RATES OF SOLUTION OF DIFFERENT ROCKS AFFECT CROSS SECTIONS

Water has several roles in structure and tectonics. Here I discuss differing rates of solution of different minerals and different grain sizes. In a typical stack of passive margin rocks, during the development processes of a fold-and-thrust belt different parts of the stack will lose widely differing volumes from solution. Trying to account for the disparity in amounts that are typically not measured in detail leaves so much uncertainty that a cross section properly balanced using the remaining rock volumes might not lead to a realistic retrodeformation reconstruction.

Here is a hypothetical case. A quartzite conglomerate glued to the basement might lose only a few percent of its volume in the same time that the overlying calcareous shale loses several tens of percent. The third stratigraphic unit, limestone, might have 20% gone, give or take 20%. The salt on top deforms so interestingly that it surely deserves Bill Thomas's word MUSHWAD, but how much volume is gone is indeterminate. The top unit, quartz siltstone, dissolves several times faster than the basal quartzite conglomerate, but that leaves tens of percent in doubt. These dissolution rates and amounts can be measured, at least in some places in some tectonic belts and at least to a first approximation, and thus should be evaluated in reconstructing and trying to retro-deform thrust belts.