CLAYS IN THE CAMP RICE FORMATION AT RINCON HILLS, NEW MEXICO

Rincon Hills in south-central New Mexico was formed by Rio Grande rift-related faults. The Pliocene-aged Camp Rice Formation, which consists of alluvial, fluvial, and lacustrine sediments, and paleosols, is well-exposed at Rincon Hills on the east side of the fault zone. These sediments were silicified near the fault zone by hydrothermal activity. Clay samples taken from the Camp Rice Formation in and around the hydrothermally altered zone consisted predominantly of well-ordered dioctahedral smectite. Other clays in these sediments included interstratified illite-smectite and kaolinite. In most samples, the dioctahedral smectite was montmorillonite. Iron-rich smectite (possibly nontronite) was present in thin clay-rich layers immediately above and below an opal tufa-like deposit just east of Rincon Hills. Associated with montmorillonite were nanometer-sized spherical gel-like particles. The well-formed films and laths of dioctahedral smectites in and around the alteration zone are probably authigenic clays formed by hydrothermal solutions.