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Tertiary volcanic rocks and the potassium content of Gulf Coast shales—The smoking gun

¹ Scealu Modus, 2617 Cutler Ave. NE, Albuquerque, New Mexico 87106
² Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada
³ Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia

The majority of Tertiary volcanic rocks of the western United States and Mexico are alkaline in composition and may contain as much as 50 wt% equivalent K-feldspar. Emplacement of these volcanic strata is coeval with Tertiary shale deposition in the Texas Gulf Coast, and they previously have been identified as likely sources of sediment for Gulf Coast shales. Evaluation of chemical trends in Gulf Coast shales, particularly K₂O, indicates changes in sediment composition in the lower Eocene, Oligocene, and near the Oligocene-Miocene boundary. In particular, there is a 250% increase in K₂O content from 2 wt% to 5 wt% from the late Eocene to the early Oligocene. Gulf Coast shale bulk-rock compositions are consistent with a Tertiary volcanic source. Estimates of erosion and mass balance calculations suggest that in the south Texas Gulf Coast, the Oligocene Frio Formation may contain between 60% and 85% volcanic detritus, and coeval Frio shales to the north contain 25%. Vertical and lateral compositional variations highlight variable abundances of source detritus and the effects of weathering and depositional processes on Gulf Coast shale composition. Trends of increasing K₂O content with depth in Gulf Coast shales previously have been interpreted to result from open-system diagenesis and K-metasomatism at depth. The data presented herein suggest instead that these trends result from variable provenance and the influx of large volumes of Tertiary alkaline volcanic material. Therefore, diagenetic models that invoke a homogeneous initial shale composition and open-system behavior may be invalid.

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