SUBSURFACE STRATIGRAPHY OF THE DEVONIAN DUNDEE FORMATION, MICHIGAN BASIN, USA - A LOG BASED APPROACH

A distinct hard ground surface separates two disparate facies tracts in numerous Middle Devonian, Dundee Formation cores in the Michigan basin subsurface. This sharp stratigraphic contact is distinguished by scour and/or dissolution of a partially lithified surface, which is commonly bored and/or eroded, and overlain by rip up clasts. This contact probably represents both subaerial and subaqueous exposure surfaces and a subsequent period of slow sediment accumulation. Diverse supratidal to shallow marine, shoal-water carbonate facies occur below this contact, basin wide. More lithologically homogeneous, fossiliferous, mudstone-wackestone facies overlie the hard ground surface in core and are indicative of transgression to more distal, open marine conditions.

Analysis of hundreds of wireline logs throughout the basin reveals a widespread gamma ray marker (grm) coincident with the hard ground/marine flooding surface observed in core. Although present across much of the basin, the grm does not always occur apparently due to local variability of carbonate lithofacies, especially in more open marine Dundee successions in the eastern basin. A corresponding sharp decrease in porosity, inferred from lithodensity logs, commonly coincides with the grm and is typically present even when the grm is not.

Formal basin lithostratigraphy does not subdivide the Dundee Formation. This investigation supports the idea that the Rogers City Limestone, recognized in outcrop, is actually a laterally extensive unit that can be differentiated from the underlying Dundee (aka “Reed City equivalent”) Formation throughout the Michigan basin subsurface. Log-based, stratigraphic subdivision of the Rogers City - Dundee succession is important in understanding the primary depositional history, genesis, and distribution of highly productive, secondary dolomite reservoirs in the Rogers City.