over the past 100 yr. Also, Richards (1954) noted dissolved oxygen 

 levels in the gulf were depressed in areas characterized by high organic 

 sediment loading. 



Sections of the eastern part of the Texas-Louisiana continental 

 shelf ecosystem have long been known to have characteristics similar to 

 a freshwater lake or closed marine system (e.g. Black Sea) in that 

 stratification and isolation of bottom waters, particularly in 

 conjunction with algal blooms, can result in anoxic conditions over the 

 bottom due to degradation and respiration processes. Whether the 

 frequency and extent of such areas have only recently increased and 

 expanded from Louisiana southwestward to central Texas in response to 

 increased pollution loads, or whether these areas have historically 

 experienced hypoxia on a regular basis is not known. Given the 

 implications of the former premise, research leading to an understanding 

 of dissolved oxygen dynamics of the Texas-Louisiana continental shelf 

 should be of high priority. 



Topography .and. Sediments 



The continental shelf of the Gulf of Mexico within the study area 

 is primarily a generally smooth, gently sloping, sediment-covered plain 

 extending from the shoreline out to the shelf break. Its typically flat 

 relief is only occasionally broken by low-profile banks, or swells, 

 usually representing the surface expression of salt domes or other 

 relict features associated with ancient coastlines. Although low-relief 

 shell and gravel ridges are exposed in some areas, most of these 

 features are blanketed by sediment. The source of the sediment is 

 freshwater runoff with recent sediments from the Mississippi Drainage 

 Basin. During the mid-tertiary period, the rivers of Texas supplied 

 much of the load and the large sedimentation of the Eocene was mainly 

 from the Rio Grande. The small submarine canyon along the shelf south 

 of Louisiana (Mississippi Trough, Figure 2), represents the Pleistocene 

 outlet of the river. The shoreward portion of this canyon has been 

 filled with sediment but it can be traced underground to the vicinity of 

 Houma, Louisiana (Hedgpeth 1954). 



A generalized sediment map of the gulf is shown by Figure 14 (after 

 Grady 1970). In the eastern portion of the study area, sediments are 

 predominantly clay and silt, whereas offshore eastern and central Texas, 

 sandy sediments predominate. In south Texas, clays and sand-silt clays 

 predominate. 



The most conspicuous topographic feature associated with the shelf 

 is the series of prominent submarine banks or highs which rise abruptly 

 from the smooth bottoms along the shelf break (Figure 15). The best 

 known of these are the East and West Flower Garden Banks, which are 

 considered to be the northernmost, thriving shallow- water, hermatypic 

 coral reefs on the eastern coast of North America. Sedimentary rock 



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