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UNIVERSITY OF DELAWARE GEOLOGICAL SURVEY REPORT OF INVESTIGATIONS NO. 42

SEISMIC LINE NO. SHOT POINT NO. / DGS I / DS 5 s 300 200 400 300 200

SEA FLOOR 3 -I ::E ____-<::~_--=--=~-- BASE OF COLUMBIA 2c o 0.1 ~ ~s;;::---_ --.:.. - 2b -- ~ A~~~iII1"~~~~ fiJ fTl-l 0;:u OF 20 ~:l> CCilnl~r>JlLCHESAPEAKE < POS -- 0.2 fTl r BASE OF S1 MARYS (?) -I s: fTl SCALE o 12 24, 0.3 (IN FEET xlOOOl

BY A. Seon ANDRES

STATE OF DELAWARE NEWARK, DELAWARE AUGUST 1986 STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE POST-CHOPTANK CHESAPEAKE

by A. Scott Andres Delaware Geological Survey

August 1986 CONTENTS

Page ABSTRACT •• · . 1 INTRODUCTION 2 Purpose and Scope 2 Previous Investigations 2 Acknowledgments •• 5

METHODS OF INVESTIGATION 5 General ..•.•• 5 Well Log Analysis • 5 Seismic Stratigraphy. 6 Seismic Analysis 8

ONSHORE DATA •••••••• 8 Stratigraphy and . ·. 8 Depositional Framework•• 15

OFFSHORE SEISMIC STRATIGRAPHY AND CORRELATIONS WITH ONSHORE CONTROL. • 18 External Form and Gross Correlations. 18 Internal Form and Correlations. 22 Structures. • 24 Seismic Facies. 25

DEPOSITIONAL HISTORY 28

CONCLUSIONS. 30 REFERENCES • ·. 36

ILLUSTRATIONS

Figures 1. Well and seismic line location map 4 2. Idealized well log facies. 7 3. Cross-section A-A' 9 4. Cross-section B-A' ••• 10 5. Cross-section C-C' ••• 11 6. Base of st. Marys(?) Formation structure contour map •••• 16

i Page

Figures 7. St. Marys(?) Fo:r:mation isopachous contour map . . . 17 8. Base of Manokin·formation··· · ···structure contour map . 19 9. Manokin and Bethany· ··formations····· isopachous contour map. • 20 10. Manokin and Bethany formations· · cumulative sand percentage contour map • 21 11- Generalized geologic and seismic cross-section 23 12. Possible faults····and structural·· · · style 26 1 3. Seismic facies . 27 1 4a. Depositional history-Subsequence· · ······ 2a basin. 31 14b. Depositional history-Subsequence 2b basin. 32 14c. Depositional history-Subsequence 2c basin. 33

TABLES

Tables 1­ Geologic and Hydrologic Units••.••••• 3 2. Sequences, seismic facies, and depositional environments.•••••••••••.•• 34

ii STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE POST-CHOPTANK CHESAPEAKE GROUP

ABSTRACT

Onshore and offshore geological and geophysical data were used to investigate the , seismic stratigraphy, and depositional history of the late Tertiary age post-Choptank Chesapeake Group rocks in Sussex County, Delaware and adjacent counties in . The results of this investigation suggest that the st. Marys!?) Formation and the sandy interval of which the Manokin aquifer is a part, are distinct lithostratigraphic units. The Manokin formation is proposed as an informal lithostratigraphic unit that refers to the sandy interval of which the Manokin aquifer is a part. On a regional scale, the section containing the Ocean City and Pocomoke aquifers and adjacent and intervening confining beds is best treated as a single undifferentiated lithostratigraphic unit. The Bethany formation is proposed as an informal lithostratigraphic unit that refers to this section.

The seismic data suggest that the post-Choptank Chesapeake Group consists of at least two depositional sequences that are separate from the underlying older Chesapeake Group and overlying depositional sequences. The complex internal structure of the seismic sequences demonstrates that, in Delaware, the lithostratigraphy of the post-St. Marys!?) Chesapeake Group is not correlative with its seismic stratigraphy.

The post-Choptank Chesapeake Group was deposited as a wave and fluvial energy-dominated delta complex. The delta complex was deposited in three phases. During the first two phases the delta advanced as two separate lobes onto a shallow marine shelf. During the third phase, a small basin that formed north of the first two lobes was filled. The Manokin formation was deposited in a delta front to lower delta plain setting, probably as wave reworked distributary channel sands and distributary mouth bars The Bethany formation was deposited in a locally variable setting, ranging from delta plain to delta front.

1 INTRODUCTION

Purpose and Scope

This report documents the results of an investigation of the post-Choptank Chesapeake Group, defined herein as the st. Marys(?) Formation, and the overlying Upper Miocene Aquifer Complex as described by Hansen (1981). The Aquifer Complex has traditionally been subdivided into the Manokin, Ocean City, and Pocomoke aquifers, and intervening fine-grained aquitards (see Table 1). The Aquifer Complex currently supplies water to many of the coastal communities and is the probable source of water for future development. An understanding of the geologic framework will aid future hydrogeologic investigations by more clearly defining the relationships between aquifers and confining beds and by identifying potential hydrologic boundaries. A map of the study area with well and seismic line locations is shown in Figure 1.

Onshore and offshore geologic and geophysical data were used to investigate the lithostratigraphy, seismic stratigraphy, and depositional history of the late Tertiary age post-Choptank Chesapeake Group in Sussex County, Delaware and adjacent counties in Maryland. The data include three 24-fold common depth point (CDP) stacked seismic reflection profiles collected for the Delaware, Maryland, and U. S. Geological surveys, drill-hole geophysical and lithologic logs, and published and paleontologic evaluations. The combination of seismic and well­ log data provides a perspective of the subsurface not available from either data set alone.

Previous Investigations

Previous work has largely been water resources oriented and has focused on the occurrence and availability of fresh ground water (e.g., Rasmussen and Slaughter, 1955; Sundstrom and Pickett, 1969; Miller, 1971; Cushing et al., 1973; Weigle and Achmad, 1982; Hodges, 1984). Geologic~ata were usually obtained as a by-product of water resources investigations. As a result of this emphasis on hydrogeology, the core samples necessary for completing detailed geologic evaluations have not been available, and much of the geologic knowledge is limited to interpretations of the distribution of the fresh water-bearing aquifers. This approach has evolved a confusing hybrid stratigraphy, in which units are defined according to hydrologic and lithologic criteria.

2 r. U Hydrologic Units 0 Geologic Units a. (Weigle and Achmad,1982) U.I (used in this report) Unnamed beds of I CD ~c marine, estuarine, and OCD ::J:U continental origin ('-- IIII 11111111111111111 CD C 0 CD Omar Formation U :Qa. 0 e:::S III :d~ -CD oC) Beaverdam Formation Pleistocene aquifer U Q. ------Upper confining Pocomoke aquifer Bethany formation Lower confining bed Ocean City aquifer Lower confining bed CD c CD Manokin formation Manokin aquifer U 0 -:E Sf. Marys (1) Formation ? I ?"'IIIIIIII'? Frederica aquifer ~------_.. ------

Calvert Formation

Table 1. Geologic and hydrologic units.

