Sedimentary Facies Patterns and Geologic History of a Holocene Marine Transgression

JOHN C. KRAFT Department of Geology, University of Delaware, Newark, Delaware 1971

ABSTRACT dark gray lagoonal muds on the bay side. It Studies of Holocene sediments in coastal appears that distinctive sedimentary structures Delaware show complex sediment distribution and sediment size-sorting relationships, such as patterns resulting from lateral and vertical those that characterize the larger, more com- movement of successive environments of depo- mon sedimentary units of the coastal area, may sition over a Pleistocene unconformity. These be formed in miniature at the very thin edge of sediments are infilling a drowned topography transgression and may lead to considerable con- with a local relief of 70 ft and possibly up to 12 5 fusion in the interpretation of sediments of this ft eroded on highly variable Pleistocene sedi- type in the geologic record. ments. Identification of the Pleistocene surface remains a problem. However, it may be recog- INTRODUCTION: THE HOLOCENE nizable at the unconformity as a soil zone or GEOLOGIC SETTING intermixture of firm marsh clay-silts with Pleis- With the waning of the latest Wisconsin tocene sands, as well as on the basis of radiocar- glaciation, sea level began to rise and to form bon dates. a transgressive sequence of sediments across Larger depositional features forming around the Atlantic continental shelf. Evidences from eroding Pleistocene headlands and infilling the the area of study show a sea level rise from 90 estuaries include characteristic shoreline envi- ft below present sea level at a steadily decreas- ronments, such as spits, dunes, baymouth barri- ing rate, from early mid-Holocene to the pre- ers, an intermeshing network of tidal deltas, sent. Initially, sea level advanced rapidly across nearshore marine erosional-depositional sands the coastal plain from 300 to 350 ft below the and gravels, and lagoons or estuaries with fring- present sea level at rates of greater than several ing Spartina, Distichlis, and Phragmites marshes, feet sea level rise per century. From approxi- which form the westernmost edge of the trans- mately 8,000 yrs B. P. relative sea level rose at gressive units. The thickness and areal extent of a continuing rate of about 1 ft per century. the sedimentary bodies are to a large degree From about 3,700 yrs before present, relative controlled by the morphology of the Pleisto- sea level rise has been at a rate of slightly under cene unconformity. A large portion of the .5 ft per century based on evidence from the Holocene sedimentary units is being eroded by Delaware coastal area. The relative rate of sea the transgressing Atlantic Ocean. level rise may include effects of compaction and Cores of sediment under the shallow la- tectonic subsidence, as well as eustatic rise. goons, such as Rehobeth, Indian River, and One can make direct correlations from the Assawoman Bays, and in the fringing marsh surface sedimentary facies into the subsurface environment, show that the depositional units lithosomes of the late and middle Holocene are thin, highly irregular in areal extent, ex- Epoch in a continuing correlation back to about tremely variable in thickness, and difficult to 10,000 yrs B. P. Acccordingly, this study is project. Sedimentary processes active in the presented as a model to be used to develop an shallow bays include shoreline marsh erosion intensive analysis of the late Holocene Epoch as and the formation of thin, possibly ephemeral, evidenced by shoreline sequences along the en- beach-dune washover complexes consisting of tire Atlantic coast. clean, well-sorted sand, with typical beach and Mid-Atlantic coastal Delaware lies on the washover sedimentary structures. These wash- west flank of the Atlantic continental margin over beaches are an anomaly completely sur- geosynclines (Fig. 1). The present shoreline has rounded by Spartina marshes on the landward transgressed approximately two-thirds across side and extremely muddy sands grading into the Atlantic continental shelf-coastal plain to its

Geological Society of America Bulletin, v. 82, p. 2131-2158, 26 figs., August 1971 2131

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THE ATLANTIC CONTINENTAL MARGIN SEOSYNCLINES

Figure 1. The Atlantic continental margin geosyn- the Baltimore Canyon Trough in relation to the underly- cline showing the area of study on the western flank of ing Quaternary, Tertiary, and Mesozoic sediments.

present position. The area of study includes a part controlled by a secondary base level major coastal embayment, Delaware Bay, formed by a barrier ridge sequence. which is in part the drowned estuary of the This study is based on over 80 cores taken Delaware River. Some characteristics of the with a plastic pipe (Oaks, 1964), 300 6 in. Delaware Bay suggest that it may be a product probe samples, and 30 drill holes by truck of local, minor tectonic subsidence. Evidence mounted rig. presented suggests that the Cape May ridge and other features in Delaware may be relict barrier LATE WISCONSIN-EARLY ridges from a pre- or mid-Wisconsin high sea HOLOCENE PALEOGEOGRAPHY stand. These pre-Holocene barrier ridge-like AND PALEOGEOLOGY features may have localized the drainage system The identification of the erosion surface on in the Delaware Bay area so that streams spread Pleistocene coastal sediments that is herein out in a fan upstream from the area in which the designated the pre-Holocene surface is based Delaware River pierced the barrier sequence in on a series of qualified characteristics (Kraft, a line from Cape May to southern Delaware. If 1969a). so, the broad shallow southern part of Dela- In identifying the pre-Holocene surface, par- ware Bay may in fact be an erosion remnant, in ticular emphasis is placed upon projection of

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TABLE 1. IDENTIFICATION OF ments. Radiocarbon age determinations have PRE-HOLOCENE EROSION SURFACE been made on approximately 40 samples. (PRE-PRESENT SEA LEVEL RISE) These include C14 dates on mollusk shells such as Crassostrea virginica in growth position, on 1. Change in sediment characters such as marsh grass peats, pine and cypress stumps in mottling and oxidation of borings, plant growth position and, in a few cases, on indeter- debris, and other sediment features. minate organic material. Three of the dates 2. The more compact nature of the muds listed are greater than 39,000 yrs B. P. and and their varicolored nature (gray and were derived from an organic material resem- dark green overlying unconformity sur- bling gyttja under the Broadkill marsh. face; brighter green, orange, tan, yel- The presently identifiable Pleistocene coastal low, and gray under unconformity sur- environments have been studied in detail along face). the Lewes-Rehoboth Canal at Rehobeth Beach, 3. The lack or low quantity of decaying organic materials such as marsh grass Delaware, and along the Assawoman Canal be- and wood fragments under the surface. tween Indian River and Assawoman Bay in the 4. Direct correlation with areal dis- southern portion of the area of study. Addi- tribution of surface Holocene and tional data has been obtained by drilling Pleistocene sediment types and patterns. through the pre-Holocene surface into the 5. Radiocarbon age determination of or- Pleistocene sediments and from various scat- ganic matter from the Holocene sediments tered sandpits, drainage ditches, and inland and from a limited number of the Pleisto- dune fields in the area of study. Sediments iden- cene sediments. tified as Pleistocene in outcrop occur in eleva- tions up to 25 ft above present sea level along the Assawoman Canal and have lineation fea- the presently eroding trellis-dendritic drainage tures that are in line with ridges farther inland pattern surface into the subsurface under the on the Delmarva Peninsula and in the subma- Holocene coastal environments. The control rine offshore area (Fig. 2). These barrier ridges morphology can be projected into the adjacent are a product of higher sea stand in mid-Wis- subsurface, thus lending a strong validity to the consin or Sangamon time. Sedimentary envi- interpretation. In addition, the older sediment ronments identified in the White Creek and the characteristics, in particular the mottling and Assawoman Canal vicinity include a dune area, oxidation and the more compact or firm nature now covered by a mature forest, characterized of the sediments, contrast greatly with the typi- by well-sorted fine to medium sand with long, cal soft dark gray and medium gray sediments sweeping cross-bedded units. Adjacent to this of the Holocene coastal lagoonal and estuarine dune field the presently cultivated area is un- sequences and the light gray to tan and non- derlain by well-sorted, clean beach sands with oxidized sands of the Holocene coastal environ- low angle (2 degrees) parallel lamination. In

WHITE NW ASSAWOMAN -*>SE CREEK CANAL PRE-HOLOCENE

PRE-HOLOCENE BARRIER RIDGE

PRE-HOLOCENE LAGOON-MARSH

300 600 METERS

Figure 2. An interpretive cross section along White derlying and surrounding Pleistocene sediments Creek and the Assawoman Canal showing the relation- (compare with Fig. 3). ship of Holocene sedimentary environments to the un-

