Late Quaternary stratigraphy and sedimentology of the inner part of southwest Joseph Bonaparte Gulf

Joseph Bonaparte Gulf is a large embayment on the northwestern continental margin of Australia. It is approximately 300 km east-west and 120 km north- south with a broad continental shelf to seaward. Maximum width from the sou thernmost shore of Joseph Bonaparte Gulf to the edge of the continental she lf is 560 km. Several large rivers enter the gulf along its shores. The cli mate is monsoonal, sub-humid, and cyclone-prone during the December-March w et season. A bedrock high (Sahul Rise) rims the shelf margin. The sediments within the gulf are carbonates to seaward, grading into clastics inshore. A seaward-thinning wedge of highstand muds dominates the sediments of the i nner shelf of Joseph Bonaparte Gulf. Mud banks up to 15 m thick have develo ped inshore. Coarse-grained sand ridges up to 15 m high are found off the m outh of the Ord River. These overlie an Upper Pleistocene transgressive lag of mixed carbonate and gravelly siliciclastic sand. Four drowned strand-li nes are present on the inner shelf at depths of 20, 25, 38 and 30 m below d atum. These are interpreted as having formed during stillstands in the Late Pleistocene transgression. Older strandlines at great depths are inferred as having formed during the fall in sea-level following the last highstand. For the most part the Upper Pleistocene-Holocene marine sediments overlie an erosion surface cut into older Pleistocene sediments. Incised valleys cu t into this erosion surface are up to 5 km wide and have a relief of at lea st 20 m. The largest valley is that cut by the Ord River. Upper Pleistocene sediments deposited in the incised valleys include interpreted lowstand fl uvial gravels, early transgressive channel sands and floodplain silts, and late transgressive estuarine sands and gravels. Older Pleistocene sediments are inferred to have been deposited before and during the 120 ka highstand (isotope stage 5). They consist of sandy calcarenites deposited in high-en ergy tide-dominated shelf environments. Still older shelf and valley-fill s ediments underlie these. The contrast between the Holocene muddy elastic se diments and the sandy carbonates deposited by the 120 ka highstand suggests that either the climate was more arid in the past, with less fluvial trans port, or that mud was more effectively trapped in estuaries, allowing devel opment of carbonate depositional environments inshore.