Granular disintegration of marble in nature: A thermal-mechanical origin for a grus and corestone landscape

Abstract

Here we document for the first time in detail a natural landscape dominated by granular disintegration of marble and the associated production of marble grus sediment and corestones. We provide several lines of evidence for a thermal-mechanical origin for the observed granular disintegration and resulting corestones. In the San Bernardino Mountains of southern California, calcite marble outcrops whose average grain diameter is greater than approximately 0.5mm are weathering into grus sediment and exhibit a corestone morphology. Minimal soil development in thick grus colluvium combined with results from a simple erosion experiment indicates that grus production and erosion is rapid. Our data indicate that unlike granite, marble granular disintegration and corestone development is a uniquely sub-aerial process. Environmental Scanning Electron Microscope (ESEM) and micro-probe analyses of rock samples collected in recent quarry exposures indicate that detachment of grain boundaries occurs primarily within the upper 2–20cm of the outcrop surface. There is minimal evidence of secondary calcite dissolution or precipitation at grain boundaries below the upper-most few centimeters of the rock surface, despite widespread intergranular fracturing. At depths greater than 20cm below the natural ground surface, granular disintegration and corestone weathering are not evident, and dissolution and precipitation of secondary calcite is only observed within a few centimeters of joint planes. Corestone morphology appears to form through the erosion of loosened grus debris at sharp edges of subaerially exposed joints. The resulting form represents a morphologic equifinality to that of granite corestones and results from different processes than those which have been proposed for granite. An 8-month, once-per-three minute temperature record from a thermocouple embedded into a marble corestone indicates that the upper 2.5cm of the corestone regularly achieves temperatures greater than 40°C, the temperature which is thought to be sufficient to produce permanent strain in calcite. All of these results are consistent with theoretical and experimental studies that indicate that marble is susceptible to mechanical weathering by thermal processes. Overall, this study provides new documentation that solar heating and cooling can be an important driver of physical weathering in natural systems of the Earth's surface.