BURIAL DIAGENESIS OF ARGILLACEOUS SEDIMENT, SOUTH TEXAS GULF-OF-MEXICO SEDIMENTARY BASIN - A REEXAMINATION

Cuttings from a well through a thick section of Miocene-Oligocene mudr ocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochem ically. On the basis of whole-rock chemical analyses, the deepest samp les have lost approximate to 18 wt% (and approximately vol%), mostly a s 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 L i. K2O and Rb have been added to the deeper rocks. The large chemical changes are accompanied mineralogically by loss of detrital calcite, k aolinite, 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-ric h interval almost certainly require advection of water (free convectio n?) to accomplish the mass transfer. Initial variation in sediment com position 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 variou s particle size fractions in the sediments; (2) deep Frio shales are u nlike Quaternary Gulf of Mexico sediments or average shales; and (3) u nreasonable primary mineralogic compositions would be necessary to exp lain the chemical composition of the deep samples. These results indic ate that burial diagenesis of argillaceous sediment can be a considera bly more open chemical process than is conventionally assumed, that it can account for the two major chemical cements (calcite and quartz) i n associated sandstones, and that it mirrors secular changes in shales throughout geologic time.