The Geochronology and Stratigraphic Evolution of Neogene Sedimentary Rocks in Jackson Hole, Wyoming

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1 University of Wyoming National Park Service Research Center Annual Report Volume 8 8th Annual Report, 1984 Article The Geochronology and Stratigraphic Evolution of Neogene Sedimentary Rocks in Jackson Hole, Wyoming Douglas W. Burbank University of Southern California Follow this and additional works at: Recommended Citation Burbank, Douglas W. (1984) "The Geochronology and Stratigraphic Evolution of Neogene Sedimentary Rocks in Jackson Hole, Wyoming," University of Wyoming National Park Service Research Center Annual Report: Vol. 8, Article 8. Available at: This Grand Teton National Park Report is brought to you for free and open access by Wyoming Scholars Repository. It has been accepted for inclusion in University of Wyoming National Park Service Research Center Annual Report by an authorized editor of Wyoming Scholars Repository. For more information, please contact scholcom@uwyo.edu.

Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment THE GEOCHRONOLOGY AND STRAriGRAPHIC EVOLUTION OF NEOGENE SEDIMENTARY ROCKS IN JACKSON HOLE, WYOMING Douglas W.

2 Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment THE GEOCHRONOLOGY AND STRAriGRAPHIC EVOLUTION OF NEOGENE SEDIMENTARY ROCKS IN JACKSON HOLE, WYOMING Douglas W. Burbank Department of Geological Sciences University of Southern California Los Angeles Objectives The Miocene s:rliments of the Jackson Hole area constitute a unique sequence of terrestrial sediments. While much of the surrounding terrain was undergoing denudation during the Miocene, over 4000 m of volcaniclastic, lacustrine, and fluvial sediments accumulated in the vic.inity of Jackson Hole. Recently completed paleontological and palynological studies have served to delineate complex bi.a:rt:ratigraphic and climatic histor:ies. The present research project has several goals. Chronologies are being developed for the Miocene sediments through the use of magnetic-pa1arity stratigraphies and f.ission-track dating. Sedimentation histories are be:ing studied by combining lithologic data with chronologie information. The results from the first year and a half of this project have raised some provocative questions. For example, sedimentation in the Teewinot Lake was very rapid, and yet the clastic influx into the lake appears minimal. Why was the detrital component so small? Where were the Tetons 10 m.y. ago, when the Teewinot Lake was flourishing? If there was no high relief to the west of the lake, when did the Tetons begin rising to their present configuration? In order to address these questions, an additional phase of study is be:ing undertaken. Through the use of fission-track dating of mineral separates (zircon and apatite) obtained from the crystalline rocks of the Teton Range and Gr as Ventre Range, uplift histories will be examined. When considered all t.ogether, the chronologies, stratigraphic histories, climatic record, and uplift histories will permit a detailed reconstruction to be made of the Miocene gealdgic history of Jackson Hole. M ethodalogy The development of a reliable magnet.astrat:igraphy usually requires successive sampling traverses. Initially, samples are spaced in such a way as to detect as much as p=ssible of the overall structure ae the magnetic record. Sutsequently, the sampling is designed to provide more detail and increase the.interval of dated s:rliments through sampling of additional sections. In later traverses, more attention is focused on the lithological characteristics and tectonic implications of the s:rliments being studied. Selected volcanic ashes have been sampled for fusion-track dating in order to determine their time of eruption and the age of the adjacent sediments. Published by Wyoming Scholars Repository,

University of Wyoming National Park Service Research Center Annual Report, Vol. 8 [1984], Art. 8 The uplift studies are based on dating different mineral separates (zircon and apatite) from bulk sa.

