arkose - teaching hand specimen of medium-grained gray-white arkosic sandstone from the Walker Formation of California
Arkoses are immature sandstones, non-marine, and derived from the weathering of feldspar-rich granitic or metamorphic rocks. The fragments or clasts, from the Greek klastos for broken, are angular and contain at least 25% feldspar, a result of short transport and rapid burial before the feldspars weathered to clay. In this case, the source of the sediments was the rapidly rising nearby Sierra Nevada batholith, largely composed of granodiorite.
This arkose is the major component of the Walker Formation, 2000 feet of arkosic sandstone, conglomerate and minor gritty clay. A rhyolitic tuff located 365 meters below the top of the Walker yielded a K-Ar age of 21.4 + 0.6 million years, putting it in the Lower Miocene. North of the Bakersfield arch, where it overlies basement, the Walker is Oligocene in age. These specimens were collected along Caliente Creek in Kern County, California.
Typically, arkoses are reddish and cemented by calcite. This arkose is gray-white and has a silica cement, so it would be useful for your students to see examples of several arkoses to understand that color and a calcite cement are not the defining factors. They should note the angular nature of the clasts, which identify it as immature and having only short transport of the sediment from the source. If advanced in the study of geology, students ought to be able to pick out the feldspar and note that it is relatively abundant. It would be instructive to compare this immature sandstone with a fluvial valley-fill sandstone or a marine sandstone, where quartz is the dominant clast type and most if not all of the feldspar has been removed by weathering, or with a sandstone such as the Navajo Sandstone, entirely wind-blown sand and with the clasts entirely frosted quartz.
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