ABSTRACT

A reef complex in the Cairn Formation of Late Devonian (Frasnian) age is exposed in the Rocky Mountain Front Ranges near Jasper, Alberta.

Study of this reef indicates a complex array of sediment-faunal types from which a number of biofacies can be recognized. These are: 1. Unstable Sand facies, 2. Reef facies, 3. Bulbous-Irregular Stromatoporoid facies, 4. Encrusting facies, 5. Algal facies, and 6. Basinal facies. Each of these biofacies has faunal and sedimentary characteristics which reflect a particular set of environmental conditions, and from which these conditions may to some extent be deduced.

Observations on the reef morphology, reef-to-basin transitions, and fauna indicate that bathymetric and ecologic zonation across the reef and into the basin were minimal at first, but increased progressively throughout Cairn time.

On the other hand, probably because of changing rates of deposition, several changes in depth and environment of the reef surface apparently took place during its development. Four main phases can be recognized. Reef growth began in water depths only slightly shallower than the surrounding basin, but gradual shallowing by more rapid reef growth took place during the Platform Phase. The succeeding short-lived Transgressive Phase saw a relative deepening of the waters above the now-established reef, and basinal sediments encroached to a large extent over reefal sediments. The Lagoonal Phase which followed was of considerably longer duration, and from the beginning certain marginal parts of the reef grew high enough above wave base to effect some restriction of the wave and current forces, resulting in somewhat lower energy conditions in the developing lagoon behind it.

Finally, in the Mature Phase, some relative deepening of the sea with respect to the reef occurred, and coalescing patch reefs began to fill the lagoon as highenergy currents, now unhindered, swept across it.