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Figure 3. Two barrier ridges exposed in the Quil- the nearshore marine environment, immediately adja- len's Point area near Bethany Beach to the south of In- cent. Figure 2 shows an interpretive cross section dian River in coastal Delaware. The barrier ridges shown through these barriers in the vicinity of the Assawoman are interpreted to be late Sangamon or mid-Wisconsin in Canal in the south and west of the photograph. Air photo age and rise approximately 20 ft above present sea level. by U. S. Department of Agriculture. They are associated with parallel ridge-like features in

the Assawoman Canal, outcrops of gray-green ded sands and gravels. These sands and gravels firm clay with orange mottles that give the ap- occur in a cross section of a linear feature which pearance of root-like structures and possibly ox- extends for several miles to the northeast and is idized masses of grass are interpreted to in line with offshore lineation features (Fig. 3). represent a marsh environment. Overlying and This is interpreted to be a tidal delta sequence lateral to the interpreted marsh sediments in in a coastal barrier with an associated adjacent the Assawoman Canal is a nonconformable rela- lagoon to the west, formed during a mid- or tively thick sequence of high angle cross-bed- pre-Wisconsin higher sea stand. A similar inter-

pretation of a pre-Holocene barrier and back REH060TH HIGHLANDS barrier lagoon was made by K. Bick (in New- man and Munsart, 1968) regarding the barrier ridge, Bradford Neck, in the Virginia eastern shore to the south. The lineation features, herein called "barriers," are parallel to many other features of a similar nature in the offshore region. Further- more, an outcrop of lagoonal sediments occurs at about the same height above sea level, approximately 12 mi to the west of these ridges at Pepper Creek Ditch, near Frankford, Delaware (Jordan, 1967). These Pleistocene lagoonal sediments include a dark gray to bluish-gray to black muddy sand with a predominantly El- phidium sp. Foraminiferida fauna intermixed with a Crassostrea virginica and other molluscan fauna in a shell bank. Attempts at radiocarbon dating of the Crassostrea virginica shells found in

growth position at Pepper Creek Ditch failed 0123 with questionable dates of greater than 35,000 yrs B. P. These dates possibly correspond with Figure 4. An interpretive cross section of Pleistocene dates from organic material in the Broadkill coastal sediment in the Lewes-Rehoboth Canal at Reho- beth Beach, Delaware, related to an interpretive cross marsh area to the north at a much greater section showing Holocene coastal sedimentary environ- depth, and clearly show that a mid-Wisconsin, ments overlying the deeply incised pre-Holocene topog- if not pre-Wisconsin, time, is involved in these raphy in a section parallel to the Delaware Atlantic coast. Pleistocene coastal features. Pleistocene coastal sedimentary units are well exposed in the Lewes-Rehoboth Canal and The Pleistocene surface under the spit complex in a seacliff on Thompsons Island at the north is irregular, indicating cross sections of chan- end of Rehobeth Bay (Fig. 4). Pleistocene nels. These channels can be lined up with drill coastal units cropping out in this canal are diffi- hole data and projected into the emerged sur- cult to correlate laterally as is typical with the face area to the west of the line of section. They Holocene coastal environments studied herein. form a logical continuation of a trellis-dendritic However, one can identify sands and gravels drainage pattern, which appears to be typical of which are similar to those occurring in the pre- the pre-Holocene surface. sent shoreline area; that is, dune sands, beach- A geological interpretation of a section be- berm sands, and underlying firm greenish-gray tween a pre-Holocene highland near Lewes, marsh muds with an unconformable relation- through the Cape Henlopen spit and dune area, ship to the overlying sand section. The seacliff across the mouth of Delaware Bay, thence over eroded along the north shore of Rehobeth Bay the shoal area to Cape May, New Jersey, is includes cross-bedded units reminiscent of a shown in Figure 5. The pre-Holocene surface tidal delta overlying a tan, hard, clay-silt. identified is based in part on drilling and in part Deeper drilling in the area penetrates units on seismic profiling. Moody and Van Reenan similar to those exposed in the Rehobeth Lewes (1967) noted a relatively deep pre-Holocene Canal. As shown on Figure 4, a deeply incised ancestral Delaware River channel which occurs erosion surface with local relief of up to 140 ft as an incised narrow valley about 180 ft below can be identified in a north-south section along present sea level in the area just north of Cape the present Atlantic coast. Deeply incised river Henlopen. A continuing seismic program by R. valleys occur under Indian River Bay and un- Sheridan, at the University of Delaware, has der the baymouth barrier to the south of Reho- verified this identification elsewhere along the beth Beach, Delaware. The Holocene Delaware coast. The shallow marine-estuarine environment sequence projected to the north unit in the cross section (Fig. 5) is reminiscent of the Rehobeth highlands toward Cape Hen- of the sediments to be expected in the mouth of lopen is more complex as a spit overrides a a large embayment such as Delaware Bay. As lagoonal and shallow marine or estuarine unit. Cape Henlopen advanced to the north and west

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MOUTH OF DELAWARE BAY

CAPE HENLOPEN

PROJECTED SEA LEVEL TEARS SCFORE PRESENT

KS&j F-M SAND-DUNES ANCESTRAL DELAWARE RIVER CHANNEL,AFTER MOODY t Bggg LAOOONAL SILT VAN KEENAN hi^J MARSH-MUD, PLANT DE8RIS

|.7.y.)SMALLO« MARINE -ESTUARINE

C::::l»t«CHE» AND SPIT SANOS a GRAVELS

Figure 5. An interpretive cross section showing showing the ancestral Delaware River channel and the Holocene sedimentary environments in the Cape Henlo- relationship of the pre-Holocene Delaware River valley pen spit-dune-marsh and lagoon area between Lewes and to the Cape Henlopen area and Cape May, New Jersey, Cape Henlopen with a projection into the submarine shoal area.

in middle and late Holocene time, a beach and submarine surface. The trellis drainage pattern spit system prograded over the shallow estua- is partially based upon onshore barrier ridges rine sediments. which are parallel to offshore lineation features A modified trellis and dendritic drainage pat- in the shallow submarine area. Lineation fea- tern has been interpreted for the late Wisconsin tures that show this striking parallelism are topographic surface (Fig. 6). This interpreta- probably shoreline barriers deposited by a fall- tion is based on a large number of borings, ing sea at the end of the Sangamon interglacial projection of present drainage patterns into the or during a lowering of sea level in mid-Wis- subsurface, a limited amount of offshor seismic consin time, if a mid-Wisconsin high sea ex- evidence, and the morphology of the nearshore isted.

••PRESENT MARSH E;-!;;;;-! PRESENT LAND PRESENT BOTTOM CONTOURS (30' INTERVAL) CONTOURS ON PRE- HOLOCENE TOPOGRAPHY 130' INTERVAL)

ACTUAL 8 PROBABLE PRE-HOLOCENE LINEAMENTS LAND

Figure 6. Pre-Holocene topography showing the submarine ridge-like features, as evidenced by present trellis-like drainage pattern associated with the ancestral submarine morphology interpreted to be parallel coastal Delaware River at approximately 12,000 to 15,000 yrs B. barrier features formed in a regressing pre- or mid-Wis- P. Note the relationship between onshore linear and par- consin shoreline. allel coastal beach barrier features and nearshore marine

The offshore submarine morphologic fea- suggests that at least the loci of formation of the tures are called coastal barriers in view of their parallel offshore ridges must be related to pre- parallelism to onshore linear coastal barriers. existing topography. Possibly accumulation Swift (1969) suggested that similar linear off- and movement of sand occurs across the relict shore features in the False Cape area, south of parallel ridges in the nearshore environment Cape Henry, Virginia, are sand waves devel- near Bethany Beach, Delaware. These subma- oped by present oceanic current systems. Sand- rine ridges are also in line and parallel to coastal ers (1963) suggested that lineation features ex- features in the New Jersey area. Positive iden- tending from relatively straight shorelines into tification of the time of formation, herein pro- small marine areas are remnants of a pre-exist- jected to a falling sea level at the end of the ing topography. Moody (1964) suggested a Sangamon interglacial or to mid-Wisconsin shallow marine origin for linear offshore ridges time, remains to be proven. at False Cape north of Bethany Beach along the An interpretation of paleogeography of the Delaware shore and produced evidence that Delaware coastal area, approximately 7,000 yrs the sand features had moved during a storm. B. P., is shown in Figure 7 (Kraft, 1969b). This However, a remarkable parallelism with im- map is based on environment projection from mediate adjacent onshore pre-Holocene bar- the subsurface of the Delaware coastal area, a rier features can clearly be shown in the limited amount of offshore seismic evidence Bethany Beach-False Cape area (Fig. 3). This and submarine morphologic patterns of the

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ANCESTRAL CAPE MAY 7,000 B.P.

Figure 7. Paleogeography of the Delaware coastal area approximately 7,000 yrs B. P.

nearshore marine area. The position of the At- The delineation of Delaware's shoreline into lantic Ocean sea level at that time has been the early Holocene Epoch remains in doubt. determined from the relative sea-level rise Conflicts in evidence herein presented with that curve shown in Figure 25. Precise shape of the of Emery (1967) indicate that significant prob- coastal lagoons and barrier features cannot, of lems remain in terms of projecting shorelines to course, be determined by these methods. On 11,000 to 15,000 yrs B. P. on the outer edge the other hand, the positions and shapes can be of the continental shelf. Solution to these prob- logically projected into the prsesent area by lems lies in the submarine area. coincidence of submarine morphologic pat- HOLOCENE SEDIMENTARY terns with surface morphologic features. The ENVIRONMENTS IN A submarine morphology in the nearshore ma- BARRIER-LAGOONAL COAST rine area off South Jersey and Delaware is strongly reminiscent of present coastal features. Recent sedimentary environments in the A very strong case may be made for projecting Delaware coastal area are summarized in Fig- the spit-dune-marsh triangle of Cape Henlopen ure 8 (Kraft, 1968b). This index to the area of 17 km to the southeast to the position shown in study emphasizes the nature of the drainage Figure 7. The shoal area delineated to the south surface that is being inundated by the present of ancestral Cape May is hypothetical but based Holocene sea level rise. Sedimentary environ- on present submarine morphology. Similar pat- ments include (1) baymouth barriers and their terns can be seen on the present Overfalls, Mid- subenvironments; (2) shallow coastal lagoons; dle, North, Prissy Wicks, and Eph shoals to the (3) Spartina-Distichlis marshes surrounding the southwest of present-day Cape May. lagoons; (4) spit and dune sequences such as

