- Email: [email protected]
Mainland influence on coastal transgression: Delmarva Peninsula
Marine Geology, 63 (1985) 19--33
19
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Chapter II. M a i n l a n d coastal e l e m e n t MAINLAND INFLUENCE ON COASTAL DELMARVA PENINSULA
TRANSGRESSION:
JAMES M. DEMAREST and STEPHEN P. LEATHERMAN
Exxon Production Research Co., P.O. Box 2189, Houston, TX 77001 (U.S.A.) Department of Geography, University of Maryland, College Path, MD 20742 (U.S.A.) (Accepted for publication May 17, 1984)
ABSTRACT Demarest, J.M. and Leatherman, S.P., 1985. Mainland influence on coastal transgression: Delmarva Peninsula. In: G.F. Oertel and S.P. Leatherman (Editors), Barrier Islands. Mar. Geol., 63: 19--33. The morphology of present-day barrier beaches and lagoons along Delmarva Peninsula is controlled by wave climate, tidal energy, sediment texture, and sand supply. However, morphology is also affected by coastal plain physiography, largely the product of Pleistocene sea-level change over the last million years. Five distinct transgressive coastal systems have been identified on Delmarva Peninsula using geomorphic and subsurface data. Each was produced during interglacial high sea levels and range in age from over one million years to 60,000 yrs B.P. Eroding pre-Holocene sediments represent major sand sources for present-day beaches. Baymouth barriers that formed adjacent to eroding headlands, such as Bethany Beach, Delaware, are characterized by infrequent inlets and robust dune systems. The mainland lagoonal shoreline follows the contours of the incised paleodrainage system. To the south, a long continuous barrier with infrequent inlets is present where littoral drift maintains a high sediment supply. Further south and away from this primary source of sand, short drumstick islands exist. This barrier chain is the result of the present transgression of a pre-existing 60,000 yr old shoreface. The mainland lagoonal shoreline follows the trend of Pleistocene beaches, which have experienced only minor stream dissection due to their young age. Along the southern part of the Peninsula, sea-level rise will eventually cause Holocene barriers to be welded onto the ancient barrier shore (the mainland). Whereas to the north, baymouth barriers will persist as the well-developed paleodrainage system is flooded farther inland. INTRODUCTION Investigations of Pleistocene shorelines on the Delmarva Peninsula illuminate some of the causes of present,lay barrier configuration. Holocene transgressive shorelines between Cape Henlopen, Delaware and Cape Charles, V i r g i n i a , c o n s i s t o f f o u r m a i n t y p e s : r e c u r v e d s p i t s a n d b a y m o u t h b a r r i e r s in Delaware, long continuous barrier beaches in Maryland, and drumstick ( s h o r t , b u l b o u s ) b a r r i e r s in V i r g i n i a ( F i g . l ; F i s h e r , 1 9 6 7 ) . A d i s t i n c t m o r p h o logic relationship exists between the modern coastal systems and the Pleistocene shorelines of the mainland that developed over the last million years. 0025-3227/85[$03.30
© 1985 Elsevier Science Publishers B.V.
20
L///_ /
'
/R__EHOBOTI ....BE'A( _ _.
,"\,jr
;LL
BETHANY BEACH
FIG. 3 ~ . \
- 3 8 ° 30'
!
DEL. --~D~
I
' •
. "~:~
CIRCA ~_ 1 my BP(
SALISBUI~Y
0.6 my BF
t'OCEAN CITY 0.06 my BP
MD~ VIR. /
"38°
HINCOTEAGUE
/
/
#
#
~A.i~,AMORE ISLAND ix~
EXMORE
.
/
/
KILOMETERS
o 75" 30'
K)
20
30
75"
21 Present-day m o r p h o l o g y can be u n d e r s t o o d by investigating the cross-cutting relationship between ancient and m o d e r n shoreline trends and the topography of the land surface transgressed by the present barrier system.
Previous work Regional geologic studies of the Delmarva coastal plain by Jordan (1962), Owens and Denny (1979), Belknap (1979), and Mixon et al. (1982) have shown surfacial sediments to consist of Pleistocene age paralic deposits which in general are y o unge r toward the south. Demarest et al. (1979, 1981), working in a small area in southeastern Delaware, showed that these paralic deposits consist of transgressive barrier systems preserved as highstand shorelines. Demarest and Belknap (1980) and Demarest (1981) also investigated the stratigraphic relationships and regional e x t e n t of some of these shorelines (Fig.l) and suggested a relationship between m o d e r n and Pleistocene shorelines.
Basic concept During major shoreline transgression sediment supplied t o the coastal system comes from shoreface erosion if updri ft headlands are absent (Swift, 1968). Present Delmarva beaches receive no coarse-grained sediment from rivers (Meade, 1969) although rivers do c o n t r i b u t e i m port ant quantities of fine-grained material to the lagoons and estuaries (Kelley, 1983). Although the exact nature of sediment t r a ns por t at the shoreface is still controversial, it is clear th at shoreface erosion is an i m p o r t a n t source of sediment for maintenance o f beach trends. Two classes o f sediment are supplied to the beach through shoreface erosion. First, sediment that was recently deposited in bays and estuaries is exposed at the shoreface with barrier retreat. This Holocene sediment was deposited within the same coastal system when the shoreline was farther seaward and does not represent new sediment to the barrier system. The second t y p e of sediment supplied to the beach is material eroded from preH olo cen e units t hat crop out on the shoreface. This material is an i m p o r t a n t supplier of " n e w " sediment to the coastal system. Any part of the shoreface that is erosional has the potential to c o n t a c t pre-Holocene units and thus becomes a source area for new sediment. The location and t e x t u r e of sedim e n t supplied to the coastal zone by this mechanism largely determines sediment supply to and equilibrium configuration of the coastline. Where the equilibrium configuration of the shoreline diverges seaward of
Fig.1. Pleistocene shoreline trends of Delmarva Peninsula are traced east of the drainage divide between Cape Henlopen and Cape Charles. The approximate age of each ancient shoreline is indicated (from Demarest and Belknap, 1980). The lines labelled A through D are lines of section schematically illustrated in Fig.7. The "arc of erosion" is caused by sediment starvation south of Assateague.
