Bituminous-laminated (or "varvitic") carbonates, which grade upward into laminated anhydrites, are of particular interest to the petroleum geologist, because the carbonates may act as source, carrier, and reservoir rocks whereas the overlying anhydrites form impermeable caprocks. Commonly, such sediments exceed 10 ft in thickness and are traceable over thousands of square miles, but have been interpreted as of upper intertidal (algal mat) and supratidal (sabkha) origin.

Carbonate-anhydrite laminites occur as a basin facies of the Middle Devonian Winnipegosis Formation in Saskatchewan and are located on the basin floor between large carbonate banks. The laminites grade downward into a thin (1-10 ft) unit of black, highly bituminous, laminated mudstone containing a pelagic fauna. Near carbonate banks this mudstone thickens and includes carbonate beds interpreted as turbidites or mud-slide

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deposits. The overlying laminites also are interpreted as turbidites (or deposits of salinity-driven bottom currents). Calcium carbonate and sulfate precipitated over shoals (the former banks) were transported basinward. Initially, basin-floor waters were undersaturated with calcium sulfate and only carbonate laminites persisted. With increasing basin-floor hypersalinity the sulfate persisted and carbonate-anhydrite (later, anhydrite) laminites formed.

Identical carbonate-anhydrite laminites occur within the Ordovician Red River, Stony Mountain and Stonewall Formations, forming the upper members of carbonate-evaporite cycles. Starved basin bituminous mudstones are absent or poorly developed. The thickness, lateral persistence (across and along depositional strike), and the absence of structures indicating emergence suggest that Ordovician laminites did not form in intertidal-supratidal environments.

The Ordovician laminites are believed to have accumulated within local depressions (perhaps quite shallow) which collect hypersaline brines. Brines were generated on neighboring platforms, sites of essentially no net deposition. Carbonates and gypsum, precipitated on the platform, were episodically introduced into depressions by high-energy events (storms, turbidite? flows).

In both Devonian and Ordovician laminites, mass-movement of sediment downslope, the incompetency of sulfate laminae, partial or extensive replacement of carbonate by anhydrite, and possible dehydration of primary gypsum to anhydrite caused local deformation and disruption of laminated sediments, converting them to rocks superficially resembling those formed in sabkha environments.

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