Fossils at Naracoorte.

A huge shell fossil from the Naracoorte member of the Gambier Limestone. This limestone formed in a shallow temperate sea. The environment appears to have pretty low biodiversity as evidenced by the small variety of fossils preserved in the rock. This large shell is unusual as the Gambier Limestone seems to be dominated by sea urchin flukes and spines.

Structural geology - road cut at Hope Downs

This is a lovely cross section through a beautiful open fold set located on the access road to Hope Downs. The folded beds are part of the Wittenoom Formation which is interbedded shales and cherts (pictured) sandwiching calcitic dolomite. The formation is dated to 2600-2500 million years old. Careful examination of this outcrop provides important evidence for the structural development of the region.

By tracing the Anticline and Syncline hinges we can see that the hinge lines are not parallel and the folds themselves lack symmetry. This suggests there has been a second deformation after the initial folding event which has rotated and further tightened the hinges. There is the added complication of rheological factors. The rheological difference between the shale unit (white) and the BIF-shale unit (stained red) means that they respond to strain differently. The shales (ex-muds) flow better, forming tighter folds without faulting while the more resilient BIF beds form open folds. Looking further afield, we can examine a cross section through the Hope Downs mine published in D.F. Lascelles 2006 paper titled 'The Genisis of the Hope Downs Iron Ore Deposit, Hamersley Province, Western Australia' published in the Journal of Economic Geology. The cross section shows a series of south dipping shallow thrust faults which steepen the norther limbs of the anticlines and provides pathways for mineralising fluids. While not 100% structural geology, the paper is well worth a read for those interested in the development of iron ore deposits in the eastern Pilbara. The next post will examine the rocks on the eastern side of the road and see what 3D conclusions can be drawn from two 2d sections.

Places I've been - Rottnest Island.

I recently visited Rottnest Island for a short holiday. The island is a small outcrop of limestone a short 30 minute ferry trip from Perth, Western Australia. The island consists of multiple limestones and is a great example of landscape evolution over time. The Geological Survey of Western Australia has published a short handbook on the geology of the island and it can be downloaded in PDF here. One of my favorite outcrops from this trip is the tiny limestone stack pictured below. There are two key features of this photo. 1) the ospray nest with both parents purched onto and 2) the meter high cross beds in the limestone which indicate the eolian origins (windblown sand dunes) of this specific limestone.

You cannot visit Rottnest Island without meetin a Quokka. This marsupial is only found on a few select islands off the coast of Western Australia as it is susceptible to predation by dingos, cats and dogs. In a way, the island is named after the Quokkas by the white colonialists who thought they were a large specimen of rat. Though they are nocturnal, a few insomniacs can be found around the main human settlement begging tourists for illicit snacks and thieving anything they can get their cute little paws on.

Colours of the Pilbara - Joffre shales

These lovely rocks are shales from the Joffre Member in the Brockman Formation of the Hamersley Group. The shales are approximarlty 2459 million years old. They are not usually these lovely blue, gold and purple colours. They display a slight hydrothermal alteration and it is likely their proximity to the BIF within the Brockman Formation has resulted in the precipitation of minerals leached from the iron ore.

The big bolder in the centre of the image is approximatly 1.5 feet long preventing me from rock-nabbing it and taking it home for my collection.

Fossils - Pecten fossil

Pecten from the Naracoorte Limestone - photographed with an iphone through a 10x hand lense.

This tiny tiny shell was found by one of my students from JMSS while we were doing a palaeontology practical exercise at the Naracoorte Caves. He was sorting through some cave sediments which has been retrieved from a cavity found in a nearby quarry. Due to the nature of the discovery, dating these items through stratification was impossible, so they have been used to give the location an assemblage database and are used to educate school groups. Most of the fossils come from small mammals and reptiles with the occasional Megafauna bone or tooth as well. The cave fossils are less than half a million years old.

This pecten comes from the Naracoorte Limestone, which is a member of the Mt Gambier Limestone Formation. The MtG has been dated to between 24 - 33 million years old. It's amazing to think this pecten is still is almost perfect condition and has survived this long!

Structural geology - Folding in the Pilbera

 

The Pilbera has undergone two significant phases of compression, N-S resulting in E-W folds and NNE-SSW -> NE-SW resulting in ENE-WNW (ish) folding. The overprinting of these two deformations creates some lovely interference patterns. I rather like the M fold in this hillside as erosion and the inclination of the bedding has conspired to turn it into a heart shape. M folds are typically found in a fold hinge of an F1 (first deformation fold) but can form on the limbs of an F1 when it is refolded by D2 (Deformation #2) creating F2 folds which are typically non-cylindrical due to the prior orientation of the bedding.

If you're interested in how interference patterns look with at a variety of orientation, plunge and dips visit the wonderful page by the Fault Analysis Group at the Univeristy College of Dublin.

Fossil tree - Inverloch, Victoria

Cliff panorama showing a fossilised tree stump in-situ in the center left.

Close up of the tree showing bedding of sands above and below it. All beds tilt approximately 10 degrees to the East.

Zoom in of the stump showing the roots still twining through the now lithified sediments. Floods can do some amazing things and it's pretty awesome that this tree has been preserved like this!