Diffusional mass transfer processes in pitted pebble conglomerates

The diffusional mass transfer of soluble components in pitted pebble conglomerates from the Alpine Molasse and the Carboniferous of northern Spain has been studied using electron microprobes. Two categories of pitted pebble conglomerates are distinguished depending on the presence or absence of an affected zone within the pebble which has suffered volume loss. In these affected zones there is a reduction of mobile components—Ca, Ba, Sr, and sometimes Si, and a concentration of immobile components—Mg, Al, Fe, Mn, Si, K, and Ti. The changes in relative abundances of different elements across these zones are shown to be dependent on the mineralogy of the dissolving pebble. This is interpreted as being partly due to the modification of the stress fields, developed within pebbles due to their composition. Theoretical predictions of the rates of pebble pitting are shown to be in reasonable agreement with natural examples. Most of the deformational structures in these conglomerates were produced by a diffusional mass transfer process such as pressure solution.