STRATIGRAPHY AND DEPOSITIONAL HISTORY

OF THE

POST-CHOPTANK CHESAPEAKE GROUP

by

A. Scott Andres

Delaware Geological Survey

August 1986

CONTENTS

ABSTRACT • •

INTRODUCTIONPurpose and ScopePrevious InvestigationsAcknowledgments • •

METHODS OF INVESTIGATIONGeneral . . • . • •Well Log Analysis •Seismic Stratigraphy.Seismic Facies Analysis

ONSHORE DATA • • • • • • • •Stratigraphy and Lithology.Depositional Framework••

OFFSHORE SEISMIC STRATIGRAPHYAND CORRELATIONS WITH ONSHORE CONTROL. •

External Form and Gross Correlations.Internal Form and Correlations.Structures. •Seismic Facies.

DEPOSITIONAL HISTORY

CONCLUSIONS.

REFERENCES •

ILLUSTRATIONS

· .

· .

· .

Page

1

2225

55568

88

15

1818222425

28

30

36

Figures 1.2.3.4.5.6.

Well and seismic line location mapIdealized well log facies.Cross-section A-A'Cross-section B-A' •••Cross-section C-C' •••Base of st. Marys(?) Formationstructure contour map • • • •

i

479

1011

16

Page

Figures 7. St. Marys(?) Fo:r:mation isopachouscontour map . · · · · · · · · . . 17

8. Base of Manokin formation structurecontour map . · · · · · · · · 19

9. Manokin and Bethany formationsisopachous contour map. • · · 20

10. Manokin and Bethany formations cumulativesand percentage contour map • 21

11- Generalized geologic and seismiccross-section · · · · · · · · 23

12. Possible faults and structural style 261 3. Seismic facies · · · · · · · · . 271 4a. Depositional history-Subsequence 2a basin. 3114b. Depositional history-Subsequence 2b basin. 3214c. Depositional history-Subsequence 2c basin. 33

TABLES

Tables 1­2.

Geologic and Hydrologic Units••.•••••Sequences, seismic facies, and depositional

environments .•••••••••••.••

ii

3

34

STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THEPOST-CHOPTANK CHESAPEAKE GROUP

ABSTRACT

Onshore and offshore geological and geophysical data wereused to investigate the lithostratigraphy, seismic stratigraphy,and depositional history of the late Tertiary age post-ChoptankChesapeake Group rocks in Sussex County, Delaware and adjacentcounties in Maryland. The results of this investigation suggestthat the st. Marys!?) Formation and the sandy interval of whichthe Manokin aquifer is a part, are distinct lithostratigraphicunits. The Manokin formation is proposed as an informallithostratigraphic unit that refers to the sandy interval ofwhich the Manokin aquifer is a part. On a regional scale, thesection containing the Ocean City and Pocomoke aquifers andadjacent and intervening confining beds is best treated as asingle undifferentiated lithostratigraphic unit. The Bethanyformation is proposed as an informal lithostratigraphic unit thatrefers to this section.

The seismic data suggest that the post-Choptank ChesapeakeGroup consists of at least two depositional sequences that areseparate from the underlying older Chesapeake Group and overlyingdepositional sequences. The complex internal structure of theseismic sequences demonstrates that, in Delaware, thelithostratigraphy of the post-St. Marys!?) Chesapeake Group isnot correlative with its seismic stratigraphy.

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

1

INTRODUCTION

Purpose and Scope

This report documents the results of an investigation of thepost-Choptank Chesapeake Group, defined herein as the st.Marys(?) Formation, and the overlying Upper Miocene AquiferComplex as described by Hansen (1981). The Aquifer Complex hastraditionally been subdivided into the Manokin, Ocean City, andPocomoke aquifers, and intervening fine-grained aquitards (seeTable 1). The Aquifer Complex currently supplies water to manyof the coastal communities and is the probable source of waterfor future development. An understanding of the geologicframework will aid future hydrogeologic investigations by moreclearly defining the relationships between aquifers and confiningbeds and by identifying potential hydrologic boundaries. A mapof the study area with well and seismic line locations is shownin Figure 1.

