ABSTRACT: Clay Mineralogy of the Monterey Formation, Santa Maria Basin Area, California

T. Netratanawong, J. S. Compton

The clay mineralogy of the Miocene Monterey Formation was determined for onshore and offshore sections in the Santa Maria basin area. The fine clay size fraction (< 0.2 µm) of clayey diatomites (opal A), siliceous mudstones (opal CT), and dolostones from the Point Pedernales area consists of nearly pure smectite (< 10% illite layers). The underlying Tranquillon Volcanics, the occurrence of several bentonite layers in these sections, unaltered volcanic ash layers elsewhere in the basin, the presence of zeolite minerals, and the highly smectitic composition of these clays suggest they formed from the alteration of volcanic ash. Alteration of volcanic ash during early diagenesis may have promoted dolomite precipitation by raising pH and releasing Mg; altered the S isotope composition of the pore waters; and provided an additional source of silica to these predominantly biogenic siliceous rocks. Detrital illite and mica occur in the coarse clay size fraction, but no mixed-layer illite/smectite (I/S) or kaolinite was found.

The < 0.2-µm fraction of quartz-grade siliceous rocks from the Lions Head and offshore sections is far more variable, containing random and ordered I/S, kaolinite, and discrete illite. Corrensite was found near the base of these sections. Illitization of smectite during burial diagenesis increases the percent illite layers from 20 to 80% in the random I/S. Two bentonite layers contain ordered I/S. The abundance of plagioclase feldspar compared to K-feldspar and the absence of kaolinite from rocks not having undergone illitization suggest that limited availability of K resulted in smectite alteration to kaolinite as well as illite.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990