Concretions found in Tertiary Sands Along Ecleto Creek in Southern Texas




Wilson, Dustin

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Cemented nodules, or concretions are fairly common in the geologic record. Nonetheless they are important proxies for the understanding of processes occurring in Earth’s critical zone. Histories can be complex because of overprinting by multiple diagenetic processes. Previously un-described decimeter scale spheroidal – ellipsoidal concretions have been found in Tertiary aged strata (Reklaw Formation) within Ecleto Creek, in Wilson County south of Seguin, Texas. These concretions eroded out of the cut bank side of the creek after heavy rains and high water flow. Thin section petrography, cathodoluminescence, and electron microscopy are used to describe the fabric, lithology as well as the nature of the cement. The concretions are banded with four apparent zones, of variable petrography, and are hollow; there is no evidence of a nucleus other than some very loose fine-grained, sub rounded, frosted quartz. The interior void of the concretions vary in size from one to the next ranging from 5-15 cm. The outer most part has smaller nodules of hematite on the surface of the concretion itself. The next layer consists of mostly angular to sub-angular quartz grains, with trace amounts of other minerals such as glauconite and fluorite; it is also rich in a black non-crystalline material, resembling spherules, which is yet to be confirmed. The third layer is extremely thin and has darker brown bean shaped grains, which may be hematite; as well as sub-rounded quartz grains. The fourth layer contains angular quartz grains more of the black non-crystalline material and yellowish limonitic cement. These concretions are unusual in the sense that they have no obvious nucleus and are hollow. They have petrographically distinct layers. Nodules such as these have application to the understanding of sedimentary diagenesis on Mars, which has an iron-rich waters and documented spherules.