Abstract

Organic reefs occur at several stratigraphic levels in the Devonian of western Canada. Economically most important are the biohermal build-ups that grew on biostromal platforms of considerably greater horizontal extent. Some reefs never grew beyond the biostromal stage; conversely, some bioherms do not have a recognizable basal platform.

The biohermal reefs lend themselves best to morphological analysis. Several groups of reefs are distinguished within each formation, each presenting different morphological characteristics. These characteristics depend on such external conditions as marine currents, wind direction, water depth, food supply, rate of subsidence temperature and salinity of the water, and concentration of clastic sediments, and on such internal factors as the reef-building organisms involved and reef geochemistry. The main considerations in analyzing and predicting reef morphology are thus hydrological climatological, biological and tectonic.

Hydrological influences, mainly in the nature of waves and cur rents, are responsible for many aspects of the reef form. Like modern reefs, ancient reefs include fringing reefs, barrier (ribbon reefs, atolls, and platform (table) reefs. Most Devonian so-called “atoll” reefs are in reality “pseudo-atolls” that owe their elevated rims to post-depositional compaction of the fine-grained sediments of their cores.

Linear trends may probably be traced to deep-seated faulting. Such faulting might have caused differences in sea-bottom elevation prior to Devonian deposition, thus creating certain area where water depths were within the limits that encouraged growth of reef-building organisms, flanked by areas either too deep or too shallow for reef growth. Some part of the morphological surface of certain reefs as seen at present may have been caused by structural cross-folding unrelated to the reef-building process.

Phenomena of hydrological origin exhibited by ancient reefs include reef channels, horns, inter-channel islands, surge channels, “buttresses” and cross-reef channels. Of interest also is the angle of the reef slope and its interruption by “terraces”. The top of the reef may have been exposed to subaerial influences, both erosional and depositional (dunes). Horns and buttresses may be interpreted to provide data concerning the directions of marine currents and winds. Inter-channel islands appear to be a function of the width of reef channels.