DEVONIAN FOSSILS IN THE COLUMBIA MARBLE OF THE CALAVERAS COMPLEX, COLUMBIA, TUOLUMNE COUNTY, CALIFORNIA

The Calaveras Complex of the western Sierra Nevada is an areally extensive, metamorphosed, chaotic oceanic complex (likely a subduction complex) lying structurally below the lower Paleozoic Shoo Fly Complex and structurally above the Jurassic Don Pedro terrane. Based on sparse fossils, the Calaveras has traditionally been considered to contain late Paleozoic to Triassic components. A very large mass of marble in the Columbia area (up to 6 km by 50 km), the largest such marble body in the Sierra Nevada, and either a series of olistoliths or thrust slices, has not previously yielded identifiable fossils. Within a large quarry operated by Blue Mountain Minerals (including the former Columbia Marble Quarry), C. Haughy first recognized possible fossils in graphitic marble in 1989. Samples were provisionally identified as amphipora, a Late Devonian stromatoporoid, by C. Stevens in 1989 and recently this identification has been confirmed by R. Shapiro. Haughy, retired geologist previously with Blue Mountain Minerals, has recognized these fossils in 134 drill holes within the quarry, covering an area of 1,200 by 1,800 feet (370 by 550 m) and a vertical interval of 300 feet (90 m).

Significance: the Columbia Marble has previously been interpreted as the shallow-water cover of an ancient seamount, incorporated into chert-argillite-rich melanges of the Calaveras. This fossil discovery implies that such a seamount (and underlying oceanic crust) would likely have been pre-Late Devonian in age, significantly older than previously thought, and that the melanges themselves contain mid-Paleozoic and possibly older ocean-floor rocks. This discovery also raises the possibility that fossils may yet be preserved in other presumed barren, highly deformed, marble units within the North American Cordillera.

We gratefully acknowledge the contributions of Cal Stevens and Russell Shapiro, and Blue Mountain Minerals for assisting in this discovery.