ABSTRACT: Open System Diagenesis and the Loss of Provenance Signatures: Isotopic and Elemental Data from Sandstones, Frio Formation, South Texas

L. S. Land, L. E. Mack, G. L. MacPherson, K. L. Milliken

Oligocene Frio sandstones in which detrital feldspars are completely altered typically contain authigenic minerals that exceed 30% of the rock volume. Elemental and isotopic analyses of whole rocks show that authigenesis is accompanied by both gain and loss of material.

Subsurface sandstones for elemental and isotopic analysis were sampled from approximately 3000-14,000 ft (0.9-4.3 km). Over this depth range, dissolution and minor albitization of detrital K-feldspar is accompanied by loss of 2 to 3 wt. % of K₂O from the whole rock. Loss of about 50% of the initial strontium also correlates with alteration of detrital feldspars. ⁸⁷Sr/⁸⁶Sr for whole-rock and silicate fractions change with depth as strontium characteristic of the detrital fraction is replaced by more radiogenic strontium derived from deeper in the basin. Aluminum, titanium, phosphorous, manganese, and barium also exhibit trends compatible with net export, but uncertainties about initial variation remain. Trends for silicon, sodium, iron, boron, and lithi m are equivocal.

Magnesium gain accompanies precipitation of authigenic chlorite. Whole-rock calcium values are dominated by calcite that includes both detrital and authigenic components. Significant dissolution of the detrital carbonate component does not occur, and calcium in authigenic carbonate exceeds by about five times the calcium released during destruction of detrital silicates, thus requiring considerable calcium and CO₂ import.

Material transfer on the scale observed suggests that simple burial diagenesis is not a closed-system process for several major elements and introduces a major obstacle to the use of elemental data for provenance interpretation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990