T316 Earth History in the Broadest Context—Ophiolites, Global Tectonics, and Public Awareness of Earth Science II: Celebrating the Contributions of Eldridge Moores

Wednesday, 28 September 2016
1:30 PM - 5:30 PM
Room 203

EXPANSION BRECCIAS IN CARBONATE ROCKS

Paper number: T316-6
Program: Technical Sessions
Time: 2:50 PM – 3:05 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-282404

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