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• Rosa Bieber-Stanley
• Malin Just
• Sonia Nicholson
• Sofie Prieto

Rosa Bieber-Stanley

Advisor: Arlo Weil

Investigating Features of Laramide Deformation and Shortening on the Colorado Plateau

The Rocky Mountain system of Western North America, formed primarily by the subduction of the Farallon plate under the North American plate, is defined by two major tectonic systems, the Sevier and Laramide belts, which overlap temporally and spatially. The Laramide belt has a thick-skinned style with basement cored arches of various orientations that were uplifted along high angle reverse faults. The Laramide is unusual because it involved the transfer of stress far inboard into the continent. While various hypotheses, including flat slab subduction, have been posited for driving Laramide uplifts, the causal relationship between subduction style and the style of Laramide deformation is not yet fully understood.

The Colorado Plateau of the Four Corners region acts as a semi-rigid body within the North American plate that records a number of subtle Laramide structures. To help develop a more robust understanding of Laramide deformation, we will travel to the Colorado Plateau and surrounding areas to measure deformation structures such as minor faults and tectonic stylolites. We will also collect rock cores for paleomagnetic analysis. Data collected from analysis of these field and laboratory measurements will be used to establish shortening directions for Laramide deformation. However, to properly interpret these past shortening directions in the context of Laramide tectonism, the presence of vertical axis rotation must be assessed using paleomagnetic analysis.

To do this, rock cores will be drilled in the field from Mesozoic red bed sandstones that contain abundant ferromagnetic minerals. In the laboratory, thermal demagnetization techniques will be used to iteratively strip away overprinting magnetic signals to identify the characteristic magnetic component that was recorded contemporaneously with the rock's formation. The magnetizations will then be used to quantify the distribution of relative vertical axis rotation between various Laramide arches in the Colorado Plateau region, and ultimately to correct our interpreted shortening directions to their orientations at the time of Laramide tectonics convergence. In order to properly interpret the significance of these results, rock magnetic experiments will be done to identify the mineralogy and granulometry of the samples to better understand the origin and significance of the magnetic remanence carriers. Ultimately, these data will further our understanding of how Laramide-related subduction resulted in the deformation styles observed in the Rocky Mountain system.

Malin Just

Advisor: Pedro Marenco

Investigating the temporal and spatial variation in the physical characteristics of mud-mounds in Early to Middle Ordovician reefs of the Ibex Succession, Utah, USA.

Reefs as we know them today are at risk due to ocean acidification and coral bleaching. To better understand how these reef environments might change as a result of these processes, researchers can gain insights from fossil reefs. The fossil record reveals multiple transitions from times during which reefs were largely built by microbial activity to those when reefs were constructed by animals. Fossil reefs located in the Ibex region of west central Utah represent different reef-building intervals during the Ordovician Period. These reefs were formed about 480 million years ago during the Early to Middle Ordovician and are characterized by dome-shaped build-ups of mud known as mud-mounds, some of which may have been built by microbial communities. The physical dimensions, spacing, frequency, and shape of the mounds vary between reef units.

In order to properly visualize the evolution of Ordovician reefs, it is helpful to map the physical characteristics of each unit. During this project, I intend to investigate how the frequency, physical dimensions, and spacing of mud-mounds vary temporally and develop a model to represent Early to Middle Ordovician reef-building environments in west central Utah to answer the following questions: How do the physical characteristics of mud-mounds vary in the Ibex Succession? What might these patterns reveal about Early to Middle Ordovician reef building environments in the Ibex Region?

To create the model, I will use GPS points of mounds from seven outcrops at three distinct geographical sites covering a time period stretching from the Early to Middle Ordovician to geocode the sites of each mound and mound unit. We mapped transects containing about four mounds each and then expanded those transects using drone footage to gain a wider view of the entire unit. I will use this footage to compliment the mapped dimensions of mounds and mound spacing. This data represents seven temporally unique instances of reef growth within the Early to Middle Ordovician. After quantifying and digitizing field data and observations and metadata from the drone footage, I will link the data for each mound to GPS points of the mounds. I will then map all of the data using ArcGIS. Previously collected petrographical and geochemical data for the mounds will be added to the geocoded points to create as high of a resolution model as possible. Using the GIS maps, drone footage, and digitized field notes, I will create a model for changing reef geometries and the depositional environments during the Early to Middle Ordovician Period. Comparing the results of my project to the development of modern reefs in the age of ocean acidification will help us understand what modern reef environments may look like without living corals.

