DETAILED CHARACTERIZATIONS OF THE CRETACEOUS TUNUNK SHALE, NORTH HENRY BASIN, UTAH, USA: A HIGH-RESOLUTION STUDY OF MUDSTONE PARASEQUENCES INTEGRATING SEDIMENTOLOGY AND PETROGRAPHY WITH HIGH-FREQUENCY PXRF

In the context of recently reassessed regional sequence stratigraphy (Li and Schieber, 2017), this study provides a detailed inventory of sedimentological, petrographic, and geochemical features for selected coarsening-upward packages (parasequences; PS’s) to characterize processes active during mudstone PS development. Eleven microfacies differentiate subtle changes in depositional energy by characterizing grain-size, BI and (trace) fossil assemblages, sedimentary structures, and composition. High-frequency pXRF data track chemical changes in sedimentological logs and are also used to discriminate microfacies.

Calcareous mudstones of transgressive and low-stand PS’s in early Tununk deposits (Sequences 1 & 2) show clastic dilution largely by ash-derived materials. They are also mixed with terrigenous lithics that were probably shed from (chemically similar) Sevier arc terrains. Younger deposits in (latest Sequence 3) high-stand PS’s show a complex assemblage of clay-mineral-dominated clasts in the silt to fine-sand size fraction. “Muddy” grains are comprised by suite of rip-ups, shale lithics, weathered volcanic lithics (VRF's), and flocculated mud. These aggregate clasts dominate both coarse-grained event beds and "fine-grained" lower-energy intervals. Presumably, this characteristic is also responsible for the muddy-weathering appearance and instability of these rocks in outcrop.

Geochemical proxies – established via pXRF – show that highly bioturbated facies have smooth trend lines due to homogenized bedding, whereas heterolithic intervals show distinct chemical variations between bedding features. Up-section increasing Si, Ti, Zr, K, Rb, Si/Al, and K/Al show negative correlation with Ca, Mn, and Sr, implying an upward-increase of depositional energy and clastic dilution. Si/Al and K/Al ratios appear unaffected by weathering, suggesting their efficacy as sedimentation-rate proxies in fine-grained successions. Compositional similarities between ash-fall and hinterland/arc-derived VRF’s – the apparent clastic sources of lower Tununk deposits – make determining provenance with pXRF difficult without petrographic verification.