Dolomitization in Cambrian and Upper Ordovician Sediments of the Intracratonic Michigan Basin: Diagenesis and Paleofluid Migration

Abstract

Petrologic and geochemical investigations on the eastern flank of the Michigan Basin in Southwestern Ontario encountered thin, laterally continuous strata-bound dolomitization within the upper Ordovician carbonates of the Trenton-Black River Groups, and secondary mineralization within underlying Cambrian clastics. Analyses of selected rock core samples from the dolomitized horizons included petrography, stable and Sr isotopes, and secondary mineral fluid inclusion assessment to determine paleofluid temperatures and composition. Combining these measurements have provided a unique insight into the migration and nature of the fluids that modified these rocks. While the geographic area of the study has recently been increased, current results indicate that the formations were subjected to temperatures higher (i.e., ~20-30C) than can be explained by burial history alone. This implies hydrothermal fluid migration and influence, possibly occurring during geologic forcing associated with Paleozoic orogenesis. Within the 200 m thick near horizontally layered carbonate and clastic sequence, dolomite and calcite fracture infill isotopic and fluid inclusion data reveal two distinct diagenetic fluid systems: i) an underlying permeable Cambrian system with a more radiogenic, cooler and saline signature; and ii) an overlying Ordovician system characterized by a less radiogenic, higher temperature and hypersaline signature. These two distinct fluid regimes coupled with the occurrence of thin (i.e., 1-5 m) near horizontal strata-bound dolomites and trace quantities of saddle dolomite strongly suggest that diagenesis, as a result of episodic hydrothermal fluid migration, was neither pervasive in volume or extent within the Huron Domain. The results demonstrate that for this region of the Michigan Basin, the style of diagenesis is presents differently to adjacent regions such as the Chatham Sag, Manitoulin Island and the Appalachian Basin.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018