STABLE ISOTOPE ANALYSES OF LIMESTONE BEDS IN THE LATE CRETACEOUS IRON SPRINGS FORMATION, SOUTHWEST UTAH

In the thrust proximal Sevier foreland basin, the ~1000m thick mid to Upper Cretaceous Iron Springs Formation (ISF) at The Three Peaks, Iron County Utah, is characterized by abundant sandstone and conglomerate, distinct but sparse limestone beds, and ash-fall tuff. The basal Marshall Creek Breccia (MCB) limestone unconformably overlies the Jurassic Carmel Formation (CF; ~165 Ma). Syndepositional deformation of thin laminated limestones interbedded with the MCB breccias demonstrate deposition by gravity flows, likely generated by slip along the nearby Iron Springs fault. The MCB is overlain by the 100 Ma Three Peaks Tuff member. Multiple, laterally discontinuous, massive, limestone beds crop out above the tuff throughout the lower ~100 m of the ISF and are overlain by thick sandstone and cobble conglomerate beds. We integrate field observations, petrography, and δ¹⁸O and δ¹³C stable isotopes to constrain the depositional history of these carbonate units.

35 total samples from the MCB and the massive limestones were selected for stable isotope analyses. 4 additional samples from the CF, a potential source for breccia clasts, were analyzed. 3 CF and 2 ISF samples from Parowan Gap (~12 km away) were analyzed to test possible effects of the Miocene Three Peaks laccolith. Apart from obvious altered samples (color mottling, concretions, mineral veins) stable isotopes are largely unaffected by diagenetic or hydrothermal processes. The results confirm marine deposition of all CF samples and ISF limestones at Parowan Gap; all ISF limestone beds at The Three Peaks are lacustrine. Carbonate clasts within the MCB appear to be derived from intraformational sources. Overall, MCB (δ¹⁸O -7.99 to -15.88; δ¹³C -0.2 to -4.62) and ISF (δ¹⁸O -13.46 to -20.43; δ¹³C -1.61 to -7.04) samples indicate a humid, hydrologically open lacustrine setting. Variation in the d¹⁸O values for the MCB clasts and laminated sediments may represent a variety of local sources, seasonal changes, or changes in source hydrology. Overall, δ¹⁸O values are highly depleted, consistent with a combination of high altitude, snow and ice drainage, carbonate rocks composing the drainage basin, and minor alteration during early diagenesis. The termination of the lacustrine lakes by coarse clastics is consistent with an increasingly wetter climate and tectonic activity.