129-27 Chemostratigraphy of the Uppermost East Mountain Shale, North-Central Texas, and Interpretation of Depositional Topography (Desmoinesian, Pennsylvanian)

Abstract:

North-Central Texas was characterized by a westward-prograding delta complex during the Pennsylvanian Period. The East Mountain Shale is a primarily marine shale deposited along the offshore margins of the delta. Geochemical measurements and observations of lithology were used to correlate two outcrops of the East Mountain Shale. The EM180-2 outcrop is approximately six meters thicker than the EM180-1 outcrop despite only being separated by a two-lane highway. To determine if the shortening at EM180-1 was the result of a slower depositional rate or of erosional loss, geochemical profiles were generated throughout the sections. Data was collected at 10-centimeter intervals using a Gamma Surveyor II handheld gamma ray spectrometer and a KT-20 handheld magnetic susceptibility meter. A third section (EM180-3) situated between the primary two sections was later added to the study to develop stronger correlation using both a lower sandstone marker bed and an upper calcareous shale marker bed known as the Bath Bend Bed. At EM180-1 potassium (K), uranium (U), and thorium (Th) concentrations generally increase upward. The lowest readings were from the sandstone marker bed, and the highest readings were from the shale just above the Bath Bend Bed. The Bath Bend Bed stands out due to a sharp decrease in K, U, and Th. These trends are also observed in the other two sections. EM180-1 shows relatively stable magnetic susceptibility with the only notable increase occurring into and above the Bath Bend Bed. The Bath Bend Bed showed similarly high readings at the other two outcrops. The data suggest that all three outcrops experienced similar depositional rates. The sandstone is equally thick between EM180-1 and EM180-2, and the Bath Bend Bed is equally thick between EM180-1 and EM180-3. Geochemical events were also equivalent in size and spacing between outcrops. The thick upper shale at EM180-2 is not preserved in the shorter outcrops, indicating missing material. This supports the interpretation of an erosional event prior to the deposition of the overlying Lake Pinto Sandstone.

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