the study area. Most can be related to past transgressions and regres- 

 sions of the sea across the shelf, and many deeply incised stream channels 

 are evident. These channels record the positions of ancient rivers which 

 once flowed across the shelf during low stands of sea level. All of the 

 channels were subsequently filled with sediment and several show evidence 

 of postfill erosion. (It is usually impossible to trace a channel from 

 one seismic profile to an adjacent profile because line spacing is too 

 great.) Ancestral shelf surfaces had low relief and the rivers meandered 

 across broad flood plains, much as they do today. Most of the channels 

 appear to be oriented perpendicular to the modem shore and several show 

 alinement with subaerial channels onshore. The combined Trinity-San 

 Jacinto River channel which underlies Galveston Inlet is several kilo- 

 meters wide and the largest in the area. Its size indicates that the 

 Trinity and San Jacinto Rivers are at least 100,000 years old and have 

 occupied only one channel on the shelf. Other sizeable channels which 

 cut into the Pleistocene surface are located just west of High Island, 

 immediately east and offshore of Crystal Beach on Bolivar Peninsula, and 

 just west of Freeport Inlet (the ancestral Brazos channel) . These chan- 

 nels are generally filled with Holocene fine-grained cohesive sediments. 

 A prominent reflector in the area marks the contact between the Pleisto- 

 cene erosion surface (top Beaumont Formation) and overlying Holocene 

 sediments. This reflector is a good mappable horizon in all parts of 

 the area except in the vicinity of Galveston Inlet, San Luis Pass, and 

 Freeport Inlet where distinctions between Holocene and Pleistocene sedi- 

 ments are difficult because of deep fluvial erosion and subsequent filling. 

 The depth of the reflector on the records correlates well in many cores 

 with the contact between greenish-gray and reddish-brown indurated clay 

 and softer grayish-brown overlying modern sediments. This reflector also 

 correlates well with the top of reported Pleistocene elevations and with 

 stratigraphy contained in boring logs from Galveston Inlet, San Luis Pass, 

 and Freeport Inlet. 



b. Faults. Faults are evident on many seismic profiles in the east- 

 ern part of the area but none were observed west of Galveston Inlet. Most 

 of the fault surfaces are vertical and maximum displacement is about 2 to 

 3 meters (6 to 10 feet). Tracing a fault between adjacent seismic lines, 

 even though the lines are only several kilometers apart, is difficult 

 which suggests the faults are minor flexures possibly related to the salt- 

 dome tectonics of High Island. All faults seem restricted to Pleistocene 

 sediments and many extend to the sea floor where Holocene cover is absent. 

 No evidence of displaced Holocene sediments or fault relief on the sea 

 floor was observed to suggest that movements along these faults occur at 

 present or have occurred in the recent past. 



3. Primary Sediment Classes . 



a. General . Data from the 34 cores collected during this study, 

 along with information on the TBEG map showing surficial sediment distri- 

 butions based on several hundred grab samples, provide the ground-truth 

 information on surficial and shallow subbottom sediments within the 

 study area. The seismic profiles were used to extrapolate the sediment 



