Cape

Charles

Virginia

Beach Norfolk Norfolk Virginia

Beach

Chesapeake

Bay

Atlantic

Ocean James River

York River

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1622

The Effects of the Chesapeake Bay Impact Crater on the

Geologic Framework and the Correlation of Hydrogeologic 

Units of Southeastern Virginia, South of the James River
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U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 

THE EFFECTS OF THE CHESAPEAKE 

BAY IMPACT CRATER ON THE 

GEOLOGIC FRAMEWORK AND THE 

CORRELATION OF HYDROGEOLOGIC 

UNITS OF SOUTHEASTERN VIRGINIA, 

SOUTH OF THE JAMES RIVER 

1622

Prepared in cooperation with the

HAMPTON ROADS PLANNING DISTRICT COMMISSION

2000

By David S. Powars
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

The use of firm, trade, and brand names in this report is for 

endorsement by the U.S. Government.

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Library of Congress Cataloging in Publications Data:

identification purposes only and does not constitute

 Charles G. Groat, Director

0-607-93693-2
Contents III

CONTENTS

Abstract ................................................................................................................................................................................. 1

Introduction ........................................................................................................................................................................... 2

Purpose and Scope ...................................................................................................................................................... 4

Description of Study Area........................................................................................................................................... 4

Previous Investigations................................................................................................................................................ 5

Acknowledgments ....................................................................................................................................................... 5

Methods of Investigation....................................................................................................................................................... 6

Compilation of Lithologic Data from Cores and Well Cuttings ................................................................................. 6

Analysis of Borehole Geophysical Logs..................................................................................................................... 8

Structural Setting of Southeastern Virginia........................................................................................................................... 8

Pre-impact Structural History...................................................................................................................................... 8

Syn-impact Structural History..................................................................................................................................... 22

Post-impact Structural History .................................................................................................................................... 25

Geologic Framework of Southeastern Virginia, South of the James River........................................................................... 26

Basement Rocks .......................................................................................................................................................... 26

Cretaceous Deposits .................................................................................................................................................... 27

Potomac Formation - Lower and Upper Cretaceous Deposits .......................................................................... 28

Upper Cretaceous Deposits Younger Than Pollen Zone III.............................................................................. 29

Upper Cenomanian Beds (Pollen Zone IV) ............................................................................................ 29

Glauconitic Sand Unit ............................................................................................................................. 31

Upper Cretaceous Red Beds (Pollen Zone V) ........................................................................................ 31

Upper Cretaceous and/or Paleocene Beds............................................................................................... 32

Tertiary Deposits ......................................................................................................................................................... 32

Pre-impact Tertiary Deposits (Paleocene and lower and middle Eocene) ........................................................ 32

Syn-impact Deposits (upper Eocene)................................................................................................................ 34

Post-impact Deposits......................................................................................................................................... 35

Upper Eocene, Oligocene, and lower Miocene Post-impact Deposits.................................................... 36

Middle and upper Miocene, Pliocene, and Quaternary Post-impact Deposits........................................ 36

Correlation of Geologic Units to Hydrogeologic Units ....................................................................................................... 38

Summary and Conclusions.................................................................................................................................................... 43

References Cited ................................................................................................................................................................... 45

Appendixes............................................................................................................................................................................ 49

1. List of boreholes used in this report........................................................................................................................ 50

2. Altitudes of the tops of stratigraphic units .............................................................................................................. 52

PLATE [in pocket at back of report]

1. Stratigraphic columns of three key boreholes

FIGURES

1. Map showing location of study areas (Phase I and Phase II), the Chesapeake Bay impact crater (CBIC), and significant 

regional physiographic and geologic features........................................................................................................... 3

2. Generalized geologic section of Chesapeake Bay impact crater. ............................................................................. 4

3. Map showing location of boreholes and stratigraphic cross sections. ...................................................................... 7

4. Map showing distribution of Cretaceous pollen zones across the Virginia Coastal Plain (A), and generalized 

stratigraphic cross section of Virginia Coastal Plain (B).......................................................................................... 12

5-8. Maps showing:

5. Structure contours of the top of the Upper Cretaceous deposits............................................................................... 14
IV Contents

6. Structure contours of the top of the pre-impact Lower Tertiary deposits. ................................................................ 15

7. Isopachs of the Upper Cretaceous deposits younger than pollen zone III showing abrupt truncation along 

the northern part of the study area, irregular thickening and thinning in the northwestern part, and a southeastthickening wedge. ..................................................................................................................................................... 16

8. Isopachs of the pre-impact Lower Tertiary deposits showing a pattern of thickening and thinning. ....................... 17

9-11. Stratigraphic cross sections:

9. B-B', Dismal Swamp to Newport News, showing the entire thickness of the Virginia Coastal Plain deposits and the top 

of the basement rocks (A), and details of the upper section of Coastal Plain deposits (B). ..................................... 18

10. C-C', Chesapeake Northwest River WTP to Fort Monroe, showing the entire thickness of the Coastal Plain deposits 

and the top of the basement rocks (A), and details of the upper section of Coastal Plain deposits (B). .................. 20

11. Fault interpretation of section B-B', Dismal Swamp to Newport News, same section as figure 9A (A), and fault 

interpretation of same section as figure 9B (B) ........................................................................................................ 23

12. Map showing isopachs of upper Cenomian beds (pollen zone IV) showing thickening, thinning, and truncation 

apparently related to the pre-impact James River structural zone and the Chesapeake Bay impact crater. ............. 30

13. Correlation of geologic and hydrogeologic units in three key boreholes—the Dismal Swamp corehole (A), the Fentress 

corehole (B), and the Chesapeake Northwest River WTP borehole (C)—showing the variability in unit correlation 

across the study area. ................................................................................................................................................ 41

14. Map showing relation of dissolved-solids concentrations in the middle Potomac aquifer to the location of the 

Chesapeake Bay impact crater. ................................................................................................................................. 44

TABLES

1. Correlation of stratigraphic units, including Cretaceous pollen zones of the Mid-Atlantic states. .................................. 9

2. Correlation of geologic units to hydrogeologic units....................................................................................................... 39

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

For readers who prefer to use metric units, conversion factors for terms used in this report are listed below:

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived

from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above or below sea level (bsl).

Concentrations of chemical constituents in water are reported in milligrams per liter (mg/L).

Age designations: The time of a geological event and the age of an epoch boundary are expressed as Ma (mega-annum), which means

1,000,000 years, or years × 106. 

Multiply By To obtain

inch (in.) 2.54 centimeters

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

cubic inch (in3) 16.387 cubic centimeter (cm3)

pound per square inch (lb/in2) 6.895 kilopascal
Abstract 1

The Effects of the Chesapeake Bay Impact Crater on the Geologic 

Framework and the Correlation of Hydrogeologic Units of 

Southeastern Virginia, South of the James River 

By D.S. Powars 

Abstract

About 35 million years ago, a large comet or meteor

slammed into the shallow shelf on the western margin

of the Atlantic Ocean, creating the Chesapeake Bay

impact crater (CBIC). Virginia Coastal Plain

sediments, the southern part of the Chesapeake Bay,

and a small part of the Atlantic Ocean now cover the

crater. The impact apparently affected pre-impact

structures near the CBIC. Subsequent structural

adjustments of these structures likely were influenced

by the crater and by the regional post-rift stress regime

typical of the passive margin scenario described for the

Atlantic Coastal Plain. Structural adjustments

disrupted pre-impact sediments and basement rocks in

the southern Chesapeake Bay region and influenced

subsequent deposition, erosion, and preservation of

sediments. Correlations of litho- and biostratigraphic

data from borehole cores and cuttings and geophysical

logs were used to identify the location and geometry of

the CBIC and possible pre-impact structures. This

report focuses on the Virginia Coastal Plain south of the

James River and complements a recent study of the

CBIC’s effects on the geologic framework beneath the

lower York-James Peninsula. 

Pollen data indicate that only Lower Cretaceous

deposits are present in the subsurface north of the

James River, whereas on the south side of the river,

both Lower Cretaceous (pollen zones I and II) and

Upper Cretaceous (pollen zones III, IV, and V) deposits

are present in the subsurface. Extreme variations in

thickness of these deposits across the river, combined

with an angular unconformity that separates these

deposits from overlying, pre-impact lower Tertiary

deposits, provide evidence for a pre-impact James

River structural zone. The distribution of sediment

suggests that the region north of the river was relatively

depressed during pollen zone I, and relatively elevated

for the remainder of Cretaceous time. By contrast, the

region south of the James River was relatively

depressed during pollen zones II, III, IV, and V.

South of the James River, Upper Cretaceous

deposits younger than pollen zone III form a

southeastward-thickening wedge across the southeastern half of the study area, but are thin and

irregularly distributed across the northwestern half.

The pre-impact lower Tertiary deposits dip eastward to

northward and are structurally higher and thinner along

the southern side of the study area, near the VirginiaNorth Carolina border, than along the northern side. 

The complex distribution of post-impact units across

the James River provides evidence for post-impact

adjustments of this James River structural zone

initiated by the emplacement of the impact crater, and

subsequent burial. The truncation of many earlier postimpact (upper Eocene to middle Miocene) deposits

indicates that the area south of the James River

structural zone was relatively elevated during that time

or at least prior to deposition of the upper Miocene St.

Marys Formation. The presence of thicker post-middle

Miocene deposits south of the James River, compared

to those north of the river, indicates downwarping of

the area south of the river during this period and a

complex structural history of adjustments to the CBIC.

The structural and stratigraphic features created by

the impact, and the consequent structural adjustments

of the James River structural zone, have influenced the

hydrogeology, ground-water flow system, and water

quality of a large part of the Virginia Coastal Plain.

Regional flow paths apparently were altered by

emplacement of the possibly low permeability,

lithologically heterogeneous Exmore tsunami-breccia

deposits, as well as by subsequent deposition of

primarily very fine grained deposits in the CBIC’s

structural low. The buried CBIC created a large region

where differential flushing of seawater from the
2 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Coastal Plain sediments in and around the crater possibly

resulted in Virginia's "inland salt-water wedge." The

outer rim of the crater appears to act as a boundary and/or

mixing zone separating ground water of high salinity

inside the outer rim from lower salinity water outside the

outer rim. The James River structural zone apparently

abruptly offsets stratigraphic units that have been

preserved differently north and south of the James River. 

INTRODUCTION

The discovery of a large impact crater beneath Chesapeake Bay has prompted a revision of the structural,

stratigraphic, and hydrogeologic framework of a large

part of the Virginia Coastal Plain. The 56-mile-wide

Chesapeake Bay impact crater (CBIC) is located beneath

the lower Chesapeake Bay, its surrounding peninsulas,

and a small part of the Atlantic Ocean east of the lower

part of the Delmarva Peninsula (fig. 1); the approximate

center of the crater is beneath the town of Cape Charles,

Va. The CBIC was formed when a large comet or meteorite crashed into shallow-shelf waters of the western

Atlantic Ocean approximately 35 million years ago (Ma).

The impact produced an inverted, sombrero-shaped,

complex crater that was immediately filled with chaotically mixed sediments and rim-collapse material and

eventually buried by younger sedimentary deposits. A

disruption boundary separates the impact rubble from

primarily undisturbed sediments and rocks (fig. 2).

Walled terraces, central peaks, and flat floors characterize complex craters (Melosh, 1989), and the CBIC has all

these features.

The CBIC impactor slammed into part of an apparent

structural zone, herein referred to as the James River

structural zone, that traversed the Coastal Plain east to

west beneath the eastern part of the present-day James

River Basin. Deformational processes such as faulting,

folding, and igneous intrusion created the unique structural features of this area. The emplacement of the crater

appears to have caused adjustments to this James River

structural zone, dramatically disrupted the pre-impact

sediments and rocks in the southern Chesapeake Bay

region, and influenced subsequent sediment deposition

and distribution patterns. Powars and Bruce (1999)

described how the impact resulted in several regional

anomalies: (1) a large crater, partly filled by impact and

collapse debris; (2) an impact tsunami-breccia consisting

of a mixture of sea water and sediments of Lower Cretaceous, Upper Cretaceous, Paleocene, and lower and

upper Eocene age; (3) a large area of anomalous water

quality (Virginia’s "inland salt-water wedge"); (4) transformation of the depositional environment from inner

neritic (shallow-shelf) to bathyal (deep-water) depths

within the crater, in which fine-grained, low permeability

sediments accumulated; and (5) a regional depression

that persisted because of post-impact loading and differential compaction. The existence of the CBIC, combined

with structural adjustments of a pre-impact structural

zone, helps explain the distribution of saline water in the

Virginia Coastal Plain aquifers. This information needs

to be considered in revisions of the hydrogeologic framework and ground-water-flow models of the aquifer system. 

In July 1997, the U.S. Geological Survey (USGS), in

cooperation with the Hampton Roads Planning District

Commission (HRPDC), initiated a multi-phase study to

refine the geologic and hydrogeologic frameworks of the

Coastal Plain sediments in and near the CBIC. This

information will be the basis for revisions of the groundwater flow models that are used to guide water-supply

management decisions. The York-James Peninsula was

the focus of Phase I (Powars and Bruce, 1999; fig. 1).

This report presents the results of Phase II of the study,

which focused on an area in southeastern Virginia, south

of the James River. Additionally, this report integrates

Phase II results with the Phase I evaluation of the YorkJames Peninsula.

The discovery of the buried CBIC, and its apparent

effect on a pre-impact structural zone, revealed the inadequacy of the multi-aquifer conceptual model currently

being used to represent the ground-water systems of the

Virginia Coastal Plain. The existing hydrogeologic

framework and ground-water models were built upon a

geologic framework that described the Virginia Coastal

Plain as an eastward dipping and thickening wedge of

unconsolidated sediments, readily subdivided into aquifers and confining units. Knowledge of the formation of

the crater and of the structural adjustments of a preimpact structural zone has disrupted this model, however,

leading to the need for a re-appraisal of the hydraulic

properties, ground-water flow, and geochemistry of the

aquifer system. First, the physical features created and

affected by the impact crater must be defined and

described–in other words, the geologic framework must

be established–in order to understand and refine the

hydrogeologic framework in this region. 
Introduction 3

VIRGINIA

VIRGINIA

MARYLAND

WEST

VIRGINIA

VIRGINIA

COASTAL PLAIN

NORTH CAROLINA

MARYLAND

DELAWARE

DELAWARE

NEW JERSEY

VA

MD

Washington, D.C.

FALL

LINE

FALL LINE

CHESAPEAKE

Potomac

River

77o 76o

 75o 74o 73o 72o

39o

38o

37o

BALTIMORE CANYON TROUGH

HATTERAS BASIN

TOMS

CANYON

SHELF EDGE

BASE OF CONTINENTAL SLOPE

D E L M A R V A P E N I N S U L A

SALISBURY

EMBAYMENT

0 25 50 MILES

0 25 50 KILOMETERS

ATLANTIC OCEAN

Richmond

James River

LOCATION OF

PHASE STUDY AREA II

VIRGINIA

NORTH CAROLINA

?

?

Rappahannock

River

York

River

STUDY AREA

(Phase ) II

STUDY AREA

(Phase )I

CBIC

EXPLANATION

IMPACT CRATER

ESTIMATED DISTRIBUTION OF PRESERVED 

EJECTA AND TSUNAMI-BRECCIA DEPOSITS ó

Darker shade used to emphasize crater

BURIED TRIASSIC (?)

RIFT BASIN

APPROXIMATE NORTH END OF CAPE FEAR-NORFOLK 

STRUCTURAL BLOCK (EQUIVALENT TO THE JAMES RIVER

STRUCTURAL ZONE)

APPROXIMATE BOUNDARY BETWEEN BALTIMORE 

CANYON TROUGH AND HATTERAS BASIN

JURASSICYORK-JAMES PENINSULA

BAY

SUFFOLK SCARP

Cape 

Charles

Base from U.S. Geological Survey

Digital Line Graph, 1:2,000,000, 1990

Susquehanna

River

MAP AREA

Figure 1. Location of study areas (Phase I and Phase II), the Chesapeake Bay impact crater 

(CBIC), and significant regional physiographic and geologic features.
4 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

CRETACEOUS SEDIMENTS

CRETACEOUS

SEDIMENTS

DISRUPTION BOUNDARY DISRUPTION BOUNDARY

KIPTOPEKE

COREHOLE

(2,000 FT)

NEWPORT NEWS

COREHOLE

(748 FT)

WEST EAST

UPPER MIOCENE TO QUATERNARY

MIDDLE MIOCENE

PEAK RING

CENTRAL PEAK

PEAK RING

PROTEROZOIC AND PALEOZOIC

CRYSTALLINE BASEMENT ROCKS

EXMORE TSUNAMI-BRECCIA

NORFOLK

RIFT

BASIN

MEGABLOCKS

MEGABLOCKS

LOWER MIOCENE

AND OLIGOCENE

CHICKAHOMINY FM. (UPPER EOCENE)

CHESAPEAKE BAY IMPACT CRATER

ANNULAR TROUGH ANNULAR TROUGH

0 SEA

LEVEL

3,200

23,000-

25,000

10,000-

12,000

1

2

3

TWO-WAY TRAVELTIME,

IN SECONDS

DEPTH EQUIVALENCE,

IN FEET (ESTIMATED)

PALEOCENE AND

EOCENE

PALEOCENE, EOCENE

30 MILES

0 30 KILOMETERS

0

EXPLANATION

POSSIBLE TRANSIENT CRATER SHAPE

FRACTURED AND FAULTED BASEMENT ROCKS

Figure 2. Generalized geologic section of Chesapeake Bay impact crater. (From Powars and Bruce, 1999.)

Purpose and Scope

This report refines the geologic framework of southeastern Virginia, south of the James River, in and near the

CBIC, and presents evidence for the existence of a preimpact James River structural zone that was reactivated

by the impact. Lithologies of cores from exploratory

holes are correlated with borehole geophysical logs to

characterize the physical properties of the stratigraphic

units and their geophysical signatures. The correlation of

cores, cuttings from wells, and borehole geophysical logs

provides the building blocks for compilation of stratigraphic cross sections. To further develop our understanding of this structurally complex area, these cross

sections are tied into the geologic framework of the lower

York-James Peninsula as refined by Powars and Bruce

(1999). The correlation of geologic units to hydrogeologic units is an important step towards the refinement of

the hydrogeologic framework.

Description of Study Area

The study area encompasses the southeastern part of

the Virginia Coastal Plain south of the Chesapeake Bay

and the James River and extends to just west of the Suffolk scarp (fig. 1). Most of this area from the Atlantic

Ocean to the Suffolk scarp has low relief, generally less

than 30 ft. The scarp marks where the relief jumps up to

100 ft and is one of a succession of step-like terraces and

intervening scarps that trend either parallel to the coast or

to the major streams and were created by Pleistocene sealevel oscillations. These terraces become progressively

lower in altitude and younger in age toward the coast and

rivers; the younger the terrace surface, the less dissected

it is. The buried outer rim of the crater is geomorphically

expressed by its alignment and concentric parallelism

with various Pleistocene wave-cut scarps. 

The study area covers part of the north flank of the

Cape Fear-Norfolk block (a structural basement high).

Figure 1 shows that part of the area is underlain by buried

rift basins that have been regionally dated as Jurassic

and/or Triassic. Other major regional structural features

of the basement include the Salisbury Embayment, the

Baltimore Canyon Trough (a major structural low), the

Hatteras Basin, and buried Jurassic-Triassic rift basins

outside the study area. 
Introduction 5

Previous Investigations 

 In the late 1930’s and early 1940’s, the need to

develop ground-water resources in the southeastern Virginia region was heightened by water demands for the

military bases and by shortages caused by a severe

drought. These pressures led to Cederstrom's (1945a, c)

hydrogeologic studies of the southeastern Virginia

region. Cederstrom’s studies provide lithologic logs

from water-well cuttings, including those he logged himself. His reports contain biostratigraphic data analyzed

by J.A. Cushman (USGS), and water-quality data, including the initial delineation of Virginia’s inland salt-water

wedge and its associated Eocene basin north of the James

River. Cederstrom first postulated the existence of a

James River fault zone to explain his interpretation of the

erratic distribution and abrupt thickening of various strata

north and south of the James River. Until the late 1980’s,

the subsurface geology of this region was interpreted primarily on the basis of water-well cuttings and geophysical logs (Brown and others, 1972; Meng and Harsh, 1988;

Hamilton and Larson, 1988). Coch (Coch, 1968, 1971),

Oaks (Oaks and Coch, 1973), and Johnson (Johnson,

1969, 1976; Johnson and Peebles, 1985; Johnson and

others, 1985, 1987) provided detailed surficial mapping

of southeastern Virginia south of the James River. A

regional compilation of the surficial units at 1:250,000

scale was prepared by Mixon and others (1989). 

The combined efforts of the USGS, the HRPDC, and

the Virginia Department of Environmental Quality

(VDEQ) over the last 14 years have greatly changed our

understanding of the geologic framework of southeastern

Virginia (Powars and others, 1987, 1990, 1991, 1992;

Poag and others, 1992; Powars and Bruce, 1999). These

investigations document the existence of the Chesapeake

Bay impact crater (Powars and others, 1993; Poag,

Powars, and Mixon, 1994; Poag, Powars, Poppe, and

Mixon, 1994); the crater's association with the inland

salt-water wedge (Powars and others, 1994, 1998; Bruce

and Powars, 1995); and the crater's structural and stratigraphic effects on post-impact sediment distribution and

on the development of the present-day landscape (Powars

and others, 1993, 1998; Poag, Powars, and Mixon, 1994;

Poag, Powars, Poppe, and Mixon, 1994; Poag, 1996,

1997c; Johnson and Powars, 1996; Riddle and others,

1996; Johnson, Kruse, and others, 1998; Johnson,

Powars, and others, 1998). Most recently, Powars and

Bruce (1999) refined the geologic framework of the

Lower York-James Peninsula and described the effects

that the CBIC structure has had on the southeastern Virginia Coastal Plain.

