A Summary for the Sis

At 8PM on the day after my sister Alison's birthday, I called her and sang a round of Happy Birthday.  This meant that in the States it was 6:30AM on her actual birthday.  At one point, we started talking about this blog and when I asked her what she thought, she she was brutally honest.  "You need to be more clear on what you are actually doing in Australia."  I was befuddled thinking that is exactly what I had been doing, but I suppose there was not cohesive run down on the daily objectives.  So with less than a week left in the field season, I will attempt to answer her question and summarize the work we have been completing.  Catherine, Adam, and whoever else are welcome to enhance, comment, and correct this post as they see fit.

First day in the field!

First of all, it should be mentioned that this is Catherine's last field season.  So in many ways, she is tying up loose ends and pulling all of the unfinished pieces together.  As a result, much of the work seems disconnected in a variety of different locations throughout Southern Australia.  Many of the field sites produced less than ideal outcrops to investigate.   From Catherine's perspective, the dead-ends and disappointing quality have been frustrating at times.  However, it has been great from my perspective as it means that I can see many different areas of South Australia all the way from the southern Flinders Mountain Ranges north to a small outpost called Marree.

A good example of the outcrop we were studying.  Notice how the distinctive layers continue into the distance like linear ribbons.  However, this bedrock exposure is not vertically continuous as modern day sediments cover the area in between the beds.

Catherine's PhD focuses on the NeoProterozoic (end of the Precambrian) rocks.  Her main focus is on the formations Trezona, Elatina , and Nuccaleena which were previously described in another post.  She has spent countless hours hunting down these rocks, recording their positioning, collecting samples, and analyzing their chemical composition.  All of these efforts are done with the hope of better understanding the paleoclimate at the time the rocks were deposited.  While understanding any time period in the Earth's history creates a more complete understanding of the transitions that the planet has experienced, these particular formations are especially interesting due to the drastic and rapid (geologically speaking) climate changes that the rocks suggest.   Trezona - warm ocean, Elatina - glacial ice deposits, Nuccaleena - another warm ocean.  In addition, these formations occurred right around one of Earth's greatest biological transformation.  A division in the geological time scale is placed between the Proterozoic and Phanerozoic because of this notable change in life.   Prior to the Phanerozoic, only bacteria thrived on our presently biologically rich planet.  Bacteria did quite well for themselves dominating the seas for over three billion years.  At the end of the Proterozoic and beginning of the Phanerozoic, the diversity in life exploded.  No longer was it only bacteria, but life on Earth also included many multi-celled animals and organisms.  This transition occurred approximately 500 million years ago.  The Trezona rocks formed about 650m.y.a. on the eve of this major transformation.  Catherine's research adds additional insight in the events that may possibly have led to this remarkable change.  

So how do we begin to complete the story of a time so long ago?  Well first of all, we need to find the ideal location to complete the research.  Australia stands out as a preferable country to work in for a number of reasons both politically and geologically.  As a relatively safe, English speaking country, with a stable government, and welcoming people, it is an easy landscape to navigate as foreigners interested in learning about their rocks.  The rocks themselves in Australia are also advantageous for study since the continent has remained tectonically stable with minimal deformation over the past billion years.  In addition, the Outback is an arid climate which results in relatively slow rates of weathering and erosion.  Such conditions provide an ideal situation for the preservation of some of the world's oldest rocks. 

Despite the comparative quality of Australia's rocks to those in other parts of the world, the formations are still over a half- billion years old.  Things are never as fresh and straight forward after so many years of abuse.  This means the rocks we are looking for are often broken with younger rocks covering them.  The atmospheric elements and tectonic disturbances have distorted and altered the rock surface that we look at; therefore, Catherine must navigate through the modern landscape looking for places where these ancient rocks poke through the more recent deposits.  Once this concrete data is collected, the areas not containing outcrops must be elaborated upon and filled in to create a complete picture of the Earth's past and attempt to explain the changes occurring over a half a billion years ago.  Catherine's research uses her meticulous detailed work to explain the disorganized incompleteness of nature that is observed in the field.  She looks at seemingly uninteresting chunks of rock that poke out of flat Australian plains and provides a story and explanation for their existence and placement.  She considers these rock outcrops on both a macro and micro level.  In the field, we focus primarily on two  techniques - mapping and stratigraphic sections.  In the lab, she will spend countless hours considering the geochemical composition of the samples we have collected. 