3 ! DELAWARE \ tbl!5-Z9• BAY i /14,22:-4• \ i ~eI3-3 i, 'GREENWOOD NC4a-2• Nf\4-2 Ng1-17 70'4"! O.I~-I 3.'40'+ i Od24·1 • Of 34.2 09l2-3 Oc~-3 •• Od22-1 \ ••0\131"1 i GEOflGETOWN Pc211-14 PI24~'• \ Pc\4!5-3 Pc •43- P'i4-4 \ .LAUf;lEL Q142-2 \ Q,52-2 c'.l • • O

Wore,3'• WIOd23• 7S'4!5' 75'00' +38'15' +38'15' EXPLANATION • DATA POINT Som :93 'No.OdGD• 111115-1 WELL/BORING NUMBER 200 ~_JU~.JJ 1SEISMIC LINE AND t;J IOoj SHOTPOINT NUMBER ---- STATE BOUNOARY

Som Oe28 A I--IA' LINE OF CROSS SECTION • FIGURE I. WELL AND SEISMIC LINE LOCATION MAP Figure 1. Well and seismic line location map. Several recent interpretations of the of the Chesapeake Group are presented in Owens and Denny (1979), Hansen (1981), Newell and Rader (1982), Kidwell (1984), Ward (1984), and Mixon (1985). Woodruff (1977) presents preliminary interpretations of the seismic reflection data used in this report. One interpreted marine multichannel seismic reflection profile used in this report is presented in Weigle and Achmad (1982). Field (1979) presents the results of a single channel sparker and acoustipulse seismic reflection investigation.

Acknowledgments

Thanks are expressed to Richard N. Benson, Kenneth D. Woodruff, John H. Talley, and Robert R. Jordan of the Delaware Geological Survey for their discussions of the interpretation of geologic and geophysical data. Harry J. Hansen (Maryland Geological Survey), and Kenneth D. Woodruff, John H. Talley, Richard N. Benson, and Robert R. Jordan (Delaware Geological Survey) are thanked for critically reviewing the manuscript. sandra Dunn drafted the figures for this report.

METHODS OF INVESTIGATION

General

The principles and procedures of sedimentary basin analysis presented in Miall (1984) and Galloway and Hobday (1983) were used to evaluate the bedding geometry and spatial relationships within and among lithologic units of the post-Choptank Chesapeake Group. The approach generates information that can be used to predict the subsurface distribution and gross textural characteristics of sand bodies. This information is useful in ground-water expforation.

Well Log Analysis

Lithostratigraphic units were defined based on the occurrence of similar geophysical log characteristics and both in individual drill holes and on a regional scale. There was no chronostratigraphic intent in the designation of the lithostratigraphic units.

5 Semi-quantitative measurements of percentage were calculated for natural gamma logs from the equations and graph in Asquith and Gibson (1982, p. 91-95). The 10 percent shale value was used for separating clean sands from muddy sand. Gamma logs were checked against lithologic and electric logs to account for the effects of glauconite, "organic" material, and potassium feldspar. Sand thickness and sand percentage were calculated for each log and a sand percentage contour map was constructed for a selected stratigraphic interval.

Well log facies were identified as aggradational, erosional, mixed, or progradational using the methods described in Galloway and Hobday (1983). Idealized eXamples of log facies are shown in Figure 2. Log facies are indicative of depositional energy conditions (Galloway and Hobday, 1983).

Structure contour, isopachous, and sand percentage maps, cross ections, and well log facies were used to evaluate the depositional framework of the post-Choptank Chesapeake Group.

Natural gamma radiation logs were the primary sources of well log data, because the gamma tool is not affected by changes in water quality as electric logs are; and, they show more detail than lithologic logs constructed from ditch samples. Hansen (1981) reports that the st. Marys(?) Formation is a distinct lithostratigraphic unit, recognizable by a distinctive gamma log signature and its clayey lithology. Hansen (1981) uses the base of the st. Marys(?) as a marker horizon because it is one of the few markers recognizable on a regional scale. For these reasons, wells with gamma logs that penetrate to the base of the st. Marys(?) were evaluated first, then shallower wells and wells with other types of logs (lithologic, electric) were compared to them. Available core and ditch samples from Delaware were analyzed for gross textural and lithologic content.

Seismic Stratigraphy

The seismic stratigraphy of the post-Choptank Chesapeake Group and overlying deposits was based on the analysis of 24-fold CDP stacked high-resolution seismic reflection data. Depositional sequences and sequence boundary types were identified according to the principles and procedures of seismic stratigraphy outlined by Mitchum et al. (1977b) and Mitchum and Vail (1977). ----

6 LOW ENERGY MIXED AGGRADATIONAL PROGRADATIONAL

HIGH ENERGY AGGRAOATIONAL EROSIONAL

Figure 2. Idealized well log facies.

7 Seismic facies were defined for each sequence, interval velocities were calculated at several points per line by using root mean square stacking velocity plots and the Dix equation (Dix, 1955) and time-to-depth conversions were calculated. The calculated depths were then used to correlate sequence boundaries, reflectors and seismic facies with lithologic changes and formation boundaries in onshore borings. Additionally, Weigle and Achmad's (1982) picks of formation and aquifer boundaries on lines GD1 and GD2 were traced northward along line DS5 and the DGS series lines, and compared with this study.

Seismic Facies Analysis

Information regarding depositional environments and lithology can usually be inferred from the geometry of reflection packages and the character of the reflections (Mitchum et al., 1977a). Identification of seismic facies and interpretations of deposi­ tional environments are based on the procedures presented in Sangree and Widmier (1977) and Brown and Fisher (1980). Lithofacies penetrated in onshore borings compare favorably with offshore seismic facies, suggesting that there are no major changes in lithologies and sedimentary facies between the coast and nearby offshore locations. This allows a comparison of the two data sets and provides a view of the subsurface not available from either data source alone.

Reflection amplitude and continuity were subjectively designated poor, moderate, or high. Amplitude ratings were judged relative to the range of amplitudes observed in the data. Continuity ratings were based on the distance a reflection event can be traced. In this report they were defined as: poor, continuity less than 2,500 feet (approximately 760 meters); moderate, continuity between 2,500 and 10,000 feet (approximately 760-3050 meters); and, high, continuity greater than 10,000 feet.

ONSHORE DATA

Stratigraphy and Lithology

Geologic and hydrologic units are shown in Table 1. Figures 3, 4, and 5 are cross sections showing the distribution of the units. It should be noted that, on the Delmarva Penninsula, the name St. Marys Formation is not acceptable in strict stratigraphic terms because correlation with the st.

8 ~

TO 250 {; I~ 'CI I-.S 1300 TO ... .r I TO Manokin I~ ~ ~ 335 I formalion ~I I '" ••• '" • 400 ..- ~ ~ ~ I Tn .., -. I ~ TO TO " - 419 730 ------'1 TO 1 \.. I ..J.. Hoo TO 505 TO 965 Sf, Ma SCALE rys (?) EXPLANATION , F'orrnafion I ..;J ~ 600 0 1 2 5 ChoPfank Mh41-6 Drill hole number IN MILES F'orrnaf' 'on Suggested correlation VERTICAL EXAGGERATION 105X 1[" Hoo -- Gamma log, radiation increases to right TO ~ 1212

Figure 3. Cross-section A-A' . B A' t 0013-1 0134-2 P1i=3 Pg53-14 Ph51-19 Qhj Q$-4 Ri15-1 WorAh-6 ~ NGVO $I} 4 )\ post- 11

Bethany fOr"'Ofion ~ 200

~ 300 _ TO "'onokin a TO fOr"'afio CD 300 n CD TO 332 ~ 968 400 TO EXPLANATION 423 St. I:. Oe13-1 Drill hole number "'orh 1'1 500 ChoPfonk· or"'Ofio Suggested correlation -~,n SCALE For"'Ofion' Gamma logs, radiation 600 increasing to right o I 2 5 } IN MILES TO 970 VERTICAL EXAGGERATION 105 X 700

TO 1212

Figure 4. Cross-section B-A'. Nc13-3 c' c Qd52-2 Pc45-3 Oc34-3 Pc35-14 I 1;;::~S~-~eSOpeOkA ~ f ~r _~if; ? b> ~ I~;~~

100 - -co 200 =- s~ 1010 j ~ I ~ C\\o\lIOl\l< fill. ... ? >"-l TO TO TO 280 414 1500 j 300 TO 390 SCALE EXPLANATION I I o I 2 5 TO IN MILES 1042 Qd52-2 Drill hole number Suggested correlotion VERTICAL EXAGGERATION 105X / Fault Gamma log I radiation } increases to right

Figure 5. Cross-section C-C'. Marys Formation at its type locality (west of Chesapeake Bay) can not be rigorously demonstrated. For this reason, Rasmussen and Slaughter (1957) and Hansen (1981) have d~fined the fine-grained beds overlying the Choptank Formation on the Delmarva Peninsula as the St. Marys(?) Formation.

