Abstract
Fluvial, deltaic, and marine-shelf sediments were deposited in a 100- by 200-mi, north-south oriented trough centered around Sydney, where up to 12,000 ft accumulated during the Permo-Triassic. A basin analysis was made using depositional models based on rock outcrops and closely spaced boreholes. These depositional models were used to form a best fit-of-environment correlation along three lines of section through the basin using 21 deep boreholes as control. Patterns of sedimentation can be delineated from the correlation charts at several intervals through the sequence. Lower Permian sediments were derived mainly from the north and, except for minor delta-plain facies in the nearshore area, were principally deposits of a marine shelf and delta front with an abundant fauna of thick-shelled pelecypods, brachiopods, and bryozoans. Most of these marine sandstones and siltstones contain predominantly lithic fragments but some, apparently representing subaqueous bars, are distinctly quartzose. From the Middle Permian into the Early Triassic, the sediments were deposited in a southward prograding delta and were derived mainly from the north. Two fluvial wedges formed which merged southward into a deltaic plain facing a shallow sea. Close borehole control within the fluvial system establishes channel-bar sandstones and conglomerates 50–200 ft thick grading laterally into levee and lacustrine silt-stones and back-swamp coal beds. Alluvial deposits grade seaward into delta-plain sediments consisting of distributary-mouth bar sandstones 30–50 ft thick which interfinger laterally with interdistributary bay sequences. These bay sequences commonly grade from fine to coarse upward and commonly are capped by intensely burrowed sandstones or root claystones and coal beds. Delta-front sandstones flank the delta-plain deposits and merge with mottled gray siltstones of the open shelf. Several episodes of delta outbuilding separated by periods of marine transgression can be delineated with the last most widespread progradation followed in the Triassic by an equally extensive transgression of shallow marine water. Fluviodeltaic deposits in this latest episode lack coal, and the sedimentary sequence is dominated by an orfhoquartzitic barrier bar-tidal delta system locally 1,000 ft thick which grades laterally into marine red claystones and gray tidal-flat siltstones and fine sandstones.
