Abstract: Active geothermal systems in fluvial-deltaic sediment of the Salton Trough typically develop a thick, carbonate-cemented sandstone caprock which shows a regular progression of carbonate minerals, mineral reactions, rock-fluid mass transfer, and physical properties on increasing temperature. The Sinclair 3 well contains calcian ankerite at < 175 °C, ankerite from 175 to 195 °C, calcite+minor dolomite from 195 to about 250°C, and calcite at higher temperatures. The carbonate content of sandstone decreases on increasing temperature from > 45% at < 140 °C to about 10% at > 250 °C, The most abrupt decrease occurs in the 140 to 170°C range where significant compaction of sandstone occurs as carbonate is reduced to < 25%, and kaolinite reacts to form chlorite. The overall result is the loss of significant Ca, Fe, Mg, and CO₂ from sandstone to the fluid phase on increasing temperature. Reaction ofcalcian ankerite to ankerite near 175 °C, and of ankerite to calcite and minor dolomite in the 195 to 245°C range, takes place on a constant carbonate-volume basis by direct replacement of one carbonate by another. The latter reaction produces significant chlorite, with Al derived from solution of detrital feldspar and from the smectite-to-illite transformation. The equilibrium coexistence of calcite with dolomite and ankerite near 200 °C has allowed construction of an isothermal section in the Ca-Mg-(Fe+Mn) carbonate phase diagram and provided a low-temperature constraint on the calcite limb of the calcite-dolomite solvus.