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DUMBS

BRACKISH-SALT MARSH©

BAYMOUTH BAR (B)

TIDAL DELTA ©

ERODING PLEISTOCENE HEADLAND JcS TIDAL CREEK - * *=3rc'^

INDIAN RIVER LAGOON (CLAY-SILT) ©

REHOBOTH LAGOON

ASSAWOMAN LAGOON

HEN & CHICKENS- SHOAL OFFSHORE BAR

SUBSEA DEPTHS IN FEET

012 MILES

0 I 2 KILOMETE

beach and dune sand E^ tidal delta, barrier, and shoreline sands j^ clay-silt

Figure 8. Index map to recent sedimentary environ- tion and interpretive diagrams within this report, ments of the Delaware Coast showing lines of cross sec-

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Cape Henlopen; (5) estuarine washover bars; clay-silts characterized by Elphidium sp. and (6) tidal creeks; (7) a relatively steep and erod- Crassostrea virginica with abundant molluscan ing submerged beach face; (8) a large ebb tide faunas; (7) submerged back barrier sand lobes; shoal, Hen and Chickens Shoal; (9) nearshore (8) on the eastern shorelines of the lagoon at marine sands and gravels and shell debris; (10) the edge of the back barrier a Spartina patens a large shoal area representing the retreating and Spartina alterniflora marsh followed by pine form of Cape May; and (11) the Delaware Estu- forests developed on the slope of the back of ary. the baymouth barrier; (9) linear dunes parallel- The sediments of coastal lagoons, such as ing the coast; (10) the berm-washover area; Rehobeth and Indian River Bays, are domi- (11) the beach face; (12) the submerged beach nantly sand and silt, with significant clay admix- face at the edge of the presently eroding trans- tures (Fig. 9). Large fans of sand occur in the gression; and finally (13) the shallow marine southeastern corner of Rehobeth Bay and sands and shells forming a lag deposit as the across the eastern portion of Indian River Bay. transgression proceeds. These sand fans are deposited by tidal deltas The sedimentary environments developed in which have existed in this area throughout late a coastal lagoon-barrier area such as illustrated, Holocene time. Inlet migration has occurred are highly complex as regards sediment trans- over a north-south area of 6 mi over the past portation agents. Sediment is directly eroded thousand years in this area. from the Pleistocene surface being submerged The surficial transgressive sequence, de- by coastal erosion and sea level rise. Some sedi- scribed from land to sea, is as follows: (1) the ment is moving down the tidal creeks from sur- submerged Pleistocene highland with a trellis- face erosion inland. Tides move sediment in dendritic drainage pattern; (2) the fringe of a water wedges of variable salinities back and marsh characterized by tree stumps, roots, and forth across the lagoon during the tidal cycle. a marsh flora with a mixture of sand, clay, and The eroding marsh shorelines of the lagoons silt; (3) typical lagoonal margin Spartina alter- provide sediment to a thin, muddy sand area in niflora and Spartina patens marshes; (4) thin, the nearshore area of the coastal lagoons. Major and possibly ephemeral, sand beaches on the amounts of sediment move through the inlets in westward sides of the coastal lagoons; (5) near- the tidal currents that sweep in and out and shore lagoonal sediments comprised of a mix- form large tidal deltas in the lagoons. A smaller ture of sand and mud; (6) typical lagoonal amount of sand is dumped in the deeper and very small tidal delta in the offshore area. Large amounts of sand are blown from the berm and deposited in the form of dunes parallel to the coastline. Very large amounts of sand and gravel are washed across the barrier during the frequent storms along the coast ("northeasters" and hurricanes). Some of the beach-berm material is washed back into the shallow marine area during these storms. Large amounts of sediment move in the littoral drift stream along shore northward in the area studied. As a result, it can be seen that an extremely complex pattern of current movement, sediment erosion, and sediment transport and deposition are involved in the relatively small coastal area de- picted. Cumulative grain size percent curves vary significantly over relatively small geographic areas (Fig. 10). The western lagoonal washover beaches have similar sorting to the sands of the Atlantic beach. However, the sands of the western lagoonal beaches are coarser. This is proba- Figure 9. Sediment facies patterns in the coastal envi- bly because they have been derived in part ronments of the Rehobeth and Indian River lagoonal areas, coastal Delaware. Sediment facies are represented from sands winnowed from the nearby Pleisto- in a triangle diagram ratio pattern. cene surface. Tidal delta sands in the high cur-

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the ancestral Delaware River, which may be traced through the Delaware Estuary and partially across the continental shelf in the form of a submarine valley (Figs. 6 and 8). Littoral drift along the Delaware beaches is dominantly to the north along a 16 mi stretch. Turner (1968) presented an estimate that 450,- 000 cu yds per yr of sand and gravel are moving from the area of Bethany Beach northward to-

p/4 i/e I/2S6MM ward Cape Henlopen as averaged over the past -*VC ' C ' M I F ' VF ••- SILT -»-CLAY 100 yrs. At the same time, Cape Henlopen has GRAIN SIZE ANALYSIS-COASTAL DELAWARE been estimated to be accreting on the north and northwest margin at greater than 135,000 cu Figure 10. Typical cumulative sediment size curves for sedimentary environments in the Rehoboth-Indian yds per yr. The net loss in sand unaccounted for River lagoonal areas. by the rapid advance of Cape Henlopen is believed by the author to be selectively winnowed rent areas near the inlets are relatively well out at the tip of Cape Henlopen, caught in an sorted as compared with tidal delta sands far- ebb tidal process, and transported seaward onto ther into the lagoon. Extremes of poor sorting the Hen and Chickens Shoal. In addition, large result at the fringes of tidal delta penetration portions of the sand in littoral transport are into the lagoon. A typical bay fringe mixture trapped in tidal deltas and washover features of may occur in the areas where tidal deltas are in the baymouth barriers and in the large wash- contact with deposition in the coastal lagoons. over features and high dunes which are form- Processes involved in the formation of a mix- ing south of the Cape Henlopen area. Sand is ture of sediment from coarse sand to mud size derived both from the shallow marine shelf in are fairly complex. They indicate a mixture by the nearshore area and from eroding Pleisto- tidal intrusion and tidal currents moving back cene headlands. However, a question remains and forth across coastal lagoons at the fringes of as to the ultimate source of sand. The sand mov- tidal deltas. Similarly, mixtures of sediments ing to the beach from offshore may be derived can occur at the point where baymouth barrier from a storm-expanded offshore bar and simply washover and eolian sands mix with lagoonal return to equilibrium with post storm beach sediments. In this case, a similar broad range of repair. The net movement of the beach-berm grain sizes from coarse sand to clay result. The face of the coastal barriers of Delaware is shore- sediments of the central portion of the coastal ward, with coastal erosion occurring at rates of lagoons are nearly entirely clay-silts. These 300 to 1,200 ft per century, as shown by his- muds are reformed into sand sized fecal pellets toric sequences of U. S. Coast and Geodetic not shown in the cumulative curve. The fecal Survey Maps. pellets are probably formed in place by burrow- In a building cycle, the berm is comprised of ing organisms and are not transported out of horizontally laminated well-sorted, medium to their environment of origin. A very large coarse beach sands. The emerged beach face in amount of sediment mixture occurs within the the intertidal area has thin laminae parallel to coastal lagoonal areas as a result of the great the beach face. With the occurrence of the com- number of separate moving-fluid transport mon "northeaster" and rarer hurricanes of the mechanisms. North Atlantic coast, normal cycles of littoral drift change direction sharply and the beach Baymouth Barriers face is rapidly and sharply cut away (Fig. 11). In sand barriers in the area of study, the In storms, the majority of the sand is dumped beach face tends to be formed at a higher angle into the shallow marine area just off the sub- (4 to 8 degrees) to the berm than is common merged beach face. Depending on the intensity for other Atlantic beach faces. This rather steep of the storm, large amounts of beach and berm beach face continues seaward of a narrow off- sand may also be washed across the barrier into shore bar to a subsea elevation of minus 30 ft. the coastal lagoons (Fig. 12). Hundreds of From that point seaward, the nearshore marine thousands of cubic yards of sand may be moved bottom surface varies from 30 to 60 ft deep in one storm by this process. Accordingly, the over a distance of up to 5 mi offshore. How- barriers, when examined in the subsurface, ever, a major exception occurs in the path of should exhibit both normal beach laminae,

