NOT-SO-BORING BASS: C-ISOTOPE STRATIGRAPHY AND FACIES ANALYSIS OF THE MESOPROTEROZOIC BASS FORMATION, GRAND CANYON, AZ

The Mesoproterozoic Bass Formation (60-100 m thick) is a rich archive of siliciclastic and carbonate strata that provides insight into the carbon cycle near the end of the ‘boring billion’, ca. 1250-1230 Ma. The new carbon, oxygen, and ⁸⁷Sr/⁸⁶Sr isotope stratigraphy for the Bass Fm is constrained by an ash near the base that yields an ID-TIMS U-Pb zircon age of 1254 ± 2 Ma. Together, these foster age-calibrated insights into the limited middle-late Mesoproterozoic global reference curve, and permit assessment of the Bass basin’s connectivity with the ocean, its diagenetic history, and insights into the region’s carbon-burial and weathering rates. Associated facies analysis provides a framework for understanding the depositional history of the Bass Fm.

Geochemical results from measured sections throughout Grand Canyon are consistent with other middle-late Mesoproterozoic successions around the globe. Over 200 carbonate samples were analyzed for δ¹³C and δ¹⁸O, and five samples for ⁸⁷Sr/⁸⁶Sr. δ¹³C values from Hance Rapids, Clear Creek, Bright Angel Creek, and Bass Canyon localities range from -1.87 to 4.86 ‰ (VPDB), with an average shift of 3.69 ‰ per section. δ¹⁸O values from these sections range from -16.07 to -1.67 ‰ (VPDB), with an average shift of 8.97 ‰ per section. ⁸⁷Sr/⁸⁶Sr ratios range from 0.70660 to 0.70807. Chemostratigraphic analysis of the South Kaibab Trail and Vishnu Canyon localities is currently in progress.

The Bass Fm contains six carbonate and six siliciclastic facies. Carbonate facies include dolomitic boundstone with domal and conical stromatolites, laminated dolomicrite, silty dolostones, evaporite dolostones, massive dolomicrite, and silty dolomitic intraclastic packstone. Siliciclastic facies include red to purple mudstones, fine- to coarse-grained sandstone, and two conglomerates. Sandstones include litharenite, arkosic arenite, and subarkosic arenite. Clast-supported conglomerates contain granite, schist, and quartzite clasts up to cobble size from the underlying basement. Diamictite contains intraformational angular dolostone boulders up to tens of cm in diameter. All facies interfinger and indicate a tectonically active system with continent-derived fluvial-alluvial environments supplying sediments to a shallow to restricted marine carbonate system.