Toward a magnetic polarity stratigraphy of upper Cretaceous coarse-grained strata from the proximal cordilleran foreland basin, northeastern Utah

To provide an independent refinement of the inferred age of the coarse-grained proximal deposits of the Cordilleran foreland basin, suitable specimens were collected from northeastern Utah to define a magnetic polarity stratigraphy for these rocks. The proximal coarse-grained deposits of the Echo Canyon (EC) and the Weber Canyon (WC) conglomerate are intercalated with fine-grained, mostly laminated reddish siltstone and silty-shale intervals, our primary targets for magnetic polarity study. Sampling sites were established in the upper part of the Echo Canyon conglomerate and almost the entire section of the Weber Canyon conglomerate for polarity information. The mid-Cretaceous proximal deposits are mostly likely of the age of the Chron (C) 34n-33r interval and younger C33r and C33n interval, the former approximates the Santonian-Campanian stage boundary. Demagnetization results show that hematitic, fine-grained intervals in the EC and WC sequences yield well-defined, interpretable magnetizations. Typical natural remanent magnetization intensities are between ~0.3 and 3 mA/m, for fine-grained deposits in both sequences. Thermal demagnetization data from the upper EC conglomerate are exclusively of normal polarity; those from the younger WC conglomerate show normal polarity in the lower section and normal and reverse polarity in the higher part of the sequence. The normal polarity of the EC conglomerate is tentatively correlated with the long C34n. The normal polarity of the lower WC conglomerate likely correlates to the long C34n; the younger reverse, normal and reverse magnetozones likely correlate to C33r, C33n and C32r, respectively. Thus; our polarity stratigraphy work on these proximal deposits may provide an improved temporal resolution for these orogenic-related strata in the Western Cordillera.