ENVIRONMENTAL DISTRIBUTION OF BOTTOM-DWELLING ANIMALS IN A PERMIAN FRESHWATER SYSTEM, ANTARCTICA

The amount of bioturbation was used as a proxy for infaunal activity in eight fluvial and lacustrine facies of the Weller Coal Measures, exposed in the Transantarctic Mountains within 200 km of McMurdo Station on Ross Island. Bioturbation was assessed semi-quantitatively on bedding plane and vertical exposures using pattern recognition methods (Droser and Bottjer, 1986; Miller and Smail, 1997); the data set consists of 4862 discrete observations. Fluvial channel, fluvial crevasse splay, lacustrine slump/subaqueous channel, and lacustrine sand lobe facies show no evidence of benthic life; these facies dominate the formation. Bioturbation is most abundant, although not ubiquitous, in the fluvial abandoned channel facies and lacustrine prodelta facies, where 67% and 50%, respectively, of observation indicate some burrowing. Bioturbation is present, but less common, in fluvial point bar facies (16% of observations) and delta mouth bar facies (30%). Rate of sedimentation and frequency of sedimentation events controlled the establishment of benthic infaunal communities, as reflected by bioturbation. In the fluvial abandoned channel and prodelta facies, influxes of sand were sufficiently rare and minor to allow colonization. In unburrowed facies, presence of soft-sediment deformation features, climbing ripple lamination, and paucity of clay drapes suggest rapid and relatively continuous deposition. Disparity of bioturbation within the delta mouth bar facies at different localities -- 100% with some bioturbation at one locality vs. 0% with any bioturbation at two others -- probably reflects differences in rates of sedimentation related to position relative to the focus of deposition. Although it has been established that ecospace use and rate of sediment overturn was less in Paleozoic freshwater aquatic environments than in contemporaneous marine environments, this study demonstrates that similar factors controlled the distribution of the extant benthic animals regardless of salinity.