Friday, August 13, 2010

Reading Core, Seeing Thrombolites

7/19: A cloudy Monday morning in chilly Perth, although the day promised to clear.  I’d arranged to spend two days examining drill core at the Geological Survey of Western Australia’s Core Library.  A core library?  Drilling core is one of the key ways that companies and researchers explore in areas where rock exposures are poor, or where what’s underground is of interest.  A rig drills a hole with a special diamond bit that leaves a long tube of rock at its center: core.  The core is periodically extracted, analyzed, and systematically stored in trays.  The average tray holds four to six meter-scale core pieces.  Cores are commonly several kilometers in length: a lot of trays.  Drilling core is expensive, and is mostly done by industry.  In Western Australia, the GSWA and companies have arranged for selected cores to be preserved: hence the Core Library, which makes core available for everyone to examine. 

The Library is in Carlislie, north of Perth City Centre.  I’d been there before, but didn’t quite remember how to get there.  Anticipating this, I’d rented a car (a Holden Barina, more on this later) which had a GPS.  Unfortunately, this Tom Tom unit was a bit confused about Perth and giving directions.  An example of its directions; “keep right, while bearing left”.  Louis eventually used it as an elaborate street guide.  This made getting to the library into another round of hunt and find.  We were good at this by now.  I prefer this game on dirt tracks rather than city streets. 

Once the Library was found, we signed in, went through the inevitable induction and quiz, and got to work.  I’d requested parts of four cores to examine, where I hoped to find the Paraburdoo Layer.  They were all from parts of the Hamersley Basin with poor outcrop; if we found the Layer in any of them, it would add greatly to understanding the Lauer.  Two cores were drilled by industry.  One was from a government project, part of a groundwater investigation.  The final core was actually a research hole, partially funded by NASA. 

The GSWA Core Library: that's "my" core in the center

The Core Library is essentially a very large warehouse with an adjacent examination area.  My four cores were laid out on several racks in the latter.  This is a Cadillac facility; the core was on racks that were at a non-back breaking height, the lighting was good, water and spray bottles (for wetting the core; textures show up much better this way) were available, and there was tea and coffee in the break room.  The only drawback was temperature.  The room wasn’t heated, since it was open to the warehouse.  Perth was a mite chilly in the mornings

We set to work.  Core was new to Louis.  I’ve not looked at it for a decade or so, but the technique came back rapidly.  It’s first a matter of staying oriented, i.e., knowing stratigraphic up and down, as one moves from box to box.  The real challenge is that rock in core looks very different from outcrop.  The core hasn’t been weathered.  For example, the tan to brown dolomites of the Paraburdoo in outcrop were gray in core, and their sugary outcrop texture (an indication of crystal size) wasn’t apparent in core.  In addition, as it grinds, the drill bit scores the outside of the core, which obscures and confuses the real features of the rock.  Core samples that have been cut with a diamond saw – the standard sampling method – can be similarly cryptic if the saw blade is worn or out of alignment.  So looking at the rocks was initially relearning and reorienting my mind to a new set of clues.  Louis picked the method up easily: no surprise.

We had two days to look at the four cores.  From reading core logs – the record made by the on-site geologist when the core was drilled – and talking with Bruce, I had a pretty good idea of where to search for the Paraburdoo Layer.  We set to work, starting on what I’ll call the NASA core, which had been nicely cut in half – with few saw cut marks.  Looking at core goes like this:
  •  Spray the core with water. 
  •  Bend over: the rules clearly stated that picking up core was not permitted, lest the pieces get out of order. 
  •  Look at every layer with a handlens. 
  •  Record observations in a progressively wetter notebook.
  •  Try to stay warm through regular infusions of tea and hot water.
  •  Repeat until the last box is reached. 

Louis looking at core
Actually, this is what I made Louis do, since he is a good observer with younger eyes.  There are much more sophisticated and technologically intense ways to look at core, but using eyes and keen minds was what we required.