Onshore and offshore geologic and geophysical data were usedto investigate the lithostratigraphy, seismic stratigraphy, anddepositional history of the late Tertiary age post-ChoptankChesapeake Group in Sussex County, Delaware and adjacent countiesin Maryland. The data include three 24-fold common depth point(CDP) stacked seismic reflection profiles collected for theDelaware, Maryland, and U. S. Geological surveys, drill-holegeophysical and lithologic logs, and published outcrop andpaleontologic evaluations. The combination of seismic and well­log data provides a perspective of the subsurface not availablefrom either data set alone.

Previous Investigations

Previous work has largely been water resources oriented andhas focused on the occurrence and availability of fresh groundwater (e.g., Rasmussen and Slaughter, 1955; Sundstrom andPickett, 1969; Miller, 1971; Cushing et al., 1973; Weigle andAchmad, 1982; Hodges, 1984). Geologic~ata were usually obtainedas a by-product of water resources investigations. As a resultof this emphasis on hydrogeology, the core samples necessary forcompleting detailed geologic evaluations have not been available,and much of the geologic knowledge is limited to interpretationsof the distribution of the fresh water-bearing aquifers. Thisapproach has evolved a confusing hybrid stratigraphy, in whichunits are defined according to hydrologic and lithologiccriteria.

2

r.Hydrologic UnitsU

Geologic Units0(Weigle and Achmad,1982)a.

(used in this report)U.I

I CDUnnamed beds of

~c marine, estuarine, andOCD

continental origin::J:U

('-- I I I I 11111111111111111CDC 0 Omar FormationCD :Qa.U0 e:::S- :d~III Beaverdam Formation Pleistocene aquiferCD oC)

UQ. ----------

Upper confining bedPocomoke aquifer

Bethany formation Lower confining bedOcean City aquiferLower confining bed

CDc Manokin formation Manokin aquiferCDU0- Sf. Marys (1) Formation:E

? I ?"'IIIIIIII'?

Choptank Formation Frederica aquifer~---------_..-----------

Calvert Formation

Table 1. Geologic and hydrologic units.

3

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Several recent interpretations of the geology of theChesapeake Group are presented in Owens and Denny (1979), Hansen(1981), Newell and Rader (1982), Kidwell (1984), Ward (1984), andMixon (1985). Woodruff (1977) presents preliminaryinterpretations of the seismic reflection data used in thisreport. One interpreted marine multichannel seismic reflectionprofile used in this report is presented in Weigle and Achmad(1982). Field (1979) presents the results of a single channelsparker 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 DelawareGeological Survey for their discussions of the interpretation ofgeologic and geophysical data. Harry J. Hansen (MarylandGeological Survey), and Kenneth D. Woodruff, John H. Talley,Richard N. Benson, and Robert R. Jordan (Delaware GeologicalSurvey) 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 analysispresented in Miall (1984) and Galloway and Hobday (1983) wereused to evaluate the bedding geometry and spatial relationshipswithin and among lithologic units of the post-Choptank ChesapeakeGroup. The approach generates information that can be used topredict the subsurface distribution and gross texturalcharacteristics of sand bodies. This information is useful inground-water expforation.

Well Log Analysis

Lithostratigraphic units were defined based on theoccurrence of similar geophysical log characteristics andlithologies both in individual drill holes and on a regionalscale. There was no chronostratigraphic intent in thedesignation of the lithostratigraphic units.

5

Semi-quantitative measurements of shale percentage werecalculated for natural gamma logs from the equations and graph inAsquith and Gibson (1982, p. 91-95). The 10 percent shale valuewas used for separating clean sands from muddy sand. Gamma logswere checked against lithologic and electric logs to account forthe effects of glauconite, "organic" material, and potassiumfeldspar. Sand thickness and sand percentage were calculated foreach log and a sand percentage contour map was constructed for aselected stratigraphic interval.

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

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

Natural gamma radiation logs were the primary sources of welllog data, because the gamma tool is not affected by changes inwater quality as electric logs are; and, they show more detailthan lithologic logs constructed from ditch samples. Hansen(1981) reports that the st. Marys(?) Formation is a distinctlithostratigraphic unit, recognizable by a distinctive gamma logsignature and its clayey lithology. Hansen (1981) uses the baseof the st. Marys(?) as a marker horizon because it is one of thefew 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 wellswith other types of logs (lithologic, electric) were compared tothem. Available core and ditch samples from Delaware wereanalyzed for gross textural and lithologic content.

