Zawacki, Emily E., Amanda B. Clarke, J Ramón Arrowsmith and Daniel J. Lynch. Reconstructing an explosive basaltic eruption in the Pinacate volcanic field, NW Sonora, Mexico. American Geophysical Union. Fall 2017 Meeting. Poster #T23B-0615.
Tephra deposits from explosive volcanic eruptions provide a means to reconstruct eruption characteristics, such as column height and erupted volume. Parameters like these are essential in assessing the explosivity of past eruptions and associated volcanic hazards. We applied such methods to a basaltic tephra deposit from one of the youngest eruptions in the Pinacate volcanic field (NW Sonora, Mexico). This roughly circular tephra blanket extends 13 km E-W and 13 km N-S, and covers an area of at least 135 km2. The source vent of this eruption is hypothesized to be the Tecolote volcano (lat 31.877, long -113.362), which is dated to 27 ± 6 ka (40Ar/39Ar). Fifty-three pits were dug across the extent of the tephra deposit to measure its thickness, record stratigraphy, characterize grain size distribution, and determine maximum clast size. Isopleth and isopach maps were created from these data to determine the column height (>9 km), estimate mass eruption rate (>2.1×106 kg/s), and calculate the erupted volume (>4.2×10-2 km3). Stratigraphic descriptions support two distinct episodes of tephra production. Unit A is dispersed in an approximately circular pattern ( 6.5 km radius) with its center shifted to the east of the vent. The distribution of Unit B is oblate ( 9.5 km major axis, 4.5 km minor axis) and trends to the southeast of the vent. Lava samples were collected from each of the seven Tecolote flows for XRF and ICP-MS geochemical analyses. These samples were compared to geochemical signatures from a Tecolote bomb, tephra from Units A and B, and cinder from the La Laja cone, which is the youngest dated cone in the field at 12 ± 4 ka (40Ar/39Ar). The La Laja sample is geochemically distinct from all Tecolote samples, confirming that it did not contribute to the two tephra units. Tephra from Unit A and Unit B have distinct signatures and fit within the geochemical evolution of the Tecolote lavas, supporting two explosive episodes from the Tecolote volcano, which has two cones. To provide a stronger age constraint on the eruption, samples for optically stimulated luminescence (OSL) dating were collected from the sandy silt unit below the tephra in two pits. Data for these dates are being analyzed.
Zawacki, Emily E., J Ramón Arrowsmith, Christopher J. Campisano, Manfred R. Strecker, Alan L. Deino, Craig S. Feibel, John D. Kingston, Helen M. Roberts and HSPDP Team Members. Modeling post-depositional accumulation of 10Be in sandstones from the Hominin Sites and Paleolakes Drilling Project (HSPDP) cores for in situ cosmogenic radionuclide paleoerosion rate analyses. Geological Society of America Abstracts with Programs, vol. 49, no 6. Poster.
Sandstones from drill cores collected in Ethiopia and Kenya by the Hominin Sites and Paleolakes Drilling Project (HSPDP) present a novel opportunity to reconstruct paleoerosion rates utilizing in situ cosmogenic radionuclides (CRNs). Concentrations of CRNs such as Be indicate the amount of time a material has been exposed to cosmic radiation at or near the earth’s surface. CRNs are produced in the watershed while the material is actively being eroded and transported, and decay at a fixed rate over time upon burial and shielding from further CRN accumulation. We collected 20 total samples from the Baringo Basin/Tugen Hills, Chew Bahir, Northern Awash and West Turkana drill cores for paleoerosion rate analyses to investigate global climate variation, regional orography, local faulting and fluvial network reorganization within the watersheds of the drill sites. In addition, there is a unique opportunity to tie the inferred paleoerosion rates to the rich paleoenvironmental proxies determined by the HSPDP. However, an important consideration when calculating paleoerosion rates is the amount of Be accumulated after sediments have been deposited. Material will only be shielded from significant nuclide accumulation when buried by ~3–5 m of additional sediment. The simplest scenario assumes no significant post-depositional Be accumulation, but sediment burial histories are likely more complex and varied. We model the potential range of post-depositional Be accumulation in the HSPDP sandstone samples to examine its importance when calculating paleoerosion rates. We utilize sedimentation rates derived from age models of the cores to assess post-depositional nuclide accumulation. Because the drill cores are predominantly lacustrine, we also identify sandstone units overlain by lacustrine sediments and model post-depositional Be accumulation for sediment shielding under a water column. Given that the cores range in age from 3 Ma, we finally consider the amount of Be lost to radioactive decay and its impact on post-depositional Be accumulation.
Zawacki, Emily E. and Marcia G. Bjørnerud. A previously unrecognized impact structure at Brussels Hill, Door County, Wisconsin: Brecciation and schok-metamorphic features. Geological Society of America Abstracts with Programs. Vol. 47, No. 5, p.82. Poster.
