Off-Reef Salina Deposition (Silurian), Southern Michigan Basin Implications for Reef Genesis
J. Frederick Sarg, 1982. "Off-Reef Salina Deposition (Silurian), Southern Michigan Basin Implications for Reef Genesis", Depositional and Diagenetic Spectra of Evaporites - A Core Workshop, C. Robertson Handford, Robert G. Loucks, Graham R. Davies
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The basinal off-reef carbonate and evaporite rocks of the Niagara-Lower Salina sequence in the southern Michigan Basin were deposited in a subaqueous mesosaline to hypersaline environment laterally adjacent to growing pinnacle reefs. The basinal carbonate rocks reflect a progressive restriction of the basin during deposition and include three lithofacies: 1) a lower platform crinoid wackestone facies on which developed the basin slope pinnacle reefs, and which is overlain by two mesosaline facies; 2) a dominant anhydritic mudstone facies; and 3) a reef-proximal peloid/"rod" packstone facies. Three anhydrite lithofacies occur interbedded with the basinal carbonate facies and include: 1) bedded massive to mosaic anhydrite (most abundant); 2) mosaic anhydrite; and 3) allochthonous dolomitic anhydrite breccias. Bedded nodular mosaic anhydrite (A-2 Evaporite) was deposited over reef crests in a supratidal sabkha environment following cessation of reef growth.
Stratal correlation surfaces and facies association divide the Niagara-Lower Salina rocks into four depositional units deposited during a relative highstand of sea level. The early highstand phase is characterized by: 1) an initial platform stage followed by; 2) a period of major pinnacle reef growth (coral-crinoid mound) surrounded by deeper mesosaline/hypersaline water; and 3) an abrupt shoaling upward phase (stromatoporoid-algal boundstone and crestal stromatolite stages). Hypersaline hiatuses in reef growth ended each phase of reef growth. A late highstand evaporite stage (A-2 Evaporite) ended reef growth and onlapped the basin margin.
Excellent porosity (5 to 15 percent) is present throughout the coral-crinoid, stromatoporoid-algal boundstone, and stromatolite stages of reef growth. Dolomitization has resulted in intercrystalline and micro-vug porosity with good permeability. Solution of skeletal fragments has created moldic porosity and remnant interparticle porosity has been enlarged by solution. Episodic evaporite replacement and solution during the late shoaling stages of reef growth have resulted in significant secondary vug and fracture porosity.
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Geologists do not often have an opportunity to examine evaporites, whether in outcrops as badly weathered exposures, or in the subsurface, where evaporites are not as frequently cored as other rock types. Nevertheless, evaporites are important economically (mineral resource, seals for hydrocarbons, disposal sites for radioactive wastes, etc.) and geologists are, by necessity, becoming more aware of their origins. This workshop is intended to increase awareness and provide useful information for comparison to other evaporites, all of which should eventually benefit geologists in their efforts to understand the depositional and diagenetic spectra of evaporites.