Diagenetic Origins of Dolomitized Reservoirs and Limestone Tombstones in the Madison Limestone, Northern U.S. Rockies

Katz, David¹, Peter Swart², Gregor Eberli³, Matthew Buoniconti⁴, Langhorne Smith⁵ (1) University of Miami/RSMAS, Miami, FL (2) University of Miami, Miami, FL (3) University of Miami, RSMAS, Miami, FL (4) Chevron Exploration Technology Company, San Ramon, (5) New York State Museum, Albany, NY

Integrated sequence stratigraphy and geochemistry from the Mississippian Madison Limestone has implications for understanding the origins and distribution of strataform dolomite reservoirs in carbonates deposited during continental transgressions and in semi-arid climates. Dolomitized reservoirs of the Madison Limestone occur in mid-to upper ramp locations in Wyoming and correlate with the transgression of the 2nd-order supersequence, whereas outer ramp locations in Montana are composed almost entirely of non-porous limestone. The results from this study show predictable cross-ramp dolomite-&delta¹⁸O and &delta¹³C trends that are preserved in the sequence stratigraphic hierarchy of 2nd-order, composite scale and 3rd-order cycles.

Strataform dolomite has two origins and is divided accordingly into finely-crystalline (2-75 µm wide) dolomud-wackestone lithologies and medium-crystalline (75-200 µm wide) dolopack-grainstones. The &delta¹⁸O, &delta¹³C, ⁸⁷Sr/⁸⁶Sr and trace element results from finely-crystalline dolomite suggest it formed from Mississippian seawater. Geographic variation in the &delta¹³C values from transgressive and regressive systems tracts of 2nd-order, composite and 3rd-order sequences document a trend towards depleted values in a landward direction suggesting that mid-to upper ramp waters had higher alkalinity which favors dolomitization. Furthermore, the interpretation of restriction and increased salinity in mid-to-upper ramp waters is indicated by cross-ramp variation in &delta¹⁸O values which show a trend towards more enriched values in a landward direction.

The paragenesis of the medium-crystalline dolomite is somewhat different. After initial formation from seawater with normal marine &delta¹⁸O and &delta¹³C values, it was slightly modified having developed radiogenic ⁸⁷Sr/⁸⁶Sr ratios during chemical compaction diagenesis of limestone beds in the shallow to moderate burial environment. In places, there is evidence of deep burial diagenesis which significantly improved porosity by calcite dissolution.