The name Yorktown and Cohansey Formations(?) has previously been used for the predominately sandy section between the st. Marys Formation and the Columbia Group (Rasmussen and Slaughter, 1955). Recent work questioned the age equivalency of the Yorktown Formation and the Cohansey Formation (Owens and Denny, 1979), and appears to show that the Yorktown and Cohansey Formations(?) is older than, and stratigraphically distinct from, the Yorktown Formatidn (Mixon, 1985). For these reasons and because of new data, the Yorktown and Cohansey Formations(?) is revised in favor of two new lithostratigraphic units, the Manokin formation and the Bethany formation. These are considered informal units pending the availability of a completely cored section.

In Delaware, the basal lithostratigraphic unit of the post­ Choptank Chesapeake Group is the St. Marys(?) Formation. Available biostratigraphic data suggest that the st. Marys(?) is late Miocene in age and is younger than the type st. Marys, perhaps coeval to the Claremont Manor Member of the Eastover Formation (Hansen, 1981). At the type locality, the st. Marys Formation is middle to late Miocene in age (Ward, 1984).

As noted in Maryland by Hansen (1981), the base of the St. Marys(?) Formation in Delaware is usually characterized on gamma logs by a sharp clay on sand "kick" (see figures 3-5). The "kick" is less distinct, but still recognizable close to the subcrop area (wells Mh41-6, Nc43-2, Oe13-1, Of34-2). Evidence of a thin phosphatic zone, noted in two Maryland wells by Hansen (1981), was not observed on any of the Delaware logs.

At the outcrop area, both the Choptank-St. Marys and Choptank-Eastover contacts have been identified as disconformities (Gernant et al., 1971; Kidwell, 1984; Ward, 1984). In Delaware, there-iS-no definitive evidence for a disconformity between the Choptank and st. Marys(?) although this may be due to the less detailed coverage of well log data. Alternatively, Newell and Rader (1982) suggest that the Choptank-St. Marys contact is transitional, and that the st. Marys is part of the depositional sequence that includes the Calvert and Choptank formations. It may be that the nature of the Choptank-St. Marys contact varies within the basin.

12 In onshore borings, the st. Marys(?) consists of fossili­ ferous, glauconitic, and lignitic, gray, olive-gray, and blue­ gray clay, silt, fine sand, and silty and sandy clay. The st. Marys(?) appears to contain more sandy beds in updip areas. Generally, well log facies are characterized by low energy aggradational facies. Some logs show small scale erosional and/or progradational facies (mixed facies). Gypsum(?) crystals have been found in core samples from well Og31-1. Paleontologic data suggest that the st. Marys(?) was deposited on a shoaling marine shelf (water depths of 265 to less than 100 feet, 81 to 30 meters) (Hansen, 1981).

The top of the st. Marys(?) is a 10 to 50 foot (3 to 15 meters) thick coarsening upward (progradational) section that appears to grade into the Manokin, suggesting that the St. Marys(?) and Manokin were deposited as part of the same depositional sequence (see Figures 3-5). In a few wells, especially in the high sand percentage areas, the transition is replaced by a sharp sand on clay break, indicating an erosive contact. The top of the St. Marys(?) is arbitrarily set at the point in the progradational or erosive section where the lithology is 50 percent clay/silt, 50 percent sand.

Previous workers have recognized that the Manokin aquifer is the first major sand unit above the st. Marys(?) Formation (Rasmussen and Slaughter, 1955; Miller, 1971). A lithostrati­ graphic equivalent of the Manokin aquifer has not been recognized west of Chesapeake Bay; however, time equivalents of the Manokin probably are present west of Chesapeake Bay (Mixon, 1985). Present information indicates a late middle to early late Miocene age (Hansen, 1981).

Of the three aquifer units, the Manokin appears to be the only areally persistent and lithologically distinct unit. The term Manokin formation is proposed as an informal lithostratigraphic unit that refers to the sandy interval of which the Manokin aquifer is a part. The name Manokin formation is used because the Manokin formation corresponds to the lithologic section previously defined as the Manokin aquifer by Rasmussen and Slaughter (1955).

In Delaware, the Manokin formation is predominately gray to olive-gray, fine to coarse sand, and silty and clayey sand, with some beds of gravel, and local clay/silt, lignitic, shelly beds. The Manokin generally consists of a lower progradational section and an upper aggradational or mixed section. High energy ~ggradational facies are more common in updip areas. Low energy

13 aggradational facies become more common in downdip areas. Paleontologic data indicates that took place in middle neritic to marginal marine environments (Hansen, 1981).

At most locations, the top of the Manokin formation is placed at the base of a 5 to 30 foot (1.5 to 9 meters) thick clay/silt unit. The contact usually is sharp. In a few drill holes (especially in high sand percentage areas) this clay/silt unit is absent, and it is difficult to distinguish between the Manokin and overlying units.

The Ocean City and Pocomoke aquifers occur within a section of lithologically complex lensing sandy and clayey units which lie stratigraphically above the Manokin formation (see Figures 3-5). The Pocomoke aquifer lies stratigraphically above the Ocean City aquifer (Hansen, 1981), and has been dated late Miocene by Owens and Denny (1979). More recent work by Mixon (1985) in the Accomack County, Virginia-Somerset County, Maryland area suggests that the Pocomoke aquifer may correlate in part with the late Miocene Eastover Formation and Pliocene Yorktown Formation, units he dated on the basis of molluscan assemblages.

As noted in Maryland by Hansen (1981), the Pocomoke and Ocean City aquifers in Delaware are part of a complex of lensing sandy and clayey units, and are not discrete sand bodies that can be regionally correlated. Not only do the sandy units pinch out, but the number of sandy units is not consistent from drill hole to drill hole. It is not surprising that there is considerable confusion in the literature regarding the distribution of the Pocomoke and Ocean City aquifers. To avoid this problem, the Bethany formation is proposed as an informal lithostratigraphic unit that refers to the rocks containing the Ocean City and Pocomoke aquifers, and the basal, intervening, and overlying clay/silt (confining) beds. The name Bethany formation is taken from its occurrence in the Bethany Beach 7.5 minute topographic sheet, in well Qj32-14, 125 to 320 feet (38 to 97 meters) below land surface.

The Bethany formation is a lithologically complex section characterized by beds of predominately gray blue-gray, or olive­ gray, fine to very coarse sand, interlayered with beds of predominately gray, olive-gray and blue-gray clay/silt. Lignit±c, glauconitic, oxidized, gravelly or shelly beds are common. Carbonate (siderite?) concretions are found in many of the clay/silt beds. The Bethany shows mixed well log facies, although individual logs may have a greater thickness of progradational or aggradational facies. In general, the Bethany

1 4 formation contains more clay/silt than the Manokin formation, indicating overall lower energy conditions. Individual sand or clay/silt sections range from less than 1 to nearly 40 feet (0.3 to 12 meters) thick, and gamma logs show a characteristic "sawtooth" pattern indicating frequent changes in energy conditions. For example, in some drill holes, clay/silt and gravel are interbedded. Paleontologic evidence indicates that deposition took place in middle neritic to marginal marine environments (Hansen, 1981).

