PRELIMINARY SEDIMENTOLOGICAL AND STRATIGRAPHIC ANALYSIS OF THE BASAL GLACIGENIC WYNYARD FORMATION (WYNYARD, TASMANIA, AUSTRALIA)

The Wynyard Fm. (Permo-Carboniferous; Wynyard coastline, Tasmania) was deposited during the Late Paleozoic Ice Age (LPIA) within the South Polar Circle. The emerging view of the LPIA attributes glaciation to regional-scale ice caps and ice sheets, rather than a single Gondwanan ice sheet. Therefore, characterizing regional glaciers is crucial to understanding the climatic significance of LPIA glaciation.

The Wynyard Fm. consists primarily of massive diamictites with m-scale interbeds of sandstones, clast-supported conglomerates, and lonestone-bearing, laminated mudrocks. Since the Wynyard Fm. is unequivocally glacigenic, most previous work focused on describing ice flow directions, clast provenance, and age. This focus has led to depositional processes (subglacial, glaciomarine, and/or mass transport processes) being proposed without a complete, detailed examination of the section’s sedimentology and stratigraphy. Hypothesized sources for Tasmanian glaciers include ice caps nucleated on regional highs (based on clast composition) or an ice sheet sourced in North Victoria Land (Antarctica; based on flow directions).

We present preliminary results from an analysis of the basal 400 m of the Wynyard Fm. These strata rest on an angular unconformity with the Neoproterozoic Burnie Fm., which, contrary to previous reports, bears no indication of subglacial erosion. The section is primarily massive to crudely-bedded sandy, clast-rich diamictites that frequently include discontinuous, 10 cm – 2 m thick bodies of coarse sandstones and conglomerates, suggestive of an ice-proximal environment. Mass transport deposits within the diamictites have a mean transport direction of 086°. Three boulder pavements with aligned striations, indicative of grounded ice, were identified throughout the measured section (mean striae trend 006°). One 16 m thick portion of the section contains laterally continuous cross bedded gravels, asymmetrically rippled sands, and thinly laminated to thinly bedded, alternating silt and clay with outsized clasts (mean paleocurrent direction of 000°). This succession represents at least one advance/ retreat cycle during a much longer “Wynyard” glaciation.

Detrital zircon provenance analysis will be used to help establish the source area(s) of sands within the basal Wynyard Fm.