29 



of the Gulf of California has been compiled by Robert L. Fisher, Gene 

 A. RusNAK and Francis P. Shepard (van Andel, etai, in press), based 

 on new soundings obtained on the various "Vermilion Sea" expeditions. 

 A simplified version of Fisher's chart is shown in fig. 6. The bathymetry 

 of the continental slope and abyssal regions from California to Panama is 

 shown on figure 3. 



Sediments of the Gulf of California. 



A general description of the sediments of the Gulf of California, based 

 on 310 samples taken by Scripps Institution on the "E. W. Scrip ps Cruise" 

 and by various cruises of the Allan Hancock Foundation, is given in Byrne 

 and Emery (1960). A more detailed map of the sediments has been com- 

 piled by van Andel et al. (in press). Both are in relatively close agreement 

 as far as the gross sedimentary characteristics of the Gulf are concerned, 

 although many more sample analyses are involved in van Andel's version 

 and the one prepared by him for this paper (fig. 7). Curray in van Andel 

 et al. (in press) has drawn up detailed charts of the sediments for the two 

 areas most important to the biological studies (Costa de Hermosillo and 

 Costa de Nayarit). These charts are based on grain-size analyses and other 

 sedimentary properties (fig. 8 reproduces one of these). Using these charts, 

 it is possible to trace the close correlation between certain macrofaunal 

 communities or assemblages and sediment type. 



According to Byrne and Emery (1960, pp. 996), the sediments of the 

 subtidal portions of the Gulf can be described in the following manner. The 

 extreme northern end in the vicinity of the old Colorado River mouth is 

 characterized primarily by silty sand (based on a nomenclature devised 

 by Shepard, 1954). Proceeding south to a point midway between the two 

 large islands in the Gulf, the sediments change to silty clay, the typical 

 sediments for shrimp fishing grounds both in the Gulf of Mexico and Gulf 

 of California. The area which marks the transition between the northern 

 and southern Gulf in the vicinity of the Tiburon Basin is predominately 

 sand. More detailed sampling as a result of the Vermilion Sea ex- 

 peditions (in part, fig. 8) in the area shows a great range of sediment 

 types, ranging from rock and gravel to silty clay, although there is a 

 preponderance of sandy sediments. The rest of the central and southern 

 portion of the Gulf below 200 meters is characterized by both Byrne and 

 Emery and van Andel as silty clay. There are a few patches of clayey 

 silt on some of the topographic highs, and portions of the Guaymas Basin 

 are covered with diatomaceous sediments. 



Detailed sediment maps of the shelf portions of the Gulf south of 

 Tiburon Island and Angel de la Guarda Island are almost impossible to 



