SPHAEROSIDERITE d18O COMPOSITION: A PROXY FOR PALEOPRECIPITATION, CENOMANIAN DUNVEGAN FORMATION, ALBERTA AND B.C., CANADA
D. F. Ufnar, P. J. McCarthy, G. A. Ludvigson, T. S. White, L. A. Gonzalez, and R. L. Brenner
Meteoric sphaerosiderite lines (MSLs) are used as an oxygen isotope proxy for Cretaceous paleoprecipitation (Ludvigson et al., 1998, Geology, 26:1039-1042). Reconstructing mid-Cretaceous d18O paleolatitudinal gradients from meteoric sphaerosiderite compositions requires data from high latitude deposits such as the Dunvegan Formation paleosols. Recent research results by our group has shown that MSLs in paleosols from the Cordilleran and cratonic Cretaceous Western Interior Basin (KWIB) margins have similar values at the same paleolatitudes. Numerous sphaerosiderite-bearing, sequence-bounding, interfluve paleosols developed in coastal plain facies have been documented in the Dunvegan Alloformation in the Peace River area of Alberta and British Columbia (McCarthy and Plint 1998, Geology 26:387-390). Close spacing of wells and outcrops in the Dunvegan make it possible to trace transitions between interfluve and valley fill deposits in detail. This provides an unique opportunity to study the paleohydrology and paleoclimatology of KWIB western margin coastal lowland precipitation from a high paleolatitude (estimates range between approximately 55º-65º N). Data from this investigation permit us to: (1) further constrain Albian-Cenomanian d18O paleolatitudinal gradients; and (2) to generate an oxygen isotope chemostratigraphy through middle Cenomanian time.
Dunvegan Formation sphaerosiderites yield d18O compositions that define an MSL of -11.40 ± 0.43% PDB. These are from the highest paleolatitudes yet sampled in the KWIB. The values are consistent with a steeper Albian-Cenomanian paleolatitudinal d18O gradient, interpreted to result from increased rainout effects and greater precipitation flux during mid-Cretaceous time.