Morphology , Faunas and Genesis of Ordovician

Brett, C. E. and Brookfield, M. E., 1984. Morphology, faunas and genesis of Ordovician hardgrounds from southern Ontario, Canada. Palaeogeogr., PaIaeoclirnatol., PalaeoecoI., 46: 233-290. We have used associations of different rnicrofacies to define facies (or microfaties associations) which fom reasonably welldefined sequences, which we infer, from amlogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scde topographic differentiation into shoal, slope and basinal environments. Shoal environments are characterized by typically cross-bedded, well-sorted b iock t i c sands, with intershoal areas consisting oP interbedded bioclastic sands and heavily bioturbated f iner-grained carbonates. Slope and "basinal" environments are typically represented by "proximal" and "distal" cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water elastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed ta migration of channels on the fans and to the effect of funnelling of storm surges dawn the channels, Mthough hardground morphology and faunas are mostly related to local effects rmch as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renerved sedirnentation Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biota . Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flaw fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assernklages formed after redeposition. 'Present address: Centre of Advanced Study in Geology, Panjab University, Chandigarh 160014 (India). 0031-0182/841$03.00