The top of the Bethany formation is placed at the base of the Columbia Group. On gamma logs the contact usually is a sharp sand on clay trace shift. It is lithologically marked by a change from the olive-gray or gray, fossiliferous clayey or silty Pocomoke to the brown non-fossiliferous, poorly sorted sands and gravels of the Columbia Group. The contact is more difficult to pick where the upper part of the Bethany is non-fossiliferous, sandy, or oxidized, or where the basal part of the Columbia is gray in color and/or fine-grained.

The stratigraphic relationship between the Manokin and Bethany formations and the overlying Columbia Group is the subject of considerable debate. Jordan (1974) and Hansen (1981) argue that the overlying Columbia Group unconformably overlies the Manokin and Bethany formations. Owens and Denny (1979) argue that the "Manokin" and "Pocomoke" beds are the marine equivalents of the Pensauken Formation (equivalent to the fluvial Columbia Formation). Although conclusive paleontologic evidence is lacking, the structural and stratigraphic arguments presented by Hansen (1981) demonstrate that the Columbia Group is younger than the Manokin formation and the basal clay/silt bed of the Bethany formation. However, the stratigraphic relationship between the Columbia Group and the remainder of the Bethany is not as conclusively demonstrated by Hansen (1981). Similarly, in Delaware, well log data does not conclusively demonstrate the stratigraphic relationship between the Bethany and Columbia.

Depositional Framework

Figure 6 is a structure contour map of the elevation of the base of the st. Marys(?) Formation. Seismic reflection data were used to extend the structure contours offshore. Figure 7 is an isopachous map of the St. Marys(?). In general, the St. Marys(?) is a southeasterly thickening wedge resting on a southeasterly sloping surface. The increased slope and thickness near Rehoboth Beach indicate that there are structural complications in this

15 \00

o • '0 15 MILES

00 <; N~

~ '"

EXPLANATION Stat. boundary Data point showing elevation of the -100 bas. 01 the SI. Marys (1) Formation in 1.01 bolaw NGVD ~ Se1smlc line location __100 ""'" Contour line showing elevation of the base of th. S1. Marys (,) in I••t b.law NGVD Approximate updlp oxtont 01 SI. Marys (?) ~ Formation I( ~ Faul' trace-D=down, U=up ('0 / .--~

Figure 6. Base of st. Marys(?) Formation structure contour map. \ i • o \ .M • • 5 10 15 MILES \ o i i 50 ,~ 10 0 N~ ,.•...1 • 38·4!5,~Oj ;<:'.,:) I <) + '0 ./ /' \. ·e4 ·eo / i ~\ ° / I ·.l(roo el05 1/ r •

~150 ~ 1~5 n~ oJ' • J -' ~ ------1 .... / 0' ,0 / 130 125 115 • • 135 / • •

el60 -140

75-45' ns·oo· 38-15' + 38-15' EXPLANATION 50+ _.....;"50 .--,,---- State boundary tP~ b. ~145 150 150 Data point showing thickness of "II __ ISer----!SO • el60 '100 St. Marys (1) Formation in feet ~ Seismic line location

.--'J-rj Fault trace· D=down, U=up Derivat'lve data poInt showing th'lckness el80 6. .--'1-00 100 of Sf. Marys (1) Formation

Figure 7. st. Marys!?) Formation isopachous contour map. area. The decreased thickness trend in central Delaware is accompanied by a thickening of the overlying Manokin and is suggestive of post-depositional or a facies change at the top of the st. Marys(?).

Figure 8 is a structure contour map of the base of the Manokin. Figure 9 is an isopachous (thickness) map of the combined Manokin and Bethany formations. In general, the Manokin and Bethany formations form a southeasterly thickening wedge resting on a southeasterly sloping surface. The thickness map is complicated by post-depositional erosion. In general, the amount of missing section is greater along Delaware Bay. An anomolously thick section trends northwest-southeast from drill hole Nf34-2 to drill hole Og31-1 and corresponds to a thin area in the underlying St. Marys(?).

A sand percentage map (see Figure 10) for the combined Manokin and Bethany interval shows several distinct sand-prone trends. One is parallel to strike and follows the subcrop of the Manokin aquifer as mapped by Pickett (1976) and Hansen (1981). Two dip-oriented "lobes" appear to originate near drill hole Oe13-1 and radiate southeastward and southward. The sand percentage generally decreases in a downdip direction. Comparison of Figures 8 through 10 seems to show a fluvial/distributary channel system and associated shallow shelf environments. The lobate pattern is indicative of a wave energy-dominated delta complex (Galloway and Hobday, 1983). Additional well coverage is needed to refine the shape of the sand prone trends.

OFFSHORE SEISMIC STRATIGRAPHY AND CORRELATIONS WITH ONSHORE CONTROL

External Form and Gross Correlations

Time-to-depth conversion of seismic reflection data and projection of the base of St. Marys(?) structure contours to offshore locations indicates that the base of the st. Marys(?) (i.e., st. Marys(?)-Choptank contact) is represented on the seismic reflection profiles by a strong areally continuous positive reflector. This reflector truncates several underlying reflectors and appears to be a boundary between the underlying relatively continuous reflectors and overlying generally discontinuous reflectors. As such, the reflector correlated with

18 1 \ \ • I • o • 10 15 MILES \ \ • \ N~ 75-45'\ 38-45'+ \ ". + / '" (/ \ ~ f \ ,f \ I ,(1 f ·75

~ \0

EXPLANATION 75-00' State boundary +38-'5' Data point showinq elevation of bose of • ManokIn formation in feet below NGVD

'-----, Seismic line location ~-.JLJ-J..J o ~ Fault trace - D=Down, U=Up <> ~ /' Contour line showing elevation of base ...200 of Manokin formation in feet below NGVD 245 • Approximate updip extent at Manokin ~ formation

Figure 8. Base of Manokin formation structure contour map. I \ 1 • i • \ o 5 10 15 MILES i • i,~". N~ 7~·4~,,I ~ ••".5'+ \ ~ )'. /·55 50 / ~ / / / ! / :85 / 35· / '"a

(~ EXPLANATION 7~·OO' State boundary +38-,5' Data point showing thickness • of Manokin and Bethany formations 250 240. ~ Seismic line location ~..JU~J.J Fault trace - 0= down, U=up A-300 -Y Contour line showing thickness of " .....2.00...... - Manokin and Bethany formations Derivative data point showing thickness o/ .6 200 .,0··250 of Manokin and Bethany formations .. .,0 Figure 9. Manokin and Bethany formations isopachous contour map. I \ ! • \ i • o • 10 15 MILES i • \ N~ 750 45'\ ••·4.'+ \ ~

tv

'3D ~207!5·00' +38°15' EXPLANATION State boundary Data point showing cumulative sand • percentage of Manokin and Bethany formations Seismic line location Cs00 '------. __ ~o"""'" Contour line showing cumulative sand percentage of Manokin and Bethany Approximate updlp extent of Manokin ~ formation

Figure 10. Manokin and Bethany formations cumulative sand percentage contour map. the base of the St. Marys(?) is a depositional sequence boundary. The nature of the st. Marys(?)-Choptank contact is not apparent.

