The northwest landward extension of the Gulf of California structural depression is filled with fine-grained sandstones and siltstones of the Colorado River delta. All are late Cenozoic and average 20,000 feet thick. Provenance is primarily from Mesozoic sedimentary rocks of the upper Colorado drainage basin. Wide ranges in both major and minor elements can be correlated with grain size and clay content of the samples. Lead isotope data from delta samples indicate that the mean age is 1700 m.y., and strontium isotope data indicate the mean age is greater than 500 m.y. In view of the small percentage of exposed Precambrian rocks in the upper Colorado River drainage basin, the bulk of the acid-soluble detritus (as measured and cited above) probably consists of reworked detritus of Precambrian age.

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