EXPANSION BRECCIAS IN CARBONATE ROCKS

We describe breccias in limestone or dolomite that appear to have been expanded with no shearing. In examples from the Lower Cretaceous Maiolica pelagic limestone in the Apennines and the Mississippian Madison Limestone in Wyoming, they are made of angular carbonate fragments from <1 mm to 10 cm dimensions. The fragments show no rounding of corners, their outlines often match, and they are separated by sparry calcite veins. This is seen in all three dimensions, showing that the breccias have been expanded. The Apennine breccias are more resistant than undeformed Maiolica, so the breccia may stand up in topographic walls cutting diagonally across the axes of 10-km-scale anticlines.

The origin of the expansion breccias appears to require an expanding fluid to break up the carbonate rock and a fluid to deposit the sparry calcite. Both tasks could have been achieved by water carrying dissolved CO₂ as HCO₃^– together with Ca²⁺. Chemical balance would require an input of CO₂ from outside the system to dissolve nearby carbonate rock and source the sparry calcite; both Wyoming and the Apennines are regions of known CO₂ accumulations. Stable (δ¹⁸O, δ¹³C) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotopes are compatible with derivation of the ingredients of the calcite cement from nearby parts of the carbonate stratigraphy. Expansion of CO₂to gas from a liquid or supercritical-fluid phase would take place at a depth of a few hundred meters, suggesting that this occurred during erosional unroofing of the anticlines; breccia formation would thus be a late event, which agrees with the lack of younger structures cutting the breccias.

These breccias may provide insights relevant to geological carbon sequestration, particularly the risk of CO₂ escape from underground traps and its permanent storage by mineralization to CaCO₃.