Year

2003

Degree Name

Master of Science - Research

Department

School of Geosciences

Abstract

Provenance and spatial distribution of heavy minerals have been investigated in the Minnamurra estuary and the adjacent shelf in NSW , Australia. Heavy mineral assemblages in the sand fractions (63-250µm) of 124 sediment samples were assessed using microscopic and microprobe analyses. In addition to the dominant opaque minerals, twelve translucent heavy minerals species were identified. The translucent assemblage is dominated by pyroxene, zircon, tourmaline and hornblende. The heavy mineral assemblage identifies multiple sources that are not restricted to the local geology. The proportion and occurrence of heavy minerals from non-local source rocks reflect reworking of quartz sand from the outer-shelf to the inner-shelf and coastal environments during the post-glacial marine transgression. These minerals were originally derived from the Precambrian craton in southeastern and central Australia, and from the Lachlan Fold Belt. The fold belt would have contributed both reworked older grains of ultrastable (ZTR) heavy minerals as well as some primary minerals from the igneous rock units. Pyroxene mineral were derived from the coastal Permian latite units as well as Tertiary basalts and Mesozoic tinguaites on the escarpment (Minnamurra falls) of the southern Sydney Basin. Chlorite and metaluminous epidote were derived from hydrothermal alteration phases in the latite units.

The main factors that control the spatial variability of heavy minerals are transport, hydraulic sorting processes, coastal erosion, and the embayment morphology. This can be seen clearly inside the estuary and on the shelf area. Denser heavy minerals are concentrated in the outer part of the inner-shelf whereas the lighter platy heavy minerals become more prominent in the mid-shelf sediments (low energy area). Coastal and subaerial erosion has concentrated augite in the inner part of the inner-shelf. Augite and hornblende show downstream increasing patterns within the estuary, while ultrastable (ZTR) heavy minerals remain as lag deposits in the area adjacent to terraces on the northwestern bank of the river in the upper fluvial part of the estuary, as well as lag deposits of zircon and tourmaline in the mid-estuary (Rocklow Creek area). Erosion of the Minnamurra spit concentrates rutile within the inlet area, as well as supplying relatively abundant zircon. The absence of aegirine-augite and titanaugite from the coastal latite units accounts for their lower proportions on the shelf and in the lower part of the estuary. The enrichment of the lower part of the estuary with augite, hornblende and the ZTR minerals has also resulted in lower relative concentrations of aegirineaugite and titanaugite within the lower part of the estuary.

Statistical cluster analysis of heavy minerals and textural data revealed the existence of five mineralogical facies: the upper fluvial part of the estuary, the Minnamurra spit and elevated inner sand terraces, the estuary inlet and the inner part of the inner-shelf, a combined group consisting of the outer part of the inner-shelf and mid-estuary (Rocklow Creek), and the mid-shelf facies. In fact, the distribution of these mineralogical facies is controlled by spatial variability of heavy mineral assemblages rather than the textural characteristics of the sediment. The similarities in facies between the mid-estuary and the outer part of the inner-shelf are attributed to two reasons: reworking of marineinfluenced sand sheets in the Rocklow Creek catchment and/or hydraulic sorting processes. River terraces probably represent an older barrier deposit but could have formed by a large marine incursion (tsunami) in the late Holocene or by the action of wind and storms whereby spit sediments were reworked and dumped into the upper fluvial part of the estuary.

Share

COinS