quite similar and somewhat warmer than at present. The record also indicates that there may not have been a hiatus in deposition at the Tertiary-Quaternary boundary at the Cape May site.

Only one sample was obtained from the Windsor Formation of Early Pleistocene age. As the preservation of the palynomorphs is very poor, a reliable count is considered impossible. However, the arboreal assemblage is essentially a Quercus-Carya-TCT- Liquidambar one, with little Pinus and Cyrilla. The nonarboreal pollen are mainly those of the Compositae, followed in frequency by Gramineae and Chenopodiaceae. A warm-temperate climate and an estuarine environment are indicated by this assemblage.

DISCUSSION

The palynological record of the late Miocene, Pliocene and Early Pleistocene sediments of the middle Atlantic area is still rather meager. Only a few samples of most of the formations described in this paper have been analyzed, except those of the Omar, Bethany, and Beaverdam formations, with more than ten analyses each (data in Groot et al., 1990, and in the files of the Delaware Geological Survey). It is expected that a greater number of samples from more localities will provide a more detailed palynological and paleoclima-

tic record than is described in this paper. It must also be remembered that periods of sedimentation during the Pliocene alternated with periods of erosion, and the paleontologic and palynological record is bound to be incomplete therefore.

Perusal of Tables 2-5 reveals that the arboreal pollen assemblages of late Miocene to Early Pleistocene time are mostly composed of Quercus, Pinus, and Carya. Indeed, it appears that from early Miocene time until the present, the major elements of the forest vegetation in the study area have changed little, at least on the generic level, except during the Pleistocene continental glaciations in North America. In order to deduce from the palynological record changes in climate, it is necessary, as mentioned earlier, to rely on the presence or absence of minor constituents, e.g. Taxodium, Cyrilla, Symplocos, Pterocarya, Picea, Tsuga, etc. Interpretations largely based on the occurrence of taxa of low frequency could possibly be misleading, because such elements may have been transported some considerable distance. However, the consistent presence of a low-frequency taxon in several samples from the same formation is likely to represent an element of the vegetation of the area near the site of deposition. For instance, Taxodium is present in all samples but one in the late Miocene-early Pliocene sediments, and Pterocarya in all (Table 2). It is assumed that these two genera were present in the vegetation of the study area, and not transported from afar. Therefore they are considered to be climate indicators.

The interpretation of paleontological and palynological data in terms of, respectively, marine and terrestrial climates does not necessarily coincide. A detailed study of the ostracode assemblages of core 16-905, (Fig. 2), a few miles off the Maryland coast in about 17 m of water, revealed the presence of two zones, the lower one (Zone 2, correlated with Substage 5d) suggesting a cold climate with water temperatures in winter between 3 and 6°C and an upper one (Zone 3) suggesting a milder climate, with an average water temperature of about 71/2°C in winter (Toscano et al., 1989). The pollen assemblages (Table 6) of Zone 2 are characterized by Pinus and Picea, indicating a cold climate. A sample taken a few inches above the Zone 2-3 boundary also suggests a cold climate but a slight amelioration is suggested by the presence of 4% Carya. Samples from the middle part of Zone 3 suggest a further improve- ment of the climate, as is shown by a decreasing percentage of Picea and an increasing frequency of Quercus. But only the uppermost sample, with 7% Quercus, 1% Carya and 1% Picea (the remainder being mostly Pinus and Sphagnum) can be interpreted in terms of a relatively mild climate. It appears that, although the general trend of climatic amelioration is indicated by both the micropaleontological and palynological data, the latter are more conservative, or the terrestrial climate change came somewhat later than the marine one. A similar situation may pertain to the Yorktown Formation, with a warmer climate suggested

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by the marine fossils than by the pollen assemblages. A likely reason for this apparent difference between the marine and terrestrial climates is the influence of ocean currents transporting warmth from the tropics.

The palynologic data of Tables 2-5 (inclusive) are summarized in Table 7. This table also includes the Oxygen Isotope record of the North Atlantic (Zimmerman et al. , 1984), showing the sharp temperature decline at 2.4 Ma. The deterioration of climate indicated by the pollen assemblages of the Bacons Castle Formation is correlated with that of the deep sea record, as there is no other published evidence with regard to its age, except that it is younger than the Chowan River. The cooling phase may also be contemporaneous with the deterioration of the climate in the pre-Tiglian of western Europe (Zagwijn, 1974), a brief cool period followed by the warmer Tiglian at the end of the Pliocene and the beginning of the Pleistocene.

The pollen assemblages of the Chowan River and Yorktown formations indicate a climate not very different from that of the present, except perhaps a warming trend at the time of deposition of the Colerain Beach Member of the Chowan River. Cronin et al.

(1989, p. 5) stated that " . . . the Yorktown Formation was deposited during a time when summer and winter temperatures were substantially warmer than those off the southeastern Virginia coast today and these differences most likely signify the enhanced influence of the Gulf Stream during the middle Pliocene." The higher temperatures referred to by Cronin pertain to water temperatures, not to air temperatures. The latter

T A B L E 6. P a l y n o m o r p h a s s e m b l a g e s o f s o m e s a m p l e s o f co re 16-905, e x p r e s s e d in p e r c e n t a g e s o f the po l l en s u m

S a m p l e n u m b e r

58050 58051 58052 58053 58054 58055

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A r temis ia 2 - - C h e n o p o d i a c e a e 1 - - C o m p o s i t a e 2 1 G r a m i n e a e P - - C y p e r a c e a e - - - - E r i c a c e a e - - - - N y m p h a e a c e a e 1 - - L y c o p o d i u m - - 2 O s m u n d a - - - - P o l y p o d i a c e a e 1 2 S p h a g n u m 32 20

P o o r p r e s e r v a t i o n . M o s t l y P i n u s ; also p r e s e n t ; Picea, Q u e r c u s , Carya , A l n u s , A r t e m i s i a , a n d E r i c a c e a e

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appear to lag behind the former, as in core 16-905 in the Quaternary.

On the basis of the limited data available, it is tentatively concluded that:

(1) Warmer-than-present terrestrial climates in the middle Atlantic Coastal Plain prevailed during late Miocene to earliest Pliocene (6-5 Ma) and during the latest Pliocene and Early Pleistocene (2 to about 1.5(?) Ma);

(2) A colder-than-present interval occurred in the Late Pliocene (2.3-2 Ma);

(3) The remainder of the Pliocene appears to have had a warm-temperate climate, not very different from that of the present, with a warming trend at the time of deposition of the Colerain Beach Member of the Chowan River Formation (about 2.4 Ma);

(4) During the relatively warm intervals, the latitudinal temperature gradient was probably small.

ACKNOWLEDGMENTS

Thanks are due to T.M. Cronin and R.S. Thompson of the U.S. Geological Survey, A. Traverse of the Pennsylvania State University, and R.N. Benson and K.W. Ramsey of the Delaware Geological Survey for critically reviewing the manuscript. The encouragement received from R.R. Jordan, State Geologist and Director is gratefully acknowledged. This research was supported, in part, by the Minerals Management Service, U.S. Department of the Interior, under MMS Agreement No. 14-12-0001-30432-DE.

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palynological evidence for late miocene, pliocene and early pleistocene climate changes in the...

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