Abstract: Trace Fossils in Relation to Water Depth in Permian Delta, Sydney Basin, Australia

Bruce McCarthy

Different associations of trace fossils are present in the middle Permian Gerringong Volcanics of the southern Sydney basin, eastern Australia. They are in red and green sandstones, collectively known as the Budgong Sandstone, which are interbedded with intermediate volcanic flows. Zoophycos is the main form in the deeper water sediments. These rocks also contain large, well preserved spiriferoid brachiopods, univalved mollusks, and bryozoans. Shallower water sedimentary rocks contain Rosselia, Cylindrichnus, Planolites, probable Thalassinoides, and escape burrows. Body fossils are uncommon. In contrast to older sediments, which were derived from a metamorphic and igneous terrain west of the Sydney basin, the Budgong Sandstone was derived by weathering of coeval lava flows which were xtruded from the east. The sandstones are distinctively red or green-gray, and the distribution of the colored sands is related to the thickness of the underlying flows. Red sandstones have trace fossils and sedimentary structures typical of shallow water environments, and were deposited under oxidizing conditions, whereas the green-gray sandstones were deposited in a deeper water reducing environment. Lateral color changes from red in the east to green-gray in the west, with corresponding changes in the trace-fossil associations, occur as the flows become thinner. Similarly, green-gray sandstones below the flows, and red sandstones above, reflect a decrease in depth caused by the outflow of the lavas. Although absolute depth differences are difficult to measure between the sediments con aining different associations of trace fossils, relative relations can be determined.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC