ELEMENTAL MOBILITY IN SANDSTONES DURING BURIAL - WHOLE-ROCK CHEMICAL AND ISOTOPIC DATA, FRIO FORMATION, SOUTH TEXAS

The deepest(> 3.5 km) and most altered Oligocene Frio sandstones from south Texas typically contain authigenic minerals that exceed 30 volum e percent of the rock. Elemental and isotopic analyses of whole rocks confirm conclusions based on petrography that authigenesis is accompan ied by substantial material transfer involving both import to and expo rt from the sandstone. From petrographic observations, Zr is apparentl y 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 b e strictly immobile, and Zr was once uniformly distributed, then volum e loss of approximately 38% from the silicate fraction of the deepest sandstones is suggested. Over the range of burial depths for samples u sed in this study (0.9-4.3 km), elemental trends controlled by feldspa r reactions are the most readily documented. Dissolution and replaceme nt 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. Sr-8 7/Sr-86 ratios for whole-rock and silicate fractions increase with dep th as loss of nonradiogenic strontium characteristic of the volcanic-r ich detrital fraction is overwhelmed by more radiogenic strontium deri ved from deeper in the basin. Relatively constant Na is compatible wit h feldspar alteration in which dissolution exceeds albitization. Chang es in Nd concentrations and epsilon(Nd) also closely track the progres s of alteration of detrital feldspar. Magnesium gain accompanies preci pitation of authigenic chlorite and very minor ferroan dolomite. Whole -rock calcium values are dominated by calcite that includes both detri tal and authigenic components. Significant dissolution of detrital car bonate grains does not occur, and calcium in authigenic carbonate ceme nt exceeds by about 5 times the calcium released by dissolution of det rital silicates, thus requiring considerable import of calcium and CO2 . Material transfer on the scale observed demonstrates that burial dia genesis is an open-system process for several major elements, and intr oduces a major obstacle to use of elemental data for provenance interp retation. Defining the sources of materials transported into sandstone s, and the fate of materials exported, is key to documenting the ''sca le of system closure'' in sedimentary basins and requires a better und erstanding of diagenetic processes in mudrocks.