Depositional and Diagenetic Effects on Sulfur Isotope Compositions of Cambro-Ordovician Knox Group Evaporates, East Tennessee

Francis C. Furman, Jay M. Gregg, Kevin L. Shelton

Sulfur isotope compositions of Knox Group evaporates in the Joy #16 Berry core, taken from the Powell River thrustsheet, east Tennessee, display a systematic trend across the Cambro-Ordovician boundary. Their ^dgr³⁴S values from 22 to 37^pmil, generally increasing in ^dgr³⁴S value with depth.

Anomalously low ^dgr³⁴S values of gypsum (21 to 23^pmil) at the top of the Knox Group are likely due to incorporation of Knox hydrocarbon sulfur by fresh water washing, oxidization, and gypsification. Minor amounts of hydrocarbons have been produced from the Knox in the nearby Sneedville and Rose Hill oil fields. Associated sulfides have low ^dgr³⁴S (-6 to +7^pmil) and carbonates have low ^dgr¹³C (-4 to 0^pmil). Anomalously high ^dgr³⁴S values of anhydrite (37^pmil) at the base of the Knox are likely the result of incomplete open-system thermochemical sulfate reduction. Powell River Mississippi Valley-type (MVT) district evaporates have similar high values (35 to 41^pmil), and ^dgr³⁴S values for late-stage sphalerite and anhydrite in the Copper Ridge MVT district are also high (35^pmil).

Exclusion of the anomalous sulfate values reveals a trend of increasing ^dgr³⁴S over 1,000 meters depth, from 26 to 33^pmil. This trend may be further divided into a Lower Ordovician upper Knox (26 to 31^pmil) group and an Upper Cambrian lower Knox (28 to 33^pmil) group. These two ranges in ^dgr³⁴S values correspond to similar ranges for the two major east Tennessee MVT sulfide groups hosted in the upper Knox.

The Joy #16 core is significant in showing that Knox Group evaporates are abundant, and that there is a statistically significant trend of increasing ^dgr³⁴S values with increasing depth, across the Cambro-Ordovician boundary. Dissolution of these evaporates controls the interstratal evaporite karstification and provided a source for MVT ore fluids.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995