Mapping:

Mapping focuses on the big picture.  We covered many miles, navigating our way from one outcrop to the next identifying the rock type, determining its orientation (which direction is it tipping and pointing), and drawing  their position onto mylar paper that overlays an aerial photo.  Once the actual locations of outcrop are recorded on the photo, it is like completing a connect the dots picture.  The known points of rock outcrop are connected creating a more complete picture of the rock layer positions across the landscape.

Aerial photo with mapped rock formations.

Mylar is like thick tracing paper that can overlay the aerial photo.

Catherine filling in the aerial photo map while in the field.  

Stratigraphic Section:

Stratigraphic sections look at one particular area in incredible detail.  We find a location where there is considerable quality outcrop, preferably including all of the different formations we are studying.  We try to walk perpendicularly to the rock layers, generally from the oldest layer towards the youngest layer.  As we walk, there are a number of things to consider: thickness of the layer, changes in the lithology (rock type), and any other interesting observations.  The thickness of layers are determined by using a large, L-shaped, metric stick.  We start at the first outcrop of the oldest formation we are studying.  This point on the ground is considered 0 meters.  By estimating the dip of the beds, the stick is tilted so that the measurement represents the thickness of the bed prior to deformation.  By eyeballing the line created by the top of the stick, you sight the furthest measured point.  Then you move to the next location and repeat the process.  This technique is easy where the rock exposure is of high quality and the beds are dipped directly perpendicular to our path.  The challenge is when the outcrop is obscured by sediments or tilted at different angles from the direction of our cross-section line.      

Catherine measures a nicely exposed, nearly vertical section of rock beds.  She will work her way up, stratigraphically measuring the thickness of the layers.

Catherine working her way through a lousy section.  Modern sediments have covered the outcrop in this particular section  Notice the outcrop behind her.  You will have to believe us that there is more rock exposure ahead of her.  

As we move along the section recording any notable observations and changes occurring, we also collect various rock samples which are then shipped back to Princeton.  Why ship rock samples half way around the world?  These seemingly insignificant chips of rock carry with them varying ratios of isotopes.  Elements such as carbon are made up of neutrons, protons and electrons.  An isotope is a version of an element that has more or less neutrons than one would expect to find.  The ratio of particular isotopes within a sample provides clues about the climate at the time the rocks were deposited.  And so, after the field season is over, we visit DHL with buckets of rocks that will make their way over the Pacific Ocean and United States and await Catherine's undivided attention and analysis back in the Princeton lab.

Trezona beds up close.  We sampled the carbonate rocks which are the layers that are recessed and lighter in color.

Me attempting to break a piece of carbonate free.

Catherine holding a bag of samples as well as one of the sampled carbonates.  The number written on the rock tells us how high in the stratigraphic section the rock was located.

Clast Counts:

The final piece of field work that we focused on was clast counts within a stratigraphic section.  Clasts are rock chunks that have broken off from some other larger rock.  Glaciers are infamous for ripping rocks apart, transporting these pieces, and then haphazardly dropping the clasts at some distant location.   Rock that has been deposited by glaciers reminds me of a Jello salad.  You know those really unappetizing jiggling side dishes that always show up on the Thanksgiving table filled with canned fruit cocktail and held together by the gelatin.  In this analogy, the Jello would be the fine grained matrix of the glacial rock and the fruit cocktail would be all of the different types of clasts suspended within the matrix.  The clasts we looked at included granites, limestones, basalts, sandstones, and quartzites to name a few.  We would measure the size of the clasts and note what type of rock they were.  If possible we would note the direction the clasts were oriented.  Understanding the clasts within these glacial deposited rocks helps Catherine understand and explain the movements of the glaciers that occurred so long ago.

Glacial formed rock (tilite)

 Tilite up close

Ultimately, we have spent the past six weeks scouring areas in Southern Australia with these particularly aged rocks.  Once all of the data from this field work (and Catherine's last four years) is compiled, the hope is that a more complete and conclusive story of the Precambrian Earth's overall environment and Australia's geological landscape will present itself.  This is our Australian field work is a very small, limited nutshell.  How did I do Alison . . . Catherine?

Last day in the field