POWRE: Strontium Isotopes to Gauge Climatic vs. Tectonic Effects on Carbonate Sedimentation in Continental Basins

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0074647Gierlowski-KordeschSr isotopes are a powerful tool in determining provenance (source area) for understanding sedimentation patterns within a basin; this information is important for reconstructing past history as well as resource exploration. Applied in large part to the marine realm, the usefulness of Sr isotopes in solving continental geologic problems has not been totally utilized by the geologic community. Because the Sr signal within basinal sediments can be dominated by the carbonate rocks in the source area (with no significant change in hydrology/climate), the application of this technique to nonmarine carbonates might be used to solve many geologic problems. A topic usually addressed in continental research is the differentiation of the main controlling factors in sedimentation: climate vs. tectonics. This has proven quite difficult to do because of the lack of time resolution across continental sequences within a basin development suggests that tectonics is more important than climate in controlling sediment architecture in basins. To test this theory, Sr isotopes from nonmarine carbonate rocks can be used to determine their origin. There are seven possible mechanisms to produce primary nonmarine carbonate sediments. The identification of carbonates formed from the overland transport of material through bedload, suspended load, and dissolved load (clastic carbonates) is crucial to assessing a link between a sediment and its source area. Layers of carbonate within siliciclastic sequences have long been identified as "arid climatic events". Large quantities of carbonates, however, can be deposited in any climate as long as the source area for the basin contains calcium-bearing rocks, especially carbonates. The occurrence of carbonate beds in a rock sequence may instead be linked to tectonic changes. In addition, the textures of a clastic carbonate rock are indicative of the type of climate in which it is deposited. Comparing the textures and stable isotopes of nonmarine carbonates within a sequence, one can determine whether climatic change affected deposition across a sequence. If not, then one could test through Sr isotopic signatures whether the rocks are indeed directly sourced from the drainage basin through comparison with the Sr signal of the source rocks. The appearance of a carbonate bed in a sequence might indicate a change in the source area, not in climate; the effects of climate vs. tectonics could then be differentiated. In order to test and refine this technique, a modern example would be sampled rigorously before application to an ancient example. The modern example is Kelly Lake in British Columbia - a lake containing a carbonate bench and associated with a river, which directly drains from a source area containing Permian limestones. The ancient example involves nonmarine carbonates found in the northern Appalachian basin within the Conemaugh and Monongahela Group of the Pennsylvanian, components of the so-called "cyclothems", with the source area postulated to be the Cambro-Ordovician carbonates if Virginia. If successful, this technique may change the perception and use of nonmarine carbonates in the sedimentary record.