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Deep-Water Interpretation of Upper Wilcox Sandstones from Core Study, Katy Field, Texas
Robert R. Berg (2), Richard L. Findley (2)
Lower Eocene Wilcox sandstones were studied in a continuous core from a depth of 10,357-10,607 feet. The sandstones are 3 to 30 feet in thickness and are very fine-grained (0.11 mm), generally thinly-laminated, and consist of quartz, 52%; feldspar, 16%; rock fragments, 7%; other grains, 7%; and clay matrix, 18%. Interbedded shales are dark gray to black, massive, and only rarely silty and bioturbated.
Sandstones are composed of thin beds that are 1 to 3 feet thick and commonly show a vertical bedding sequence: (1) a thin, basal zone that contains small, siltstone clasts that are 1 to 5 mm in long diameter, (2) a dominant middle zone of inclined laminae that dip at low angles of about 5°, and (3) a thin upper zone of horizontally-laminated siltstone. Quartz size decreases upward from 0.16 mm to 0.05 mm within thin beds. These units appear to represent a turbidite sequence corresponding to a basal graded unit (A), a middle laminated unit (B), and upper laminated unit (D). A thin pelite unit (E) may occur at the top, but the ripple-laminated unit (C) is missing. Contorted bedding is present in several intervals. A minor amount of sandstone also occurs in ripple lenses that are thinly interlaminated with dark gray, pelagic shales. A deep-water origin is postulated for this section because of the turbidite-like bedding sequence and graded texture, scarcity of organic reworking and regional location beyond the limits of mapped Wilcox deltas.
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