Eocene and Oligocene otay-type waxy bentonites of San Diego County and Baja California: Chemistry, mineralogy, petrology and plate tectonic implications

Otay-type waxy bentonites of San Diego County are illite-smectite (I-S) wit h 85% dioctahedral smectite mixed with dioctahedral illite showing a Reichw eite of 0. Primary or secondary waxy bentonite exposures are found in all E ocene and Oligocene formations of southwest San Diego County and western Ba ja California north of Ensenada. Primary waxy bentonites formed when hot vo lcanic ash fell into quiet marine or brackish coastal water. The transforma tion from glass to bentonite occurred within hours or days but consolidatio n of the bentonite into its waxy consistency took longer. Primary waxy bent onite consists of 95 wt% I-S with the remainder consisting of volcanic glas s shards, sanidine fourling twins, hexagonal biotite crystals and amorphous manganese oxides and hydroxides. Secondary waxy bentonite is primary waxy bentonite that was mixed with nonvolcanic detritus either before consolidat ion or after consolidation and subsequent erosion. The hydrophobic characte r of primary waxy bentonite allows it to reflect, accurately, the chemistry and petrology of the original volcanic material. Chemical analysis of prim ary waxy bentonites shows that the original lava was subduction-related and exhibited petrologic variations nearly identical to those of the modern Ca scades of the northern Pacific coast of the lower United States. Primary an d secondary waxy bentonites as well as smectites derived from the weatherin g of volcanic ash that fell outside the waxy bentonite-producing environmen ts are the previously unrecognized products of extensive Eocene and Oligoce ne subduction-related volcanic activity. Baja California exposures of waxy bentonite demonstrate pre-Pliocene subduction tectonics that gave way to ri fting tectonics.