APPLICATION OF SULFUR ISOTOPE RECORDS TO THE RECONSTRUCTION OF RELATIVE SEA LEVEL CHANGES IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The stratigraphic record of the mid-Cretaceous Western Interior Seaway (WIS) is characterized by evidence of relative sea level (RSL) oscillations at scales ranging from long-term “cyclothems” to orbitally-forced parasequences. Both tectonic and eustatic forces have been invoked to explain migrations of the paleoshoreline, which in turn, modulated sediment supply to the basin (with transgressions flooding alluvial valleys and starving offshore reaches of fine-grained detrital material, and regressions supplying mud to the distal basin). Some studies contend that sedimentologic evidence for RSL cycles can be traced from the western paleoshoreline (Utah) to the eastern cratonic platform (Kansas). Although these observations collectively imply relatively significant changes in sediment accumulation rates spanning such cycles, at all scales, this concept has not been directly tested to determine magnitudes of change, or the distances over which such effects persist.

Building on prior studies that established detailed litho-, bio-, and chemostratigraphic frameworks for the Cenomanian-Turonian Greenhorn Cyclothem, and the Coniacian-Campanian Niobrara Cyclothem, we examine the relationship between changes in RSL and offshore (fine-grained) sediment supply using a relatively new proxy: the isotopic composition of authigenic pyrite sulfur (δ³⁴S_pyr). The basis for this proxy, recently applied to Pleistocene and Holocene sediments, is derived from the influence of sedimentation rate upon the early diagenesis of organic carbon by sulfate reduction, with sulfide ultimately captured in sedimentary pyrite. High rates of sedimentation (regressions) isolate pore waters more rapidly, resulting in a reservoir effect that drives progressively heavier δ³⁴S_py values, whereas decreased supply of fine-grained sediment (transgressions) allows fractionation to approach theoretical limits (~70‰), resulting in much more negative d³⁴S_pyr values. Testing this hypothesis at two different scales – the short-term (parasequence) scale and the long-term (cyclothem) scale, we observe clear evidence for consistent negative d³⁴S_pyr excursions during transgressive episodes, and a less pronounced effect in progressively more distal sites where the sedimentation rate contrast is less pronounced.