ABSTRACT

Interior sediments of the Kaybob reef can be differentiated into two major lithofacies: massive and fenestral. The massive lithofacies was deposited in a low-energy, shallow-subtidal and possibly lower-intertidal environment characterized by poor circulation. Sediments of the fenestral lithofacies are of a low-energy and upper-intertidal to supratidal origin. The lithofacies are cyclical, a complete cycle consisting of a subtidal to lower-intertidal unit overlain by an upper-intertidal to supratidal unit that documents upward shoaling conditions. Successive cycles are separated by discontinuities.

A hypothesis for cyclicity must satisfactorily account for the large number of cycles present (twenty cycles in 26 m of section) and the localization of cyclicity in the reef interior. This can be explained by variations in the rate of carbonate production. Under conditions of steady subsidence and stable sea level, optimum carbonate production occurred in the subtidal areas of the reef interior; subsequently, carbonate sedimentation outpaced subsidence. Progressive infilling of the subtidal areas occurred by the outward progradation of intertidal to supratidal sediments from topographic highs. Subtidal areas were, therefore, gradually replaced by intertidal to supratidal areas. The result was a gradual decrease in the extent of the subtidal areas, increasing restriction of water circulation, and a net reduction of carbonate sediment production by organisms. With decreased carbonate production, subsidence outpaced sedimentation and subtidal conditions were brought back to supratidal and intertidal areas. Repetition of this sequence produced cyclic sediments.