Virtual Field Trip– Ebagoola Case study

Aim

To demonstrate the use of gamma-ray imagery for mapping regolith materials, bedrock and understanding geomorphic and weathering processes.


Datasets and Layout

This interactive VRML interface allows the user to rotate, zoom and navigate through a series of thematic datasets including a terrain model, ternary gamma-ray image, individual radioelement channels (K, Th and U), Landsat TM imagery and geology maps. These datasets can be turned off and on using the surface toggle drop down menu. The VRML viewer drapes the gamma-ray images over a digital elevation model (DEM) to create a series of 3D perspective views. This type of data presentation is an excellent way of understanding geomorphic/weathering relationships and separating bedrock and regolith gamma-ray responses.

A series of embedded spheres flag field site locations that are linked with pictures and text descriptions.

The remainder of the VRML models in this module operate in a similar fashion to this case study.

Datasets include:

Location, Climate, Vegetation and Landuse

The study area covers the Ebagoola 1:250,000 map sheet located on the east coast of Cape York Peninsula, west of Princess Charlotte Bay. The area has a warm monsoonal climate with distinct dry and wet seasons. Average annual rainfall is 1117 mm with 95% falling in the wet season between November and April. Vegetation is mainly eucalyptus, open woodlands and forests. Extensive mangrove communities occur along stretches of the coastline. Land use is restricted to broad-scale cattle grazing, owing to the prolonged dry season.

 

Landforms and Geology

Landforms (see Landsat TM image) vary with depositional and erosional plains over the western side of the image, moderate to high relief over the central area and generally low relief erosional and depositional plains over the eastern third of the image. The most prominent landform feature is the Great Escarpment (up to 300 m relief) that separates the central belt of hills from broad alluvial and coastal landform facets (see annotation layer).

The geology of the Ebagoola study area can be divided into three main rock groups (see Geology layer). These groups include a central zone of Palaeozoic granites (granodiorite, muscovite-biotite and alkali feldspar granites) and folded metamorphic rocks (phyllite, schist and gneiss) with gently dipping Mesozoic sediments (sandstones and siltstones). Mesozoic sediments of the Carpentaria and Laura Basins dip at low angles westwards and eastwards, respectively from the central Coen Inlier. The Coen Inlier is characterized by hills of the central uplands with elevation 150 - 200 m above sea level.

 

Gamma-ray Interpretation

Over the eastern coastal plain, alluvial sediments forming braided river deposits are recognised (eastern side of the ternary gamma-ray image). The source rocks of individual sediment packages can be identified with red and orange hues (high K) reflecting the presence of mica and feldspar derived from weathering of granites, and blue/green hues associated with sediments derived from weathering of metamorphic rocks (schist and phyllite). A black line on the imagery shows beach sands that trace out an older strand-line up to 1 5km inland from the present coast (see annotation layer for location).

Relatively young sediments on the coastal plains contrast sharply with deep in situ chemical weathering on the western plains. Here sandy soils covering weathered bedrock appear in shades of black to greenish blue, reflecting the surface accumulation of residual sand (quartz) and to a lesser extent heavy minerals such as zircon. Bauxitic plateaus are recognised by blue/green hues indicating relatively high Th/U and low K. High Th and U are associated with clays, Fe/Al oxyhydroxides and heavy minerals (e.g., zircon, sphene and monazite) in the soil profile.

The central uplands consist mainly of erosional landforms with typically thin regolith over either granitic or metamorphic bedrock. Granitic rocks appear in bright reddish hues, reflecting their high K-feldspar and mica content. Metamorphic rocks (quartzites, schists, phyllites and greenstones) appear in green/blue hues, reflecting the higher Th and U contents relative to K. The Colman and Holroyd Rivers that are sourced from the central uplands are characterised by sandy channel and silty overbank deposits. These sediments appear in white and greenish/blue hues in the ternary gamma-ray image reflecting the geochemistry of the source rocks and modification of the sediments due to weathering.

The edge of the Great Escarpment is clearly visible on the gamma-ray spectrometric imagery. The escarpment separates deeply weathered regolith above the scarp from steep actively eroding slopes with shallow regolith along the scarp face. The relationships between gamma-ray response and geomorphic process across the scarp are summarised in the diagram below. See the text layer drop down menu for the location of the cross-section.


Click on image to enlarge