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

Abstract

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 southernmost shore of Joseph Bonaparte Gulf to the edge of the continental shelf is 560 km. Several large rivers enter the gulf along its shores. The climate is monsoonal, sub‐humid, and cyclone‐prone during the December–March wet 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 inner shelf of Joseph Bonaparte Gulf. Mud banks up to 15 m thick have developed inshore. Coarse‐grained sand ridges up to 15 m high are found off the mouth of the Ord River. These overlie an Upper Pleistocene transgressive lag of mixed carbonate and gravelly siliciclastic sand. Four drowned strandlines are present on the inner shelf at depths of 20, 25, 28 and 30 m below datum. 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 cut into this erosion surface are up to 5 km wide and have a relief of at least 20 m. The largest valley is that cut by the Ord River. Upper Pleistocene sediments deposited in the incised valleys include interpreted lowstand fluvial 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‐energy tide‐dominated shelf environments. Still older shelf and valley‐fill sediments underlie these. The contrast between the Holocene muddy clastic sediments and the sandy carbonates deposited by the 120 ka highstand suggests that either the climate was more arid in the past, with less fluvial transport, or that mud was more effectively trapped in estuaries, allowing development of carbonate depositional environments inshore.

A Holocene record of coastal landscape dynamics in the eastern Kimberley region, Australia

“…Marine sediment records from the Joseph Bonaparte Gulf (JBG, Fig. 1) imply that postglacial sea-level rise reached modern datum either around 6 or 4.7k cal a BP (Clarke and Ringis, 2000;Yokoyama et al, 2000;Lewis et al, 2013). Stratigraphic and geomorphological data from the region have been interpreted as indicators of a 1-2-m highstand during the mid-Holocene (Jennings, 1975;Lees, 1992;Lessa and Masselink, 2006).…”

“…To date, evidence for a mid-Holocene 'big swamp'-phase in the north-west is sparse and consists of poorly preserved pollen, unspecified 'mangrove muds' and buried mangrove trees (Jennings, 1975;Lees, 1992;Clarke and Ringis, 2000;Clarke et al, 2001). Enhanced summer monsoon rainfall between 7.5 and 4.5k cal a BP probably supplied more freshwater and sediment to the coast, conditions that may have facilitated widespread mangrove coverage (Denniston et al, 2013).…”

Holocene freshwater wetland and mangrove dynamics in the eastern Kimberley, Australia

“…During the early parts of the middle Holocene, when postglacial sea level was still rising (Clarke and Ringis, ; Yokoyama et al ., ; Lewis et al ., ), the pollen assemblages of all cores show that a highly biodiverse mangrove forest was widespread throughout the study area. At the upstream location of KR02, mangroves grew on‐site whereas at KR01, located in a downstream position along a small tidal tributary of the Cambridge Gulf West Arm today, an upper intertidal/back mangrove setting prevailed.…”

“…remained and an increase in salt marsh and grass pollen implies that an open, hypersaline environment adjacent to the very landward edge of the mangrove forest had established. The middle Holocene mangrove contraction observed at the King River sites is probbaly reflecting a reduction in accommodation space under sea‐level stabilization at modern datum around 6k cal a BP (Clarke and Ringis, ; Yokoyama et al ., ; Lewis et al ., ; Solihuddin et al ., ). In addition, a reduction in summer monsoon strength between 7.0 and 5.5k cal a BP probably led to a decline in freshwater runoff and thus a reduction in mangrove habitat in the region (Denniston et al ., ).…”

Physical sedimentary controls on subtropical coastal and shelf sedimentary systems: Initial application in conceptual models and computer visualizations to support archaeology

“…The modern bathymetry indicates that the tidal channels, which contain very large mobile sandy bedforms, extend well out into the subtidal zones to depths of 30 m and beyond, where they have a greater silt component (Clarke & Ringis, ; Przeslawski et al., ). The system's present and past dynamics (Clarke & Ringis, ; Lees, ,b) indicate that for this section of coastline at least, the tidally dominated regime and the sedimentary environments have probably been similar for the last 8–5 ka. Therefore, there has been thousands of years of strong reworking of inner‐shelf and coastal sediments and hence of any associated archaeology.…”