Acknowledgments

The stratigraphic and lithologic interpretations presented in this report benefited from several years of

research by the author and his colleagues. Special thanks

are extended to USGS colleagues for the biostratigraphic

data on dinoflagellates (Lucy E. Edwards), Foraminifera

(C. Wylie Poag; Thomas G. Gibson, now with the Smithsonian Institution; and Scott E. Ishman), Tertiary mollusks (L.W. Ward, now with the Natural History Museum

of Virginia), Cretaceous mollusks (Norman F. Sohl,

deceased), ostracodes (Thomas M. Cronin), calcareous

nannofossils (Laurel M. Bybell), and pollen (Ronald J.

Litwin and Norman O. Frederiksen). 

Thanks to the Hampton Roads Planning District Commission (HRPDC) and its Directors of Utilities Committee, which made this study possible. The HRPDC

represents the Cities of Chesapeake, Franklin, Hampton,

Newport News, Norfolk, Poquoson, Portsmouth, Suffolk,

Virginia Beach, and Williamsburg, Va., and the Counties

of Gloucester, Isle of Wight, James City, Southampton,

Surry, and York. 

Much appreciation also to Gregory S. Gohn (USGS),

T. Scott Bruce (VDEQ), Gerald H. Johnson (College of

William and Mary), and Scott R. Emry (HRPDC) for

their helpful scientific reviews, and to Martha Erwin for

editorial review of this report. Special thanks to Theodore B. Samsel III and Brent Banks for their help in the

digitization of many figures, and to Wendy Danchuck for

cartographic assistance. The assistance of George E.

Harlow, Jr., in the location and proper labeling of many

wells is greatly appreciated.

Finally, special appreciation is extended to USGS

drillers Donald Queen, Dennis Duty, Gene Cobbs, and

Gene Cobbs III, and VDEQ drillers John Creason and Jay

Owens for their determination in obtaining continuous

core samples that provide the ground truth needed for this

kind of investigation, discovery, and report. Appreciation

is given to Sydnor Hydrodynamics, Inc., for providing

geophysical logs for this study. 
6 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

METHODS OF INVESTIGATION

The geologic framework of southeastern Virginia,

south of the James River, was refined by analyzing stratigraphic and lithologic data from cores, well cuttings, and

borehole geophysical logs. Selected core intervals were

sampled for mineralogic and biostratigraphic analysis.

The more recent data were combined with re-evaluations

of previously published data to provide new interpretations that account for the effects of the CBIC, including

its apparent effects on the pre-impact James River structural zone.

Compilation of Lithologic Data from Cores 

and Well Cuttings

The borehole-numbering system used in this study

refers to a location number on figure 3—printed in bold

in the text, for example, (12)—and in appendixes 1 and 2.

To maintain continuity with Powars and Bruce (1999),

appendix 1 includes the local reference number, such as

the USGS Ground-Water Site Inventory (GWSI) number

or the well number assigned in other reports (Cederstrom,

1945a, b; Brown and others, 1972). The GWSI is based

on a system in which Virginia's 7-1/2-min quadrangles

are numbered 1 through 69 from west to east, and lettered

A through Z (omitting I and O) from south to north; wells

are identified and numbered serially within each 7-1/2-

min quadrangle. As an example, well 58A76 is in quadrangle 58A and is the 76th well in that quadrangle for

which the location and other data were recorded by the

USGS. Appendix 2 lists the altitudes of the tops of stratigraphic units used in this report. 

Litho- and biostratigraphic data derived from continuous coreholes with high recovery rates (fig. 3) provided

the stratigraphic control for this investigation. Nine cores

were obtained between 1986 and 1995 by the USGS and

the VDEQ as part of their cooperative research efforts

(Powars and others, 1992; Powars and Bruce, 1999).

These cores are stored at the USGS core-storage areas in

Reston and Herndon, Va., or at the VDEQ in Richmond,

Va. Corehole names are derived from nearby geographic

features and include (listed in the order drilled) Exmore,

Dismal Swamp (2), Jenkins Bridge, Fentress (12), Kiptopeke (64), Newport News Park 2 (63), Windmill Point,

Airfield Pond, and Jamestown (61). (Only those coreholes assigned a borehole number are discussed in this

report.) The Dismal Swamp (2) and Fentress (12) coreholes are located in the southern part of the study area,

and the Newport News Park 2 (63) and Kiptopeke (64)

coreholes, which lie just outside the northern boundary of

the study area, were used for control across the northern

part. An additional corehole was drilled for the City of

Chesapeake as part of its Western Branch Aquifer Storage and Recovery Project and is labeled 21 on figure 3.

This corehole is referred to as MW4-1 (the original field

designation) in Powars and Bruce’s 1999 report, but is

herein given the name Chesapeake-Portsmouth Airport

corehole. This core also is stored at the USGS core-storage area in Herndon, Va. In this study, the Dismal Swamp

(2), Fentress (12), and Chesapeake-Portsmouth Airport

(21) coreholes provided the key lithostratigraphic and

biostratigraphic data for borehole log correlation. 

 Lithologic and biostratigraphic data from selected

cored intervals in test holes drilled in the 1970’s by the

VDEQ and by the U.S. Department of Energy and the

Virginia Polytechnic Institute and State University (VPI)

provided additional stratigraphic control. Subsurface

data consisting primarily of lithologic data from cuttings,

borehole geophysical logs, and selected cored intervals

became available from a Brackish Groundwater Development Project recently conducted by the City of Chesapeake at the Northwest River Water Treatment Plant. The

deepest borehole drilled at this site is herein referred to as

the Chesapeake Northwest River Water Treatment Plant

(WTP) borehole (3). 

Descriptions of borehole cuttings were interpreted by

correlation to the coreholes, resulting in many reinterpretations of stratigraphic units described by Cederstrom

(1945a, b) and by Brown and others (1972). The biostratigraphic data in these earlier reports were emphasized, while noting the potential for down-hole

contamination. The detailed Virginia Division of Mineral Resources (VDMR) lithologic descriptions of

washed samples are from mud-rotary drilled wells and

clearly reflect down-hole contamination. Therefore, the

descriptions were carefully scrutinized for the first occurrences of stratigraphically significant lithologic components, for example, shells and glauconite for marine

deposits; and feldspar, gravel, lignitized wood, and oxidized, multicolored clays for deltaic and fluvial deposits.

Where available, decreasing or increasing percentages of

the various lithic components were also used to help

define stratigraphic horizons. 

When conflicting data were encountered, either within

a single borehole (for example, when lithologic descriptions did not agree with the geophysical log) or between

wells, priority was given to cuttings descriptions that

were made by an on-site geologist (primarily D.J. Ceder-
Methods of Investigation 7

Base from National Oceanic and Atmospheric Administration/National Ocean Service

Medium Resolution Coastline, 1:70,000, 1994 

32 29

30 31

51

Virginia

Beach

Chesapeake

Hampton

Newport Poquoson

News

James City County

Suffolk

Isle of Wight

County

Southampton

County

Surry

County

Bacons

Castle

Smithfield

Hog Island York

County

Northampton

County

Norfolk

1

1

12

2

9

10

11

37

38

57

56

76o30' 76o

37o

36o30'

0 2 4 6 8 10 MILES

02468 10 KILOMETERS

55

54

ATLANTIC

OCEAN

James River

A

A'

B

NORTH CAROLINA

VIRGINIA

19

20

39

40

48

42 41

49

62, 63

61

64

53

50

36

52

60

58, 59

46

44 45 43

CHESAPEAKE

BAY

B' C'

3

4

5 6

7 8

12

13

14

15

17

18

22

23

24

25

26

27

28

33

34 35

47

C

16

OUTER RIM OF CRATER

INNER RIM OF CRATER

EXPLANATION

A A'

Lake

Drummond

Dismal Swamp Canal

Burwell

Bay

63 MILES

21

CROSS SECTION

CHESAPEAKE BAY IMPACT CRATER RIM

STUDY AREA

BOREHOLE AND NUMBER

CONTINUOUS COREHOLE AND NUMBER

Figure 3. Location of boreholes, continuous coreholes, and stratigraphic cross sections. See Appendixes 1 and 2 for 

data on boreholes.
8 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

strom, and T.S. Bruce, VDEQ). In some cases, emphasis

was placed on the interpretation of geophysical logs from

boreholes that were similar to and located near one of the

three key coreholes. Emphasis was also placed on any

biostratigraphic data that were included. Data from wells

that were drilled by the cable tool method also were given

priority over rotary-drilled wells because rotary methods

tend to produce greater mixing than cable methods. 

Analysis of Borehole Geophysical Logs

 Borehole geophysical logs were interpreted by establishing geophysical signatures for the various units

defined in several continuous coreholes. These geophysical signatures were then correlated to those of other logs

gathered for this investigation. Interpretation for each

borehole was an iterative process because the detail of the

lithic descriptions ranged from generalized drillers’ logs

to microscopic descriptions of samples. Correlations

also were made to other nearby borehole lithologic logs

published by Cederstrom (1945a, b), and by Brown and

others (1972), and to unpublished VDMR, VDEQ, and

USGS data. Powars and Bruce (1999) found that conflicting data were encountered most often around the

outer rim of the CBIC, especially for the uncored boreholes located far from one of the continuous coreholes. 

The number and type of geophysical logs varied

greatly from borehole to borehole. Single-point resistance and natural gamma logs were the most numerous

and therefore were used for establishing the geophysical

signatures. Correlation also was made with multipoint

resistivity, 6-foot lateral resistivity, and spontaneous

potential logs. The cross sections in this report are based

on interpretation of the lithic descriptions and geophysical logs.

STRUCTURAL SETTING OF 

SOUTHEASTERN VIRGINIA

In contrast to the regional post-rift thermal- and loaddriven subsidence typical of the “passive margin” scenario described for the Atlantic Coastal Plain (Grow and

Sheridan, 1988), the basement rocks and sediments of the

Coastal Plain of Virginia record a complex geological

history, including the effects of the catastrophic upper

Eocene comet or meteorite impact. This impact produced

the CBIC, the largest impact crater found so far in the

United States. The southwestern portion of the outer rim

of the crater extends from the Newport News and Hampton area to the northern Norfolk and Virginia Beach area

(fig. 3).

This section subdivides the structural history into preimpact, syn-impact, and post-impact sections and presents evidence for the existence of a pre-impact James

River structural zone and the CBIC’s effects on this zone.

Table 1 shows the geologic units described in this report

and their correlation with geologic units of key previous

investigations, as well as Cretaceous pollen data. This

table provides the stratigraphic nomenclature and position of the geologic units discussed in this section. A

more detailed description of the geologic units is given in

the Geologic Framework section. 

Pre-impact Structural History

The basement rocks beneath the Coastal Plain of Virginia have been interpreted to be an assembly of various

tectonostratigraphic terranes that were accreted to the

North American continent during Paleozoic continental

collisions (Horton and others, 1991). Most of the central

to outer Coastal Plain of Virginia is underlain by the

Chesapeake Block, which has been interpreted as African

Archean to Lower Proterozoic rocks that were left

accreted on to the North American continent during the

Mesozoic opening of the present Atlantic Ocean (Lefort

and Max, 1991). Recently, Sheridan and others (1999)

presented a rubidium/strontium date from rocks beneath

the southeastern New Jersey Coastal Plain that suggests a

Middle Proterozoic age for the Chesapeake Block. Late

Paleozoic granitic plutons also are present. It appears

that some major structural zones are present in the crystalline basement rocks, some of which likely were zones

of weakness (faults) that became involved with Mesozoic

rift basin formation. 

The last breakup of North America, Eurasia, and

Africa began with Triassic rifting and was associated

with an extensional stress regime that pulls apart and

thins the crust. This rifting continued into the Early

Jurassic and produced crustal instability and asymmetrical, down-dropped, sediment-filled basins (grabens) with

wrench and transform faults, tilting, folding, igneous

intrusion, and widespread volcanism (see fig.1 for location of rift basins beneath the Coastal Plain). Part of the

study area is underlain by two of these rift basins, both

having northern boundaries south of the CBIC and

extending southward 10 to 20 miles into North Carolina
Structural Setting of Southeastern Virginia 9

SYSTEM SERIES Geologic units this report Cederstrom (1957) Brown and others (1972) Mixon, Berquist, and others (1989)

QUATERNARYHolocene Pleistocene Alluvium, swamp, beach Tabb Formation Shirley Formation Chuckatuck Formation

U

M

L

Windsor Formation

Bacons Castle Formation

Pliocene

UL

Miocene

MU

L

CHESAPEAKE GROUP

Yorktown Formation Eastover Formation

St. Marys Formation

Calvert Formation4 Calvert Beach Member Plum Point Member Fairhaven Member

Recent beach

Columbia

Group

(Quaternary)

YorktownFormation(Miocene)

St. Marys

Formation(Miocene)

Calvert

Formation(Miocene)

Moore House Member

Mogarts Beach Member

Rushmere Member

Sunken Meadow

Member

Cobham Bay

Member

Claremont Manor

Member

Unnamed beds3

Rocks of

post Miocene

age

Rocks of

late Miocene

age

Rocks of

middle Miocene

age

Coastal barriers, lagoons

alluvial, swamp, eolian

Tabb Formation

Shirley Formation

Chuckatuck Formation

Charles City Formation Charles City Formation

Windsor Formation

Bacons Castle Formation

Chowan River Formation

YorktownFormation

Eastover

Formation

Pliocene sand + gravel Miocene sand + gravel

Calvert

Formation

Choptank Formation

St. Marys Formation

CHESAPEAKE GROUP Choptank Formation

(not present in study area)

(East of the Chesapeake 

Kent Island Formatio

Wachapreague Form

Nassawadox Formati

Omar Formation

U

M

Moorings unit

sand

Choptank Forma

not present ea

and south o

Chesapeake 

Oligocene Old Church Formation

TERTIARY

? ?

? ? U Old Church Formation

?

2

Newport News unit

?

1Follows Brenner (1963) and Owens and Gohn (1985). 

2Chowan River Formation.

3Powars, D.S. and Cronin, T., U.S. Geological Survey, unpub. data, 1995.

4Not present south of James River.

?

? ?

? ?

Table 1. Correlation of stratigraphic units, including Cretaceous pollen zones of the Mid-Atlantic states. Modified from Powars and Bruce[Upper, upper; M, middle; L, lower; Fm, formation]
10 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River Rocks of

L Middle Miocene age (?)

Old Church Formation

Delmarva beds Oligocene

Eocene

Chickahominy

Formation

Exmore tsunami-breccia

Piney Point

Formation4

NanjemoyFormation

Marlboro Clay

Aquia

Formation

BrightseatFormation

Paleocene

Chickahominy

Formation

NanjemoyFormation

(Eocene)

(upper Eocene)

Aquia

Formation

Pamunkey Group

MattaponiFormation

? ? U

U

U

L

L

L

M

?

Rocks of

Jackson age

Rocks of

Claiborne

age

Rocks of

Sabine

age

Old Church Formation

Delmarva beds5

Chickahominy

Formation

PineyPoint

Formation

NanjemoyFormation

Marlboro Clay

Aquia

FormationBrightseatFormation

Rocks of Midway

age

Exmore beds5

Upper

Cenomanian

beds6

Unnamed6

Potomac

Formation

Rocks of unit ARocks of unit BRocks of unit C

Rocks of unit D

Rocks of unit ERocks of unit F

Rocks of unit GRocks of unit H

Potomac

Formation

Exmore megablock beds

Upper Cenomanian b

Glauconitic sand uni

Red beds5 

Lower Mesozoic rift-basin

deposits

Basement rocks

Woodstock Mem

Potapaco Memb

Pasapotansa MemPiscataway Mem

(Eocene)

Potomac

Group

Potomac

Group

(Upper

Cretaceous)

(Lower

Cretaceous)

(Paleocene +

Upper

Cretaceous)

?

?

Upper

Lower

Miocene Newport News unit of Calvert Fm Calvert Formation

SYSTEM SERIES

TERTIARY CRETACEOUS

Geologic units

this report

Cederstrom

(1957)

Brown and

others (1972)

Mixon, Berquist,and others (1989)

Calvert Formation

(Miocene)

JURASSIC-TRIASSIC

PALEOZOIC AND PROTEROZOIC

Red beds6 Glauconitic sand unit 6

6Not present north of James River.

1Follows Brenner (1963) and Owens and Gohn (1985). 

2Chowan River Formation.

3Powars, D.S. and Cronin, T., U.S. Geological Survey, unpub. data, 1995.

4Not present south of James River.

5From Powars and others (1992).

7Glauconitic sand unit not studied.

? ?

?

? ?

? ?? ?

Table 1. Correlation of stratigraphic units, including Cretaceous pollen zones of the Mid-Atlantic states. Modified from Powars and Bruc[Upper, upper; M, middle; L, lower; Fm, formation]
Structural Setting of Southeastern Virginia 11

(fig. 1). The major border faults of these rift basins generally are parallel to old Appalachian lineaments. 

Deposition of the Coastal Plain sediments began in the

Late Jurassic with the opening of the Atlantic Ocean and

the beginning of sea floor spreading. The initial stages of

accumulation of post-rift sediments in southeastern Virginia occurred in the Early Cretaceous with deposition of

fluvial and deltaic deposits by streams and rivers. As the

Atlantic Ocean widened, the environment of deposition

was influenced by regional-scale tectonism that involved

gentle subsidence of the entire Atlantic continental margin and major alternating marine transgressive and

regressive phases (the passive margin scenario), as well

as local independent structural movement that created a

transverse arch-basin structural configuration. The axes

of these arches and basins trend in an easterly or southeasterly direction transverse to the northeast- southwest

strike of the Atlantic continental margin. This arch-basin

configuration has been explained, using tectonic models,

as block-like structures bounded by zones of weakness in

the crystalline basement rocks or resulting from the possible movement of the landward parts of oceanic transform faults. The structural blocks would have moved up

or down relative to each other in response to non-uniform

loading or basement rock tectonics. Brown and others

(1972) suggested that structural blocks were bounded by

basement faults that created recurrently reversing vertical

movement along wrench faults during deposition to

explain the stratigraphic thinning and thickening of Cretaceous- and Tertiary-age formations associated with

these arches and basins.

The Norfolk arch trends eastward across southeastern

Virginia just south of the James River and appears to represent a basement and stratigraphic structural high that is

part of the north end of the Cape Fear-Norfolk block (see

fig. 1). North of the arch is a major tectonic downwarped

basin referred to as the Salisbury Embayment; south of

the arch, in the outer Coastal Plain of North Carolina

within the Cape Fear-Norfolk block, is a minor basin

known as the Albemarle Embayment. The Albemarle

Embayment is located on the northern side of Albemarle

Sound, just south of the study area. The Norfolk arch

appears to coincide with (1) the north end of shallow rift

basin deposits (indurated red beds) that overlie granite

and (2) the north end of the Cape Fear-Norfolk block,

which also aligns with the onshore extension of the Norfolk Fracture zone. These factors point to the existence

of a structural zone in the vicinity of the James River

Basin. Cederstrom (1945a) originally suggested that a

basin controlled by basement faulting occupied the area

immediately north of the present James River from

Hampton Roads northwestward to at least Hog Island, in

Surry County. Powars and Bruce (1999) interpreted

much of this postulated James River fault zone as part of

the outer rim of the crater, but also suggested some preimpact structural involvement. The distribution of Cretaceous pollen zones, which extends outside of the crater

and its preserved ejecta, also supports the existence of a

pre-impact James River structural zone (fig. 4). This

James River structural zone coincides with the north side

of the Norfolk arch. A series of isopach maps by Hamilton and Larson (1988) show several confining units that

dramatically thin or pinch out northward across or in

proximity to the postulated James River structural zone. 

Structure contour maps of the top of the Upper Cretaceous and the top of the pre-impact lower Tertiary deposits show east to southeast deflections of the contours,

possibly reflecting faulted zones (figs. 5 and 6); the main

deflection coincides with a previously postulated fracture

zone (Johnson and others, 1998). Figure 5 also shows the

truncated distribution of the red beds, which may be fault

controlled, and that in the Norfolk to Virginia Beach area

adjacent to the crater, Upper Cretaceous deposits are

truncated and overlain by the Exmore tsunami-breccia.

An isopach map of the Upper Cretaceous, younger than

pollen zone III deposits (upper Cenomanian beds, glauconitic sand unit, red beds, and Upper Cretaceous and/or

Paleocene) combined shows a southeastward-thickening

wedge that thickens from 30 to 500 ft across the study

area (fig. 7). Some of this thickening occurs because

more Upper Cretaceous units are preserved on the southeast side of the Norfolk arch. An isopach map of the preimpact lower Tertiary deposits indicates that these deposits thicken north of the James River and west of their truncation by the CBIC (fig. 8). These deposits thicken and

thin across the rest of the study area with their erratic distribution apparently caused primarily by syn- and postimpact erosional and structural influences of the CBIC on

the pre-impact James River structural zone. Part of the

pattern also appears to reflect radial fault systems that

were created by the CBIC.

Three north-south stratigraphic cross sections show

the complex stratigraphy encountered across this region

(figs. 4, 9, and 10). Figure 9A shows the entire section of

Coastal Plain sediments and the top of the basement

rocks in a cross section that extends from near the Dismal

Swamp corehole (2) to the City of Newport News City

Hall Complex borehole (49). This section is within the

disruption boundary that separates pre-impact units from

impact debris or syn-impact units. Figure 9B shows the
12 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Richmond

FALL

LINE

FALL LINE

38o 

37o 

77o 76

POTOMAC

RIVER

JAMES

RIVER

YORK

RIVER

RIVER

RAPPAHANNO

CK

VIRGINIA

NORTH CAROLINA

DELAWARE

MARYLAND

MARYLAND

VIRGINIA

ATLANTIC

OCEAN

CHESAPEAKE BAY

0 25 MILES

0 25 KILOMETERS

OUTER 

RIM

OF CRATER

X

X

X X

NO DATA

NO DATA

NO DATA

12F

2

DS

65AP

61

66M

62NNP1

57

40

43

15

Drewrys

Bluff

68H

67

69OG

A'

A

EXPLANATION

 DS - Dismal Swamp 2

 OG - Oak Grove 69

F

 F - Fentress 12

 AP - Airfield Pond 65

 H - Haynesville 68

A A' STRATIGRAPHIC CROSS SECTION

Zone 

Zone 

Zone 

Zone 

Zone 

V

IV

III

II

I

X

CONTINUOUS COREHOLE (AND NUMBER) WITH 

POLLEN ANALYSIS

(A)

62NNP1 - Newport News Park 1 66M - Mann Tract Monitor well #1 

67 - Taylor #1, oil test well (includes 

 Cretaceous deposits younger 

 than pollen zone ) V

43 BOREHOLE AND NUMBER

OUTCROP AT DREWRYS BLUFF 

(Owens and Gohn, 1985)

Southampton County

Franklin

Isle of Wight

County

Surry

County

Hanover

County

Cenomanian

Coniacian 

to Santonian

Lower 

Cretaceous

Upper 

Cretaceous

BURIED JURASSIC- (?)