The gross geometry of the post-Choptank Formation section is a southward and eastward thickening wedge unconformably overlain by a complex sheet that thickens toward the north, east, and south. There are at least three depositional sequences within the section studied (numbered 1-3 in Figure 11). A minimum of two depositional sequences are present in the post-Choptank Chesapeake Group. Three subsequences (2a-2c) have been identified in the upper sequence of the post-Choptank Chesapeake Group. Additional seismic coverage is needed to determine the continuity of these subsequences.

Sequence 1, the basal sequence, is thickest in the northern part of the study area. It thins rapidly at SP 320-400, Line DGS1, in the vicinity of Indian River Inlet, where the slope of the upper bounding reflector becomes steeper. The sequence thickens again to the south and east by divergence of reflectors. The top of the sequence is well defined in the southern part of the study area by a moderate to strong positive reflector. In the area north of Indian River Inlet, the reflector marking the upper boundary fades in and out, suggesting frequent facies changes either above or below the boundary. The upper boundary appears to be concordant except near Indian River Inlet.

Sequence 2 is a southward and eastward thickening wedge. The boundaries of the subsequences are usually discordant with Figure 11 truncation, toplap- and lapout- (onlap and downlap) type boundaries (as defined in Mitchum et al., 1977b) present. At several locations the reflectors bounding the subsequences fade in and out, suggesting frequent facies changes either above or below the boundaries.

The base of Sequence 3 truncates underlying reflectors of Sequences 1 and 2, and marks the upper boundary of the post­ Choptank Chesapeake Group. This surface appears to represent an and may correlate with one of the post-Chesapeake Group age, low sea level stands when drainage extended out onto the present continental shelf.

Internal Form and Correlations

Time-to-depth conversions of seismic reflection data and projection of onshore data indicate that Sequence 1 includes the St. Marys(?) Formation and, in northern areas, the st. Marys(?)-

22 SEISMI,. LINE NO. SHOT POINT NO. N OGS 3 OGS 2 OGS I / OS 5 s 0 0 0 0 0 0 400 300 200 0.01 39 , 29 • I 39 • 29 ••I. 39 • 29 • I. .. ! ! • to.O SEA FLOOR

3 ~ - BASE OF COLUMBIA o~ ~====:::::::::===--==:;~;-;;;;~~ 2c 0.1~~' 0.' - - - - -2'- - "TOp 0' "ANOK.. " w'" BASE 01' -l>< 0.2 MANOkiN 0.2 f:!! BASE OF S1 MARYS (1) --'__ ~ --t 3:: ITI SCALE 0.3 o 12 24 0.3 (I N FEET x1000)

Figure 11. Generalized geologic and seismic cross-section. Manokin transition. In northern areas, Sequence 2 includes the upper part of the Manokin and the Bethany formations. In southern and eastern areas, Sequence 2 includes the Manokin and Bethany and the upper part of the St. Marys(?). Correlations with onshore well logs and comparison with the structure contour maps of the base of the Columbia Group in Field (Figure 15, 1979) and Sundstrom and Pickett (Figure 9, 1969) indicate that Sequence 3 includes the Columbia Group (as defined by Jordan, 1974) and younger . Sequence 3 appears to truncate the underlying Chesapeake Group. This indicates that in the coastal area, the Columbia Group is not the time equivalent of the post-st. Marys(?) Chesapeake Group as suggested by Owens and Denny (1979), but rather unconformably overlies the Chesapeake Group.

Data are not detailed enough to distinguish between the individual units of the Columbia Group and the younger sediments. Considering the complex post-Chesapeake Group depositional history of the area, Sequence 3, as recognized here, should consist of several depositional sequences. For example, Field (1979) recognized four areally continuous reflectors within the post-Chesapeake section that are not evident in the records studied for this report.

The complex internal structure of Sequence 2 indicates that the traditional (hydrologic) subdivisions and the lithostratigraphy of the Aquifer Complex are inadequate for describing its seismic stratigraphy. Figure 11 shows that the three subsequences within Sequence 2 (2a, 2b, 2c) do not correlate with the lithostratigraphic units. In fact, the subsequences can include parts of one, or both of the lithostratigraphic units and/or parts of the underlying St. Marys{?) Formation. Additionally, individual subsequences are not continuous on the regional scale over which the lithologic or aquifer units have been mapped. For example, Subsequence 2c, which includes part of the Manokin and Bethany in northern areas, pinches out to the south. Subsequence 2b, which includes only about 20 feet (6 meters) of the Manokin at Shotpoint (SP) 380, Line DGS1, thickens to the south and includes part of the St. Marys{?) and all of the post-St. Marys{?) Chesapeake Group section. • Structures

In general, few structures are evident in the post-Choptank Chesapeake Group. Evidence for normal faults (or warping) is present on lines DGS2, SP320-360 and 120-150, and DGS3, SP101-140

24 (one example is shown in Figure 12). These possible faults occur in the area where structure contour maps indicate structural complications (see Figure 7). In general, these possible faults do not truncate or offset reflectors. It may be that the offset is beyond the resolution of the data. These faults appear to terminate within the post-Choptank Chesapeake Group and apparently persist down to the pre-Mesozoic basement as mapped by Benson (1984), suggesting that older basement structures were active and influenced the location of deposition during the middle to late Miocene.

Another possible normal fault is shown on Figure 4. This fault apparently has a greater displacement than those offshore. Additional borehole control and paleontologic evaluation is needed to confirm the existence of this fault and other possible faults. Coincidently, a linear feature was mapped in this area on LANDSAT imagery by Spoljaric (1979).

Seismic Facies

There are two seismic facies in Sequence 1 (see Figure 13): variable continuity (high to low), transparent to high amplitude, and parallel (Type 1-1); and, variable continuity (moderate to high), moderate to high amplitude, and parallel to hummocky (Type 1-2). The hummocky reflectors appear to be thin progradational sequences. Facies type 1-1 is common in the lower third of the sequence to the north of the clinoform and throughout the sequence to the south of the clinoform. There is a gradational change between facies located at DGS2, SP101-140, in the area just north of the clinoform.

There appear to be three seismic facies within Sequence 2 (see Figure 13). The dominant type (Type 2-1) exhibits variable continuity (poor to moderate), variable amplitude (poor to moderate), and hummocky to parallel reflectors. Many of the hummocky reflectors appear to be cut-and-fill features, while the remainder appear to be thin prograding sequences. The second (Type 2-2) exhibits variable continuity (poor to high), moderate to high amplitude, and parallel reflectors. Type 2-2 most commonly occurs in the sections that correlate with the St. Mary(?) Formation, and on line GD 1 SP 100-200. The third facies (Type 2-3) is present only on lines GD1 and GD2, SP 200-275 and 150-100 (a distance of approximately 1.5 miles, 2.4 kilometers) respectively. Weigle and Achmad (1982) tentatively identify the feature as a relict island. The feature extends from approximately 0.125 seconds (within subsequence 2b) to the

25 LINE DGS 2 Northwest ...... 1/2 mi.kl~ shot point Southeast number 350 340 330 320 310 300 290 280 11""""""'~~~~=~~=~~~~~~

0.1 base of 0.2 St. Mary s {?}

Eocene 0.5 Oligocene Possible faults and direction Tertiary of motion upper

1.0

-tf) "0 lower Cretaceous c 0 <.> CII.) -tf) CII.) E 1.5 +-

CII.) ::> 0 Sub-coastal plain "- 00- unconformity >- 0 3t I 0 Synrift (?) 3t I- Rocks 2.0

Pre- Mesozoic crystalline basement

2.5

Figure 12e Possible faults and structural stylee Time stratigraphic correlations from Benson et ale (1986). 26 DGS2 DGSI

