OXYGEN ISOTOPIC COMPOSITIONS OF LOCAL vs. REGIONAL RECHARGE ON ALBIAN COASTAL PLAINS, PROXIMAL FORELAND BASIN, WESTERN CANADA
The Geological Society of America
1998 Annual Meeting
Toronto, Ontario, October 26-29, 1998
1998 Abstracts with Programs, v. 30, no. 7, p. A-316
Most freshwater carbonates in the proximal foreland of the Cretaceous Western Interior Basin have extremely depleted d18O values (ca. 20 PDB) reflecting dominance of regional recharge by alpine snowmelt from orogenic highlands. Stable isotopic studies on sphaerosiderites from coastal plain paleosols of the proximal foreland in western Canada indicate that these features provide proxy records of the mean annual d18O of local paleoprecipitation, as has already been shown on the cratonic basin margin. The principle source of d18O variation in sphaerosiderites from Albian paleosols is paleoprecipitation at a given paleolatitude, with sites including: (1) Dakota Fm of Iowa, Nebraska, and Kansas (34-36° N); (2) Mattagami Fm of Ontario (41° N); (3) Swan River Fm of Manitoba (46° N); (4) Bow Island Fm of Alberta (48° N); and (5) Boulder Creek Fm of British Columbia (52° N). Meteoric sphaerosiderite lines (MSLs) range from 2.86 at the southern end of the transect to 9.89 at the northern end. The latitudinal MSL d18O gradient of the Albian "Greenhouse World" was steeper than the modern, possibly because of increased rainout effects from higher global precipitation flux.
Sphaerosiderites in the Boulder Creek Fm paleosols of Leckie et al. (1989, Sedimentology) have very different d18O values than the early diagenetic calcite and siderite cements with meteoric end-member values of 18.2 and 16.4 , respectively, reported by Bloch (1990, Bull. Can. Petrol. Geol) from the immediately underlying prodelta deposits of the Hulcross Fm. The groundwater system of the Hulcross evidently integrated recharge from a much larger drainage basin than the Boulder Creek, one that intercepted significant volumes of highland snowmelt. Leckie and Reinson (1993, GSC SP39) noted that reduced sedimentation during paleosol development on the Boulder Creek coastal plain was coeval with fluvial incision, a process which likely confined regional runoff and caused Cordilleran snowmelt to bypass the coastal plain.