RIFT BASIN

STUDY AREA

TRIASSIC

middle to

late Albian

Barremian

to early 

Albian

Base from U.S. Geological Survey 

Geologic Map, 1:250,000, Mixon, Berquist, and others, 1989

APPROXIMATE EXTENT OF POLLEN ZONE

Figure 4. Distribution of Cretaceous pollen zones—these deposits were removed or disturbed inside the 

crater—across the Virginia Coastal Plain (modified from Owens and Gohn, 1985) (A) , and generalized stratigraphic cross section of Virginia Coastal Plain (B). See Appendix 2 for data on boreholes outside study area.
Structural Setting of Southeastern Virginia 13

POST-IMPACT

 TERTIARY NORTHWEST James River SOUTHEAST

A

(B)

PRE-IMPACT TERTIARY

CRYSTALLINE 

BASEMENT 

ROCKS

"CHESAPEAKE BLOCK"

(HORTON AND OTHERS, 1991)

JURASSICTRIASSIC (?)

RIFT BASIN

II

I

II

15

43

40

57 61 OG

DS

SEA 2

LEVEL

-200

-400

-600

-800

-1,000

-1,200

-1,400

-1,600

-1,800

-2,000

 CRYSTALLINE 

BASEMENT ROCKS

"CHESAPEAKE BLOCK"

(HORTON AND OTHERS, 1991)

JURASSICTRIASSIC (?)

RIFT BASIN

 MANN TRACT

MONITOR 

WELL #1

Granite

? Position of contact uncertain 

EXPLANATION

Inconclusive or no pollen data

JURASSICTRIASSIC (?) 

RIFT BASIN

+ Newport News Park 1 pollen data (I) projected into line of section

III(?)

A'

I

V

0 25 MILES

0 25 KILOMETERS

ALTITUDE, IN FEET

Indurated red beds

NORFOLK ARCH

JAMES RIVER STRUCTURAL ZONE

IV

+

III

VERTICAL SCALE GREATLY EXAGGERATED

?

?

?

Figure 4. Distribution of Cretaceous pollen zones—these deposits were removed or disturbed 

inside the crater—across the Virginia Coastal Plain (modified from Owens and Gohn, 1985) (A) , 

and generalized stratigraphic cross section of Virginia Coastal Plain (B). See Appendix 2 for 

data on boreholes outside study area. —Continued.
14 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Chesapeake

Hampton

Newport Poquoson

News

Suffolk

Isle of Wight

County

Surry

County

York

County

Northampton

County

Norfolk

76o30' 76o

37o

36o30'

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

ATLANTIC

OCEAN

James River

VIRGINIA

CHESAPEAKE

BAY

OUTER RIM OF CRATER

INNER RIM OF CRATER

Lake

Drummond

Dismal Swamp Canal

EXPLANATION

Southampton

County

Virginia

Beach

-815

-608

-564 -687

-308

-378

-346 -407

-334 -397

-336

-415

-313

-381

-371

-423

-433

-449

-338

-294

-313

-294

-324

-294

-252

-251

-263

-300

-232

-295

-283

-247

-280 -276

-272

-250

-292

-495

-504

-572

-750

-515

-487

-524

-859

-350

-300

-400

-450

-500

-550

-600

-650

-700 -750

-800

-850

-250

-250

-900

-300

-350

-705

-701

-294

-397

STUDY AREA

APPROXIMATE SOUTHERN LIMIT OF 

PRESERVED EXMORE TSUNAMI-BRECCIA

CHESAPEAKE BAY IMPACT CRATER RIM

APPROXIMATE LIMIT OF UPPER

CRETACEOUS DEPOSITS

DISTRIBUTION OF RED BEDS

AREA WHERE UPPER CRETACEOUS DEPOSITS 

ARE ABSENT (OUTSIDE LIMIT OF PRESERVED

EXMORE TSUNAMI-BRECCIA)

STRUCTURE CONTOUR—Contour interval 50 feet. 

Datum is sea level

BOREHOLE—Number is altitude of top of unit, in feet

below sea level

CONTINUOUS COREHOLE—Number is altitude of

top of unit, in feet below sea level

-100

NORTH CAROLINA

Hog Island

James City County

-258

Burwell

Bay

Figure 5. Structure contour map of the top of the Upper Cretaceous deposits. East to southeast deflections of the contours and the truncated distribution of the red beds suggest possible faulting.
Structural Setting of Southeastern Virginia 15

Virginia

Beach

Chesapeake

Hampton

Newport Poquoson

News

James City County

Suffolk

Surry

County

York

County

Northampton

County

Norfolk

76o30' 76o

37o

36o30'

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

ATLANTIC

OCEAN

James River

VIRGINIA

64

CHESAPEAKE

BAY

OUTER RIM OF CRATER

INNER RIM OF CRATER

Lake

Drummond

Dismal Swamp Canal

Isle of Wight

County

Southampton

County

-242

-269

-620

-738

-555

-464

-430

-351

-441

-495

-408

-382

-379

-367

-316 -344

-340

-282

-269

-260

-253

-215

-260

-231

-241

-180

-190

-208 -230

-218

-175

-216

-245

-225

-218

-215

-152

-398

-302

-400

-350

-300

-200

-250

-650

-600

-700

-750

-500

-550

-450

NORTH CAROLINA 

Hog Island

-300

EXPLANATION

STRUCTURE CONTOUR—Contour interval 50 feet. 

Datum is sea level

BOREHOLE—Number is altitude of top of unit, in feet below sea level

CONTINUOUS COREHOLE—Number is altitude of top of unit, in feet below sea level

CHESAPEAKE BAY IMPACT CRATER RIM

MARKS TRUNCATION BY CBIC (EQUIVALENT TO APPROXIMATE 

LIMIT OF PRESERVED EXMORE TSUNAMI-BRECCIA)

STUDY AREA

-441

-269

-300

Burwell

Bay

Figure 6. Structure contour map of the top of the pre-impact Lower Tertiary deposits (equals top of Nanjemoy

Formation except for northwest corner where equals top of the Piney Point Formation).
16 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Virginia

Beach

Chesapeake

Hampton

Newport Poquoson

News

James City County

Suffolk

Surry

County

York

County

Northampton

County

Norfolk

76o30' 76o

37o

36o30'

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

ATLANTIC

OCEAN

James River

VIRGINIA

CHESAPEAKE

BAY

OUTER RIM OF CRATER

INNER RIM OF CRATER

Lake

Drummond

Dismal Swamp Canal

250

EXPLANATION

LINE OF EQUAL THICKNESS OF UNIT—Interval 50 feet. 

Queried where inferred

BOREHOLE—Number is thickness of unit, in feet (>, greater than)

CONTINUOUS COREHOLE—Number is thickness of unit, in feet

CHESAPEAKE BAY IMPACT CRATER RIM

APPROXIMATE LIMIT OF UPPER CRETACEOUS DEPOSITS

STUDY AREA

APPROXIMATE SOUTHERN LIMIT OF PRESERVED

EXMORE TSUNAMI-BRECCIA

Isle of Wight

County

Southampton

County

297

252

NORTH CAROLINA

Hog Island

135

117

76

100

143

153

163

95

>80

146

127

97

120

142

118

155 52 55

81

105

>130

>39

>61 >41

56

112

112

33

60

84

0

0

0

0

158

172

112 120

252

297

>184

>304

119

155

164 348

251

449

0

0

0

0

0

0

70

76

400

450

300

350

250

200

150

100

50

100

50

100

150

100

50

?

Burwell

Bay

?

?

Figure 7. Isopach map of the Upper Cretaceous deposits younger than pollen zone III showing abrupt truncation along 

the northern part of the study area, irregular thickening and thinning in the northwestern part, and a southeastthickening wedge. The 449-feet thickness in the Chesapeake Northwest River WTP borehole (3) includes 107 feet of 

Cretaceous and/or Palocene (?) deposits.
Structural Setting of Southeastern Virginia 17

36o30'

Virginia

Beach

Chesapeake

Hampton

Newport Poquoson

News

James City County

Suffolk

Surry

County

York

County

Northampton

County

Norfolk

76o30' 76o

37o

ATLANTIC

OCEAN

James River

VIRGINIA

CHESAPEAKE

BAY

OUTER RIM OF CRATER

INNER RIM OF CRATER

Lake

Drummond

Dismal Swamp Canal

100

50

EXPLANATION

LINE OF EQUAL THICKNESS OF UNIT—Interval 25 feet

BOREHOLE—Number is thickness of unit, in feet

CONTINUOUS COREHOLE—Number is thickness of unit, in feet

CHESAPEAKE BAY IMPACT CRATER RIM

MARKS TRUNCATION BY CBIC

STUDY AREA

50

39

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

67

77

53

63

100

39 87

50

38

63

64

60

77

79

71

73

67

64 47

40

36

57

77

62 47 35

44

53

31 68

53

57

82

45

70

115

137

106

65

67

75

50

75

50

100

125

50

100

Isle of Wight

County

Southampton

County

Hog Island

NORTH CAROLINA

Burwell

Bay

Figure 8. Isopach map of the pre-impact Lower Tertiary deposits showing a pattern of thickening and thinning. This pattern is apparently related to the effects of the Chesapeake Bay impact crater on the James River structural zone.
18 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

-300

 SEA

LEVEL

100

FEET

-600

-900

-1,200

-1,500

-1,800

-2,100

SOUTH NORTH

B B'

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

(A)

2

PRE-IMPACT TERTIARY DEPOSITS

Dismal Swamp

corehole

21 24 25

 Newport News

City Hall Complex

borehole

49

Chesapeake-Portsmouth

 Airport

 corehole

 VPI

 geothermal

 testhole NAN-P-13

mouth of

Nansemond

River

James River

?

?

?

?

?

LOWER CRETACEOUS DEPOSITS

UPPER CRETACEOUS DEPOSITS

YOUNGER THAN POLLEN ZONE III IMPACT DEBRIS

POST-IMPACT DEPOSITS

 CRYSTALLINE

BASEMENT ROCKS

JURASSIC-TRIASSIC (?)

RIFT BASIN ROCKS

granite

indurated red beds

 JURASSIC-TRIASSIC (?)

OR LOWER CRETACEOUS (?)

POST-IMPACT DEPOSITS

UPPER CRETACEOUS DEPOSITS (POLLEN ZONE III)

Potomac

Formation

? ?

VERTICAL SCALE GREATLY EXAGGERATED

Figure 9. Stratigraphic cross section B-B' , Dismal Swamp to Newport News, showing the entire thickness of the 

Virginia Coastal Plain deposits and the top of the basement rocks (A), and details of the upper section of Coastal Plain 

deposits (B). Cross section location shown in figure 3.
Structural Setting of Southeastern Virginia 19

EXPLANATION

Qt Quaternary deposits 

Ty Yorktown Formation (upper and lower Pliocene)

Tcm Calvert Formation (middle Miocene)

Tlmo Newport News unit of Calvert 

Formation (lower Miocene), Old 

Church Formation (upper 

Oligocene), and Delmarva beds 

(lower Oligocene), undivided

Tn Nanjemoy Formation (lower Eocene)

Tm Marlboro Clay (Eocene and Paleocene?)

Ta Aquia Formation (upper Paleocene)

 SEA

LEVEL

100

-100

-200

-300

-400

-500

-600

-700

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

St. Marys Formation

Eastover Formation

(B)

Yorktown Formation

Ty

Qt

Ta

Tm

upper Cenomanian beds

Tcm

Tlmo

Tn

Ta

Tcm

Potomac Formation

Tlmo

Cretaceous

 red beds

 Cretaceous

glauconitic sands

 Exmore

tsunami-breccia

Chickahominy

 Formation

Tm

Qt

Qt

B

NORTH

B'

SOUTH

FEET

James River

mouth of

Nansemond

River

49 21 24 25

Chesapeake-Portsmouth

Airport

corehole

VPI

geothermal

testhole

Dismal Swamp

corehole

2 NAN-P-13

Newport News

City Hall 

Complex

borehole

James River

paleochannel

VERTICAL SCALE GREATLY EXAGGERATED

Figure 9. Stratigraphic cross section B-B' , Dismal Swamp to Newport News, showing the entire thickness of the 

Virginia Coastal Plain deposits and the top of the basement rocks (A), and details of the upper section of Coastal Plain 

deposits (B). Cross section location shown in figure 3—Continued.
20 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

-2,700

-2,400

-2,100

-3,000

-1,800

-1,500

-1,200

-900

 SEA

LEVEL

-600

-300

300

0 1 2 5 MILES 3 4

0 1 2 3 4 5 KILOMETERS

EXMORE

TSUNAMI-BRECCIA

LOWER CRETACEOUS DEPOSITS

CBIC

MEGABLOCK

BEDS

UPPER CRETACEOUS DEPOSITS

YOUNGER THAN POLLEN ZONE III

PRE-IMPACT TERTIARY DEPOSITS

POST-IMPACT DEPOSITS

CRYSTALLINE BASEMENT ROCKS

UPPER CRETACEOUS DEPOSITS

 POLLEN ZONE III

Potomac

Formation

NOR-T-12

 VPI

Willoughby

 Spit

 Fort

Monroe

32 50

Fentress corehole 53 12

Chesapeake

Northwest River WTP

3 James River

FEET

SOUTH NORTH

C C'

(A)

? ?

VERTICAL SCALE GREATLY EXAGGERATED

Figure 10. Stratigraphic cross section C-C ' , Chesapeake Northwest River 

WTP to Fort Monroe, showing the entire thickness of the Coastal Plain 

deposits and the top of the basement rocks (A), and details of the upper 

section of Coastal Plain deposits (B). Cross section location shown in

figure 3.
Structural Setting of Southeastern Virginia 21

Yorktown Formation

Eastover Formation

St. Marys Formation

Exmore tsunami-breccia

Chickahominy

 Formation

Tcm

Upper Cretaceous red beds

Upper Cretaceous

 glauconitic sands

upper Cenomanian beds Potomac Formation

Qt

Ta

Ta

Tlmo

Tm

Tcm

Tcr

 Upper Cretaceous

and (or) Paleocene (?)

Quaternary deposits

100

 SEA

LEVEL

-100

-200

-300

-400

-500

-600

-700

-800

-900

Tlmo

Tn

-1,000

-1,100

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

SOUTH

3

Chesapeake

Northwest River WTP 12 32 Fentress corehole NOR-T-12

 VPI

Willoughby

 Spit

 Fort

Monroe

NORTH

50 53

C C'

FEET

James

River

(B)

VERTICAL SCALE GREATLY EXAGGERATED

EXPLANATION

Qt Quaternary deposits 

Tcr Chowan River Formation (upper Pliocene)

Tcm Calvert Formation (middle Miocene)

Tlmo Newport News unit of Calvert 

Formation (lower Miocene), Old 

Church Formation (upper 

Oligocene), and Delmarva beds 

(lower Oligocene), undivided

Tn Nanjemoy Formation

 (lower Eocene)

Tm Marlboro Clay

 (Eocene and Paleocene?)

Ta Aquia Formation

 (upper Paleocene)

Figure 10. Stratigraphic cross section C-C ' , Chesapeake Northwest 

River WTP to Fort Monroe, showing the entire thickness of the Coastal 

Plain deposits and the top of the basement rocks (A), and details of the 

upper section of Coastal Plain deposits (B). Cross section location shown 

in figure 3—Continued.
22 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

upper part of this line of section and shows the angular

unconformity between the Upper Cretaceous deposits

younger than pollen zone III and pre-impact Tertiary

deposits and some possible upwarping of pre-impact

deposits that are close to the outer rim of the impact crater. Figure 10 is similar to figure 9, except that the line of

section shown is farther to the east and the northwestern

end steps into the crater across the outer rim escarpment.

As suggested by Powars and Bruce (1999) and shown

graphically in figure 4, only Lower Cretaceous deposits

are present in the subsurface of the Virginia Coastal Plain

north of the James River. In contrast, both Lower Cretaceous and Upper Cretaceous deposits are present on the

south side of the James River. The available pollen data

indicate that a very thick section of Lower Cretaceous

deposits assigned to pollen zone I is present in the subsurface north of the James River, west of the impact crater,

and south of the northern part of Hanover County to the

mouth of the Rappahannock River. In contrast, on the

south side of the river, both Lower Cretaceous deposits

(pollen zones I and II) and Upper Cretaceous deposits

(pollen zones III, IV and V) are present in the subsurface.

The Upper Cretaceous deposits younger than pollen zone

III are absent in the Hog Island area and locally in the

Burwell Bay area (figs. 4 and 5). These deposits do not

appear to thin or feather out, but end abruptly, supporting

the existence of a pre-impact James River structural zone.

These Upper Cretaceous deposits are truncated to the

north of the James River and west of the preserved limit

of impact debris by an apparent structural upwarping

prior to the impact; farther east, they are truncated by the

impact crater across the lower Chesapeake Bay region

(figs. 4 and 5).

The extreme variations in thickness in the various Cretaceous units across the James River, combined with the

angular unconformity with the overlying pre-impact

lower Tertiary deposits and coincidence with a highly

probable basement structural zone, provide evidence for

the existence of a pre-impact James River structural zone.

In figure 11, an alternative interpretation to that shown

in figure 9, faults are shown cutting across the basement

and Coastal Plain sediments. The interpretations

depicted in both figures 9 and 11 indicate that this region

has experienced a combination of structural events

involving pre-impact movements of the James River

structural zone, syn-impact faulting and fracturing of the

region, and post-impact adjustments to the recovery

phase of the CBIC. 

Given that sediment deposition and preservation are

greater on the down-dropped side of a structural zone, it

would appear that the region north of the James River was

relatively depressed in pollen zone I time and relatively

elevated for the remainder of Cretaceous time. By contrast, the region south of the James River was relatively

depressed during pollen zones II, III, IV, and V time. Part

of this distribution has been attributed to a southward

shift in the depositional basin during Late Cretaceous

time along with a reduction in the rate of subsidence

across the region (Johnson, 1976). 

During Late Cretaceous and Early Tertiary time,

oscillations of sea level up to 400 ft produced major

transgressions and regressions across the study area.

North and south of the James River, the Paleocene to middle Eocene pre-impact deposits vary in their distribution

and thickness. The middle Eocene Piney Point Formation is found outside the disruption boundary, primarily

north of the James River, where it is underlain by sections

of the upper Paleocene Aquia and lower Eocene Nanjemoy Formations that are thicker here than south of the

James River. The limited extent of Piney Point strata

south of the James River is probably caused by a combination of syn-impact erosional processes and post-impact

uplift and removal by post-impact transgressions (Powars

and Bruce, 1999). The distribution of the basement

rocks and the pre-impact Cretaceous and Tertiary deposits indicates the existence of a generally east-to-westtrending pre-impact James River structural zone beneath

the lower James River Basin and appears to correspond to

the north side of the Cape Fear-Norfolk block. 

Syn-impact Structural History

The CBIC impactor sliced through the water column,

penetrated the full thickness of the existing Coastal Plain

sediments, slammed into the basement rock, and vaporized, creating a catastrophic explosion that generated a

series of gigantic tsunamis and sent tremendous amounts

of steam and ejecta into the atmosphere. The debris was

spread over most of the U.S. Atlantic shelf and coastal

areas (the coastline was then west of the present-day Fall

Line; Poag, Powars, Poppe, and Mixon, 1994, Powars

and Bruce, 1999). The high-velocity impact left an

immense crater that is almost 1.2 mi deep and is partly

filled with debris and tsunami deposits. The Coastal

Plain sediments and basement rocks lining the crater cavity were melted, and the basement rocks in a region

beneath and around the crater were faulted and fractured

(fig. 2).
Structural Setting of Southeastern Virginia 23

-300

 SEA

LEVEL

100

FEET

-600

-900

-1,200

-1,500

-1,800

-2,100

SOUTH NORTH

B B'

2

PRE-IMPACT TERTIARY DEPOSITS

(A)

Dismal Swamp

corehole

21 24 25

 Newport News

City Hall Complex

corehole

49

Chesapeake-Portsmouth

 Airport

 MW1-4 corehole

 VPI

 geothermal

 testhole NAN-P-13

mouth of

Nansemond

River

James River

LOWER CRETACEOUS DEPOSITS

UPPER CRETACEOUS

DEPOSITS YOUNGER

THAN POLLEN ZONE III IMPACT DEBRIS

POST-IMPACT DEPOSITS

 CRYSTALLINE

BASEMENT ROCKS

JURASSIC-TRIASSIC (?) 

RIFT BASIN ROCKS

granite

 CRYSTALLINE

BASEMENT ROCKS

indurated red beds

JURASSIC-TRIASSIC (?)

OR LOWER CRETACEOUS (?)

POST-IMPACT DEPOSITS

UPPER CRETACEOUS

DEPOSITS 

(POLLEN ZONE III)

?

?

Potomac

Formation

VERTICAL SCALE GREATLY EXAGGERATED

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Figure 11. Fault interpretation of stratigraphic cross section B-B' , Dismal Swamp to Newport News, same

section as figure 9A (A), and fault interpretation of same section as figure 9B (B). Cross section location 

shown in figure 3.
24 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

 SEA

LEVEL

100

-100

-200

-300

-400

-500

-600

-700

Qt

Ta

Tm

upper Cenomanian

 beds

Qt

Tcm

Tlmo

Tn

Tn

Tm

Ta

Tcm

Tm

Potomac Formation

Tlmo

Cretaceous red beds

 Cretaceous

glauconitic sands

Qt

Ty

Chickahominy

 Formation

 Exmore

tsunami-breccia

Potomac Formation

upper Cenomanian

 beds

Ta

Potomac Formation

Tn

 Eastover Formation

St. Marys Formation

 Eastover Formation

 Eastover Formation

St. Marys Formation

St. Marys Formation

Yorktown Formation

B

NORTH

B'

SOUTH

FEET

James River

mouth of

Nansemond

River

49 21 24 25

Chesapeake-Portsmouth

Airport 

corehole

 VPI

 geothermal

 testhole

Dismal Swamp

corehole

2 NAN-P-13

Newport News

City Hall 

Complex

corehole

(B)

VERTICAL SCALE GREATLY EXAGGERATED EXPLANATION

Qt Quaternary deposits 

Ty Yorktown Formation (upper and lower Pliocene)

Tcm Calvert Formation (middle Miocene)

Tlmo Newport News unit of Calvert 

Formation (lower Miocene), Old 

Church Formation (upper 

Oligocene), and Delmarva beds 

(lower Oligocene), undivided

Tn Nanjemoy Formation (lower Eocene)

Tm Marlboro Clay (Eocene and Paleocene?)

Ta Aquia Formation (upper Paleocene)

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Figure 11. Fault interpretation of stratigraphic cross section B-B' , Dismal Swamp to Newport News, same section as figure 9A 

(A), and fault interpretation of same section as figure 9B (B). Cross section location shown in figure 3—Continued.
Structural Setting of Southeastern Virginia 25

The formation of the CBIC truncated and excavated

large quantities of Eocene to Cretaceous Coastal Plain

sediments and the underlying Proterozoic, Paleozoic

crystalline, and possibly Jurassic- Triassic rift-basin

basement rocks, creating the CBIC megablock beds and

the Exmore tsunami-breccia deposits. The CBIC is a

complex peak-ring crater with an inner and outer rim, a

slumped terrace zone, and a relatively flat-floored annular trough that encircles a deep central depression into the

basement (fig. 2). The central depression is also referred

to as the inner basin and contains a series of concentric

valleys and ridges that surround a central uplift (Powars

and Bruce, 1999). 

The seismically defined syn-impact CBIC megablock

beds are a product of large-scale slumping into the crater

that occurs during an early stage of crater filling; these

deposits covered the basement surface of the annular

trough (Poag, 1996, 1997; Powars and Bruce, 1999).

A narrow band of preserved Exmore tsunami-breccia

deposits surrounds the crater’s outer rim; these deposits

also are probably offset by the bounding outer rim fault

zone and other faults apparently produced or adjusted by

the impact. The location of the preserved limit of the

Exmore tsunami-breccia beneath the river, however, is

uncertain.

A zone of normal-faulted slump blocks characterizes

the highly irregular outer rim of the CBIC (fig. 2). These

features encircle and are downthrown into the annular

trough, forming a buried escarpment. On seismic reflection data, this escarpment is easily distinguished from the

nearly flat-lying Coastal Plain deposits outside the disruption boundary. The relief of the escarpment ranges

from 1,000 to nearly 4,000 ft and its width varies from

about 0.5 to 2 mi (Poag, 1996; Powars and Bruce, 1999).

Its geometry is characterized as a steep wall in places;

elsewhere, the escarpment stair-steps down into the annular trough. This variation is expected because the escarpment was produced by a combination of complex

catastrophic events, including the collapse of the crater’s

outer wall interacting with subsequent gigantic tsunamis.

This variation also is related to the unconsolidated nature

of the sediments involved. Section C-C ' shows the tremendous relief at the escarpment of the outer rim and

how the post-impact deposits drape over the outer rim

and thicken into the crater (fig. 10).

Post-impact Structural History

Post-impact deposits from numerous marine transgressions across the lower mid-Atlantic Coastal Plain

have now buried the crater with about 1,300 to 1,600 ft of

sediment. In the study area, south of the crater, the postimpact deposits form an eastward-dipping wedge of sediments 225 to 750 ft thick. Seismic data show that most

of these deposits dip concentrically into the crater, especially across the outer and inner rims (Powars and Bruce,

1999).

The post-impact upper Tertiary deposits outside the

crater and south of the James River are thicker than comparable units north of the river. The area south of the river

apparently was downwarped during this period. This

downwarping appears to be related to structural adjustments of the James River structural zone. Two features

indicate that structural instabilities persisted in the areas

adjacent to the crater’s outer rim, at least through late Tertiary time: (1) the stratigraphic anomalies found in upper

Tertiary deposits outside the disruption boundary, and (2)

faults that displace basement rocks and Cretaceous

through upper Tertiary sediments observed in the seismic

data north of the crater where the faults extend outside the

disruption boundary (Powars and Bruce, 1999). These

structural instabilities likely are produced by several factors: (1) faulting caused by the initial impact and structural adjustments to older faults; (2) post-impact

structural readjustment of the basement; (3) post-impact

differential compaction; and (4) post-impact differential

movement of fault blocks.

The Oligocene to middle Miocene post-impact deposits exhibit a patchy distribution pattern south of the James

River. These deposits appear to be preserved adjacent to

the outer rim and inside the crater along the northern

boundary of the study area, as well as downdip along the

Atlantic coastline. The thinness and local absence of the

Oligocene and lower Miocene strata in the boreholes

make it very difficult to correlate these deposits without

data from a corehole. The absence of the Calvert Formation south of the river suggests that large-scale structural

readjustments to the impact, such as movement along

faults in the James River structural zone, were still occurring in the middle Miocene. 

 The very clayey, upper Miocene, post-impact St.

Marys Formation was the first unit distributed and preserved across the entire region and therefore serves as an

easily recognized marker unit. 
26 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

The juxtaposition of the pre-impact deposits with the

syn-impact deposits marks the disruption boundary and,

except for the narrow, relatively thin band of ejecta, coincides with the outer rim of the crater. Relatively thin

post-impact deposits outside the crater abruptly thicken

across the outer rim of the crater and are up to ten times

thicker inside the crater. The absence of many early postimpact deposits (upper Eocene to middle Miocene) south

of the James River and the presence of relatively thick

upper Miocene and Pliocene shallow-marine deposits

suggest that the impact crater is responsible for structural

adjustments to the pre-impact James River structural

zone. The distribution of strata suggests that these structural adjustments have caused the area south of the river

to be uplifted or downwarped at various times. In this

transgressive-regressive depositional setting, the uplift

causes erosion of strata or little deposition whereas the

downwarp allows deposition and preservation if sediment is available. 

Borehole data indicate that most of these post-impact

deposits are overall coarser grained (including Miocene

and Pliocene marine bioclastic sands) along the outer rim

and become finer grained toward the interior of the crater

and farther outside and away from the outer rim (Powars

and Bruce, 1999). 

Pliocene to Quaternary deposits show complex lithofacies distribution and thickness patterns that include thin

to thick and fine to coarse beds within 12.5 miles of the

crater’s outer rim (Johnson, Powars, and others, 1998;

Powars and others, 1998). Pliocene deposits, which dip

radially away from the center of the impact structure over

areas several miles in width, exhibit dips that are discordant from the typical eastward regional dip of Tertiary

strata. These dip reversals are generally less than one

degree and commonly include fan-like inter- and intraformational angular unconformities, indicating that deformation and deposition were synchronous and a product

of post-impact deformation related to slump-block

motion near the outer rim of the crater (Johnson, Kruse,

and others, 1998; Johnson, Powars, and others, 1998;

Powars and others, 1998; Powars and Bruce, 1999).

The CBIC has also had a major effect on the

development of the regional drainage to the sea of the

mid-Atlantic rivers (from the Susquehanna River to the

James River) that converge on the crater. The location of

the outer rim of the crater and its relation to the course of

the lower James River is apparent, as the river turns

sharply northeastward as it crosses the outer rim and into

the crater. 

The Holocene transgression, along with higher subsidence rates over the crater, has produced generally the

highest measured rates of subsidence in the mid-Atlantic

region (Nerem and others, 1998), which possibly account

for the abundant swamps that border the lower Chesapeake Bay.

GEOLOGIC FRAMEWORK OF 

SOUTHEASTERN VIRGINIA, SOUTH 

OF THE JAMES RIVER

The following sections describe the lithology, distribution, and borehole geophysical signatures for each

Coastal Plain stratigraphic unit. The basement rocks are

also discussed briefly.

Basement Rocks

Within the study area, Coastal Plain deposits are

underlain by igneous and metamorphic Paleozoic and

Upper Proterozoic crystalline basement rocks and by

Jurassic-Triassic rift basin sedimentary rocks. The crystalline basement rocks beneath much of the mid-Atlantic

Coastal Plain, including the study area, have been

mapped by Horton and others (1991) as part of the Chesapeake Block (a tectonostratigraphic unit) and were

defined as rocks of undetermined affinity east of the

Alleghanian “Chesapeake Bay suture” of Lefort and Max

(1991). Lefort and Max (1991) suggested that the Chesapeake Block includes African rocks left behind in the

Mesozoic breakup of Africa and North America. Late

Paleozoic granitic plutons are included in this block (for

details, see Horton and others, 1991).

Four boreholes penetrated basement rocks within the

study area. Two geothermal test holes, #26 Isle of Wight

(38) (top of granite at –1308 ft) and #25 Suffolk (24) (top

of granite at –1772 ft), encountered 210 to 250 ft of

mostly indurated sediments overlying granite and greenstone basement rocks. The cuttings from these indurated

sediments consist of a mixture of reddish siltstone, shale

and feldspathic and quartz sand and represent either

Jurassic-Triassic rift basin deposits or Lower Cretaceous

deposits. Jurassic-Triassic rift basin rocks were also

encountered in the Dismal Swamp corehole (2) (top of

rock at –1817 ft) and at the VDEQ borehole (1) (top of

rock at –1822 ft).
Geologic Framework of Southeastern Virginia, South of the James River 27

Structure contours of the top of the basement surface

show an overall eastward dip and stepping beneath Virginia Beach (Brown and others, 1972; Meng and Harsh,

1988; Powars and others, 1992). A basement high (an

arch or ridge) trends east to west from Norfolk to the

northeastern corner of Southampton County and appears

to be coincident with the north end of the two JurassicTriassic rift basins within the study area (figs. 1 and 4).

The #26 Isle of Wight (38) and #25 Suffolk (24) boreholes provide the main evidence for this arching. The

arching is reflected even if the indurated sediments are

interpreted as Lower Cretaceous. It is likely that the

basement high is a product of faulting, possibly a part of

Jurassic-Triassic rift basin faulting, which is connected to

Paleozoic fault systems. 

Two boreholes north of the study area on the lower

York-James Peninsula, just inside the outer rim of the

impact crater, penetrated a few feet of white granite.

These are the NASA-Langley Air Force Base borehole

(top of rock at –2084 ft; for location and details see

Powars and Bruce, 1999), and the Fort Monroe borehole

(53) (top of rock at –2251 ft).

Cretaceous Deposits 

Within the study area, the Cretaceous sediments consist of Lower and Upper Cretaceous deposits that range in

age from about 120 to 85 Ma. An interval in the Chesapeake Northwest River WTP borehole (3), however, possibly represents some younger Upper Cretaceous

sediments (85 to 65 Ma). Updip (westward), the Lower

Cretaceous deposits (Potomac Formation) consist of a

complex array of fluvial-deltaic deposits that intertongue

downdip (eastward) with thin glauconitic sands typical of

shallow-shelf deposits. The Lower Cretaceous deposits

extend across the study area, except where disturbed by

the impact, forming an east- to-southeast-dipping wedge

that ranges from around 725 ft thick over an apparent

basement high on the western side of the study area to

around 2,400 ft thick along the coastline at Virginia

Beach. If the indurated sediments are interpreted as

Lower Cretaceous deposits, then the deposits are 935 ft

thick on the western side of the study area. 

Regionally, the Cretaceous section includes Upper

Cretaceous deposits that are relatively thick south and

southwest of the crater. The Upper Cretaceous deposits

form a wedge that thickens from 377 ft in the Dismal

Swamp corehole to 628 ft in the Fentress corehole. The

Upper Cretaceous deposits younger than pollen zone III

thicken from 252 ft thick in the Dismal Swamp corehole

(2) (pl. 1) to 350 ft thick in the Fentress corehole (12)

(pl. 1) to 449 ft thick in the Chesapeake Northwest River

WTP borehole (3) (pl. 1). These deposits are absent

north of the pre-impact James River structural zone.

Lower Cretaceous core samples (at -455 ft and -648 ft)

from the Newport News Park 1 borehole (62) documented that pollen zone I is present nearly to the top of

the Cretaceous section (L.A. Sirkin, Adelphi University,

written commun. to A. Meng, formerly with the USGS,

1983). This implies that the entire Cretaceous section at

the Newport News Park 1 borehole (62) is represented by

a 1300-ft-thick interval of pollen zone I; however, Lower

Cretaceous deposits that are older than zone I were found

in the downdip Taylor #1 oil testhole, located on the Delmarva Peninsula (Robbins and others, 1975), and equivalent deposits may be present in the lower part of the

Newport News Park 1 borehole (62). Brenner (1963)

reported that an outcrop of the Potomac Formation at

Drewrys Bluff on the James River southeast of Richmond

contained pollen indicative of zone I. The author has

found no documentation of Upper Cretaceous deposits

containing pollen zones III, IV, or V in Virginia west of

the Chesapeake Bay, north of the James River, and south

of the Potomac River. The apparent absence of Lower

Cretaceous pollen zone II deposits west of the Chesapeake Bay, north of the James, and south of the northern

part of Hanover County to the mouth of the Rappahannock River (fig. 4), suggests the need to re-evaluate Meng

and Harsh’s (1988) hydrogeologic subdivision and

regional correlation of Lower Cretaceous deposits across

the entire Coastal Plain, outside the outer rim of the crater. Correlation of the geologic to hydrogeologic units

outside the study area, however, is beyond the scope of

this report. The reader is referred to Meng and Harsh

(1988) for a detailed and comprehensive discussion of

log correlations and identification of depositional patterns and settings that guided the delineation of the

regional hydrogeologic units of the Potomac Formation. 

Powars and Bruce (1999) concluded that the unit

mapped in the southeastern Virginia Coastal Plain by

Cederstrom (1945b) as the Mattaponi Formation actually

represents Lower Cretaceous to lower Miocene deposits

outside the preserved limit of Exmore tsunami-breccia

deposits. Furthermore, inside this preserved limit and

inside the outer rim of the crater, the Mattaponi

Formation is equivalent to the Exmore tsunami-breccia

deposits. 
28 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Potomac Formation - Lower and Upper 

Cretaceous Deposits

Within the study area, the Potomac Formation consists

of fluvial-deltaic deposits of Early Cretaceous (Barremian (?) to Albian) and Late Cretaceous (early Cenomanian) age. These deposits extend across the study area,

forming an eastward-thickening wedge that ranges from

about 1,000 ft thick beneath the Hog Island to Bacons

Castle area to about 1,700 ft thick at Fentress to 2,400 ft

at Virginia Beach. The Potomac Formation variously

overlies metamorphic and igneous rocks of the crystalline basement and red siltstones, shales, and sandstones

of Jurassic-Triassic rift basins.

These deltaic deposits are highly variable in their

lithology and thickness and probably represent stacked

deposits of meandering streams, braided streams, and

river- and wave-dominated delta-plain and delta-front

facies (Glaser, 1969; Reinhardt and others, 1980; Owens

and Gohn, 1985; Meng and Harsh, 1988). These deposits

are difficult to correlate and subdivide because of their

lateral and vertical heterogeneity, lithic similarities, and

the paucity of biostratigraphic data; pollen is the only

biostratigraphic indicator that is found consistently in

these deposits. Recent development and refinement of a

pollen zonation for these deposits (Brenner, 1963; Robbins and others, 1975) has provided a basis for subdividing the sequences into units of temporal and possibly

genetic significance (Reinhardt and others, 1980; Meng

and Harsh, 1988; Powars and others, 1992). Some investigators (Glaser, 1969; Hansen, 1969; Brown and others,

1972) suggest that a correlation exists among the lithologic and depositional patterns, the five major pollen

zones (labeled I, II, III, IV, V), and their corresponding

“formations” (table 1). Meng and Harsh (1988) based

their hydrogeologic subdivision of the Potomac Formation primarily on geophysical log interpretations of lithologic characteristics, mode of deposition, available

palynostratigraphic zonation data, and hydrologic data. 

The fining-upward fluvial-deltaic deposits consist primarily of light-gray to pinkish to greenish-gray to green,

in part mottled red, brown, and yellow, poorly sorted, fine

to coarse, quartzose and feldspathic sand and gravel,

which grade up into silt and clay. The sands vary from

being thick-bedded and trough crossbedded to interbedded with thin- to thick-bedded clay-silts to thick-bedded

clays. Locally, the sands also contain clay-clast conglomerates and lignitic material (finely disseminated to

wood chunks to logs). The finer grained beds range from

gray to dark-gray, finely laminated, carbonaceous clays

interbedded with thin, sandy clay beds to highly oxidized, multicolored (reds, browns, purples, and yellow),

laminated to thick-bedded clays. The highly oxidized

clays include intervals that represent stacked paleosols

typical of channel-overbank deposits that have characteristic pedotubules (cracks and fractures) and abundant

iron-rich glaebules (nodules and concretions). 

Within the crater, all deposits traditionally mapped as

Lower Cretaceous (Potomac Formation) are now interpreted as sediments disturbed by the impact and were

informally named by Powars and Bruce (1999) as the

“Chesapeake Bay impact crater megablock beds” (CBIC

megablock beds). These deposits are considered an

Eocene stratigraphic unit because the slump blocks were

transported and rapidly emplaced by impact cratering

processes that most likely mixed them with Exmore tsunami-breccia and possible basement fragments and

melted rocks from the initial blast.

South of the James River and the Chesapeake Bay,

relatively thick Upper Cretaceous marine and fluvialdeltaic deposits overlie fluvial-deltaic deposits of the

Potomac Formation. The Upper Cretaceous fluvialdeltaic facies exhibit a sporadic, patchy distribution and

intertongue with the marine facies, which have a more

consistent distribution. 

The resistivity and gamma logs reflect the litho-stratigraphic differences of the thin-bedded, glauconitic,

shelly, marine Upper Cretaceous deposits that overlie the

thicker bedded, fluvial-deltaic Potomac Formation.

Where Upper Cretaceous deltaic deposits overlie Potomac fluvial-deltaic sediments, however, the logs do not

reflect these differences; therefore, lithic log and data

from nearby wells must be analyzed. Generally, the

marine lower Tertiary (Aquia Formation) and Upper Cretaceous deposits have consistently higher gamma values

(deflection to the right) and are thinner bedded than the

Potomac fluvial-deltaic deposits. The fact that both of

these units underlie the consistently clayey Chickahominy and Nanjemoy-Marlboro units throughout the area is

also helpful. Overall, the deposits of the Potomac Formation have a blockier, thicker stratified resistivity and

gamma log signature. Within this overall blocky pattern,

the resistivity and gamma logs of the Potomac deposits

also show numerous gradational fining-upward

sequences (about 5- to 100-ft-thick) that typically have

sharp contacts between the tops of the clays and the basal

sands of the overlying sequence. These logs reflect largescale, fining-upward cycles (about 100- to 200-ft-thick)

that are typical for the Potomac deposits. The sawtoothed appearance of the resistivity and gamma logs of
Geologic Framework of Southeastern Virginia, South of the James River 29

the Potomac deposits reflects their highly stratified, commonly relatively thin-bedded nature. The quartzo-feldspathic sands and tough, multicolored clays of the

Potomac deposits contrast lithologically with the glauconitic, shelly Aquia Formation and Exmore tsunami-breccia deposits. 

Because of the paucity of borehole and geophysical

log data from inside the crater and because of the lithic

similarities between undisturbed Potomac deposits and

the CBIC megablock beds, these units were differentiated

primarily on the basis of seismic reflection data by

Powars and Bruce (1999). A subtle dampening of the

resistivity and gamma-log signatures for the deposits

interpreted as CBIC megablock beds distinguish them

from undisturbed Potomac deposits.

Upper Cretaceous Deposits Younger Than Pollen 

Zone III

Upper Cretaceous deposits (upper Cenomanian to

Santonian) south of the James River younger than pollen

zone III are present in the subsurface south and southwest

of the outer rim of the crater (Powars and others, 1992;

Powars and Bruce, 1999). As shown in figure 4A, these

deposits extend eastward from central Southampton

County and eastern Surry County to Virginia Beach; they

form a wedge that thickens from 200 to 500 ft across the

southeastern half of the study area but are thinner, from

30 to 150 ft, have a irregular distribution across the northwestern half, and are abruptly truncated on the northernmost part of the study area. 

Powars and others (1992) subdivided the Upper Cretaceous deposits into three lithic units on the basis of data

from the Dismal Swamp (2), Fentress (12), and Chesapeake-Portsmouth Airport (21) coreholes. In ascending

order, they are (1) upper Cenomanian beds consisting of

marine and deltaic deposits, (2) a glauconitic sand unit,

and (3) oxidized red-bed fluvial-deltaic deposits that

include multiple paleosols. Evidence for the presence of

Upper Cretaceous pollen zone III was only recently

obtained from the Fentress corehole (12) (N. Frederiksen,

USGS, written commun., 1999) and was therefore not

included in Powars and others’ (1992) subdivision.

Lithic and geophysical log data obtained from the Chesapeake Northwest River WTP borehole (3) (pl. 1) indicate the presence of 107 ft of Upper Cretaceous and(or)

lower Paleocene deltaic sands and clays overlying the

red-bed unit. These deposits are interpreted to be overlain by the upper Paleocene Aquia Formation and underlain by the red bed unit, which is Coniacian-Santonian in

age (pollen zone V) in the Fentress corehole (12) (N. Frederiksen, USGS, written commun., 1999). These deposits may be partially or completely equivalent to the Black

Creek Formation or Peedee Formation of North Carolina,

and they also may be partially equivalent to the lower

Paleocene Brightseat Formation that was documented in

the Dismal Swamp corehole (2) (Powars and others,

1992). Throughout the study area, relatively thin (less

than 75 ft thick) pre-impact Paleocene and lower Eocene

marine strata locally overlie these various Upper Cretaceous deposits south of the James River. 

 Upper Cenomanian Beds (Pollen Zone IV)

The upper Cenomanian beds form a southeastwarddipping wedge that ranges from 33 ft (54) to 212 ft (3) in

thickness. An isopach map of this unit shows zones of

thinning and thickening that are interpreted to be a product of pre-impact faulting (fig. 12). These beds are found

throughout the study area where they form the oldest

lithic unit of the Upper Cretaceous units younger than

pollen zone III. In the Chesapeake-Portsmouth Airport

corehole (21), only the upper Cenomanian part of these

younger Upper Cretaceous units is preserved. The upper

Cenomanian beds in the Dismal Swamp (2) and Fentress

(12) coreholes contain a pollen assemblage indicative of

pollen zone IV (R.J. Litwin, USGS, written commun.,

1988; N. Frederiksen, USGS, written commun., 1999).

