Measuring a stratigraphic section

Note: It may be helpful to read the Project Summary before reading this post.

Within every stack of sedimentary carbonate rock lies a record of the evolving environment and climate of the Earth, locked up in isotopic signatures that are only deciphered after analysis in the lab. To fully interpret these signals, we must first understand the physical settings in which the carbonates were deposited. This kind of information can only be uncovered through fieldwork, and measuring stratigraphic sections is one important way of determining the sedimentary succession of different rock types and observing subtle clues left behind by the environment.

The Wonoka formation, which is our main focus, is composed of siliciclastic siltstone interbedded with layers of carbonate. During the Ediacaran, these carbonates deposited flat on the ocean floor on top of an ancient continental shelf, precipitating like sea salt and preserving information about the sea water it formed in. At the same time, the continent eroded sediments into the ocean, leading to the formation of distinct layers of two different kinds of sediment. Long after these carbonates and siliciclastics solidified into rock and were buried by other deposits, the flat layers became jumbled up in a continental collision, exposing themselves as tilted beds in the Flinders mountains where we see them outcrop today.

Some places have better outcrop than others; luckily, instead of wandering around aimlessly throughout Australia, we can use satellite imagery and geologic maps to help spot potential locations with near 100% exposure of the Wonoka. Unfortunately, the vegetation in satellite photos can be an obstruction to seeing the rock, and oftentimes can be highly misleading, so its usually necessary to do some reconnaissance work on foot at the site of interest to determine in the section is worthy for measurement.

If we find lots of carbonate beds and the outcrop looks relatively continuous, it's hammertime. Starting from the bottom of the Wonoka (i.e. the oldest rock), we walk perpendicular to the plane of bedding with Jon measuring our stratigraphic height and taking notes on the lithology, grain size, and sedimentary structures of the rock. As we come across carbonates, it is my job to smash them with my rock hammer and take a sample for carbon isotope analysis. This way, a stratigraphic height in the Wonoka can be matched to a unique isotopic value.

The process may sound straight forward, but the seemingly obvious task of differentiating between carbonate and siltstone can actually be quite tricky. Oftentimes we rely on distinctive weathering patterns. For instance, carbonate is less resistant to weathering than siltstone and thus frequently appears as a recessive layer between more resistant, protruding siltstone. Carbonate also exhibits karst texture, which I will poorly describe to you as sharp-edged ripples. When surface features fail to readily identify a rock unit, we'll hammer a rock open to reveal a fresh surface and look for signs of carbonate's crystalline, sugar-like texture.  As a last resort, we can try and fizz the fresh surface with a drop of 5% HCl solution, though sometimes this test can be misleading due to annoying things like carbonate-cemented siltstone.

To shed light on the physical conditions during deposition, we look for several sedimentary structures captured in time within the rock.  Small, symmetrical ripples suggest that the sediments deposited on a shallow shore rather than in the stormy deep ocean, and stromatolites indicate that they formed within the photic zone.  And larger more assymmetrical ripples can tell us a thing or two about the direction of paleocurrents and the energy of sediment transport.

Since we are collecting samples on a meter scale resolution, depending on how much carbonate we find, measuring a section can take just one afternoon or two whole days.  So far, we've measured three sections, two of which were more than 1000 meters long (but only one of them took forever). Sometimes, the carbonates are found in nice 200-meter thick packages while other times, you'll only find boring siltstone for what seems like miles and miles without end (in reality, it's just a bunny hill or two).

Of course, science is not without its sacrifices. In the name of research, I have taken rock shrapnel to the face, cut myself on karsted carbonate, kneeled on a bed of thorns, been trapped by the webs of palm-sized spiders, and attacked by ants crawling up my pants (more on all that in a week).  It's all worth it though--I mean, just look at this sunrise: