Some Questions about the Development of Upper Devonian Carbonate Buildups (Reefs), Western Canada.

Abstract

ABSTRACT Some of the key difficulties concerning the development of Upper Devonian carbonate buildups ('reefs') in the Alberta Basin are: recognizing facies and setting up members and formations that reflect as closely as possible the primary depositional patterns. establishing local and regional time markers. determining the causes of carbonate buildup initiation and localization. duration and significance of diastems during carbonate buildup development. determining the factors responsible for termination of the carbonate buildups. style of basin sedimentation and filling, including factors controlling the Duvernay-Perdrix anoxic basin. In order to help resolve these problems and arrive at suitable buildup and basin models, information from both the subsurface and Rocky Mountain outcrops must be utilized in conjunction with data and concepts derived from Holocene, Pleistocene and other Phanerozoic reefs. The Swan Hills and Leduc carbonate buildups of central and western Alberta have a similar pattern of development: beginning with a shallow-water carbonate platform often above an unconformity or abrupt shift in facies, followed by biohermal and reef complex growth on local carbonate shoals. This pattern represents a major transgressive sequence starting with a slow rise of sea level, followed by a more rapid rise, and finally a period of sea-level stability or still-stand. Carbonate sedimentation kept pace with sea-level changes and probably exposure was frequent, resulting in more gaps than rock record within individual buildups. The main factor that controlled carbonate buildup location and development appears to be sea-level fluctuations. The major changes in sea level seem to be basin wide and probably are epeirogenic or eustatic in origin. Later the amount of terrigenous and carbonate sediment deposited in the basin, controlled by both rates of supply and sea-level changes, also had a major influence on the location and development of the carbonate buildups as irregular carbonate shelf sequences of limited and irregular distribution extended successively basinward. The dominant north-northeast-south-southwest trends of carbonate buildup and platform margins suggest the presence of linear flexure zones across which slight differential subsidence probably occurred in the underlying crystalline basement. End_Page 315------------------------