Diagenesis, thermal maturation and burial history of the Upper Cambrian Bonneterre dolomite, southeastern Missouri; an interpretation of thermal history from petrographic and fluid inclusion evidence

Abstract

In this report, fluid inclusion, thermal alteration index (TAI) and petrographic data are used to interpret thermal maturation and burial history of the Bonneterre Formation of southeastern Missouri. TAI visual estimates, based on indigenous Upper Cambrian acritarchs, indicate maximum thermal exposure into the mid-upper peak gas window. This interpretation is in agreement with maximum fluid inclusion homogenization temperatures which average 235°C (455°F or an approximate vitrinite equivalent of 1.7% R 0). Thin-section petrography and fluid inclusion evidence further indicate that three separate episodes of burial have occurred. Initial burial, of unknown depth or thermal exposure, culminated in a period of uplift and karstic subaerial exposure possibly during Late Ordovician time. Maximum burial and thermal exposure to the mid-upper peak gas window followed, and is represented in the rock by a high-temperature (235°C), randomly oriented two-phase fluid inclusion population in matrix dolomite. Subsequent uplift, possibly coincident with Ouachita overthrusting, resulted in a variety of diagenetic fabrics, including late burial cements and fractures. Secondary two-phase fluid inclusions within these late calcite cements and those occurring in linear trains that cut across matrix dolomite record lower average homogenization temperatures than their unoriented precursors (average T h = 170°C, or an approximate equivalent of 1.3% R 0), further indicating that a significant amount uplift has occurred since maximum burial. Present-day bottom hole temperatures of only 74°C (an approximate equivalent of 0.7% R 0) indicate that uplift has occurred after this second burial episode. This event probably coincides with the development of the New Madrid Fault system during the Tertiary, which still active today. These results demonstrate the utility of fluid inclusion microthermometry complex burial/thermal histories.