ABSTRACT

An investigation of degree of kaolinitic weathering alteration of feldspar grains present in samples of Domengine (Eocene; northern California) sandstone resulted in recognition of the order: orthoclase-more-weathered-than-andesine. Geological evidence indicates a common weathering history for these minerals. Interpretation of this order--the reverse of that adhered to in the classic literature of relative weathering of feldspar species--depends on two factors: (1) a knowledge of the structural details of the mineral phases; (2) an understanding of the influence of the weathering enviroment on these phases at the microchemical level. Under active leaching conditions, Na⁺ emigration rates from plagioclase feldspar of intermediate (andesine-labradorite) composition exceed K+ rates from orthoclase. If soil water in areas of Domengine source rock was characterized by a high Na⁺/H⁺ ratio relative to K⁺/H⁺, and leaching (as differentiated from weathering) was retarded by stable high water tables, the orthoclase-more-weathered-than-plagioclase order could be explained. Low relief, high non-seasonal rainfall, and subtropical temperatures would produce the required microchemical conditions. Sodium ionic activity would be higher in soil water than K⁺ because of continuous incorporation into plant tissue of K⁺ released by mineral weathering, thus inhibiting release of sodium from plagioclase. Paleobotanical and paleogeographical evidence indicate a maximum elevation of approximately 700 meter in the Domengine source terrane, during the Eocene Epoch, with weathering taking place under conditions of abundant, essentially non-seasonal rainfall, and a subtropical climate.

A preliminary examination of feldspar from the Modelo (Miocene; southern California) sandstone showed less total weathering, plagioclase-more-weathered-than-orthoclase, and development on orthoclase of potassium hydromica rather than kaolinite. This may be interpreted as indication of greater relief (lower soil water levels), lower temperatures, and seasonal rainfall which would expose potassium to leaching during release from perennial plants. Paleobotanical evidence indicates increased floral provinciality during the Miocene due to greater climatic diversity, with lower mean temperatures, and restricted, seasonal rainfall. The greater climatic diversity was a function of both increased relief and lower mean temperature.

This study supports the contention that under favorable circumstances, gross climatological characteristics of ancient terranes may be induced from mineralogical evidence. Classical methods of establishing the stability of minerals under soil weathering conditions are inadequate because: (1) the effects of structural details of the host minerals on weathering reaction rates have not been given sufficient consideration: (2) direct host-guest mineral relationships have not been established, with the result that inhibitory or accelerative effects of metastable weathering products on host degredation are unclear; (3) micrometeorological influence on the chemical environment at grain boundaries has not been carefully investigated. It is urged that recently-developed analytical tools such a the electron microprobe and scanning electron microscope he more effectively employed in solution of these problems.