PRELIMINARY INVESTIGATION OF THE OCCURRENCE OF MARBLE GRUS AND CORESTONES IN THE SAN BERNADINO MOUNTAINS, CALIFORNIA

Saprolite, corestone weathering and grus formation by granular disintegration are all common features in a range of environmental settings. While these features and the processes that contribute to their formation have been described extensively for granites and similar igneous rocks, few studies have described them for marbles. Essentially pure carbonate marbles exposed along the northern flank of the San Bernadino Mountains, California, commonly weather by granular disintegration. Most outcrops of marbles in the area that bear crystals greater than approximately 0.5 mm in maximum dimension disintegrate readily into well-sorted, mono-crystalline granules and can be shoveled easily up to depths of a half meter. Mining exposures reveal that marbles friable at the surface retain a solid crystal fabric below about 2 meters depth. Finer-grained marbles (<0.5 mm) appear to weather primarily by dissolution rather than granular disintegration.

Marble corestones are also common and range in diameter from 0.2 to 1 m as a function of joint spacing. Joint surfaces between corestones commonly exhibit rinds of reprecipitated calcite that, in some cases, clearly penetrates between grain boundaries. Root traces preserved along these rinds indicate that they formed prior to exhumation of the corestone. This secondary calcite surface acts as a local capstone that prevents granular disintegration of the underlying friable marble. In the absence of these rinds, however, marble outcrops erode rapidly by physical weathering, as evidenced by the lack of dissolution features. Such dissolution features are common on late Holocene-aged alluvial fan surfaces (Eppes, 2002). Preliminary soil pits show accumulation of decimeters to meters of unweathered, well-sorted grusy calcite crystals on hillslopes. Soil development is minimal with only incipient pedogenic carbonate precipitation on larger clasts and very weak structure development. Many explanations (e.g. alteration of plagioclase, biotite expansion) proposed to explain this style of physical weathering in granitic igneous rocks fail to explain similar weathering in carbonate. Further study of rock micromorphology, sediment production and marble grus soils of the region should provide insights into the characteristics and rates of processes acting on this landscape.