CALCITE-SIDERITE OXYGEN ISOTOPIC PALEOTEMPERATURE ESTIMATES FROM THE UPPERMOST SWAN RIVER FORMATION, LATE ALBIAN, CRATONIC MARGIN, WESTERN INTERIOR BASIN
P.L. Phillips, G.A. Ludvigson, L.A. González, and B.J. Witzke
The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-65
Early calcite and siderite cements of the Swan River Formation are interpreted to have precipitated from a meteoric groundwater system with fluid d18O of approximately -14 SMOW, at a mean annual temperature of approximately 14.8° C. Portions of the Manitoba Energy and Mines Pine River Core (M8-78), west-central Manitoba, include carbonate-cemented sandstone with mm-scale mudstone interbeds and were likely deposited in a fluvial to marginal marine setting. Early cementation of these strata is evident by coexisting calcite and siderite cements that occlude 38 to 40% minus-cement porosity. Calcite cements are pervasive in the sandstone interbeds, while siderite cements are prevalent in the mudstone interbeds. Carbon and oxygen isotope (C-O) plot of the siderite cements shows a positive covariant trend which is interpreted to record mixing between meteoric phreatic and marine phreatic pore fluids. d18O compositions for siderite range from -9.8 to -6.2 VPDB. Invariant d18O values in a C-O plot define a meteoric calcite line of approximately -12.8 + 0.3 VPDB. Light (meteoric) end-member d18O compositions of siderite (-9.8 VPDB) and calcite (-13.4 VPDB) were evaluated with published oxygen fractionation equations for calcite and siderite under the premise that both cements precipitated from the same groundwater system. The resulting paleotemperature estimate is in accord with published climate model (GENESIS) predictions for the mid-Cretaceous.