The upper Cenomanian deposits also contain index

macro- and microfossils that allow local and regional correlation of this unit. For example, the presence of the

mollusks Inoceramus arvanus and Exogyra woolmani

indicates a late Cenomanian age (N.F. Sohl, oral commun., 1988) and suggests correlation with unit E of

Brown and others (1972) and sequence 2 of Gohn (1988).

In the Fentress corehole (12) (pl. 1), the basal 50 ft of

the upper Cenomanian beds consists of two finingupward sequences. Abundant wood fragments suggest a

nearshore-shelf to deltaic depositional environment. The

base of the upper Cenomanian deposits here is marked by

a sharp lithic contact at -1,035 ft elevation where an

olive-gray to light-gray, micaceous, lignitic, loose,

medium to very coarse, glauconitic quartz sand overlies

the Potomac Formation. The Potomac Formation consists of greenish-gray, silty and sandy clay that grades

downward into medium to coarse, crossbedded sand. At

Fentress (12) these nearshore and deltaic deposits are

overlain by 115 ft of laminated to thick-bedded, olivegray to dark-gray silt, clay, and fine to coarse sand containing variable amounts of glauconite, mica, pyrite,
30 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Virginia

Beach

Chesapeake

Hampton

Newport Poquoson

News

James City County

Suffolk

Isle of Wight

County

Surry

County

York

County

Northampton

County

Norfolk

76o30' 76o

37o

36o30'

James River

VIRGINIA

CHESAPEAKE

BAY

OUTER RIM OF CRATER

INNER RIM OF CRATER

Lake

Drummond

Dismal Swamp Canal

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

50

NORTH CAROLINA

ATLANTIC

OCEAN

EXPLANATION

LINE OF EQUAL THICKNESS OF UNIT–Dashed where

approximately located. Interval 50 feet. Queried where

inferred

BOREHOLE—Number is thickness of unit, in feet

CONTINUOUS COREHOLE—Number is thickness of unit, in feet

CHESAPEAKE BAY IMPACT CRATER RIM

APPROXIMATE LIMIT OF UPPER CENOMANIAN BEDS

STUDY AREA

151

142

102

164

199

212

161

51

70

76

155 119

89

153

135

117

163

95

>80

143

100

76

52

81

118

155

118

158

172

112

120

97

112

33

56

105

127

113

84

151

142

150 100

200

?

150

100

150

150

100

50

100

100

50

?

Burwell

Bay

Hog Island

Figure 12. Isopach map of upper Cenomian beds (pollen zone IV) showing thickening, thinning, and truncation apparently 

related to the pre-impact James River structural zone and the Chesapeake Bay impact crater.
Geologic Framework of Southeastern Virginia, South of the James River 31

shells, microfossils, wood, and burrows that indicate a

marine origin. This section contains numerous finingupward sequences. The sandier, coarser grained beds at

the bases of sequences are generally more glauconitic and

shelly than the overlying sediments. Many of the sandier

shell beds and shell hashes (representing storm deposits)

are cemented by calcium carbonate. These indurated beds

are generally reflected on resistivity logs by thin, sharp,

high-resistivity spikes. 

In the Dismal Swamp (2) corehole (pl. 1), the lowest

upper Cenomanian beds consist of glauconitic sands,

which grade upward into shelly, glauconitic silt, clay, and

sand of marine origin. The base of the upper Cenomanian

is an erosional unconformity; dark-gray to black, clayey,

glauconitic, phosphatic, pyritic, carbonaceous, pebbly

quartz sand overlies and is burrowed down into the Potomac Formation, consisting of white to light-gray clay

showing some yellow mottling. Within a few feet, these

Lower Cretaceous beds grade downward into multicolored sands and clays, which are oxidized red, purple,

brown, and yellow. At Dismal Swamp, 128 ft of cyclic

Cenomanian marine beds were penetrated. Abundant

finely disseminated lignitic material indicates nearshore

deposition. This marine unit is overlain by a section of

dark gray, very fine grained, thinly laminated, very micaceous clayey silt to muddy sand containing abundant

wood fragments that indicate a deltaic-lagoonal depositional environment. This section is 21.5 ft thick in the

Dismal Swamp corehole (2) and 33.9 ft thick in the Fentress corehole (12). The very abundant mica imparts a

greasy feel to the sediments and enhances recognition of

the unit in well cuttings.

The resistivity and gamma logs generally reflect

numerous fining-upwards sequences, which are represented in the cores as glauconitic shelly sands that grade

upwards into clayey silts to silty clays. Differentiation of

these sequences from overlying similar marine Paleocene

or Eocene deposits is very difficult. By contrast, it is relatively easy to distinguish the Upper Cretaceous marine

sequences from the underlying thicker bedded, fluvialdeltaic Potomac deposits using resistivity and gamma-log

responses. In general, the resistivity logs of the upper

Cenomanian beds indicate an overall thick, fine-grained

section with numerous thin, indurated or slightly sandier

layers. The upper Cenomanian log signature contrasts

well with the blocky, more distinct sand and clay logs of

the Potomac fluvial-deltaic deposits. The gamma logs

also show consistently higher values (deflection more to

the right) for the upper Cenomanian beds as compared to

the Potomac deposits.

Glauconitic Sand Unit

The Upper Cretaceous glauconitic sand unit is loose

sand that is found only across the southern part of the

study area. The Dismal Swamp and Fentress coreholes

penetrated 54.5 ft and 59 ft of this unit respectively, with

almost no recovery. The lack of clay or silt and the apparent well-sorted nature of the sands appear to be responsible for the poor core recovery. The small amount of

recovered core, the drill cuttings, and the geophysical

logs indicate that most of the unit is fine to very coarse

glauconitic quartz, marine sand. In the Dismal Swamp

corehole, the basal 0.7 ft of this unit was recovered along

with contact with the underlying deltaic-lagoonal upper

Cenomanian unit. This contact is an erosional unconformity where light-green to olive-gray, poorly sorted, pebbly, muddy sand sharply overlies dark-gray, finely

laminated, very micaceous, lignitic clayey silt. The basal

bed of the glauconitic sand unit contains rip-up clasts

from the underlying micaceous silt.

Upper Cretaceous Red Beds (Pollen Zone V)

The distribution of the Upper Cretaceous red beds is

similar to that of the glauconitic sand unit and is shown in

figure 5. The red beds are found throughout the southern

half of the study area but have a patchy distribution across

the northern half. The red beds were not encountered in

the Chesapeake-Portsmouth Airport corehole (21). The

thickness of the red beds is 53.4 ft in the Dismal Swamp

corehole, 91.3 ft in the Fentress corehole (12), and 80 ft

in the Chesapeake Northwest River WTP borehole (3).

The unit has a heterogeneous lithology including uniform

gray and green and bright red, mottled purple, yellow,

orange, and brown sequences of interbedded oxidized

clay, silty clay, silty fine sand, and pebbly coarse sand.

Some beds contain scattered mica, carbonaceous

material, wood chunks, mud cracks, and rootlets. Several

paleosols occur at the top of fining-upward sequences.

The bases of the fining-upward sequences typically

consist of muddy pebbly sand that overlies the clay to

silty clay or sandy clay of an underlying paleosol. At the

base of the red beds in the Dismal Swamp corehole (2) is

a gray to multicolored, interbedded, micaceous clayey

sand to sandy clay, which sharply overlies the glauconitic

sand unit. This lower unit consists of dark-green to graygreen, fairly well sorted, fine to coarse quartz sand

containing small amounts of glauconite and phosphate.

The extensive development of these red beds indicates a

fluvial to upper-delta-plain depositional environment.
32 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Recent pollen analysis (N. Frederiksen, USGS, written

commun., 1999) of samples from the red bed unit in the

Fentress corehole (12) indicates assignment to Coniacian-Santonian pollen zone V, which is equivalent to the

Amboy Stoneware Clay Member of the Magothy Formation in New Jersey and the Black Creek or Peedee Formations in North Carolina. This analysis provides, for the

first time, a definitive age for the red-bed unit, and for the

first time, deposits of this age have been documented in

Virginia. The glauconitic sand unit is still not definitively

dated; however, it is positioned between the ConiacianSantonian age red-bed unit above and late Cenomanian

strata below, and therefore may be Cenomanian to Santonian in age. This investigation found that both the glauconitic sand unit and red beds correlate with unit D of

Brown and others (1972) and sequence 3 of Gohn (1988).

Upper Cretaceous and/or Paleocene Beds

In the southeastern corner of the study area, the Chesapeake Northwest River WTP borehole (3) (pl. 1)

encountered a 107-ft-thick section of deltaic sands and

clays that are interpreted to be underlain by the Coniacian-Santonian age red-bed unit and overlain by the

upper Paleocene Aquia Formation. These deposits can

be subdivided into three units: 51-ft-thick lower glauconitic sand overlain by 15-ft-thick organic-rich clay that is

capped by 41-ft-thick clayey-silty quartz sand. Their

position between the red bed unit and the Aquia Formation indicates that the deposits are either Upper Cretaceous (pollen zone V or younger) and/or Paleocene

deposits.

Tertiary Deposits

Interpretations of Tertiary deposits are based on correlations between corehole and borehole data, with emphasis placed on correlations between the Dismal Swamp

(2), Chesapeake-Portsmouth Airport (21), and Fentress

(12) coreholes. Powars and Bruce (1999) provided the

geologic framework adjacent to and including the northwestern to central part of the present study area

(figs. 1 and 2). The Jamestown (61) and Newport News

Park 2 (63) coreholes provide lithostratigraphic interpretations for the northwestern part of the study area; on the

northeastern side of the study area, the Kiptopeke corehole (64) gives stratigraphic control inside the crater.

Chronostratigraphic control is based on fauna and flora

analyses of core samples. Biostratigraphic information is

included on the Dismal Swamp (2) and Fentress (12)

corehole stratigraphic columns (pl. 1). 

Following Powars and Bruce (1999), the Tertiary

deposits beneath southeastern Virginia are grouped into

pre-impact, syn-impact, and post-impact deposits. The

pre-impact deposits consist of shallow-shelf to marginalmarine facies that are characteristically thinly stratified,

partly shelly, glauconitic, clayey sands and silts. These

deposits include the Brightseat Formation (lower Paleocene), Aquia Formation (upper Paleocene), Marlboro

Clay (which straddles the Paleocene-Eocene boundary),

Nanjemoy Formation (lower Eocene), and Piney Point

Formation (middle Eocene). The syn-impact deposits are

represented by the instantaneously deposited Exmore

tsunami-breccia that has a highly variable mixture of

autochthonous sedimentary intraclasts (from Early Cretaceous to late Eocene age) and by the CBIC megablock

beds, defined by Powars and Bruce (1999) on seismic

data. The upper Eocene to Middle Miocene post-impact

deposits consist of bathyal, shallow shelf and marginalmarine sediments that have progressively filled much of

the upper part of the crater. The upper Miocene to

Pliocene shallow shelf, and marginal-marine deposits

filled the upper part of the crater and blanketed the entire

region, thereby burying the crater. Post-impact deposits

include, in ascending order: the very clayey Chickahominy Formation (upper Eocene); the glauconitic, phosphatic, and partly shelly Delmarva beds (lower

Oligocene) and Old Church Formation (upper Oligocene); the shelly and sandy beds of the Calvert Formation (lower Miocene); the primarily siliciclastic, finegrained, Calvert (middle Miocene), St. Marys (upper

Miocene), and lower Eastover (upper Miocene) Formations; the siliciclastic, locally glauconitic, fine- to coarsegrained fossiliferous upper Eastover (upper Miocene),

Yorktown (lower and upper Pliocene), and Chowan River

(upper Pliocene) Formations; and fluvial to estuarine

Quaternary units.

Pre-impact Tertiary Deposits (Paleocene and lower 

and middle Eocene)

Within the study area, the pre-impact Tertiary deposits

consist of shallow-shelf to marginal-marine facies that

are characteristically thinly stratified, shelly, glauconitic,

clayey sands and silts, and include the Brightseat, Aquia,

Nanjemoy, and Piney Point Formations and the Marlboro

Clay. The pre-impact lower Tertiary deposits dip eastward to northward and are higher and thinner near the

Virginia-North Carolina border.
Geologic Framework of Southeastern Virginia, South of the James River 33

The Brightseat Formation is a muddy, glauconitic sand

only a few feet thick (Powars and others, 1992). It is biostratigraphically documented only in the Dismal Swamp

corehole (2) (pl. 1) on the basis of Foraminifera (T.G.

Gibson, formerly USGS, written commun., 1988), calcareous nannofossils (L.N. Bybell, USGS, oral commun.,

1988), and dinoflagellates (L.E. Edwards, USGS, written

commun., 1988). 

 The Aquia Formation (upper Paleocene) consists of

massive to thinly stratified, black to greenish-black to

light-greenish-gray clayey and silty, fine to coarse glauconitic quartz sands, with variable amounts of shells,

microfossils, mica, pyrite, lignitic material, and calciumcarbonate cemented layers and concretions. The glauconitic sand typically is found floating in a clay-silt matrix,

and some intervals have abundant burrows. Quartz and

phosphatic pebbles and very coarse glauconitic quartz

sand mark the base of the unit. South of the James River,

a few hard streaks of shells or thin “rock” layers are fairly

abundant and show up as sharp peaks on the resistivity

logs. Drillers’ logs use a variety of lithic descriptions for

this unit such as a “black sand” or “black pepper sand” or

“marl” or “clay and shell” or “shell rock.” 

South of the James River, the Aquia Formation ranges

in thickness from 30 to 60 ft at Hog Island (60, 58) to

21 ft in the town of Rescue (45), 13 ft in the ChesapeakePortsmouth Airport corehole (21), 19 ft in the Dismal

Swamp corehole (2), 42 ft in the Fentress corehole (12),

and 24 ft in the Chesapeake Northwest River WTP borehole (3). The regional distribution of the Aquia Formation is similar to that of the overlying Marlboro Clay and

Nanjemoy Formations (Powars and Bruce, 1999). 

A distinctive suite of micro- and macrofossils (for

example, the brachiopod Oleneothyris harlani) found in

the lower part of the Aquia in the Fentress corehole indicates a late Paleocene age and equivalence to the Piscataway Member (Powars and others, 1992). Dinocyst data

(analyzed by L.E. Edwards) also establish that the entire

unit in the Dismal Swamp corehole and the upper part of

the Aquia in the Fentress corehole (12) are equivalent to

the updip outcrop Paspotansa Member of the Aquia

(Powars and others, 1992). A summary of the chronostratigraphic data is included on the stratigraphic columns of the Dismal Swamp (2) and Fentress (12) coreholes (pl. 1) and on the stratigraphic column of the

Chesapeake-Portsmouth Airport corehole (21) (Powars

and Bruce, 1999, MW4-1 corehole, pl. 5). These Aquia

micro- and macrofossil assemblages are also found as

intraclasts in the Exmore tsunami-breccia deposits. 

The resistivity log signatures from the three key coreholes (2, 12, and 21) within the study area are typical for

the Aquia Formation and reflect the sandy nature of the

section. Thin calcium-carbonate-cemented layers are

reflected on the resistivity log by sharp peaks. The

gamma readings reflect the variable amount (20-75 percent) of glauconite (and some phosphate) found in the

Aquia strata, which generally causes deflections to the

right, indicating increasing radiation. 

It is difficult to differentiate the Aquia strata from the

underlying Upper Cretaceous marine deposits south of

the James River. Differentiating the Aquia from the

Exmore tsunami-breccia outside the outer rim also is difficult because these units are thin and have similar lithologic and borehole signatures. Powars and Bruce (1999)

suggested that differentiation might be aided by noting

the vertical stacking order of the units. They pointed out

that the Marlboro Clay and Nanjemoy Formation are

most likely underlain by the Aquia Formation whereas

the Chickahominy Formation is most likely underlain by

the Exmore tsunami-breccia. The distinctive, nearly flat

resistivity signature of the homogeneous, clayey Chickahominy Formation has thus far only been found overlying

the Exmore tsunami-breccia. This signature contrasts

well with the more irregular, curvy resistivity signature of

the Nanjemoy and Marlboro Clay logs above the Aquia

Formation; this irregular, curvy signature is caused by

variations in the sand content and the sand-filled burrows

in the Marlboro Clay.

The Marlboro Clay (upper Paleocene and lowermost

Eocene) ranges in thickness from 8 ft in the ChesapeakePortsmouth Airport corehole (21), to 16 ft in the Dismal

Swamp corehole (2), 10 ft in the Fentress corehole (12),

and 14 ft in the Chesapeake Northwest River WTP borehole (3). It consists of light-gray to pinkish-gray and reddish-brown kaolinitic clay and is found consistently

outside the crater between the glauconitic Aquia and

Nanjemoy Formations. The clays are massive to thinly

laminated with silt and very fine, micaceous-rich sands.

The contact between the Marlboro Clay and underlying

Aquia is gradational, whereas a sharp, burrowed contact

exists between the Marlboro Clay and the overlying Nanjemoy beds. The contrast in lithology and color between

the Marlboro Clay and the underlying and overlying

glauconitic units facilitates identification. Resistivity

logs reflect the very clayey nature of the Marlboro; however, the lowest Nanjemoy beds are often described as

gray clay, making differentiation between these two units

difficult.
34 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

The Nanjemoy Formation (lower Eocene) ranges in

thickness from about 3 to 40 ft south of the James River.

Powars and Bruce (1999) described its regional distribution pattern as similar to those of the Aquia and Marlboro

Clay Formations. The Nanjemoy Formation consists of

massive to thin-bedded, dark-olive-gray, greenish-gray,

and olive-black, variably clayey, silty, fine to coarse glauconitic-quartz sand, with varying amounts of shells,

microfossils, mica, lignitic material, pyrite, and goethite.

The unit is characteristically intensely burrowed (including clay-filled, clay-lined, and sand-filled types) and contains several fining-upwards sequences that are usually

capped with a sandy clay-silt. South of the James River,

the Nanjemoy section contains thin deposits of both the

Woodstock and Potapaco members (Powars and others,

1992). The Nanjemoy has a sharp, burrowed contact with

the underlying Marlboro Clay.

The Nanjemoy Formation contains a distinctive suite

of early Eocene micro- and macrofossils (Powars and

others, 1992). Dinocyst data (analyzed by L.E. Edwards,

USGS) from the Dismal Swamp corehole (2) indicate

that the Nanjemoy Formation includes Beds A and B of

the Potapaco Member (Powars and others, 1992).

Dinocyst data (also analyzed by L.E. Edwards) from the

Fentress corehole (12) indicate that the Nanjemoy Formation consists of 8 ft of the Woodstock Member, which

consists of poorly sorted, black, fine to coarse, rounded,

glauconitic sand in a muddy matrix.

At Hog Island in the northwestern corner of the study

area, the Nanjemoy Formation appears to be overlain by

the shelly, glauconitic Piney Point Formation. A highresistivity signature is characteristic of the Piney Point

and distinguishes it from the underlying Nanjemoy.

Along the northern side of the study area (adjacent to and

stepping into the CBIC), the Nanjemoy is overlain by a

patchy distribution of shelly, sandy, Oligocene and/or

lower Miocene deposits that have distinctively highresistivity and high gamma-log signatures. 

In the Fentress corehole (12) (pl. 1), the Nanjemoy is

overlain by 15 ft of coarser glauconitic sand containing

middle or late Eocene or early Oligocene dinocysts (L.E.

Edwards, written commun., 1989). The middle and late

Eocene dinocysts are probably reworked into lower Oligocene deposits (Powars and others, 1992). The very

clayey upper Miocene St. Marys Formation overlies the

Nanjemoy farther west, in the Dismal Swamp corehole

(2). The clayey nature of these upper Miocene deposits

exhibits a fairly uniform low resistivity signature; therefore, they are readily distinguishable from the Nanjemoy.

The thin sandy basal beds of the Calvert and St. Marys

Formations often contain phosphate, which causes a

high-radiation peak on the gamma log. The Nanjemoy's

variable percentage of glauconite and phosphate (20 to 70

percent) creates high-radiation gamma-log signatures,

similar to the Aquia Formation and the thin basal beds of

the Oligocene to middle Miocene deposits. 

The Piney Point Formation (middle Eocene) is a richly

fossiliferous, olive-gray to grayish-olive-green, poorly

sorted, medium to coarse, glauconitic quartz sand that

commonly contains interbedded calcium-carbonatecemented sand to shelly sand and moldic limestone (hard

"shell rock" with voids). The sand contains varying

amounts of clay, silt, shells, microfossils and glauconite

(25 to 50 percent). The Piney Point Formation is preserved mainly north of the James River, and its present

distribution is due to the immense erosional power of the

impact blast and subsequent train of tsunamis that largely

shaped the upper surface of the Piney Point. The Piney

Point is present only in the northwestern corner of the

study area, beneath Hog Island. Its thickness varies from

7 to 17 ft in wells on Hog Island. On the north side of the

James River, 6.6 ft of the Piney Point strata were documented in the Jamestown corehole, which represents the

only biostratigraphically dated section of this unit close

to the study area (Powars and Bruce, 1999).

High resistivity-log signatures are characteristic of the

interbedded sand and limestone of the Piney Point Formation (Powars and Bruce, 1999). Piney Point strata at

Hog Island appear to be overlain by Oligocene glauconitic sands and shelly sands and are underlain by lower

Eocene glauconitic clayey sands of the Nanjemoy Formation. These interpretations are based largely on correlation with the Jamestown corehole. 

Syn-impact Deposits (upper Eocene)

The syn-impact deposits are divisible into two principal depositional units: the Exmore tsunami-breccia (the

upper deposit) and the CBIC megablock beds (the lower

deposit). The Exmore tsunami-breccia overlies either the

CBIC megablock beds or the relatively undisturbed or little disturbed Upper and Lower Cretaceous deposits

(fig. 2). The Exmore tsunami-breccia has a highly variable lithology, which consists of an overall finingupwards sequence of pebble- to boulder-size intraclasts

in a gray, shelly, clayey and silty, fine to granular, glauconitic sand matrix (partially sublithified). The abundant

intraclasts contain fauna and flora from Lower Cretaceous (Albian), Upper Cretaceous (Cenomanian, Santonian, Campanian, and Maestrichtian), Paleocene, and
Geologic Framework of Southeastern Virginia, South of the James River 35

lower, lower middle, and upper Eocene deposits (Powars

and others, 1992; Powars and Bruce, 1999). The clasts

consist of a wide variety of lithologies and sizes and are

rounded to angular, mostly deformed fragments up to

6.5 ft in diameter. They occur isolated to amalgamated,

and are composed of soft, friable, marine to fluvial-deltaic sands and clays, hard silty clays, and indurated sands

and bioclastic limestones. As expected, these clasts display a wide variety of colors, from black to various grays

and greens to oxidation colors (red, purple, yellow, and

brown). 

Core samples of the Exmore tsunami-breccia contain

trace amounts of shocked quartz, a high-pressure mineral

produced only by the high pressures created by impact

events (Powars and Bruce, 1999). Koeberl and others

(1996) also found abundant, centimeter-sized fragments

of melt rock and scattered clasts of crystalline basement

that contain many quartz and feldspar deformation features in core samples.

The syn-impact Exmore tsunami-breccia ranges in

thickness from 30 to 110 ft outside the outer rim of the

crater to about 125 to 664 ft a short distance inside the

outer rim (50, 51, 53). Farther into the crater, seismic

data (recorded in two-way traveltime) indicate that the

unit is 1,200 to 1,300 ft thick and up to approximately

2,300 ft thick inside the crater’s inner basin (Powars and

Bruce, 1999). It abruptly thickens across the faulted

outer rim of the crater and across the faulted peak ring of

the inner basin but generally thins above the variably

uplifted peak ring (fig. 2)

The Exmore tsunami-breccia is overlain by the clayey

post-impact Chickahominy Formation, which also is

found only inside the disruption boundary that separates

pre-impact units from syn-impact units. Core data indicate that this stratigraphic contact is gradational and that

the Exmore tsunami-breccia deposits are capped by a thin

(2 to 20 ft), olive-gray to brownish-purple, very clayey

unit that contains abundant, coarse-grained to fine pebble-size intraclasts and mixed fauna similar in lithology

to the Chickahominy Formation. Except for this thin clay

cap, the characteristic flat resistivity signature of the

Chickahominy strata is easily differentiated from the

irregular resistivity signature typical of the rest of the

Exmore tsunami-breccia (Powars and Bruce, 1999).

Within the study area, the Exmore tsunami-breccia

deposits are underlain by pre-impact Upper and Lower

Cretaceous deposits outside the outer rim of the crater

and by the CBIC megablock beds inside the outer rim.

The location of the disruption boundary beneath the

irregular, near-surface outer rim is uncertain, and undisturbed Lower Cretaceous deposits may extend far into the

annular trough. Powars and Bruce (1999) pointed out

that differentiating the Exmore tsunami-breccia from the

Lower Cretaceous fluvial-deltaic deposits is difficult, but,

in general, the Lower Cretaceous deposits have blockier,

thicker stratified, resistivity and gamma log signatures

than the more subdued signatures of the thinner stratified

Exmore tsunami-breccia deposits.

Powars and Bruce (1999) defined the CBIC

megablock beds as a seismic-stratigraphic unit found

inside the disruption boundary. This unit represents all

the deposits (700 to 2,500 ft thick) found in the annular

trough beneath the Exmore tsunami-breccia and above

the basement rocks. The syn-impact CBIC megablock

beds are interpreted to consist primarily of Lower Cretaceous fluvial-deltaic deposits that slumped into the crater

during an early stage of crater filling and covered the

floor (basement surface) of the annular trough (Poag,

1996, 1997; Powars, and Bruce, 1999). 

Post-impact Deposits

The post-impact deposits consist of approximately

1,300 to 1,600 ft of upper Eocene to Holocene deposits

that buried the crater and the syn-impact deposits (fig. 2;

Powars and Bruce, 1999, pl. 7a). Except for some Quaternary fluvial-estuarine deposits, most of the postimpact deposits are marine clays, silts, and very fine to

very coarse sands that may include diatomaceous, glauconitic, shelly, and thin calcium carbonate indurated intervals. Microfauna and macrofauna indicate a marine to

restricted marine origin. 

Post-impact deposits include, in ascending order: the

very clayey Chickahominy Formation (upper Eocene);

the glauconitic, phosphatic, and partly shelly Delmarva

beds (lower Oligocene) and Old Church Formation

(upper Oligocene); the shelly and sandy beds of the Calvert Formation (lower Miocene); the primarily siliciclastic, fine-grained Calvert (middle Miocene), the St. Marys

(upper Miocene) and lower Eastover (upper Miocene)

Formations; the siliciclastic, locally glauconitic, fine- to

coarse-grained fossiliferous upper Eastover (upper

Miocene), Yorktown (lower and upper Pliocene), and

Chowan River (upper Pliocene) Formations; and various

fluvial to estuarine Quaternary and Holocene units. 
36 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Upper Eocene, Oligocene, and lower Miocene 

Post-impact Deposits

The upper Eocene Chickahominy Formation is the

first post-impact deposit and conformably overlies the

Exmore tsunami-breccia. Within the study area, this unit

ranges in thickness from 55 ft (32) to 227 ft (51). The

Chickahominy Formation consists of massive to thinbedded, olive-gray, very compact, dry, micaceous, clayey

silt to silty clay (Powars and Bruce, 1999). It contains

variable amounts of fine-sand to silt-sized, primarily

black to dark green glauconite, shells including solitary

corals, abundant iron sulfides, and pyrite. The abundant

microfauna impart a white-speckled appearance to some

intervals. The entire unit contains a wide variety of

locally scattered to extensive burrows. The lower part of

the section coarsens downward to very fine-to-fine sand

and contains pebbles and reworked microfauna from

Upper Cretaceous through middle Eocene deposits

(Powars and others, 1992).

The Oligocene to middle Miocene post-impact deposits exhibit a patchy, very thin distribution pattern south of

the James River. These deposits appear to be preserved

adjacent to the outer rim and to thicken inside the crater

along the northern boundary of the study area (Powars

and Bruce, 1999). In the Fentress corehole (12) (pl. 1),

dinocyst data (L.E. Edwards, USGS, written commun.,

1989) indicate the presence of 15 ft of lower Oligocene

Delmarva beds consisting of black, very fine, glauconitic

and phosphatic sand in an olive-brown clay-silt matrix

(Powars and others, 1992). Therefore, the unit is most

likely locally present along the Virginia Beach coastal

area. 

The upper Oligocene deposits are found in the

Jamestown corehole (61) and probably are also present in

the northwest corner of the study area. However, they

were not encountered in the Dismal Swamp (2) or Fentress coreholes (12) and, therefore, probably are absent

over most of the study area outside the crater.

Powars and Bruce (1999) presented biostratigraphic

and strontium-isotope data that document the presence of

lower Miocene marine bioclastic sand deposits in the Virginia Coastal Plain, which they informally named the

Newport News unit of the Calvert Formation. Strontiumisotope analysis of shells from these deposits indicates

that they range in age from about 20.1 to 17.1 Ma. 

Within the study area, the thinness and local absence

of the Oligocene and lower Miocene strata, combined

with their general lithologic similarity, make it difficult to

correlate these deposits between cores. Therefore, in this

report the thin lower Oligocene Delmarva beds are combined with the overlying upper Oligocene Old Church

Formation and lower Miocene Newport News unit of the

Calvert Formation into a single stratigraphic unit. The

shelly, sandy, Oligocene and/or lower Miocene deposits

characteristically have high-resistivity and high gammalog signatures, which contrast with the characteristic

low-resistivity signatures (deflection to the left) of the

underlying fine-grained Nanjemoy or Chickahominy

Formations and overlying finer grained Calvert or St.

Marys Formations. 

 Middle and upper Miocene, Pliocene, and 

Quaternary Post-impact Deposits

Post-impact deposits from numerous marine transgressions and regressions across the lower mid-Atlantic

Coastal Plain have covered the study area with a generally eastward-dipping wedge of sediments that ranges

from 225 to 750-ft-thick outside the crater. These deposits thicken and dip into the crater, becoming up to 1,300

to 1,600 ft thick. Miocene and Pliocene shallow-shelf to

nearshore marine deposits account for most of the section

and are overlain by relatively thin Quaternary fluvialestuarine deposits. 

Most of the post-impact middle and upper Miocene

deposits cored in Virginia’s central to outer Coastal Plain

have been found to consist of fining-upward sequences

that have thin basal sands overlain by thick, very fine

grained clays, silts, and sands with scattered shell material. These deposits consistently display a low-resistivity

signature except along and adjacent to the outer rim of the

crater, where the deposits become coarser grained, which

results in higher resistivity signatures. In general, the

uppermost part of the upper Miocene Eastover Formation

and parts of the Pliocene Yorktown Formation have

shelly sands that produce high-resistivity signatures

throughout the study area.

The middle Miocene Calvert Formation is absent from

the Dismal Swamp corehole (2) and consists of 12 ft of

shelly sand in the Fentress corehole (12); northeast of the

study area (within the crater) it thickens to 458 ft of primarily very fine grained sediments in the Kiptopeke corehole (64). A thin section of Calvert Formation is,

therefore, locally present along the eastern side of the

study area. The thicker beds in the crater contain sparse

to abundant diagnostic diatoms that are easily visible

using a 10X hand lens, and were not visible in the Fentress core. Similar to the other thin pre-impact and post-
Geologic Framework of Southeastern Virginia, South of the James River 37

impact units, differentiation of thin units is very difficult

between cores.

The St. Marys Formation is the oldest post-impact unit

that extends across the entire study area and progressively

onlaps and truncates older units west to southwest of the

crater. The St. Marys appears to be thickest in the southern part of the crater and south of the crater in the Norfolk

to Virginia Beach area (Powars and Bruce, 1999). In the

Dismal Swamp corehole (2), Foraminifera (T. Gibson,

written commun., 1987) from a thin, benthic-foraminifera-rich, brownish gray, sandy clay indicates that the

upper Miocene St. Marys Formation overlies the Nanjemoy Formation. The St. Marys consists of up to 330 ft

(51) of mostly muddy, very fine sand and sandy clay and

silt containing scattered shells, abundant iron sulfide, and

finely disseminated organic material. 

Ostracode (T. Cronin, USGS, written commun., 1995)

and Foraminifera (S. Ishman, formerly with the USGS,

written commun., 1995) data from the upper part of the

Eastover Formation in the Dismal Swamp (2) and Fentress (12) coreholes indicate the presence of a thick

younger section that has no correlative units in outcrop

(Powars and Bruce, 1999). Powars and Bruce (1999)

reported strontium-isotope ages for the St. Marys strata

that range from about 6.7 to 5.5 Ma, which extrapolates

to foraminiferal zone N17.

The Eastover Formation (upper Miocene) also extends

across the entire study area and ranges in thickness from

140 ft in the Dismal Swamp corehole (2) to 174 ft in the

Fentress corehole (12). The Eastover consists of darkgray to bluish-gray to greenish-gray, muddy fine sand

interbedded with finer and coarser grained beds. The

Eastover is sparsely to abundantly shelly, contains shell

hashes and indurated beds, and is locally glauconitic and

micaceous. Macrofauna and microfauna indicate a shallow-water, marine to restricted-marine depositional environment. The pearly luster, tabular mollusk Isognomon

maxillata is a common species in the upper part of the

Eastover Formation, but not present in the overlying

Yorktown Formation. Throughout the study area, the

lower part of the Eastover Formation consists of a clayey,

fine-grained facies and the upper part is a shelly, coarser

grained, sandy facies. Powars and Bruce (1999) pointed

out that regionally the lower, finer grained facies has

characteristically low resistivity-log signatures (deflection to the left) that show an upward-coarsening trend

into the upper shelly, coarse-grained facies that has characteristically high resistivity-log signatures (deflection to

the right).

The Yorktown Formation (lower and lower upper

Pliocene) extends across the entire study area except

where it has been cut out by Quaternary James River

paleochannels (see Powars and Bruce, 1999). The Yorktown deposits overlie the Eastover strata throughout the

study area and locally are overlain by shallow-marine

deposits of the Chowan River Formation (late Pliocene).

The Yorktown consists of bluish-gray, greenish-gray, and

dark greenish-gray, very fine to coarse sand, in part glauconitic and phosphatic, commonly very shelly and interbedded with gray and blue-gray sandy and silty clay. The

Yorktown also contains abundant microfauna and locally

includes cross-bedded biofragmental lenticular sand bodies.

Subdivision of the Yorktown Formation into its four

members (from oldest to youngest: Sunken Meadow,

Rushmere, Mogarts Beach, and Moore House) is beyond

the scope of this report. Within the study area, however,

outcrops of the Yorktown Formation found along much

of the James River shoreline in Isle of Wight County

expose the upper part of the Rushmere Member and the

Mogarts Beach and Moore House Members. The Yorktown has been documented to range in thickness from

42 ft in the Dismal Swamp corehole (2) to 107 ft in the

Fentress corehole (12). Ostracode data (T. Cronin, written commun., 1995) from these two coreholes indicate

that the Yorktown consists of the Sunken Meadows,

Rushmere, and Mogarts Beach Members. The youngest

Moore House Member may be present at the Fentress

corehole (12) site but was not resolved because of poor

core recovery from the upper section of the Yorktown. 

The Chowan River Formation (upper Pliocene) has a

very limited, irregular distribution across the southeastern part of the study area. It consists of interbedded, silty,

fine sand, clayey silt, and bioclastic sand. The Chowan

River Formation (upper Pliocene) was reported (Gibson,

1983) in the Moores Bridge well (31) at depths from 90

to 115 ft and consists of blue-green fossiliferous clayey

fine sand. It is also reported (Johnson and others, 1987)

1ocally only in a few borrow pits (the Gomez and Yadkin

pits) and in the subsurface of southeastern Virginia.

Exposure of the Chowan River Formation in relatively

shallow borrow pits less than 5 miles south of the NORT-12 well (32) suggests that either a steep gradient exists

between these sites (possibly caused by structural effects)

or that identification was incorrect. A bioclastic, silty,

fine to medium sand found in the Fentress corehole from

-51 to -65 ft elevation is assigned to the Chowan River

Formation. Where present, this unit unconformably

overlies the Yorktown Formation and is truncated by
38 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Quaternary fluvial to estuarine and marginal-marine

deposits. The lithology of the Chowan River is so similar

to the underlying Yorktown Formation that biostratigraphic differentiation is required. No attempt was made

to differentiate the Chowan River from the Yorktown,

however, because of the lack of biostratigraphic data, the

Chowan River’s patchy distribution, and their similar

hydrogeologic characteristics. 

Quaternary strata in the study area include fluvial,

estuarine, marginal-marine, and nearshore-shelf sediments deposited during the Pleistocene epoch. The early,

middle, and late Pleistocene deposits form a step-like

succession of terraces and intervening scarps that parallel

the coast (or the buried outer rim of the crater) and major

streams, thereby dominating the topography of the

Coastal Plain (Johnson and Ramsey, 1987). These terraces decrease in elevation and age towards the coast and

major streams. The Pleistocene and Holocene deposits

consist of light- to dark-gray, blue-gray, to oxidized variegated (brown, yellow, orange, red), interbedded sand,

gravel, silt, clay, and locally include shells and peat. The

estuarine and coast- facing deposits east of the Suffolk

scarp commonly include scattered to dense shell accumulations. The Holocene deposits include estuarine, marsh,

swamp, dune, alluvial and colluvial sediments. 

Across the study area, Pleistocene scarps and paleochannels cut into the older units (Johnson, 1969; Johnson

and Ramsey, 1987; Powars and Bruce, 1999). Near the

mouth of the James River, a paleochannel cuts down to

nearly -160 ft beneath the modern James River (fig. 9B).

Paleochannel fills of the Atlantic Coastal Plain generally

are transgressive sequences consisting, from bottom to

top, of coarse fluvial gravel and sand, estuarine muds,

and fossiliferous lower estuarine to open-bay sand and

sandy mud. The channel fills are overlain by nearshore

shelf and barrier-spit deposits representing deposition

during interglacial highstands (when the sea level is high

due to melting of continental glaciers and partial melting

of the polar ice caps). 

The Pleistocene scarps were formed by fluvial and

estuarine erosion (valley-facing scarps) and shoreline

erosion (coast-facing scarps) caused by changes in sea

level that occurred during the glacial-interglacial period.

The proximity and parallelism of the coast-facing scarps

to the outer rim of the crater, and the stacked nature of

some of the scarps near the outer rim, indicate an influence by episodic differential movement around the buried

outer rim and continued higher subsidence rates inside

the crater. In addition, late Pleistocene and Holocene

deposits appear to be the only surficial units found inside

the buried outer rim of the crater (Powars and Bruce,

1999).

Subdivision of the Quaternary deposits is beyond the

scope of this report. For more information about individual Quaternary units, the reader is referred to the geologic

map of the Virginia Coastal Plain (Mixon, Berquist, and

others, 1989), which shows the surficial distribution and

briefly describes the lithology of the Quaternary units.

CORRELATION OF GEOLOGIC 

UNITS TO HYDROGEOLOGIC UNITS

On the basis of lithology, biostratigraphy, and borehole geophysical logs, the geologic units described in this

report are correlated with the hydrogeologic units of the

Virginia Coastal Plain described by Meng and Harsh

(1988) as part of the Regional Aquifer-System Analysis

(RASA) program (table 2). The 56-mi-wide CBIC truncated the aquifers and confining units (Lower Potomac

through the lower part of the Chickahominy-Piney Point)

that were identified by Meng and Harsh (1988). The St.

Marys-Choptank aquifer, an important aquifer in the

Maryland Coastal Plain, is not present in this area (Laczniak and Meng, 1988), and the Virginia Beach aquifer is

present only south of the James River and Chesapeake

Bay (Hamilton and Larson, 1988). 

The variability of correlation of the geologic and

hydrogeologic units across the study area is shown in figure 13. For example, the Virginia Beach aquifer is correlated to (1) the top of the upper Cenomanian beds, (2) the

glauconitic sands unit, (3) the red beds unit, and (4) the

lower part of the Upper Cretaceous and/or Paleocene (?)

unit. In some places, very clayey or fine-grained strata

correlate with intervals assigned to aquifers instead of

confining units. These correlations call for the revision

of the hydrogeologic units of Meng and Harsh (1988) and

Hamilton and Larson (1988).

Various structural and stratigraphic complexities

related to the CBIC and its burial have altered the hydraulic characteristics of the aquifers and confining units

inside and adjacent to the crater. Regional ground-water

flow paths apparently were altered by emplacement of

the lithically heterogeneous, seawater-saturated Exmore

tsunami-breccia and its burial by fine-grained postimpact deposits in the structural low. The combined

effect apparently resulted in differential flushing of freshwater over and around the primarily fine-grained deposits

inside the crater. The distribution of Virginia's inland
Correlation of Geologic Units to Hydrogeologic Units 39

SYSTEM SERIES

QUATERNARY TERTIARY

Geologic units

this report

Holocene

Pleistocene

Alluvium, swamp, + beach

Tabb Formation

Shirley Formation

Chuckatuck Formation

U

M

L

Windsor Formation

Bacons Castle Formation

Pliocene

UL

Miocene

M

U

L

CHESAPEAKE GROUP

Yorktown Formation Eastover Formation

St. Marys Formation

Calvert Formation Calvert Beach Member Plum Point Member Fairhaven Member

Moore House Member

Mogarts Beach

Member

Rushmere Member

Sunken Meadow

Member

Cobham Bay

Member

Claremont Manor

Member

Unnamed beds2

Charles City Formation

Choptank Formation

(not present in study area)

Old Church Formation

Oligocene

U

Hydrogeologic units

Meng and Harsh

(1988)

Hamilton and Larso

(1988)

Columbia

aquifer

Virginia

RASA

model unit1

Yorktown confining unit

Yorktown-Eastover

aquifer

St. Marys

confining unit

St. Marys-Choptank

aquifer

Calvert

confining unit

AQ10

CU9AQ9

CU8

AQ8CU7

Columbia

aquifer

Yorktown confining u

Yorktown-Eastove

aquifer

St. Marys

confining unit

Calvert

confining unit

Newport News unit

?

ChickahominyPiney Point

aquifer

AQ7 Chickahominy- Piney Point

aquifer

1Meng and Harsh (1988).2Powars, D.S. and Cronin, T., U.S. Geological Survey, unpub. data, 1995.

L Delmarva beds

Table 2. Correlation of geologic units to hydrogeologic units. Modified from Powars and Bruce, 1999[U, upper; M, middle; L, lower; RASA, Regional Aquifer-System Analysis; Fm, formation; AQ, aquifer; CU, confining unit]
40 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

L

Fairhaven Member of the Calvert Fm

Old Church Formation

Delmarva beds Oligocene

Eocene

Chickahominy

Formation

Exmore tsunami-breccia

Piney Point

Formation

NanjemoyFormation

Marlboro Clay

Aquia

Formation

BrightseatFormation

Paleocene

UU

U

L

L

L

M

upper

Cenomanian

beds

Potomac

Formation

Exmore megablock beds

Rift-basin deposits

ChickahominyPiney Point

aquifer

NanjemoyMarlboro Clay

confining unit

Aquia aquifer

Brightseat

confining unit

Brightseat aquifer

Upper Potomac

confining unit

Upper Potomac

aquifer

Mid-Potomac confining unit

Mid-Potomac aquifer

Lower Potomac confining unit

Lower Potomac aquifer

AQ7CU6AQ6

CU3

AQ3

CU3

AQ3

CU2AQ2CU1

AQ1

ChickahominyPiney Point

aquifer

NanjemoyMarlboro Clay

confining unit

Aquia aquifer

Not present

in study area

Virginia Beach

CU4 Virginia Beach confining unitAQ4

Upper Potomac confining u

Upper Potomac aquifer

Mid-Potomac confining u

Mid-Potomac aquifer

Lower Potomac confining u

Lower Potomac aquifer

aquifer

1Meng and Harsh (1988).2Powars, D.S. and Cronin, T., U.S. Geological Survey, unpub. data, 1995.

