Volcanic ash beds have been shown to be reliable stratigraphic marker beds because they can be radiometrically dated using magmatic minerals: e.g. zircon, sanidine, etc. This can be utilized in any region containing ash beds and can be especially helpful where beds are laterally discontinuous, such as in the Ogallala Formation in western Kansas and Nebraska and overlying Pliocene and Pleistocene strata. These contain abundant volcanic ashfall beds, but regional correlations of these have so far been limited, due to their non-continuous outcrops and complex stratigraphy. U-Pb dating of zircon by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at The University of Kansas from several ash outcrops has confirmed the correlation of these ashes with Yellowstone hotspot track (YHT) eruptions.
Volcanic glasses from individual eruptions exhibit unique geochemical signatures, therefore lending each eruption its own distinct geochemical fingerprint. This tephrochronological approach yields robust correlations between the ash beds in the Great Plains strata of Kansas and Nebraska and their YHT source eruptions. The YHT eruptions Lava Creek B (0.6 Ma) and Huckleberry Ridge (2.1 Ma) of the Yellowstone Plateau and the Ibex Hollow (11.93 Ma) eruption of the Bruneau-Jarbidge (10.5-12.7 Ma) eruptive center on the Snake River Plain have previously been correlated to ash beds in western and central Kansas as well as northeastern Nebraska by major and trace element concentrations. Ash samples were collected in Norton, Smith, Jewell, and Meade Counties in Kansas and the Ashfall Fossil Beds State Historical Park in northeastern Nebraska. Trace element data trends of the Norton and Smith county, KS ashes can be correlated to the Ibex Hollow eruption (11.93 Ma) of the Bruneau-Jarbidge YHT eruptive center when compared to published values. Trace elements in volcanic glass shards from an ash bed in Jewell county correlate to Lava Creek B (0.6 Ma). Samples from different locations in Meade County, KS can be correlated with Huckleberry Ridge (2.1 Ma) and Lava Creek B (0.63 Ma), respectively. The sample taken from the basal unit of the deposit in Ashfall Fossil Bed State Historical Park, NE correlates with the Ibex Hollow eruption of the Bruneau Jarbidge eruptive center. Utilizing single shard analysis to generate bivariate elemental plots, chemical trends, and multi-element correlation coefficients proved to be more useful for determining correlations as opposed to comparing average measured values to published average values. The insight from this research into the provenance of volcanic glass helps to further refine the understanding of dispersal of volcanic ash and other minerals from their associated eruptive centers, and the chronostratigraphy of High Plains, especially the Ogallala Formation. Research discussed here strives to demonstrate advantages in both processing time and overall analytical cost compared to traditional U-Pb techniques. This technique is also applicable to deposits that are too young to be reliably dated using U-Pb dating. The demonstrated high efficiency of the geochemical fingerprinting approach will allow researchers to have a denser coverage of the hundreds of ash deposits throughout the Central United States, and elsewhere, to improve stratigraphic correlations.
|Commitee:||Ludvigson, Greg, McLean, Noah|
|School:||University of Kansas|
|School Location:||United States -- Kansas|
|Source:||MAI 81/8(E), Masters Abstracts International|
|Subjects:||Geochemistry, Petrology, Geology|
|Keywords:||Geochemical Fingerprinting, High Plains, LA-ICP-MS, Ogallala Formation, Tephrochronology, Yellowstone Hotspot Track|
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