UNRAVELING REGIONAL FROM GLOBAL CARBON ISOTOPIC SIGNATURES IN THE ANTLER FORELAND BASIN

Carbon isotopes (n = 800) of fossil allochems and bulk rock provide a high-resolution record of ocean chemistry in the Antler foreland basin of western Laurentia during the Late Mississippian transition from greenhouse to icehouse conditions. A concomitant meter-scale sequence stratigraphic framework constrained by detailed conodont biostratigraphy and strontium isotopes documents the facies response to regional changes in tectonic subsidence, paleogeography, eustasy and climate. Three distinct sedimentation episodes are delineated by 3‰ excursions in the isotopic record and by facies changes in the stratigraphy. Lower Chesterian strata record an abrupt transition from shallow shelf packstones to a thick succession of deep subtidal mudstones. This facies change is accompanied by a 1‰ negative and subsequent 2‰ positive carbon isotope excursion, following which δ¹³C values remain constant. European and mid-continent data sets confirm the initial negative excursion, but not the following abrupt positive excursion, suggesting a regional origin for the latter. Increased tectonic subsidence and enhanced circulation of open marine waters may explain this stratigraphic and isotopic trend. Reduced accommodation in the Middle Chesterian results in more frequent siltstone- capped shallowing upward carbonate facies cycles. A prominent 4‰ negative isotope excursion corresponds to one of these sands, and may indicate meteoric water influence during sea-level lowstand. Differences in resolution complicate a comparison to published data sets, but a tentative western European negative excursion of similar magnitude implies eustatic control on this sea-level change. Moderate amplitude fluctuations characterize the remaining Middle Chesterian interval. Thin bedded skeletal grainstone and carbonate-cemented sandstone produced by moderate-amplitude sea level fluctuations dominate the Upper Chesterian and mid-Carboniferous boundary. A positive isotope excursion (3‰) near the mid-Carboniferous boundary may be coeval with one identified in the US mid-continent and in western Europe suggesting that the observed facies and isotopic shifts may have developed in response to the onset of Gondwanan glaciation and eustatic sea-level fall.