A HYDROGEN ISOTOPE AND MINERALOGICAL STUDY OF BENTONITE CLAY FROM THE PORTUGUESE BEND LANDSLIDE, RANCHO PALOS VERDES, CALIFORNIA

Sliding surfaces for the Portuguese Bend Landslide are linked to a bentonite clay layer in the Portuguese Tuff, a distinct unit in the Altamira Shale member of the Monterey Formation. The role of expanding clays on these slip surfaces is addressed through a mineralogical and hydrogen isotope study of the altered Portuguese Tuff within the Portuguese Bend landslide complex. Bentonites are of variable mineralogical composition, resulting in differences in the ability of clay mineral surfaces to absorb water. These differences cause clays to expand, weaken, and become impermeable to H₂O, causing an increase in the velocity of the landslide along its base as the bentonite layer weakens and the overlying material becomes heavier due to the accumulation of groundwater. PIMA infrared spectroscopy identified montmorillonite in most samples, but a few contained a smectite/chlorite mixture. Amorphous silica chalcedony was found to occur in all bentonites, suggesting mixing between bentonite and Monterey shale during movement. Powder XRD reveals zeolite minerals heulandite, clinoptolite, and chabazite and expandable clays montmorillonite, nontronite, and saponite, along with minor occurrences of kaolinite, quartz, dolomite, and opal. With the exception of two low-dD kaolinite samples (~–85‰), whole rock δD values range between –36 and –63‰. Water contents for zeolite and expandable clay-rich samples range from 7.9 to 11.8%. Dolomite and quartz-dominated samples that contain zeolite have wt.% H₂O between 2.9 and 3.3%. δD values of zeolite and expandable clay-bearing samples decrease with decreasing H₂O content, indicating variable degrees of dewatering. Water δD values (–20 to +5‰) in equilibrium with bentonite at 25°C indicate that water vapor having a seawater source recharged the zeolite/clay during expansion. Waters in equilibrium with the kaolinite-bearing bentonites have δD values similar to local meteoric water (-–50 to –30‰). Our results from the direct analysis of the Portuguese Bend bentonite in the slip surface show that meteoric water vapor directly derived from seawater was absorbed by zeolite and expandable montmorillonite group clay minerals, resulting in a weakened slip surface and impermeable barrier that hinders draining of the landslide.