The Winnipegosis carbonates were deposited in three episodes during the initial transgressive-regressive cycle of the Kaskaskia sequence. The first episode is represented by a normal, open marine environment. During the second episode, the basin differentiated into two regions: the shallower shelves and a bathymetrically “deeper” basin. This deeper basin was comprised of a restricted environment with large pinnacle reefs. Shallow marine, patch reef, lagoon, and tidal flat environments became established in the shelf region. During the third episode, supratidal stromatolites and lime mudstones were deposited on top of the shelf and pinnacle reefs while stromotolites, dolomites, and starved-basin anhydrites accumulated in the inter-reef areas. The pinnacle reefs contain four Hthofacies: the Stromatoporoid, Tabulate Coral Boundstone; the Codiacean Algae, Calcisphere, Peloid Packstone; the Porous Dolomite; and the Pisolite Dolomite Lithofacies. The patch reef is composed of the Stromatoporoid, Tabulate Coral Boundstone Lithofacies.
The pinnacle reefs exhibit a three-stage successional biofacies development: (I) pelmatozoan packstones which represent an intrinsic preparation of the substratum for reef growth; (II) establishment of reef-building domal and dendroidal tabulate corals; and (III) alternating zones of more massive thamnoporid corals and stromatoporoids. Stromatoporoid-algal boundstones are also common. During Stage III a lateral zonation also developed with higher energy environments on an outer reef rim and a lower energy backreef lagoon. The backreef deposits are composed of codiacean algae, calcispheres, peloids and Amphipora.
Patch reefs show a four-stage, shallowing-upward succession: (I) pelmatozoan skeletal sand shoals; (II) growth of domal and branching tabulate corals; (III) increasing size, number of genera, and growth forms of the reef building taxa, with growth shapes indicative of a higher energy, above mean wave-base environment predominating; and (IV) large massive, hemispherical stromatoporoids which dominate in turbulent high energy environments.
Diagenetic fabrics in the pinnacle reefs include partial to extensive dolomitization. The extent of dolomitization and the destruction of the allochems and textures decreases downward within the reef core. The diagenesis of the patch reef is minor in comparison with that of the pinnacle reef. Diagenetic events include isopachous calcite and equant calcite spar cements, stylolites, local dolomitization of allochems and the mudstone matrix and rare bladed or blocky anhydrite.
