GM18 Geologic Map of the Bethany Beach and Assawoman Bay
Total Page：16
File Type：pdf, Size：1020Kb
DELAWARE GEOLOGICAL SURVEY DELAWARE GEOLOGICAL SURVEY University of Delaware, Newark GEOLOGIC MAP OF THE BETHANY BEACH AND ASSAWOMAN BAY QUADRANGLES, DELAWARE David R. Wunsch, State Geologist GEOLOGIC MAP SERIES NO. 18 6' 75° 4' 2' 75° 0' 0" W 75° 7' 30" W 38° 37' 30" N EXPLANATION 38° 37' 30" N 10 Qd Qscy Qm Delaware Seashore State Park LONG NECK RD f Qm 1 FILL OMAR FORMATION Qo Qd Qm f 3 Man-made and natural materials (sand, gravel) emplaced in stream valleys or marshes to Light-gray to gray, silty clay to silty, very fine sand with scattered shell beds. The Omar Qd Qscy f 3 Formation consists of up to 5 ft of light to dark-gray, basal, pebbly, coarse to very coarse L O N G N E C K 7 Qss bring the topography above grade, usually in road beds, dams, or construction near a 20 shoreline. Fill deposits include sediment dredged from the marshes and offshore in sand that grades upward into 1 to 3 ft of gray to very dark-gray, fine to coarse silty sand Tbd f Qm Qm Indian River Bay and emplaced on the uplands. with scattered laminae to thin beds of peat composed of sand to gravel-size plant Steels Qbw 40 Qm Big Ditch fragments. The sands are overlain by 3 to 5 ft of very dark-gray to black organic-rich Cove f Qfs Qscy Qns sandy silt to silty clay. Above this organic-rich zone, in the areas where the Omar is 6 Qqw of Holocene lagoon (Ql) at depth thickest, 10 to 40 ft of greenish-gray, compact, silty clay to clayey silt is common. The 10 ml Little Ditch f MODIFIED LAND extension clayey silt grades upward into a gray, to light greenish-gray, silty, gray clay to silty fine Qsh Bottom Hills Drain 5 f 3 Qm Qd Offshore Areas of land where the surficial deposits have been modified due to human activity to sand with scattered laminae to thin beds of Crassostrea (oyster) shell. In places, there is 3 4 Qm 4 Qscy the point that surficial deposits can no longer be reliably determined. The unit is mapped a 2- to 10-ft thick bed of medium to coarse relatively clean sand separating the compact 6 Qsh Qsh Qm in areas large enough to be shown on the map and does not include local disturbances on clayey silt from the overlying shelly, silty clay. The overall thickness of the Omar 5 Qm Formation in the map area ranges from 2 ft thick along the surficial contact with the 4 10 the scale of an individual housing lot or shallow disturbances such as large parking lots or Delaware Seashore 40 retail areas. An example of modified land is a golf course where numerous ponds and Ironshire Formation to about 75 ft thick just south of Ocean View. The Omar Formation 4 f State Park 7 f 30 3 40 embankments have been constructed. fills an east-west oriented paleovalley and is comprised of swamp, marsh, and lagoonal 5 Qets sediments deposited during a sea-level highstand at approximately 400,000 yrs B.P. and 5 Indian River possibly reworked during the subsequent highstand at 320,000 yrs B.P. (Ramsey, 2010a). 5 30 Middle Pleistocene. 5 Inlet NEARSHORE DEPOSITS offshore Qns paleovalley 40 5 10 30 f f 10 Pale-yellow to light-gray, cross-bedded, very fine, silty sand to fine to coarse sand with Tbd 6 5 laminae to thin beds of very coarse sand, granules, and pebbles. Shells are a common BEAVERDAM FORMATION I N D I A N R I V E R constituent of the sands and range from fragmented, granule-size pieces to whole shells. 6 Ql Nearshore deposits are mapped from the shoreline to the break in slope where water Heterogeneous unit ranging from very coarse sand with pebbles to silty clay. The Qm Delaware Seashore State Park depths increase from less than 10 ft to greater than 20 ft. Texture of nearshore sediments predominant lithologies at the land surface are white to mottled light-gray and reddish- 6 Qss 6 Qftd A is related to texture of underlying deposits from which they are reworked. Nearshore brown, silty to clayey, fine to coarse sand. Laminae and beds of very coarse sand with Qns 36' 36' deposits consist of fine, silty sand north of Bethany Beach to fine to coarse sand with pebbles to gravel are common. Laminae and beds of bluish-gray to light-gray silty clay Qss 40 6 Qm lagoon (Ql) at depth T shells reworked from the underlying Sinepuxent Formation off Bethany Beach. To the are also common. In a few places near the land surface, but more commonly in the of Holocene B A Y south of Bethany Beach, nearshore deposits range from medium to coarse sand. Near- subsurface, beds ranging from 2 to 20 ft thick