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truncated beach laminae, and all of the charac- ure 13 shows a mat of vegetation composed teristic washover sedimentary structure fea- mainly of pine needles and pine cones. This tures. forest floors surface lies just above the root sys- When extreme erosion occurs during a tem of pine trees variably exposed as stumps. "northeaster" or a hurricane along the beach Radiocarbon dates plus estimates of rate of face on the baymouth barriers, the stumps and shoreline advance in this area suggest that these flora of an ancient forest are frequently ex- forests were here at the time of the first arrival posed. The exposed forest surface is normally of Europeans to the Delaware coast. There re- buried 5 to 7 ft under the beach-berm system mains the possibility, accordingly, that Euro- (Fig. 13). This extensive forest area has been pean settlers in the 17th and 18th centuries may reported along the entire Delaware coast with have been responsible for the destruction of important exceptions at the truncated Pleisto- these forests. On the other hand, close examina- cene highlands. The forests and marsh peats tion has not revealed evidences of saws or axes. exposed in the surf are mainly comprised of However, this could be unlikely in view of the pine and are typical of the natural flora of pre- fact that the stumps are continually exposed to sent back barrier areas. In some cases, cypress the sanding action of the surf-sand mixture with and white cedar are also found in lesser each "northeaster" or hurricane that occurs in amounts in these fossil "back barrier" forests. the area. Large numbers of "peat rollers" are Radiocarbon dates indicate that the forests vary formed from outcropping Spartina rhizomes from 250 to 350 yrs B. P. Newman and Mun- embedded in marsh clays and silts in the erod- sart (1968) reported similar outcrops of trunks ing shoreline area and are transported in the of trees and Spartina marsh fragments with a littoral drift. These Spartina clay-silts are typical radiocarbon date of 200 yrs B. P. derived from of the back barrier marshes that are widespread ajuniperus sp. fragment in their study of the along the edge of the coast of lagoons on the Wachapreague lagoon in Virginia, to the south backside of the baymouth barriers. The "fossil" of this area of study. back barrier forests and marshes being trun- Close examination of the forest floor in Fig- cated by the advancing Atlantic Ocean are con- clusive evidence of coastal transgression. With the end of a major storm, repairs to the beach immediately proceed in the form of a

ERODINl CYCLE

BUILDING CYCLE

Figure 11. Beach-berm profiles on a baymouth barrier showing sand lamination relative to building and eroding cycles. STORM EROSION-HURRICANE OR NORTHEASTER

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/8/2131/3443095/i0016-7606-82-8-2131.pdf by guest on 29 September 2021 Figure 12. Baymouth barrier south of Dewey Beach, fans and entire destruction of the normal beach-berm- Delaware, immediately after an extremely destructive dune system. Air photo by U. S. Coast and Geodetic "northeaster" in March, 1962. Note the large washover Survey.

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shallow bar advancing across the eroded berm the effect of man. Jetties and groins have caused area. Sedimentary structures formed at this normal coastal buildup in the "upstream" lit- time include thin, high-angle cross-laminated toral drift area with resulting rapid shoreline units facing toward the land, and thin laminae erosion caused by refraction of waves around parallel to the base of the bar at angles of 1° or the jetties and groins in the "downstream" lit- 2°, building landward. Within 6 months, in the toral drift area. Exceptional bulges of the coast, absence of other destructive storms, the beach such as the convex seaward bulge opposite face and berm surface are built back to their Rehobeth Bay, may be caused in part by an former level of extreme high tide. Sediment increased resistance to erosion of outcropping lost in washover fans and spits is not recovered; lagoonal and marsh clays in the submerged accordingly, the net advance of the Delaware eroding beach face, or may be caused in part by shore is landward in the form of a coastal trans- a program of dumping sand on the beach by the gression. U. S. Army Corps of Engineers and the Dela- The shoreline includes irregularities of sev- ware State Highway Department. Far to the eral hundred yards width (Fig. 8). These ir- north, a 90 ft high dune is being truncated by regularities reflect the nature of the surface the advancing Atlantic Ocean. The massive being transgressed. Opposite Pleistocene high- amount of sand being contributed to the littoral lands (10 to 25 ft above present sea level), the drift in this area causes a slight bulge just to the shoreline normally bulges toward the ocean. south of Cape Henlopen. Opposite the drowned valley areas between the The baymouth barriers include both linear Pleistocene highlands the shoreline is com- barrier and tidal delta sequences. Stabilized monly indented slightly along the barriers. tidal deltas covered by a Spartina marsh with a However, exceptions occur. These are in part fan-like arrangement of tidal creeks are trun-

Figure 13. The Atlantic shoreline area immediately foreground. An examination of the soil of the forest floor after a "northeaster" in the spring of 1969. The forest of showed it to be a sand with very low organic content and stumps exposed at mean low tide are comprised mainly a large number of pine needles and pine cones dispersed of pines which are interpreted to have grown in a back throughout. barrier forest. The forest floor is shown in the lower left

cated by the beach-berm sands which closed an motion landward. Accordingly, vertical se- inlet 40 yrs ago (Fig. 14). As may be seen on quences are similar to horizontal sediment se- Figure 9, the actual barrier of sand-sized sedi- quences (Fig. 15). If so, then potential outcrops ments in the area of the tidal inlets and tidal of landward sedimentary environments should deltas is much broader than that of the more occur under a baymouth barrier. A possible la- narrow and linear exposed barrier area. The goonal outcrop in the submerged beach face is baymouth barrier is a largely submerged sand shown in Figure 15. Coastal maps of the area of body with only the smaller portion exposed study made during the past 100 yrs, frequently above the tidal level. The surficial environ- reported black mud outcrops in the nearshore ments evident in Figure 15 from bay to ocean area. However, during the time of this study, include: (1) shallow lagoonal clay-silts; (2) la- shallow probing encountered no mud outcrop. goonal margin submerged baymouth barrier Drill holes through the barrier encountered, in sands; (3) back barrier surface of Spartina vertical sequence: (1) back barrier sands and marsh and, formerly (18th century), a pine for- washover areas; (2) an underlying back barrier est; (4) a dune area parallel to the ocean; (5) marsh with Spartina rhizomes and trunks of the berm; and (6) the relatively steep emerged pine trees; (3) lagoonal back barrier sands and and submerged beach face. washover features; (4) high-angle cross- A transgressing baymouth barrier is part of a laminated sand units reminiscent of tidal delta sequence of coastal environments in continual sequences; (5) coastal lagoonal muds with an enclosed fauna of Crassostrea virginica and other mollusks; (6) a Spartina marsh mud; (7) a marsh fringe sequence of muddy sand with roots and stumps; and (8) the submerged Pleistocene surface. An exceptional sequence occurs in those areas of baymouth barriers in which a tidal delta has formed (Fig. 16). Normal lagoonal facies patterns occur across the western third of Reho- beth Bay. However, tidal delta sequences ex- tend in a widespread fan of sediment deposited over the Rehobeth Bay lagoonal silts. A trunca- tion surface impinged on a buried pre-Holo- cene hill occurs under the delta area and the normal sediment sequences expected under a baymouth barrier are not encountered. Two borings encountered detrital peat balls, derived from the lagoonal marsh area, at the Paleisto- cene unconformity. Accordingly, an exceptional radiocarbon date sequence occurs. The 2,900 yr and 2,660 yr B. P. dates encountered against the unconformity in the eastern part of the section are derived from rounded detrital balls of material, washed along the unconformity as the transgression continued. These balls are believed to result from the scouring action of tidal channels in extant several thousand years ago. The majority of the cores taken through the tidal delta area encountered cross- bedded sands and gravels with the dominant cross-bedding direction toward Rehobeth Bay. Similar cross-bedding of the sands and gravels (10 to 20 degrees) has been interpreted by Figure 14. Air photo mosaic of Rchoboth Bay and some geologists to be fluviatile in origin where Indian River Bay, coascal Delaware, showing relation- encountered in Pleistocene outcrops in the ship between baymouth barrier features and adjacent coastal lagoons and highlands. Air photo by U. S. De- vicinity. partment of Agriculture. The present inlet into the Rehoboth-Indian

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10'

30' TAN a GREEN-GRAY FIRM GRAY-SREEN F CLAY; ORANGE-BROWN MM -" SAND MOTTLING 40' i>>-:-:-:-:1 LAGOONAL SILT

1^3 MARSH-MUD, PLANT DEBRIS 'J53S1 MARSH-SANDY MUD 50'

2000' 4000' 6000'

Figure 15. An interpretive cross section of a bay- cene environments and associated radiocarbon dates, mouth barrier showing the vertical sequence of Holo-

20' REHOBOTH BAY TIDAL DELTA ATLANTIC OCEAN

10'

TIDAL DELTA - SAND 8 GRAVEL

LAGOONAL SILT

MARSH-MUD, PLANT DEBRIS-PEAT

FRINGING MARSH-SANDY MUD

Figure 16. Cross section showing the relationship be- tion of Holocene environments associated with the tidal tween present sedimentary environments in a tidal delta delta area and the occurrence of the detrital organic area to Holocene coastal environments encountered by material in the form of peat "balls" located at a lower the drill under the tidal delta-barrier. Note the trunca- horizon than marsh peats of an older date.