Seismic Stratigraphy

The seismic stratigraphy of the post-Choptank ChesapeakeGroup and overlying deposits was based on the analysis of 24-foldCDP stacked high-resolution seismic reflection data.Depositional sequences and sequence boundary types wereidentified according to the principles and procedures of seismicstratigraphy outlined by Mitchum et al. (1977b) and Mitchum andVail (1977). ----

6

LOW ENERGYAGGRADATIONAL PROGRADATIONAL MIXED

HIGH ENERGYAGGRAOATIONAL

Figure 2. Idealized well log facies.

7

EROSIONAL

Seismic facies were defined for each sequence, intervalvelocities were calculated at several points per line by usingroot mean square stacking velocity plots and the Dix equation(Dix, 1955) and time-to-depth conversions were calculated. Thecalculated depths were then used to correlate sequenceboundaries, reflectors and seismic facies with lithologic changesand formation boundaries in onshore borings. Additionally,Weigle and Achmad's (1982) picks of formation and aquiferboundaries on lines GD1 and GD2 were traced northward along lineDS5 and the DGS series lines, and compared with this study.

Seismic Facies Analysis

Information regarding depositional environments and lithologycan usually be inferred from the geometry of reflection packagesand the character of the reflections (Mitchum et al., 1977a).Identification of seismic facies and interpretations of deposi­tional environments are based on the procedures presented inSangree and Widmier (1977) and Brown and Fisher (1980).Lithofacies penetrated in onshore borings compare favorably withoffshore seismic facies, suggesting that there are no majorchanges in lithologies and sedimentary facies between the coastand nearby offshore locations. This allows a comparison of thetwo data sets and provides a view of the subsurface not availablefrom either data source alone.

Reflection amplitude and continuity were subjectivelydesignated poor, moderate, or high. Amplitude ratings werejudged relative to the range of amplitudes observed in the data.Continuity ratings were based on the distance a reflection eventcan 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 (approximately760-3050 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 distributionof the units. It should be noted that, on the DelmarvaPenninsula, the name St. Marys Formation is not acceptable instrict stratigraphic terms because correlation with the st.

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Marys Formation at its type locality (west of Chesapeake Bay) cannot be rigorously demonstrated. For this reason, Rasmussen andSlaughter (1957) and Hansen (1981) have d~fined the fine-grainedbeds overlying the Choptank Formation on the Delmarva Peninsulaas the St. Marys(?) Formation.

The name Yorktown and Cohansey Formations(?) has previouslybeen 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 theYorktown Formation and the Cohansey Formation (Owens and Denny,1979), and appears to show that the Yorktown and CohanseyFormations(?) is older than, and stratigraphically distinct from,the Yorktown Formatidn (Mixon, 1985). For these reasons andbecause of new data, the Yorktown and Cohansey Formations(?) isrevised in favor of two new lithostratigraphic units, the Manokinformation and the Bethany formation. These are consideredinformal units pending the availability of a completely coredsection.

In Delaware, the basal lithostratigraphic unit of the post­Choptank Chesapeake Group is the St. Marys(?) Formation.Available biostratigraphic data suggest that the st. Marys(?) islate Miocene in age and is younger than the type st. Marys,perhaps coeval to the Claremont Manor Member of the EastoverFormation (Hansen, 1981). At the type locality, the st. MarysFormation 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 gammalogs by a sharp clay on sand "kick" (see figures 3-5). The"kick" is less distinct, but still recognizable close to thesubcrop area (wells Mh41-6, Nc43-2, Oe13-1, Of34-2). Evidence ofa 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 andChoptank-Eastover contacts have been identified asdisconformities (Gernant et al., 1971; Kidwell, 1984; Ward,1984). In Delaware, there-iS-no definitive evidence for adisconformity between the Choptank and st. Marys(?) although thismay be due to the less detailed coverage of well log data.Alternatively, Newell and Rader (1982) suggest that theChoptank-St. Marys contact is transitional, and that the st.Marys is part of the depositional sequence that includes theCalvert and Choptank formations. It may be that the nature ofthe Choptank-St. Marys contact varies within the basin.

12

In onshore borings, the st. Marys(?) consists of fossili­ferous, glauconitic, and lignitic, gray, olive-gray, and blue­gray 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 energyaggradational facies. Some logs show small scale erosionaland/or progradational facies (mixed facies). Gypsum(?) crystalshave been found in core samples from well Og31-1. Paleontologicdata suggest that the st. Marys(?) was deposited on a shoalingmarine shelf (water depths of 265 to less than 100 feet, 81 to 30meters) (Hansen, 1981).

