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ABSTRACT: Formation of Intercalated Illite and Kaolinite in the Meteoric Environment

Sharon A. Stonecipher

Intercalations of illite and kaolinite are common diagenetic products in many sandstones. Most literature references suggest that these intercalations formed by the progressive growth of illite within preexisting vermiform kaolinite. This process of illitization has been related to the progressive release of potassium from feldspars and micas during middle- to late-stage burial diagenesis. The application of similar time and process connotations to all intercalations of illite and kaolinite is not justified, however.

Petrographic studies of sands of different origins have revealed intercalations of kaolinite and illite which appear to have resulted from the progressive growth of kaolinite on or in between the layers of preexisting clay or mica sheets. Expansion of degraded micas in confining pore spaces produced accordion-like structures. Similarly oriented layers in the accordion were infilled with kaolinite producing pseudovermiform booklets. In each case, the kaolinite appears to have formed early in the sand's diagenetic history before much compaction took place. Examples of this fabric will be shown from diverse settings. Provenance, age, and geographic location vary widely; however, in all cases, the sands were deposited in deltaic to shoreline environments which were subjected to extensive eteoric flushing during deposition and early burial. The kaolinite appears to have formed in response to leaching and dissolution of micas and clays in the meteoric environment.

This interpretation raises a caveat for petrologists; the determination of which came first, the kaolinite or the intercalated clay, has serious implications not only for the timing and origin of the kaolinite, but also for the depositional setting of the enclosing sand.

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