Are we in the Smith River Allochthon?: A geologic trope through Gladstone

This week I posed as a Gladstone Gladiator (my research advisory cohort of 4 structural geologists) in the Gladstone quadrangle to gain field experience and help with data collection for their USGS EDMAP project. The Gladiators’ goal is to map the 7.5 minute quadrangle’s geologic components over the course of the next year, and my goal was to practice my field methods over the course of a week. There was no diamictite evident, but most rocks exhibited a fair amount of strain and that satisfied my research theme of the summer just fine.

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Gladstone Gladiators (plus the Honorary Omani [me on the left]) prior to the first traverse!

Our journey in the Piedmont was based from the James River State Park each day. We had a solid primitive camp setup along the James River, which we left early in the morning and returned to around early evening each day. After reviewing the topographic maps and previously published geologic maps of the area each morning, the W&M Geo minivan and the ’97 Forerunner would head to the trail head and trail end, respectively, and the Gladiators would begin the traverse. We were armed with Brunton compasses, GPS’s, topographic maps, moderately recent Google Earth aerial imagery, hand lenses, field books, and Chuck’s homemade clipboards (of which he needs to patent soon). Each of the 4 original Gladstone Gladiators had a specific area of interest within the confines of the quadrangle, so each traverse was engineered to cover a broad swath of that geologic focus. I was worried that my inexperience with fieldwork/mapping and lack of knowledge of the area would be a large detriment, but I soon caught the hang of the process and was able to quiz my field mates on their areas of expertise to build my knowledge of the regional geology.

The traverses ranged from bush whacking to upstream rapid paddling, and stopping often to log a new outcrop and check for ticks. Not only was I a strong contender in the Tour de Tick category for number of bites in 4 days (9 bites), but I also learned a great deal about geologic mapping techniques, and rock identification specifically. We used the Universal Transverse Mercator (UTM) system for locating ourselves via GPS and topographic map and Brunton compass for taking structural measurements, which the responsible parties would note and later use for formal mapping.

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The Old Goat and Amadan bushwhacking and collecting ticks at the front of the pack.

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CF, Amadan, and the Old Goat examining the greenstone and preparing to collect samples (pre swim)

Most of the rocks we came upon fell into 1 of 4 categories: phyllite, a metamorphosed mudstone (we were often swimming in a sea of the stuff); carbonates, mostly limestones and marbles that react to hydrochloric acid; metavolcanics, deformed volcanogenics that often had magnetite(!!); and diabase, an igneous rock that crops out in multi-meter sites which indicated dikes. On one of our ridge hikes, we were treated to a different sedimentary rock pictured below.

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This conglomerate sandstone is so extraordinary a sample that we may need to return and claim it for our WM Rock Garden! The top half of the boulder (above the cyan line) is a sandstone, and the bottom half is a blue quartz meta-conglomerate. It’s so special because of the obvious facing indicators, obvious shear, and potential inclusion in the Smith River Allochthon.

This rock was about 3-5m from the mother rock, but we could also determine that it was not in situ because of the facing indicators. Geologists use facing indicatorsto determine the original orientation of rocks, which have implications for the forces that move these rocks. While this boulder was likely just move by a flow (instead of structural forces) we can tell that the rock is overturned.

The first clue is the grain size within the conglomerate. These grains were likely deposited at the bottom of a river chanel, and should display a fining upward sequence, simply meaning that the rocks at the bottom are larger than those at the top. We can see, though, that the grains in this conglomerate near the cyan line are in fact larger than those near the compass.

If you’re not convinced by the conglomerate, though, you can look at the beddingwithin the sandstone (shown in white). Bedding are the layers of a sedimentary depositional sequence that indicate how the sediment was initially collected on a surface. We assume the principle of original horizontality: that gravity causes sediment to settle horizontally on a surface. But this sandstone also exhibits cross bedding (shown by the cyan lines), or bedding that truncates other layers. We can infer that the original bedding was tilted and eroded in some manner, and then the cross bedding was superimposed (emplaced on top of the original bedding).

The meta-conglomerate gets the meta because of the deformed and aligned grains that dip approximately 45° to the right of the photo and the sinistral (or left handed) sense of shear. If I were to perform a strain analysis on this rock, likely using the Center to Center method as opposed to Rf/ phi, I’d throw a rough estimate toward a strain ratio of ~2 (not very deformed), but we know there was some amount of deformation because of the general uniformity expressed by the grain alignment.

The final point of grandeur within this sample (at least for my argument) is the depositional environment from which this rock came. How would a sandstone/meta-conglomerate boulder from a river bottom make its way to the top of a mountain ridge? Thats a question the Gladiators will work on answering later throughout the summer, but I’m willing to bet a thrust belt was involved.

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