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

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

Of the three aquifer units, the Manokin appears to be theonly areally persistent and lithologically distinct unit. Theterm Manokin formation is proposed as an informallithostratigraphic unit that refers to the sandy interval ofwhich the Manokin aquifer is a part. The name Manokin formationis used because the Manokin formation corresponds to thelithologic section previously defined as the Manokin aquifer byRasmussen and Slaughter (1955).

In Delaware, the Manokin formation is predominately gray toolive-gray, fine to coarse sand, and silty and clayey sand, withsome beds of gravel, and local clay/silt, lignitic, shelly beds.The Manokin generally consists of a lower progradational sectionand 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 deposition took place in middleneritic to marginal marine environments (Hansen, 1981).

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

The Ocean City and Pocomoke aquifers occur within a sectionof lithologically complex lensing sandy and clayey units whichlie stratigraphically above the Manokin formation (see Figures3-5). The Pocomoke aquifer lies stratigraphically above theOcean City aquifer (Hansen, 1981), and has been dated lateMiocene by Owens and Denny (1979). More recent work by Mixon(1985) in the Accomack County, Virginia-Somerset County, Marylandarea suggests that the Pocomoke aquifer may correlate in partwith the late Miocene Eastover Formation and Pliocene YorktownFormation, units he dated on the basis of molluscan assemblages.

As noted in Maryland by Hansen (1981), the Pocomoke and OceanCity aquifers in Delaware are part of a complex of lensing sandyand clayey units, and are not discrete sand bodies that can beregionally correlated. Not only do the sandy units pinch out,but the number of sandy units is not consistent from drill holeto drill hole. It is not surprising that there is considerableconfusion in the literature regarding the distribution of thePocomoke and Ocean City aquifers. To avoid this problem, theBethany formation is proposed as an informal lithostratigraphicunit that refers to the rocks containing the Ocean City andPocomoke aquifers, and the basal, intervening, and overlyingclay/silt (confining) beds. The name Bethany formation is takenfrom its occurrence in the Bethany Beach 7.5 minute topographicsheet, in well Qj32-14, 125 to 320 feet (38 to 97 meters) belowland surface.

The Bethany formation is a lithologically complex sectioncharacterized by beds of predominately gray blue-gray, or olive­gray, fine to very coarse sand, interlayered with beds ofpredominately gray, olive-gray and blue-gray clay/silt.Lignit±c, glauconitic, oxidized, gravelly or shelly beds arecommon. Carbonate (siderite?) concretions are found in many ofthe clay/silt beds. The Bethany shows mixed well log facies,although individual logs may have a greater thickness ofprogradational 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 orclay/silt sections range from less than 1 to nearly 40 feet (0.3to 12 meters) thick, and gamma logs show a characteristic"sawtooth" pattern indicating frequent changes in energyconditions. For example, in some drill holes, clay/silt andgravel are interbedded. Paleontologic evidence indicates thatdeposition took place in middle neritic to marginal marineenvironments (Hansen, 1981).

The top of the Bethany formation is placed at the base of theColumbia Group. On gamma logs the contact usually is a sharpsand on clay trace shift. It is lithologically marked by achange from the olive-gray or gray, fossiliferous clayey or siltyPocomoke to the brown non-fossiliferous, poorly sorted sands andgravels of the Columbia Group. The contact is more difficult topick where the upper part of the Bethany is non-fossiliferous,sandy, or oxidized, or where the basal part of the Columbia isgray in color and/or fine-grained.

The stratigraphic relationship between the Manokin andBethany formations and the overlying Columbia Group is thesubject of considerable debate. Jordan (1974) and Hansen (1981)argue that the overlying Columbia Group unconformably overliesthe Manokin and Bethany formations. Owens and Denny (1979) arguethat the "Manokin" and "Pocomoke" beds are the marine equivalentsof the Pensauken Formation (equivalent to the fluvial ColumbiaFormation). Although conclusive paleontologic evidence islacking, the structural and stratigraphic arguments presented byHansen (1981) demonstrate that the Columbia Group is younger thanthe Manokin formation and the basal clay/silt bed of the Bethanyformation. However, the stratigraphic relationship between theColumbia Group and the remainder of the Bethany is not asconclusively demonstrated by Hansen (1981). Similarly, inDelaware, well log data does not conclusively demonstrate thestratigraphic relationship between the Bethany and Columbia.

