Lab Work, Literature, and Finishing Up

Upon returning to Williamsburg, around June 16th, I began the process of interpreting the data I collected in the field. With the help of my advisor, I spent the day organizing data, preparing samples, and setting up what I was to work on for the rest of the summer, until I returned to campus again.

Geologists take detailed descriptions of the rock formations they find in the field, noting characteristic such as color, rock type, mineralogy, primary and secondary structures, and measurements such as orientation of bedding and/or foliation. These descriptions are important not only to the geologist that measured them, but other scientists as well, as they help future researchers continue what previous scientists started. With the assistance of a microscope, I studied the samples I previously collected in the field in greater detail, ensuring that my descriptions were as accurate and thorough as possible. I recorded any new observations along with the ones I had previously made in my field notebook.

After revisiting my rock descriptions, Chuck Bailey, my advisor, and Ben Weinmann, a recent graduate of W&M, taught me how to cut my samples with a rock saw, so I could use them to measure density and prepare them for observation under a microscope in thin section. Using the rock saw proved to be a bit of a challenge due to the nature of my samples. Most of my collected samples were fault rocks or heavily foliated metamorphic rocks, all fractured to some degree. Because of this, they were prone to crumbling when sawed. Even while going quite slowly to apply the least amount of stress to the samples, I was only able to get a cube of one fault rock sample and one metamorphic sample, and thin sections from those samples plus one additional fault rock. One of my other metamorphic samples, a chlorite breccia, was so heavily fractured I didn’t bother with cutting it. The other sample, a chlorite schist, which was also my oriented sample, I was too nervous to cut, so my advisor offered to attempt cutting it himself later.

While waiting for samples to dry off after cutting them, I met with Chuck in his office to do some dimensional analysis with Google Earth. Using UTM coordinates and strike and dip measurements taken during our traverse, we plotted the waypoints from the traverse as well as made a rough diagram of the fault and its geometry. Surprisingly, the fault looked wavy from the data I collected. Previously, I had only though of faults as having flat scarps, so to see otherwise was quite fascinating.

The challenges with cutting samples proved to not hinder my research terribly, as I had both metamorphic and fault rock samples that had cut well. With the cubic cut samples, I took density measurements of the fault rock (limestone breccia) and metamorphic rock (amphibolite). I found the breccia to have a density of 2.75±0.15 g/cm³ and the amphibolite 3.1±0.2 g/cm³. These values were to be expected since they are upper crustal rocks that have undergone some degree of compressional/shearing force.

After I had completed all this in the lab, Chuck helped me begin to search through the literature for more information on the Castle Cliff Fault. For the rest of my summer, I spent the bulk of my research time parsing through academic literature to understand what leading structural geologists have to say about the Castle Cliff Fault and surrounding region. The mechanics of the fault have been disputed since the mid-twentieth century, so as I read, I tried to gain an understanding of how opinions on the kinematics of the Castle Cliff Fault have changed over the decades. In recent years, geologists have come to believe this fault to be either a detachment fault or gravity-slide event. Based on data presented in the literature, I currently favor the gravity-slide theory, but it won’t be until I have a chance to look at the thin sections I previously prepared that I can say anything conclusively on where I stand on the matter.

Once the thin section slides come back to the lab, my advisor and I will examine them in order to further understand the forces at play in the Castle Cliff Fault. Once I have this data, I will be able to draw some conclusions as to which kind of normal fault applies to the Castle Cliff Fault, whether it is the result of a gravity-slide event or detachment fault. I’m excited to see what information my slides yield, and to present my conclusions later in the coming month.