by the modern sea floor (identified as Aj in Fig. 13). Excluding the 

 steeply sloping shoreface region and topographic irregularities (shoals, 

 swales), the modern shelf floor has a slope of about 1 on 800. Internal 

 reflection horizons, A 2 , A 3 , and A 4 are flatter than either bounding 

 surface and have slopes of 1 on 1,500, 1,600, and 1,000, respectively. 

 Horizon A 2 lies at between 12 and 18.3 meters and therefore is present 

 only in the inner half of the study area. The seismic reflection data, 

 especially that obtained with the 3 .5-kilohertz seismic profiler, show 

 that in low areas (swales between linear ridges) horizon A 2 "crops out" 

 or is exposed at the water sediment interface. Seaward of about the 18- 

 meter (60 feet) contour, the horizon is no longer present in the subsur- 

 face. A structure contour map on the surface of reflector A 2 (Fig. 14) 

 shows that the reflector has a north-northeast strike and is generally 

 smooth and featureless, with the exception of several broad depressions 

 and highs exhibiting about 1.5 meters of relief. The comparison between 

 this surface and the sea floor (horizon A}) is quite marked. 



Sufficient data were collected from the 3. 5-kilohertz profiler to 

 construct a contour map of the A3 and A 4 subsurfaces (Field, 1976). 

 In general, horizon A 3 is flat-lying. The slope gradient varies from 

 1 on 1,200 to 1 on 3,500. Horizon A^ is a persistent mappable reflector 

 lying between 18.3 and 30 meters (60 and 100 feet) below sea level in the 

 study area. Its slope is fairly consistent at about 1 on 1,000 to 1 on 

 1,500, and there are no discernible perturbations in the surface. 



2. Correlation of Reflector Surfaces with Stratigraphic and Hydrologic 

 Units . 



Acoustic reflections are generated by a change in acoustic impendence 

 within the sediment column. This change is normally attributed to some 

 lithologic change such as grain size, matrix content, degree of cementa- 

 tion, water content, etc. Therefore, as a working assumption, reflecting 

 surfaces revealed by the records are considered stratigraphically signif- 

 icant. These changes often relate to previous lithologic changes mapped 

 onshore as formational boundaries or provide the contrasts that define 

 aquifer-aquiclude contacts. The age of different substrata is difficult 

 to accurately label, since the stratigraphy of Delmarva is presently being 

 revised by the U.S. Geological Survey; this revision includes reassignment 

 of some ages. A major aspect of the revision includes reevaluation of 

 criteria delineating the Quaternary-Tertiary boundary. Earlier studies 

 placed the eroded upper surface of the Tertiary (Yorktown Formation, 

 Miocene age) at about 30 meters below sea level at Ocean City (e.g., 

 Rasmussen and Slaughter, 1955) . Since new terminology or ages have not 

 been formally proposed, the stratigraphic nomenclature of Rasmussen and 

 Slaughter (1955) and hydrologic nomenclature of Weigle (1974) are used 

 here for correlation. 



The top of the St. Marys Formation is a well-defined boundary in bore- 

 holes and well logs in the Maryland coastal area. At Ocean City this sur- 

 face lies at about 146 meters (480 feet) below sea level and is beyond the 

 depth penetration obtainable by the acoustic equipment used in this study. 



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