Stromatoporoid growth forms and Devonian reef fabrics in the Upper Devonian Alexandra Reef System, Canada - Insight on the challenges of applying Devonian reef facies models

摘要

Existing facies models for Devonian reef systems can be divided into high-energy and low-energy types. A number of assumptions have been made in the development of these models and, in some cases, criteria that distinguish important aspects of the models are poorly defined. The Upper Devonian Alexandra Reef System contains a variety of reef fabrics from different depositional environments and is ideal for studying the range of environments in which stromatoporoids thrived and the facies from these different environments. A wide variety of stromatoporoid growth forms including laminar, tabular, anastamosing laminar and tabular, domal, bulbous, dendroid, expanding conical, concave-up whorled-laminar, concave-up massive tabular and platy-multicolumnar are present in the Alexandra Reef System. The whorled-laminar and massive tabular concave-up growth forms are virtually undocumented from other Devonian reefs but were common in the reef front of the Alexandra, where they thrived in a low-energy environment around and below fair-weather wave base. In contrast, high-energy parts of the reef margin were dominated by bioclastic rubble deposits with narrow ribbon-like discontinuous bodies of laminar stromatoporoid framestone. In the lagoon, laminar stromatoporoids formed steep-sided sediment-dominated bioherms in response to sea-level rise and flooding. Relying mostly on the different reef facies in the Alexandra system, a new classification scheme for Devonian reef fabrics has been developed. Devonian reef fabrics can be classified as being: (i) sediment-laden metazoan dominated; (ii) metazoan-microbial dominated (boundstone); (iii) metazoan dominated (framestone); or (iv) metazoan-marine cement dominated. Distinction of these fabrics carries important sedimentary and palaeoecological implications for reconstructing the depositional environment. With examples from the Alexandra Formation, it is demonstrated that reef facies accumulated in a range of depositional environments and that the simple observation of massive stromatoporoids with or without microbial deposits does not automatically imply a high-energy reef margin, as otherwise portrayed in a number of the existing facies models for these systems.