River lagoonal system has been stabilized by Pleistocene sequence are comprised of beach the Corps of Engineers at Indian River Inlet by and inlet sediments and their erosion contrib- means of a jetty system. Accordingly, the natu- utes a large amount of medium and coarse sand ral environment has been disrupted. Historical into the lagoon. Accordingly, sand-silt mixtures and geological records show that the inlet has occur throughout the lagoonal areas wherever migrated back and forth across the mouths of a Pleistocene surface is being eroded by wave Rehobeth and Indian River Bays throughout action. Topographic features, such as Thomp- the past 3,000 yrs as the Holocene transgres- son's Island, are usually first surrounded by sion continued in the area of study. Spartina marsh and tidal creeks in the ongoing transgression. Later, as sea level rises, open wa- Coastal Lagoons and Fringing Marsh, ter may surround the hills and form a true is- Rehobeth and Indian River Bays land. Wave attack ultimately destroys these The formation of coastal lagoons is primarily islands as they submerge as sea level rises. Big controlled by the shape of the drowned river Piney Island, on the west-central side of Reho- topography on the sub-Holocene surface. The beth lagoon, is an example of a buried hill. fringing Spartina alterniflora, Spartina patens, Here a Spartina alterniflora and Phragmites com- and Phragmites communis marshes form an en- munis marsh occurs on a very small (less than trapment surface in which large amounts of one acre) island. Historical records show that sediment are deposited. The surface of the peri- this island at one time was much larger and had pheral marshes of the coastal lagoons is proba- an orchard and a building upon it. Drill cores bly being built upward with the sea level rise of through the marsh sequence showed a sub- the ongoing Holocene transgression. Marsh merged pre-Holocene surface approximately 6 surfaces identified in the subsurface can, there- ft below present sea level. This Pleistocene fore, be used as indicators of relative "sea sediment surface under Big Piney Island is simi- level" position. The extreme tides associated lar to that of Thompson's Island to the north with storms and spring tides put salt water in (Fig. 18). contact with the forest cover of the Pleistocene An intensive coring program in the shallow uplands. At this point, the normal mature forest areas of Rehobeth Bay showed evidence that a vegetation is killed off by the salt water intru- drowned topography similar to that of the ex- sion. This line forms the outer edge of the Spar- posed land surface to the west lies under Reho- tina patens and Distichlis spicata marshes that beth and Indian River Bays. The tidal streams surround the lagoons and tidal creeks, and is at the heads of the coastal lagoons can be traced the leading edge of the Holocene transgres- into this drowned topography and into the sion. nearshore submarine area of the Atlantic shelf As the marsh surface builds upward, the la- and used to form a paleogeographic interpreta- goonal shore of the Spartina marshes undergoes tion. Bennett (1935, unpub. data) called atten- erosion. Wave slap against the edge of the tion to the similarity between the shape of a marsh causes a continuous erosion of the sand- surface probed under southern New Jersey mud mixture of the marsh edge, with a result- coastal lagoon sediments and that of the den- ant deposition in part on the lagoonal floor in dritic drainage pattern in the uplands surround- the nearshore area and transport of sediment ing the lagoon. The situation is similar in across the top of the marsh where the sediment coastal Delaware. In fact, it appears possible deposits and builds up the marsh surface itself. that nearly all lagoonal islands may in fact be Accordingly, a continuum can be seen in a developed by inundation of hills in a rising sea transgression accompanied by sea level rise. A and are, accordingly, remanents of the balance may develop whereby the coastal la- drowned topography in the ongoing transgres- goon may remain over a long period of time as sion. a part of a transgressive geological sequence. Bottom sediments throughout the coastal la- The Pleistocene surface being submerged by goons have been totally reworked by burrow- the ongoing Holocene transgression is some- ing organisms. Almost no original sedimentary times exposed in sea cliffs such as "Thompson's structures have been encountered and the silts Island" on the north side of Rehobeth Bay and clay silts have been intensively reformed (Fig. 17). Waves developed by easterly and into fecal pelleted muds. No evidence was ob- southerly winds impinge on a Pleistocene high- served that the pellets are in transport motion. land and have formed an eroding cliff, approxi- However, further study is required of the very mately 15 ft high. The sands and gravels of this dense arthropod, annelid, molluscan, and

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Figure 17. An eroding sea cliff on the northern shore exposed are Pleistocene barrier and tidal delta sands and of Rehobeth lagoon at Thompson's Island. Sediments gravels, and marsh clay-silts.

foraminiferal fauna which is constantly turning Spartina and Iva marsh, the sands of the over the bottom sediments and destroying the washover beach, a shallow nearshore lagoonal mud laminae that probably form after every sand-mud mixture and typical lagoonal clay-silts period of turbulence and surface wave activity. underlain by either vestiges of the transgressing A diving program initiated to further study the lagoonal beach-barrier or the Spartina marsh bottom sediment surface in the coastal lagoons mud, or both, underlain by the marsh fringe, failed because turbidity within the lagoons is underlain by the inundated Pleistocene surface extremely high when sediment is in motion. (Fig. 20). These sequences are, in miniature, Lagoonal washover beaches frequently form reminiscent of those expected on a normal on the westerly and southwesterly side of the transgressive baymouth barrier. Accordingly, coastal lagoon (Fig. 14). These beaches are be- although thin and possibly ephemeral, they lieved to be a product of winnowing and may in fact create an anomaly that would be washover action by waves generated by difficult to interpret in the subsurface records of northeasterly winds dominant during storms. ancient sediments. They are similar to cheniers but never occur in parallel, land-locked series. They occur as a bar- Tidal Creeks rier between Spartina and Iva marshes and The landward limits of submergence of the muddy nearshore lagoon sediments. These thin pre-Holocene surface are the deep penetration washover beaches (1 to 3 ft thick) are com- of tidal creeks into the trellis-dendritic drainage prised of clean, coarse sands with thin laminae, system being inundated. These tidal creeks are frequently formed by layers of organic debris, also the landward limit of the salt water intru- parallel to the beach surface. They are under- sion that has killed off the normal flora and lain by truncated marsh muds. The center and allowed the development of fringing marsh rear of these features include washover sands muds. The fringing marsh of the tidal creeks with cross-lamination toward the marsh varying has a slightly different flora from that of the salt from 9 degrees to 30 degrees (Fig. 19). The marshes fringing the coastal lagoons. Although sands are moderate to well sorted and tend to Spartina alterniflora and Spartma patens are pre- be slightly coarser than average beach sand on sent along the tidal creeks, the marsh grass Dis- the baymouth barriers to the east. The horizon- tichlis spicata becomes a dominant form. Within tal sequence of the washover beaches consists of the tidal creek centers, clay-silts that are very inundated Pleistocene surface, marsh fringe, similar to those of the coastal lagoon are depos-

ANGOLA NECK ESTUARINE WASHOVER BARRIER ATLANTIC DROWNED OCEAN TIDAL BIG PINEY ISLE COASTAL CREEK (BURIED HILL) LAGOON

BURIED MARSH (BACK BARRIER 3OO YEARS B P)

F-M SAND LAGOONAL SILT MARSH-MUD, PLANT DEBRIS SANDY MUD

4 KILOMETERS

Figure 18. Regional cross section across coastal sedi- in the subsurfaces by drill and the relation of these sedimentary environments in the Rehoboth lagoonal area. mentary environments to the pre-Holocene surface The diagram shows the projected relation between pre- eroded into the coastal Pleistocene sediments, probably sent sedimentary environments and those encountered of late Sangamon-Early Wisconsin age.

ited. Salinities within the tidal creeks are common in the tidal creek as compared with an monly much lower than those of the open Elphidium sp. calcareous fauna more common coastal lagoons. Accordingly, the tidal creek in the coastal lagoon areas (Kraft and Margules, faunas differ. Foraminifera studies suggest that 1968). axiAmmobaculites arenaceous fauna is more com- Examination of drill hole data and outcrops along tidal creeks and canals such as the Assa- woman Canal (Fig. 2), shows that Pleistocene SW REHOBOTH BAY SOUTHWEST REHOBOTH BAY

•us-

' SPARTINA PATENS

Figure 20. A diagrammatic cross section of the Figure 19. A diagrammatic cross section of a la- southwest corner of Rehoboth Bay showing the lagoonal goonal washover barrier showing sedimentary struc- washover beach in relation to the nearby Pleistocene tures and grain size analysis and the relationship of the highland and associated marsh fringe, Spart/na marsh, barrier to its associated Spartina marsh and nearshore nearshore lagoonal muddy sands, and lagoonal clay-silt lagoonal muddy sands. environments.