22 the point of sea-level impingement on the pre-existing topography, lagoons are formed. Thus the morphology and evolution of lagoons are also closely tied to the pre-existing geomorphology. PLEISTOCENE HIGHLANDS Shorelines on Delmarva Peninsula currently intersect Pleistocene headlands at two locations: Bethany Beach and Rehoboth Beach, Delaware (Kraft, 1969, 1971b; Fig.l}, Two ancient barriers are currently cropping out in the shoreface in the vicinity of Bethany Beach, Delaware. The ancient shorelines appear as linear trends in cultivation on aerial photographs (Fig.2). Demarest et al. (1981) referred to these as the Cedar Neck and Bethany Barriers. The stratigraphy of these barriers (Fig.3) has also been studied by Demarest (1981) and McDonald (1981). Detailed examination of the map pattern of streams and topographic contours indicates the approximate location of ancient barriers and the relative ages of the surface (Demarest and Belknap, 1980). Linear trends in stream orientation and reduced stream density (Fig.4) coincide with the position of barrier sediments as determined by drilling in southeastern Delaware (Fig.3). These stream trends also parallel linear trends in topographic contours. These shorelines, referred to as the Bethany, Cedar Neck and White Neck barriers and dated at 0.06, 0.6 and 1.0 m . y . B . P . , respectively (Belknap, 1979; Demarest et al., 1981), can be morphologically traced to the south into Maryland and Virginia (Fig.5). In the Virginia sector of the Peninsula (Fig.l), a different topographic and drainage pattern is present. The oldest identifiable shoreline trend is thought to be equivalent in age to the Cedar Neck shoreline. This shoreline forms the prominent Mapsburg Scarp (Fig.6). Seaward of this scarp two linear trends are evident; one is a lower, less prominent scarp that follows the 10-ft contour and is the position of major stream intersections near Locustvitle, Virginia. This shoreline has been tentatively dated at 0.3 m.y.B.P. (Belknap, 1979, Demarest and Belknap, 1980). Bradford Neck is the youngest shoreline trend and forms a peninsula (Fig.6) because it is being inundated by the Holocene rise of sea-level. The shoreline forming Bradford Neck is thought to be age-equivalent to the Bethany Barrier (circa 0.06 m.y.B.P.). Since Pleistocene shorelines are oriented at an angle to the present Delaware coast (Fig.l), Bethany is the only place where the modern shoreline intersects the ancient barriers. The Cedar Neck barrier is relatively old (circa 0.6 m.y.B.P.) based on amino acid dating of lagoonal molluscs (Belknap, 1979; Demarest, 1981). These data along with work by McDonald (1981) indicate that the Bethany Barrier is much younger (about 60,000 yrs B.P.). Therefore, the surface being transgressed north of Bethany Beach is much older than the surface south of Bethany. The significance of the age of the surface being transgressed lies in the difference in maturity of the drainage system being flooded by rising sea level. North of Bethany Beach, Delaware, large dendritic rivers drain major portions
23
Fig.2. P l e i s t o c e n e age (ca. 0.6 m . y . B . P . ) b a r r i e r s h o r e l i n e s are e v i d e n t o n this I R aerial p h o t o g r a p h o f t h e B e t h a n y Beach, D e l a w a r e area. T h e linear t r e n d s in f a r m fields are t h e high, well-drained, s a n d y areas associated w i t h t h e high sea level interglacial shorelines.
24
FRANKFORD
- BETHANY
WHITE NECK PARALIC UNIT CEDAR NECK PARALIC UNIT
if- :
;6 t~o,6
HD'4A A ,
Ol43-2 QI44-1
/I"--
LG
Z
MN.2,JOBB-1 MN'2
Qi45 1.TB-3.9,16
~ ~ O' Z <1(,¢~ BETHANY PARAL~C U N N ~0 ¢p ~p "~
QP3
LG QP1
PRE QP1
Qp" E • EOLIAN NS - NEARSHORE HB - H I G H L A N D BEACH B • BARRIER RB - RIVER BED
CH - TIDAL CHANNEL LG - LAGOON "
QP5 G QP 1
PARALIC
?NIT NAME
,J
AMIND ACID AGE (AVG. OIL LEUCINE
ESTIMATED RELATIVE SEA LEVEL ~iA)
MOLOCENE
O-'Oka (0.151
0
QPE
BETHANY
80 lO0ka (0.241
3TOO
PEPPER CREEK
120 - 130 ka (0.33}
QP4
UNDIFFERENTIATED UNITS (S) CEDAR NECK
5 TO 6 -5 TO -15
500 -900 k~ (0.55)
2,3 TO 2.5
i
,ap2
UNDIFFERENTIATED UNtTS
QP1
WHITE NECK
-0 TO -12
1000 - leOO k= f0.64)
3 TO E
PRE.Opl I BEAVERDAM FM. OF JORDAN. l g ~ l
J
Fig.3. East--west cross-section west of Bethany Beach, Delaware, showing the ages and stratigraphic relationships o f the Pleistocene transgressive barriers. Note that Qp5, the Bethany Barrier, is currently being eroded in the shoreface supplying "new" sediment to the littoral system (from Demarest, 1981).
of the coastal plain; the drainage divide is located far to the west and, where flooded, large estuaries in the form o f Delaware Bay, Indian River, and St. Martins River fill the lower reaches of the river valleys (Fig.l). South of Bethany Beach, Delaware, numerous streams drain smaller areas and have narrower channels. In addition, the drainage divide is much closer to the present coast (Fig.6). Up-stream of the ancient shoreline trends, the drainage pattern again becomes dendritic. Fig.4. The trends o f the White Neck, Cedar Neck and Bethany barriers are illustrated on the present-day drainage pattern to show the control of these shorelines on stream orientation. Also note that from the intersection of the Bethany barrier south, the Holocene barrier is transgressing a young shoreface which has been dissected during only one sealevel fall. From the intersection of Cedar Neck Barrier north, the surface is much older.
25
58'55"-
ANY 4
38*25'-
,J
58°20t
4
26
Fig.5. Landsat image of the southern part of Delmarva Peninsula illustrating the same linearity of cultivation evident in Fig.2. These trends along with detailed analysis of stream orientation and topography were used by Demarest and Belknap (1980) to trace southwards the Pleistocene shorelines found in Delaware. These geomorphic characteristics correlate well with relative age differences between surfaces being transgressed by the Holocene coastal system. The 60,000 yrs B.P. shoreline south of Bethany Beach, Delaware, generally forms the most seaward lagoonal headland (Figs.4 and 6). Sinepuxent Neck, opposite northern Assateague Island, is the most prominent geomorphic expression of this Pleistocene shoreline in Maryland. In Virginia this ancient barrier shoreline forms a prominent scarp until it again forms a peninsula at Bradford Neck, extending into the Holocene lagoon (Fig.6). The antecedent topography which is being transgressed seaward of the Bethany shoreline has been dissected by streams during only one glacial low sea level. North of Bethany and therefore landward of both the Cedar Neck and Bethany shorelines, the surface has not been reworked by shoreface processes for at least 600,000 yrs and possibly over a million years. As a result, the depth of
27 I
;"
j)
I
ACCOMAC~
)
y /
/
37°L
"E¥C~E~i// 37°? qECK
EXMORE
?
s7025'-
o
KM
4
7~1155' Fig.6. The Pleistocene and Holocene barrier configurations in the Virginia sector of Delmarva showing that the present shoreline is separated from the ancient barriers by considerable distance. The Virginia barrier islands are distinctly different from the barrier systems developed farther north, where the 60,000 yrs B.P. shoreline is being eroded in the beachface.
