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2015-2016

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  • Thumbnail for Branching out of the Bighorn Basin: Records of Early Eocene Hyperthermals in the Piceance Basin of Western Colorado
    Branching out of the Bighorn Basin: Records of Early Eocene Hyperthermals in the Piceance Basin of Western Colorado by Hazen, Fischer Lee

    The Eocene Thermal Maximum 2 (ETM2) and Hyperthermal 2 (H2) events are Paleogene hyperthermal events occurring ~53.7 mya and ~53.6 mya, respectively, and are characterized by increased global temperatures and an influx of isotopically light carbon to the exogenic carbon reservoir. Unlike the much better studied Paleocene Eocene Thermal Maximum (PETM), no descriptions of environmental change during these events have been published, and to date only one record of the ETM2 and H2 has been reported from terrestrial sections. In this study, carbon isotope chemostratigraphy is used to identify records of the ETM2 and H2 in the Piceance Basin of western Colorado, and relate them to known section in the Bighorn Basin of Wyoming. Oxygen isotope records, weathering indices from major element concentrations in paleosols, and sedimentological changes are then used to study environmental and sedimentation changes associated with those events in the Piceance Basin. In particular, it is suggested that hyperthermal events are associated with a noted decrease in weathering intensity, decrease in soil moisture, enhanced seasonal precipitation, and a drastic change in the nature of sedimentation. In the Piceance Basin, there is evidence for a decrease in weathering during the ETM2 event, a decrease in soil moisture, seasonal precipitation, and a drastic change in sedimentation flux. The importance of having a second terrestrial record of the ETM2 and H2 in the Piceance Basin is revealed through comparison with the Bighorn Basin records. There are obvious geochemical and sedimentological differences between the two basins. Consequently, careful consideration must be applied when extrapolating environmental impacts from one locality through time or space in order to characterize the nature of terrestrial environmental change during hyperthermal events.

  • Thumbnail for Clastogenesis as a Result of Reactivation of Agglutinated Spatter
    Clastogenesis as a Result of Reactivation of Agglutinated Spatter by Hightower, Erin Jessica

    Clastogenic flows form by coalescing from accumulated spatter. Due to the viscous nature of spatter, agglutinate remobilization behaves according to the Bingham flow law. The two main factors controlling the onset of clastogenesis are yield strength and basal shear stress. These factors are dependent on concomitant factors including accumulation rate, cooling rate, cone height, and topographic slope. To model clastogenesis, yield strength and basal shear stress can be plotted together against time. When basal shear stress exceeds yield strength, clastogenesis may occur. Four spatter cones were investigated at Krafla, Iceland. Measurements of height and topographic slope from the Krafla cones were used to calculate basal shear stress. Yield strength was calculated as a function of the spatter cone’s internal temperature using the equation in Dragoni (1989), which itself is a function of the cooling rate times time. Experimental results from Rader and Geist (2015) were used to derive a linear statistical relationship between accumulation rate and cooling rate, since higher accumulation rates means greater insulation of the deposited clasts and thus slower cooling rates. The exterior of a spatter clast quenches in the air before landing, creating a glassy skin that binds the molten interior. Thus, a strength parameter that accounts for the strength of the vitreous interclastic framework formed by these rinds is added to the yield strength. Major oxide data was used to calculate the viscosity of the spatter as a function of temperature using the method of Giordano et al. (2008). These viscosities along with a vesicle strain analysis conducted using Geological Image Analysis Software (GIAS) were used to determine the amount of strain preserved in vesicles and to estimate the strain rates needed to initiate melt mobilization (Beggan and Hamilton, 2010; Rust et al., 2003). Only one of the four cones exhibited convincing evidence of clastogenesis, with a slope of 40° and a height of 3.5 m to 4 m. Major oxide and trace element data from XRF analysis indicate there is no geochemical control on clastogenesis. Viscosities calculated using Giordano et al.’s (2008) method were averaged across all the samples from each cone to construct a general viscosity versus temperature curve for Krafla lava. Compaction of vesicles is evident in thin section but there is no quantitatively consistent increase in strain towards the bottom of the cone according to GIAS. However, strain may be accommodated by deformation via viscous flow within the deposit (Grunder and Russell, 2005). The cones that did not produce clastogenic flows actually have slightly higher strain rates for given viscosities than the cone that did produce a clastogenic flow. In general, accumulation rates greater than or equal to 11 m/hr on 40° slopes, 14 m/hr on 30° slopes, and 18 m/hr on 20° slopes are capable of producing clastogenic flows. On 40° slopes, failure heights range from 4.5 m to 4.8 m depending on the accumulation rate, which are comparable to the height measured at the clastogenic cone at Krafla. Based on the strain rates for the clastogenic cone calculated from GIAS and those predicted by the model using the Bingham flow law, it appears that vesicles only accommodate a maximum of about 0.017% of the total strain. This model assumes a constant accumulation rate and constant linear cooling rate; however, real rates are likely not constant or linear. The cone will cool as a function of time, height, and accumulation rate, and the accumulation of material may occur episodically and still be high enough to generate clastogenesis. Future work on this model should address variations in these rates. Modeling the heat diffusion out of individual clasts and how it can potentially remelt the glassy rinds between clasts is also key to understanding the stability of a spatter cone and should be investigated in future studies. In general, the model presented is consistent with field observations.

