BASALTIC PILLOW LAVAS AND PILLOW BRECCIAS, AND EARLY LAKE LEVEL FLUCTUATIONS IN NW TAHOE BASIN, CALIFORNIA

Basaltic pillow lavas and pillow breccias (PLPB), 50 m to over 120 m thick, crop out over a distance of more than 4 km on the slopes and cliffs north of Tahoe City. Pillowed flows occur at elevations ranging from current lake level at 1900 m (6229 ft) to 2070 m (6800 ft). The pillows exhibit both rounded and elongate pillow forms, glassy palagonitized margins, and radial cooling joints. Pillows range in size from 0.5 m to >2 m. Pillow breccia is distinguished by a palagonitized hyaloclastite matrix enclosing isolated and/or broken pillows. Outcrops of PLPB are directly overlain by cliff-forming columnar jointed basalt with columns up to 2 m diameter. Glassy margins occur rarely on columns. PLPB indicate passive eruption or flow of lava into an ancestral Lake Tahoe. Columnar jointed basalt suggests the initiation of subaerial eruptions. The rare glassy margins along columnar joints may have resulted from upward streaming of volatiles from the cooling subaqueous pillows below.

Near the easternmost basaltic outcrop, PLPB overlie Upper Pliocene diatomaceous lake sediments. Near the westernmost exposure, at the same basaltic outcrop dated by Dalrymple (1964) at 2.0 Ma, basaltic lapilli tuff overlies the diatomaceous lake sediments. This indicates that the initiation of basaltic volcanism in the Tahoe Basin was explosive in some areas (tuff) and passive in others (PLPB). Since tuff and pillows are at the same elevation (within 30 m), lava effusion rates may have controlled eruption characteristics, with high effusion rates forming more passive eruptions. Lava effusion rates may have also controlled the formation of pillow lava (rapid eruption) vs hyaloclastite–rich pillow breccia (slower eruption), as postulated by Smith and Batiza (1989).

Upper Pliocene diatomaceous sediments exposed near the modern shoreline in the NW Tahoe basin contain the taxa Pliocaenicus which are indicative of eutrophic conditions and suggest that the lake at that time was shallow and subject to a high rate of influx of nutrients (Starratt, personal comm., 2005). By the time of eruption of the PLPB described here, the lake had reached at least 120 m depth (maximum thickness of pillows) in this part of the basin. This increase in depth was probably caused by the damming of the lake outlet by basalt flows (Birkeland, 1962,1963).