28 erosion and extent of dendritic development of the drainage topography are greater north of Bethany Beach, Delaware than to the south where the surface currently being transgressed is a relatively young shoreface. HOLOCENE SHORELINES Holocene barrier beaches along Delmarva can be divided into four segments: Cape Henlopen to Rehoboth, R e h o b o t h to South Bethany, South Bethany to Chincoteague and Chincoteague to Cape Charles (Fig.l). Each of these segments has a distinct morphology which is the product of interactions with the different land surfaces being transgressed. Cape Henlopen to R e h o b o t h Beach is the northern terminous of the Atlantic barrier systems of Delmarva Peninsula. Here sediment eroded from South Bethany and R e h o b o t h headlands is deposited at the entrance to Delaware Bay where wave energy is insufficient to continue carrying the sediment along the bayshore (Kraft, 1971a). The segment from R e h o b o t h to South Bethany is a b a y m o u t h barrier anchored at either end by eroding headlands. Indian River Inlet is the only break in this shoreline, and barrier stratigraphy is primarily inlet-dominated (Carey, 1981). The barrier is characterized by well-developed dunes and is rarely overwashed by storm waves. The mainland shoreline of the lagoon is highly variable in shape and dependent upon the contours of the incised valley. From South Bethany to Chincoteague, the barrier shoreline is interrupted by one inlet at Ocean City, Maryland. However, historical records show that numerous inlets were present in the recent past (Truitt, 1968). This barrier segment is connected at one end to an eroding headland which presumably supplies much of the sediment to the beach. Sand supply from the headland appears to be a relatively recent occurrence, and Assateague Island is currently prograding to the south faster than it is retreating landward. It has prograded past a pre-existing Holocene barrier (Chincoteague Island; Goettle, 1978). U.S.G.S. topographic maps from the early 1900's provide evidence for the recent intersection of the barrier with the headland at South Bethany. At that time a lagoonal marsh separated the beach at Bethany from the mainland. Barrier demise later occurred as the shoreface intersected the headland at Bethany. The lagoonal shoreline follows an incised valley in the north part but becomes relatively linear where it follows a Pleistocene shoreline in the southern part of this segment (Fig.l). Intersection of the eroding barrier with the sandy Pleistocene barriers at Bethany Beach, Delaware, could explain both the rapid progradation of Assateague Island and the closing of all but one tidal inlet (even Ocean City Inlet is kept open artificially). The y o u n g age of the Bethany Barrier, which is being eroded at Bethany Beach, suggests that a relict shoreface is being flooded south of Bethany. Since headlands did n o t persist seaward of the present Assateague Island, this barrier in its present configuration is a relatively recent development.
29 South of Assateague Island is a long chain of discontinuous, short barrier islands, which have been termed drumstick barriers by Hayes (1979). The barriers largely consist of fine-grained sand and are frequently overwashed during storms (Rice et al., 1976). Each island is separated from adjacent islands by large, fairly stable tidal inlets that feed a complex network of tidal channels. The backbarrier lagoons are largely filled by tidal marshes and mud flats. Although tectonic activity, sediment texture and supply, tidal range and wave climate account for some of the difference between the barriers along Delmarva shoreline (Leatherman et al., 1982), the antecedent topography is believed to have had a major influence on barrier morphology. The position of major tidal inlets along the Virginia barrier islands generally coincides with topographically low areas on the Pleistocene erosional surface (Morton and Donaldson, 1973). Some of these low areas appear to be associated with paleodrainage valleys eroded during lower sea level. Halsey (1979) noted that the streams tend to have a parallel drainage network which would give rise to fairly evenly spaced tidal inlets. Sea-level submergence of incised valleys determine the location of major tidal prisms. These tidal prisms are primarily confined to tidal channels which appear to be relatively stable during transgression. Thus, tidal inlets at the m o u t h of tidal channels tend to be confined to the area of ancestral drainage channels. Additionally, the interfluve areas with greater relief have the potential of supplying new sediment to the transgressive system. These ancient drainageways are not incised as deeply as valleys on the landward side of the Pleistocene barriers and thus are much lower relief features than those farther north in Delaware. This difference in valley depth is believed to be a function of age differences of the pre-Holocene surfaces. The present drumstick shape of the Virginia barrier islands is related to the relative stability of major tidal inlets and the largely infilled lagoons. Flow characteristics of tidal currents (Boon, 1975) coupled with relatively stable inlet channels have resulted in the evolution of large ebb-tidal deltas; these sand lobes have a pronounced effect on barrier dynamics and ultimately island morphology (Hayes et al., 1970; Oertel, 1977). This type of island was originally described for mesotidal environments (2--4 m tidal range; Hayes, 1979), and it is hypothesized that Pleistocene topography has had a role in establishing the requisite conditions for development of such drumstick barriers along an ostensible microtidal shoreline. The apparent lack of headlands in the foreshore updrift of the drumstick barriers suggests that new sediment is supplied from shoreface erosion and moved onshore to replace sediment lost to littoral drift or inlet-related sinks. The mainland shoreline landward of the backbarrier lagoon is abutting the steep portion of the Pleistocene shoreface that formed about 60,000 yrs B.P. As a result, the mainland lagoonal shore is linear and exhibits a sharp contact along older Pleistocene shorelines south of Bradford Neck. Because the lagoonal beach rests on the steep portion of the relict shoreface, it will migrate landward by submergence more slowly than the drumstick barriers with continued sea-level rise.
30 SUMMARY AND CONCLUSIONS During the Holocene pre-existing topography has played an important role in development of transgressive barriers and backbarrier lagoons on Delmarva Peninsula. Much of the present morphologic differences between northern and southern Delmarva may be explained in terms of the geomorphic influence of the underlying surface (Fig.7). The pre-existing topography can be manifested in two different ways; the surface is primarily a subaerial drainage system characterized by deeply incised valleys cut during low sea level, or it is a relict shoreface formed seaward of a Pleistocene transgressive shoreline. All of the stream valleys east of the Delmarva drainage divide show a consistent relationship between drainage maturity and elapsed time since the last transgression. This suggests that several cycles of downcutting are required to develop deeply incised valleys, although some downcutting will occur seaward of even the youngest Pleistocene shoreline as demonstrated by Morton and Donaldson (1973). Bethany Beach, Delaware, serves as a transition area in the present coastal trend along the Delmarva Peninsula. There is an order of magnitude (0.6 vs. 0.06 m . y . B . P . ) difference in age between the surfaces which are being transgressed north and south of Bethany Beach, Delaware. Age differences of the pre-Holocene surface being flooded determines whether b a y m o u t h barriers form across incised valleys that dissect the surface (to the north) or barrier beaches are disconnected from the mainland along unaltered shorefaces (to the south). In addition, two Pleistocene barriers are currently being eroded at Bethany Beach, providing a good source of sand to the littoral drift system. The lack of headlands to supply the southern beaches with sediment implies that the bulk of the beach sand is derived from offshore. Therefore, some of the morphologic differences between northern and southern Delmarva Peninsula may reflect differences in scouring of the shoreface. The northern barriers are primarily nourished by longshore transport away from eroding headlands, while southern barriers are fed by onshore transport from presumed subcrops in the shoreface. The position of inlets, particularly along the Virginia barriers, appears to be influenced by incised valleys on the antecedent topography, even where the valleys are low relief well below the active depositional surface. However, these valleys are still of secondary importance in shoreline dynamics since n o t all inlets coincide with valleys. The primary control appears to be whether or not sufficient relief exists between the base of the valley and the interfluves to produce headlands in the beachface. Control of relief is primarily a function of age since the last transgression. Differences in antecedent topography will cause the future evolution of the backbarrier systems to follow different pathways. With no headlands present or likely to be intersected in the near future, the drumstick barriers will continue to migrate landward across a gently sloping relict shoreface while the lagoonal shoreline remains relatively stable along the steep portion
~" ~"
--
~
~
-
C
~
,,
.
.
.
.
• ,, ,, ..,,. ,,.,
.
° :'-'.','.~:.:... ".
V
~ Barrier !:+:':-:-:-I Sand
BARR,ER
"
~
~
"
Lagoonal Mud
I-~
Lagoonal Marsh
: ~ " : : : : : : : " : : : : : " " " : : : ' " .................... . : . ~ . .
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
'
•
....
