--> Abstract: Evidence for 60 M Base Level Change of the Colorado River in the Yuma Area, Since the Early Pliocene, by J. D. Nations and W. R. Betts; #90958 (1995).

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Abstract: Evidence for 60 M Base Level Change of the Colorado River in the Yuma Area, Since the Early Pliocene

J. Dale Nations, W. Richard Betts

Fluvial terraces of probable early Pliocene age border the eastern margin of the lower Colorado River extensional corridor approximately 40 kilometers northeast of Yuma, Arizona. The terraces are 60 m above present river level, and are capped by Colorado River cobble gravel. The gravel interfingers with the Mio-Pliocene Bouse Formation and provides evidence of timing of the progradation of the Colorado River system into the Gulf of California. Clast compositions and textures indicate that the river was draining the Colorado Plateau and that it had a much steeper gradient than at present, which may have tectonic implications.

Abundant silicified tree trunks and branches as much as 20 cm in diameter and 3 m in length are contained within the gravel. Preliminary identification indicates taxonomic composition of palm and other angiosperms. More precise identification of the wood will allow for an interpretation of the Pliocene paleoclimate in the lower Colorado River corridor. Oxygen isotope analysis of the replacement silica is planned to further interpret early Pliocene climatic conditions. Sedimentology, contemporaneous tectonics, paleoclimatic changes and archeological information are integrated to analyze the causes and timing for the 60 meter change in base level of the Colorado River since the early Pliocene.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California