I did a quick assessment of all four cores.  I soon discovered that two of them would be useless.  One started too far below the surface, i.e., below the level where the Paraburdoo Layer should be.  We’d visited the site where this core was drilled in the Hamersley Basin.  I’d actually stood on the capped well head.  The outcrop did not go low enough in the stratigraphy for the layer to be visible.  And now I knew that the core started too deep!  It was not surprising, but frustrating, to know that the Layer was probably buried a few meters below where we were working and camping.  Nothing for it.  The other core was a mess; the upper portion, where the Layer was likely to be, was a solution breccia.  The originally solid rock had been dissolved by groundwater into loose pieces, which had compacted together.  The open space between these pieces was filled with coarse crystals of calcite and quartz.  A very pretty rock, but useless for my purposes.  All sense of original stratigraphy and bedding was gone.  The core probably started too deep as well.

We motored along.  From my overview, I recognized marker beds in both “good” cores that I’d seen in the field; beds that were largely composed of chert (fine-grained quartz) rather than dolomite.  These were hints to the location of the Paraburdoo Layer.  It had to be lower in the core.  We looked hard.  I began to doubt my assessment and looked for the marker beds again.  They were there.  We looked hard a second time.
Lunch at a local lunch bar.  The Core Library was on the edge of a light industrial area.  Andy, the core librarian, had told us there were equidistant lunch bars along the road in front of the facility.  I picked left; out of the LIA and into residential Carlslie.  A sudden surrounding by small brick houses; the usual suburban variation in appearance, lawn decoration (no spinifex), and vehicles (no Patrols).  A quiet time of day.  I felt out of place; the house seemed small, and as I walked down the street, they seemed to converge.  Ah, the post-field work adjustment.  We came to the lunch bar, run by a happy Malaysian family.  More dislocation; I had not ordered a lunch in weeks.  A curry soothed.

I may have made the core examination process sound straightforward in my earlier description.  However, it’s often tricky.  Core is always fragmented.  This may happen for many reasons.  It may fracture as it comes out of the drill rig.  It may be broken intentionally to fit in the core trays.  It may split naturally along bedding surfaces or changes in composition.  Finally, it might be dropped when someone is handling it.  Not us; we were good, although for easier examination by Monday afternoon we surreptitiously started picking up pieces.
Louis marked interesting beds with pieces of paper towel.  In between my own examinations, I looked at these.  We were both finding the same thing: beds of dolomite that were the right thickness, and composed of tantalizingly, excitingly spherical-looking carbonate crystals.  I knew from my previous work that the spherules would really stand out.  None of these beds was right.  It was time for dinner.

7/20: We’d had the examination area to ourselves on Monday, except for one area blocked off by little orange safety cones.  The core trays beyond the cones were covered with paper.  Andy had asked us to stay away from this “confidential project”.  This of course made me curious.  On Tuesday, the paper was gone, and two people were working on the cores.  They were polite but distant.  I looked at the visitor’s log.  They were from one of the WA petroleum companies, which suggested to me that the core was from the Northwest Shelf of Australia, where there’s lots of natural gas.  As the day progressed, the Library staff gradually filled the rest of the tables with more core trays.  Based on their names, many of these also were from localities related to also petroleum. 

I was glad the petroleum geos were there.  They had gotten the Library staff to turn on a couple of large space heaters.  We got a pair too.  While being cold had sharpened my focus, it was making me too sharp.  The heat kept my head warm, at least.

We looked at the cores for a third time.  I revisited my stratigraphic assignments for a second time.  We looked at the core that was too deep.  No luck, no dice: the day was over, and we had a dinner date. 
Alas, no impact layers.  Well, the cores covered the right intervals.  We could not have looked any harder.  I’ve written earlier about why rocks don’t outcrop where they “should”.  Some of the same things may happen with core: removal by deformation or dissolution, nondeposition, or just bad luck.  In addition, since the core is broken, it’s possible a bed as thin as the Paraburdoo Layer may have fallen out of a core tray, or been destroyed in the drilling process.  I don’t know which of these possibilities were factors in my core reading. 

So went the official end of field work for Australia Fifteen.  I’d arranged to have dinner with my friend Carol in Fremantle.  I met Carol and her husband Shawn in New Zealand in 2006 when I was there for an Aikido seminar/holiday.  They’re both geologists; they moved to Perth to work in the minerals industry.  Shawn was unfortunately in the field. 