V
METOMKIN ISLAND
Itr
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
T
D
~' vf"
= ~**,* "*
Xe.%--------__~2
BASED IN PART ON DATA FROM KRAFT (1971)AND JOHN (1977)
~
T
,
METOMKIN INLET
-
iiiiiiiiiiiiiiiiiii:i:i:iiiiiiiiiiiiiiiiiiiiiiiii:i::::::::':'""'""'"" ~ ~
CEDAR ISLAND
( CIRCA 600,000 my BP )
~
B
STRIKE
INDIAN RIVER INLET .~__~.:.....~. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
X-SECTIONS
Fig.7. Schematic cross-sections from northern and southern Delmarva Peninsula illustrating the difference between the two types o f barrier systems. In sections A and B, irregularity of the pre-existing topography, particularly in strike section, is the result o f over 500,000 yrs of valley incision during lowered sea level. In sections C and D, the pre-existing topography in front of the Bradford Neck barrier is primarily a relict shoreface formed about 60,000 yrs ago. The young age has precluded extensive valley incision comparable to that found to the north. This difference in pre-existing configuration is an important factor in determining the present-day morphology and future trends of the barrier--lagoon system during transgression.
•
.
CEDAR ISLAND (RECENT)
AFTER KRAFT (197!)
,.,',,,'.,.-,,'.%,-,.
~
Y,T. ""-.'.'.:.'..'.'..:...'...'.?.'...
~.vBRADFOR NECK D BARRIER ( CIRCA 60,000 YR BP )
.,
BASED IN PART FROM DATA FROM MORTON & DONALDSON (1973)
\--
A
DIP
SCHEMATIC
32
of the Pleistocene shoreface (scarp). If the transgression continues, the lagoons will narrow, causing a decrease in total wetlands area. North of Bethany Beach, Delaware, the lagoons will not show a systematic change in area with continued sea-level rise and transgression. Some lagoons may expand in area while others will shrink, depending on the slope of the mainland surface being flooded and the rate of barrier transgression (Field and Duane, 1976). Headland erosion will tend to decrease the overall rate of transgression. Furthermore, continued erosion increases the length of shore that is headland, and therefore sand supply to littoral drift will also increase. This may further slow the rate of shoreline retreat, allowing the lagoons to further expand with mainland submergence. In summary, pre-existing topography plays an important role in the morphologic development of barrier shorelines. Proper evaluation of these effects permits a more precise assessment of the important process variables controlling coastal evolution. To ignore or underestimate the fundamental legacy of the geologic past in shaping present-day barriers can lead to false deductions a b o u t the morphodynamics of coastal depositional systems. REFERENCES Belknap, D.F., 1979. Application of amino acid geochronology to stratigraphy of late Cenozoic marine units of Atlantic Coastal Plain. Ph.D. Diss., Department of Geology, University of Delaware, Newark, Del., 580 pp. Boon, J., 1975. Tidal discharge asymmetry in a salt marsh drainage system. Limnol. Oceanogr., 20: 71--80. Carey, W.L., 1981. Surfacial morphology and subsurface stratigraphy of the flood tidal deltas on the Atlantic Coast of Delaware. M.S. Thesis, College of Marine Studies, University of Delaware, Newark, Del., 187 pp. Demarest, J.M., 1981. Genesis and preservation of Quaternary paralic deposits on Delmarva Peninsula. Ph.D. Diss., Department of Geology, University of Delaware, Newark, Del., 240 pp. Demarest, J.M. and Belknap, D.F., 1980. Quaternary Atlantic shorelines on Delmarva Peninsula -- chronology and tectonic implications. Northeast Geol. Soc. Am., Abstr. with Programs, 12: p.30. Demarest, J.M., Biggs, R.B. and Kraft, J.C., 1979. The interpretation of Pleistocene Epoch coastal barrier complexes with Holocene models, southeastern Delaware. AAPG--SEPM Annual Meeting, Houston, Texas, pp.75--76 (abstract). Demarest, J.M., Biggs, R.B. and Kraft, J.C., 1981. Time-stratigraphic aspects of a formation: interpretation of surficial Pleistocene deposits by anology with Holocene paralic deposits, southeastern Delaware, Geology, 9: 360--365. Field, M. and Duane, D., 1976. Post-Pleistocene history o f the United States inner continental shelf: significance to origin of barrier islands. Geol. Soc. Am. Bull., 87: 691-702. Fisher, J.J., 1967. Origin of barrier island chain shoreline, middle Atlantic states. Geol. Soc. Am., Spec. Pap., 115: 66---67. Goettle, M.S., 1978, Geological development of the southern portion of Assateague Island, Virginia. M.S. Thesis, Department of Geology, University of Delaware, Newark, Del., 187 pp. Halsey, S., 1979. Nexus: New model of barrier island development. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, N.Y., pp. 185--210. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, N.Y., pp. 1--27.
33 Hayes, M.O., Goldsmith, V. and Hobbs, C., 1970. Offset coastal inlets. Proc. 12th Coastal Engineering Conf., pp.1187--1200. Jordan, R.R., 1962. Stratigraphy of the sedimentary rocks of Delaware, Del. Geol. Surv. Bull., 9, 51 pp. Kelley, J.T., 1983. Composition and origin of the inorganic fraction of southern New Jersey coastal mud deposits. Geol. Soc. Am. Bull., 94: 689--699. Kraft, J.C., 1969. Pre-Holocene paleogeography and paleogeology in the Delaware coastal area. Geol. Soc. Am. Abstr. with Programs, Northeast Section Annual Meeting, Albany, N.Y., p.34. Kraft, J.C., 1971a. Sedimentary facies patterns and geologic history of a Holocene trans~ gression. Geol. Soc. Am. Bull., 82: 2131--2158. Kraft, J.C., 1971b. A guide to the geology of Delaware's coastal environments. College of Marine Studies, University of Delaware, Newark, Del., 220 pp. Leatherman, S.P., Rice, T.E. and Goldsmith, V., 1982. Virginia barrier island configuration: a reappraisal. Science, 215: 285--287. McDonald, K.A., 1981. Three-dimensional analysis of Pleistocene and Holocene coastal sedimentary units at Bethany Beach, Delaware. M.S. Thesis, University of Delaware, Newark, Del., 204 pp. Meade, R.H., 1969. Landward transport of b o t t o m sediments in estuaries of the Atlantic Coastal Plain. J. Sediment. Petrol., 39: 222--234. Mixon, R.B., Szabo, B.J. and Owens, J.P., 1982. Uranium-series dating of mollusks and corals, and age of Pleistocene deposits, Chesapeake Bay area, Virginia and Maryland. U.S. Geol. Surv., Prof. Pap., 1067-E, 18 pp. Morton, R.A. and Donaldson, A.C., 1973. Sediment distribution and evolution of tidal deltas along a tide-dominated shoreline, Wachapreague, Virginia. Sediment. Geol., 10: 285--299. Oertel, G.F., 1977. Geomorphic cycles in ebb deltas and related patterns of shore erosion and accretion. J. Sediment. Petrol., 47: 1121--1132. Owens, J.P. and Denny, C.S., 1979. Upper Cenozoic Deposits of the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Surv., Prof. Pap., 1067-A, 28 pp. Rice, T., Niedoroda, A. and Pratt, A., 1976. Coastal processes and geology, Virginia barrier islands in Virginia Coast Reserve Study. The Nature Conservancy, Arlington, Va, pp.109--382. Swift, D.J.P., 1968. Coastal erosion and transgressive stratigraphy. J. Geol., 67: 444-456. Truitt, R., 1968. High winds and high tides. Natural Resources Institute Educational Ser. 77, University of Maryland, College Park, Md., 35 pp.
