Modes of holocene coastal progradation : Gulf of Carpentaria

Abstract

Chenier and beach-ridge plains are both examples of prograded coasts, and various models have been proposed for their development• In the case of the chenier plain, the Louisiana coast has provided the prime example where deposition is linked to variation in sediment supply from the Mississippi delta. The study of beach-ridge plains has been more widespread, although Galveston Island and the Nayarit (Mexico) examples have been important in the understanding of such coasts. In most examples of prograded coasts, relative sea level and climatic change have been suggested as important factors controlling coastal deposition• Holocene chenier and beach-ridge plains in the Gulf of Carpentaria, a shallow epi-continental sea in northern Australia, exhibit a wide range of depositional environments distributed over broad coastal plains. The depositional environments produce distinctive facies which are preserved in the stratigraphy of the prograded coast. Some of these facies have a well defined relationship to sea level, and also contain datable shells. Therefore, a record of upper Holocene sea level changes remain preserved in the morphostratigraphic record of these prograded plains. Relative sea level change during the last 6000 radiocarbon years has been different for the chenier and beach-ridge plains. The chenier plain, which occurs on the southern margin of the Gulf, shows a relative fall of 2.4m since the mid-Holocene. In contrast, the beach-ridge plain, on the eastern margin of the Gulf, has experienced only l.5m of fall during the same period. These differences appear attributable to hydro-isostatic response to water loading of the continental shalf during and after the Postglacial Marine Transgression. Coastal deposition since the mid-Holocene has occurred in an episodic manner on both the chenier and beach-ridge plains. The modes of deposition are different with mudflats constructed on the chenier plain and beach ridges developed on the beach-ridge plain during periods of high terrigenous input. During periods of lower terrigenous input, cheniers are constructed on the chenier plain whilst a period of non-deposition occurs on the beach-ridge plain· In the Gulf of Carpentaria, such a variation in sediment supply has occurred at least four times since 6000 years B.P. In areas outside the Gulf of Carpentaria, similar episodic phenomenon can be shown in areas such as Broad Sound, Queensland, where sufficient radiocarbon dates have been obtained on coastal sediments. Climatic change in the form of variation in rainfall. intensity (or storminess) is suggested as the forcing function in this model. At present, insufficient data on upper Holocene climatic change exist within

Australia for regional correlation of such events. However, there appears to be a correlation between progradation of the beach-ridge plain during the period 6000-4800 years B.P. and increased cyclogenesis accompanied by a pluvial in eastern and southern Australia· On a global scale, glacial advances in the northern hemisphere also appear to be in phase with this increase of sediment supply to the Gulf of Carpentaria· Furthermore, there is a general agreement between a wetter period in southern Australia after 2000 years B.P. and a long stage of beach-ridge. progradation in the Gulf of Carpentaria during the time 2300-600 years B. P. Show more