Depositional Framework

Figure 6 is a structure contour map of the elevation of thebase of the st. Marys(?) Formation. Seismic reflection data wereused to extend the structure contours offshore. Figure 7 is anisopachous map of the St. Marys(?). In general, the St. Marys(?)is a southeasterly thickening wedge resting on a southeasterlysloping surface. The increased slope and thickness near RehobothBeach indicate that there are structural complications in this

15

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area. The decreased thickness trend in central Delaware isaccompanied by a thickening of the overlying Manokin and issuggestive of post-depositional erosion or a facies change at thetop of the st. Marys(?).

Figure 8 is a structure contour map of the base of theManokin. Figure 9 is an isopachous (thickness) map of thecombined Manokin and Bethany formations. In general, the Manokinand Bethany formations form a southeasterly thickening wedgeresting on a southeasterly sloping surface. The thickness map iscomplicated by post-depositional erosion. In general, the amountof missing section is greater along Delaware Bay. An anomolouslythick section trends northwest-southeast from drill hole Nf34-2to drill hole Og31-1 and corresponds to a thin area in theunderlying St. Marys(?).

A sand percentage map (see Figure 10) for the combinedManokin and Bethany interval shows several distinct sand-pronetrends. One is parallel to strike and follows the subcrop of theManokin aquifer as mapped by Pickett (1976) and Hansen (1981).Two dip-oriented "lobes" appear to originate near drill holeOe13-1 and radiate southeastward and southward. The sandpercentage generally decreases in a downdip direction.Comparison of Figures 8 through 10 seems to show afluvial/distributary channel system and associated shallow shelfenvironments. The lobate pattern is indicative of a waveenergy-dominated delta complex (Galloway and Hobday, 1983).Additional well coverage is needed to refine the shape of thesand prone trends.

OFFSHORE SEISMIC STRATIGRAPHYAND CORRELATIONS WITH ONSHORE CONTROL

External Form and Gross Correlations

Time-to-depth conversion of seismic reflection data andprojection of the base of St. Marys(?) structure contours tooffshore locations indicates that the base of the st. Marys(?)(i.e., st. Marys(?)-Choptank contact) is represented on theseismic reflection profiles by a strong areally continuouspositive reflector. This reflector truncates several underlyingreflectors and appears to be a boundary between the underlyingrelatively continuous reflectors and overlying generallydiscontinuous reflectors. As such, the reflector correlated with

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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 isa southward and eastward thickening wedge unconformably overlainby a complex sheet that thickens toward the north, east, andsouth. There are at least three depositional sequences withinthe section studied (numbered 1-3 in Figure 11). A minimum oftwo depositional sequences are present in the post-ChoptankChesapeake Group. Three subsequences (2a-2c) have beenidentified in the upper sequence of the post-Choptank ChesapeakeGroup. Additional seismic coverage is needed to determine thecontinuity of these subsequences.

Sequence 1, the basal sequence, is thickest in the northernpart of the study area. It thins rapidly at SP 320-400, LineDGS1, in the vicinity of Indian River Inlet, where the slope ofthe upper bounding reflector becomes steeper. The sequencethickens again to the south and east by divergence of reflectors.The top of the sequence is well defined in the southern part ofthe study area by a moderate to strong positive reflector. Inthe area north of Indian River Inlet, the reflector marking theupper boundary fades in and out, suggesting frequent facieschanges either above or below the boundary. The upper boundaryappears to be concordant except near Indian River Inlet.

Sequence 2 is a southward and eastward thickening wedge. Theboundaries of the subsequences are usually discordant with Figure11 truncation, toplap- and lapout- (onlap and downlap) typeboundaries (as defined in Mitchum et al., 1977b) present. Atseveral locations the reflectors bounding the subsequences fadein and out, suggesting frequent facies changes either above orbelow the boundaries.

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

Internal Form and Correlations

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

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Manokin transition. In northern areas, Sequence 2 includes theupper part of the Manokin and the Bethany formations. Insouthern and eastern areas, Sequence 2 includes the Manokin andBethany and the upper part of the St. Marys(?). Correlationswith onshore well logs and comparison with the structure contourmaps of the base of the Columbia Group in Field (Figure 15, 1979)and Sundstrom and Pickett (Figure 9, 1969) indicate that Sequence3 includes the Columbia Group (as defined by Jordan, 1974) andyounger sediments. Sequence 3 appears to truncate the underlyingChesapeake Group. This indicates that in the coastal area, theColumbia 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 theindividual units of the Columbia Group and the younger sediments.Considering the complex post-Chesapeake Group depositionalhistory of the area, Sequence 3, as recognized here, shouldconsist of several depositional sequences. For example, Field(1979) recognized four areally continuous reflectors within thepost-Chesapeake section that are not evident in the recordsstudied for this report.