19
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Chapter II. M a i n l a n d coastal e l e m e n t MAINLAND INFLUENCE ON COASTAL DELMARVA PENINSULA
TRANSGRESSION:
JAMES M. DEMAREST and STEPHEN P. LEATHERMAN
Exxon Production Research Co., P.O. Box 2189, Houston, TX 77001 (U.S.A.) Department of Geography, University of Maryland, College Path, MD 20742 (U.S.A.) (Accepted for publication May 17, 1984)
ABSTRACT Demarest, J.M. and Leatherman, S.P., 1985. Mainland influence on coastal transgression: Delmarva Peninsula. In: G.F. Oertel and S.P. Leatherman (Editors), Barrier Islands. Mar. Geol., 63: 19--33. The morphology of present-day barrier beaches and lagoons along Delmarva Peninsula is controlled by wave climate, tidal energy, sediment texture, and sand supply. However, morphology is also affected by coastal plain physiography, largely the product of Pleistocene sea-level change over the last million years. Five distinct transgressive coastal systems have been identified on Delmarva Peninsula using geomorphic and subsurface data. Each was produced during interglacial high sea levels and range in age from over one million years to 60,000 yrs B.P. Eroding pre-Holocene sediments represent major sand sources for present-day beaches. Baymouth barriers that formed adjacent to eroding headlands, such as Bethany Beach, Delaware, are characterized by infrequent inlets and robust dune systems. The mainland lagoonal shoreline follows the contours of the incised paleodrainage system. To the south, a long continuous barrier with infrequent inlets is present where littoral drift maintains a high sediment supply. Further south and away from this primary source of sand, short drumstick islands exist. This barrier chain is the result of the present transgression of a pre-existing 60,000 yr old shoreface. The mainland lagoonal shoreline follows the trend of Pleistocene beaches, which have experienced only minor stream dissection due to their young age. Along the southern part of the Peninsula, sea-level rise will eventually cause Holocene barriers to be welded onto the ancient barrier shore (the mainland). Whereas to the north, baymouth barriers will persist as the well-developed paleodrainage system is flooded farther inland. INTRODUCTION Investigations of Pleistocene shorelines on the Delmarva Peninsula illuminate some of the causes of present,lay barrier configuration. Holocene transgressive shorelines between Cape Henlopen, Delaware and Cape Charles, V i r g i n i a , c o n s i s t o f f o u r m a i n t y p e s : r e c u r v e d s p i t s a n d b a y m o u t h b a r r i e r s in Delaware, long continuous barrier beaches in Maryland, and drumstick ( s h o r t , b u l b o u s ) b a r r i e r s in V i r g i n i a ( F i g . l ; F i s h e r , 1 9 6 7 ) . A d i s t i n c t m o r p h o logic relationship exists between the modern coastal systems and the Pleistocene shorelines of the mainland that developed over the last million years. 0025-3227/85[$03.30
© 1985 Elsevier Science Publishers B.V.
20
L///_ /
'
/R__EHOBOTI ....BE'A( _ _.
,"\,jr
;LL
BETHANY BEACH
FIG. 3 ~ . \
- 3 8 ° 30'
!
DEL. --~D~
I
' •
. "~:~
CIRCA ~_ 1 my BP(
SALISBUI~Y
0.6 my BF
t'OCEAN CITY 0.06 my BP
MD~ VIR. /
"38°
HINCOTEAGUE
/
/
#
#
~A.i~,AMORE ISLAND ix~
EXMORE
.
/
/
KILOMETERS
o 75" 30'
K)
20
30
75"
21 Present-day m o r p h o l o g y can be u n d e r s t o o d by investigating the cross-cutting relationship between ancient and m o d e r n shoreline trends and the topography of the land surface transgressed by the present barrier system.
Previous work Regional geologic studies of the Delmarva coastal plain by Jordan (1962), Owens and Denny (1979), Belknap (1979), and Mixon et al. (1982) have shown surfacial sediments to consist of Pleistocene age paralic deposits which in general are y o unge r toward the south. Demarest et al. (1979, 1981), working in a small area in southeastern Delaware, showed that these paralic deposits consist of transgressive barrier systems preserved as highstand shorelines. Demarest and Belknap (1980) and Demarest (1981) also investigated the stratigraphic relationships and regional e x t e n t of some of these shorelines (Fig.l) and suggested a relationship between m o d e r n and Pleistocene shorelines.
Basic concept During major shoreline transgression sediment supplied t o the coastal system comes from shoreface erosion if updri ft headlands are absent (Swift, 1968). Present Delmarva beaches receive no coarse-grained sediment from rivers (Meade, 1969) although rivers do c o n t r i b u t e i m port ant quantities of fine-grained material to the lagoons and estuaries (Kelley, 1983). Although the exact nature of sediment t r a ns por t at the shoreface is still controversial, it is clear th at shoreface erosion is an i m p o r t a n t source of sediment for maintenance o f beach trends. Two classes o f sediment are supplied to the beach through shoreface erosion. First, sediment that was recently deposited in bays and estuaries is exposed at the shoreface with barrier retreat. This Holocene sediment was deposited within the same coastal system when the shoreline was farther seaward and does not represent new sediment to the barrier system. The second t y p e of sediment supplied to the beach is material eroded from preH olo cen e units t hat crop out on the shoreface. This material is an i m p o r t a n t supplier of " n e w " sediment to the coastal system. Any part of the shoreface that is erosional has the potential to c o n t a c t pre-Holocene units and thus becomes a source area for new sediment. The location and t e x t u r e of sedim e n t supplied to the coastal zone by this mechanism largely determines sediment supply to and equilibrium configuration of the coastline. Where the equilibrium configuration of the shoreline diverges seaward of
Fig.1. Pleistocene shoreline trends of Delmarva Peninsula are traced east of the drainage divide between Cape Henlopen and Cape Charles. The approximate age of each ancient shoreline is indicated (from Demarest and Belknap, 1980). The lines labelled A through D are lines of section schematically illustrated in Fig.7. The "arc of erosion" is caused by sediment starvation south of Assateague.
