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Dusty dendrites

Pseudo-dendrites formed in iron rich dust.

This pseudo-dendrite has formed from a combination of water precipitation and very fine dust. During an outback drive, the morning mist has precipitated and dripped on the slightly rounded black plastic surface. The continued movement of the car has shaken another few drips loose and kicked up sufficient fine dust to stick to the water trails. Lighter trails indicate areas which were wet longer and able to trap a thicker dust layer.
Interestingly, the water trails only diverge and never appear to merge. My current hypothesis is that the water trails were pushing tiny iron rich dust particles in front and to the sides of them. When being transported, these would rotate to align with the earths magnetic field but as they dried, this ability would be reduced. A cross section of a dried trail would look a lot like a brontosaurus, thin and pointy at both ends and much thicker in the middle. Not only would this present too sharp an incline compared to the black plastic surface, the dust would have a very minor magnetic field which would contribute to guiding the actively moving droplet away.

Mafic dykes cross cut a granite

Mafic (basalt) dykes cross cut thegranite of the Burnside Batholith at Lowlands Beach.

I was very taken with the two planes of weakness highlighted here by right angled jogs in the intruding dyke. The locations where the dykes, which are around 25cm wide, pinch out and step across suggest to me a more sheared or rotational strain field.

Iron dust

The interpretation of magnetic anomalies has been an enjoyable part of my job as a geologist for many years now. I'm currently working in the Iron Ore industry in the Pilbara which makes this especially challenging. The Archean rocks of the Pilbara are variably magnetic, with fresh Banded Iron Formation (BIF) creating magnetic highs and remnant magnetic fields creating magnetic lows. Neutral magnetic signatures can come from enriched iron deposits where the Fe is no-longer magnetic or from overlying detrital packages. Detritals can also be enriched in magnetic pisolites which create shallow, highly variable anomalies.

 

Shown above is a standard geological probe with a small magnet imbedded on the top. I have run the magnet through the dust next to the car and then rinsed the result in my water bucket. As you can see, I'm left with very fine grains of magnetic material which form pseudo-crystals as they self organise in the magnetic field.

With current supplies, the soil and BIF is not deemed to be sufficiently enriched in iron (it's less than 45%) to offset the work of removing gangue material such as the silica/chert.

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.