AN INVESTIGATION OF THE DISTAL FACIES OF THE TABLE MOUNTAIN LATITE AT KNIGHTS FERRY, CALIFORNIA

We are examining the distal facies of the Table Mountain Latite, a high potassium volcanic rock that purportedly erupted from the Little Walker Center near Sonora pass, central Sierra Nevada. While there are 23 flows near the source, at least one lava flow flowed 80 miles through the paleo-Stanislaus river channel to Knights Ferry, California. Today, the Table Mountain Latite displays inverted topography defining the sinuous Table Mountain. The latite lava flows, which recent studies from the Sierran crest have dated at 10.02-10.25 Ma (Busby et al. 2008) have a high-K geochemistry that is distinct from the andesitic volcanism erupted in the same location between 16-6 Ma as part of the ancestral Cascades arc. This high potassium volcanism is important because the eruptive causes are currently unknown.

In the proximal facies of the high Sierra, glacial erosion has exposed the interiors of the flows and their contact with underlying granites and andesitic volcanic rocks; in the distal facies of the Sierran foothills only the top flow is exposed. In preparation for a water tunnel for the Oakdale Irrigation District, Condor Earth Technologies drilled five 2.5 inch diameter cores, three of which contain Table Mountain Latite. These cores provide a complete vertical section of the distal facies and the underlying andesitic volcaniclastic rocks. The cores were spaced 1,500 feet apart and drilled at an oblique angle to the inferred direction of flow.

The first author, Gorny, performed an in depth analysis of the Table Mountain Latite portion of the cores. The most proximal (relative to the eruption site at the crest) has three vesiculated flows averaging 12 ft thick, with micro-breccia altering into clay weathering horizons topping each flow. Underlying the latite is andesitic coarse-grained-lithic sandstone. The next core has a 144ft thick flow, underlain by a minor flow which may represent a lobe of the large flow. Again the latite is underlain by andesitic coarse-grained-lithic sandstone. The most distal core consists of alternating vesiculated latite to microbrecciated latite altering into clay. Each sequence is 1-2 ft thick, with fine grained sediment penetrating the top and bottom of the latite sequence. We interpret this to be a result of steam explosions occurring when latite lava came into contact with lake water.