Meteoric sphaerosiderite lines (MSL's; Gonzalez et al., 1996, GSA Abst. with Prog. 28:7:A36), the stable isotopic signatures extracted from meteoric phreatic sphaerosiderite nodules in gleyed wetland paleosols of coastal plain successions of the Cretaceous Western Interior Basin, are providing new insights into the paleolatitudinal d¹⁸Ogradients of mean annual rainfall in the basin. Early meteoric phreatic diagenetic phenomena are known to integrate the mean annual oxygen isotopic composition of local precipitation and the mean annual temperature (Hays and Grossman, 1991, Geology 19:441-444), and the d¹⁸Ovalues of MSL's can potentially be used to estimate paleotemperatures and groundwater d¹⁸Oof ancient continental environments. Sphaerosiderite horizons in Late Albian strata in the Iowa-Nebraska area formed at about 36°N, and their MSL's range between -4.7 to -2.9 PDB. Using the inorganic ¹⁸Ofractionation relation of Carothers et al. (1988; GCA 52: 2445-2450), and assuming mean annual temperatures ranging between 10-30°C, these MSL's yield estimated groundwater d¹⁸Ovalues that range between -10 to -5 SMOW. Sphaerosiderite horizons in Valanginian strata in southwest Saskatchewan (Leckie et al., 1997, Can. Soc. Petrol. Geol., Memoir 18, p. 211-262) formed at about 46°N, and their MSL's range between -8.3 to -7.5 PDB. Over mean annual temperatures ranging between 5-25°C, ¹⁸O fractionation relations yield estimated groundwater d¹⁸O values that range between -15 to -10 SMOW. The different d¹⁸Ovalues from sphaerosiderites at these two locales is interpreted to primarily reflect differing ¹⁸O compositions of mean annual rainfall at their respective paleolatitudes Collections of sphaerosiderites from the Dakota Formation of Kansas, Nebraska, and Iowa, the Swan River Formation of Manitoba-Saskatchewan, and the Mattagami Formation of the Moose River Basin in Ontario are now being analyzed to characterize latitudinal ¹⁸Ogradients of Late Albian rainfall in North America from about 33° to 47°N paleolatitude. The purpose of this investigation is to develop paleogeographic information on atmospheric tracers in order to provide a feedback to GCM experiments simulating the Albian climate system.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah