THE PORTUGUESE TUFF AND ITS RELATIONSHIP TO GROUND MOVEMENT IN THE PALOS VERDES PENINSULA, CALIFORNIA

The Portuguese Tuff, of Miocene age, is a part sedimentary, part ash-fall unit found at shallow depths in the south-central part of the Palos Verdes Peninsula and the near-offshore area. The unit is poorly exposed, but has been penetrated by exploratory borings. Thickness of the Tuff is 40-70 feet and the unit is massive. The parent material was fine, volcanic ash. This material was devitrified to clay minerals. The Portuguese Tuff is conformable within a thick section of Miocene sedimentary rocks. These units are thin, rhythmically bedded layers of shale and claystone. Both lithologies are clay-rich, although the shales are harder than the claystones. The seaward-sloping, south central Palos Vedes Hills are marked by numerous landslides. The presence of a thick and extensive clay unit beneath the slide-prone region has led some geologists to conclude that most failure surfaces occur within the Tuff. Drilling logs indicate this is not the case. Above sea level, the Portuguese Tuff is an aquiclude. Bucket-auger borings logged through the Tuff reveal a firm, dry material similar to candle wax. Locally, perched water has been found above the unit, and fresh water, under hydrostatic pressure, has been found directly beneath it. The boring logs also show that the vast majority of the landslide failure surfaces are conformable features within the shale/claystone section stratigraphically above the Protuguese Tuff. Failure surfaces within the Tuff are rare and occur only near sea level where the clay minerals have undergone attack by sea water.