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• 21. [Article] Quaternary Chronology and Stratigraphy of Mickey Springs, Oregon

  Mickey Springs in the Alvord Desert, southeast Oregon, is analogous to other Basin and Range hydrothermal systems where the requisite conditions of heat source and permeable pathways are met through crustal ...

  Citation × Citation Citation Abstract Copy Title: Quaternary Chronology and Stratigraphy of Mickey Springs, Oregon Author: Mowbray, Leslie Allen Year: 2015 Mickey Springs in the Alvord Desert, southeast Oregon, is analogous to other Basin and Range hydrothermal systems where the requisite conditions of heat source and permeable pathways are met through crustal thinning due to normal faulting. This study examines the morphology and lifespan of near-surface spring features through use of ground penetrating radar, thermoluminescence (TL) dating, and elevation modeling. Duration of hydrothermal activity at Mickey Springs has not previously been determined, and age determinations of sinter at the site are conflicting. The reason for and timing of this change in silica saturation in the hydrothermal fluid has not been resolved. Three morphologies of silica sinter deposition have been identified at Mickey Springs. These are (1) well-sorted, fine-grained sandstone with ripple marks, cross beds and preserved root casts, to poorly-sorted conglomerate of primarily basalt clasts, both cemented by coeval silica deposition, (2) large depressions (12-32 m diameter) rimmed with sinter, characterized by fine silt and clay blanketing a sinter apron and infilling the central depression, and (3) quaquaversal sinter mounds identified by outcropping pool-edge sinter typically surrounding a shallow depression of loose sediment. Silica-cemented sandstone and conglomerate were the first features formed by coeval hydrothermal processes at the site, and were emplaced prior to 30 kya as suggested by structural and stratigraphic relationships. Structure between two interacting fault tips may have constrained the extent of silica cementation. By 30 kya, a left-stepping fault oriented roughly north/south further constrained the near-surface permeable zone. TL dates from sediment stratigraphically below and above sinter aprons around mounds and depressions (former spring vents) indicate sinter deposition between 30 and 20 kya. Location of these features was dictated by development of the left-stepping fault. As pluvial Lake Alvord filled at the end of the Pleistocene, lake sediment filled most vents, which were largely inactive, with fine-grained silt and clay. Today, hydrothermal activity persists in two modes: (1) The current high-temperature springs, steam vents and mudpots concentrated in a 50 x 50 m area south of the sinter mounds and depressions, and (2) scattered springs and steam vents that exploit previous permeable pathways that once provided the hydrothermal fluid which precipitated the sinter aprons. Currently there is no active silica sinter deposition at Mickey Springs. Structures and stratigraphic relationships identified through this study favor a transport-limited and structurally controlled model of fluid transport. Sinter deposition is determined to have occurred before the most recent highstand of pluvial Lake Alvord. A climate driven model, where groundwater recharge from pluvial Lake Alvord circulates to a deep heat source and enhances spring discharge, is not supported by these findings, as no evidence was found for sinter precipitation after the drying of the lake. Future studies of other hydrothermal systems in the Basin and Range may reveal that permeable pathways along local structures are the primary drivers in this region. Title: Quaternary Chronology and Stratigraphy of Mickey Springs, Oregon Author: Mowbray, Leslie Allen Year: 2015 Mickey Springs in the Alvord Desert, southeast Oregon, is analogous to other Basin and Range hydrothermal systems where the requisite conditions of heat source and permeable pathways are met through crustal thinning due to normal faulting. This study examines the morphology and lifespan of near-surface spring features through use of ground penetrating radar, thermoluminescence (TL) dating, and elevation modeling. Duration of hydrothermal activity at Mickey Springs has not previously been determined, and age determinations of sinter at the site are conflicting. The reason for and timing of this change in silica saturation in the hydrothermal fluid has not been resolved. Three morphologies of silica sinter deposition have been identified at Mickey Springs. These are (1) well-sorted, fine-grained sandstone with ripple marks, cross beds and preserved root casts, to poorly-sorted conglomerate of primarily basalt clasts, both cemented by coeval silica deposition, (2) large depressions (12-32 m diameter) rimmed with sinter, characterized by fine silt and clay blanketing a sinter apron and infilling the central depression, and (3) quaquaversal sinter mounds identified by outcropping pool-edge sinter typically surrounding a shallow depression of loose sediment. Silica-cemented sandstone and conglomerate were the first features formed by coeval hydrothermal processes at the site, and were emplaced prior to 30 kya as suggested by structural and stratigraphic relationships. Structure between two interacting fault tips may have constrained the extent of silica cementation. By 30 kya, a left-stepping fault oriented roughly north/south further constrained the near-surface permeable zone. TL dates from sediment stratigraphically below and above sinter aprons around mounds and depressions (former spring vents) indicate sinter deposition between 30 and 20 kya. Location of these features was dictated by development of the left-stepping fault. As pluvial Lake Alvord filled at the end of the Pleistocene, lake sediment filled most vents, which were largely inactive, with fine-grained silt and clay. Today, hydrothermal activity persists in two modes: (1) The current high-temperature springs, steam vents and mudpots concentrated in a 50 x 50 m area south of the sinter mounds and depressions, and (2) scattered springs and steam vents that exploit previous permeable pathways that once provided the hydrothermal fluid which precipitated the sinter aprons. Currently there is no active silica sinter deposition at Mickey Springs. Structures and stratigraphic relationships identified through this study favor a transport-limited and structurally controlled model of fluid transport. Sinter deposition is determined to have occurred before the most recent highstand of pluvial Lake Alvord. A climate driven model, where groundwater recharge from pluvial Lake Alvord circulates to a deep heat source and enhances spring discharge, is not supported by these findings, as no evidence was found for sinter precipitation after the drying of the lake. Future studies of other hydrothermal systems in the Basin and Range may reveal that permeable pathways along local structures are the primary drivers in this region. Close

• 22. [Article] Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane

  For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits ...

