CHANGES IN THE HYDROGEOLOGY OF PLEISTOCENE LAKE BONNEVILLE, UTAH DEFINED BY SHIFTS IN MINERALOGY AND SR-ISOTOPE COMPOSITION

Significant shifts in the mineralogy and Sr-isotope composition of authigenic carbonate minerals in marl deposits from Lake Bonneville mark major hydrogeologic events in the transgressive history of the lake. Cores of the transgressive interval from the two major subbasins of Lake Bonneville, the Sevier and Great Salt Lake subbasins, were sampled every 10 cm. Mineral percentages were determined by X-ray diffraction analyses. Scanning-electron microscopy verified an authigenic origin for calcium carbonate minerals, based on crystal morphology and pristine condition. Marl from the Sevier core shows two major shifts in mineralogy and one in Sr-isotope composition. Calcite is the only calcium carbonate mineral in the basal 40 cm, with an ⁸⁷Sr/⁸⁶Sr of 0.71083. The overlying 130 cm consists of almost pure aragonite, with minor calcite. The ⁸⁷Sr/⁸⁶Sr ratios in this interval range from 0.71137 to 0.71183 (n=4). The one sample above this interval is calcite, with minor aragonite and an ⁸⁷Sr/⁸⁶Sr of 0.71129. Marl from the Great Salt Lake core also shows two major shifts in mineralogy and one in Sr-isotope composition. Aragonite dominates in most of the basal 110 cm (15-80%) and in the top 20 cm (70-80%), but is absent in the intervening 60 cm. Calcite is present in all samples, and percentages are high where those of aragonite are low and vice versa. The greatest shift in ⁸⁷Sr/⁸⁶Sr occurs near the base and is concomitant with a fluctuation in aragonite percentage. Within 20 cm, the ⁸⁷Sr/⁸⁶Sr and aragonite percentage drop from 0.71285 and 80%, respectively, to 0.71175 and 15%. Although aragonite percentage increases abruptly to 70% in the overlying sample, the ⁸⁷Sr/⁸⁶Sr changes little (0.71183). The two ⁸⁷Sr/⁸⁶Sr values in the interval where aragonite is absent are 0.71127 and 0.71146. In the top of the core where aragonite abruptly increases to 80%, the ⁸⁷Sr/⁸⁶Sr increases slightly to 0.71170. The greatest shift in ⁸⁷Sr/⁸⁶Sr occurs near the base in both cores, concomitant with major changes in mineralogy. This change is interpreted to result from mixing of waters between the two subbasins after they joined at the threshold separating them. The major shift in mineralogy, accompanied by minor changes in ⁸⁷Sr/⁸⁶Sr, at the tops of the cores is interpreted to result from the abrupt change in lake volume caused by the Bonneville flood.