Mapping breccias

In an earlier post, I discussed how measuring a stratigraphic section can show us how rocks change from top to bottom. The end result is a one-dimensional, vertical chart of stacked boxes representing different rock types, but rocks rarely behave like perfect layers of a cake--more often than not, rocks change from side to side. For example, if we followed one rock layer down its length, it may change in lithology, sedimentary structure, or thickness, perhaps indicating changes in sea level at the time of deposition. To capture and understand this variability, we strive to create a geologic map.

In the last two places we've been, Jon and I have set out to create such a map, combining old methods with new fancy-schmancy technology. At Oodnapanicken, we split up and each mapped out the paleocanyon separately; Jon used the newly acquired Trimble technology while I mapped the old fashioned way, by hand with paper and pencil. 

For me, old-school mapping is a throwback to my preschool days of scribbling within the newsprint sheets of connect-the-dots coloring books. In the same way that those numbered black dots form a picture (or some semblance of one), mapping provides a two-dimensional visualization of the outcrop and the interpretations made from it. The only difference is that we are not given the black dots on the page; those known values are exactly the things we seek out while mapping.

My coloring book is my clipboard full of 1-to-1000 scale Quickbird satellite images of the area, each overlaid with a thin mylar sheet. Instead of chubby crayons and washable markers, my arsenal consists of a GPS, a fine set of retractable colored pencils, and my eyes. As I walk through the paleocanyon and come across a layer or two of rock poking out from the ground, I identify it and then locate my position on the satellite image with the help of my GPS. Once I've found my spot on the map, I use the colored pencil designated to the rock type of the outcrop in question and make a dot on the mylar overlay. 

This process continues until I have many dots of multiple colors on the mylar sheet, and soon patterns emerge. With enough dots, boundaries between rock units become apparent and viola!--a geologic map begins to form. The more dots on the map, the easier it is to draw conclusions regarding unit boundaries, however sometimes things like dirt, bushes, and trees obscure the outcrop enough to cause large empty gaps in the map. Small-scale faults can also complicate the position of layers, and you often won't know that there is a fault until the unit you're following suddenly disappears and comes up again tens of meters up or down stratigraphically. If you're lucky, you'll only come across just one type of rock unit for hundreds of meters, allowing you to scribble across an entire mylar sheet in one color with a satisfying flourish. 

At Saint Ronan, Jon and I switched roles, and I had my first real taste of Trimble-ing. The beauty of the Trimble and the rest of the GEOXT 6000 Explorer ensemble is that it allows us to map units to nearly 30 centimeters of precision--30 centimeters! With the Trimble, we can map on a resolution much finer than would be possible by hand, which can make a difference when certain rock units are less than a meter thick. For instance, numerous carbonate breccias are found within the paleocanyon at Saint Ronan, but these layers can be as thin as a tenth of a meter (in fact, most of them were). Using the traditional method of mapping by pencil and paper, the breccia layers would be amassed together into one unintelligible unit, but the Trimble allows us to differentiate them.

So how does the Trimble work? Well, more-or-less, the Trimble is just a super fancy, Martian-looking set of GPS equipment that is able to correct for drift, the main cause of inaccuracy in most hand-held GPS devices. If you were to stand in a single spot for an entire day and record GPS points, you would find that your position according to your GPS would change, affected by factors such as the number of connected satellites and cloud cover. This scattering of points is called drift. In order to correct for this phenomenon and get our amazing 30-centimeter precision, we first set up a base station up on a ridge of a hill near the center of our area of interest. For approximately seven hours on the day before we begin mapping, we allow the station to log data points on its GPS location every five seconds such that when the day is through, we (or rather, the computer) can calculate the average of all those GPS points to give us extremely precise coordinates of the base station's location (as in 6-places-after-the-decimal-point precise). The next day, using a two-meter tall carbon fiber staff with an antenna mounted on the top, we collect GPS locations of the breccias, inputting information such as thickness and bedding orientation into the Trimble whenever possible. While doing this, the base station continues to collect data on its GPS location in present conditions, but this time compares it to its previously determined location to figure out how much drift is occurring. At the end of the day, the computer differentially corrects our breccia data according to the amount of drift occurring at the time each data point was collected, and tada! Supertastic precision.

Mapping has been my most favorite task in the field so far, perhaps because I am on my own, fully responsible for what gets accomplished that day (not that Jon isn't great company). I can work at my own pace, taking Crunchy Nut Nutty granola bar breaks whenever pangs of hunger strike, and I am forced to use my brain without having a knowledgeable grad student around to answer my questions. It's a cool feeling, knowing that you are one of the only people around for perhaps hundreds of kilometers, yet the sense is not one of isolation. Rather, I find that I can sing and hum anything and everything I want, as loudly or as ridiculously as I care.