Abstract

Cuttings from a well through a thick section of Miocene–Oligocene mudrocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochemically. On the basis of whole-rock chemical analyses, the deepest samples have lost ≈18 wt% (and approximately vol%), mostly as CaCO3, mineral-bound H2O, and SiO2, but including additional Ca, as well as Sr, light rare earth elements (REE) (La, Ce, Nd, Sm), Fe, and Li. K2O and Rb have been added to the deeper rocks. The large chemical changes are accompanied mineralogically by loss of detrital calcite, kaolinite, K-feldspar, Ca-plagioclase, and muscovite, gain of chlorite and albite, and continued reaction of smectitic illite/smectite (I/S) to more illitic (and K-rich) compositions throughout the entire depth interval of the well. The large chemical changes in this thick mud-rich interval almost certainly require advection of water (free convection?) to accomplish the mass transfer. Initial variation in sediment composition is ruled out as a cause of the observed compositional changes with increasing depth because (1) a variety of “immobile” elements (Al2O3, TiO2, Zr, Hf, heavy REE [Er, Yb], Th, and Sc) remain constant relative to each other despite their uneven distribution across various particle size fractions in the sediments; (2) deep Frio shales are unlike Quaternary Gulf of Mexico sediments or average shales; and (3) unreasonable primary mineralogic compositions would be necessary to explain the chemical composition of the deep samples.

These results indicate that burial diagenesis of argillaceous sediment can be a considerably more open chemical process than is conventionally assumed, that it can account for the two major chemical cements (calcite and quartz) in associated sandstones, and that it mirrors secular changes in shales throughout geologic time.

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