3 University of Wyoming National Park Service Research Center Annual Report, Vol. 8 [1984], Art. 8 The uplift studies are based on dating different mineral separates (zircon and apatite) from bulk sa.mples collected at known elevations and structural pa3itions within the ranges of interest. Because these minerals anneal at different temperatures, their ages reflect the geo-thermal history of the individual sample. When combined with data on geothermal gradients, these dates can be used to interpret the timing and magnitude of uplift. About a dozen sa.mples are being callected for fission-track uplift studies from the Teton and Groo Ventre Ranges. Research Results In 1983, four weeks of field work were completed as part of the effort to develop a chronology for the Teewinot Formation. In the National Elk Refuge, about 900 m of Teewinot sediments were measured and described. Additional sections were measured along Kelly Road and at Haybarn HilL In the main sequence, 65 paleomagnetic sampling sites were p1aced, and at each 3 specimens were collected 2 dozen lithalogical samples were collected for sedimentological study, and bulk samples were collected at 3 sites for radiometric dating. The results to date from the magnetic sb..ldies in the Elk Refuge and at the Kelly Road site are shown in Figures 1 and 2. In the Elk Refuge section, only about 1/3 of the sites yielded reliable magnetic data (Class I sites). As a result, many gap:; are present in the magnetic-polarity information. lt. appears that most of the unreliable magnetic data are due to the failure to sample und:ist.urbed bedrock. Therefore, in 1984, sampling was designed to fill the gap in the previous year's record, and only outcrop:> that were clearly in place were sampled. One K-Ar date of m.y. was determined for the Elk Refuge section. Two additional fission-track dates are presently being processed. The Kelly Road site yielded rather good magnetic data, and a tentative correlation with the global magnetic time scale is shown in Figure 2. If this interpretation.is correct, it implies a much longer duration than was previously suggested for this 7Q-m-thick p:rt.i.on of the Teewinot Formation. The field work in 1984 was designed to clarify ambiguities in the magnetic record and to reconfirm the p1acement of this section in the context of the global reversal time scale. Sampling was also carried out for lithological analysis, ostracod collections, and additional f.is;ion-track dating. Thus far, over 8000 magnetic measurements have been made on 230 samples collected in An additional 100 samples were collected from the Elk Refuge and Kelly Road sections in August, These are presently being analyzed. The base of the Elk Refuge section.is poorly expa:;ed and does not include the depa:ri.tional base of the Teewinot Formation. To try to delineate the timing and nature of sedimentation in the early stages of deposition of the Teewinot, a new section was measured and sampled in 1984 near the Teton Science SchooL These samples are also being analyzed at the present time. 2

Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment 800 GLOBAL SAMPLE VGP LOCAL MAGNETIC SITES LATITUDE MPS POLAR I TY '83 '84-90-o O-o 90o TIMESCALE 700 ' 8.98 4xx.

4 Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment 800 GLOBAL SAMPLE VGP LOCAL MAGNETIC SITES LATITUDE MPS POLAR I TY '83 '84-90-o O-o 90o TIMESCALE 700 ' xx.<X ASH TO BE DATED :.,-... 0:: <:( w ' XX > :.: w... z _J _j u... - :E: :r: ' 200 I I I X'<XXX ASH TO I BE DATED CLASS I DATA 0 SITE S Figure 1. Teewinot magnetic stratigraphy from the Na ti onal Elk Ref uge. 0 Published by Wyoming Scholars Repository,

70 60 University of Wyoming National Park Service Research Center Annual Report, Vol. 8 [1984], Art. 8 SAMPLE SITES '83 '84 VGP LATITUDE -90 oo GLOBAL MAGNETIC POLAR I TY TIME LE LOCAL.

5 70 60 University of Wyoming National Park Service Research Center Annual Report, Vol. 8 [1984], Art. 8 SAMPLE SITES '83 '84 VGP LATITUDE -90 oo GLOBAL MAGNETIC POLAR I TY TIME LE LOCAL.49 MPS / 900 / I / 50 ' ' ' ' ' w z: u :::r:: ' ' ' ' 0:.: ' 8.87 >- z 0... j j XXXXXXXX ASH TO BE DATED 0 CLASS I DATA SITE CLASS II DATA SITE F i g u r e 2. T e ew i not magnet i c s t rat i g rap hy from Kelly Road sect i on

Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment Con elusions The results of the magnetic studies to date have been encouraging.

6 Burbank: The Geochronology and Stratigraphic Evolution of Neogene Sediment Con elusions The results of the magnetic studies to date have been encouraging. The stable magnetic properties of many of the samples permit their characterist.ic remanence directions to be determined quite confidently. More work is needed to verify and enlarge the present results. The present interpretation of these sequences suggests that there have been ma:pr changes in the rates of sediment accumulation during the late Miocene. These changes may have resulted from the initial pulses of uplift of the Teton Range. This pa;sihility is now being.investigated by developing preliminary uplift histories for several ranges based on fiffiion-track dating. Despite the need for further chronologie work on the Teewinot, the present results indicate that the bulk of Teewinot sediments accumulated J:::etween about 10 and 8.5 m.y. ago. During this time, ma:pr changes occurred in the style of sedimentation, the compcsition of the lake sediments, and the geometry of the dep:si.tional basin. A full description of the Miocene paleoenvironments encompassed by the Teewinot Formation must await forthcoming research results. Published by Wyoming Scholars Repository,

Contemporary Tectonics and Seismicity of the Teton and Southern Yellowstone Fault Systems- Phase I

University of Wyoming National Park Service Research Center Annual Report Volume 4 4th Annual Report, 1980 Article 16 1-1-1980 Contemporary Tectonics and Seismicity of the Teton and Southern Yellowstone