Texture and Sedimentary Structures of Pleistocene Lava Creek Ash,
Shoshone, Southern California
SUNY Geneseo Department of Geological Sciences
Madeline Ess, Yvonne Bennett, Supervisor: Dr. D.J. Over
Methods
Abstract Results & Discussion Conclusion
The latest Yellowstone eruption was massive and deposited ash up to 2000 The hotspot of Yellowstone in Wyoming has been the source of the three The strata consists of massive beds of weakly consolidated, fine grained to sand sized particles of white ash. The bedding ranged from thinly kilometers away from the Yellowstone caldera (Figure 4). The Lava Creek largest volcanic eruptions in recent Earth history. The latest eruption, laminated to thick massive beds. However, the composition was uniform in both texture and grain size. In Shoshone, there was approximately 3 Formation was typically deposited to the east, suggesting west and occurring 0.63 million years ago, was the second largest and deposited a thin meters of ash deposited from the Yellowstone eruption. Cross laminations and ripple marks were present due to wave action in the lake during north-western winds. Areas in the isopach with higher thicknesses were layer of ash, known as the Lava Creek Formation. The ash traveled up to two time of deposition. Flame structures were also found in one particular bed (Figure 2). Flame structures form as a result of differentiation efficient depositional areas like that of Lake Tecopa. Lake Tecopa was very thousand kilometers in atmospheric winds across the western and central between two sediment layers. The lighter layer pushes upward as a result of a heavier layer placed on top. Convolute bedding is another saline and had an alkaline pH, making it inhospitable for life. The ash when United States. In some depositional environments, like lakes and basins, the sedimentary structure found in the ash deposit which is indicative of seismic activity during time of deposition (Figure 3). ash is from one to ten meters thick. A sample of the Lava Creek Formation settling in Lake Tecopa was reworked and retained convolute bedding, ripples, was hand collected near Shoshone, California from Pleistocene Lake Tecopa. and flame structures. The Lava creek Formation is defined by its hyalotuff The ash particles exhibit hyalotuff texture, which is concordant with the texture which is concordant with the explosive eruption style of Yellowstone. explosive, phreatomagmatic style eruption of Yellowstone volcanism. A lack Today the Lava Creek Formation is visible due to the erosional work of the of fossils indicate Lake Tecopa was alkaline and had a high salinity, which Armagosa River, which was once the source waters for Lake Tecopa. made it lacking in life, but sedimentary structures observed suggest that wave Geological History action was similar to that of a present day lake.
Future Study Geologic History Figure 2. Flame structure highlighted by blue circle Figure 3. Convolute bedding Due to the discrepancy of the ash dispersal there is many different ideas to In the past 2 million years there have been three major Yellowstone eruptions. The Lava Creek Ash Formation deposited across the United States from the Yellowstone Caldera, located in northwest Wyoming. It would be explore. Some topic questions include: The first Yellowstone eruption was the largest, depositing 600 cubic miles of expected that as the ash moved further from the eruption, it would have a consistent decrease in depositional thickness. However, thickness ash about 2 million years ago. The next eruption was the smallest of the three data from localities where the Lava Creek Formation was recorded does not agree with this, as seen on the isopach map in Figure 4. There is a • For areas across the country that have thicker deposits of the Lava and it occurred 1.3 million years ago and produced 67 cubic miles of ash. The thick 10 meter deposit in the pan handle of Oklahoma surrounded by much thinner deposits This is likely due to the ash falling into a Creek Formation, what depositional environment existed at that eruption that deposited the Lava Creek Formation was the most recent sedimentary sink where the ash could be accumulated. The dispersal of ash suggests a west and north-western wind direction during the time location? eruption, 0.63 million years ago, producing 240 cubic miles of ash which was of