Sequence stratigraphic analysis of early and middle Triassic alluvial and estuarine fades in the Sydney Basin, Australia

Abstract

Early to Middle Triassic sediments of the Sydney Basin (up to 1350 m thick) have hitherto been regarded as an alluvial and lacustrine succession unaffected by sea‐level induced base‐level changes. Here it is proposed that the dominantly alluvial sediments were deposited on an alluvial plain which graded seawards to an estuarine coastal plain. Hence, the succession had hydrological connection with the sea and, therefore, deposition may have been controlled by changes in relative sea‐level. A sequence stratigraphic framework, established in the downstream alluvial plain/coastal plain transition, can be carried landward to varying degrees, even to the basin margins. The intercalation of lowstand, coarse‐grained alluvial‐plain sediments with transgressive and highstand fine‐grained, coastal‐plain sediments was influenced by high‐frequency, 4th‐order changes in relative sea‐level which were responsible for the overall depositional character of the succession. These cycles constitute seven 3rd‐order sequences. During a low‐frequency, 2nd‐order highstand in the Early Triassic, dominantly fine‐grained coastal plain sediments were deposited, whereas during a lowstand in the Middle Triassic, dominantly coarse‐grained alluvial‐plain sediments were deposited. Interbedded alluvial and estuarine sediments were deposited in the more rapidly subsiding northeastern and central parts of the basin. Landwards, in the direction of slower subsidence towards the western basin margin, shaly coastal plain intervals thinned to zero and coarse‐grained alluvial‐plain sediments amalgamated vertically to form thick sandstone successions. Alluvial‐plain sediments are characterised by upward‐fining fluvial sandstones which intercalate seawards with grey, green, and red floodplain/estuarine shales. Further seawards these sediments grade to finer grained sandstones and multicoloured shales in upward‐coarsening estuarine bayhead deltas and central bay facies. Extensive redbed deposition, considered to indicate oligotrophy, commenced during the second 3rd‐order sequence above the Permian/Triassic boundary, continued during the deposition of three sequences, and finished at the end of the Early Triassic.