MARBLE CORESTONE AND GRUS WEATHERING IN HUMID AND ARID CLIMATES

Corestone weathering and grus formation by granular disintegration are common in granites for a range of environmental settings. Eppes and Griffing (2010), however, described similar features within essentially pure carbonate marbles exposed in a desert scrubland along the semiarid northern flank of the San Bernardino Mountains of California (CA). Marble corestone and grus formation were attributed to: 1) subsurface formation of protective weathering rinds, and 2) diurnal thermal cycling of calcite grains, which facilitates granular disintegration, rapid erosion and the formation of calcite grus. Here we build on this work by examining rounded coarse-grained marble exposures that superficially resemble the California marble corestones, but are located in forested, humid hot and humid cold settings in the eastern US.

Field examination and preliminary petrography of marble found in both quarry and naturally-exposed outcrops within the Murphy Marble Belt of northern Georgia (GA) and in the Adirondack lowlands of New York (NY) reveal both similarities and differences between each other and their more arid counterparts. All three areas contain natural outcrops with rounded, corestone morphology. The origins of the corestone rounding, however, contrast between humid sites (edges rounded by subsurface dissolution along joint boundaries), and arid sites (edges rounded by subaerial granular disintegration). In the GA quarry walls, these subsurface features are also accompanied by deep clay-filled solution pipes that bear small granularly disintegrating rounded relict marble cobbles. We observe the accumulation of several centimeters of marble grus at base of natural outcrops in the CA and NY sites, but not the natural GA site. In the GA site, microfractures are limited to the uppermost few millimeters of natural surfaces and are commonly filled by calcite with/without goethite. At all sites, lichens are found on natural surfaces, but in greater abundance in GA and NY than CA. In CA, we note growth of filamentous endolithic microorganisms within and at the base of surface rinds, that are absent in humid sites. Ultimately, we believe these observations will lead to important insights into the relative role of unroofing/exposure rates, thermal cycling, moisture, freezing temperatures in the weathering of marble.