Abstract

In the shelf belt surrounding the Michigan basin, the A-1 Unit at the base of the Upper Silurian Salina Group comprises in stratigraphic succession the A-0 Carbonate, the A-1 Anhydrite and the A- l Carbonate subunits, The unit consists of six lithofacies representing three depositional environments: microlaminated mudstone and pelletal wackestone deposited in a shallow subtidal environment; planar stromatolite deposited in an intertidal environment; nodular anhydrite and indurated dolomite crusts formed on supratidal fiats and intraformational breccia and flat pebble conglomerate, formed on supratidal and intertidal flats and occasionally distributed to the nearby subtidal zone. With few exceptions, all carbonate facies consist of dolomicrite, are completely devoid of skeletal remains, contain early diagenetic nodular and acicular anhydrite and authigenic framboidal pyrite. With the overlying A-2 Anhydrite, the A-1 Unit records the superposition of four successive sabkha cycles. At the beginning of Salina time (late Wenlockian-early Ludlovian?) intensive evaporation caused the gradual withdrawal of the sea to the central part of the basin and the progressive exposure of the circumferential Guelph carbonate banks and pinnacle reefs. The basal part of the Salina Group in the basin center area was deposited in a water depth of about 800 ft. The evaporative withdrawal led to the near complete desiccation of the basin during which time potash salts were deposited. The subsequent shallow water evaporite deposits of the middle part of the Salina gradually filled the basin. These deposits successively onlapped the basin margins along a surface which represents a regional unconformity. The A-2 Carbonate was the first Salina unit which eventually transgressed the southern margin. It is most probably correlative to the Greenfield Dolomite on the Indiana-Ohio platform. The Salina B, D and F Salts were also formed in shallow water. Their deposition was contingent upon synsedimentary basin subsidence. Probably most of this subsidence took place during the relatively long time spans represented by the abundant carbonate horizons within and between the main evaporite cycles. Abundance of organic matter of algal origin and the prevalence of reducing conditions soon after deposition make ancient sabkha carbonates interbedded within thick evaporite deposits potential source beds for hydrocarbons. The environmental interpretation of the A-1 Unit provides insight into the problem of the origin of evaporite-encased reefs. What appears to be contemporaneous deposition of incompatible reef and evaporite facies is best resolved when viewed as a three-stage evolution comprising a reef-building period, an evaporative withdrawal phase and an evaporite deposition period.

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