Getting to Carol’s house involved more GPS agony.  The Tom Tom failed to recognize both the district where she lived and her street name.  Louis navigated us to the general vicinity: then both missing localities suddenly appeared.  Maybe this is the place to comment on the Holden Barina.  Holden is Australia’s native car company.  It’s part of Ford at present.  The Barina is their bottom of the line compact car.  Ours was brand new, and it was a fine small vehicle with plenty of space for tall Americans.  However, it had two quirks.  First, the trunk had a propensity to open at random, which made backing down Carol’s winding driveway at night somewhat challenging, but amusing.  Less fun was the arrangement of brake and accelerator pedals.  They were at almost the same level.  More than once, I attempted to brake, and pushed the accelerator at the same time.  If the brakes had not been new, and I not an alert driver, this could have been dangerous.  I plan to tell Holden about this.

A satisfying Thai dinner helped to hatch a plan for the next day. Carol was free, and we had time and a car full of gas.  Three geologists and a car: what to do but something geologic?  I wanted to be tired for the long trip home; my flight left at 12:30am. 

7/21: Back at Carol’s house.  She’d consulted some friends.  Their suggestions made the day’s activity obvious: go see the thrombolites at Lake Clifton. 
Using a word like thrombolites means it’s time for me to get geologic again.  Thrombolites are a type of stromatolite, so let me start with the general.  Stromatolites are layered sedimentary structures formed by the combined action of cyanobacteria (photosynthetic blue-green algae and their brethren) and sediment deposition.  Basically, the cyanobacteria grow in lake or ocean water within the photic zone, forming a mat on the basin floor.  As sand and mud grains are deposited, or as minerals precipitate out of water, they are trapped in the mat, and in some cases also adhere to the cyanobacteria.  As they become buried by sediment, the bacteria grow upwards, trapping a layer of sediment.  Repeat and you get a layered structure: a stromatolite.  Thrombolites are a variation on this theme; they form the same way, but their layering is poorly defined. 
I’ve always liked stromatolites.  The word - stro-ma-to-lite - is fun.  Moreover, like iron formations, they are a remnant of an earlier earth.  Fossil cyanobacteria are among the oldest forms of life on Earth so far discovered.  These earliest fossils are a bit over 3.5 billion years in age.  They’re in stromatolites from the Pilbara, of course.  Three point five billion years ago – a billion years before the Hamersley Basin.  Until around 650 million years ago, when modern life forms begin to appear, cyanobacteria and stromatolites were the main players on the stage of life.  That’s all there was: simple, mostly unicellular plants, no animals. 
Given this, imagine an ocean shoreline three billion years ago.  The land would be barren; no life there, so no soil, just loose sediment and bedrock.  The rocks would be weathered, maybe slowly rusting; too little oxygen in the atmosphere (yet).  The sky would have been a different color (less blue?) for the same reason.  The only sound would be wind blowing among the rocks.  The beach itself would be coated with stromatolites, growing from the mean high tide line to below the surface.  The surf would be muted, the waves filtered as they coursed through the stromatolites.  That would be it.  A narrow band of life, taking advantage of sunlight and nutrients in the water.  I’ve always wondered what color the stromatolites were.  Probably dull greens to blacks: early plant colors.  Painting this sort of mental picture is what first thrilled me about geology. 
Stromatolites show a diversity of shapes.  I’ve implied a flat, matted appearance in my description above.  Stromatolites can also form domes, cones, or columns.  These forms can evolve into each other.  I’ve seen small cones on larger domes, and mats that morph into columns.  Stromatolites can be small- palm-sized – or large- bigger than an average car. 
Today, stromatolites are largely extinct.  They effectively vanish from the geologic record shortly after animals appear and diversify.  The paleontologists who have worked on this hypothesize that this disappearance is due to the evolution of grazing forms of life.  These animals ate cyanobacteria.  All those stromatolites must have provided free lunches for a while.  This is arguably the first mass extinction of life on earth.
With time, interest and exploration, remnant stromatolite occurrences have been discovered.  A surprising number of these are in Western Australia.  I’ve visited four of the five publicly accessible stromatolite populations, including Lake Clifton.  But hey, Carol had not been there.  Neither had Louis, and we just seen samples of the ancient Pilbara stromatolites.  I’d only been to Lake Clifton once, when the weather was windy.  I hoped for better viewing conditions.
Lake Clifton is south of Mandurah, an exurb south of Perth.  Getting there proved to be the final round of hunt and find for Australia Fifteen.  Like the Pilbara, having been there previously I had a vague memory of where to go.  There was one main road leading south from Perth, just like a dirt track in the Pilbara.  The navigational aids were vaguely useful.  The Tom Tom GPS covered the area, but refused to acknowledge the existence of Lake Clifton.  Carol’s road atlas stopped at Mandurah.  Again like field work, it was easy to know the way.  Keep the ocean to the right, stay on the main road.  Mandurah came and went.  I got a bit twitchy, as the coastal scrubland began to look familiar.  We were in need of water, junk food, and restrooms.  I sneaked a peek at a road map.  That’s right, Lake Clifton was in Yalgorup National Park, and it looked to be only about another twenty five kilometers to the south.  Relief.
Driving further.  A road sign: Yalgorup National Park, quickly followed by another: “Lake Clifton” with a right pointing arrow.  I turned on to a suddenly familiar steep road.  I knew where I was.  Right at the first T-junction, left at the second, go past the funky farmhouse, into the parking area.  A typical national park; dirt road, vague pullouts, chemical toilet, and a kiosk with maps and signage.  None of this had changed since my last visit. 
Having made use of all of these amenities, we followed the trail to the lake.  Lake Clifton is spring fed.  It’s networked to the cave system which underlies much of the coastal plain south of Perth.  In other words, no streams flow into it, so it is an isolated body of water.  I presume this is how thrombolites have survived here; grazers have not been able to get into the lake.  It’s possible that the water chemistry helps as well.
A nice wooden boardwalk extended into Lake Clifton.  The wind was light, the water fairly placid.  I didn’t get very far before stopping.  Instead of the accumulation of detritus (organic, inorganic, and human-deposited) that usually rings a lake, Lake Clifton’s margin was encrusted with thrombolites.  At the water’s edge, the thrombolites had broad circular shapes, and showed internal ring-like layering.  They looked truncated: I suspect that these are erosional remnants of no longer active thrombolites, which formed when the lake was a bit larger.  They reminded me of similar structures in Shark Bay, one of the other West Australian stromatolite sites.  I took pictures.  The light was good, sun low to the west.  Glad for the nth time to have a good circular polarizing filter on my Nikon lenses.  
Thrombolites close up - note the flat tops and concentric layering.

