Depositional and Stratigraphic Features of Littoral Algal Bioherms and Basin Facies: ABSTRACT

Abstract

The Devonian Nubrigyn Reef Complex, Australia, consists of a littoral and a sublittoral facies. The former is composed of numerous abruptly intertongued lithologies: impure algal detritus, andesite pebble lenses, Devonian erosional remnants of andesite lava flows, and over 300 pure algal atoll-like bioherms (some fringing the andesite hills) and biostromes with lagoonal codiaceae-calcarenite. Numerous depositional and diagenetic features are suggestive of a turbulent intertidal environment. The sublittoral deposits consist of uniformly-bedded detrital algal limestones with interbedded claystones. Most of the former are graded and are believed to be turbidites. There are scattered, thick, lens-like End_Page 363------------------------------ dump--deposits composed of an unsorted mixture of clay and limestone fragments. These are thought to be fluxoturbidites or slumps. Intermittent subsidence of the Nubrigyn shelf caused periodic transgression of basinal sediments over the littoral accumulations with the result that a complex interfingering of both facies developed near the former shoreline. This seems to indicate that the eastern limit of the reefs was not determined merely by a sudden increase in gradient of the shelf. The intimate association of cross-bedding and grading in the littoral units suggests that grading can form under relatively shallow-water conditions, and one should be circumspect in using it as a depth indicator. Application of the Law of Minimum in Environmental Reconstruction is more reliable in establishing conditions of sedimentation. Inasmuch as the basinal carbonate detritus is composed of material derived from the littoral bioherms, the syngenetic chemical composition of the material is an unreliable parameter for the discrimination between shallow- and deep-water limestones. Only if characteristic diagenetic differentiation occurred may chemical composition be useful in this regard. End_of_Article - Last_Page 364------------