The diagenetic alteration of volcanic rock fragments in the Stevens Sandstone, San Joaquin Basin, California

Abstract

The Upper Miocene Stevens Sandstone of the San Joaquin Valley, California was deposited in an active forearc basin. Compositionally the Stevens is characterized by a high content of feldspar and volcanic rock fragments. These constituents are altered under the conditions of burial diagenesis to form fine-grained aggregates of clay minerals and oxides. The alteration products can have a large effect on the physical properties of the rock. This alteration process was studied in the Stevens Sandstone and in a series of hydrothermal experiments in an attempt to understand the mechanisms involved. The alteration products in the Stevens were characterized by petrographic, x-ray, and electron microscopic methods. This alteration occurred in isolated microenvironments. The most important parameter controlling the alteration appears to be contact with the water present in the formation. Because of this control some of the framework grains are highly altered while others remain unaltered. Volcanic rock fragments are altered to an interstratified clay mineral containing saponite, vermiculite, and chlorite. The feldspars are altered to a well-crystallized kaolinite and some of the biotite present shows evidence of chloritization. These alteration products are confined to the sandstones; the enclosing shales show no evidence of being affected by the processes which acted on the sandstones.