SR ISOTOPE EVIDENCE FOR A LACUSTRINE ORIGIN FOR THE UPPER MIOCENE TO PLIOCENE BOUSE FORMATION, LOWER COLORADO RIVER TROUGH, AND IMPLICATIONS FOR TIMING OF COLORADO PLATEAU UPLIFT

The upper Miocene to Pliocene Bouse Formation in the lower Colorado Ri ver trough, which consists largely of siltstone with basal tufa and ma rl, has been interpreted as estuarine on the basis of paleontology, Th is interpretation requires abrupt marine inundation that has been link ed to early rifting in the Gulf of California and Salton trough. New s trontium isotope measurements reported here from carbonates and invert ebrate shells in the Bouse Formation reveal no evidence of marine wate r, but are consistent with deposition in a lake or chain of lakes fed by the Colorado River, Furthermore, the absence of a southward decreas e in Sr-87/Sr-86 within the Bouse Formation does not support the estua rine model in which low Sr-87/Sr-86 marine Sr would have dominated the mouth of the hypothetical Bouse estuary, Elevation of originally mari ne Sr-87/Sr-86 in the Bouse Formation to its present level, due to pos tdepositional interaction with ground water, is unlikely because Sr fr om secondary calcite above, below and within the Bouse Formation is co nsistently less radiogenic, not more, than Bouse marl and shells. In c ontrast to Bouse Sr, strontium from mollusks in tidal-flat and delta-f ront paleoenvironments in the contemporaneous Imperial Formation in th e Salton trough and from the subsurface south of Yuma was derived from sea water and confirms the dominance of marine strontium near or at t he mouth of the late Miocene to early Pliocene Colorado River, Inferre d post-early Pliocene uplift of the Bouse Formation from below sea lev el to modern elevations of up to 550 m has been used to support a late Cenozoic uplift age for the nearby Colorado Plateau, This constraint on uplift timing is eliminated if the Bouse Formation is lacustrine.