Tuesday, June 23, 2009

Undisclosed Location

June 20, second day of field work. After a warm swag out at Weeli Wolli Springs, we headed west and somewhat north to my next target site. After 35 kilometers of corrugated dirt track and a several times that on the Great Northern Highway (all of two lanes), the site came into view. It's a nice steep hill of strata .

I’d been to this location before. The plan was similar to much of the research emphasis of this trip: make repeated vertical measurements of stratigraphy (sedimentary layering and its characteristics) at regular intervals along the outcrop zone and to study the spherule-rich zones in particular. I’m trying to do this all over the Hamersley Range, as I described in an earlier post.

I knew from satellite imagery that there was turn off from the main road onto a dirt track that would pass close in front of the hill. The road looked familiar from space; it seemed to be the same track I’d traveled on in 1989 and 1996. The turn appeared as expected. I hauled the steering wheel left and the Landcruiser headed onto dirt. The track was new. Same exit point off the main highway, but it cut at a different angle across the bush. A sign appeared: “Warning Buried Fibreoptic Cable”. Hmm, must be a communications line for the West Angelas iron mine. No worries, the track was in good shape and headed where I wanted to go.

Eventually we reached the point of closest passage to the hill. Another left turn, over the berm at the edge of the track. A short cross country bounce, till I felt we were close enough to walk, climb the hill, find the layer, and still get enough exercise to feel virtuous.

I was pleased to see an extensive outcrop of limestone at the base of the hill. The geologic unit which I am studying at present is the Wittenoom Formation. It’s named after the Wittenoom family, who were early Dutch homesteaders or the equivalent in the Pilbara. The Wittenoom has two parts. The lower part is the Paraburdoo Member. It’s basically all dolomite – a type of limestone containing magnesium in addition to calcium. This dolomite also has lots of iron in it. It is thus a pleasant yellow to brown colors in weathered outcrops. It’s named after the mining town of Paraburdoo; I’ve read that Paraburdoo means “meat feathers” in the local Aboriginal language. The Paraburdoo is what I was seeing at the base of the hill.

The upper part of the Wittenoom Formation is called the Bee Gorge Member. It’s named after Bee Gorge, and I haven’t a clue who or what “Bee” was. There are certainly plenty of bees around here when wildflowers are blooming. I know the Bee Gorge Member very well. Not only does the impact layer occur in these strata, but the rocks on which I did my PhD research occur here.

The Bee Gorge sequence is like an old friend I visit regularly, who always shows me new and exciting things. It’s a diverse suite of rocks. The basic framework is shale, limestone, chert (very fine grained quartz), and ferruginous chert that represent the slow accumulation of fine grained sediment in the ocean through time. Intermixed with these are volcanic sandstones, the impact layer, sandstones, and other limestones that were deposited suddenly and episodically, interrupting the normal quiet conditions. This is the part that attracts me; the abrupt events when something exciting happens. Puzzling out what happened in these events is fun and creative work. It’s also rewarding to look at the Bee Gorge because it’s never the same in two places. The general sequence of rock types and units is constant, but the details - the charming part - keep changing.

We climbed the hill. After some casting about, I found a bed that I thought should be the impact layer. Louis eventually found spherules at its base, confirming that we were in the right place. We did our work. The spherule-rich part of the bed – the five or so centimeters at its base – also contained lots of small platy particles about the size and shape of a fingernail. These chips were bits of the sea floor under the impact layer that were eroded when it was deposited. They’re badly rusted; they were probably pyrite in the seafloor at the time they were eroded. The layer was just crammed with them. It was really unusual, and a challenge to try and understand how the cramming happened. I am still working on it.

We were done by late afternoon: time to head back to the truck and camp. I pointed us down through the Paraburdoo Member. One narrow drainage looked to have close to 100% exposure. It was a good place for a quick scan in search of a mythical missing impact layer. It was hard to walk down hill through steep, rubbly outcrop and look for tiny spherical particles.

I carefully descended a steep smooth face of delicately brown and tan banded limestone. I thought, great photograph; the light was good. I stopped at the base to get out my camera. As I turned, a loose slab of rock caught my eye. It had petroglyphs on it! Almost the whole surface had Aboriginal imagery: whirls and what I think were representations of throwing sticks. Time indeed for a few pictures. This was exciting; I’ve found petroglyphs only once before in the Pilbara. It was a natural place for Aboriginal art; there was a spring in the middle of the drainage, and actually a couple tiny pools of standing water. Spring fed pools were new to me in this area. It made sense that they would be here. Being limestone, the Paraburdoo Member would be dissolved by rainwater and form caves, i.e., underground plumbing systems for groundwater. This spring was a spot where groundwater was leaking onto the surface. The other Aboriginal site I have “found” also showed signs of springs.

We searched around the drainage looking at all the flat rocks, but could not find any additional art. It’s a place I could well imagine Aboriginals using. In addition to surface water (a rare commodity), there was food, evidenced by plenty of wallaby scat and a couple of plants that could provide small berries and fruit.

Further down the drainage, I found a piece of nautilus shell. I can’t imagine how this piece of a sea animal got here. It was not a fossil. In any case, the rocks under it are a factor of ten times too old to have such fossils in them. It’s of course exciting to imagine an Aboriginal transporting the shell here hundreds or thousands of years ago. It’s also possible that some Westerner dropped it in the drainage recently. There were some test bore holes nearby. Still, another exciting discovery.

So, for the second time on this trip, I’ve had an experience which has brought me a step further in knowing Aboriginal senses of place. Tnorala was explained; this site, as far as I know, is unreported. I’ll investigate that back in Perth.

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