1. Jackson Hole, WY using Detrital Zircon U/Pb Geochronology
Age and Provenance of proximal Tertiary Quartzite Cobble Conglomerates near
Jackson Hole, WY using Detrital Zircon U/Pb Geochronology
RAPPE, Jason, Geography-Geology, Illinois State University, 100 N University St, Normal, IL 61761, MALONE, David, Geography-Geology,
Illinois State University, 100 N University St, Normal, IL 61761, and CRADDOCK, John, Macalester College, 1600 Grand Ave, St. Paul, MN 55105
Abstract
Quartzite cobble conglomerates of Late Cretaceous to Eocene in age outcrop in deposits up to 1-4 km thick in areas of Sublette and Teton Counties, WY.  Our goal in this research is to characterize the age and provenance of these clasts using detrital zircon U/Pb geochronology.  As part of this study, we sampled three different formations (n=475; zircon analysis total) at six localities.  The Harebell Formation was sampled at along Buffalo Fork (n=85) and Pacific Creek (n=83).  The Pinyon Formation was sampled along Pacific Creek (n=43), in the Gros Ventre Mountains (n=98) and at Hominy Peak (n=82).  The Hominy Peak Formation was sampled just south of Hominy Peak (n=84).  Meso- and Paleoproteroic zircons dominated these spectra, ranging from 71-91%.  Grenville and Archean zircon populations also are present, ranging from 1-15% and 5-18%, respectively.  The overlap and similarity indices ranged from and , respectively.  These data indicate that the protolith sediment of these quartzites were generated mainly in the Yavapai-Mazatzal orogenic belt to the south and east.  This sediment was transported to the Neoproterozic continental margin of ID and MT and then metamorphosed.  These quartzites were then uplifted during the Sevier Orogeny, were eroded and the clasts were transported eastward into the Western Interior Basin.  It is likely that much of this sediment could have been reworked and recycled into younger conglomeratic units.  The quartzite clasts studied here have similar detrital zircon spectra to quartzite bearing conglomerates of the Fort Union, Willwood and Wapiti Formations to the east.
Regional Setting and Problem Definition
The Late Cretaceous Harebell Formation is exposed in and around Jackson Hole. It is primarily sandstone with interbedded quartzite conglomerates unconformably overlain by the Pinyon Formation consisting of quartzite conglomerates with thin interbedded fine grained sandstones. Above is the Hominy Peak Formation separated by an unconformity. This is part of the Absaroka Volcanic Supergroup and consists of quartzite conglomerates at the base with volcaniclastic conglomerates, tuffs, and claystones above these, which spans from Late Cretaceous to Eocene in age, is found north of the Teton Range as well as Jackson Hole and North of the Gros Ventre Range. Because of their similarity, the Harebell and Pinyon formations are considered to have the same depositional source, and are frequently studied collectively. The stratigraphic relations of the Harebell, Pinyon, and Hominy Peak formations in the Teton Range region are shown in Figure 1. Photographs of some of the sampling localities are shown in Figures Figure 5 is a geologic map of the region that indicates sampling localities.
For this study, six samples were collected to determine ages of the Harebell, Pinyon, and Hominy Peak Formations in these locations. The goal of this research was to determine, what, if any, variability of zircon provenance exists between quartzite clast populations in conglomerates that range span more than 50 Ma in age. If there is significant variability, this would mean that the source area and materials evolved over time through the Sevier to Laramide transition. If there is little variability over time, this would indicate that the quartzite source was stable, and supplied sediment for a considerable amount of time. That, or, there was significant recycling of quartzite clasts from one unit to another.
A secondary goal was to determine the provenance of the quartzites themselves, and to relate these to the provenance to other Cordilleran Neoproterozoic quartzites.
Figure 6: Histogram of Harebell Clasts at Buffalo Fork.
Figure 7: Histogram of Harebell Clasts at Pacific Creek.
Figure 8: Histogram of Pinyon Clasts at Pacific Creek.
Figure 9: Histogram of Pinyon Clasts at Hominy Peak.
Methodology
Six samples (n=477 total zircons) of conglomerate matrix from the Hominy Peak (n=84), Upper Pinyon (n=83), Middle Pinyon (n=98), Lower Pinyon (n=44), Upper Harebell (n=83), and Lower Harebell (n=85) Formations were collected for the study. Quartzite clasts were sampled randomly, with no bias toward color, size or degree of rounding. Approximately 20 quartzite cobbles were sampled from each locality.
The samples were crushed and pulverized using a chipmunk crusher and a disk mill. Lower density grains were removed using a Wilfely table and heavy liquids. The remaining heavy mineral residues were separated with a vertical and a sloped Franz magnetic separator.
Large populations (500+) of zircon from each sample were mounted on epoxy pucks with the “Peixe” and “FC-1” standard and imaged using cathodoluminescence (CL) imaging. Approximately , randomly selected zircons were analyzed from each sample for U-Pb isotope ratios. In-situ analysis of the zircons was accomplished using Laser ablation ICP-MS at the University of Arizona LaserChron facility using the methods and analytical procedures of Gehrels et al. (2006) and Gehrels et al. (2008).
Analyses that are >10% or >5% reversely discordant were excluded from further consideration. The 207Pb/206Pb isotope ratios were used to calculate the age of crystallization of each zircon and construct the Histograms using Isoplot (Ludwig, 2003) seen on the left side of figures The same dataset was used to construct the population distribution pie charts (Figures 6-11) to see the relative proportions of different age ranges using the Excel macro of Gehrels et al., (2008).
Figure 2: Photo at collection site for Pinyon Matrix Hominy Peak 2 sample
Figure 3: Photo of collection site for Pinyon Matrix Gros Ventre Mountains, WY.