The complex internal structure of Sequence 2 indicates thatthe traditional (hydrologic) subdivisions and thelithostratigraphy of the Aquifer Complex are inadequate fordescribing its seismic stratigraphy. Figure 11 shows that thethree subsequences within Sequence 2 (2a, 2b, 2c) do notcorrelate with the lithostratigraphic units. In fact, thesubsequences can include parts of one, or both of thelithostratigraphic units and/or parts of the underlying St.Marys{?) Formation. Additionally, individual subsequences arenot continuous on the regional scale over which the lithologic oraquifer 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 onlyabout 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 Groupsection.

•Structures

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

24

(one example is shown in Figure 12). These possible faults occurin the area where structure contour maps indicate structuralcomplications (see Figure 7). In general, these possible faultsdo not truncate or offset reflectors. It may be that the offsetis beyond the resolution of the data. These faults appear toterminate within the post-Choptank Chesapeake Group andapparently persist down to the pre-Mesozoic basement as mapped byBenson (1984), suggesting that older basement structures wereactive and influenced the location of deposition during themiddle to late Miocene.

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

Seismic Facies

There are two seismic facies in Sequence 1 (see Figure13): variable continuity (high to low), transparent to highamplitude, and parallel (Type 1-1); and, variable continuity(moderate to high), moderate to high amplitude, and parallel tohummocky (Type 1-2). The hummocky reflectors appear to be thinprogradational sequences. Facies type 1-1 is common in the lowerthird of the sequence to the north of the clinoform andthroughout the sequence to the south of the clinoform. There isa 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 variablecontinuity (poor to moderate), variable amplitude (poor tomoderate), and hummocky to parallel reflectors. Many of thehummocky reflectors appear to be cut-and-fill features, while theremainder appear to be thin prograding sequences. The second(Type 2-2) exhibits variable continuity (poor to high), moderateto high amplitude, and parallel reflectors. Type 2-2 mostcommonly 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 and150-100 (a distance of approximately 1.5 miles, 2.4 kilometers)respectively. Weigle and Achmad (1982) tentatively identify thefeature as a relict island. The feature extends fromapproximately 0.125 seconds (within subsequence 2b) to the

25

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surface, where it forms a bathymetric high. Lateral boundarytypes are both concordant and discordant. Discordant boundarytypes exhibit toplap, baselap, and onlap. Laterally boundingfacies are Type 2-1 to the west and Types 2-1 and 2-2 to theeast. The base of the feature appears to be discordant, andcould be an erosional surface. Internal reflectionconfigurations include both high amplitude and parallel, andmoderate amplitude and hummocky. Many of the hummocky reflectorsappear to be cut and fill structures.

The subsequence geometries and complex interfingering ofseismic facies are indicative of a complicated depositionalsystem that has characteristics of both deltaic and shelfsystems. The interpretation of the data is included in a latersection.

DEPOSITIONAL HISTORY

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

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

2. In general, higher but variable energy, shallower water(inner neritic) environments (seismic facies 1-1) werepresent in the northern part of the st. Marys(?) basin,whereas lower, uniform energy, deeper water (middleneritic) environments (seismic facies 1-2) were presentin the southern part. The St. Marys(?)-Manokintransltion marks the southward progradation of higherenergy deltaic environments over the low energy openshelf environments. The section that includes the topof the st. Marys(?) and basal part of the Manokin wasdeposited as part of a time transgressive, continuousdepositional event that becomes younger in a downdipdirection. The St. Marys(?)-Manokin transition occurswithin Sequence 2 in southern areas.

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

28

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

1. The traditional (hydrologic) subdivisions and thelithostratigraphy of the post-St. Marys(?) ChesapeakeGroup are inadequate for describing its seismicstratigraphy. It appears that the sandy interval ofwhich the Manokin aquifer is a part, may be retained asa lithostratigraphic unit, the Manokin formation. Thelithologically complex section that includes the OceanCity and Pocomoke aquifers is best treated as a singleundifferentiated lithostratigraphic unit, the Bethanyformation.

