Distribution of 2:1 Dioctahedral Mica Polytypes of Smackover Formation, Manila Embayment, Southwestern Alabama

Bruce R. Wilcoxon, Ray E. Ferrell, Jr., William C. Wade, Roger Sassen

Dioctahedral 2:1 micas commonly occur as 1M and 2M polytypes. X-ray powder diffraction techniques can be applied to determine the percentage of the 2M polytype (2M/1M + 2M). Results from 14 samples of shaly partings in eight cores of the Smackover Formation in the Manila embayment, southwestern Alabama, reveal a systematic increase in the 2M micas over the depth interval 3,249-5,651 m. The 2M polytype values average 12.2 ± 1.78% to a depth of about 3,800 m. Below this depth, the 2M polytype content becomes more variable but increases by approximately 35%/km to an average of 75% below 5,600 m.

The distribution in the Smackover Formation could be accounted for by a gradually increasing detrital muscovite component (presumably 100% 2M) beginning at about 3,800 m. However, no large increase in the muscovite contribution is suspected from the source of Smackover clastic material to the north. In addition, hydraulic sorting of the large muscovite flakes and the smaller illitic material should have decreased the muscovite component in the more distal, down-dip (deeper) Smackover samples. The relatively constant 2M polytype content down to 3,800 m suggests that about 12% of the less than 2 µm material may be detrital muscovite (assuming most detrital illite to be the 1M polytype).

Calculated geothermal gradients indicate a temperature of 80°C for the beginning of the percent 2M polytype increase in the Smackover Formation. The percent polytype/temperature relationship in the Smackover corresponds reasonably well with other polytype data in the literature. Corresponding equivalent vitrinite reflectance values range from 0.59 to 1.01%. Combined use of thermal history indicators provides better insight to the thermal evolution of the basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.