Date of Award

Summer 8-4-2017

Degree Type

Report

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Susanne Janecke

Second Advisor

John Shervais

Third Advisor

Tammy Rittenour

Abstract

The objective of this research project in Gem Valley graben, southeast Idaho is to identify the location, age and history of the Quaternary diversion of the Bear River into paleo-Lake Thatcher, and then into the Lake Bonneville basin from the Columbia River basin. Mapping, geochemical analysis, cross-cutting relationships, and five new age determinations, together with prior published research, shed new light on the complex history of interaction between the volcanic rocks, rivers, lakes, and faults in Gem Valley. Research goals were to test the hypothesis that local faulting and volcanism drove the diversion of the Bear River and controlled the geomorphic evolution of Gem Valley.

Mapping of cinder cones and maars shows that some of the Pleistocene Gem Valley-Blackfoot volcanic field erupted through active normal faults along the East Gem Valley fault zone, and fault-related fissures. Following these eruptions, local and regional volcanic and structural topography guided the path of the Bear River in Gem Valley as it flowed in the low areas between the growing shield volcanoes emerging from the Alexander, Tenmile, and Niter groupings of volcanic centers. Subsequently, the paleo-Bear River assumed a northwesterly flow across the flat bottom of central Gem Valley and produced sizable meander bends southeast of Bancroft, Idaho, as it flowed toward the Portneuf River and then the Snake River drainage. Younger basalt lava flowed into and along the meandering channel producing a shoestring-shaped basalt flow with a meandering geometry in map view. This “meandering basalt flow” diverted the Bear River into southern Gem Valley. 40Ar/39Ar dating of the groundmass of the meandering basalt flow produced a late Pleistocene date of 169 ± 14 ka indicating the position of the Bear River prior to that time.

Janecke and Oaks (2014) hypothesized that the meandering basalt flow was responsible for the final diversion of the Bear River into the Bonneville Basin at ~60-50 ka. New 40Ar/39Ar age control from the meandering basalt flow and four optically stimulated luminescence (OSL) ages from loess suggests instead that emplacement of the meandering basalt flow at 169 ± 14 ka is evidence for an incompletely understood penultimate diversion of the paleo-Bear River, that predates the final diversion by > 100 ky. Sedimentology and age control from tephrochronology of the Main Canyon Formation suggest the path of the paleo-Bear River was towards the Columbia River Basin through the Portneuf Range, and not into the Thatcher basin between 110ka and 60-50 ka. This is evident from OSL sample USU-2145 taken from white to off-white sediment from the Main Canyon Formation located in the Thatcher basin, in a faulted wall of a maar, dated to 90.4 ± 12.2 ka in the Niter shield. The lack of red sediment suggests a lack of sediment contribution from the Bear River catchment. The final diversion is constrained at about 55 ± 5 ka by sediment dated in southern Gem Valley showing the high stand of Lake Thatcher, change in deposition style, and the incision of the Oneida narrows after this time (Pederson et al., 2016).

With the meandering basalt flow ruled out as the cause of the final diversion of the paleo-Bear River into the Bonneville basin, a group of flows mapped around Soda Point in the Alexander Shield volcano are the only other basalts close to the river and young enough to be involved in the final diversion of the lower Bear River. An OSL age of 32.6 ± 5.2 ka obtained from >2-4 m thick section of loess and sediment on top of a basalt flow in eastern Gem Valley north of the Alexander cinder cone cluster suggests that underlying basalt might be the correct age to have produced the final diversion of the Bear River at 60- 50 ka.

OSL age determinations of 37.9 ± 9.0 ka and 32.6 ± 5.2 ka taken from loess bracketing cinders in the North cinder pit road cinder cone of the Niter cluster show that the Niter shield had the most recent volcanic activity in the area. This is also evident in the Niter shield with tuff rings and maars showing phreatomagmatic interaction with water in the Thatcher Basin.

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