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sediments which underlie the Holocene are An offshore bar or shoal called Hen and also comprised of coastal environment sedi- Chickens Shoal, extends from the tip of Cape ments. Accordingly, an extremely complex lat- Henlopen toward the southeast. This shoal is eral facies arrangements exists. Pleistocene interpreted to be an ebbtide shoal formed by a dunes, barriers, lagoon-marsh, and tidal delta highly winnowed and selectively eroded por- sequences surround Holocene lagoons, mar- tion of the littoral drift sediment transported to shes, and tidal creeks. Logical interpretation of the tip of Cape Henlopen. This well-sorted fine facies sequences like this would be nearly im- winnowed sand body extends from the tip of possible if one could not differentiate the Holo- Cape Henlopen 10 mi to the southeast in a cene sediments from the Pleistocene sediments. narrow linear shoal which never emerges The situation is obvious along the White Creek- above sea level. At its shallowest depths, it is Assawoman Canal and may be used as a model within the wave breaker zone. Historic maps and warning to investigators as to the nature show that Hen and Chickens Shoal has always and complexity of facies interpretation in an been associated with the tip of Cape Henlopen area where a coastal sequence is transgressing even though the tip has moved over a mile over a previously formed coastal sediment se- toward the northwest within the past 125 yrs. quence. As can be seen from Figures 14 and 18, Accordingly, the formation of the shoal must be the underlying submerged Pleistocene surface closely associated with the formation of the tip forms a strong control over the present deposi- itself. tional environment features. For instance, the The surface features of the spit, Cape Henlo- location of Herring Tidal Creek to the west of pen, include a surficial beach face-berm and Rehobeth Bay is controlled by Angola Neck, a dune system and back barrier tidal flat with ridge-like portion of the trellis-dendritic drain- giant sand waves. A harbor developed in the age pattern. Big Piney Island, which is the small Cape Henlopen area in the early 1800s has marsh covered island shown in the center of the since shoaled with sediment moved by littoral Figure 18 is a remanent of a drowned Pleisto- currents generated by refracting waves around cene hill. Accordingly, the water between Big the point of Cape Henlopen. Sand moves in the Piney Island and Angola Neck probably covers form of giant sand waves along the tidal flats a submerged tidal creek valley. and into the harbor formed by a large breakwater. A resultant shoreline accretion occurs on Cape Henlopen Spit Complex the Delaware Bay side of the spit and shoreline To the north of Rehobeth and Indian River in the Cape Henlopen complex. This shoreline lagoons and to the north of a Pleistocene head- advance can be shown to have been relatively land being truncated by the eroding Atlantic continuous at least over the past 1,000 yrs by Coast at Rehobeth Beach, Delaware, lies a evidence derived from the drill, morphology of complex spit-dune-marsh-washover barrier beach ridge patterns, archaeological remains, tract ending in Cape Henlopen, which is rap- and historical records. A large number of thin, idly accreting to the north and west. The shore- narrow, dune ridge-beach lines parallel with line area to the north of the Pleistocene the present Delaware Bay shoreline have headland at Rehobeth is one of a dominant formed over the past 300 yrs and are now being littoral drift to the north with frequent storm truncated by the advancing Atlantic Ocean. washovers forming a washover barrier and a In addition, a high dune (89 ft) lies perpen- back barrier Spartina marsh. Cape Henlopen dicular to the Atlantic shoreline and is being itself is a complex of dunes on a prograding truncated by the eroding Atlantic. This 80 to 90 spit. This erosional and depositional feature is ft structure, known as the "Galloping Dune of one in which erosion of the beach face can be Cape Henlopen" is formed by predominantly shown by historic maps to be occurring at the northeasterly winds and can be shown to have rate of up to 12 ft per yr averaged over the past come into existence early in the 19th century. 150 yrs. The sands of Cape Henlopen are erod- Cape Henlopen has historically always been an ing on the seaward side into the littoral drift area of dunes. However, the present large and system, moving to the tip of Cape Henlopen relatively stable dune can be seen from histori- and depositing into the estuary-ocean mouth in cal drawings to have formed between 1820 and water 60 ft deep. Accordingly, a thick sand- 1830. Evans (1958, unpub. thesis, Delaware gravel spit sequence is forming and extending Univ.) pointed out that the sequence of events itself into a major estuarine environment, the leading to the rapid transgression and forma- Delaware Bay. tion of the large Cape Henlopen dune complex

can be correlated with a time of intense build- that of the southern Delaware Bay of today. ing activity on a coastal breakwater in approxi- Accordingly, the estuarine sequence presently mately 1824. At that time, the first breakwater encountered to the north of Cape Henlopen harbor was constructed at Lewes, Delaware. today may be projected back in time to the The workmen very likely stripped off the forest south and east of the area of study. cover which existed in the immediate back bar- An air photograph of the Cape Henlopen rier area. Deforestation may have led to an ero- area with shorelines interpreted from U. S. sion cycle and development of the large Coast and Geodetic Survey records for 1910 Henlopen dune. Intensive study, based on 30 and 1843 is shown in Figure 22. It can be seen drill holes, of the Cape Henlopen area is con- that rapid coastal erosion has occurred in the tinuing and a paper on spit complex facies is in Atlantic coastal area with accompanying shore- preparation. line advance in the southern Delaware estuary- A large Spartina salt marsh lies behind the bay area and rapid shoreline erosion on the washover barriers and dune areas south of Cape estuary shoreline to the west of the area of Henlopen. This Lewes Creek presently floods study. By 1910, Cape Henlopen's spit tip had in extreme storm tides and forms a shallow advanced to the north with minor erosion of body of water over 5 mi long and several miles the shoreline in the southern bay area. By wide. Archeological evidence shows that an in- I960, the date of the photograph shown, still tensive Indian farming and shoreline shellfish further erosion had occurred in the Atlantic culture existed around the Lewes Creek marsh shoreline area with an acceleration in the rate previous to 1600 A. D. This Indian settlement of spit advance to the north and west. Once culminated in the Townsend Phase of the Late again, the shoreline from Cape Henlopen to Woodland Period of American Indian culture. the town of Lewes began to advance toward the A number of shell mounds comprised mainly of north in a slow shoreline accretion movement. Crassostrea virginica (oyster), Mercenaria merce- Thus, the sedimentation area between Reho- naria (clam), and Busycon carica (knobbed beth Beach to the south, Cape Henlopen to the whelk), occur in the fringing area of the Lewes north, and Lewes to the west can be shown to Creek marsh. These shell mounds occur up to be a triangle of sedimentary environments in 5 mi from the Delaware Bay, the most logical which the dune-washover barrier shoreline present source of this large quantity of shellfish. area is transgressing rapidly to the west, the spit Shallow core investigations show that the Cape Henlopen is accelerating in rate of ad- Lewes Creek Spartina alterniflora and Spartina vance to the north and west, the southern Dela- patens marsh forms a very thin veneer which ware Bay shoreline is slowly accreting overlies clay-silts of a thick lagoonal sequence, northward, and the edge of the Delaware Bay which once existed behind the coastal barriers to the west of the area of Study is eroding rap- of the Cape Henlopen area. It, accordingly, ap- idly inland. It is hypothesized that a similar pears most logical that the Indian farming and coastal movement sequence of a triangular spit shellfish culture which developed around the complex has existed over the past 7,000 yrs. Lewes Creek marsh in fact developed around Ancestral Cape Henlopen, shown on Figure 7, the shoreline of a coastal lagoon which was very has migrated northwest approximately 17 km much like that of present day Rehobeth Bay and 60 ft upward with the relative rise of sea and Indian River Bay (Fig. 21). This is verified level of the past 7,000 yrs. by Crassostrea virginica shells and other mi- To the north and west of the area of study, crofauna typical of a coastal lagoon, found in along the shore of Delaware Bay, a rapidly boreholes in the Lewes Creek Marsh (Jeanne transgressing washover bar has developed. This Ritzman, oral commun.). washover bar is comprised mainly of sands and Deeper drilling in the spit-dune area encoun- gravels derived from the eroding Pleistocene tered a complex sequence of beach and spit headlands and winnowed out from the shore- sands and gravels interbedded with shallow ma- line muds of Delaware Bay. The washover bar- rine-estuarine silts (Fig. 21). The topography riers of Delaware Bay are very thin and rapidly on the underlying pre-Holocene surface has a advancing. Bayward of the sand barrier, dead local relief of -£ 140 ft in this area. The shallow Spartina alterniflora marsh muds outcrop be- marine-estuarine sediments were in part iden- tween the washover bar and typical lagoonal tified by the presence of Anomia sp., Ensis sp., oyster (Crassostrea virginica) mud sediments. and Pholas sp., which are interpreted to be typi- Landward of the washover bars the entire Dela- cal of the open estuarine environment such as ware Bay area is fringed by a bordering Spartina

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HEN a COASTAL DUNE- CHICKENS WASHOVER BARRIER SHOAL

LEWES CREEK EBB TIDAL MARSH SHOAL SEA LEVEL IN YEARS BEFORE PRESENT

3.000

'5,000

fv/Xv/Xj F-M SAND-DUNES

[2223 LAGOONAL SILT 10.000 MARSH-MUD, PLANT DEBRIS

|;T— S. | SHALLOW MARINE -ESTUARINE ^*J • • t::::l BEACHES AND SPIT SANDS a E - ' f^oA\ict e

Figure 21. An interpretive cross section showing archaeologic remains of the Townsend Phase of the Late Holocene sedimentary environments in relation to the Woodland Period of American Indian culture are Lewes Creek marsh. The location of shell middens and related to the Lewes Creek marsh.

marsh covered with numerous dendritic and countered include all the sedimentary environ- meandering tidal creeks. ment facies encountered in horizontal order in presently depositing environments. Fischer (1961) first formulated an interpre- RETENTION OF THE SEDIMENT tive diagram showing potential retention of the RECORD IN A MARINE coastal sediment record in a transgressing sea. TRANSGRESSION Coastal environments, as herein interpreted, A complex environmental sequence, from can be expected in vertical sequence under the tidal creeks to ocean shore, is shown on Figure baymouth barriers. Accordingly, the retention 18. Accordingly, the encountering of any one of the sediments of these depositional environ- single coastal environment by drill is not ments will be dependent on the nature of sea enough to position oneself correctly in relation level rise and coastal erosion as the transgres- to a shoreline of coastal transgression. How- sion continues (Fig. 24). Total retention may ever, one can discern an orderly sequence of be expected if sea level rise is relatively rapid events including transgressive fades patterns as the beach face will be exposed for only a going from the submerged pre-Holocene un- short time before burial. If, in fact, little or no conformity surface, through marsh fringe, Spar- sea level rise is occurring and a transgression is tina marsh, lagoonal shorelines, lagoonal ongoing, then the transgression is purely by clay-silts, the barrier complex, and nearshore coastal erosion. The submerged beach face shallow marine. This sequence is frequently en- should be an outcrop area of all of the sedimen- countered by drilling through a baymouth bar- tary environment units being formed at the thin rier (Fig. 15). A schematic diagram showing a edge of the Holocene transgression. Accord- typical Holocene transgressive sequence is ingly, the drill, in a shallow marine area oppo- shown in Figure 23. Vertical sequences en- site a Holocene lagoon-barrier coast with no

