238-7 Petrological Trends in Llandoverian Dolomites and Implications for Platform-Scale Dolomitization, Michigan Basin, USA

Session: Recent Advances and New Voices in Marine and Coastal Geoscience

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Individual petrological data are routinely used to interpret the fluid sources, mechanisms, and relative timing of platform-scale dolomitization, all of which are highly debated in the rock record.

The current study integrates a suite of sedimentological, mineralogical, elemental, isotopic, and textural data from three cores positioned along a dip profile of a Llandovery-aged carbonate ramp platform in the Michigan Basin to evaluate vertical and lateral trends in petrographic data, determine the relative timing of platform-scale dolomitization, as well as the mechanisms and fluids responsible. Previous research on the Llandovery succession in the Michigan Basin indicates that dolomite abundance within the carbonate ramp platform decreases from the upper-ramp down dip and grades to zero laterally towards the basin center, suggesting a reflux mechanism. The focus of the current study is to evaluate petrological patterns and trends in the mid- to upper-ramp, where dolomite is most abundant.

Sedimentological and petrographic data provide clear evidence of carbonate deposition in a near-shore restricted tidal flat and supratidal setting, including a lack of marine fauna, abundant mud cracks, rip-up clasts, and algal laminations. Cross-cutting relationships indicate that dolomitization occurred very early, likely during evaporative depositional conditions. However, mineralogical and geochemical data from these intervals show that dolomites are Ca-rich and moderately ordered (n=205), with d¹⁸O and d¹³C values (n=98) generally falling in the expected range for carbonates formed from normal Silurian seawater. In contrast, clumped isotope data indicates D⁴⁷ dolomite crystallization temperatures of ~60-75°C, suggesting recrystallization during burial at depths greater than 1 km.

Collectively, these observations lead to a model of early, syndepositional dolomitization in a restricted tidal flat and supratidal setting of the upper carbonate ramp. Subsequent recrystallization occurred during burial, which reset the isotopic and geochemical signatures of the dolomite, resulting in elevated D⁴⁷ temperatures. This study suggests that multiple superposed dolomitization mechanisms contributed to the formation of the Silurian dolomites of the Michigan Basin, and that integration of multiple proxies provides the most reliable approach for working out diagenetic timing and conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9417

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