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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 56 (1986)No. 1. (January), Pages 89-98

Diagenesis of Plio-Pleistocene Sediments of the Colorado River Delta, Southern California

Scott Jennings, Graham R. Thompson


Interbedded mudrocks and fine-grained sandstones from three wells in the Plio-Pleistocene Colorado River delta in the Imperial Valley, southern California, were examined by X-ray powder-diffraction methods, chemical analyses, and scanning electron microscopy (SEM) to determine the extent and course of low-temperature metamorphic reactions. This sequence was compared to Tertiary rocks from the U.S. Gulf Coast in an effort to explore the kinetics of low-temperature reactions in natural systems.

Mineralogical changes observed are 1) the progressive transformation of mixed-layer illite-smectite to pure illite over the temperature range 70°C to 210°C; 2) the apparent appearance of chlorite as a diagenetic mineral at 180°C to 194°C coincident with the onset of R (symbol) 3 ordering in the illite-smectites; 3) the loss of kaolinite above approximately 210°C; and 4) a progressive decrease in the amount of detrital potassium feldspar over the same temperature interval that brackets the illitization reaction.

Comparison of bulk chemical analyses with analyses of the <0.1-µm fraction indicates that the clay mineral reactions are associated with redistribution of components from the coarse fraction to the fine, clay-rich fraction.

Comparison of Tertiary sequences with this Plio-Pleistocene sequence indicates that kinetics significantly influence the appearance of clay mineral assemblages at temperatures below approximately 175°C. At temperatures above 175°C, temperature, rather than time, appears to govern the reactions.

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