Sonia Nicholson

Advisor: Arlo Weil

Analyzing Laramide Deformation and Paleo-Stresses in the Colorado Plateau

The Rocky Mountain tectonic system formed as a result of the eastward subduction of the Farallon Slab beneath the North American Plate between about 120 and 50 million years ago. This long-lived phase of subduction drove deformation far into the Rocky Mountain foreland basin, forming the Sevier and Laramide belts, which overlap temporally and spatially. The thick-skinned Laramide deformation, which is today located far inboard from the paleo-plate margin, contrasts with the earlier thin-skinned Sevier deformation, which is thin-skinned in style and located closer to the paleo margin. By studying Laramide structures exposed in the Colorado Plateau region, we will contribute to the understanding of the shortening history of the region, and ultimately how the resultant stresses produced by Laramide-style subduction caused the deformation styles observed in the Colorado Plateau/ Four Corners region. 

This project aims to further quantify the shortening direction for Laramide deformation in the Colorado Plateau/ Four Corners region. To do this the Anisotropy of Magnetic Susceptibility (AMS) will be used on Mesozoic red beds collected in the field later this summer. In undeformed rocks, the easy-axes of magnetic susceptibility are generally randomized. However, once a rock starts to undergo tectonic shortening, the magnetic susceptibility axes tend to rotate into a preferred orientation that is indicative of the applied stress. Therefore, the AMS measurements can be used as a proxy to infer shortening directions. In addition to AMS, structural deformation indicators, such as minor faults, widespread fracture and vein sets, and tectonic stylolites will be measured in the field and analyzed to quantify the orientation and spatial distribution of paleo-shortening directions. These structural deformation indicators have previously been interpreted to record Laramide-related layer-parallel shortening and therefore can be used to infer paleo-stress directions from the time of Laramide tectonism. Combined with ongoing paleomagnetic analysis to determine the degree of vertical axis rotation across the sampled region, our interpreted shortening directions will be used to test existing models that link Laramide-style subduction to the structural styles observed today across the Laramide system.

Sofia Prieto

Advisor: Katherine Marenco

Investigating Lower-Middle Ordovician Reef Fabrics Using Grid Analysis and Petrography

Reefs during the Early-Middle Ordovician Period (~485 to 460 million years ago) experienced a significant transition from microbialite dominance to increased contributions from metazoans (animals) such as sponges. My summer research is part of a broader project which aims to quantify the contribution of microbial communities to reef-building from the Early Ordovician into the Middle Ordovician in an effort to understand how ancient reef ecosystems evolved in response to environmental and biological changes. I aim to test the hypothesis that sponges became more significant contributors to reef fabrics over time.

We traveled to the Ibex region of western Utah, which contains multiple reef-bearing rock units from the Early-Middle Ordovician Period. We used a grid sampling technique in which a grid frame was placed on an exposed mound surface in order to systematically record the presence/absence and spatial distribution of sponges, receptaculitids (a sponge-like animal), suspected microbialite textures, and fossil fragments within a 25 cm x 25 cm area. This summer I will digitize the grid results, investigate patterns in our new data, and compare them with previously collected data. With these additional grid results, we have grid data from every mound interval in the Lower through Middle Ordovician succession in the Ibex area. In addition, physical samples were collected from representative mounds. In the lab, I will prepare thin sections from a subset of these samples for petrographic analysis. I will use a petrographic microscope to examine the microbialite fabrics and sponge fossils on a smaller scale. My goal is to compare data from each unit in order to identify changes in overall reef fabric, suspected microbialite textures, and the abundance of sponges within the reefs. This research will contribute to our understanding of the underlying biological and environmental dynamics that facilitated the transition from microbial to metazoan dominance in Early-Middle Ordovician reefs and set the stage for further metazoan involvement in reef frameworks.