Eocene and Oligocene otay-type waxy bentonites of San Diego County and Baja California: Chemistry, mineralogy, petrology and plate tectonic implications
Rw. Berry, Eocene and Oligocene otay-type waxy bentonites of San Diego County and Baja California: Chemistry, mineralogy, petrology and plate tectonic implications, CLAY CLAY M, 47(1), 1999, pp. 70-83
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.