A PLIOCENE-PLEISTOCENE BASALTIC PHREATOMAGMATIC VENT ON THE WEST SHORE OF LAKE TAHOE, CALIFORNIA: IMPLICATIONS FOR EARLY TAHOE BASIN

Plio-Pleistocene (~2 Ma) basaltic rocks in the NW part of the Tahoe Basin locally interacted with lake waters and provide insights into the processes that created Lake Tahoe. Eagle Rock (ER), a basaltic promontory that rises over 70 m above the canyon bottom, is located on the west shore of Lake Tahoe at the mouth of Blackwood Canyon. ER is a basaltic phreatomagmatic vent that erupted through stream/beach gravels that were either water saturated or in shallow lake water as discussed below.

ER is composed of basaltic tuff breccia with abundant juvenile clasts up to 50 cm. Accidental clasts of rounded andesitic pebbles and cobbles up to 30 cm are common. On the W, N, and E sides of ER, distinct inward dipping stratification of the tuff breccia indicates intra-vent deposition. In the central vent (south side of ER), a mega-breccia with juvenile clasts up to 3 m fills a diatreme-like structure. One juvenile clast encloses a 1 m andesitic cobble. The vent area exposes multiple vertical basaltic dikes up to 2 m wide and irregularly shaped intrusive bodies. One vertical dike shows textbook flaring and progressive brecciation toward its top. The basalts of ER are most likely correlative with the 2.0 Ma and younger basalts in the Tahoe City area.

ER was overridden by Tahoe and Tioga age glaciers (Schweickert and Lahren, 2002) which case-hardened the top surface of the tuff, deposited >50 granitoid glacial erratics on top of ER, and gave ER its roche moutonnee shape.

The relatively smooth top surface of ER has led to speculation that ER is a tuya – a sub-glacial vent. However, ER lacks the typical tuya stratigraphy (pillow lavas overlain by outward dipping beds of hyaloclastites overlain by subaerial lava flows) and its smooth top is more readily explained by glacial scouring.

ER and other basaltic hydromagmatic outcrops in the NW Tahoe basin (e.g. Skylandia beach (Kortemeier et al., 2005)), indicate eruptions occurred in a basin with water-saturated soils and/or shallow lake water. This is consistent with other evidence that faulting and formation of the Tahoe basin commenced before ~2 Ma and that an ancestral Lake Tahoe existed before basaltic flows dammed the Truckee River near present-day Tahoe City. Ongoing field and laboratory investigations of the basalts should elucidate the intertwined history of magmatism and lake formation.