GEOLOGY ALONG THE AMADOR CENTRAL RAILROAD: SPEEDER CARS TO ACCRETION

For the last three years geology field trips have been conducted via speeder cars on the historic Amador Central Railroad (ca1905), Amador County, CA. These field trips are an outgrowth of the California Geological Survey’s (CGS) efforts to conduct geologic mapping in the central Sierra Nevada foothills. The preserved railroad tracks between Ione and Martell, the current end of track, are approximately 10 mi (16 km) in length, and from west to east, pass through the Salt Spring slate, Copper Hill volcanics, serpentinite, mélange, and Logtown Ridge Formation which are predominantly Jurassic in age and accreted to North American during that time.

The many superb outcrops along the railroad grade offer an unprecedented view of most rock units of the Sierra foothills metamorphic belt with many lithologies, structure and contact relationships not seen at any other locations. The exposures along the railroad tracks have assisted in recent geologic mapping by the CGS as part of the 1:24,000 scale mapping efforts in the San Andreas 30’ x 60’ quadrangle. A major improvement to geologic mapping in this portion of the Sierra Nevada foothills is the division of the Copper Hill volcanics into lithologies that correspond to the four stages of arc building and arc destruction proposed by Springer and Day (2005). The mapping performed by the CGS supports previous evidence that the Copper Hill volcanics is overturned in the field area.

Also exposed along the tracks are the mostly undeformed Cenozoic nonmarine deposits in the form of Eocene-Oligocene Ione Formation and Miocene Mehrten Formation. One key railroad cut exposes portion of a thick tropical soil profile that developed on the mélange unit during Paleogene time. This soil profile illustrates the effects of the intense chemical weathering that occurred during the lead up to the Early Eocene Climatic Optimum.

This field trip on the preserved historic railroad route offers an unprecedented opportunity to view the diverse geology of the Sierra Nevada foothills that would otherwise be inaccessible.