Earth and Planetary Sciences ETDs

Publication Date

Winter 12-16-2019

Abstract

Chapter 1: Westernmost Grand Canyon

Conflicting hypotheses about the timing of carving of the Grand Canyon involve either a 70 Ma (“old”) or < 6 Ma (“young”) Grand Canyon. This paper evaluates the controversial westernmost segment of the Grand Canyon where the following lines of published evidence firmly favor a “young” Canyon. 1) North-derived Paleocene Hindu Fanglomerate was deposited across the present track of the westernmost Grand Canyon, which therefore was not present at ~55 Ma. 2) The 19 Ma Separation Point basalt is stranded between high relief side canyons feeding the main stem of the Colorado River and was emplaced before these tributaries and the main canyon were incised. 3) Geomorphic constraints indicate that relief generation in tributaries and on plateaus adjacent to the westernmost Grand Canyon took place after 17 Ma. 4) The late Miocene-Pliocene Muddy Creek Formation constraint shows that no river carrying far-traveled materials exited at the mouth of the Grand Canyon until after 6 Ma.

Interpretations of previously-published low-temperature thermochronologic data conflict with these lines of evidence, but are reconciled in this paper via the integration of three methods of analyses on the same sample: apatite (U-Th)/He ages (AHe), 4He/3He thermochronometry (4He/3He), and apatite fission-track ages and lengths (AFT). “HeFTy” software was used to generate time-temperature (t-T) paths that predict all new and published 4He/3He, AH, and AFT data to within assumed uncertainties. These t-T paths show cooling from ~ 100 ℃ to 40-60 ℃ in the Laramide (70-50 Ma), long-term residence at 40-60 ℃ in the mid-Tertiary (50-10 Ma), and cooling to near-surface temperatures after 10 Ma, and thus support a “young” westernmost Grand Canyon.

A subset of AHe data, when interpreted alone (i.e., without 4He/3He or AFT data), are better predicted by t-T paths that cool to surface temperatures during the Laramide, consistent with an “old” Canyon. This inconsistency, which mimics the overall controversy, is reconciled by optimizing cooling paths so they are most consistent with multiple thermochronometers from the same rocks and adjusting parameters to account for model uncertainties. We adjusted model parameters to account for uncertainty in the rate of radiation damage annealing during sedimentary burial in these apatites and thus possible changes in He retentivity. In the westernmost Grand Canyon, peak burial conditions (temperature and duration) during the Laramide were likely insufficient to fully anneal radiation damage that accumulated during prolonged, near-surface residence since the Proterozoic. The combined AFT, AHe, and 4He/3He analysis of a key sample from Separation Canyon can only be reconciled by a ‘young’ Canyon, but thermochronologic uncertainties remain large for this geologic scenario. Additional new AFT (5 samples) and AHe (3 samples) data in several locations along the canyon corridor also support a “young” Canyon and suggest the possibility of variable mid-Tertiary thermal histories beneath north-retreating cliffs. We conclude that application of multiple thermochronometers from common rocks reconciles conflicting thermochronologic interpretations and is best explained by a “young” westernmost Grand Canyon.

Chapter 2: Formation of the Grand Staircase

Dutton’s (1882) “Great Denudation” involved the lateral erosion of Mesozoic strata northwards from the rim of Grand Canyon via cliff retreat. He described a “great stairway” of alternating benches of erodible strata and cliffs of resistant strata now known as the Grand Staircase. We analyze 52 samples from across the southwestern Colorado Plateau and use linear inverse modeling of apatite thermochronometric data in HeFTy to determine continuous thermal histories for each sample. Results from these histories were then passed through a MATLAB code to extract temperatures of the weighted mean paths every 5 Ma, combined with geologic constraints on dated paleosurfaces, and interpolated across the study area. These interpolations show reconstructed cooling patterns of the top contact of the Kaibab Limestone through time since 70 Ma and can be used to estimate the geometry and rates of lateral cliff retreat and resulting erosional volumetric loss through time. We find three main times of denudational cooling, each associated with major through-going river systems. The Laramide orogeny (70-50 Ma) and incision of Paleocene fluvial valleys is associated with 55,750 cubic kilometers of volume loss above the Kaibab datum. The 35-15 Ma ignimbrite flare-up, initiation of Basin and Range extension, and incision of the East Kaibab paleocanyon is associated with 37,000 cubic kilometers of volume loss above the Kaibab datum. Post-6-Ma river integration of the Colorado River system through Grand Canyon and probable young uplift is driving ongoing deep incision and is associated with a minimum of 15,500 cubic kilometers of volume loss above the Kaibab datum, with at least 4,166 additional cubic kilometers of volume loss from the Grand Canyon alone. Cliff retreat rates are highly lithology dependent and range between 1.8 km/Ma for the Chocolate Cliffs, 2.4 km/Ma for the Jurassic (Vermillion) Cliffs, and 1.8 km/Ma for the Grey Cliffs. We conclude that three periods of accelerated denudation of the southwestern Colorado Plateau, most likely driven by epeirogenic uplift, represent punctuated periods of broad-scale cliff retreat that are coeval with evidence for significant fluvial systems. The Colorado Plateau region is thus a type example of epeirogenic controls on geomorphic re-shaping of plateau landscapes.