22 the point of sea-level impingement on the pre-existing topography, lagoons are formed. Thus the morphology and evolution of lagoons are also closely tied to the pre-existing geomorphology. PLEISTOCENE HIGHLANDS Shorelines on Delmarva Peninsula currently intersect Pleistocene headlands at two locations: Bethany Beach and Rehoboth Beach, Delaware (Kraft, 1969, 1971b; Fig.l}, Two ancient barriers are currently cropping out in the shoreface in the vicinity of Bethany Beach, Delaware. The ancient shorelines appear as linear trends in cultivation on aerial photographs (Fig.2). Demarest et al. (1981) referred to these as the Cedar Neck and Bethany Barriers. The stratigraphy of these barriers (Fig.3) has also been studied by Demarest (1981) and McDonald (1981). Detailed examination of the map pattern of streams and topographic contours indicates the approximate location of ancient barriers and the relative ages of the surface (Demarest and Belknap, 1980). Linear trends in stream orientation and reduced stream density (Fig.4) coincide with the position of barrier sediments as determined by drilling in southeastern Delaware (Fig.3). These stream trends also parallel linear trends in topographic contours. These shorelines, referred to as the Bethany, Cedar Neck and White Neck barriers and dated at 0.06, 0.6 and 1.0 m . y . B . P . , respectively (Belknap, 1979; Demarest et al., 1981), can be morphologically traced to the south into Maryland and Virginia (Fig.5). In the Virginia sector of the Peninsula (Fig.l), a different topographic and drainage pattern is present. The oldest identifiable shoreline trend is thought to be equivalent in age to the Cedar Neck shoreline. This shoreline forms the prominent Mapsburg Scarp (Fig.6). Seaward of this scarp two linear trends are evident; one is a lower, less prominent scarp that follows the 10-ft contour and is the position of major stream intersections near Locustvitle, Virginia. This shoreline has been tentatively dated at 0.3 m.y.B.P. (Belknap, 1979, Demarest and Belknap, 1980). Bradford Neck is the youngest shoreline trend and forms a peninsula (Fig.6) because it is being inundated by the Holocene rise of sea-level. The shoreline forming Bradford Neck is thought to be age-equivalent to the Bethany Barrier (circa 0.06 m.y.B.P.). Since Pleistocene shorelines are oriented at an angle to the present Delaware coast (Fig.l), Bethany is the only place where the modern shoreline intersects the ancient barriers. The Cedar Neck barrier is relatively old (circa 0.6 m.y.B.P.) based on amino acid dating of lagoonal molluscs (Belknap, 1979; Demarest, 1981). These data along with work by McDonald (1981) indicate that the Bethany Barrier is much younger (about 60,000 yrs B.P.). Therefore, the surface being transgressed north of Bethany Beach is much older than the surface south of Bethany. The significance of the age of the surface being transgressed lies in the difference in maturity of the drainage system being flooded by rising sea level. North of Bethany Beach, Delaware, large dendritic rivers drain major portions
23
Fig.2. P l e i s t o c e n e age (ca. 0.6 m . y . B . P . ) b a r r i e r s h o r e l i n e s are e v i d e n t o n this I R aerial p h o t o g r a p h o f t h e B e t h a n y Beach, D e l a w a r e area. T h e linear t r e n d s in f a r m fields are t h e high, well-drained, s a n d y areas associated w i t h t h e high sea level interglacial shorelines.
24
FRANKFORD
- BETHANY
WHITE NECK PARALIC UNIT CEDAR NECK PARALIC UNIT
if- :
;6 t~o,6
HD'4A A ,
Ol43-2 QI44-1
/I"--
LG
Z
MN.2,JOBB-1 MN'2
Qi45 1.TB-3.9,16
~ ~ O' Z <1(,¢~ BETHANY PARAL~C U N N ~0 ¢p ~p "~
QP3
LG QP1
PRE QP1
Qp" E • EOLIAN NS - NEARSHORE HB - H I G H L A N D BEACH B • BARRIER RB - RIVER BED
CH - TIDAL CHANNEL LG - LAGOON "
QP5 G QP 1
PARALIC
?NIT NAME
,J
AMIND ACID AGE (AVG. OIL LEUCINE
ESTIMATED RELATIVE SEA LEVEL ~iA)
MOLOCENE
O-'Oka (0.151
0
QPE
BETHANY
80 lO0ka (0.241
3TOO
PEPPER CREEK
120 - 130 ka (0.33}
QP4
UNDIFFERENTIATED UNITS (S) CEDAR NECK
5 TO 6 -5 TO -15
500 -900 k~ (0.55)
2,3 TO 2.5
i
,ap2
UNDIFFERENTIATED UNtTS
QP1
WHITE NECK
-0 TO -12
1000 - leOO k= f0.64)
3 TO E
PRE.Opl I BEAVERDAM FM. OF JORDAN. l g ~ l
J
Fig.3. East--west cross-section west of Bethany Beach, Delaware, showing the ages and stratigraphic relationships o f the Pleistocene transgressive barriers. Note that Qp5, the Bethany Barrier, is currently being eroded in the shoreface supplying "new" sediment to the littoral system (from Demarest, 1981).
of the coastal plain; the drainage divide is located far to the west and, where flooded, large estuaries in the form o f Delaware Bay, Indian River, and St. Martins River fill the lower reaches of the river valleys (Fig.l). South of Bethany Beach, Delaware, numerous streams drain smaller areas and have narrower channels. In addition, the drainage divide is much closer to the present coast (Fig.6). Up-stream of the ancient shoreline trends, the drainage pattern again becomes dendritic. Fig.4. The trends o f the White Neck, Cedar Neck and Bethany barriers are illustrated on the present-day drainage pattern to show the control of these shorelines on stream orientation. Also note that from the intersection of the Bethany barrier south, the Holocene barrier is transgressing a young shoreface which has been dissected during only one sealevel fall. From the intersection of Cedar Neck Barrier north, the surface is much older.
25
58'55"-
ANY 4
38*25'-
,J
58°20t
4
26
Fig.5. Landsat image of the southern part of Delmarva Peninsula illustrating the same linearity of cultivation evident in Fig.2. These trends along with detailed analysis of stream orientation and topography were used by Demarest and Belknap (1980) to trace southwards the Pleistocene shorelines found in Delaware. These geomorphic characteristics correlate well with relative age differences between surfaces being transgressed by the Holocene coastal system. The 60,000 yrs B.P. shoreline south of Bethany Beach, Delaware, generally forms the most seaward lagoonal headland (Figs.4 and 6). Sinepuxent Neck, opposite northern Assateague Island, is the most prominent geomorphic expression of this Pleistocene shoreline in Maryland. In Virginia this ancient barrier shoreline forms a prominent scarp until it again forms a peninsula at Bradford Neck, extending into the Holocene lagoon (Fig.6). The antecedent topography which is being transgressed seaward of the Bethany shoreline has been dissected by streams during only one glacial low sea level. North of Bethany and therefore landward of both the Cedar Neck and Bethany shorelines, the surface has not been reworked by shoreface processes for at least 600,000 yrs and possibly over a million years. As a result, the depth of
27 I
;"
j)
I
ACCOMAC~
)
y /
/
37°L
"E¥C~E~i// 37°? qECK
EXMORE
?
s7025'-
o
KM
4
7~1155' Fig.6. The Pleistocene and Holocene barrier configurations in the Virginia sector of Delmarva showing that the present shoreline is separated from the ancient barriers by considerable distance. The Virginia barrier islands are distinctly different from the barrier systems developed farther north, where the 60,000 yrs B.P. shoreline is being eroded in the beachface.
28 erosion and extent of dendritic development of the drainage topography are greater north of Bethany Beach, Delaware than to the south where the surface currently being transgressed is a relatively young shoreface. HOLOCENE SHORELINES Holocene barrier beaches along Delmarva can be divided into four segments: Cape Henlopen to Rehoboth, R e h o b o t h to South Bethany, South Bethany to Chincoteague and Chincoteague to Cape Charles (Fig.l). Each of these segments has a distinct morphology which is the product of interactions with the different land surfaces being transgressed. Cape Henlopen to R e h o b o t h Beach is the northern terminous of the Atlantic barrier systems of Delmarva Peninsula. Here sediment eroded from South Bethany and R e h o b o t h headlands is deposited at the entrance to Delaware Bay where wave energy is insufficient to continue carrying the sediment along the bayshore (Kraft, 1971a). The segment from R e h o b o t h to South Bethany is a b a y m o u t h barrier anchored at either end by eroding headlands. Indian River Inlet is the only break in this shoreline, and barrier stratigraphy is primarily inlet-dominated (Carey, 1981). The barrier is characterized by well-developed dunes and is rarely overwashed by storm waves. The mainland shoreline of the lagoon is highly variable in shape and dependent upon the contours of the incised valley. From South Bethany to Chincoteague, the barrier shoreline is interrupted by one inlet at Ocean City, Maryland. However, historical records show that numerous inlets were present in the recent past (Truitt, 1968). This barrier segment is connected at one end to an eroding headland which presumably supplies much of the sediment to the beach. Sand supply from the headland appears to be a relatively recent occurrence, and Assateague Island is currently prograding to the south faster than it is retreating landward. It has prograded past a pre-existing Holocene barrier (Chincoteague Island; Goettle, 1978). U.S.G.S. topographic maps from the early 1900's provide evidence for the recent intersection of the barrier with the headland at South Bethany. At that time a lagoonal marsh separated the beach at Bethany from the mainland. Barrier demise later occurred as the shoreface intersected the headland at Bethany. The lagoonal shoreline follows an incised valley in the north part but becomes relatively linear where it follows a Pleistocene shoreline in the southern part of this segment (Fig.l). Intersection of the eroding barrier with the sandy Pleistocene barriers at Bethany Beach, Delaware, could explain both the rapid progradation of Assateague Island and the closing of all but one tidal inlet (even Ocean City Inlet is kept open artificially). The y o u n g age of the Bethany Barrier, which is being eroded at Bethany Beach, suggests that a relict shoreface is being flooded south of Bethany. Since headlands did n o t persist seaward of the present Assateague Island, this barrier in its present configuration is a relatively recent development.
29 South of Assateague Island is a long chain of discontinuous, short barrier islands, which have been termed drumstick barriers by Hayes (1979). The barriers largely consist of fine-grained sand and are frequently overwashed during storms (Rice et al., 1976). Each island is separated from adjacent islands by large, fairly stable tidal inlets that feed a complex network of tidal channels. The backbarrier lagoons are largely filled by tidal marshes and mud flats. Although tectonic activity, sediment texture and supply, tidal range and wave climate account for some of the difference between the barriers along Delmarva shoreline (Leatherman et al., 1982), the antecedent topography is believed to have had a major influence on barrier morphology. The position of major tidal inlets along the Virginia barrier islands generally coincides with topographically low areas on the Pleistocene erosional surface (Morton and Donaldson, 1973). Some of these low areas appear to be associated with paleodrainage valleys eroded during lower sea level. Halsey (1979) noted that the streams tend to have a parallel drainage network which would give rise to fairly evenly spaced tidal inlets. Sea-level submergence of incised valleys determine the location of major tidal prisms. These tidal prisms are primarily confined to tidal channels which appear to be relatively stable during transgression. Thus, tidal inlets at the m o u t h of tidal channels tend to be confined to the area of ancestral drainage channels. Additionally, the interfluve areas with greater relief have the potential of supplying new sediment to the transgressive system. These ancient drainageways are not incised as deeply as valleys on the landward side of the Pleistocene barriers and thus are much lower relief features than those farther north in Delaware. This difference in valley depth is believed to be a function of age differences of the pre-Holocene surfaces. The present drumstick shape of the Virginia barrier islands is related to the relative stability of major tidal inlets and the largely infilled lagoons. Flow characteristics of tidal currents (Boon, 1975) coupled with relatively stable inlet channels have resulted in the evolution of large ebb-tidal deltas; these sand lobes have a pronounced effect on barrier dynamics and ultimately island morphology (Hayes et al., 1970; Oertel, 1977). This type of island was originally described for mesotidal environments (2--4 m tidal range; Hayes, 1979), and it is hypothesized that Pleistocene topography has had a role in establishing the requisite conditions for development of such drumstick barriers along an ostensible microtidal shoreline. The apparent lack of headlands in the foreshore updrift of the drumstick barriers suggests that new sediment is supplied from shoreface erosion and moved onshore to replace sediment lost to littoral drift or inlet-related sinks. The mainland shoreline landward of the backbarrier lagoon is abutting the steep portion of the Pleistocene shoreface that formed about 60,000 yrs B.P. As a result, the mainland lagoonal shore is linear and exhibits a sharp contact along older Pleistocene shorelines south of Bradford Neck. Because the lagoonal beach rests on the steep portion of the relict shoreface, it will migrate landward by submergence more slowly than the drumstick barriers with continued sea-level rise.
30 SUMMARY AND CONCLUSIONS During the Holocene pre-existing topography has played an important role in development of transgressive barriers and backbarrier lagoons on Delmarva Peninsula. Much of the present morphologic differences between northern and southern Delmarva may be explained in terms of the geomorphic influence of the underlying surface (Fig.7). The pre-existing topography can be manifested in two different ways; the surface is primarily a subaerial drainage system characterized by deeply incised valleys cut during low sea level, or it is a relict shoreface formed seaward of a Pleistocene transgressive shoreline. All of the stream valleys east of the Delmarva drainage divide show a consistent relationship between drainage maturity and elapsed time since the last transgression. This suggests that several cycles of downcutting are required to develop deeply incised valleys, although some downcutting will occur seaward of even the youngest Pleistocene shoreline as demonstrated by Morton and Donaldson (1973). Bethany Beach, Delaware, serves as a transition area in the present coastal trend along the Delmarva Peninsula. There is an order of magnitude (0.6 vs. 0.06 m . y . B . P . ) difference in age between the surfaces which are being transgressed north and south of Bethany Beach, Delaware. Age differences of the pre-Holocene surface being flooded determines whether b a y m o u t h barriers form across incised valleys that dissect the surface (to the north) or barrier beaches are disconnected from the mainland along unaltered shorefaces (to the south). In addition, two Pleistocene barriers are currently being eroded at Bethany Beach, providing a good source of sand to the littoral drift system. The lack of headlands to supply the southern beaches with sediment implies that the bulk of the beach sand is derived from offshore. Therefore, some of the morphologic differences between northern and southern Delmarva Peninsula may reflect differences in scouring of the shoreface. The northern barriers are primarily nourished by longshore transport away from eroding headlands, while southern barriers are fed by onshore transport from presumed subcrops in the shoreface. The position of inlets, particularly along the Virginia barriers, appears to be influenced by incised valleys on the antecedent topography, even where the valleys are low relief well below the active depositional surface. However, these valleys are still of secondary importance in shoreline dynamics since n o t all inlets coincide with valleys. The primary control appears to be whether or not sufficient relief exists between the base of the valley and the interfluves to produce headlands in the beachface. Control of relief is primarily a function of age since the last transgression. Differences in antecedent topography will cause the future evolution of the backbarrier systems to follow different pathways. With no headlands present or likely to be intersected in the near future, the drumstick barriers will continue to migrate landward across a gently sloping relict shoreface while the lagoonal shoreline remains relatively stable along the steep portion
~" ~"
--
~
~
-
C
~
,,
.
.
.
.
• ,, ,, ..,,. ,,.,
.
° :'-'.','.~:.:... ".
V
~ Barrier !:+:':-:-:-I Sand
BARR,ER
"
~
~
"
Lagoonal Mud
I-~
Lagoonal Marsh
: ~ " : : : : : : : " : : : : : " " " : : : ' " .................... . : . ~ . .
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
'
•
....
