An Examination of the Geochemistry of the Thermopolis Shale, Central WY
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The Thermopolis Formation was deposited in an inland sea on the east side of the Rocky Mountains approximately 105 million years ago. The formation is divided into three members; a lower black shale, a middle sandstone, and an upper black shale. Because the lower and upper shale members appear nearly identical lithologically, this project investigated geochemical attributes in order to understand similarities and differences between the shales that could indicate differences in paleoenvironment. Sixteen shale samples were collected from localities in Thermopolis, Wyoming. They were analyzed for TOM using loss on ignition, and for trace metal concentrations using x-ray fluorescence. Calculated sedimentation rates for the lower and upper shales indicated that the lower member has a lower sedimentation rate than the upper. Therefore, the lower member should have higher TOM values than the upper member. Loss on ignition did not show a significant difference in TOM between members. Literature suggests the lower member was deposited during rising sea level indicating a negative Ce/Ce* and the upper member was deposited during falling sea level indicating a positive Ce/Ce*. Both the lower and upper members measured a negative Ce/Ce* indicating that both members were deposited during rising sea level in an anoxic environment. TM concentrations tend to correlate with TOM levels, therefore TM should be higher in the lower member. V was significantly higher in the lower than the upper but U and Ni values were not significantly higher. Al, Ti, and Si are indicators of source area change. Al, and Ti values were significantly not equal between members but Si values were not. The two shale members showed some geochemical difference with each other, which indicated that they were deposited in different environments.