Fires in high-elevation subalpine forests have been rare, making estimates of fire-return intervals and influences of climate on fire in these forests difficult. Lake sediment charcoal provides an opportunity to extend fire records into the past and to compare them with long-term climate reconstructions. Here, we reconstruct fire histories from two high-elevation subalpine lakes that are surrounded by fragmented spruce-fir ribbon forests. We then compare the fire histories to independent temperature and moisture reconstructions. Fire episodes at the two lakes have been rare for the last millennium, but were more frequent when the climate was warm and dry, a period from ∼1000 to 3000 Before Present (BP). Variations in fire-episode frequency at individual lakes rarely exceeded the stochastic range of variability estimated by resampling the fire-episode distributions, although variations at a site with few topographic firebreaks were more significant than at a site in rough terrain. When fire-episode frequencies from both lakes were summed, fire-episode frequencies declined significantly relative to the stochastic range when the climate was cool and wet, suggesting that climate exerts a more meaningful influence at larger spatial scales than individual lake records (>3000 ha). Temperature and moisture were significant predictors of fire frequency, but, overall, climate had a weak influence on burning; regression showed that the two climate variables significantly explained 34% of the variance in the summed frequency record. Based on the results, climate change is an important driver of fire frequency in high-elevation forests, but stochastic influences may overprint the climate controls and determine patterns at local spatial scales.
{"title":"High-elevation fire regimes in subalpine ribbon forests during the Little Ice Age and Medieval Period along the Continental Divide, Colorado, U.S.A.","authors":"W. J. Calder, C. Stopka, B. Shuman","doi":"10.2113/GSROCKY.49.1.75","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.75","url":null,"abstract":"Fires in high-elevation subalpine forests have been rare, making estimates of fire-return intervals and influences of climate on fire in these forests difficult. Lake sediment charcoal provides an opportunity to extend fire records into the past and to compare them with long-term climate reconstructions. Here, we reconstruct fire histories from two high-elevation subalpine lakes that are surrounded by fragmented spruce-fir ribbon forests. We then compare the fire histories to independent temperature and moisture reconstructions. Fire episodes at the two lakes have been rare for the last millennium, but were more frequent when the climate was warm and dry, a period from ∼1000 to 3000 Before Present (BP). Variations in fire-episode frequency at individual lakes rarely exceeded the stochastic range of variability estimated by resampling the fire-episode distributions, although variations at a site with few topographic firebreaks were more significant than at a site in rough terrain. When fire-episode frequencies from both lakes were summed, fire-episode frequencies declined significantly relative to the stochastic range when the climate was cool and wet, suggesting that climate exerts a more meaningful influence at larger spatial scales than individual lake records (>3000 ha). Temperature and moisture were significant predictors of fire frequency, but, overall, climate had a weak influence on burning; regression showed that the two climate variables significantly explained 34% of the variance in the summed frequency record. Based on the results, climate change is an important driver of fire frequency in high-elevation forests, but stochastic influences may overprint the climate controls and determine patterns at local spatial scales.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"75-90"},"PeriodicalIF":0.0,"publicationDate":"2014-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.75","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312953","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The headwaters of the Rocky Mountains contribute considerable water resources to the growing populations of the western United States. Hydroclimatic variations of the past provide context for the potential ranges of moisture availability in the future. For example, paleoclimatic reconstructions from lake levels and pollen data indicate extremes in moisture availability (both wet and dry periods) in the Rocky Mountains over the past 6,000 years. The concept of a north–south dipole to describe variations of past precipitation within the Rocky Mountains may not capture the range of synoptic processes that can lead to extreme wet or dry conditions. Modern precipitation data are used to identify recent anomalously wet and dry periods in the Rocky Mountains for use as modern climate analogs for past extreme hydrologic conditions. Using the North American Regional Reanalysis data set (32-km resolution), the surface and atmospheric climatic controls of extreme wet and dry periods can be assessed to provide context on the first-order precipitation dynamics that may have contributed to changes in past moisture availability during the Holocene. A hierarchy of controls led to extremes in moisture availability in the region. Large-scale atmospheric anomalies associated with the shape and position of the jet stream influence secondary atmospheric mechanisms like rising (leading to wet conditions) and sinking motions (leading to dry conditions). Simultaneously, small-scale controls such as topography and soil moisture anomalies also influence such extremes. The various scales of climatic controls during modern extreme wet and dry periods can be used as analogs for past extremes to contextualize the range of variability for such events in the past as well as for potential extremes in the future.