Upper

Lower

Basement rocks

Newport News unit of the Calvert Fm

Miocene

M

SYSTEM SERIES

TERTIARY CRETACEOUS

Geologic units

this report

Meng and Harsh

(1988)

Hamilton and Larson

(1988)

Virginia

RASA

model unit1

Calvert

confining unit

CU7 Calvert

confining unit

Hydrogeologic units

Plum Point Member of the Calvert Fm

JURASSIC-TRIASSIC

PALEOZOIC AND PROTEROZOIC

Glauconitic sand unit

Red beds

Unnamed

Table 2. Correlation of geologic units to hydrogeologic units. Modified from Powars and Bruce, 1999—Continued
Correlation of Geologic Units to Hydrogeologic Units 41

(A) DISMAL SWAMP COREHOLE

ALTITUDE

(IN FEET)

Sea

level

33

-418

 upper

Cenomanian

 beds

Glauconitic

 sands

Red beds

 Potomac

Formation

-560

-361

Aqiua Formation

Marlboro Clay

-308

-289

-273

-269

-167

St. Marys

Formation

 Eastover

Formation

 Yorktown

 confining unit

-13

-50

 Yorktown 

Formation

Shirley Formation

-222

-167

Nanjemoy Formation

Yorktown-Eastover

 aquifer

 Calvert

confining unit

Chickahominy-

 Piney Point

 aquifer

Tn-Tm conf. unit

-288

-308

upper Potomac

 aquifer

upper Potomac

 confining unit

-684

-600

-422

-381

-366

 Aquia

aquifer

Virginia Beach

 aquifer

VA Beach conf. unit

Geologic units

 this report

 Hydrogeologic units

Hamilton and Larson, 1988

-27

15

-900

-500

-100

-200

-800

 Columbia

 aquifer

Figure 13. Correlation of geologic and 

hydrogeologic units in three key boreholes—

the Dismal Swamp corehole (A), the Fentress 

corehole (B), and the Chesapeake Northwest 

River WTP borehole (C)—showing the variability 

in unit correlation across the study area. 

Hydrogeologic unit contacts include estimates 

from structure contour and isopach maps 

of units by Hamilton and Larson (1988).
42 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

-600

Tabb

 Formation

Chowan River Formation(?)

Yorktown

Formation

Eastover

Formation

St. Marys

Formation

Calvert Formation

Delmarva beds

Nanjemoy Formation

Marlboro Clay

Aquia Formation

Red beds

Glauconitic

 sands

upper

 Cenomanian beds

ALTITUDE

Sea (IN FEET)

level

-172

-346

-65

-52

?

-600

-612

-620

-635

-645

-687

-778

-836

-1,035

15

FENTRESS COREHOLE

Geologic units

 this report

 Hydrogeologic units

Hamilton and Larson, 1988

Yorktown-Eastover

 aquifer

Yorktown conf. unit

VA Beach conf. unit

 Potomac

Formation

 upper

Potomac aquifer

upper Potomac

 confining unit

Virginia Beach

 aquifer

-1,012

-820

-720

-710

-603

Chickahominy-

 Piney Point

 aquifer

-353

 Calvert

confining unit

-54

-79

CHESAPEAKE NORTHWEST 

RIVER WTP BOREHOLE

Yorktown Formation

Eastover Formation

St. Marys Formation

Calvert Formation

Delmarva beds

Nanjemoy Formation

ALTITUDE

(IN FEET) 10

Sea

level Chowan River Formation

Upper Cretaceous

and (or)

Paleocene (?)

Tabb Formation

Aquia Formation

Marlboro Clay

Potomac Formation

upper

Cenomanian

beds

Red beds

Glauconitic sands

-1,057

-845

-795

-715

-608

-584

-570

-555

-540

-521

-306

-170

-30

-70 Yorktown

confining unit -100

-356

 YorktownEastover aquifer

 Calvert

confining unit

-536

-650

Chickahominy-

 Piney Point

 aquifer

-678

VA Beach conf. unit

-875

-1,100

upper Potomac

confining unit

 Virginia

Beach aquifer

upper Potomac

 aquifer

 Hydrogeologic units

Hamilton and Larson, 1988

Geologic units

 this report

-900

-500

-100

-1000

-800

-400

-200 -200

-400

-600

-800

-1000

Columbia aquifer Columbia aquifer

(B) (C)

Tn-Tm Nanjemoy-Marlboro

conf. confining

EXPLANATION

Figure 13. Correlation of geologic and hydrogeologic units in three key boreholes—the Dismal 

Swamp corehole (A), the Fentress corehole (B), and the Chesapeake Northwest River WTP borehole 

(C)—showing the variability in unit correlation across the study area. Hydrogeologic unit contacts 

include estimates from structure contour and isopach maps of units by Hamilton and Larson 

(1988)—Continued.
Summary and Conclusions 43

salt-water wedge coincides with the location of the CBIC.

Prior to the discovery of the CBIC, Focazio and others

(1993) compiled contour maps depicting the quality of

ground water across the Virginia Coastal Plain. Figure 14

shows that an increase in the concentration of total dissolved solids in the middle Potomac aquifer coincides

with the outer rim of the crater. The Brightseat-upper

Potomac and Chickahominy-Piney Point aquifers have

similar contour patterns (Focazio and others, 1993).

The location of the CBIC with respect to the transverse

arch-basin configuration apparently provides a relatively

easy downslope flow pathway around the northeastward

side of the crater toward the axis of the Salisbury Embayment. In contrast, the Norfolk arch, which is close to the

crater on the south side, possibly inhibits flow southeastward around the crater. The abrupt juxtaposition of dissimilar stratigraphic sections north and south of the river

produced by the James River structural zone, and the

indurated red beds associated with the Norfolk arch, probably also inhibits flow southeastward around the crater by

adding barriers to flow paths or by creating more vertical

flow paths. In addition, if the CBIC partially truncated a

portion of the pre-existing structural zone, as Powars and

Bruce (1999) have suggested, then the area along and

adjacent to that truncation would be more faulted and

fractured than other areas and therefore would possibly

provide more barriers or vertical flow paths. 

 Increasing our knowledge of the complex geologic

framework in this area is important for water-resource

management. Alternate water supplies are lacking, and

water utilities in this region have begun to develop

projects that withdraw brackish ground water from just

outside the outer rim of the crater. The results of this study

indicate that the area south of the lower James River is

structurally more complex than other areas adjacent to the

crater because of the CBIC’s effects on the James River

structural zone. These structural complexities apparently

have disrupted the pre-impact aquifers and confining

units; as a result, the physical properties of the groundwater flow system have been altered significantly.

SUMMARY AND CONCLUSIONS

The influence of the Chesapeake Bay impact crater

(CBIC) on the structural, stratigraphic, and hydrogeologic

framework of southeastern Virginia’s Coastal Plain is

complicated by the presence of a pre-impact James River

structural zone. The emplacement of the CBIC and subsequent structural adjustments to this James River structural zone help explain the complex distribution patterns

of post-impact deposits found across the region. 

Litho- and biostratigraphic data from cores and well

cuttings and borehole geophysical log data were compiled

and analyzed to refine the geologic framework of southeastern Virginia south of the James River. Cross sections

were constructed to show the stratigraphic and structural

configuration west of the crater and the configuration of

the south to southwest side of the crater. The western

cross section, which traverses a region that apparently was

little affected by the impact crater, was constructed to

show the existence of the pre-impact James River structural zone. The other two cross sections connect the geologic framework south of the James River to that of the

lower York-James Peninsula; these sections illustrate the

structural and stratigraphic relations of geologic units

inside and adjacent to the outer rim of the impact crater.

The abrupt truncation and variations in thickness of the

various fluvial, deltaic, and marginal marine Cretaceous

units across the James River, combined with the angular

unconformity with the overlying pre-impact lower Tertiary marine deposits, provide evidence of a pre-impact

James River structural zone. Given that sediment deposition and preservation are greater on the down-dropped

side of structural zones, it would appear that the region

north of the James River was relatively depressed during

pollen zone I and relatively elevated for the remainder of

Cretaceous time. In contrast, the region south of the

James River was relatively elevated during pollen zone I

and relatively depressed during the time periods represented by pollen zo nes II, III, IV, and V.

The Upper Cretaceous deposits consisting of pollen

zones III, IV, and V form a wedge that thickens from

377 ft in the Dismal Swamp corehole to 628 ft in the Fentress corehole. The Upper Cretaceous deposits younger

than pollen zone III thicken from 200 to 500 ft across the

southeastern part of the study area. The deposits are thinner, 30 to 150 ft, and have an irregular distribution across

much of the northwestern part of the study area. The

Upper Cretaceous deposits are abruptly truncated along

the northernmost part of the study area. On the western

side of the study area, the truncation appears to be a product of pre-impact movements of the James River structural

zone; on the eastern side of the study area, the CBIC was

responsible for the truncation. An isopach map of the preimpact lower Tertiary deposits shows that these deposits

thicken north of the James River and west of their abrupt

truncation by the CBIC. The variable thickness of these

deposits across the rest of the study area apparently is a
44 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

result of syn- and post-impact structural influences of the

CBIC that caused erosion of uplifted areas. Part of the

pattern also appears to reflect radial fault systems that

were created by the Chesapeake Bay impact crater.

Within the study area, the pre-impact lower Tertiary

deposits dip eastward to northward and are higher and

thinner near the Virginia-North Carolina border. East to

southeast deflections of structure contours on top of the

Upper Cretaceous and pre-impact Tertiary deposits show

possible structural effects; the southernmost deflections

coincide with a previously postulated fracture zone. 

Complex distributions and truncations of post-impact

units across the James River suggest post-impact structural influences of the CBIC on the James River structural zone. The truncation of many of the earlier postimpact units, such as the upper Eocene to middle

Miocene deposits, indicates that the area south of the

James River structural zone was relatively up during that

time or at least prior to deposition of the upper Miocene

St. Marys Formation. 

 The pre-impact Tertiary deposits south of the James

River consist of shallow-shelf to marginal-marine deposits, including the Brightseat Formation (lower Paleocene), Aquia Formation (upper Paleocene), Marlboro

Clay (straddles the Paleocene-Eocene boundary), Nanjemoy Formation (lower Eocene), and Piney Point Formation (middle Eocene). These pre-impact units are

characteristically thin, partly shelly, glauconitic, clayey

sands and silts that are bounded by unconformities. 

The syn-impact deposits consist of the upper Eocene

Exmore tsunami-breccia and the seismically defined

CBIC megablock beds. The Exmore tsunami-breccia

fills much of the relatively flat-floored annular trough

and central basin. The CBIC megablock beds appear to

form a concentric wedge that thins toward the center of

38o30'

76o 77o 77 30' o

37o

0 25 MILES

0 25 KILOMETERS

James

York

River

River

Franklin

Williamsburg

HAMPTON

NORFOLK PORTSMOUTH

CHESAPEAKE

BAY

VIRGINIA ATLANTIC OCEAN

NORTH CAROLINA

1,000

2,500

5,000

1,000

2,500

5,000

INNER RIM

OUTER RIM

Base from U.S. Geological Survey

Geologic Map, 1:250,000,

Mixon, Berquist, and others, 1989

EXPLANATION

1,000 LINE OF EQUAL DISSOLVED-SOLIDS

CONCENTRATION—Dashed where

approximately located. Interval, in

milligrams per liter, is variable

CRATER RIM

Figure 14. Relation of dissolved-solids concentrations in 

the middle Potomac aquifer to the location of the Chesapeake Bay impact crater. (Modified from Smith, 1999.)
References Cited 45

the crater and covers nearly all the rest of the flat-floored

annular trough.

Post-impact upper Eocene to Holocene deposition buried the crater and the syn-impact deposits with approximately 1,300 to 1,600 ft of sediment, which explains the

abrupt thickening of these stratigraphic units into the crater across the northeastern part of the study area. 

The Pliocene and Quaternary deposits have a complex

distribution of lithofacies within 12.5 miles of the crater’s

outer rim, indicating synchronous deposition and deformation. This deformation is attributed to episodic differential movement of the bounding faults associated with

the buried outer rim of the crater and the rotation of slump

blocks near the crater’s perimeter.

The structural and stratigraphic features created by the

impact and adjustments to the James River structural

zone also have influenced our understanding of the

hydrogeologic framework, ground-water flow system,

and regional water quality of the Virginia Coastal Plain.

Pre-impact aquifers and confining units were truncated

and disrupted in and adjacent to the crater. 

Regional flow paths possibly were altered by emplacement of the lithically heterogeneous, seawater-saturated

Exmore tsunami-breccia and its burial by fine-grained

post-impact deposits in the structural low. The result was

differential flushing of fresh water over and around the

primarily fine-grained deposits inside the crater. The

location of the CBIC with respect to the pre-existing

transverse arch-basin configuration also apparently has

provided a relatively easy downslope flow pathway

around the northeastward side of the crater toward the

axis of the Salisbury Embayment. The proximity of the

Norfolk arch to the south side of the crater possibly inhibits flow southeastward around the crater. The distribution

of Virginia's anomalous inland salt-water wedge coincides with the CBIC's location. An increase in concentrations of total dissolved-solids in the middle Potomac,

Brightseat-upper Potomac, and Chickahominy-Piney

Point aquifers coincides with the outer rim of the crater.

In the absence of alternate water supplies, water utilities

in this region have begun to develop projects that withdraw brackish ground water along the edge of the CBIC.

The location and geometry of the outer rim of the

CBIC beneath the lower York-James Peninsula and the

Norfolk to Virginia Beach area are poorly understood,

and additional data are needed to enhance that understanding. The outer rim coincides with an increase in

concentrations of total dissolved solids and chloride;

therefore, the outer rim separates ground water of high

salinity inside the outer rim from fresher water outside

the outer rim. To locate this boundary accurately, we

need additional land-based seismic reflection profiles,

cores, and borehole geophysical logs, especially a sonic

velocity log that would allow for accurate depth correlation between borehole and seismic data. Hydrologic data

for the western side of the crater, such as flow direction,

water quality, and permeability, are non-existent, and

information about the depositional processes associated

with such a large impactor into water-saturated, unconsolidated sediments is sparse. Obtaining cores and

installing observation wells in and around the crater

would help us understand how this impact crater has

affected the regional ground-water resources. This information also is needed to more accurately model and evaluate the ground-water flow and the potential for saltwater

intrusion in the vicinity of the impact crater. As groundwater use increases in the Hampton Roads region and as

public water utilities increasingly rely on brackish-water

aquifers as sources of drinking water, additional information about the CBIC will be needed for future management of these ground-water resources.

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North American tektite strewn field: Geology, v. 22, 

p. 691-694. 

Poag, C.W., Powars, D.S., Poppe, L.J., Mixon, R.B., Edwards, 

L.E., Folger, D.W., and Bruce, S., 1992, Deep Sea Drilling Project Site 612 bolide event—new evidence of a late 

Eocene impact-wave deposit and a possible impact site, 

U.S. East Coast: Geology, v. 20, p. 771-774.

Powars, D.S., and Bruce, T.S., 1999, The effects of the Chesapeake Bay impact crater on the geological framework and 

correlation of hydrogeologic units of the lower YorkJames Peninsula, Virginia: U.S. Geological Survey Professional Paper 1612, 82 p., 7 pl.

Powars, D.S., Bruce, S., Poag, C.W., and Mixon, R.B., 1994, 

Virginia's coastal plain inland salt water wedge-a geohydrological response to the Chesapeake Bay bolide impact 

[abs.]: Geological Society of America, Abstracts with 

Programs, v. 26, p. 410.

Powars, D.S., Johnson, G.H., and Bruce, T.S., 1998, Stratigraphic, structural, and hydrogeological complexities 

related to the outer rim of the Chesapeake Bay impact 

crater [abs.]: Geological Society of America, Abstracts 

with Programs, v. 30, No. 7.

Powars, D.S., Mixon, R.B., and Bruce, S., 1992, Uppermost 

Mesozoic and Cenozoic geologic cross section, outer 

coastal plain of Virginia, in Gohn, G.S., ed., Proceedings 

of the 1988 U.S. Geological Survey Workshop on the 

Geology and Geohydrology of the Atlantic Coastal Plain: 

U.S. Geological Survey Circular 1059, p. 85-101.

Powars, D.S., Mixon, R.B., Edwards, L.E., Andrews, G.W., 

and Ward, L.W., 1987, Evidence for Paleocene and lower 

Eocene pinch-outs on the north flank of the Norfolk Arch, 

Eastern Shore of Virginia [abs.]: Geological Society of 

America, Abstracts with Programs, v. 26, p. 410.

Powars, D.S., Mixon, R.B., Edwards, L.E., Poag, W.C., and 

Bruce, S., 1990, Cross section of Cretaceous and Cenozoic strata, Norfolk Arch to Salisbury Basin, outer coastal 

plain of Virginia [abs.]: Geological Society of America, 

Abstracts with Programs, v. 22, p. 57.

Powars, D.S., Poag, C.W., and Bruce, S., 1991, Uppermost 

Mesozoic and Cenozoic stratigraphic framework of the 

central and outer coastal plain of Virginia [abs.]: Geological Society of America, Abstracts with Programs, v. 23, 

p. 117.

Powars, D.S., Poag, W.C., and Mixon, R.B., 1993, The Chesapeake Bay "impact crater"–stratigraphic and seismic evidence [abs.]: Geological Society of America, Abstracts 

with Programs, v. 25, A-378. 

Reinhardt, J., Christopher, R.A., Owens, J.P., 1980, Lower 

Cretaceous stratigraphy of the core, in Geology of the 

Oak Grove core: Virginia Division of Mineral Resources 

Publication 20, part 3, p. 31-52.

Riddle, P.C., Vaughn, A.W., Lucey, J.K., Kruse, S.E., Johnson, 

G.H., and Hobbs, C.H., 1996, Geophysical studies of 

near-surface deformation associated with the Chesapeake 

Bay impact structure, southeastern Virginia [abs.]: Geological Society of America, Abstracts with Programs, v. 

28, no. 7, A-119.

Robbins, E.I., Perry, W.J., Jr., and Doyle, J.A., 1975, Palynological and stratigraphic investigation of four deep wells 

in the Salisbury embayment of the Atlantic Coastal Plain: 

U.S. Geological Survey Open-File Report 75-307, 120 p.

Sheridan, R.E., Maguire, T.J., Feigenson, M.D., Patino, L.C., 

and Volkert, R.A., 1999, Grenville age of basement rocks 

in Cape May NJ well: new evidence for Laurentian crust 

in U.S. Atlantic Coastal Plain basement Chesapeake terrane: Journal of Geodynamics 27, p. 623-633.

Smith, B.S., 1999, The potential for saltwater intrusion in the 

Potomac aquifers of the York-James Peninsula, Virginia: 

U.S. Geological Survey Water-Resources Investigations 

Report 98-4187, 24 p.

APPENDIXES
50 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units 

of Southeastern Virginia, South of the James River 

Appendix 1. List of boreholes used in this report

[Altitudes are in feet; latitude and longitude are in degrees, minutes, and seconds; VDEQ, Virginia Department of Environmental Quality; VPI, Virginia 

Polytechnic Institute; VEPCO, Virginia Electric Power Company; --, local number not assigned]

Borehole 

location 

number 

on

figure 3

Local 

number

Identifying name, owner, or

organization, and some references Latitude Longitude Surface 

altitude

Bottom 

altitude

1 58A2 VDEQ 36 34 08 76 35 00 58 –1,959

2 58A76 Dismal Swamp corehole (Powars and others,1992;

unpub. data, D.S. Powars, USGS, and T.S. Bruce, 

VDEQ)

36 36 55 76 33 20 33 –1,827

3 61A15 City of Chesapeake-Northwest River Water Treatment 

Plant

36 34 50 76 12 10 10 –1,769

4 62A4 VPI geothermal well # C32 36 36 25 76 00 26 7 –1,014

5 57B6 City of Suffolk 36 42 48 76 39 13 55 –662

6 58B270 VDEQ, Kilby 36 43 20 76 36 55 26 –674

7 58B115 City of Suffolk, City farm 1 36 44 52 76 35 14 30 –986

8 58B11 NAN-P-8 (Brown and others, 1972) 36 44 28 76 33 32 20 –654

9 60B1 Clay Bank Motorlodge (Cederstrom, 1945a) 36 38 11 76 22 22 17 –679

10 60B3 VDEQ 36 38 40 76 20 20 16 –984

11 60B2 CHE-P-5 (Brown and others, 1972) 36 41 49 76 20 19 14 –806

12 61B11 Fentress corehole (Powars and others, 1992; unpub. data, 

D.S. Powars, USGS and T.S. Bruce, VDEQ)

36 42 27 76 07 47 15 –2,005

13 57C17 City of Norfolk, Lake Prince 4 (Cederstrom) 36 48 10 76 39 21 30 –882

14 58C7 City of Norolk, Lake Prince 1 (Cederstrom) 36 48 38 76 37 09 43 –906

15 58C48 City of Norfolk, Well #80 (Cederstrom, 1945a) 36 48 52 76 37 30 54 –926

16 58C10 City of Suffolk, Well #1 36 46 05 76 32 24 24 –611

17 58C5 Well #37 (Cederstrom, 1945a) Drivers-Monogram Farm 36 49 04 76 32 50 20 –520

18 58C51 City of Norfolk, DR Well #1 36 49 00 76 33 10 20 –1,040

19 58C6 Well #8 (Cederstrom, 1945a) Chuckatuck-Cedarbrook 

Farm

36 51 16 76 33 26 15 –535

20 58C8 Nimmo Well, Chuckatuck, Va. 36 52 18 76 31 30 22 –563

21 59C39 MW4-1 corehole (Powars and others, 1992); ChesapeakePortsmouth Airport (this report)36 47 10 76 26 52 17 –983

22 59C28 City of Chesapeake-Bowers Hill-production well #1 36 47 02 76 24 55 21 –979

23 59C2 Virginia Division of Forestry 36 48 08 76 23 15 20 –633

24 59C40 VPI geothermal well #25 36 51 01 76 28 49 22 –1,978

25 59C13 Tidewater Water Co., NAN-P-13 (Brown and others, 

1972)

36 52 18 76 27 47 16 –639

26 60C40 City of Chesapeake IW1 36 47 00 76 22 00 20 –950

27 60C6 Lone Star Cement Corp. 36 48 53 76 17 09 5 –790

28 60C7 City of Portsmouth 36 51 15 76 19 17 10 –1,144

29 60C25 Campbell Soup Co., Well #1 36 51 30 76 18 30 5 –900

30 -- Well #9 (Cederstrom, 1945a) Lamberts Point-Norfolk & 

Western Railway Co.

