Patterns of Permo-Triassic Sedimentation, Southeastern Australia: ABSTRACT

Abstract

Fluvial, deltaic, and marine-shelf sediments were deposited in a 100 × 200 mi, north-south-oriented trough centered around Sydney where as much as 18,000 ft of strata accumulated during the Permo-Triassic. The trough is between and was filled by debris from two major blocks of older, deformed Paleozoic rocks. The northern block consists mainly of radiolarian chert, volcanic graywacke, and mudstone, intermediate-composition volcanics, and Permian granite. The southern and western block consists mainly of quartzose sediments, silicic volcanic rocks, quartzite, granite, and Permian basaltic rocks. Short transport distances and relatively slight reworking of these diverse rock types within the basin of deposition yielded a petrographically complex sequence but one in whi h contributions from southern, western, and northern sources can be distinguished readily. Lower Permian sediments apparently were derived mainly from the southern and western blocks and, except for small deltas in the nearshore area, were principally marine-shelf deposits having an abundant fauna of thick-shelled pelecypods, brachiopods, and bryozoans. Most of the marine sandstone and siltstone deposits contain a very large proportion of lithic fragments but some, apparently representing subaqueous bars, are mainly quartzose. During the Middle Permian the sequence was covered by a rapidly prograding sedimentary sequence derived from the northern block, which until Middle Triassic time provided most of the sediments to the trough. The principal sediment complexes derived from this northern system were two fluviatile wedges. One wedge each was deposited on either side of a b sement high, and the two wedges coalesced southward into a deltaic plain facing a shallow sea. Close borehole control within the fluvial system establishes the presence of channel-bar sandstone beds 50-200 ft thick. These grade laterally into levee and lacustrine siltstone and back-swamp coal beds. The alluvial deposits grade seaward into delta-plain deposits consisting of distributary-mouth bar sandstone beds 30-50 ft thick which interfinger laterally with interdistributary bay fills. Bay fill sequences generally grade from fine- to coarse-grained upward and commonly are overlain by intensely burrowed sandstone or root-clay and coal beds. Delta-front sandstone flanks the delta-plain deposits and merges with mottled gray siltstone of the open shelf. Several episodes of delta outbuilding separated by periods of marine transgression can be delineated. The last, most widespread progradation was followed in early Middle Triassic time by an equally extensive marine transgression. Fluvio-deltaic deposits which formed during this latest episode lack coal, and the sedimentary sequence is dominated by an orthoquartzitic barrier bar-tidal delta system, locally 1,000 ft thick, which grades laterally into marine red claystone and gray tidal-flat siltstone and fine-grained sandstone. The mineral composition of the latter sediments shows an increasing quantity of basaltic detritus from the south and a concurrent reduction of sediment influx from the north. End_of_Article - Last_Page 713------------