Abstract

Keg River reefs are primarily products of pervasive cementation of mechanically deposited skeletal debris. Organic binding was limited mainly to intervals of sediment stabilized by algal mats. Significant amounts of true skeletal boundstone are lacking.

During Upper Keg River time, intermittent episodes of super-salinity associated with salinity stratification occurred. Accordingly, “normal” marine, skeletal calcirudites and calcarenites and algal mats are intercalated with virtually unfossiliferous evaporitic carbonates dominated by lime mud and containing syngenetic anhydrite. The evaporitic sediments overlie pronounced diastem surfaces and occur as internal sediment in the underlying marine carbonates.

Both depositional and intrafossil porosity were high. However, composition, porosity and rock fabric of the Rainbow buildups were modified quite early; principal diagenetic processes being cementation and dolomitization. During periods of normal marine conditions, environments of high agitation of bottom waters or relatively steep topographic gradients were conducive to precipitation of copious amounts of presumed high magnesium calcite and aragonite pore cement. Cementation resulted in the host carbonate sediments becoming solidly lithified to form wave resistant rocks. Temporary subaerial exposure produced little or no meteoric calcite, rather erosion and intensive fracturing of underlying sediment occurred. The fractures were subsequently filled with internal sediment of the evaporite facies.

In contrast to lithification of the marine lithofacies by cementation, the evaporitic carbonates became lithified through early dolomitization. Dolomite replacement occurred mainly during super-saline periods at subtidal to supratidal sites. Evaporitic carbonates were selectively and completely replaced by fine crystalline dolomite. The same dolomitizing fluids spread down into the underlying marine carbonates resulting in replacement by medium to coarse crystalline dolomite. Further precipitation of dolomite and anhydrite cements in pores offset any increases in porosity by leaching in the evaporitic carbonate muds.

Late diagenetic processes after effective burial have not been as important to reservoir development as the above early processes, however, they include: (1) conversion of all metastable calcium carbonate to low magnesium calcite; (2) minor cementation and replacement by anhydrite, calcite and dolomite; (3) moderate fracturing; (4) weak stylolitization; (5) deposition of much carbonaceous material in pores.

Some of the diagenetic information seen in the cores of the Rainbow reefs may assist in establishing a better understanding of the petrology and geology of cementation reefs in general.