Earth and Planetary Sciences ETDs

Publication Date

5-1-2016

Abstract

The timing and processes of development of the high topography and high relief of the southern Rocky Mountains of Wyoming, Colorado and New Mexico has been controversial for over a hundred years. The Mesozoic and Paleozoic rocks in the region formed a 3-4 km thick sub-horizontal stratigraphy that resided at elevations of zero to minus 4 km at the end of the Cretaceous. These units have been differentially uplifted/subsided and are presently at elevations of -10 to 4 km elevations, and have been eroded from above the core of many Precambrian basement cored uplifts exposed in > 4 km peaks. This study applies several different methodologies towards understanding the timing and relative importance of the events that uplifted and shaped the Rocky Mountains. These studies include incision studies (Chapter 1), detrital zircon analysis of Cenozoic fluvial deposits resting on the Rocky Mountain Erosion Surface (Chapter 2), and low temperature apatite fission track (AFT) and U-Th/He (AHe) low temperature thermochronology to look at differential cooling histories across the Rockies (Chapter 3). Chapter 4 investigates a project in which I designed and deployed a mobile device application for enhancing the traditional geoscience field education and research experience. Incision studies (Chapter 1) investigated the timing and rate of incision of the Black Canyon of the Gunnison by the Gunnison River. Canyon geometry is shaped by a headward-migrating knickzone presently located within the Painted Wall section of the Black Canyon. Average bedrock incision rates over the last 0.64 Ma surrounding the knickpoint vary from 150 m/Ma (downstream), to 400-550 m/Ma (within), to 90-95 m/Ma (upstream), suggesting it is a transient feature. Lava Creek B ash constrains strath terraces along a paleo-profile of the river. Within the paleo-Bostwick River tributary, we determine an incision rate of 400-550 m/Ma, reflecting incision at 2-3 times regional incision rates. We interpret this to be incision response to a headward-migrating wave of transient incision, potentially initiated by downstream base level fall during abandonment of Unaweep Canyon at about 1 Ma. Rate extrapolation indicates that the ~700 m depth of Black Canyon has been eroded since 1.3 – 1.75 Ma. The Black Canyon knickpoint overlies a strong gradient between low velocity mantle under the Colorado Rockies and higher velocity mantle of the Colorado Plateau. We interpret drainage reorganization and transient incision of both the Gunnison and upper Colorado River systems to be responding to mantle-driven epeirogenic uplift of the southern Rockies in the last 10 Ma. This study (Chapter 2) investigates the timing of erosion and deposition of several fragmented and little-understood conglomerate and sedimentary units, which are then used to better understand the timing of formation of the enigmatic Rocky Mountain Erosion Surface. Detrital zircon studies reclassify the Telluride Conglomerate and Blanco Basin Formation as being of Oligocene age, with maximum depositional ages of 30±3 Ma. This requires new understanding of the uplift of the San Juan Mountain regions, as we interpret these units to be the conglomeratic/sedimentary response to pre-volcanic epeirogenic doming and unroofing. This study also expands into New Mexico to investigate the depositional timing of the Paleocene-Eocene age McDermott, Galisteo and Baca Formations, and the Oligocene-age El Rito Formation and Ritito Conglomerate; each of these units rests upon the diachronous Rocky Mountain Erosion Surface and grades upward into volcanic deposits; these units are interpreted to be a sedimentary record of the multi-stage uplift of the Rocky Mountains. At each age, these units record the repeated process of doming and unroofing of basement rocks accompanying magmatism, deposition of thin gravel-sand sheets above regional unconformities, deposition of voluminous volcaniclastic aprons. This study suggests that relief has increased through time as the Rocky Mountain headwater regions of the fluvial systems that deposited progressively younger units of this study have gained in absolute elevation, while the ultimate base levels have remained the same or very similar. The third study (Chapter 3) moves to larger scales of both time and space in order to investigate uplift and exhumation histories on the regional scale. This study included a literature review and compilation of all published low-temperature thermochronology data (apatite fission track and apatite (U-Th)/He) (AHe) for the southern Rocky Mountain region and adds new apatite (U-Th)/He data on selected conglomeratic/sedimentary units to both add richness to the data base and to attempt to better constrain Oligocene burial histories. This study reinforces interpretations of a multi-stage uplift history to the Rocky Mountains by identifying regional episodes of rapid cooling that was driven by regional exhumation and differential uplift. Laramide cooling is investigated in compiled AFT and AHe data and age-elevation transects across the region. We notice spatial preservation of Laramide ages in Rocky Mountain thrust uplifts and elevation differences between the base of the Laramide-age AFT partial annealing zone, reinforcing models suggesting that Laramide cooling was driven by thrust uplifts that have not been reheated since uplift. New AHe data show that the Oligocene fluvial deposits were covered by ~1 km of volcaniclastic debris that likely extended westward onto the Colorado Plateau, refining understanding of the geometry and extent of the San Juan volcaniclastic apron. A delay of ~10 Ma between Oligocene magmatism and regional cooling through the 110-50ºC (AFT-AHe system) implies that post-Oligocene denudation was driven by erosional denudation following Oligocene surface uplift. Case studies highlight the semi-steady cooling of Oligocene plutons from ~30-15 Ma, followed by a potential ~20-10 Ma acceleration in cooling perhaps due to local faulting and/or uplift associated with Rio Grande rifting. Post-10 Ma cooling in areas not associated with the Rio Grande rift is compatible with models that incorporate regional integration of the Colorado River and Rio Grande to cause differential incision and localized rapid cooling. The fourth study highlights a project called Field Play. Our current human interaction with these landscapes is also of importance. The communication of science and education of Rockies both academic and public populations is of great importance as we have unprecedented access to information and rapidly evolving digital technologies. This project utilizes mobile technologies (smartphones, tablets) to augment the field experience. The goal of this project is to create a personalized, scalable and interactive educational learning environment which facilitates user exploration and access to robust geologic information. This project was featured in the New Mexico Geological Society's 2014 Fall Field Conference, as well as being used in my own teaching of introductory environmental science classes. This project is ongoing.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Crossey, Laura

Second Committee Member

Weissmann, Gary

Third Committee Member

Scuderi, Louis

Fourth Committee Member

Kelley, Shari

Project Sponsors

National Science Foundation Alfred P. Sloan Minority PhD Foundation New Mexico Geological Society Colorado Scientific Society

Language

English

Document Type

Dissertation

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