PLIOCENE/PLEISTOCENE BASALTIC PILLOW LAVA AND TUFF ALONG NW SHORE OF LAKE TAHOE, CA: NEARSHORE VENT OR LITTORAL CONE?

A 150 m-thick section of basaltic lapilli tuff and pillow lava is exposed along 750 m of cliffs and shoreline at Skylandia Beach, CA. These rocks developed near the edge of a Pliocene/Pleistocene lake occupying part of the present area of Lake Tahoe and are inferred to represent part of a regionally extensive ~ 2.0 Ma basaltic magmatic event. The tuffs show 1 to 40 cm-thick beds that generally dip 15^o-35^o south toward the lake. The tuff consists of ash-coated vesicular lapilli and sparse bombs up to 75 cm in length. Some bombs exhibit cauliflower and fluid shapes, and have radial joints, indicating they were hot when deposited, and some have formed sag structures in the tuff. These tuffs extend over a kilometer offshore at this location. Outcrops of similar tuff also occur 2 km SW at Commons Beach in Tahoe City, CA.

Locally, tuffs both overlie and intrude Pliocene fine-grained, diatomaceous lake beds along a gently to steeply dipping contact. Along the intrusive contact, the tuff forms a coarse-depleted, fine-grained traction layer. In a complex 24 m-wide zone at the contact, tuff beds are vertical and are intruded by a 14 m-wide body of brecciated, vesicular pillow lava. The pillow lava also intrudes the lake beds, and the lake beds are highly disturbed in a 12 m-wide zone near the contact. Development of a post-depositional terrace above the cliffs removed tuffs east of the contact and eliminated any evidence of a possible tuff cone.

The complex zone of vertically-dipping basaltic tuff and pillow lava may represent a small vent developed near lake level along a north-striking normal fault which down-dropped sediments to the west, forming a depression in which tuffs were deposited and preserved. Tuffs are not exposed east of this proposed fault. Lavas intruded into the vent formed pillows from interaction with shallow ground water or a standing body of water.

Another possible scenario is that basaltic lava flowed into the lake and ensuing steam explosions generated surges that deposited lapilli and bombs in a series of littoral cones and ramparts. In one place, lava flowed into wet tuffs and diatomaceous sediments so that explosions were inhibited and pillow lava formed.

Continuing field and lab work on the age, character and composition of the tuff will shed light on the development and evolution of ancestral Lake Tahoe.