Late Silurian Hypersaline Seas in the Michigan Basin and Their Impact on Ancient and Modern Groundwater

大约4亿年前密歇根盆地地区高盐度海水的存在可能与上志留统厚盐层的沉积有关，与附近中志留统油气藏古咸水对孔隙度的封堵有关，也可能与极端盐度和富金属的现代地下卤水的发展有关。为了确定古咸水海是否被孤立，其表面远低于周围海的水平，将分析内部和外部碳酸盐单元的初级稳定碳和氧同位素比值。碳同位素特征将被用来推断咸海的深度，而氧同位素特征将被用来推断其区域范围和蒸发状态。通过对圈闭古流体的显微测温分析，揭示了中志留统油气藏中古咸水地下水的存在，揭示了古地下水的形成温度、总盐度和阳离子含量。最后，将通过重建密歇根盆地地区中志留统碳酸盐岩的晚期流体历史来检验最近的咸水演化为现代卤水的理论。通过分析氧同位素趋势和捕获在这些岩石中的流体包裹体的化学成分，将记录卤水发育的时间和盆地卤水流出盆地的程度。该项目将通过购买设备、岩石样本和让学生参与高中课程的开发而使本科生受益。

The presence of hypersaline ocean water in the Michigan Basin region approximately 400 million years ago can be tied to the deposition of thick upper Silurian salt sequences, to the plugging of porosity by ancient saline groundwater in nearby middle Silurian hydrocarbon reservoirs, and perhaps to the development of extremely saline and metal-rich modern groundwater brines. In order to determine whether the ancient saline sea was isolated, with its surface well below the level of surrounding seas, both internal and external carbonate units will be analyzed for primary stable carbon and oxygen isotopic ratios. Carbon isotopic signatures will be used to deduce the depth of the saline sea whereas oxygen isotopic signatures will be used to deduce its regional extent and evaporation state. The presence of ancient saline groundwaters in middle Silurian hydrocarbon reservoirs will be documented by microthermometric analysis of trapped ancient fluids, which can reveal their temperature of formation, total salinity and cation content. Finally, the recent theory that saline seawater evolved into modern brines will be tested by reconstructing the late fluid history of middle Silurian carbonates located throughout the Michigan basin region. Both the timing of brine development and the extent to which basinal brines traveled out of the basin will be documented by analyzing both oxygen isotopic trends and the chemistries of fluid inclusions trapped in these rocks. The project will benefit undergraduate instruction through acquisition of equipment, rock samples and involvement of students in development of senior these topics.