ABSTRACT

The Salton Trough--the northwest landward extension of the Gulf of California structural depression--is filled primarily with fine-grained sandstones and siltstones that constitute the Colorado River delta. These rocks have an aggregate thickness of approximately 20,000 feet and are all of late Cenozoic age. Detritus that makes up the delta was eroded primarily from Mesozoic sedimentary rocks of the upper Colorado River drainage basin. This detritus consists of dominant quartz and calcite, subordinate dolomite, plagioclase, K-feldspar (both monoclinic and triclinic), montmorillonite, illite and kaolinite, and minor magnetite, zircon, leucoxene, clinozoisite, biotite, and chlorite. Wide ranges in both major and minor elements can be correlated with grain size and clay content of the sa ples.

Lead isotope data from the acid-insoluble fraction of Colorado River delta samples from the Salton Trough indicate that the mean age of the detritus is 1700 m.y., and strontium isotope data indicate that the mean age is greater than 500 m.y. In view of the small percentage of exposed Precambrian rock in the upper Colorado River drainage basin, the bulk of the acid-insoluble detritus probably consists of reworked detritus of Precambrian age.