School of Earth & Environment5al Sciences
Conacher, Davy R., A micropalaeontological and stratigraphic study of Benandarah and Cumbralaway Creeks, Durras Lake, Southeast Australia, BSci Hons, School of Earth & Environment5al Sciences, University of Wollongong, 2016.
Microfossil are indicators of palaeoenvironmental changes especially in estuary settings such as southeast Australia. Foraminifers, ostracods and charophytes were analysed in Cumbralaway Creek delta as part of a larger stratigraphic study of the previously unstudied bayhead delta regions at Durras Lake.
A total of 12 cores were taken in the bayhead delta region, including 7 in Cumbralaway Creek and 5 in Benandarah Creek. The cores were divided into four major categories including the prodelta (DLC2), active delta front (DLC 1 and DLB4), lower delta front (DLC4, DLC3, DLSC1,DLSC2, DLSC3, DLB1, DLB3 DB5) and upper delta plain/alluvial basin (DLB2). Each core was analysed stratigraphically and X-ray diffraction was conducted on four cores (DLC4, DLC1, DLC2. DLB4) and grainsize measurements were conducted on Cumbralaway Creek cores (DLC4, DLC1, DLC2), this information was used to divide core section into facies units.
A total of 12 unique stratigraphic units were found in both Cumbralaway and Benandarah Creeks, which are divided into three major groupings. The Pleistocene terrestrial surface was dominated by oxidised cohesive sands and pebbles. Deltaic sediments are characterised by coarsening upwards grainsize, higher energy, oxidation and common organic fragments. Central basin sediments were characterised by silt dominated grainsize, estuarine shells such as Notospisula trigonella and a reducing environment. Furthermore 5 AMS and 6 AAR dates were obtained from wood and shells to provide sedimentation rates. The sedimentation rates in the prodelta (0-53-0.61 mm/a) and in delta deposits in Crumbalaway and Benandarah Creeks (0.12-0.34 mm/a) are comparable to natural sedimentation rates in other southeast Australian lagoons.
Three cores from the prodelta (DLC2), active delta front (DLC1) and lower delta plain (DLC4) in Cumbralaway Creekwere examined for their microfossil assemblages and palaeoenvironments. Seven microfossil assemblages are present. The basal Assemblage 3 in DLC1 and DLC2 is an unstable marine facies characterised by a variable fauna including the foraminifera Ammonia beccarii, Elphidium crispum crispum, E. mortonbayensis and Haynesina depressula and the ostracods dominated by Osticythere baragwanathi, with sporadic charophytes (Lamprothamnium succintum) in DLC2. Assemblage 2 is marine dominated and has distinct open and closed estuarine facies. In the prodelta area the open subassemblage is dominated by a high diversity of Elphidium species, the presence of miliolids (such as Quinquoculina poeyana) and planktonic species (such as Globigerina dutertrei) and a greater diversity of marine ostracod species such as Callistocythere keiji. The closed estrarine subassemblage has a reduced foraminiferal diversity (including Elphidium exacatum sydneyense) and high numbers of Osticythere baragwanathi. Assemblage 2 in the active delta region (DLC1) is characterised by a lower diversity assemblage compared to the prodelta assemblage with more Elphidium mortonbayensis, a few Ammobacilites foliateous, a lower number of Osticythere barawanthi and rare Lamprothamnium succintum. The uppermost Assemblage 1 also shows a gradation from a closed estuarine environment characterised by Ammonia beccarii, Elphidium excavatum sydneyense and Osticythere baragwanathi in DLC2 to a shallower assemblage characterised by agglutinated species, such as Ammobacilities foliateous, Elphidium mortonbayensis and an absence of ostracods and charophyte species in DLC1. In the lower delta plain (DLC4) Assemblage 1 becomes dominated by sporadic agglutinated forminifers, such as Trochammina inflata, and the charophyte Lamprothamnium succintum indicating a lower salinity environment.
Assemblages within the central basin units (DCL1 and DLC2 Assemblages 2 and 3) are governed by sea-level and environmental stability. In contrast the assemblages within the deltaic units (Assemblage 1 in DLC1, DLC2 and DLC4) are governed by depth, salinity, energy and oxidation. All these assemblages follow similar distributions to other microfossil distributions in southeast Australian estuaries.
The combination of stratigraphic and microfossil evidence reveal that before 8130 yr BP an alluvial valley and palaeochannel were present. Flooding of the alluvial valley at 8130 yr BP formed a unstable open estuary as indicated by a transgressive upward-fining sandy marine unit and microfossil assemblage is deposited between 8130-~5145 yr BP. From ~5145 yr BP t0 2500 yr BP an open maximum transgression layer is indicated by the prodelta and active delta front units and shown by microfossils. Since 2500 yr BP a closed estuarine environment is recorded, which reflects a fall in sea-level which is shown by restricted foraminiferal assemblages. Delta progradation is recorded in DLC2 and DLC1 from1880 yr BP while at DLB4 in Benandarah Creek delta sedimentation started at ~1050 yr BP.