Brussels Hill is an anomalous area of intensely fractured, faulted, and folded bedrock in a region of otherwise undeformed lower Silurian dolostone in Door County, WI. The area of disturbed rock coincides with a distinctive, nearly circular, flat-topped topographic high ca. 2 km in diameter, standing 40 m above the surrounding landscape and ringed by rugged tree-covered slopes. Bedding orientations vary dramatically over distances of meters. Coherent structures are difficult to discern, and fragmentation appears to have happened at multiple scales. Both mono- and polymict breccias occur, commonly as wedges that seem to have been intruded between bedding planes. Silurian dolostone is the only bedrock normally exposed in this area, but fault-bounded blocks of sandstone occur at Brussels Hill. This atypical rock likely comes from Cambrian strata that lie up to 300-400 meters below the surface. Although we have not found shatter cones in the host dolostones, we have identified shock-metamorphic planar microstructures (PMs) in the quartz grains of the sandstones. Planar fractures (PFs) typically occur in multiple parallel sets that stretch the length of the grain. In addition, these sandstones feature mechanically twinned and mosaicized grains. The age of the Brussels Hill disturbance is not well constrained but must date between post-Early Silurian and pre-Late Pleistocene as its target rocks are lower Silurian and the site lies in a recently glaciated shield area. As Brussels Hill shares many striking similarities with the central uplift of the Rock Elm impact structure in western Wisconsin—in diameter, topographic rise, stratigraphic uplift, and PMs—we conclude that Brussels Hill is likely the eroded remnant of a central peak of a larger impact structure.
Zawacki, Emily E. and Marcia G. Bjørnerud. A previously unrecognized impact structure at Brussels Hill, Door County, Wisconsin: Brecciation and schok-metamorphic features. Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.707. Talk.
Brussels Hill is an anomalous area of intensely fractured, faulted, and folded bedrock in a region of otherwise undeformed lower Silurian dolostone in Door County, WI. The area of disturbed rock coincides with a distinctive, nearly circular, flat-topped topographic high ca. 2 km in diameter, standing 40 m above the surrounding landscape and ringed by rugged tree-covered slopes. Bedding orientations vary dramatically over distances of meters. Coherent structures are difficult to discern, and fragmentation appears to have happened at multiple scales. Both mono- and polymict breccias occur, commonly as wedges that seem to have been intruded between bedding planes. Silurian dolostone is the only bedrock normally exposed in this area, but fault-bounded blocks of sandstone occur at Brussels Hill. This atypical rock may come from Cambrian or Ordovician strata that lie 100s of meters below the surface. Although we have not found shatter cones in the host dolostones or planar deformation features in the sandstone blocks, both rock types have microstructures that may reflect shock metamorphism at lower peak pressures. Some beds in the local Silurian sequence are vuggy, with cm-size cavities lined with euhedral dolomite crystals. In thin sections of samples from Brussels Hill, the infilling crystals have unusual concentric fractures, suggesting tensile spallation radially toward the interior of the cavities. These could record a high amplitude pressure wave passing through a rock with large open pore spaces. The sandstone blocks contain both rounded and highly angular grains, some with unusual concave margins unlikely to represent primary sedimentary features. A puzzling aspect of Brussels Hill is that even though it is underlain by pervasively fragmented rock and lies in an area that was glaciated only 15,000 yrs ago, it stands as a topographic high. Although Brussels Hill does not have definitive characteristics of an eroded impact structure, we conclude that an impact origin best explains its formation.
Zawacki, Emily E., Amber J. Ritchie, Lauren L. Hoffman and Lisa M. Tranel. Transport of detrital sediments in low-gradient stream sections in the Teton Mountain Range and the Guadalupe and Sacramento Mountains. Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.586. Poster.
The Teton Mountain Range of Wyoming and the Guadalupe and Sacramento Mountains of New Mexico offer differing mountain stream environments, though all three lie along the eastern margin of the Basin and Range Province. Due to the arid setting of the Guadalupes and Sacramentos, stream beds remain dry until activated by storm events, while streams actively flow year-round in the Tetons, thus transporting detrital sediment through varied processes. However, in mountain streams, obstacles such as talus accumulation, glacial incision, and vegetation may reduce local slope so that sand transport is limited. Such sediments may be useful in determining erosion rates and patterns, but results may not be faithful if the sediments are trapped in certain portions of the stream. In this study, we examine low-gradient sections of mountain streams in the Tetons, Guadalupes, and Sacramentos in order to determine whether sand sized sediments are accumulated or transported in these channels. Cross-sections were measured and sediment samples were collected during 2011 and 2012 in the Tetons and during 2013 in the Guadalupes and Sacramentos in canyons with catchment areas ranging from 0.974 – 94.8 km and 0.13 – 120.78 km respectfully. Sediment samples were then sieved using sieve classifications from <0.063 – 45 mm and sorted to determine d50 and d95values. We calculated total stream power based on cross-sectional information and determined its relation to catchment area, elevation, and annual precipitation. Despite a large decrease in total stream power in the Tetons from 2011-2012, we found that d50 particles would be transported through everyday flow based on measured average stream velocities. However, nearly all of the largest observed clast sizes require higher velocity storm flow in order to be transported. In the Guadalupes and Sacramentos, there was a negative correlation between catchment area, total stream power, and d50 grain size, with increasing areas resulting in decreasing stream powers and d50 values. Based on calculated velocities, sand sized sediment will be transported, but only when precipitation events occur. Despite the differences in climate and main mode of sediment transportation, detrital sediments are successfully transported in these streams and should offer accurate erosion information.