2. In the coastal area, an unconformity separates thepost-Choptank Chesapeake Group from the overlyingColumbia Group.

3. Depositional environments varied locally, within a trendof a southward and eastward increasing water depth. Thesubsequence geometries and distribution of well logfacies indicate a complex depositional history whichHansen (1981) referred to as, "complex stratigraphysuggestive of a locally shifting shoreline" (p. 131 l.

Deposition apparently took place in wave-energydominated delta plain to prodelta (marginal marine tomiddle neritic) environments. Sandy sections weredeposited in a wide range of environments that includedistributary channel, shore zone, crevasse splay,distributary mouth bar, and tidal delta. Seismic facies2-1 probably is indicative of variable energy innerneritic to marginal marine delta plain environments.Seismic facies 2-2 probably is indicative of relativelylower energy inner to middle neritic delta front toprodelta environments. The log facies and lithologieswithin the Manokin formation that correlate with thelarge clinoforms present in Sequences 1 and 2 areindicative of a wave energy dominated delta frontenvironment. The areal continuity of the Manokinindicates reworking by waves and/or currents.

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

29

probable depositional environment is ahas been modified by marine processes.occurs within Subsequence 2b.

delta lobe thatThis delta lobe

4. The deposition of sandy sediments and the locations ofdepocenters shifted with time, from the northern FenwickIsland area (Subsequence 2a, oldest), to the centralOcean City area (Subsequence 2b), to the north-centralDelaware Coast (Subsequence 2c, youngest). The offshoreprojections of the two southernmost sand prone trendsappear to correspond with the thicker parts ofSubsequences 2a and 2b, suggesting that the sand pronesections extend off-shore. A model of the evolution ofthe post-Choptank depositional basin is summarized inFigures 14a-14c. During deposition of Subsequences 2aand 2b the delta advanced southeastward onto a shallowmarine shelf. Maximum advance occurred duringdeposition of Subsequence 2b. This event created alocal basin north of the axis of greatest sandaccumulation. Subsidence of this area due to faultingmay have contributed to the development of the basin.The basin was filled during deposition of Subsequence2c.

Table 2 lists the seismic facies associated with thedepositional sequences.

CONCLUSIONS

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

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

30

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Table 2. Sequences, Seismic Facies, and Depositional Environments.

Sequence Unit Seismic Facies DepositionalEnvironment

1

2

St.Marys(?)

St.Marys(?)­Manokintransition

Manokin andBethany

formations

1-1. Transparent tohigh amplitude,parallel.

1-2. Variable con­tinuity, moderateto low amplitudeparallel to hum­mocky.

2-1. Variable contin­uity, low to mod­erate amplitude,parallel to hum­mocky.

2-2. Variable con­tinuity, medium tohigh amplitude,parallel.

2-3. Mounded.

34

Low energy,prodelta, deltafront, mid- toinner-shelf.

Variable energyinner shelf tomarginal marinedelta front todelta plain.

Variable energymid-shelf tomarginal marinedelta plain todelta front.

Delta plain(subaqueous).

Delta plain,distributarychannel complex.

The interpretation of seismic data shows that the post­Choptank Chesapeake Group consists of at least two depositionalsequences which are separate from the underlying (olderChesapeake Group) and overlying (Columbia Group) sequences. Thecomplex internal structure of the upper sequence demonstratesthat the lithostratigraphy of the post-st. Marys(?) ChesapeakeGroup is not correlative with its seismic stratigraphy.

The integrated interpretation of onshore and offshore data inthe context of depositional history and environments suggeststhat the post-Choptank Chesapeake Group was deposited as a waveand fluvial energy-dominated delta complex. The delta complexwas deposited in three phases. During the first two phases thedelta advanced onto a shallow marine shelf. During the thirdphase a small basin, which formed north of the first two lobes,was filled. The Manokin formation was deposited in a delta frontto lower delta plain setting, probably as wave reworkeddistributary channel sands and distributary mouth bars. TheBethany formation was deposited in a locally variable settingranging from delta plain to delta front.

The results of this investigation are applicable to otherprojects in Sussex County. They will have direct application towater resources exploration and planning. Additionally, thedepositional model can be a starting point in the geologicalevaluation of the Columbia Group.

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

35

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38

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39