  • Thumbnail for Composition and rheology of overthickened lava flow units on Banks Peninsula, New Zealand
    Composition and rheology of overthickened lava flow units on Banks Peninsula, New Zealand by Kroner, Ryan

    Exceptional coastal exposures of Miocene lava sequences on Banks Peninsula, New Zealand provide an opportunity to understand the emplacement processes of over-thickened lava flows. Field observations of three geochemically variable (nephelinite, hawaiite, and mugearite) over-thickened flows reveal 2-4 m-thick lavas inland that transition into ~50 m cliff-forming compound units toward the ocean. These cliff sections are the result of rapid emplacement of multiple flows that enable cooling as a single unit. Possible eruption rates and reconstruction of various emplacement mechanisms can determine controls on the over-thickening of the aforementioned lava examples. Flow compositions modeled against temperatures and viscosities indicate that viscosities and compositions did not exert a significant influence on the over-thickened morphologies. There is no correlation between geochemistry (especially wt. %SiO2) and the morphology of the overthickened flows, and the close relationship of viscosity to geochemistry suggests that viscosity is not a major factor in the emplacement of these overthickened flows. Computational analysis of viscosities, coupled with measurements of flow dimensions and crystal content, reveal that high effusion rates may have been a critical factor in forming these overthickened flows, with rates of between 400 and 2000 m3 s-1 being likely. Production of more precise ranges of possible rates is hindered by uncertainties associated with taking dimensional measurements of flows as old, weathered, and poorly exposed as these. Additionally, a particularly large margin of error for flow temperatures prevents the drawing of any reasonably precise conclusions with regard to effusion rates. This example illustrates the problems inherent in attempting to quantify such precise numerical parameters for ancient lava flow units. It is very likely that paleo-topography was a very influential factor on these flows’ unique morphologies. Further work should focus on significantly narrowing the margins of error for flow dimensions (thickness and width) and flow temperatures.

  • Thumbnail for Fault strand relationships of the Wairau Fault (New Zealand): Scarp diffusion modeling for slip rate evaluation
    Fault strand relationships of the Wairau Fault (New Zealand): Scarp diffusion modeling for slip rate evaluation by Chupik, Colin

    The Australian- Pacific plate boundary on the South Island of New Zealand comprises a zone of distributed deformation along active right lateral strike slip faults of the Marlborough Fault System. The four main strike slip faults that make up the Marlborough Fault System are the Hope, Clarence, Awatere and Wairau faults and of these, the Wairau Fault is minimally studied. The Wairau Fault is a northeastward extension of the Alpine Fault beyond the branch point for the distributed zone. It is prone to Ms 7.2-7.7 earthquakes with displacements of 7 meters. Slip rates vary from 3.6-6.7 mm/yr across the Wairau Fault, according to previous studies. The fault splits into two parallel traces near Wairau Valley that continue separately to Cloudy Bay. A 2006 paleoseismology study on the northern trace at Wairau Valley, by Zachariasen et al. revealed four earthquakes within the last 5321-5611 yrs years associated with 23 meters of displacement, providing a slip rate of 3.6 mm/yr – 4.7 mm/yr on that trace. LiDAR coverage reveal geomorphic landforms offset by 30 meters along the southern trace of the Wairau Fault near Wairau Valley, that haven’t been mapped and haven’t been accounted for in slip rate calculations. A shutter ridge with a measurable south facing scarp, 2.3-3.9 m in height is evident along the southern fault trace. The ridge developed from at least three earthquake events. Ages determined from scarp diffusion modeling of the shutter ridge are compatible with the earthquake time intervals from previous studies (Zachariasen et al, 2006; Barnes and Pondard, 2010). There are contrasting results of modeling of scarps along the the two traces including variations in degradation coefficients between the northern and southern fault trace. This indicates that the fault strands rupture independently and that the penultimate event occurred along the southern 6 fault trace. Diffusion ages from the shutter ridge suggest a maximum displacement of 10 m per event since 6023 yrs, and a maximum slip rate of 9.67 mm/yr for the Wairau Fault. One concern is that irregularities in the scarp morphology might obscure evidence for older events, leading to an overestimate of slip rate and displacement. Using a conservative age estimate from the diffusion age of the valley bordering the shutter ridge, the slip rate is 4.2 to 5.8 mm/yr for the Wairau Fault. An implication is that previous studies may underestimate the slip rate and seismic hazard for the Wairau Fault. Therefore, this study identifies a need for more paleoseismology research along the Wairau Fault in order to realistically assess the earthquake hazard for townships in the Marlborough region of South Island, New Zealand.