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HOLOCENE TRANSGRESSIVE SEQUENCE BAY MOUTH BARRIER AREA

F-M, WELL SORTED DUNE IPS EOLIAN CROSS BEDDING BEACH -BERM ^Se: M-VC, PEBBLY WASHOVER i^p LOW ANGLE LAMINATION PEAT-CLAYEY SAND BACK BARRIER MARSH • .'XL

TIDAL DELTA M-PEBBLY,POORLY SORTED %•& ^xx ABUNDANT CROSS BEDDING ^Pl£ • • * *

BACK BARRIER F-M, CLEAN, WELL SORTED LAGOONAL SAND RARE SILT STREAKS

— ^ LAGOON SOFT DARK GRAY CLAY- SILT ABUNDANT BORINGS — ^ BEACH «**^3,y\y\yx/\^,

BRACKISH-SALT iLC SOFT GRAY-BROWN ORGANIC MUD MARSH PEAT

^ —

Figure 22. Aerial photograph of the spit, dune, MARSH FRINGE DARK BROWN MUDDY SAND, ROOTS marsh, barrier system that has developed in the late flv4 Holocene between the towns of Lewes and Rehoboth CHANNEL GRAVELS (RARE) Beach and the spit, Cape Henlopen. The shoreline is %§? FIRM GREENISH-GRAY MUD s\s\s^s\/\* TAN MOTTLING AND SAND PATCHE shown as in 1960, with the shoreline from 1910 and 1843 T> _ shown as interpreted from historic maps of the U. S. ~~^- Coast and Geodetic Survey. The overlay lines also show PLEISTOCENE COASTAL O^ ^• TAN, ORANGE, GREENISH-GRAY an interpretation of the Cape Henlopen area and an as- • o sociated lagoon in the Lewes Creek marsh area at ap- ENVIRONMENTS proximately 600 yrs B. P. ••*•

Figure 23- Holocene transgressive sequence showing relative sea level rise, should encounter a thin a schematic interpretation of a typical Holocene trans- veneer of residual sands and gravels underlain gressive environmental sequence to be expected under a baymouth barrier in a transgressive shoreline area. by the submerged Pleistocene surface. Limited seismic evidence in the offshore area suggests that this is not the case in coastal Delaware. paleogeographic reconstruction (Fig. 25). The Partial retention may be expected if sea level formation of a relative sea level rise curve from rise and coastal erosion and transgression occur data derived from sediment facies of the com- at the same time. This appears to be the case in plexity encountered in the study area is diffi- coastal Delaware. The submerged beach face cult. The relative sea level rise curve presented contains outcrops of back barrier environments is based mainly on the position of salt marsh overlain by a thin veneer of sand derived from peat derived C14 dates. It is believed by the the beach face and spread across the shallow author that dates obtained from Spartina, Dis- marine area. Limited seismic work by Moody tichlis, and Phragmites marsh peats may be most and Van Keenan (1967) and R. E. Sheridan closly related to true position of sea level. Com- (1970, oral commun.), suggests that variable paction effect is difficult to calculate in an area thicknesses of Holocene sediments overlie a of variable thickness of sediment units. Accord- deeply incised pre-Holocene topography in the ingly, it has been left within the framework of offshore area. Accordingly, it appears that the the relative sea level rise curve constructed, as ongoing transgression in the Delaware coastal local paleogeographic reconstruction should area is leaving behind a partial record of shore- not be affected by this compaction. On the line environments covered by a thin veneer of other hand, true eustatic sea level rise informa- shallow marine sediments. tion is masked within the framework of the curve. RADIOCARBON DATING OF THE It should seriously be questioned that any sea HOLOCENE SEQUENCE level rise curves presented are in fact truly "eu- A local relative sea level rise-sediment com- static." Certainly throughout the world, the na- paction curve has been constructed for use in ture of the compacting sediment, the nature of

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RETENTION OF COASTAL SEDIMENT RECORD IN A the open Atlantic coast and up to 6 ft in the TRANSGRESSING SEA Delaware estuary. Accordingly, errors in inter- preting sea level may be up to 4 + ft in the area of study, based on tidal range variation alone. Regardless, it is my thesis that locally derived sea level curves can be used for extremely precise paleogeographic reconstruction, as it is in fact the position of the shoreline and its adjacent environments that are critical. These environments can be located with some precision in the subsurface as encountered along coastal Delaware. Should tides have fluctuated over a much greater range in the past because of some topographic constrictor or variation that cannot Figure 24. Retention of coastal sediment record in a be known at present, errors of considerably transgressing sea showing alternative situations an- ticipated with rapid sea level rise, little or no sea level larger nature could be introduced into the data. rise, and the relative sea level rise-marine transgression For instance, tidal ranges on the northern part that has occurred in the Delaware coastal area. of the Atlantic coast may vary up to 40 ft between tides of some of the narrow estuaries in the Nova Scotia area, as compared to the Dela- local tectonism, the extent of regional tecto- ware coastal area. Unrecognized tidal range nism, tidal variation, water load, and the like, variation of this nature could make eustatic sea are so highly variable that it is questionable that level rise curve construction almost worthless. any one area will truly represent "sea level On the other hand, there appears to be no rea- rise" through the Holocene Epoch. As ob- son to project these extremes of tidal variation served by Newman and Rusnak (1965), the into the area of this study. entire idea of "coastal stability" may be a myth The closest other radiocarbon data published and casts doubt on all attempts to define a truly to date is a sea level rise curve by Stuiver and eustatic sea level rise curve. The use of average Daddario (1963) from a profile of the Holo- curves does not solve the problem. Averages cene in southern New Jersey, and a series of are in fact merely a way of adding the errors C'4 dates from a basal marsh peat in the Wacha- from data derived elsewhere to those data preague Lagoon area of the Virginia eastern which might be derived from factual local C14 shore, by Newman and Munsart (1968). No analysis. The nature of the organic material evidence was observed in this study for a 5,000 being dated must be carefully analyzed. For in- yr B. P. sea level high stand or crustal upwarp- stance, many of the dates shown within Figure ing and marine regression, as reported by New- 25 clearly are derived from shells and detrital man and Munsart. However, close parallelism wood and peat deposited within the lagoonal may be seen with C14 dates from the New Jer- environment. These have been interpreted and sey profile by Stuiver and Daddario (Fig. 25). placed below the sea level curve. The proper derivation of paleogeography Peat dates are a problem in themselves. Root from relative sea level rise data may be precise contamination from the overlying younger locally, but lead to difficult regional interpreta- grasses can penetrate to depths of 4 to 10 ft. tion problems. Figure 26 shows a cross section The contamination effect of even a single root- of Indian River Bay in the baymouth area, and let can very seriously alter the date derived. In 6 mi to the west in a narrower shallower section addition, the tidal range may critically affect the of the bay. The earliest dated evidence of the true height of sea as interpreted from salt marsh Holocene transgression in the coastal area was peats. Spartina patens and Distichlis spicata grow encountered by the drill at Indian River inlet. in the upper tidal range and are limited by Here the drill encountered a poorly preserved mean high sea level. On the other hand, Spar- marsh peat with wood fragments at — 84 ft be- tina alterniflora and Pbragmites communis flour- low present sea level, dated 10,800 -£ 300 yrs ish in lower tidal ranges and are limited by B. P. The environments encountered by nu- mean low sea level. Tides vary in the coastal merous borings in the Indian River inlet area lagoons studied over a range of approximately show a vertical sequence from a basal pre-Holo- 2 ft, whereas tides vary approximately 4.5 ft on cene surface comprised of mainly Pleistocene

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YEARS BEFORE PRESENT

MARSH PEAT -CLAY-SILT (MUDDY SAND IN BACK -21 BARRIER PEATS }

TREE IN GROWTH POSITION

SHELL IN SAND

WOOD AND DETRITAL PEAT IN MUDDY SAND. MUO ft SAND

^ SHELL IN MUD

ALTERNATE CURVE IF SPL 22 -- RANGE Of DATING ERROR IS INTERPRETED TO BE UPLAND OETRITAL WOOD- POSITION OF MEAN LOW SEA LEVEL PEAT BASED ON SEA LEVEL INTERPRETED To BE ENVIRONMENT HIGHER THAN SAMPLE POSITION INTERPRETATION SEA LEVEL INTERPRETED TO BE LOWER

VIRGINIA. BASAL PEATS (NEWMAN AND RUSNAK.I965)

