--> ABSTRACT: Early Diagenetic Formation of Illite: Implications for Clay Geothermometry, by Neil S. Fishman, Christine E. Turner-Peterson, and Douglass E. Owen; #91038 (2010)

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Early Diagenetic Formation of Illite: Implications for Clay Geothermometry

Neil S. Fishman, Christine E. Turner-Peterson, Douglass E. Owen

A concentric zonation of authigenic clays occurs in altered tuff beds deposited in a saline-alkaline playa-lake complex that represents the Brushy Basin Member of the Upper Jurassic Morrison Formation in the eastern part of the Colorado Plateau. Along the margins of the playa-lake complex, randomly ordered smectitic mixed-layer clays (80-100% expandable layers) formed upon alteration of silicic volcanic ash. In contrast, stratigraphically equivalent altered tuff beds in the central part of the playa-lake complex contain ordered illitic mixed-layer clays (0-30% expandable layers). This pattern of a basinward increase in illitic layers coincides with the concentric zonation of authigenic minerals in tuff beds from smectite and clinoptilolite (playa margin) to analcime, K-feldspar, and albite (central playa).

Temperatures above 90°C have been considered necessary for transforming randomly ordered smectitic clays to ordered illitic phases. (It remains unclear whether illitic phases can precipitate directly from solution.) In the Brushy Basin Member, however, zonation of clays occurs irrespective of proximity to sources of heat (intrusive and extrusive igneous bodies) and depth of burial. Instead, the basinward increase in salinity and alkalinity of syndepositional pore waters, which is indicated by the zonation of authigenic zeolites and feldspars in Brushy Basin tuff beds, probably also controlled the basinward increase in illitic content of mixed-layer phases. Thus, illitization possibly resulted from early transformation of precursor clays, or alternatively, by early, direct precipi ation. Because pore waters at near-surface temperatures can seemingly control formation of illitic phases, caution should be exercised when using illite as a geothermometer to evaluate the thermal evolution of sedimentary sequences.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.