Petrology of the Eocene Marquez Shale Member of the Reklaw Formation, Bastrop County, Texas




Collins, Ann Marie, M.A.

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The Eocene Marquez Shale, a member of the Reklaw Formation, was studied in Bastrop County, Texas, where a complete section crops out. The unit consists of (1) lignitic, pyritic fissile claystones; (2) glauconitic fossiliferous bioturbated mudstones; (3) plant-rich, laminated, slightly rippled interbedded siltstones and mudstones; and (4) small pyrite concretions, large septarian siderite concretions, and cone-in-cone structures. The dominant clay mineral is smectite, with some intermixed kaolinite. Silt-size material includes abundant quartz, and rarer feldspar and muscovite. Glauconite, in the form of pellets, occurs in the lower Marquez, often in great abundance. Marine fossils, especially mollusks and foraminifera, are also common in the lower Marquez. Plant fragments and amorphous organic material are present throughout. Pyrite is associated with marine fossils, plant fragments and organic matter, glauconite, and concretions, and is usually framboidal. Weathering products include gypsum and hematite. The depositional setting of the lower Marquez fluctuated between a stagnant, brackish swamp or lagoon, and an oxygenated, open marine, shallow shelf area. The upper Marquez was probably delta-influenced, being perhaps an interdeltaic or interdistributary embayment.