Lake Clifton thrombolites en mass
I walked further.  The thrombolites proliferated.  Their shapes became more clearly defined; individual domes, multiple domes joined together.  As the water deepened, the domes took on more conical shapes.  They did not get much wider, just taller.  This was eerie, as this change with depth was exactly what I expected to see; as the water got deeper, there was less wave influence, so the thrombolites could grow taller.  The thrombolites tops remained flat, like the ones nearer to shore.  Why?  Hmm, maybe this was a function of wave energy in Lake Clifton.  The thrombolites could not build up into the zone of water motion, so they were spreading.  Or maybe this reflects seasonal variations in lake depth.  I didn’t know the answer, but it was a pleasure to look and speculate.  The sides of the thrombolites looked fuzzy.  Allowing for dispersion in the water, I think this was caused by a coating of cyanobacteria. 
Louis and Carol iso the ultimate thrombolite picture
I walked around, took pictures, and shot video.  Within around ten minutes, I’d tried all of the obvious and creative photo angles.  It was time to stop and simply look.  Earlier in this post, I drew a mental picture earlier of a primordial stromatolite-encrusted shoreline.  Here I was, with a real colony of thrombolites.  This was one of the few places on Earth that I could actually do this.  It was a precious place.  A glimmer of the early Earth, and evidence of the tenacity and creativity of life. 
Well, my ultimate thrombolite picture

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