NEW JERSEY (STUIVER ft. DADDARIO, 1963}

Figure 25. Local relative sea level rise curve for the each specimen for which a radiocarbon date has been Holocene Epoch in the study area mid-Atlantic coastal obtained from samples encountered by drill and borings Delaware. An attempt has been made to draw a local sea in the Delaware coastal area. No adjustment has been level rise curve by means of adjusting sea level in relation made for compaction or subsidence. to sedimentary environment and growth position of

gravels (which may be in part channel gravels shell pairs of Crassostrea virginica in growth posi- at the base of a narrow erosional Holocene tion, encountered at the top of the lagoonal river channel), overlain by a peat, overlain by mud sequence, were dated 3,430 + 170 yrs B. a fringing marsh mud, overlain by a relatively P. at approximately 35 ft below sea level. thick lagoonal sequence including the oyster, Shallow borings in the western end of Indian Crassostrea virginica, overlain by a truncated sec- River Bay, approximately 6 mi west of the pre- tion and a thick sequence of tidal inlet- sent Atlantic Ocean, show a narrow valley simi- baymouth barrier sands. Lateral facies lar in shape to the base of the valley complexity in the present baymouth area in- encountered about 70 to 90 ft below sea level cludes a tidal delta sequence at —40 ft to —60 under the present baymouth barrier. Sediment ft on the southern side of the inlet area, and a sequences in the western bay area penetrated marsh fringe sequence from approximately — by borings include shallow lagoonal clay-silts 5 ft to — 40 ft on the extreme south side of the underlain by Spartina and Phragmites marsh relatively narrow pre-Holocene valley. Several muds and the underlying pre-Holocene sur-

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NORTH SOUTH

OCEAN-BAY INLET

20'

INDIAN RIVER BAY COASTAL DELAWARE

60'

| TIDAL DELTA - SAND S GRAVEL LAGOONAL CLAY-SILT

! MARSH-MUD, PLANT DEBRIS-PEAT

- FR|NG,NG MARSH-SANDY MUD

IOC O MILES Figure 26. Cross sections of the Holocene sedimen- Note the similarity in the shape of the topography near tary environment sequences encountered by drill across the base of the Holocene unit, and the drop of approxi- the baymouth barrier at the mouth of Indian River Bay mately 60 ft in elevation of the base of the pre-Holocene and across Indian River Bay 6 mi to the west of the coast. stream valley.

face. Accordingly, Indian River Bay can be Another possibility, of course, is that the shown to be a relatively linear, narrow, deep material dated 10,800 yrs B. P. under Indian valley that has been progressively infilled by River inlet was derived from upland marsh peat coastal environments in the ongoing Holocene and washed into its present position after the transgression. ninth or tenth millenium B. P. The poorly pre- Radiocarbon dates, such as the 10,800 yr old served marsh peat and wood fragments dated marsh peat lying against the pre-Holocene un- 10,800 B. P. were located precisely at an unconformity surface, are at variance with data conformity at the base of an apparent salt-brack- and interpretations presented by Emery ish marsh mud, under a lagoonal mud, under a (1967). A large number of radiocarbon dates tidal delta sequence, all indicative of a normal derived from shells and peat have been used by transgressive sequence. However, an undated Emery, and others, to place Holocene shore- marsh peat 12 ft higher in the section is clearly lines nearly 100 mi to the east of the area of allochthonous. Therefore, the herein proposed study at about 9,000 to 10,000 yrs B. P. It is relative sea level rise curve is presented with possible that the "marsh peats" encountered by two alternates. Emery, and other researchers, in the middle It is possible that some of the drowned es- and outer edge of the Atlantic shelf adjacent to tuaries of the stream valleys that infilled as the the mid-Atlantic coastal area are partially detri- Holocene transgression continued penetrated tal. over 100 mi inland. The present Delaware es-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/8/2131/3443095/i0016-7606-82-8-2131.pdf by guest on 29 September 2021 REFERENCES CITED 2157

tuary is evidence of the extremes of shoreline States: Canadian Jour. Earth Sci., v. 5, p. 993- indentation possible. Salt-brackish marsh condi- 1010. tions can be encountered along the coast of the Emery, K. O., 1967, The Atlantic continental margin of the United States during the past 70 mil- present Delaware estuary over 120 mi inland lion years: Geol. Assoc. Canada Spec. Paper 4, from the line along the outer shore of New p. 53-70. Jersey and Delaware. Accordingly, the 10,800 Fischer, A. G., 1961, Stratigraphic record of trans- B. P. peat date in question may have formed a gressing seas in light of sedimentation on Atlan- great distance inland from the position of the tic coast of New Jersey: Am. Assoc. Petroleum shoreline at that time. In fact, the marsh peat Geologists Bull., v. 45, no. 10, p. 1656-1667. dated could have formed in a stream tributary Jordan, R. R., 1967, Atlantic Coastal Plain Geol. to the upper limits of the Delaware estuary of Assoc. Guidebook, 8th Ann. Field Conf., Dela- early mid-Holocene time. ware, 1967: 63 p. Kraft, J. C., 1968a, Transgressive facies patterns in ACKNOWLEDGMENTS the Delaware coastal area [abs.]: Am. Assoc. Petroleum Geologists Bull., v. 52, no. 3, p. 537. I wish to acknowledge the generosity of the 1968b, Coastal sedimentary environments, Shell Development Company, Houston, Texas, Lewes-Rehoboth Beach, Delaware, in Soc. who supplied many of the radiocarbon dates Econ. Paleontologists and Mineralogists Guide- and a large portion of the subsurface cores. book, Ann. Field Trip, Rehobeth Beach, Dela- Funds to support the research in field and ware, 1968, 33 p. laboratory which resulted in this paper were 1969a, Pre-Holocene paleogeography and partially provided by the National Science paleogeology in the Delaware coastal area: Ab- stracts with programs for 1969, pt. 1, Geol. Soc. Foundation, G. P. 5604, and the University of America, p. 34. Delaware Research Foundation. Funds to com- 1969b, Le Dernier mouvement quaternaire des plete the study in its present form were prov- cotes et 1'histoire de 1'augmentation du niveau ided by the Department of Defense-Project de la mer J. la region mediane de la cote atlan- Ocean Themis, Sedimentary Environments. tique de 1'Etat du Delaware: VIII Congress IN- Figures 3 and 14 are air photos from the U. S. QUA, Resume des communications, Paris, p. Department of Agriculture, Ashville, North 215. Carolina. Figure 12 is an air photo by the U. S. Kraft, J. C., and Margules, G., 1968, Correlation of Coast and Geodetic Survey. Glenn Elliott criti- Foraminifera distribution, sediment facies pat- cally read the manuscript and provided helpful terns and physical data in Indian River Bay, coastal Delaware [abs.]: Geol. Soc. America suggestions; however, responsibility for the Spec. Paper 121, p. 41. concepts presented remains mine. I also wish to Moody, D. W., 1964, Coastal morphology and pro- acknowledge the help of the following student cesses in relation to the development of subma- field assistants: William Osburn, Glenn Elliott, rine sand ridges off Bethany Beach, Delaware: William Porter, Thomas Stafford, Al Crossan, Ph. D. dissert., Johns Hopkins Univ., Bal- Dustin Dornbrook, and Linda Bell. Barbara timore, 167 p. Bimbi, Robert Crouch, Christine Dutton, and Moody, D. W., and Van Reenan, E. B., 1967, High- Robert Caulk helped with the illustrations. resolution subbottom seismic profiles of the This manuscript is based in part on a paper Delaware estuary and baymouth: U. S. Geol. presented at the Annual Meeting of the Society Survey Prof. Paper 575-D, p. D347-D252. of Economic Paleontologists and Mineralogists Newman, W. S., and Munsart, C. A., 1968, Holo- in Oklahoma City, 1968 (in Kraft, 1968a), and cene geology of the Wachapreague lagoon, eastern shore peninsula, Virginia: Marine Geology, a paper presented at the Annual Meeting of the v. 6, p. 81-105. Northeastern Section of The Geological So- Newman, W. S., and Rusnak, G. A., 1965, Holo- ciety of America-Society of Economic Paleon- cene submergence of the eastern shore of Vir- tologists and Mineralogists in Albany, 1969 ginia: Science, v. 148, p. 1464-1466. (Kraft, 1969a). Oaks, R., 1964, Post-Miocene stratigraphy and morphology, outer coastal plain, southeastern Vir- REFERENCES CITED ginia: Ph.D. dissert., Yale Univ., p. 11. Sanders, J. E., 1963, North-south trending subma- Bennett, R. S., 1935, The geology of the southern rine ridge composed of coarse sand off False New Jersey Lagoon: M. S. thesis, Princeton Cape, Virginia: Am. Assoc. Petroleum Geolo- Univ., Princeton, N. J. gists Bull., v. 46, p. 278. Drake, C. L., Ewing, J. I., and Stockard, H., 1968, Stuiver, M., and Daddario, J. J., 1963, Submergence The continental margin of the eastern United of the New Jersey coast: Science, v. 142, p. 951.

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Swift, D. J. B., 1969, Intershelf sedimentation: Pro- Whitmore, F. C, Emery, K. O., Cooke, H. B. S., and cesses and products, in New concepts of conti- Swift, D. J. B., 1967, Elephant teeth from the nental margin sedimentation: Am. Geol. Inst. Atlantic continental shelf: Science, v. 156, no. Lecture Notes, p. DS-4-1-46. 3781, p. 1477-1481. Turner, P. A., 1968, Shoreline history Atlantic coast, DelmarvaPeninsula[abs.]: 1968 Ann. Meeting, MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 17, Northeastern Sec., Soc. Econ. Paleontologists 1970 and Mineralogists, Washington, D. C. REVISED MANUSCRIPT RECEIVED FEBRUARY 10, 1971

PRINTED IN U.S.A.

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