120 110 230 220 LL1. . 0.0 1 I I ,0.0 . .. ~ .iii; j i Ilimil II1ti1 iI ~~.. ·lli>.il~ I- ..ik;· ' •.•...ii i. m..,. ~1~"I~t~Lri~,~!~~I.lllllt~~~lttttlil~~~ J .' .'," ~ ,lI: '~.!..l.~-l~.' • ..J ..••f'l' ,.;.. ~..,.;.,.,.~, ,.~!-...... ~ .~ ~ en ~uul~n l.....,. ,pt,. t, pt_"".~ ;n;;m;;~l.~ijmg?:::m=1miEe;~~!.~ri~I~\ « c ··,~tl,'!"t~ ·.'I";t~ ...... h··I···H .. ~ ..·... ' "j'.' ... '·'··,;·.'.n!l.~,.h,II....,IP.I'.... ,.)~~~I .• ' a::: z ·.·~_.u .i.:..•..llJ:!J,.!-.u~J.w.J>J... ~.:P..I.li~!..~,,·4'~~; ;~~~~ ~j'::';:1 ~~.!..:,!J!~'. U~~,~: !.~ !.!.!!..,.!..~~_~.~_~}~j~.~,~lL~~~~L 0 I I- 0 -"l' .., '.Jt'hU')ll"tn'.·~II.,., ....',." ~ , 0.1 'I~.·.tlr.;·'.t-".),· ~~: '.~ ...... ~. ")l:".1.~.I.I)PP·'IJ··t~··t·!"·"'··j ...~~ ..•.• "t··:,.f;:rHH:~: .··.'l.~.';.... r)\ .. . 11·;_·r~".:.~:"'".~T.'"l;,:I:::~·i _1il'.';·;.",-,,·.'~· .1.· '-.' ..:l.';.." .. J ..... I U ,...... d,.'~- ..?~JII: , .. ~-_ .. ~ ... ~.d,""" •. J. ~ ~ .~~!'r!!~·J!I!t!!~r::.tlr~!!..".b."J.~'!~.;!. ~..t~~'. ',~·.h~)"'-~f~;:f'::',: ~... ~'".,.... '. J.PI."!rl ...... :t~ .' , ',' -,;. _.• :.:.':'..J.._~ ;.~ .. .!";"",:,,,:.... .!..!I ·'.'.~I.·'I .,lI..,J~ "~ ::»0 - .·····., .. .. )t··, ... •..,,- .. ,~U'n"""·,~.·.r'-:t .""l",~'h ••H q,~.~ .. j Utll I ."~~'.j' ",1.' ':', !'·':'.')'l'··· ,.,;., .•.•. .,... ;.: ",.•• 'JI".'f)· .. ' .I.r·-" . .. ., . o _1 --',1,'1 l..,i).~··}.,~,J·-l"·- ... ·, )...... !.~<. '.' ' .. ...:.~ ..:u# ~.u..·.~j': .J.. ~ ;,11 ~~ •.•• ~r ~: ~!'~:!;! :.::~. "~n! ~."".', ;r; tUth II ...... ,':... ~ ... ~.H "" •. t'.'.h.'; ::,'~ ~·'~"",i9""-f·'''~""","r.mllm~ t- "?·1 .. j! U: ~?*~?"!!!~~.~.~~:~~!...... h,..~ 0.2 ~ttn.TrI"fFl:''J~~D-;.~T.1T~ff;i-ll1;-P·n.i!"i1.~~~n~;T .. ·tO.2 N _ ..!!: :!!!~nll~~hPpt"2.1>-'''~~~ -J ;,;.:: .r~" ;:r"'lt,t'hAA''''!f'1!''I!!h (SEQUENCE ,,, 00 $ SEQUENCE • , '.0.0 ~ i i . i i= ..J LL1ii) ~o ~- ~~ 0.1 (.) ? ro,1 LLI ~~~~ ~ III ./ ____ - i- _ ?:;;> --- ~ ~= ~:3S o~· '__---= _ ----- c I- 0.2 ~ 0.2 TYPE 1-2 SCALE o 1000 TYPES 2-1 AND I-I FEET

GD2 DS5 120 220 210 200 130

... ". , ."""" ," ' 0.0 ~ .·'.i•• ~mi.,."'i'il~.~~~'"~ •.-.!~~~...i.l*!!~~l ..i. i= H,,:.....' •• u.TP:J~.· .,.~l'i~,: r::_.'!:',·.. ·.·,·,-~ .. :.~:' - ~"'''i''''''i.'··,;:".hM$'N,~,,*I~'»»)Jm»»~·· ~ _ >Wi (f) tttt.t'•.m' t•.•. ,'.' .•.. , -: \:1':"" :!." ·;-Ii1));'.;·: ::;. i' _lfi1i wo •. ·.... :· _.., ··,,'·.·.•_.~.h.: ... ··_~··, .. ,:, ,~M:'.. :.!·.. .'tH"'.~·..,·,.~J 0.1 0.1 ~ ~ :~..~; ~·~~i~~;~~ (~,' m~t:~.T~·~.!·i.li.:;·:~~: ,~;;.~;.;~~ l.·.t·~~~t~!~'·r.i~i~·"t;J·,.·~... I u _LIo.Io..I.~.) __ - -- •.••. ,-. ~.I-·~~·:"TPTT"....--1..-I·-tl,f-,f-! "-, ~ .._.,...---- . .- • .. , ..• .-,~ .' :"':.. '....": .": '.'.'.•.. ' .. '1 .'.J-' '] ~ ~ ~.'!"*.',~ ~g!,~~~Hrnl1~L,l·.~t~htilrt_l.'!.f~~~~:~~~'~.i.'~rt-f.' ~~. ·.·t 0.15 __ .. ,'. r. , •••,,---- ., - \ .• , ••• ,.. ~_...•.•.•:..,. 1,»,. ~ P'ftl.;.t~f-~· 1_ ". ' .. , '!If'. ,..- .•...••.., •. , ...... ,.L.~,'. '.' ~ " •.~....Utl/.lL>'lh~!~'.u~h~ ~..mftH;;;»tHtt~ I .; '''1·-'' ••• 1. , ',,~ _ , .. ", ,.-~.,... r~ i.rp,.nr,..,.,Jt .' ~.'.t.liTrn.u).:'oMnu..,tu•• ).l.',: r . · .. u.!. o ·- .J,~1J7 ...~~ ..~~ -I-;~;'...,t~~ ....•.. ,},.'·"l""f':.,' ... .. l' 0,2 0.2 ~ ,; Ut.lfn~r~:: I).lj:.,.. :,!: ;J, :JJ}J.l:'.::JJ:.~.I:",·.,i~~~.t.•.t ... ~! ... 1 I- nl'~'\'l 'iH~,i.il.;\l'lmllmmt1J\:m't;Hl1J.immmiillitllull: , _.....uw.l.U ~~ ~..••..I •••• 0.25

• SEQUENCE

LLI ~ J'SEQUENC~ I 0.15 2i , -- ---~::::::-:::--.::----- •«0.1 i= - ---- ...Jro ~-~ We 0.2 ~z .D = ;~ 0::8 N - ,...-- I-w ------0.25 )-£1 -= - 0,2 ~ :::::====--- ;;> ~~ ~----- , ~ ~ - SCALE TYPE 2-2 TYPE 2-3 o 1000 FEET

Figure 13. Seismic facies. surface, where it forms a bathymetric high. Lateral boundary types are both concordant and discordant. Discordant boundary types exhibit toplap, baselap, and onlap. Laterally bounding facies are Type 2-1 to the west and Types 2-1 and 2-2 to the east. The base of the feature appears to be discordant, and could be an erosional surface. Internal reflection configurations include both high amplitude and parallel, and moderate amplitude and hummocky. Many of the hummocky reflectors appear to be cut and fill structures.