36 52 26 76 18 56 10 –606

31 -- Well #20 (Cederstrom, 1945a) Moores Bridge 36 52 21 76 12 13 10 –1,730

32 61C1 NOR-T-12 (Brown and others, 1972) 36 52 23 76 12 21 15 –2,563

33 62C5 VDEQ 36 47 15 76 03 15 14 –386

34 62C4 VDEQ 36 47 11 76 06 01 13 –387

35 62C2 VDEQ 36 47 15 76 03 08 14 –386

36 63C1 VB-P-3 (Brown and others, 1972) 36 52 00 75 58 51 5 –1,583
Appendixes 51

37 57D20 City of Virginia Beach, Isle of Wight 2 36 52 32 76 40 56 50 –970

38 57D28 VPI geothermal well #26,Town of Isle of Wight 36 54 29 76 42 07 75 –1,310

39 57D2 Well #81 (Cederstrom, 1945a) Smithfield Ice Plant 36 59 05 76 37 21 10 –311

40 57D1 IW-P-13 (Brown and others, 1972) 36 59 42 76 37 53 40 –414

41 58D3 Well #108 (Cederstrom, 1945a) Carrolton 36 58 02 76 34 48 8 –382

42 58D9 Tidewater Virginia Properties-Graymor Estates 36 57 27 76 31 39 15 –541

43 58D7 Town of Smithfield-Red Point Heights 36 59 12 76 36 50 35 –477

44 58D2 Well #54 (Cederstrom, 1945a) Battery Park Water Co. 36 59 32 76 29 44 13 –333

45 58D6 Rescue Water Company 36 59 39 76 33 30 22 –528

46 -- Well #25 (Cederstrom, 1945a) Lone Star Cement Co., near 

Mogarts Beach

37 00 29 76 36 24 12 –324

47 59D6 CHE-P-11 (Brown and others, 1972) 36 52 41 76 23 17 3 –597

48 59D1 Tidewater Water Co. 36 52 55 76 23 11 15 –573

49 59D20 City of Newport News-City Hall Complex 36 58 40 76 25 50 30 –870

50 60D7 VPI geothermal well #c24 -Willoughby Bay 36 57 27 76 29 19 5 1,030

51 61D5 City of Virginia Beach, ferry slip 36 54 25 76 10 50 11 –1,589

52 62D2 VB-T-4 (Brown and others, 1972) 36 57 59 76 06 47 –35 –1,500

53 60E1 Well #8 (Cederstrom, 1945b, 1957)-Fort Monroe 37 00 05 76 18 25 3 –2,251

54 57E10 VDEQ, Moonlight, Isle of Wight Co. 37 02 36 76 42 59 85 –615

55 -- Well #7 (Cederstrom, 1945a) Burwells Bay 37 03 23 76 40 13 15 –306

56 -- Well #3a (Cederstrom, 1945a) Rushmere 37 04 34 76 40 05 5 –381

57 -- Well #42a (Cederstrom, 1945a) Bacons Castle test well 37 06 10 76 44 13 70 –985

58 57F5 Hog Island Nuclear Power Plant 37 09 50 76 41 52 34 –386

59 57F26 VEPCO 37 09 51 76 41 57 35 –385

60 57F16 Hog Island (unpub. data, T.S. Bruce, VDEQ, and 

D.S. Powars, USGS)

37 11 33 76 40 53 5 –1,235

61 -- Jamestown corehole (unpub. data, D.S. Powars, USGS) 37 13 05 76 46 37 1 –272

62 58F50 Newport News Park 1 corehole (Meng and Harsh, 1988; 

unpub. data, T.S. Bruce, VDEQ, and D.S. Powars, 

USGS)

37 12 08 76 34 11 55 –1,423

63 58F67 Newport News Park 2 corehole (unpub. data, T.S. Bruce, 

VDEQ, and D.S. Powars, USGS)

37 12 08 76 34 11 52 –570

64 63F50 Kiptopeke corehole (Powars and others, 1992;

unpub. data, D.S. Powars, USGS, and T.S. Bruce,

VDEQ)

37 08 07 75 57 08 7 –1,993

651 -- Airfield Pond corehole (unpub. data, J.S. Schindler, R. 

Weems, and D.S. Powars, USGS)

36 54 48 77 01 28 91 –130

661 -- Mann Tract Monitor well #1 (unpub. data, D.S. Powars, 

USGS, and T.S. Bruce, VDEQ)

37 26 21 76 40 42 90 –1,225

671 66M1 Taylor #1, Oil test well 37 53 03 75 31 01 42 –6,237

681 -- Haynesville corehole (Mixon, Berquist, and others, 1989) 37 57 14 76 40 10 87 –469

691 -- Oak Grove corehole (Reinhardt and others, 1980) 38 10 10 77 02 19 180 –1,180

Appendix 1. List of boreholes used in this report—Continued

[Altitudes are in feet; latitude and longitude are in degrees, minutes, and seconds; VDEQ, Virginia Department of Environmental Quality; VPI, Virginia 

Polytechnic Institute; VEPCO, Virginia Electric Power Company; --, local number not assigned]

Borehole 

location 

number 

on

figure 3

Local 

number

Identifying name, owner, or

organization, and some references Latitude Longitude Surface altitude

Bottom 

altitude

1Borehole lies outside the study area and is not shown on figure 3.
52 The Effects of the Chesapeake Bay Impact Crater on the Geologic Framework and the Correlation of Hydrogeologic Units of Southeastern Virginia, South of the James River Appendix 2. Altitudes of the tops of stratigraphic units [Altitudes are in feet; ?, unit present, contact uncertain; --, unit not present; nd, no data available; ??, insufficient data; *, contact extrapolated from compilation 

Fm., Formation]

Borehole 

location 

number 

on 

figure 3

Potomac 

Fm.

Upper 

Cenomanian

beds

Glauconitic

sand

unit

Red

beds

Aquia 

Fm.

Marlboro 

Clay

Nanjemoy 

Fm.

Piney 

Point

Fm.

Exmore 

tsunamibreccia 

deposit

Chickahominy 

Fm.

Delmarva 

beds

Old 

Church 

Fm.

Calvert 

Fm. 

Newport 

News 

unit

Calvert 

Fm.

St. 

Marys 

Fm.

E

1 –589 –438 –339 –292 –262 –252 –242 -- -- -- -- -- -- -- –122 –2 –560 –418 –361 –308 –289 –273 –269 -- -- -- -- -- -- -- –167 –3 –1,057 –845 ?–795 ?–715 ?–584 ?–570 ?–555 -- -- -- ?–540 ?? ?? ?–521 ?–306 –4 nd –985 –922 –815 ?–771 ?–754 ?–738 -- -- -- ?–721 ?? ?? ?? ?–318 ?–56 –452 –294 -- -- ?–279 ?–268 ?–231 -- -- -- -- ?? ?? -- ?–118 ?7 –448 –324 -- -- –306 –290 –260 -- -- -- -- -- ?–233 -- ?–127 ?8 –550 ?–378 -- -- ?–365 ?–352 ?–340 -- -- -- -- -- ?? ?? ?–202 ?9 nd ?–599 –543 –495 –432 –420 –408 -- -- -- -- -- ?? ?–346 –205 

10 –808 –644 –602 –504 –471 –452 –441 -- -- -- ?? -- ?? ?–390 –222 11 –728 –626 -- –564 –508 –500 –464 -- -- -- ?? -- ?? ?–416 –198 12 –1,035 –836 –778 –687 –645 –635 –620 -- -- -- –612 -- -- –600 –346 –13 –375 –294 -- -- –254 –242 ?–215 -- -- -- -- -- -- -- ?–110 14 ?–365 ?–313 -- -- ?–287 ?–267 ?–253 -- -- -- -- -- -- -- ?–143 ?15 –373 ?–318 -- -- ?–286 –269 –241 -- -- -- -- -- -- ?? ?–140 ?16 –458 ?–346 -- -- ?–326 ?–300 ?–282 -- -- -- -- -- ?? ?? ?–156 ?17 –489 –334 -- -- ?? ?–295 ?–285 -- -- -- ? ? –271 -- ? 18 –454 –336 -- -- –320 ?–297 ?–269 -- -- -- -- -- ?? ?? ?–120 19 –535 –415 -- -- ?–365 ? ? -- -- -- nd nd nd nd nd 20 ?–410 ?–313 -- -- ?–278 ?–268 ?–260 -- ?? ?? ?? ?? ?? ?? ?–152 ?21 –540 –397 -- -- –384 –372 –350 -- -- -- -- ?? –339 –325 –186 –22 –526 –407 –386 –378 –367 -- -- -- ? ? –349 –330 –186 –23 –534 ?–446 ?–423 ?–388 –366 ?–352 ?? -- -- -- ?? ?? ?? ?? ?–227 ?24 *–486 –378 -- -- –342 –333 –316 -- -- -- ?? ?? ?? ?–281 ?–166 ?25 –467 ?–391 -- -- ?–360 ?–347 ?–344 -- ?? ?? ?? ?? ?–334 ?–304 ?–194 26 –604 –449 -- -- –433 –415 –382 -- -- -- ?? ?? ?? ?? ?–242 27 ?–735 ?–572 -- -- ?–525 ?–511 ?–495 -- 27 17 ?? ?? ?? ?–459 –275 ?–28 ?–640 –487 -- -- –453 –448 –430 -- ?? ?? ?? ?? ?–418 ?–403 ?–254 ?29 ?–610 –515 -- -- –475 –462 –450 -- ?? ?? ?? ?? ?–439 ?–409 ?–255 ?–30 nd ?–524 -- -- ?? ?? ?? -- ?–397 ?–387 ?? ?? ?? ?? ?–254 31 ?–775 –705 -- -- -- -- -- -- ?–655 ?–625 ?–615 ?? ? ? ?–250 32 ?–777 ?–701 -- -- -- -- -- -- ?–650 ?–610 ?–606 ?? ?–595 ?–540 ?–275 ?–33 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd ?–34 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd ?–35 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd ?–36 –1,110 ?–949 -- ?–859 -- -- -- -- ?? ?? ?? ?? ?? ?–700 –435 ?–
Appendixes 53

37 ?–363 ?–251 -- -- –226 –212 ?–180 -- -- -- -- -- -- -- –100 38 –375 –263 -- -- –231 –220 –190 -- -- -- -- -- ?? ?–172 ?–97 39 ?? –280 -- -- ? ? ?–230 -- -- -- nd nd nd nd nd 40 –356 ?–300 -- -- ?–258 ?–238 ?–218 ?? ?? ?? ?? ?? ?? ?–210 ?–124 41 ?? ?–325 -- ?–252 ? ? ? nd nd nd nd nd nd nd nd 42 ?–425 ?–298 -- -- –280 –262 –245 -- ?? ?? ?? ?–241 ?–235 ? –182 43 ?–352 ?–247 -- -- –227 –219 ?–216 -- ?? ?? ?? ?? ?? ? –144 44 ?? ?–297 -- –272 –247 ? ? nd nd nd nd nd nd nd nd 45 ?–418 ?–305 -- –276 –255 –246 –208 -- -- -- nd ? ?–191 ? –149 46 ?? ?–283 -- -- –253 ? ? nd nd nd nd nd nd nd nd 47 ?–568 ?–433 -- -- ?–413 ?–405 ?–398 -- ?? ?? ?? ?? ?? ?–373 –227 48 ?–540 ?–423 -- -- ?–390 ?–385 ?–379 -- ?? ?? ?? ?? ?–367 ?–344 –210 49 –700 -- -- -- -- -- -- -- –574 –410 –392 –378 –368 –323 –228 50 nd -- -- -- -- -- -- -- –769 –654 –623 –613 –603 –540 –240 51 -- -- -- -- -- -- -- -- –925 –739 ? ?–709 ?–698 –647 ?–317 ?52 -- -- -- -- -- -- -- -- ?–910 –830 ? ? –810 ?–508 ? –290 ? 53 ?? -- -- -- -- -- -- -- –837 –637 ?–628 ?–618 –607 ?–400 –292 54 ?–265 ?–232 -- -- ?–215 ?–204 ?–175 -- -- -- -- -- -- -- –172 55 –290 -- -- -- ? ? –245 nd nd nd nd nd nd nd nd 56 ?–355 ?–295 -- -- ? ? –225 -- -- -- ? ? ?–214 nd nd 57 –334 –250 -- -- –231 ? ? ? -- -- ? –169 –167 -- –105 58 –333 -- -- -- –270 –254 –212 –205 -- -- –196 –187 –168 ? –162 –92 59 ?–355 -- -- -- ?–293 ?–277 –223 ?–218 -- -- ?? ?? ?? ?? ?–106 60 –330 -- -- -- –300 –286 –232 –215 -- -- ? –200 –185 –166 –125 61 –258 -- -- -- –217 –203 –159 –152 -- -- -- –139 –120 ?–115 –55 63 –411 -- -- -- -- -- -- -- –357 –323 –308 –293 –279 –216 –127 64 -- -- -- -- -- -- -- -- –1,279 –1.077 –1,063 -- ?–1,028 –613 –455 –

Appendix 2. Altitudes of the tops of stratigraphic units—Continued[Altitudes are in feet; ?, unit present, contact uncertain; --, unit not present; nd, no data available; ??, insufficient data; *, contact extrapolated from compilatioFm., Formation]

Borehole 

location 

number 

on 

figure 3

Potomac 

Fm.

Upper 

Cenomanian

beds

Glauconitic

sand

unit

Red

beds

Aquia 

Fm.

Marlboro 

Clay

Nanjemoy 

Fm.

Piney 

Point

Fm.

Exmore 

tsunamibreccia 

deposit

Chickahominy 

Fm.

Delmarva 

beds

Old 

Church 

Fm.

Calvert 

Fm. 

Newport 

News 

unit

Calvert 

Fm.

St. 

Marys 

Fm.
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St. Marys 

Upper

and

lower

Yorktown 

TERTIARY

SYSTEM

AND SERIES

FORMATION OR

EQUIVALENT

DISMAL SWAMP COREHOLE (2)

QUATERNARY Pleist.

Pliocene

Shirley 

Eastover 

Upper

Miocene

Nanjemoy7

Marlboro Clay

Aquia8

Brightseat 

Eocene Lower

Lower

Upper Paleocene

Upper

Red beds

Glauconitic sand unit

Upper Cenomanian beds

CRETACEOUS

Lower Potomac

Upper

 Jurassic-Triassic (?) rift basin rocks 

Total depth = -1,827 feet

x

x

x

x

SP 16" 64"

-1,700

-1,800

LITHOLOGY1 Altitude, 

in feet

GAMMA

-RAY LOG

N18–

N209

FORAM.2

ZONE

N239

N17–

N189

N16

P6b

P6a?

P5

P1cNot zoned

-100

-200

-400

-300

-500

-600

-700

-800

-900

-1,000

-1,100

-1,200

-1,300

-1,400

Sea

level

33

-1,500

-1,600

DINOCYSTS5

POLLEN3

ZONE

zone IV

zone V

?

zone III?

zone II

zone I?

SPONTANEOUS

 POTENTIAL AND

RESISTIVITY LOGS

Not zoned

OSTRA-6

CODES

Orionina 

vaughani AZ

Pterygocythereris

inexpectata AZ

DIATOMS4

AND

Yorktown

Formation

(?)Yorktown

Formation

Barren of

diatoms

Barren of

diatoms

Paleocene Early

Paleocene

-Eocene

boundary

Eocene

Early

Miocene

SYSTEM

AND SERIES FORMATION OR EQUIVALENT LITHOLOGY1 GAMMA-RAY LOG

FENTRESS COREHOLE (12)

QUATERNARY

Pleistocene

Upper Tabb 

Upper Chowan River (?)

Upper

and

Pliocene

lower

Yorktown

Eastover 

TERTIARY

Miocene

St. Marys 

CRETACEOUS

Middle Calvert 

Delmarva beds

Nanjemoy 

Marlboro Clay

Aquia 

?

Lower

Lower

Oligocene

Eocene

Paleocene Upper

?

Red beds

Glauconitic sand unit

Upper Cenomanian beds

Upper

Lower

Potomac

N178

N17–

N188

N18–

N208

N218

N238

N13–N14

P20

P9

P6a

P5

? Not zoned

P4

Upper

-900

-200

-400

-300

-500

-600

-700

-800

-1,000

-1,100

-1,200

-1,300

-1,400

Altitude,

in feet

-100

Sea

level

FORAM.2

ZONE

DINO-4

CYSTS

POLLEN3

ZONE

-1,500

-1,600

-1,700

-1,800

-1,900

early Oligocene

early Eocene

barren

late Paleocene

near Eocene

-Paleocene

boundary

barren

zone V

zone IV

zone IV

zone III

zone III

barren

barren

barren

barren

barren

15

SP 64" 16"

SPONTANEOUS

 POTENTIAL AND

RESISTIVITY LOGS

Not zoned

16"

?

Total depth = -2,005 feet

7

6

OSTRA-5

CODES

-No diagnostic

ostracodes

X

 Orionina vaughani 

AZ

? Pteerygocythereis inexpectata AZ

CHESAPEAKE NORTHWEST RIVER WTP BOREHOLE (3)

Eastover

Formation

Not sampled

Not sampled

Not sampled

<

<

<

<

<

<

<

<<

<

<

<

<

<

MPR

MPR

SYSTEM

AND SERIES

FORMATION OR 

EQUIVALENT LITHOLOGY10

GAMMA-RAY LOG

QUATER

 -NARY

Pleistocene and

(or) Upper Pliocene

Upper

and

Pliocene

lower

Yorktown 

Eastover 

TERTIARY

Miocene

St. Marys 

CRETACEOUS

Middle Calvert 

Delmarva beds

Nanjemoy 

Marlboro Clay

Aquia 

Clayey silty quartz sand unit

Lower

Lower

Oligocene

Eocene

Paleocene Upper

Red beds

Glauconitic sand unit

Upper Cenomanian beds

Upper

Lower

Potomac 

Upper

-900

-200

-400

-300

-500

-600

-700

-800

-1,000

-1,100

-1,200

-1,300

-1,400

Altitude, 

in feet

-100

Sea

level

-1,500

-1,600

-1,700

Glauconitic quartz sand unit

Organic-rich clay unit ?

Chowan River (?)

SPONTANEOUS

 POTENTIAL AND

RESISTIVITY LOGS

Upper Cretaceous

and (or)

Paleocene (?)

Total depth = -1,769 feet

SP 64"16"

MPR Tabb 10

SPR

6 Cronin, T., USGS, written

commun., 1995

7 Potopaco Member

8 Paspotansa Member

9 Extrapolated from regional

correlation including strontium

isotope data.

10 Lithology shown is a generalized summary based on a combination 

of lithic and geophysical data from several boreholes at the Northwest 

River Water Treatment Plant (WTP) site. Multipoint (16- and 64-inch) 

and spontaneous potential (SP) logs come from borehole 3 (61A15). 

The gamma log comes from borehole 61A18. Cuttings descriptions 

from borehole 61A15 and 61A18 were used from the surface down to 

- 990 ft. Core descriptions from borehole 61A16 were used from -990 to

-1,524 ft. Drillers logs from borehole 61A2 were also used, especially 

for the interpretation shown for the base of the Tertiary. The geophysical 

logs guided a very generalized interpretation from -1,524 to -1,769 ft.

EXPLANATION

x 

AZ 

SPONTANEOUS POTENTIAL LOG—Negative to the left, positive to the right

SINGLE-POINT RESISTANCE LOG—Resistance increases to the right

MULTIPOINT RESISTIVITY LOG—16- and 64-inch multipoint resistivity log

FORAMINIFERA NEOGENE ZONE

FORAMINIFERA PALEOGENE ZONE

ASSEMBLAGE ZONE

DIATOM SAMPLE

DINOCYST SAMPLE

POLLEN SAMPLE

SP

MPR

+

SPR

<

P

N

CLAY

CLAYEY SILT

SANDY SILTY CLAY TO

SANDY CLAYEY SILT

CLAYEY SILTY SAND

VERY FINE

FINE

MEDIUM

COARSE SAND

VERY COARSE

GRANULES

PEBBLES

SHELLS

TURRITELLA 

LIGNITIC MATERIAL

INDURATED LAYER (GENERALLY 

CALCITE CEMENTED)

BURROWED CONTACT

PALEOSOLS

SHARK TOOTH

 LITHOLOGY 

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

PREPARED IN COOPERATION WITH THE

HAMPTON ROADS PLANNING DISTRICT COMMISSION

PROFESSIONAL PAPER 1622

Stratigraphic columns of three key boreholes—PLATE 1 Powars, D.S., 2000, The effects of the Chesapeake Bay impact crater on the geologic framework

and the correlation of hydrogeologic units of southeastern Virginia, south of the James River

STRATIGRAPHIC COLUMNS OF THREE KEY BOREHOLES

By

David S. Powars

2000

1 Lithology shown is a generalized summary

based on cored intervals. Dismal Swamp

continuously cored down to -950 ft, and

cored from -1,040 to -1,080 ft. Fentress

continuously cored down to -1,109 ft, and

cored from -1,275 to -1,375 ft and from 

-1,682 to -1,852 ft. 

2 Gibson, T., formerly USGS, now with

Smithsonian Institution, written

commun., 1990

3 Frederiksen, N., USGS, written

commun., 1999

4 Andrews, G., USGS, written

commun., 1989

5 Edwards, L., USGS, written

commun., 1989

Printed on recycled paper

Virginia

Beach

Chesapeake Suffolk

Norfolk

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

James River

Lake

Drummond

Dismal Swamp Canal

2

3

12

Study area

VIRGINIA

CHESAPEAKE BAY

NORTH CAROLINA

ATLANTIC OCEAN

OUTER

RIM OF

CRATER

INNER RIM OF CRATER 76o30' 76o

37o

36o30'

Location map of coreholes and borehole 

2

EXPLANATION

Borehole and number Continuous corehole and number 3