ESTABLISHING CORDILLERAN PALEOGEOGRAPHY DURING LATE MESOZOIC CONTINENTAL MAGMATIC ARC FLARE-UP EVENT (105-85MA) IN EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

The mid-Cretaceous Marble of Duck Lake in the Mount Morrison pendant of east-central Sierra Nevada, California, is the only Cretaceous carbonate known in any Sierran pendant. Previous paleoaltimetric interpretations suggest that by Cretaceous time the range had already risen 2-5 km above sea level as inferred by large magma addition rates from the last of three episodes of the Mesozoic arc magmatism. The stratigraphic section constraining the one-meter-thick lens of heterogeneous marble is dominated by thick layers of volcaniclastic tuff and breccia. The thickness and immediate stratigraphy point towards a paleoenvironment of large lake at elevation significantly above sea level. These interpretations put the depositional environment of the meta-limestone into question.

Thin section analysis of two distinct layers within the carbonate reveals differing modal percentages of large calcite zones displaying undulose extinction, which are interpreted as recrystallization, juxtaposed against fine-grained unaltered limestone. The two carbonate layers yield ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.7072 and 0.7068 and were determined to be an unreliable fit to the ⁸⁷Sr/⁸⁶Sr isotopic ratio of contemporaneous marine water. A two-point linear regression, however, compares the degree of recrystallization to ⁸⁷Sr/⁸⁶Sr isotopic ratio and suggests that an unrecrystallized limestone would have a higher initial ratio of 0.7074.

A comparison of the marble’s ⁸⁷Sr/⁸⁶Sr isotopic ratios and its stratigraphically constrained ages of 101-99 Ma nicely match with the well-established marine curve database of ⁸⁷Sr/⁸⁶Sr isotope ratio fluctuation through time. This proxy suggests that the marble’s environment of origination is marine rather than lacustrine, which suggests an alternative interpretation of the mid-Cretaceous paleogeography characterized by calm coastal ocean fingering into and around island volcanoes.