The subsequence geometries and complex interfingering of seismic facies are indicative of a complicated depositional system that has characteristics of both deltaic and shelf systems. The interpretation of the data is included in a later section.

DEPOSITIONAL HISTORY

Well-log and seismic reflection data associated with seismic Sequence 1 indicate that:

1. The base of the st. Marys(?) (base of Sequence 1) is a depositional sequence boundary and may represent a non­ depositional hiatus.

2. In general, higher but variable energy, shallower water (inner neritic) environments (seismic facies 1-1) were present in the northern part of the st. Marys(?) basin, whereas lower, uniform energy, deeper water (middle neritic) environments (seismic facies 1-2) were present in the southern part. The St. Marys(?)-Manokin transltion marks the southward progradation of higher energy deltaic environments over the low energy open shelf environments. The section that includes the top of the st. Marys(?) and basal part of the Manokin was deposited as part of a time transgressive, continuous depositional event that becomes younger in a downdip direction. The St. Marys(?)-Manokin transition occurs within Sequence 2 in southern areas.

3. Deposition took place in open shelf and prodelta to delta front (middle to inner neritic) environments. Sandy sections are probably wave-and current-deposited sand bodies.

28 Well-log and seismic reflection data associated with seismic Sequence 2 indicate that:

1. The traditional (hydrologic) subdivisions and the lithostratigraphy of the post-St. Marys(?) Chesapeake Group are inadequate for describing its seismic stratigraphy. It appears that the sandy interval of which the Manokin aquifer is a part, may be retained as a lithostratigraphic unit, the Manokin formation. The lithologically complex section that includes the Ocean City and Pocomoke aquifers is best treated as a single undifferentiated lithostratigraphic unit, the Bethany formation.

2. In the coastal area, an unconformity separates the post-Choptank Chesapeake Group from the overlying Columbia Group.

3. Depositional environments varied locally, within a trend of a southward and eastward increasing water depth. The subsequence geometries and distribution of well log facies indicate a complex depositional history which Hansen (1981) referred to as, "complex stratigraphy suggestive of a locally shifting shoreline" (p. 131 l.

Deposition apparently took place in wave-energy dominated delta plain to prodelta (marginal marine to middle neritic) environments. Sandy sections were deposited in a wide range of environments that include distributary channel, shore zone, crevasse splay, distributary mouth bar, and tidal delta. Seismic facies 2-1 probably is indicative of variable energy inner neritic to marginal marine delta plain environments. Seismic facies 2-2 probably is indicative of relatively lower energy inner to middle neritic delta front to prodelta environments. The log facies and lithologies within the Manokin formation that correlate with the large clinoforms present in Sequences 1 and 2 are indicative of a wave energy dominated delta front environment. The areal continuity of the Manokin indicates reworking by waves and/or currents.

The position of seismic facies 2-3 within Sequence 2 suggests that it was not an island as suggested by Weigle and Achmad (1982). Internal reflector configurations indicate fluvial/marginal marine and marine processes were active throughout deposition. The

29 probable depositional environment is a delta lobe that has been modified by marine processes. This delta lobe occurs within Subsequence 2b.

4. The deposition of sandy sediments and the locations of depocenters shifted with time, from the northern Fenwick Island area (Subsequence 2a, oldest), to the central Ocean City area (Subsequence 2b), to the north-central Delaware Coast (Subsequence 2c, youngest). The offshore projections of the two southernmost sand prone trends appear to correspond with the thicker parts of Subsequences 2a and 2b, suggesting that the sand prone sections extend off-shore. A model of the evolution of the post-Choptank depositional basin is summarized in Figures 14a-14c. During deposition of Subsequences 2a and 2b the delta advanced southeastward onto a shallow marine shelf. Maximum advance occurred during deposition of Subsequence 2b. This event created a local basin north of the axis of greatest sand accumulation. Subsidence of this area due to faulting may have contributed to the development of the basin. The basin was filled during deposition of Subsequence 2c.

Table 2 lists the seismic facies associated with the depositional sequences.

CONCLUSIONS

This report presents the results of an integrated study of the post-Choptank Chesapeake Group. The combination of onshore geophysical and lithologic well logs and nearshore multichannel common-depth-point seismic reflection data provide a view of the subsurface not available from either data set alone.

The results of this investigation suggest that the St. Marys(?) Formation and the sandy section that includes the Manokin aquifer may be retained as lithostratigraphic units. The Manokin formation is proposed as an informal lithostratigraphic unit that refers to the sandy interval of which the Manokin aquifer is a part. The section that includes the Ocean City and Pocomoke aquifers and adjacent and intervening confining beds is best treated as a single undifferentiated lithostratigraphic unit for regional studies. The Bethany formation is proposed as an informal lithostratigraphic unit.

30 \ • • 15 MILES \ o • '0 .' \ • I N~ 75-45'\ • • • 3.·••'+ I + \ • • I '~< I . I • ...... ~ \ . - \ ;/ 1/" \ • • • L..-..:...... _ w •• ~ • • • • • • EXPLANATION 7'-4" 75·00' +3.·,.' +38·1!~' _ .. _-- State boundary • Well/boring data point ~ • • Seismic line location - - Edge of basin

High energy t sand prone facies

Figure 14a. Depositional history-Subsequence 2a Basin. o

o o· • -IS 10 1!5 MILES \ ~ o .' \ o • N~ 1~.4S·\ • ,/S? 38-415' + 'I r'o:", (?';-"',O +~ \ 0 OJ l ?l """- o "'..~ I 0 / \ 0/0 , \ ?/ y', o L__ w

'" o o

o o EXPLANATION 715-45' 715-00' +38-115' +3S-1l5' -----.- State boundary • Well/boring data point o o ~ Seismic line location ~.JU~J...J -- Edge of basin " High energy, sand prone facies

o

Figure 14b. Depositional history-Subsequence 2b basin. I •

• /? a • 10 15 MILES \ • I N~ n~·4'·\ • 38-45'+ I \.. "\~'C I I • \• ~ \ \.. I .. • "- \ . '\.. • \.L. "~--. w •• w ..-? •• • • • • EXPLANATION 7'-45' 75·00' +38-1!5' +38-15' State boundary • Well/boring data point • '---., Seismic line location -- Edge of basin High energy, sand prone facies • .-_.

Figure 14c. Depositional history-Subsequence 2c basin. Table 2. Sequences, Seismic Facies, and Depositional Environments.

Sequence Unit Seismic Facies Depositional Environment

1 St.Marys(?) 1-1. Transparent to Low energy, high amplitude, prodelta, delta parallel. front, mid- to inner-shelf.

St.Marys(?)­ 1-2. Variable con­ Variable energy Manokin tinuity, moderate inner shelf to transition to low amplitude marginal marine parallel to hum­ delta front to mocky. delta plain.

2 Manokin and 2-1. Variable contin­ Variable energy Bethany uity, low to mod­ mid-shelf to formations erate amplitude, marginal marine parallel to hum­ delta plain to mocky. delta front.

2-2. Variable con­ Delta plain tinuity, medium to (subaqueous). high amplitude, parallel.

2-3. Mounded. Delta plain, distributary channel complex.

34 The interpretation of seismic data shows that the post­ Choptank Chesapeake Group consists of at least two depositional sequences which are separate from the underlying (older Chesapeake Group) and overlying (Columbia Group) sequences. The complex internal structure of the upper sequence demonstrates that the lithostratigraphy of the post-st. Marys(?) Chesapeake Group is not correlative with its seismic stratigraphy.

