Within unit A, reflection horizon A 4 is Pleistocene in age and 

 perhaps represents the upper surface of the Pleistocene aquifer. Horizon 

 A 3 is judged to be pre-Holocene erosion surface, A map of the pre- 

 Holocene erosion surface on the Delaware shelf prepared by Sheridan, Dill, 

 and Kraft (1974) clearly indicates that in the northern part of the study 

 area, configuration of the local drainage pattern was the dominant influ- 

 ence on pre-Holocene topography (Fig, 16). It appears that this influence 

 was obliterated during the Holocene, leaving few if any traces on the 

 modern sea floor. 



3. Secondary Reflection Horizons . 



a. Buried Channels Beneath Delaware Bay Mouth . Relict channels 

 incised into the Delmarva shelf surface can be divided into two groups: 

 the channels associated with the ancestral Delaware River drainage system, 

 and the channels which may or may not be related to existing drainage 

 patterns. The broadest and deepest channels are those in the mouth of 

 the Delaware Bay. Although survey coverage is minimal in that area, 

 several lines completely cross the estuary entrance and provide some 

 interesting information about the original channel depths and positions. 



Positions of tracklines crossing the bay mouth are shown in Figure 17 

 in relation to the position of present channels. Line 2A crosses the 

 western channel (a closed depression defined by the 24-meter contour) up 

 to the Cape May shoal platform. The reduced seismic profile of line 2A 

 shows that several episodes of channelization occurred and that the deep- 

 est channel, approximately 67 meters below sea level, lies beneath the 

 present western channel. The cut and fill substructure extends across 

 the bay mouth and beneath the shoal topography. Line 2B, a west -east 

 line on the shoal platform, also shows that construction of the platform 

 occurred on a stream-eroded topography. More than 10.7 meters (35 feet) 

 of Holocene sediment has been deposited over the channeled topography, 

 the deepest part of which lies about 49 meters below sea level . 



The three lines crossing the eastern channel show the present channel 

 to be narrower than the original channel. Flat-lying reflectors beneath 

 the channel indicate the ancestral thalweg did not extend more than 6.1 

 meters below the channel; if it was deeper but displaced, it was displaced 

 to the west side (see line B, Fig. 17). It is difficult to determine the 

 mechanism for filling the channels; however, the lines crossing the east- 

 ern channel, especially line B, give some indication, by the steeply dip- 

 ping progradational secondary reflections, that lateral movement of sand 

 has infilled the channel. This contrasts with the classical longitudinal 

 filling by fluvial processes and indicates that the majority of filling 

 has been by marine and littoral processes. South of the Delmarva Penin- 

 sula, the main channel in the Chesapeake Bay entrance has been filling in 

 by lateral migration during historic migration (Meisburger, 1972; Field 

 and Duane, 1976); the history of both of these large estuary entrances 

 may be quite similar. 



b. Buried Channels on the Inner Shelf . Excluding those channels 

 associated with the ancestral Delaware River, the northern Delmarva shelf 



44 