  • Thumbnail for Fluvial landscapes of the Cretaceous: Insights integrating stable isotope geochemistry, sedimentology and taphonomy
    Fluvial landscapes of the Cretaceous: Insights integrating stable isotope geochemistry, sedimentology and taphonomy by Evans, Erica Sarah Janecke

    This project is focused on the study of climate, hydrology, and surface processes of western North America during the late Cretaceous (~75 million years ago) in what is now southern Utah. The goal of this project is to describe in detail the hydrology of fluvial systems associated with the deposition of the Kaiparowits Formation. Differences in stable isotope ratios of gar ganoine, pedogenic carbonate, and enamel from hadrosaur and crocodile teeth, in conjunction with previously published bivalve data, indicate there are three main parts to the fluvial system: 1) FS1—large anastomosing rivers draining upland areas 2) FS2—lakes subject to episodic flooding and 3) FS3—smaller streams draining the foreland basin.  Furthermore, it is possible to infer mixing of water between these sources, in particular the mixing of FS1 and FS3 waters to form FS2 water, presumably during seasonal flooding events that were analogous to processes taking place in modern-day Tonle Sap Lake in central Cambodia. The organic content of sediment, carbon isotope ratios of paleosol carbonate, and the carbon isotope ratios of hadrosaur dentine and enamel from different sites all indicate that soils along the margin of the FS2 lakes were characterized by episodic flooding and saturation, with those closer to the margin being saturated for a longer period of time, compared to more distal localities.  Furthermore, hadrosaurs that ate vegetation located closer to the lake margin have teeth with high carbon isotope ratios, consistent with the existence of closed canopy forests in these localities.  Thus, variations in the hydrology of these fluvial systems appears to have played an important role in determining the distribution of plants over Kaiparowits landscapes, with closed canopy forests perhaps accounting for the large diversity in herbivorous dinosaurs observed in southern Utah during the late Cretaceous. 

  • Thumbnail for Shallow magmatic recharge in an intraplate volcanic complex, Akaroa, Banks Peninsula, New Zealand
    Shallow magmatic recharge in an intraplate volcanic complex, Akaroa, Banks Peninsula, New Zealand by Beckham, Emily Christine

    Lava flows of the intraplate Miocene Akaroa Volcanic Complex (AVC), Banks Peninsula, New Zealand, display a cyclical geochemical trend from picrite to benmoreite. When observed within a single stratigraphic section, flows reveal repeating patterns, or batches, of primitive to evolved magmas. Primitive flows are generally porphyritic, while the more evolved flows are consistently aphyric. Previous studies have led to a model for the AVC in which a deep reservoir (lithospheric detachment sourced) fed and replenished multiple shallow magma chambers, which then fractionated individually to produce several independently evolving magma batches. The purpose of this research is to test that model and extend it spatially across the eastern flanks of the AVC, as well as to characterize magma chamber dynamics using geochemistry and petrography. Sixty-nine samples were taken from six stratigraphically controlled transects across the eastern AVC for XRF analysis. Based on rock type and composition, samples were separated into individual batches within their respective transects. Distinct geochemical variations were observed in samples ranging from 43-59 wt. % SiO2, 0.5-7 wt. % MgO, 1-4 wt. % TiO2, and 0-270ppm V. The distinction between batches were drawn where element concentrations varied significantly within stratigraphy, further supported by petrographic distinctions (plagioclase resorbtion and sieved cores, and skeletal textures). Eleven of the sixty-nine original samples were selected for microprobe analysis of individual plagioclase, clinopyroxene, and olivine crystals. Anorthite composition in plagioclase ranged from 0.15-0.77, with both reverse and normal zoning patterns observed from core to rim. By correlating the crystallinity and textures of each flow with the bulk-rock geochemistry, this study argues that the shallow magma chambers underlying the AVC experienced cycles of magma evolution punctuated by magmatic recharge from depth. As each batch evolved, the hawaiite, benmoreite, and mugearite flows would experience crystal separation in the chamber forming a crystal mush while the residual liquid erupted aphyrically. The more primitive picrite flows, however, erupted with their phenocrysts without experiencing crystal separation, resulting in the initial, most primitive flow(s) of each cycle containing the greatest degree of crystallinity. Reverse zoning patterns coupled with resorbtion and sieve textures in the plagioclase phenocrysts within the picrite flows suggest that either the erupted flow experienced several recharge events prior to eruption, or incorporated pieces of the previous batch’s crystal mush. As such, the phenocrysts contained within the picrite flows record complex geochemical process occurring in the shallow magma chambers below the AVC.