As part of the Lawrence Livermore/Sandia National laboratories shallow drilling (19 wells) into Salton Sea sediments, four 1-m cores were recovered from 76 m subbottom depth. The cored wells are along a transect from the margin toward the interior of the Mullet Island thermal anomaly. The thermal gradients range from 0.191 ¿C/m near the margin to 0.816 ¿C/m near the interior of the thermal anomaly. Bottom-hole temperature in a cored well close to the center of the thermal anomaly is 87 ¿C. Pore water chemical composition and bulk and clay size sediment mineralogical and chemical compositions are influenced by hydrothermal reactions which intensify with increasing thermal gradient. X ray diffraction and petrographic microscopic analyses of bulk and clay size fractions of the sediments indicate that the relative proportions of smectite to kaolinite plus illite and of dolomite to calcite increase from the margin toward the interior of the thermal anomaly region. Clay mineral recrystallization and dolomitization are indicated by increased K/Al and decreased Ca/Mg molar ratios in the clay size sediment fraction. Calcium, silica, potassium, lithium, and strontium are released from sediments into pore fluids. These data suggest that even at temperatures less than 100 ¿C, hydrothermal activity significantly alters the pore water chemistry and moderately alters the sediment mineralogical and chemical composition. ¿ American Geophysical Union 1989