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<br />. <br /> <br />AA. <br />AA.a. <br /> <br />Geology <br />General <br /> <br />Tile project area is situated over a large portion of tile Colorado River delta. <br />In its broadest sense. tile delta includes tile sediments in tile Yuma area. tile Imperial <br />ami Mexicali Valleys. and tile San Luis Mesa in tile United States and Mexico, laid down <br />by tile present day and ancestral Colorado and Gila Rivers. Delta building began in late <br />Pliocene time (rougllly 5 million years agol and continued into Ilistoric time until tile <br />river was brougllt under control and tllorougllly regulated by dams. The normal flow of <br />tile Colorado River has been utilized since about 1938: as a result, sedimentation Ilas <br />bL'en in manllladl? lakes. desilting basins. ~lI1u. to a minor extent. major channels. <br />Tile delta is draped across tile boundary between two su')units of tile Basin <br />and Range physiographic province of the southwestl'rn United States and northwest <br />Mexico: the Sonoran Desert and tile Salton Trough (Figure I and Map No. I ~9~-303-1 0 II). <br />The SOllOr;ll1 Desert covc>rs mllch of southwestern ArizoniJ. sOlltheastt'rtl California. anti <br />nortllwestern Mexico. Typical of tile subunit 'He ,ubparallel. barren mountain ranges <br />separated by debris-filled basins. Tile ranges are low but rugged and stand in sllarp relief <br />above tile seemingly level alluvial basins. Tile Sonoran Desert encompasses only a small <br />part of tile Colorado River delta: tile greater part is situated in the Salton Trougll. The <br />Salton Trougll is a sediment-filled structure roughly 100 miles wide and 800 miles long. <br />It extends from tile Transverse Ranges of San Bernardino County in southern California <br />soutlleastward throu;;1l Imperial County. California and Yuma County. Arizona, into <br />Me.xico. <br /> <br />. <br /> <br />Till..' Salton Trough is a major rift bl'twecn two crustal plates. one ocennic and <br />tile other continental. (nfl'faction between tilt' two crustal plutes involves spreading as <br />wt'll :b rl'iativl' horizontal motion and gr<Jvity sliding. Strllctur:J1 deformation is. therefore. <br />more or less continllolls. involving const311t creep or stick-slip action along 3 network <br />of northwest-trending Liults known collectively as the San Andreas Fault system. Crustal <br />disturbance' in this region is indicated by tilted and. folded strat<J. sheared bedrock terranes, <br />ruptured "lluvial surfaces. "1ll1 offset c1rainageways. Right ,"teral slip along the San Andreas <br />rift has .1gg.rcgatt'd about 200 miles or more during 30 million years of Cenozoic time <br /> <br />. <br /> <br />15 <br />