ABSTRACT

The deepest (> 3.5 km) and most altered Oligocene Frio sandstones from south Texas typically contain authigenic minerals that exceed 30 volume percent of the rock. Elemental and isotopic analyses of whole rocks confirm conclusions based on petrography that authigenesis is accompanied by substantial material transfer involving both import to and export from the sandstone.

From petrographic observations, Zr is apparently less mobile than other "immobile" elements such as Al, Ti, and REE, but nonuniform initial distribution of Zr due to sorting limits its usefulness for normalizing elemental variations. If Zr is assumed to be strictly immobile, and Zr was once uniformly distributed, then volume loss of approximately 38% from the silicate fraction of the deepest sandstones is suggested.

Over the range of burial depths for samples used in this study (0.9-4.3 km), elemental trends controlled by feldspar reactions are the most readily documented. Dissolution and replacement of detrital K-feldspar is accompanied by loss of 2-3 weight percent of K₂O from the whole rock. Loss of more than half of the initial Rb, Sr, and Pb also correlate with alteration of detrital feldspars. ⁸⁷Sr/⁸⁶Sr ratios for whole-rock and silicate fractions increase with depth as loss of nonradiogenic strontium characteristic of the volcanic-rich detrital fraction is overwhelmed by more radiogenic strontium derived from deeper in the basin. Relatively constant Na is compatible with feldspar alteration in which dissolution exceeds albitization. Changes in Nd concentrat ons and _Nd also closely track the progress of alteration of detrital feldspar.

Magnesium gain accompanies precipitation of authigenic chlorite and very minor ferroan dolomite. Whole-rock calcium values are dominated by calcite that includes both detrital and authigenic components. Significant dissolution of detrital carbonate grains does not occur, and calcium in authigenic carbonate cement exceeds by about 5 times the calcium released by dissolution of detrital silicates, thus requiring considerable import of calcium and CO₂.

Material transfer on the scale observed demonstrates that burial diagenesis is an open-system process for several major elements, and introduces a major obstacle to use of elemental data for provenance interpretation. Defining the sources of materials transported into sandstones, and the fate of materials exported, is key to documenting the "scale of system closure" in sedimentary basins and requires a better understanding of diagenetic processes in mudrocks.