Investigations of the Dunes Geothermal Anomaly,Imperial Valley,California. Part II. Petrological Studies. Active Formation of Silicified Cap Rocks in Arenaceous Sands in a Low-Temperature,Near-Surface Geothermal Environment,in the Salton Trough of California, U.S.A.,

摘要

Alteration of sedimentary rocks by geothermal brines is common in a zone of high heat flow within the Salton Trough,California. A 612meter deep test-well encountered a maximum of 104C at 285meters depth in the Dunes Anomaly,at the southeast margin of the Trough. The rocks penetrated were deltaic sediments deposited by the Colorado River. It was found that the amount of silica precipitated is greatest below impermeable shale beds,however extensive hydrothermal alteration is absent in the lower 300meters of the hole,where only diagenetic processes seem to have occurred. Based upon studies of the textures it is evident that there have been various stages of hydrothermal and diagenetic alteration in each of these silicified zones. It appears that when moderately hot brines encountered colder rocks,precipitation of quartz and feldspar made the rocks impermeable. Water-dominated geothermal systems which operate in porous sandstones are essentially self-sealing. Convective overturn of the geothermal brine is influenced by the original and subsequent permeability of the system;however,the silica cap rock is a good exploration target for geophysical surveys.