Stable Carbon Isotopes for Constraining Sequence Stratigraphic and Paleogeographic Interpretations of Carbonate Successions: Fact or Fantasy?
Ruppel, Stephen C.; Rowe, Harry
Although the use of stable isotopes in carbonate systems for defining paleotemperatures and diagenetic events has been widespread for several decades, the application of δ13C for high-resolution chronostratigraphy has only gained widespread interest in recent years. Many recently published studies have argued that the δ13C signal in shallow water carbonates preserves an excellent record of global changes in seawater chemistry that can be applied for both high resolution and global correlations. Others have pointed out the inherent complexities in the δ13C signal created by diagenesis and mixing of variable carbon reservoirs that imply that global high-resolution correlations are improbable.
The potential value of a carbon isotope chronometer in carbonate successions is great, especially where temporal correlations based on sequence stratigraphic datums are absent or poorly constrained. To test the viability of this technique, we obtained whole rock powders from seven suites of cores in dolomitized carbonate platform successions in middle Permian hydrocarbon reservoirs in the Permian basin of west Texas. Samples were collected from a variety of depositional facies and from rocks known to have been diagenetically altered to test for isotope shifts tied to these processes.
Our results revealed δ13C events that appear to be correlative over relatively large areas, but also defined variances in δ13C whose causes and distribution are unclear. Whether these apparently uncorrelative δ13C signatures reflect local diagenesis or the presence of successions whose stratigraphic position is not understood or clear.
Overall, our data, though limited, suggest that (1) even diagenetically modified carbonate successions retain δ13C signals that are at least locally correlative, (2) these isotopic shifts can help refine and extend sequence stratigraphic and paleogeographic interpretations in areas of poor temporal control, (3) isotopic shifts are generally at a lower, not higher level of resolution compared to sequence boundaries, but also that (4) δ13C values are locally modified by diagenesis and or variations in carbon reservoirs, and that (5) it may require the development of local reference curves to utilize trends in δ13C for age control rather than global "secular" curves.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013