Figure 13, the clay actually crops out at the sea floor in several areas 

 (e.g., cores 4 and 7); in other areas it is covered by a relatively thin 

 mantle of Holocene muds and muddy sands. 



e. Sediment Age . Sediment properties and faunal content indicate 

 that some sediments recovered in the cores are part of the modern shelf 

 and nearshore sediment blanket, and other sediments are from various re- 

 lict deposits. Distinctions between the modern sediments and underlying 

 relict sediments are relatively clear in most cases; however, more subtle 

 gradations occur in places, such as adjacent to the inlets and in the 

 Galveston Bay dredged-material disposal area. 



Modern sediments are usually characterized in bulk by their gray to 

 grayish-brown color and low shear strength. Microscopic examination 

 shows that the principal sand-size components are subangular quartz 

 particles with various amounts of fragmented mollusk shells, echinoid 

 tests, ostracod carapaces, and forarainiferal tests. The most common 

 inorganic accessory minerals are opaques dominated by pale-olive to dark- 

 green glauconite grains. The foraminiferal fauna is a typical low diver- 

 sity marginal marine assemblage dominated by Ammonia beooarii (Linne) and 

 several species of Elphi-diim. 



Most of the relict sediments appear to belong to two widespread 

 deposits underlying the surficial modern sediment blanket. The largest 

 of these is a stiff clay, with interbedded sand layers, which is nearly 

 ubiquitous in cores 19 through 32 (Fig. 13). Typically, the clay is a 

 yellow to reddish-brown color, massive, and very stiff. Residues from 

 washing the clay through a U.S. Standard 230 sieve (0.063 millimeter or 

 4 phi) generally consist of a small amount of quartz grains. 



The sand, which appears to be interbedded with the stiff clay, is 

 usually reddish-brown in color, well sorted, and consists predominantly 

 of quartz particles. In contrast to the surficial modern sediments, 

 samples of this deposit contain little or no faunal remains "or glauconite 

 grains. Seismic data as well as lithology indicate that the stiff clay 

 and associated sand are part of the Pleistocene Beaumont Formation. 



The other extensive relict deposit occurs in the eastern part of the 

 study area (cores 1, 2, 4, 5, 6, and 7; Fig. 13). This deposit consists 

 of muddy sand with abundant shells and shell fragments of the marsh clam 

 [Rangia sp.). The sand fraction contains little or no glauconite and the 

 sparse, marginal marine foraminiferal fauna is usually dominated by 

 Ammonia beaoarii (Linne). The presence oi Rangia sp. indicates these 

 sediments were deposited in a back-barrier lagoon or estuary and not 

 in the existing open marine environment. This deposit may be either 

 Pleistocene-age or have been deposited during the Holocene transgression. 



A third group of deposits consisting of gray to grayish-brown sand and 

 mud is very similar in character to the modern sediment layer but contains 

 small percentages of glauconite and a foraminiferal fauna characterized by 

 a greater diversity as well as a larger number of Quinquelooulina sp. than 



35 



