PALEOMAGNETIC AND ROCK MAGNETIC DATA FROM MID-CRETACEOUS COARSE-GRAINED STRATA FROM THE CORDILLERAN FORELAND BASIN: A MEANS TO TEST THE TWO-PHASE TECTONOSTRATIGRAPHIC MODEL

The conventional syntectonic interpretation of coarse-grained fluvial deposits within foreland basins has often been challenged by a “two-phase tectonostratigraphic model”, which advocates that coarse deposits of the medial to the distal part of foreland basin are associated with post tectonic adjustment rather than syntectonic. High precision age control for proximal and distal strata is required to test this alternative hypothesis. The Cordilleran foreland basin in NE Utah and SW Wyoming contains thick (>1000 m) megasequences of proximal coarse-grained deposits (e.g., Echo Canyon and Weber Canyon Formations). Magnetic polarity stratigraphy and high-resolution detrital thermochronology approaches applied to these deposits may provide an excellent opportunity to test the model hypothesis. Preliminary demagnetization results confirm the presence of suitable fine-grained material within the coarse-grained proximal Echo Canyon and Weber Canyon deposits that yield well-defined, interpretable magnetizations. Laboratory unblocking temperatures up to 680°C, the saturation of isothermal remanent magnetization (IRM) at 2.8 T (tesla) or higher, and 0.2 to 0.5 T backfield coercivities of remanence all imply that hematite is the principal remanence carrying phase in these rocks. Nearly reversible susceptibility vs. temperature curves suggest a little or no mineralogic changes at high temperature. Typical NRM intensities of the finer grained materials range between ~0.3 and 3 mA/m.

Results from the Echo Canyon Formation show a well-defined magnetization of exclusively normal polarity, possibly implying deposition entirely during, yet toward the end of the mid-Cretaceous long normal interval (Chron 34N). The younger Weber Canyon Formation tentatively yields a normal then reverse polarity sequence, which we interpret to correlate to the 34N and ~ 4 Ma duration 33R polarity record, implying that at least part of this formation is of Campanian age. Future work on the complete Echo Canyon and Weber Canyon sequences should yield a high-resolution magnetic polarity stratigraphy which may provide more precise depositional ages for these rock units.