of finely laminated, very fine sand and silty Offshore extension L shore deposits are thin, usually less than 2 ft in thickness. Holocene. clay are present. The sands of the Beaverdam Formation have a white silt matrix that 3 interfluve of Holocene lagoon (Ql) at depth gives samples a milky appearance when wet. This white silt matrix is the most distin- 6 Old Basin guishing characteristic of the unit and readily differentiates the Beaverdam Formation 7 3 Qbw A 3 Cove from the adjacent cleaner sands of the Sinepuxent, Ironshire and Scotts Corners Forma- 6 Qd EBB TIDAL SAND DEPOSITS offshore Qets Qfs tions. The Beaverdam Formation is interpreted to be a late Pliocene fluvial to estuarine fshore extension 40 N T deposit (Ramsey, 2010a, b). It ranges from 50 to 100 ft thick in the map area. Late 4 Qm Qsh Of Pale-yellow to light-gray, cross-bedded coarse to fine sand with laminae of heavy 20 Qsh paleovalley Pliocene. 3 minerals and granules to pebbles. Shells and shell fragments are a rare constituent of the Qm sands. Ebb tidal delta deposits are found offshore of Indian River Inlet. The sediments 4 Qm Qsh Qsh f are partly disturbed by dredging and influenced by the location of the jetties offshore of Lookout Discussion Cem 6 Tower the inlet. Ebb tidal delta deposits range from 5 to over 25 ft in thickness. Holocene. Qm 4 I C O C E A Qm The geologic history of the surficial units of the Bethany Beach and Assawoman Qsc f Qscy y 4 Bay Quadrangles is that of deposition of the Beaverdam Formation and its subsequent Qsi FINGER SHOAL DEPOSITS offshore Qfs modification by erosion and deposition related to sea-level fluctuations during the 10 Qm Beach Pleistocene. The geology reflects this complex history onshore, in Indian River Bay and W H I T E N E C K 40 WALTERSBLUFF RD 4 Cove Gray, silty, coarse to fine, and fine to very fine sand that fines seaward to very fine sandy Assawoman Bay, and offshore in the Atlantic Ocean. Erosion during the late Pleistocene Qd f silt. In the medium sands, shell fragments and whole shells are common. Clay and sea-level lowstand and ongoing deposition offshore and in Indian River Bay during the Qscy f Tbd Pasture Point 30 clayey, silt-lined burrows are common throughout. The sands are finely laminated with Holocene rise in sea level represents the latest of several cycles of erosion and deposition. Joshia Prong 4 3 4 Cove CEDAR NECK RD opaque heavy minerals and textural laminae of very fine, silty sand and coarser sand. Qd 4 3 The deposits interfinger with the quiet-water deposits. Finger shoal deposits extend from Qi 3 The Beaverdam Formation consists of stacked, 1- to 5-feet thick beds of very Qm Qss Tbd the bathymetric break between 20 and 30 ft offshore that marks the limit of the nearshore coarse sand and gravel that commonly fine upwards to fine to medium sand and rarely to Qscy Qm deposits to water depths of about 35 ft. They have a characteristic bathymetric signature 10 very fine silty sand to silty clay. These types of deposits are typical of either fluvial or 30 6 of shore-oblique highs that extend like fingers from the nearshore. Thickness ranges Qscy Edgar Prong 4 3 f estuarine environments (Ramsey, 2010a, b). Rare burrows have been observed in the Qd Qm Tbd Qbw from less than 1 ft off of Bethany Beach to greater than 10 ft to the south. Holocene. Beaverdam Formation elsewhere in Delaware that indicate at least a marginal estuarine Qj12-01 Qj13-03 30 Qm Qsi B setting (DGS unpublished data; Owens and Denny, 1979). The Beaverdam Formation in f Tbd DERRICKSONS VIEW RD Qd the Bethany Beach and Assawoman Bay Quadrangles is exposed along the south side of SHEET SAND DEPOSITS offshore Qss Indian River Bay and on Miller Neck and Dirickson Neck. The Beaverdam underlies all f the younger deposits of the area and extends offshore (cross sections A-A' and B-B'). The Qd Qsi Qm Qsh Qj13-02 Pale-yellow, well-sorted, fine to medium, to coarse to very coarse sand with abundant age of the Beaverdam Formation is uncertain due to the lack of age-definitive fossils 10 granules, pebbles, and shell fragments. Cross-bedding is common, consisting of laminae within the unit. Stratigraphic relationships in Delaware indicate that it is no older than Qd Qscy Tbd 6 of sand, granules, and pebbles. Shells most common within sheet sand deposits are late Miocene and no younger than early Pleistocene, and is most likely late Pliocene Qsi Qd f Qm Spisula, Ensis and Anomia (few to common in abundance) and lesser amounts of sand (Ramsey, 2010a, b).