  Citation × Citation Citation Abstract Copy Title: Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane Author: Hermanns, Reginald L., Fischer, Luzia, Oppikofer, Thierry, Bøhme, M., Dehls, John F., Henriksen, H., Booth, Adam M., Eilertsen, R., Longva, O., Eiken, Trond Year: 2011 For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits of prehistoric and historic rock slope failures onshore and with help of a bathymetry. Mapping on land included structural mapping of ten sites by on-site field mapping and nine sites by remote structural mapping using terrestrial laser scanning technology (TLS). Field work also included periodic monitoring of 14 sites using differential Global Positioning Systems (dGPS) and TLS at 4 sites. Synthetic Aperture Radar was app lied for the entire county but slide velocities could only be mapped out at one locality at Osmundneset (Gloppen municipality). A large amount of work was carried out on the slope east of Flåm in the Aurland valley, and results have been reported separately (NGU 20 11 .025). Three large instabilities have been discovered or taken into the monitoring program. These are Osmundneset in Gloppen municipality, Skrednipa in Sogndal municipality, and the Ovris valley in Vik municipality. The largest movements with 1.5 cm horizontal and 1.5 cm vertical displacement were measured on the instability with a volume of approx. 1 Mm3 in Ovris valley. Opening of cracks has been measured at that site also in the 1960’s, 1970’s and 1980’s. Our data suggest a slight acceleration of this instability. Acceleration was also documented for a 100.000 m3 large instability called Lifjellet, although velocity of that site is less than half of the velocity of the block in Ovris valley. However at that site a collapse of a rockslope with a volume of 25.000 to 30.000 m3 occurred only 19 years ago. Installation of continuous monitoring and early-warning systems should be considered at both of those sites as well as at similar sites where relatively small instabilities that might fail without a long acceleration phase are positioned above settlements (Gråberget in Høyanger municipality). Similarly all other monitored instabilities in Sogn og Fjordane are in the order of mm/yr and not considered to be critical on a short term. However, periodic monitoring has to be continued. Cosmogenic nuclide dating (CN) has been applied to determine ages of rockslide deposits in Fjærlandfjord (Sogne municipality) and at the slope E of Flåm (Aurland community), resulting in Late Pleistocene and Holocene ages. CN dating has also applied to the sliding planes at Skjeringahaugane (Luste municipality). The results indicate that the movement initiated at the beginning of the Holocene and is progressive. Long term slip rates are in the same order of slip rates measured by dGPS. Title: Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane Author: Hermanns, Reginald L., Fischer, Luzia, Oppikofer, Thierry, Bøhme, M., Dehls, John F., Henriksen, H., Booth, Adam M., Eilertsen, R., Longva, O., Eiken, Trond Year: 2011 For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits of prehistoric and historic rock slope failures onshore and with help of a bathymetry. Mapping on land included structural mapping of ten sites by on-site field mapping and nine sites by remote structural mapping using terrestrial laser scanning technology (TLS). Field work also included periodic monitoring of 14 sites using differential Global Positioning Systems (dGPS) and TLS at 4 sites. Synthetic Aperture Radar was app lied for the entire county but slide velocities could only be mapped out at one locality at Osmundneset (Gloppen municipality). A large amount of work was carried out on the slope east of Flåm in the Aurland valley, and results have been reported separately (NGU 20 11 .025). Three large instabilities have been discovered or taken into the monitoring program. These are Osmundneset in Gloppen municipality, Skrednipa in Sogndal municipality, and the Ovris valley in Vik municipality. The largest movements with 1.5 cm horizontal and 1.5 cm vertical displacement were measured on the instability with a volume of approx. 1 Mm3 in Ovris valley. Opening of cracks has been measured at that site also in the 1960’s, 1970’s and 1980’s. Our data suggest a slight acceleration of this instability. Acceleration was also documented for a 100.000 m3 large instability called Lifjellet, although velocity of that site is less than half of the velocity of the block in Ovris valley. However at that site a collapse of a rockslope with a volume of 25.000 to 30.000 m3 occurred only 19 years ago. Installation of continuous monitoring and early-warning systems should be considered at both of those sites as well as at similar sites where relatively small instabilities that might fail without a long acceleration phase are positioned above settlements (Gråberget in Høyanger municipality). Similarly all other monitored instabilities in Sogn og Fjordane are in the order of mm/yr and not considered to be critical on a short term. However, periodic monitoring has to be continued. Cosmogenic nuclide dating (CN) has been applied to determine ages of rockslide deposits in Fjærlandfjord (Sogne municipality) and at the slope E of Flåm (Aurland community), resulting in Late Pleistocene and Holocene ages. CN dating has also applied to the sliding planes at Skjeringahaugane (Luste municipality). The results indicate that the movement initiated at the beginning of the Holocene and is progressive. Long term slip rates are in the same order of slip rates measured by dGPS. Close