New Mexico Geological Society Annual Spring Meeting — Abstracts

Authigenic clay minerals are useful indicators of the water/rock interactions in a clastic unit. Mixed-layer illite/smectite (l/S) clays are especially promising diagenetic indicators, because of their sensitivity to solution chemistry and temperature, and their common occurrence in sandstones and shales. The Westwater Canyon Member is a fluvially deposited, to coarse-grained, Jurassic sandstone that has hosted a variety of pore fluids, resulting in the formation of authigenic clay minerals. Also present, however, are mixed-layer clays that were deposited with this sandstone in detrital form. An analysis of diagenetic reactions based on the assumption that all of the mixed-layer clay is authigenic could be biased by the detrital clay fraction.

We have examined the composition of the fine-grained materials from core samples to determine the mineralogy of detrital clays and to determine trends in the l/S transformation. The detrital clay component in the Westwater Canyon Member occurs as prominent rip-up clasts and fine-grained interbeds in core samples. We have extracted the detrital, fine-grained portion of approximately 30 core samples, taken from 6 cores along a 30 km cross-section of the Chaco slope in the southern San Juan Basin, N.M., ranging in depth from 560 to 3264 feet. The l/S ( < 1µm size fraction) in rip-up clasts and clay rich, fine-grained layers are compared laterally and vertically across the region.

The authigenic component contains chlorite, kaolinite, and mixed-layer l/S. Clays in fine-grained samples are highly illitic. In contrast to previous work, in which sandstone samples exhibit a distinct pattern of transformation from illite to smectite, our preliminary results reveal no uniform diagenetic trend. Results indicate that the detrital component should be taken into consideration when authigenic clay minerals, such as l/S mixed-layer clays, are used as diagenetic indicators.