Chapter 3: Provenance and Pathways of the Music Mountain and Buck and Doe Paleorivers

Scattered remnants of “Rim Gravels” on the southern margin of the Colorado Plateau preserve a direct record of its fluvial and erosional history. Two paleoriver deposits containing well-described gravels occur on the far southwestern corner of the plateau, the Paleocene (55-50 Ma) Music Mountain Formation and the Oligocene-early Miocene (24-18 Ma) Buck and Doe Conglomerates (Young, 1999). Detrital zircon analysis can provide clues to potential sources and sinks of these deposits and more recently, high-precision detrital sanidine studies have been developed to tightly constrain the maximum depositional ages and provenance of Cenozoic deposits. In this study, we use a combination of new and published detrital zircon and detrital sanidine data to determine provenance and potential sinks of the Music Mountain Formation (n = 1090 U-Pb zircon ages and n = 5 detrital sanidine ages used for maximum depositional ages) and the Buck and Doe Conglomerates (n = 1028 U-Pb zircon ages and n = 6 detrital sanidine ages used for maximum depositional ages). In addition, we collected 4 samples (n = 670 U-Pb zircon ages) from formations in the Table Cliffs Basin (e.g. Markagunt and Paunsaugunt Plateau regions) to evaluate it as a potential sink for the Music Mountain Formation.

Detrital zircon age populations for the Music Mountain Formation are dominated by a 1691 Ma age peak with a smaller 1390 Ma peak and a small portion of Mesozoic ages. Grenville-aged grains (~1100 Ma) are conspicuously absent. This age distribution indicates that the Music Mountain fluvial system is sourced from the nearby Kingman uplift, where basement rocks of these ages occur. Comparison of this age distribution with coeval Laramide deposits from the Uinta Basin, San Juan Basin, and Table Cliffs Basin reveal that while none of these basins are ideal matches, connections with the Table Cliffs or Uinta Basins are unlikely. The San Juan Basin provides a potential match for age populations, particularly the significant ~1390 Ma age peak, and thus is preferred as a hypothesized sink for the Music Mountain fluvial system. The maximum depositional age via both detrital sanidine and detrital zircon is ~73 Ma, several tens of million years older than the best age estimates of 50-60 Ma and provides little new information.

Detrital zircon age populations for the Buck and Doe Conglomerate are also dominated by peaks at ~1680 Ma and 1390 Ma, although the 1390 Ma peak is significantly stronger in comparison with the older Music Mountain Formation. There are very minor peaks of Mesozoic-aged zircons and a sharp peak at ~20 Ma sourced from local volcanism, although this is only present in two of the 6 samples. These age populations suggest continued unroofing of the Kingman uplift with increased contribution from the 1390 Ma Peacock Mountain Granite and reworking of the Music Mountain Formation. Contemporaneous deposits include the lower portion of the Brown’s Park Formation from northern Utah and the Rainbow Gardens Formation in the Lake Meade region of eastern Nevada, and a sample from the Table Cliffs Basin that unexpectedly yielded a population of ~20 Ma zircon U-Pb ages. Comparison of age spectra for these units with the Buck and Doe ages indicate that a relationship with the Rainbow Gardens or the Brown’s Park Formation are unlikely, but that there is a high likelihood of a connection with the “White Claron” sample from the Table Cliffs Basin. Maximum depositional ages from detrital sanidine for three samples from the Buck and Doe Conglomerate are 18.065 ± 0.017 Ma, 25.52 ± 0.10 Ma, and 30.2 ± 0.2; the youngest of these is slightly younger than the Peach Springs Tuff (18.78 ± 0.02 Ma), which caps the type section of the Buck and Doe Formation.

The erosional history of the southern Colorado Plateau has been a topic of scientific debate for over 150 years. Key conclusions include 1) both of these fluvial systems were sourced from the Kingman uplift to the south as shown by the 1390 Ma age population, 2) a potential connection is proposed between the Music Mountain Formation and the San Juan Basin, which is also based on this 1390 Ma population, and 3) we also propose a ca. 18 Ma connection between the westernmost Buck and Doe sample and the Miocene “White Claron” of the Table Cliffs Basin, indicating there was no paleo Grand Canyon at that time. The timing of these fluvial systems is consistent with the major periods of denudation found in Chapter 2 and these paleorivers provide potential pathways for the removal of this material.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Karl Karlstrom

Second Committee Member

Shari Kelley

Third Committee Member

Matt Heizler

Fourth Committee Member

Laura Crossey

Fifth Committee Member

Thomas Turner

Language

English

Keywords

low-temperature thermochronology, apatite, detrital zircon, detrital sanidine, Grand Canyon, Colorado Plateau

Document Type

Dissertation

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