the eruption. • How does the ash of previous Yellowstone eruptions compare? deposited across the country (Izett & Wilcox, 1982). The Yellowstone hotspot • What is the economic history of the ash? Could it have uses in the is 1000 kilometers away from the small desert town of Shoshone, located in future? Inyo county, southern California. Shoshone is situated in the sedimentary • Are we in danger of another Yellowstone eruption? (of alien invasion?) remnants of Pleistocene Lake Tecopa, which in present-day is seen as highly weathered, badland topography that includes the 3 meter thick Lava Creek Formation (Hillhouse, 1987). Lake Tecopa was once part of a larger lake known as Lake Aubrey, which covered much of Inyo and San Bernardino counties. However, the larger lake drained, leaving behind a series of smaller lakes that included Lake Tecopa Acknowledgements (Figure 1). This lake was alkaline and very saline therefore, there is little to no fossils found in the lake bed for it was a hostile place to live. Lake Tecopa’s head waters include the primary sedimentary Cambrian age Shoshone Dr. Scott Giorgis and Dr. Nick Warner for organizing the department trip Mountains and the Yucca Timber Mountains (Sheppard & Gude, 1968). Also funded in part by the Geneseo Foundation found in the lake bed was a natural spring that was once contributing to the SUNY Geneseo Geological Sciences Department lake (Morrison, 1999). At the north end of the lake flowing inward, is the Amargosa River. This was a freshwater river with a watershed of 8,300 square kilometers. At the north end of the lake life was more common. Fossils like ostracods, diatoms, and snails can be found at the north end of the basin along References with vertebra fossils like horse, camel, and mammoth (Sheppard & Gude, 1968). Today the basin is empty and all that remains is hyper arid, badland topography eroded by the Amargosa River. Hillhouse, J.W., 1987, Late Tertiary and Quaternary geology of the Tecopa basin, southeastern California: doi: 10.2172/60181.
Izett, G.A., and Wilcox, R.E., 1982, Map showing localities and inferred distributions Figure 4. Isopach map of the Lava Creek Formation deposition across the United States 0.63 million years ago, shown in red. The Yellowstone Caldera, highlighted in of the Huckleberry Ridge, Mesa Falls, and Lava Creek ash beds (Pearlette family ash blue. The Shoshone study site is highlighted in green. Modified from Izett & Wilcox, USGS map 1982. beds) of Pliocene and Pleistocene age in the western United States and southern Canada: USGS I-1325. The sample of the Lava Creek Formation was hand-collected from Shoshone. It exhibits clean, light silver colored, clay to silt sized particles which weather to a pale yellow-tan. The ash is reflective and features hydrophilic properties in hand sample. Under Morrison, R.B., 1999, Lake Tecopa; Quaternary geology of Tecopa Valley, magnification the ash is vitreous and angular. Some glass shards observed retain a concentric shape and vesicularity indicative of California, a multimillion-year record and its relevance to the proposed hyalotuff texture which results from water-magma interaction (Figure 5 & 6). The concentricity and vesicularity is due to the nuclear-waste repository at Yucca Mountain, Nevada: Special Paper 333: exsolution of volatiles as ejecta solidified into glass bubbles (Tucker, 2001). As the glass cooled it fractured, resulting in the glass Cenozoic basins of the Death Valley region, p. 301–344, doi: shards observed in microphotographs (Figure 7). It is possible that the original size of the glass shards was decreased by wave action 10.1130/0-8137-2333-7.301.
reworking during deposition. Sheppard, R.A., and Gude, A.J., 1968, Distribution and genesis of authigenic silicate minerals in tuffs of Pleistocene Lake Tecopa, Inyo County, California: Professional Paper, doi: 10.3133/pp597.
Tucker, M.E., 2001, Sedimentary petrology: Oxford, Blackwell Science.
Figure 5. Circled shard exhibits vesical and Figure 6. Textbook example of Figure 7. Circled shards show angularity Figure 1. The map in the upper corner shows the Tecopa basin marked by a red point. The larger hyalotuff texture shard exhibiting hyalotuff texture due to further fracturing from reworking map is of Lake Tecopa, highlighted in yellow. Sample taken from Shoshone, highlighted in pink. (Tucker, 2001)