Surficial sands on the inner shelf have been reworked by waves and 

 currents and are inner shelf deposits only in the context of their most 

 recent history. The sands were originally deposited as fluvial, estua- 

 rine, lagoonal, barrier island, and shoreface deposits. These shelf 

 sediments that have been sorted and redeposited and are nearly in tex- 

 tural equilibrium with the shelf hydraulic regime, although still bearing 

 mineralogic or component evidence of a former, different depositional 

 environment, are neither truly relict nor truly modern sediments. Swift, 

 et al . (1971) termed such sediments "palimpsest." 



Major sediment types in the upper 15 meters of the sea floor are both 

 vertically consistent and laterally continuous, as shown by subsurface 

 sediment distributions in Figures 24 to 27. The vertical succession 

 records the passing of adjacent coastal facies during the most recent 

 transgression, from nonmarine to open marine. From the core samples 

 collected in this study a generalized sediment column for the shallow 

 subsurface of the shelf was constructed to show age, dominant lithology, 

 and depositional environment of major sediment units (Fig. 33, units A to 

 E) . The uppermost unit comprises type I fine to coarse, well-sorted sands 

 and is characterized by an undulating thickness that is related to surface 

 topography (distribution of linear thickening beneath shoals and thinning 

 or pinching out in swale areas). In general, the unit is thin on the 

 Delaware shelf and thickens to the south, with a maximum thickness of 

 about 12 meters. Unit E is nearly ubiquitous in the study area and is 

 absent only where it has been locally eroded. 



Unit D is a Holocene, very fine to fine, clean sand representing 

 a back-barrier environment. The unit is locally thick off central 

 Assateague; elsewhere it is generally less than 1.2 meters (4 feet) 

 thick and does not generally occur at the specific sediment depths map- 

 ped in Figures 25 to 27 for type II sand. Interpretation of the specific 

 environment is based on stratigraphic position, texture, and biogenic 

 constituents, which are indicative of low salinity water. The transition 

 to unit E is usually gradational . 



Unit C is a rarely occurring organic-rich silt (type V) deposited in 

 a marsh environment. Only two samples of unit C were suitably enriched 

 in organic material to permit radiocarbon dating and they were of dis- 

 tinctly different ages. The peat deposit, dated as Holocene age, may 

 have actually been deposited in a terrestrial environment. Several other 

 cores contain thin layers (<0.3 meter) of fine sand and silt with plant 

 remains but were not suitable for radiocarbon dating. 



Unit B is a clayey, sandy silt and very fine, poorly sorted sand that 

 represents sediment types III and IV. This unit is very common in the 

 shallow subsurface and is exposed on the sea floor in depressions or 

 flats between shoals. In many places it directly underlies unit E, and 

 the contact between the two is usually abrupt, with clasts of unit D 

 present in the base of unit E. The upper surface of unit D is a strong 

 acoustic reflector, identified in this study as horizon A 2 . This 

 widespread mud unit is interpreted as lagoonal and estuarine facies of 



74 



