Successive Illitization/Alteration Zones Identify with K-Ar Data

Sylvie L. Furlan

This work focuses on K-Ar dating of clay fractions with mixed-layer illite-smectite. The K-Ar ages are used to trace not only the illitization process as a function of depth but also physico-chemical changes that have affected clay minerals. Samples were selected at several depths between 250 and 4230m from the Handil field located in the Mahakam Delta (Indonesia). The K-Ar analyses were done on several clay fractions < 0.6 micrometers from both shales and associated sandstones. Calculations were made to date detrital and authigenic end-members and to identify specific variations in K and radiogenic ⁴⁰Ar amounts in the fractions with increasing depth.

Comparing the calculated and measured K-Ar ages, three illitization zones have been identify at the stratigraphic intervals of 800-1150m, 2300-2750m and about 3000m. The illitization zones are separated by "alteration" zones. In the deeper part, the relatively high amount of Ar loss is strongly influenced by Ar diffusion. Variations in the apparent K-Ar age are related to the illitization process -crystallization, transformation- and dissolution diffusion affecting the day particles during burial. Succession of illitization and alteration zones depends on geochemistry and hydrodynamics of the formation water and temperature evolution in the history of the basin. Combining the stratigraphic and mineralogical data, alterations zones can be interpreted as association with "migration zone", illitization with "maturation zones", and Ar diffusion zones with "higher paleo-temperatures" relative to the present-day conditions.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California