An in-depth look at distal Sierra Nevada palaeochannel fill: drill cores through the Table Mountain Latite near Knights Ferry

Abstract

The ∼10.4 Ma Table Mountain Latite (TML) consists of high-K trachyandesite lavas that likely erupted from the Little Walker Center near Sonora Pass and flowed 80 miles (130 km) through the palaeo-Stanislaus river channel to Knights Ferry in the Sierra Nevada foothills. Complete sections through the proximal facies of the flow stratigraphy are common in the high-Sierra and along range-front faults, but in much of its distal facies in the Sierra Nevada foothills, the internal features of the unit are poorly exposed. Condor Earth Technologies offered us the opportunity to study three complete vertical sections through the distal facies of the TML through access to three drill cores. These cores, spaced 1500′ (457 m) apart and oriented oblique to inferred flow direction by 30°, are referred to here as cores A, B, and C, in a down-palaeoflow direction.

Previous outcrop studies of the distal TML over the past century have concluded that the distal TML consists of a single flow. This is true of core B, which is dominated by a single 144′ (44 m) thick flow, underlain by a thin (∼5′, 1.5 m) flow with a vesiculated top, inferred to represent a toe of the thick flow, inasmuch as there is no baked zone or weathered contact between them. This contrasts sharply with core C, where the 50′ (15.2 m) thick TML consists of four flows 5–12′ (1.6–3.6 m) thick, each also defined by vesiculated tops, but differing in having weathered tops 1–2′ (0.3–0.6 m) thick, inferred to record an eruptive hiatus between each flow. The third flow in core C appears to be geochemically distinct from both overlying and underlying units, although all of the core samples are petrographically similar. The TML in cores C and B overlies coarse-grained andesitic volcaniclastic debris and fluvial deposits. By contrast, in core A, the TML overlies very fine-grained siltstones interpreted to record deposition in still water, probably a small lake produced by damming of the river by the thick flow in core B. The TML in core A is 20′ (6 m) thick, and is composed of eleven 1–3′ (0.3–1.0 m) thick couplets, each consisting of a clay-altered hyaloclastite breccia passing upwards into a relatively fresh, nonbrecciated, vesiculated top. We interpret these couplets to represent thin toes of a flow that generated steam explosions when they came into contact with a standing body of water.

Palaeomagnetic remanence data (inclination only) on lava flows from the cores are consistent with the Classic Table Mountain direction and inconsistent with directional results from any other Stanislaus Group lava flows published to date. Thus, by palaeomagnetic correlation, the lavas studied here were emplaced between 10.36 ± 0.06 and 10.41 ± 0.08 Ma. In addition, remanence results suggest that the three sampled flows from core C were erupted over a time period shorter than the secular variation rate (i.e. less than a few centuries) and the Knights Ferry portion of the TML was emplaced within several centuries of the average Classic TML age range spanning the Sierra Nevada. This indicates that the weathered flow tops at Knights Ferry formed very quickly.

Keywords:

• Sierra Nevada
• Ancestral Cascades arc; high-K magmatism

Acknowledgements

A very special thanks to Dr Donald Bishop and Dr John Kramer and the rest of the crew at Condor, for supervising the senior author's senior thesis research during summer 2007, and for providing continued access to their cores and data during the 2007–2008 academic year. We also thank Dr Bishop and Dr Kramer for their reviews of this manuscript. We thank Gary Jernigan at the Oakdale Irrigation District for allowing access to the cores and property, and to Albert Conlin for allowing access to his land. We owe many thanks to Professor Frank Spera and to graduate students Duane DeVecchio, Sarah Fowler, and Emily Peterman (all at UC Santa Barbara) for the assistance and advice they gave the senior author. We thank Alice Koerner for assistance on drafting figures. Funding for this project was provided by National Science Foundation grants EAR-0711276 (to Busby and Putirka) and EAR-0711181 (to Busby), and by the Undergraduate Research and Creative Activities grant (to Gorny).