Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic 10Be

Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia’s continental depocentre at Lake Eyre. We apply cosmogenic 10Be exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4±6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than ~112–121ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since ~270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period ~75–55 ka Cooper Ck changed from a bedload- dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline–anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term

Tending to tradition: Dating stone arrangement maintenance in northwest Australia using optical methods

Despite the ubiquity of Aboriginal stone arrangements around the Australian continent, there is very little data-driven information about the antiquity or development of these sites. The lack of chronological controls for these sites has limited our understanding of the timing, development, and functions of stone arrangement culture. Here, we present geochronological investigations of a stone arrangement complex near the Packsaddle Valley in the Pilbara region of Western Australia using optically stimulated luminescence (OSL) dating. Sedimentary OSL samples were collected from directly beneath the boulders that comprise the stone arrangements to estimate the timing of their emplacement. The process of boulder erection by Aboriginal people involved partial removal of the topsoil, which would only have provided the sediments with brief periods of sunlight exposure. Hence, the OSL signals of our samples were not expected to have been fully bleached during boulder erection. Consequently, the single-grain OSL equivalent dose (De) data were modelled using the unlogged Minimum Age Model to identify those grains that reflect boulder emplacement. The OSL characteristics (proportion of zero-dose grains, skewness of De distribution, and Central Age Model De) from the stone arrangement OSL samples were compared to those from natural soil samples taken adjacent to the boulders, and results show that the stone arrangement samples retain a record of boulder emplacement and do not simply reflect natural soil processes. Consequently, ages for all samples were calculated to generate a chronology for boulder emplacement at the site. Boulder emplacement proceeded from ∼1,410 years ago until ∼420 years ago, with data from four recumbent boulders indicating they collapsed ∼380 years ago. The data clearly clusters into three discrete, self-consistent phases of boulder emplacement occurring at 1,360 ± 90 years ago, 800 ± 30 years ago, and 480 ± 20 years ago. We conclude that (i) the stone arrangement complex was constructed at least 1,360 years ago, (ii) the boulders tend to fall down by natural processes and, thus, the site was subsequently visited and maintained at least twice (800 and 480 years ago) when the fallen boulders were re-emplaced, and (iii) this process ended ca. 380 years ago after which the site has not been maintained. The maintenance of the stone arrangement complex over at least 900 years demonstrates a strong connection between people, the landscape, and their imprint on the landscape

Journal of applied finance : JAF

Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia’s continental depocentre at Lake Eyre. We apply cosmogenic 10Be exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4±6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than ~112–121ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since ~270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period ~75–55 ka Cooper Ck changed from a bedload- dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline–anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term