V
METOMKIN ISLAND
Itr
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
T
D
~' vf"
= ~**,* "*
Xe.%--------__~2
BASED IN PART ON DATA FROM KRAFT (1971)AND JOHN (1977)
~
T
,
METOMKIN INLET
-
iiiiiiiiiiiiiiiiiii:i:i:iiiiiiiiiiiiiiiiiiiiiiiii:i::::::::':'""'""'"" ~ ~
CEDAR ISLAND
( CIRCA 600,000 my BP )
~
B
STRIKE
INDIAN RIVER INLET .~__~.:.....~. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
X-SECTIONS
Fig.7. Schematic cross-sections from northern and southern Delmarva Peninsula illustrating the difference between the two types o f barrier systems. In sections A and B, irregularity of the pre-existing topography, particularly in strike section, is the result o f over 500,000 yrs of valley incision during lowered sea level. In sections C and D, the pre-existing topography in front of the Bradford Neck barrier is primarily a relict shoreface formed about 60,000 yrs ago. The young age has precluded extensive valley incision comparable to that found to the north. This difference in pre-existing configuration is an important factor in determining the present-day morphology and future trends of the barrier--lagoon system during transgression.
•
.
CEDAR ISLAND (RECENT)
AFTER KRAFT (197!)
,.,',,,'.,.-,,'.%,-,.
~
Y,T. ""-.'.'.:.'..'.'..:...'...'.?.'...
~.vBRADFOR NECK D BARRIER ( CIRCA 60,000 YR BP )
.,
BASED IN PART FROM DATA FROM MORTON & DONALDSON (1973)
\--
A
DIP
SCHEMATIC
32
of the Pleistocene shoreface (scarp). If the transgression continues, the lagoons will narrow, causing a decrease in total wetlands area. North of Bethany Beach, Delaware, the lagoons will not show a systematic change in area with continued sea-level rise and transgression. Some lagoons may expand in area while others will shrink, depending on the slope of the mainland surface being flooded and the rate of barrier transgression (Field and Duane, 1976). Headland erosion will tend to decrease the overall rate of transgression. Furthermore, continued erosion increases the length of shore that is headland, and therefore sand supply to littoral drift will also increase. This may further slow the rate of shoreline retreat, allowing the lagoons to further expand with mainland submergence. In summary, pre-existing topography plays an important role in the morphologic development of barrier shorelines. Proper evaluation of these effects permits a more precise assessment of the important process variables controlling coastal evolution. To ignore or underestimate the fundamental legacy of the geologic past in shaping present-day barriers can lead to false deductions a b o u t the morphodynamics of coastal depositional systems. REFERENCES Belknap, D.F., 1979. Application of amino acid geochronology to stratigraphy of late Cenozoic marine units of Atlantic Coastal Plain. Ph.D. Diss., Department of Geology, University of Delaware, Newark, Del., 580 pp. Boon, J., 1975. Tidal discharge asymmetry in a salt marsh drainage system. Limnol. Oceanogr., 20: 71--80. Carey, W.L., 1981. Surfacial morphology and subsurface stratigraphy of the flood tidal deltas on the Atlantic Coast of Delaware. M.S. Thesis, College of Marine Studies, University of Delaware, Newark, Del., 187 pp. Demarest, J.M., 1981. Genesis and preservation of Quaternary paralic deposits on Delmarva Peninsula. Ph.D. Diss., Department of Geology, University of Delaware, Newark, Del., 240 pp. Demarest, J.M. and Belknap, D.F., 1980. Quaternary Atlantic shorelines on Delmarva Peninsula -- chronology and tectonic implications. Northeast Geol. Soc. Am., Abstr. with Programs, 12: p.30. Demarest, J.M., Biggs, R.B. and Kraft, J.C., 1979. The interpretation of Pleistocene Epoch coastal barrier complexes with Holocene models, southeastern Delaware. AAPG--SEPM Annual Meeting, Houston, Texas, pp.75--76 (abstract). Demarest, J.M., Biggs, R.B. and Kraft, J.C., 1981. Time-stratigraphic aspects of a formation: interpretation of surficial Pleistocene deposits by anology with Holocene paralic deposits, southeastern Delaware, Geology, 9: 360--365. Field, M. and Duane, D., 1976. Post-Pleistocene history o f the United States inner continental shelf: significance to origin of barrier islands. Geol. Soc. Am. Bull., 87: 691-702. Fisher, J.J., 1967. Origin of barrier island chain shoreline, middle Atlantic states. Geol. Soc. Am., Spec. Pap., 115: 66---67. Goettle, M.S., 1978, Geological development of the southern portion of Assateague Island, Virginia. M.S. Thesis, Department of Geology, University of Delaware, Newark, Del., 187 pp. Halsey, S., 1979. Nexus: New model of barrier island development. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, N.Y., pp. 185--210. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, N.Y., pp. 1--27.
33 Hayes, M.O., Goldsmith, V. and Hobbs, C., 1970. Offset coastal inlets. Proc. 12th Coastal Engineering Conf., pp.1187--1200. Jordan, R.R., 1962. Stratigraphy of the sedimentary rocks of Delaware, Del. Geol. Surv. Bull., 9, 51 pp. Kelley, J.T., 1983. Composition and origin of the inorganic fraction of southern New Jersey coastal mud deposits. Geol. Soc. Am. Bull., 94: 689--699. Kraft, J.C., 1969. Pre-Holocene paleogeography and paleogeology in the Delaware coastal area. Geol. Soc. Am. Abstr. with Programs, Northeast Section Annual Meeting, Albany, N.Y., p.34. Kraft, J.C., 1971a. Sedimentary facies patterns and geologic history of a Holocene trans~ gression. Geol. Soc. Am. Bull., 82: 2131--2158. Kraft, J.C., 1971b. A guide to the geology of Delaware's coastal environments. College of Marine Studies, University of Delaware, Newark, Del., 220 pp. Leatherman, S.P., Rice, T.E. and Goldsmith, V., 1982. Virginia barrier island configuration: a reappraisal. Science, 215: 285--287. McDonald, K.A., 1981. Three-dimensional analysis of Pleistocene and Holocene coastal sedimentary units at Bethany Beach, Delaware. M.S. Thesis, University of Delaware, Newark, Del., 204 pp. Meade, R.H., 1969. Landward transport of b o t t o m sediments in estuaries of the Atlantic Coastal Plain. J. Sediment. Petrol., 39: 222--234. Mixon, R.B., Szabo, B.J. and Owens, J.P., 1982. Uranium-series dating of mollusks and corals, and age of Pleistocene deposits, Chesapeake Bay area, Virginia and Maryland. U.S. Geol. Surv., Prof. Pap., 1067-E, 18 pp. Morton, R.A. and Donaldson, A.C., 1973. Sediment distribution and evolution of tidal deltas along a tide-dominated shoreline, Wachapreague, Virginia. Sediment. Geol., 10: 285--299. Oertel, G.F., 1977. Geomorphic cycles in ebb deltas and related patterns of shore erosion and accretion. J. Sediment. Petrol., 47: 1121--1132. Owens, J.P. and Denny, C.S., 1979. Upper Cenozoic Deposits of the central Delmarva Peninsula, Maryland and Delaware. U.S. Geol. Surv., Prof. Pap., 1067-A, 28 pp. Rice, T., Niedoroda, A. and Pratt, A., 1976. Coastal processes and geology, Virginia barrier islands in Virginia Coast Reserve Study. The Nature Conservancy, Arlington, Va, pp.109--382. Swift, D.J.P., 1968. Coastal erosion and transgressive stratigraphy. J. Geol., 67: 444-456. Truitt, R., 1968. High winds and high tides. Natural Resources Institute Educational Ser. 77, University of Maryland, College Park, Md., 35 pp.