Using Carbonates to Reconstruct Provenance and Watersheds

Elizabeth H. Gierlowski-Kordesch
Ohio University, Athens, OH

Determination of Sr isotope ratios in basinal sequences can contribute to reconstruction of paleohydrology, weathering patterns, bedrock sources, and drainage characteristics of a continental basin in the modern and fossil record.. Three key properties of strontium isotope chemistry make it an excellent geochemical tracer through continental systems. (1) Sr fractionation during physical and biologic processes on the Earth's surface is negligible. (2) Surface and groundwaters derive their Sr isotopic signature directly from the rocks along transport path. (3) Sr isotope values of continental basinal sediments are homogenized from bedrock values in the source area and catchment. Shallow groundwaters and surface waters mix with waters from the same source area in most cases so shallow diagenesis does not alter the Sr isotopic signal in the basin. Only contributions from deep geothermal waters can alter the final Sr isotopic signal in basinal sediments. Weathering of carbonates dominates the Sr isotopic signal of rivers in the modern whatever the surface area coverage of carbonate rocks or disseminated calcite in a drainage area. Therefore, only the sampling of carbonates is necessary; dolomite and calcite need not be separated for isotopic analyses.

Use of geochemical signals in carbonates has great potential for connecting Sr isotopic signatures in the source area with that of basinal sediments in a continental basin without costly analyses of all source rocks. Each fluvial input would drain through its unique watershed carbonate outcrops and deposit its Sr-enriched carbonate sediment in the distal basin. Sr isotopic values of source area carbonates could then be directly linked to basinal carbonates, allowing for reconstruction of paleowatersheds. Examples will be shown.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005