Use of Carbon Isotopes as a Chrononstratigraphic Tool from Outcrops and Subsurface Core of the Mississippian Madison Limestone, Wyoming and Montana
Katz, David A.; Buoniconti, Matthew R.; Montañez, Isabel P.; Swart, Peter; Eberli, Gregor; Smith, Taury
Coupling of carbon isotope stratigraphy with strontium isotope analysis and biostratigraphy of the Mississippian Madison Limestone defines a chronostratigraphic framework that constrains surface to subsurface sequence stratigraphy and provides considerable insight into the origin and evolution of facies variability across the Madison ramp during the Early Mississippian. The δ13Ccarb values from twelve outcrop and three subsurface cores distributed across inner ramp to slope facies exhibit a rise and subsequent fall throughout the Kinderhookian and lower Osagean reaching maximum values of up to +7.5‰ PDB; these secular variations have also been correlated with the same values documented from lower Mississippian successions in Belgium and the Urals. The peak in δ13C values coincide with least radiogenic carbonate 87Sr/86Sr values near the Kinderhookian-Osagean transition and is defined as a maximum flooding surface. Several shorter-term fluctuations in δ13Ccarb values are superimposed on this longer-term trend and are also recorded in subsurface carbonates of the inner Madison ramp. The δ13Ccarb values show no significant isotopic variability between bulk carbonate and microsampled micrite, calcitic brachiopods and marine cements and are generally independent of facies including dolomitized inner ramp and calcareous outer ramp deposits.
We interpret the long-term increase of the δ13C values and coincident decrease of the 87Sr/86Sr as a product of increased productivity and preservation of organic matter in ocean basins with decreased continental weathering rates due to flooding of previously exposed land masses during sea-level rise. The subsequent decrease of δ13Ccarb values and concomitant increase of the 87Sr/86Sr are interpreted as a product of limited productivity and oxidation of organic matter with accelerated continental weathering rates due to exposure of land masses during sea-level falls. There is also a local transramp spatial variation of the δ13Ccarb which is attributed to increased environmental restriction proximal to land, suggesting that facies of the inner ramp were covered by stranded and aged water masses. Nevertheless, the secular variations in seawater δ13C have been successfully applied as a chronostratigraphic tool to subsurface core which has resulted in considerable improvement of our knowledge regarding the lateral distribution and timing of depositional facies and evolution of Madison reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013