title: Sedimentologic, stable isotopic, and paleomagnetic analysis of Laramide synorogenic strata: unroofing of the Beartooth Range, Montana and Wyoming
Creators: name: Hart, Austin S.
Publisher: name: Western Washington University
Year: 2012

description: <p>The timing and sequence of Paleogene proximal sediments derived from the rising Beartooth Range of Montana and Wyoming and shed eastward into the western Bighorn Basin have been interpreted differently by earlier workers. Improving our knowledge of the relationships between proximal and distal strata in the western Bighorn Basin will lead to a better understanding of basin development in the northern Laramide Province. The objective of this study is to use petrographic, stable isotopic and paleomagnetic datasets from Beartooth synorogenic deposits in order to address different hypotheses concerning the timing and sequence of sedimentation off the Beartooth uplift. The synorogenic strata on the west side of the basin closest to the Beartooth Range, also called proximal deposits, are variously mapped as upper Paleocene Fort Union Formation (Lopez, 2001), uppermost Paleocene-lower Eocene Willwood Formation (Love and Christensen, 1985) and lower Eocene Wasatch Formation (Raines and Johnson, 1995). These strata form a series of range-front sequences that consist of two or more packages of strata; contacts between these packages were determined to be unconformable in the field. Seven areas were studied for this project. All sequences show more deformed strata lower in the section and closer to the range front and subhorizontal or less deformed strata higher in the section. Lower strata consist of material eroded off the top of the uplift, and the upper strata contain more fragments from the underpinnings of the orogen. Petrographic data from sandstones in the western Bighorn Basin indicate the sediments were derived from a recycled orogenic provenance. Sandstone modal compositions exhibit unroofing sequences through progressively higher strata. Lower sediments are dominated by carbonate lithic fragments and recycled quartz, whereas upper strata contain greater proportions of first-cycle granitic and meta-volcanic grains. Petrographic comparisons among synorogenic units in the western Bighorn Basin show that the proximal sediments are mineralogically similar to distal sands in late Paleocene and early Eocene deposits. Lower strata along the eastern front are compositionally similar to sediments in the upper Fort Union Formation, whereas upper synorogenic strata along the northern front have sand compositions more akin to sediments in the Willwood or Wasatch formations. Stable oxygen and carbon isotope data were analyzed from calcium-carbonate cements, bulk rock, and carbonate-clast subsamples from intervals within the North Clarks Fork section. Carbonate clasts have the highest δ18O values, -5.79‰ to 10.43‰, whereas cements have the lowest δ18O values, from -11.76‰ to -1.18‰ (V-PDB); δ13C subsample values show similar characteristics. Overlap between the isotopic compositions of cement and carbonate clast suggests that most of the cement isotope compositions were inherited from carbonate fragments. Isotopic enrichment of cements occurred from the dissolution of carbonate source rocks followed by calcium carbonate precipitation during early diagenesis and from the reprecipitation of carbonate phases during late diagenesis. The most negative values are interpreted to be derived from high elevation, isotopicallydepleted, meteoric waters that mixed with carbonate-rich waters during fluvial transport and early diagenesis. Assuming early cements formed in equilibrium with shallow groundwater and an isotopic lapse rate of 2.9‰/km (Dutton et al., 2005), the precipitation was sourced in adjacent highlands a minimum of 1.6 ± 0.6 km above the early Eocene Bighorn Basin. This estimate of relief is consistent with other isotope-based relief estimates for the early Eocene from neighboring basins in the northern Laramide Province (Fan et al., 2011). Forty-six paleomagnetic sites were sampled from the North Clarks Fork section to determine polarity; ten alpha sites have mean directions in stratigraphic coordinates (tilt corrected) that are consistent with the expected early Eocene direction. The grand-alpha site mean direction (D/I) is 162.5/-75 (α95=10.1) and within error of the expected antipodal direction for the early Eocene Bighorn Basin (169/-63; α95=2.6), based on the reference pole of Diehl et al. (1983). Well-defined components in alpha sites are interpreted to be depositional remanent magnetizations that locked in a reverse polarity field during deposition. Reverse polarity alpha sites are matched to Clarkforkian-age fossils (56.8-55.4 Ma; Gingerich et al., 1980) and the Chron 24 reverse (C24r) magnetozone of the geomagnetic polarity time scale (Cande and Kent, 1995). Coupled Clarkforkian and C24r age constraints suggest that strata in the North Clarks Fork section are correlative to late Paleocene members of the Fort Union Formation. Upper synorogenic units younger than North Clarks Fork strata are likely early Eocene deposits and closer in age to the Willwood or Wasatch formations. This interpretation is consistent with changes in sandstone compositions observed in upper synorogenic sediments from this study. The findings presented here are consistent with tectonic models that call for onset of the Laramide orogeny in the northern U.S. Cordillera during the late Paleocene (Bird, 1998; Liu et al., 2010).</p>