Figure 10: Histogram of Pinyon Clasts at Gros Ventre.
Figure 11: Histogram of Hominy Formation Quartzite Clasts.
Results
Zircon age provenance for each sample was subdivided into Archean, Paleoproterozoic, Trans-Hudson orogeny, Yavapai-Mazatzal, Midcontinent Granite-Rhyolite, and Grenville ages.
There were 168 zircon samples analyzed in the Harebell formation. The Buffalo Fork sample (Figure 6) represents the lower section of the Harebell formation. Yavapai-Mazatzal ages made up the majority of the zircons at 67%, with Archean age zircons being the next most significant source at 18%. The upper part of the Harebell formation represented by the Pacific Creek Sample (Figure 7) also was primarily Yavapai Mazatzal, 52%, with some Archean and Midcontinent Granite-Rhyolite Province aged zircons, 19% and 17% respectively.
There were 3 samples taken from the Pinyon Formation with a total of 225 zircons. The Pacific Creek sample (Figure 8) represented the basal conglomerates, The Gros Ventre sample (Figure 10) is about the middle section of the formation, and the Hominy Peak sample (Figure 9) represents the upper part of the formation. The zircons from the Pacific Creek sample were primarily Yavapai Mazatzal, 48 %, with large minorities of Proterozoic, Archean, and Granite – Rhyolite Province aged zircons, 21%, 18%, and 11% respectively. The middle section of the formation was represented by the Gros Ventre sample and was similar to the base, the largest amount of zircons were Yavapai Mazatzal, 39%, with large minorities of Archean, Proterozoic, and Granite – Rhyolite Province aged zircons, 21%, 17%, and 11% respectively. The uppermost section of the Pinyon formation, Hominy Peak sample, was dominated by Yavapai Mazatzal aged zircons, 67%.
The Hominy Peak formation was analyzed from its basal quartzite clasts with 84 zircons where Yavapai Mazatzal were the majority at 42%, with Archean, Trans Hudson, and Granite – Rhyolite Province making up 18%, 18%, and 13% respectively.
Figure 1: Stratigraphic column depicting relations of the Pinyon and Harebell Formations and sample collection intervals
Figure 4: Collection site for Harebell Matrix Buffalo Fork sample
Figure 12: Black arrows indicate sediment transport from source. Red arrow indicates sediment erosion and current deposition due to erosion of Sevier highlands.
References
Antweiler, J.C., Love, J.D., and W.L. Campbell, 1977, Gold content of the Pass Peak Formation and other rocks in the Rocky Mountain overthrust belt, northwestern Wyoming: 29th Field Conference, Wyoming Geol. Assoc., p
Dorr, J.A., Jr., Spearing, and J.R. Steidtmann, 1977, Deformation and deposition between a foreland uplift and an impinging thrust belt, Hoback basin, Wyoming. Geol. Soc. Am. Sp. Paper 177, 82 pp.
Gehrels, G.E., Valencia, V., Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, Q03017,
Gehrels, G.E., Valencia, V., Pullen, A., 2006, Detrital zircon geochronology by Laser-Ablation Multicollector ICPMS at the Arizona LaserChron Center, in Loszewski, T., and Huff, W., eds., Geochronology: Emerging Opportunities, Paleontology Society Short Course: Paleontology Society Papers, v. 11, 10 p.
Lindsay, D.A., 1972, Sedimentary Petrology and Paleocurrents of the Harebell Formation, Pinyon Conglomerate, and associated coarse clastic deposits, northwestern Wyoming: US Geol. Survey Prof. Paper 734-B.
Love, J.D., 1973, Harebell Formation (upper Cretaceous) and Pinyon Conglomerate (uppermost Cretaceous and Paleocene), northwest Wyoming: US Geol. Survey Prof. Paper 734a.
Love, J.D. and J.C. Reed, 1968, Creation of the Teton landscape –the geologic story of Grand Teton national park : Teton Natural Hisory Assoc., 120 pp.
Ludwig R. K., User’s Manual for Isoplot A geochronological Toolkit for Microsoft Excel. Berkley Geochronology Center, Special Publication No. 4.
Whitmeyer, S.J., and Karstrom, K.E., 2007, Tectonic Model for the Proterozoic Growth of North America, Geosphere, v. 3. no. 4. p
Steidtmann, J.R., 1971, Origin of the Pass Peak Formation and equivalent early Eocene strata, central western Wyoming: Geol. Soc. America Bull. 82, p
Discussion
From the results, the source of the sediment deposited forming the Harbell, Pinyon, and Hominy Peak Formation were Yavapai Mazatzal (39-67 %) with a total of 250 zircons (54%). With large minorities of Archean and Granite – Rhyolite aged zircons the source of sediment for these formations was from the southeast. This indicates that sediment dispersal patterns during the Neoproterozoic were from southeast, probably in the vicinity of Colorado, Kansas, and New Mexico northwest to the ancient continental margin of Idaho. These rocks were then metamorphosed to quartzite and eventually uplifted during the Sevier Orogeny, and were shed eastward as the Sevier highlands eroded (Figure 12). Archean zircons were probably derived from Wyoming Province basement rocks, which were more proximal to the site of deposition. All samples included Grenville ages, which were the most distally derived. The maximum age for the protolith of these quartzite clasts is about 1.0 Ga.
These data indicate that the quartzite clasts have a very consistent detrital zircon provenance. This means that the same source materials shed sediment throughout deposition of the three units or that significant recycling occurred.
Figure 5: Regional map including collection sites of Pinyon and Harebell matrix samples