--> ABSTRACT: Regionally Inhomogeneous Potash Metasomatism, Paleogene Mudrocks, Texas Gulf Coast, by L. S. Land, F. L. Lynch, L. E. Mack, K. L. Milliken; #91020 (1995).

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Regionally Inhomogeneous Potash Metasomatism, Paleogene Mudrocks, Texas Gulf Coast

L. S. Land, F. L. Lynch, L. E. Mack, K. L. Milliken

The K2O/Al2O3 ratio in Paleogene mudrocks from some, but not all, Gulf Coast wells, increases sytematically up to 50% with increasing depth. Changing provenance can not explain the increase because the sediments were rapidly accumulated, and individual wells have nearly invarient Sm/Nd model ages, consistent U-Pb and Rb-Sr sytematics, and uniform elemental ratios such as U/Th and Th/Sc. Primary differences in sediment composition as the result of sorting can be ruled out because unreasonably high initial amounts of detrital K-feldspar (approximately 20% of pure K-feldspar) would need to have been present in the most K2O-rich samples. Al2O3, TiO2, Hf, Zr, Th, and HREE all increase similarly in con entration in one intensely analyzed well, and demonstrate bulk volume loss of approximately 16%. Gain in K2O relative to all these "immobile" elements took place as CaCO3, H2O, and SiO2 were being lost. The source of K2O is not likely to be local, as an approximately equal mass of sandstones is required to supply sufficient K2O for the mudrocks, whereas the ratio of sandstones to mudrocks is only about 1/10.

We conclude that mudrocks can be open systems during diagenesis, and that compositional variations are distributed unevenly because of the intrinsic spatial inhomogeneity of subsurface fluid flow. We suggest that local zones of upwelling fluids supply potassium, at the same time permitting illite/smectite to continue to react, as is evidenced by increased crystallinity with depth. Additionally, two-way transport through mudrocks may provide a mechanism to continuously flush products of organic maturation from these relatively "lean" sediments, making them adequate source-rocks for Cenozoic hydrocarbons.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995