• Locality Amadeus Basin, 160 km SW of Alice Springs 24°30’S 133*E

• Map Henbury (SG 53-1) 1:250K

• Google Earth Petermann Creek, NT

• Photos SVY 842 Henbury Run 7 Frames 5153-55 (20-4-1950)

• Scale approximately 1:50K

Background

This exercise covers part of the Parana Hills, which consist of tightly-folded Cambro-Ordovician sedimentary rocks that form east-striking ridges, and a small exposure of Neoproterozoic sedimentary rocks in the core of an asymmetric dome. The rocks consist of thinly interbedded sandstone and siltstone, limestone and dolomite.

What to do

Right click on the image to the immediate left and save the file to your desktop . Use this photograph as the map base; it is at a scale of approximately 1:50,000. The downloaded file should cover more area than shown in the image to the left.

Using the oblique, or three-dimensional, view in Google Earth explore around the large central fold structure with a printed copy of the photobase next to you. Try to identify the dip of the various ridge-forming units, and note the slightly confusing effect where the dips become shallow and the units have seemingly have less defined form. Interpret the main geological elements, with sandstones being prominent, the white limestones recessive. and thinly bedded sandstones with distinctive ribbing.

Prepare a geological map as an overlay tracing, showing the main sets of ridges and the structure. Estimate dips of marker beds on your map, using the form of creek - ridge junctions, and remembering the effect of vertical exaggeration. A large asymmetrical dome can be recognized by tracing beds of sandstone covered in scrub - start with a distinct layer that goes all the way around the dome and trace it out completely so that the place of other units can be established to define a stratigraphy. The main unit forming the dome has a wide outcrop on the north-westrrn part of the image due to shallow dips, but restricted outcrop on the south side due to steep dips there. On the eastern side, the edge of the dome is also steep and may be faulted.

• Carefully trace out the boundaries of the different units on the overlay, using a much detail as you can. Neatness is important in these exercises as neat traces are more accurate than cursory lines. Don't (lightly) colour in the geological units until you have annotated the map with other elements, for example morphological features, bedding strike and dip (see symbols on surface interpretation)

• Don't forget to prepare a geological legend and make sure you have all the elements of a geological map. Use the outcrop habit (recessive, prominent) to infer the rock type.

• Interpret the nature of the fold structures - anticlines or synclines - and mark the major fold axes and their plunge on your map.

• Draw a north-south cross section through the western third of the map area, taking care to correlate ridge-forming sandstone units in different parts of the large folds. Use marker horizons, such as the white (dolomitic) beds to help in this regard. Note that north is towards the right-hand side of the photographs as printed in this document. Indicate the start and end of the section on the map,

• Prepare a geological history table, based on the observations you have made.

Guiding questions

• Explain the drainage pattern in the northern half of the photo cover

• In the SW corner of the stereo cover, a second structure can be defined. What type of structure is this likely to be? Look for a rock layer that can be recognized in the dome and also in the other structure - trace the units around the folds

• The older rocks in the core of the dome can be recognised by their distinct strike. What is the nature of the contact between the two units?

• From your interpretive map and stratigraphy, determine the extent of the structures and the asymmetry of the folds (which flanks are steeper).

Further reading

Branagan, D. & McNally, G., 1991. Australian Geology; Remote Sensing Exercises Manual. Geoscope, 55 pp.

Wells AT, Forman DJ, Ranford LC, Cook PJ (1970) Geology of the Amadeus Basin, Central Australia. Bureau of Mineral Resources, Australia, Bulletin 100.

Shaw RD (1991) The tectonic development of the Amadeus Basin, central Australia. In Korsch RJ, & Kennard JM (Editors). Geological and geophysical studies in the Amadeus Basin, central Australia. Bureau of Mineral Resources, Australia, Bulletin 236, 429–462.