The integrated interpretation of onshore and offshore data in the context of depositional history and environments suggests that the post-Choptank Chesapeake Group was deposited as a wave and fluvial energy-dominated delta complex. The delta complex was deposited in three phases. During the first two phases the delta advanced onto a shallow marine shelf. During the third phase a small basin, which formed north of the first two lobes, was filled. The Manokin formation was deposited in a delta front to lower delta plain setting, probably as wave reworked distributary channel sands and distributary mouth bars. The Bethany formation was deposited in a locally variable setting ranging from delta plain to delta front.

The results of this investigation are applicable to other projects in Sussex County. They will have direct application to water resources exploration and planning. Additionally, the depositional model can be a starting point in the geological evaluation of the Columbia Group.

Before the stratigraphy and depositional history of the post-Choptank Chesapeake Group can be fully understood, further work, especially biostratigraphic investigation, is needed to refine the ages and range of depositional environments present. Additional seismic coverage is needed to further define sequence geometries and boundary types, and to define the seismic stratigraphy of the post-Chesapeake Group sediments.

35 REFERENCES

Asquith, G. B., and Gibson, C. R., 1982, Basic well log analysis for : American Association of Petroleum Geologists, Tulsa, 216 p.

Benson, R. N., 1984, Structure contour map of pre-Mesozoic basement, landward margin of Baltimore Canyon Trough: Delaware Geologic Survey Miscellaneous Map Series No.2.

Benson, R. N., Andres, A. S., Roberts, J., and Woodruff, K. D., 1986 (in press), Seismic stratigraphy along three multichannel seismic reflection profiles off Delaware's coast: Delaware Geological Survey Miscellaneous Map No.4.

Brown, L. F. and Fisher, W. L., 1980, Seismic stratigraphic interpretation and petroleum exploration: American Association of Petroleum Geologists Continuing Education Course Note Series No~ 16, 125 p.

Cushing, E.M., Kantrowitz, I. M., and Taylor, K. R., 1973, Water Resources of the Delmarva Peninsula: U. S. Geological Survey Professional Paper 822, 58 p.

Darby, D. A., 1984, Origin of transgressive coarse sediments marking in the Tertiary section exposed along the Pamunkey River, Virginia: in Ward, L. W., and Krafft, K. eds., Guidebook, 1984 Field Trip, Atlantic Coastal Plain Geological Association, p. 93-110.

Dix, G. H., 1955, Seismic velocities from surface measurements: , v. 20, p. 68-86.

Field, M. E., 1979, Sediments, shallow subbottom structure, and sand resources of the inner continental shelf, central Delmarva Peninsula: U. S. Army, Coastal Engineering Research Center Technical Paper 79-2, 122 p.

Galloway, W. E •. , and Hobday, D. K., 1983, Terrigenous clastic depositional systems: Springer-Verlag, New York, 490 p.

Gernant, R. E., Gibson, T. G., and Whitmore, F. C., 1971, Environmental history of the Maryland Miocene: Maryland Geological Survey Guidebook No.3, 58 p.

36 Gibson, R. E., 1970, Late Mesozoic-Cenozoic tectonic aspects of the Atlantic coastal margin: Geological Society of America Bulletin 81, p. 1813-1822.

Hansen, H. J., 1981, Stratigraphic discussion in support of a major unconformity separating the Columbia Group from the underlying Upper Miocene Aquifer Complex in Eastern Maryland: Southeastern Geology, v. 22, p. 123-138.

Hodges, A. L., 1984, Hydrology of the Manokin, Ocean City, and Pocomoke aquifers of southeastern Delaware: Delaware Geological Survey Report of Investigations 38, 60 p.

Jordan, R. R., 1974, Pleistocene deposits of Delaware: in Oakes, R. Q., and Dubar, J. R., eds., Post-Miocene stratigraphy: State University Press, p. 30-52.

Kidwell, S. M., 1984, Outcrop features and origin of basin margin unconformities in the lower Chesapeake Group (Miocene), Atlantic coastal Plain: in Schlee, J. S., ed., Interegional unconformities and hydrocarbon accumulation: American Association of Petroleum Geologists Memoir 36, p. 37-58.

Miall, A. D., 1984, Principles of sedimentary basin analysis: Springer-Verlag, New York, 490 p.

Miller, J. C., 1971, Ground-water geology of the Delaware Atlantic seashore: Delaware Geological Survey Report of Investigations 17, 33 p.

Mitchum, R. M., Vail, P. R., and Sangree, J. B., 1977a, Stratigraphic interpretation of seismic reflection patterns in depositional sequences: in Payton, C. E., ed., Seismic stratigraphy-applications to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, p. 117-133.

Mitchum, R. M., Vail, P. R., and Thompson, S., 1977b, The depositional sequence as a basic unit for stratigraphic analysis: in Payton, ed., Seismic stratigraphy - application to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, p. 53-62.

Mitchum, R. M. and Vail, P. R., 1977, Seismic stratigraphic interpretation procedure: in Payton, C. E., ed., Seismic stratigraphy - applicationS-to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, p. 135-144.

37 Mixon, R. B., 1985, Stratigraphic and geomorphic framework of uppermost Cenozoic deposits in the southern Delmarva Peninsula, Virginia and Maryland: U. S. Geological Survey Professional Paper 1067-G, 53 p.

Newell, W. L., and Rader, E. K., 1982, Tectonic control of cyclic sedimentation in the Chesapeake Group of Virginia and Maryland: in Lyttle, P. T., ed., Central Appalachian geology: American Geological Institute, p. 1-28.

Owens, J. P. and Denny, C. S., 1979, Upper Cenozoic deposits of the central Delmarva Peninsula, Maryland and Delaware: U. S. Geological Survey Professional Paper 1067-A, 28 p.

Pickett, T. E., 1976, Generalized of Delaware: Delaware.Geological Survey Special Publication No.9, 1 p.

Rasmussen, W. C., and Slaughter, T. H., 1955, The water resources of Somerset, Wicomico, and Worcester counties: Maryland Department of Geology, Mines and Water Resources Bulletin 16, 533 p.

--~=-7' 1957, The water resources of Caroline, Dorchester, and Talbot counties: Maryland Department of Geology, Mines and Water Resources Bulletin 18, 456 p.

Sangree, J. B., and Widmier, J. M., 1977, Seismic interpretation of clastic depositional facies: in Payton, C. E., ed., Seismic stratigraphy - applications to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, p. 165-184.

Spoljaric, N., 1979, LANDSAT view of Delaware: Delaware Geological Survey Open File Report No. 12.

Sundstrom, R. W., and Pickett, T. E., 1969, The availability of ground water in eastern Sussex County, Delaware: University of Delaware, Water Resources Center, 136 p.

Ward, L. w., 1984, Stratigraphy of outcropping Tertiary beds along the Pamunkey River - Central Virginia Coastal Plain: in Ward, L. w., and Krafft, K., eds., Guidebook, 1984 Field Trip, Atlantic Coastal Plain Geological Association, p. 11-78.

38 Ward, L. w., and Blackwelder, B. W., 1980, Stratigraphic revision of upper Miocene and lower Pliocene beds of the Chesapeake Group, Middle Atlantic Coastal Plain: U. S. Geological Survey Bulletin 1492-D, 61 p.

Weigle, J. M. and Achmad, G., 1982, Geohydrology of the fresh­ water aquifer system in the vicinity of Ocean City, Maryland, with a section on simulated water level changes: Maryland Geological Survey Report of Investigations 37, 55 p.

Woodruff, K. D., 1977, Preliminary results of seismic and magnetic surveys off Delaware's coast: Delaware Geological Survey Open File Report No. 10, 19 p.

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