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The Ruff formation, the informal A-1 carbonate unit of the lower Salina Group, underlies the A-1 evaporite and overlies the A-2 evaporite in the subsurface of the Michigan basin. It basically is brown to dark grayish brown, fetid, unfossiliferous carbonate mudstone that ranges from limestone to dolomitic limestone and dolomite.

The Ruff formation can be divided into five lithofacies, based upon associations of subordinate lithologie constituents: microlaminated mudstone, leached mudstone, pelletal wackestone-packstone, thinly laminated mudstone, and nodular anhydrite. Deposition primarily was in shallow subtidal to infratidal, low-energy, hypersaline reducing environments, and secondarily on ephemeral tidal to supratidal flats.

In southeastern Michigan, the basal portion of the Ruff formation is intertidal-flat algal-laminated mudstone that was transgressive over the A-1 evaporite, a contiguous supratidal deposit. The remaining Ruff formation primarily is subtidal to infratidal microlaminated mudstone that grades into infratidal to intertidal pelletal wackestone and packstone peripheral to the Niagaran reefs. Deposition of these lithofacies was interrupted circumferential to the reefs by development of ephemeral tidal flats in which nodular anhydrite was deposited. The uppermost Ruff formation is the subtidal facies-equivalent of the supratidal deposit of the lowermost A-2 anhydrite.

Within the Ruff formation, differences in lithofacies distribution among various areas of the fore-slope-shelf region, and other stratigraphie variations {Mantek, 1973) are responses to different tectonic regimes within the basin. These variations have resulted in conflicting interpretations of the “reef-evaporite” relation in the Michigan basin, depending on the locations of studies.

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