{"title":"Climatic controls of hydrologic extremes in south-central Rocky Mountains of Colorado, U.S.A.","authors":"J. Shinker","doi":"10.2113/GSROCKY.49.1.51","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.51","url":null,"abstract":"The headwaters of the Rocky Mountains contribute considerable water resources to the growing populations of the western United States. Hydroclimatic variations of the past provide context for the potential ranges of moisture availability in the future. For example, paleoclimatic reconstructions from lake levels and pollen data indicate extremes in moisture availability (both wet and dry periods) in the Rocky Mountains over the past 6,000 years. The concept of a north–south dipole to describe variations of past precipitation within the Rocky Mountains may not capture the range of synoptic processes that can lead to extreme wet or dry conditions. Modern precipitation data are used to identify recent anomalously wet and dry periods in the Rocky Mountains for use as modern climate analogs for past extreme hydrologic conditions. Using the North American Regional Reanalysis data set (32-km resolution), the surface and atmospheric climatic controls of extreme wet and dry periods can be assessed to provide context on the first-order precipitation dynamics that may have contributed to changes in past moisture availability during the Holocene. A hierarchy of controls led to extremes in moisture availability in the region. Large-scale atmospheric anomalies associated with the shape and position of the jet stream influence secondary atmospheric mechanisms like rising (leading to wet conditions) and sinking motions (leading to dry conditions). Simultaneously, small-scale controls such as topography and soil moisture anomalies also influence such extremes. The various scales of climatic controls during modern extreme wet and dry periods can be used as analogs for past extremes to contextualize the range of variability for such events in the past as well as for potential extremes in the future.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"51-60"},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.51","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312369","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. Shuman, G. Carter, Devin D. Hougardy, K. Powers, J. Shinker
Lake-level changes since ca. 3.6 kilo-annum (ka) at Emerald Lake in the Upper Arkansas River Basin of west-central Colorado coincide in time with changes of the opposite direction at Lake of the Woods in northwestern Wyoming. The contrast provides evidence of a multi-centennial moisture dipole across the Southern and Central Rocky Mountains' region similar to one associated with the effects of the El Nino Southern Oscillation (ENSO) on annual to decadal time scales today. Cores and ground penetrating radar (GPR) profiles from Emerald Lake show that deep-water muds accumulated as extensively across the lake basin as today at ca. 3.6–3.0 and 1.4–0.8 ka, and nearly as extensively at 2.5–1.9 ka. The extensive muds indicate episodes of high water at Emerald Lake and date to times when Lake of the Woods was low. Nearshore sand layers at Emerald Lake indicate that water levels fell during the intervening centuries, including when bristlecone pine chronologies have documented repeated multi-decadal droughts in the Upper Arkansas River Basin. Water levels were also low, based on the absence of nearshore mud accumulation, before ca. 3.6 ka, and dramatically lower (>2 m in the currently 4.5-m deep lake) before a sharp rise in water levels by ca. 5.7 ka. A basin-wide change in sediment accumulation patterns, consistent with an expansion and deepening of the lake at ca. 5.7 ka, correlates with regional cooling and similar evidence of increased effective moisture at Lake of the Woods and other sites throughout central North America. The step increase in moisture availability may relate to a global-scale reorganization of climatic patterns, which developed as the mid- and high-latitudes cooled in response to a decline in summer insolation.
{"title":"A north–south moisture dipole at multi-century scales in the Central and Southern Rocky Mountains, U.S.A., during the late Holocene","authors":"B. Shuman, G. Carter, Devin D. Hougardy, K. Powers, J. Shinker","doi":"10.2113/GSROCKY.49.1.33","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.33","url":null,"abstract":"Lake-level changes since ca. 3.6 kilo-annum (ka) at Emerald Lake in the Upper Arkansas River Basin of west-central Colorado coincide in time with changes of the opposite direction at Lake of the Woods in northwestern Wyoming. The contrast provides evidence of a multi-centennial moisture dipole across the Southern and Central Rocky Mountains' region similar to one associated with the effects of the El Nino Southern Oscillation (ENSO) on annual to decadal time scales today. Cores and ground penetrating radar (GPR) profiles from Emerald Lake show that deep-water muds accumulated as extensively across the lake basin as today at ca. 3.6–3.0 and 1.4–0.8 ka, and nearly as extensively at 2.5–1.9 ka. The extensive muds indicate episodes of high water at Emerald Lake and date to times when Lake of the Woods was low. Nearshore sand layers at Emerald Lake indicate that water levels fell during the intervening centuries, including when bristlecone pine chronologies have documented repeated multi-decadal droughts in the Upper Arkansas River Basin. Water levels were also low, based on the absence of nearshore mud accumulation, before ca. 3.6 ka, and dramatically lower (>2 m in the currently 4.5-m deep lake) before a sharp rise in water levels by ca. 5.7 ka. A basin-wide change in sediment accumulation patterns, consistent with an expansion and deepening of the lake at ca. 5.7 ka, correlates with regional cooling and similar evidence of increased effective moisture at Lake of the Woods and other sites throughout central North America. The step increase in moisture availability may relate to a global-scale reorganization of climatic patterns, which developed as the mid- and high-latitudes cooled in response to a decline in summer insolation.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"33-49"},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.33","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312251","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Link, B. Crosby, Z. Lifton, Elijah A. Eversole, T. Rittenour
Geomorphic mapping coupled with optically stimulated luminescence (OSL) dating reveal the late Pleistocene history and geomorphic development of the narrow canyon of Big Creek, a major tributary to the Middle Fork of the Salmon River in central Idaho. The most prominent feature in the region is the Soldier Bar landslide, which consists of slumped and rotated blocks of Mesoproterozoic quartzite bedrock that slid northward from an arcuate headwall, damming both the east-flowing Big Creek and Goat Creek, a southdraining tributary. Water impounded behind the dam ultimately overtopped the deposit. Overflow laterally
{"title":"The late Pleistocene (17 ka) Soldier Bar landslide and Big Creek Lake, Frank Church-River of No Return Wilderness, central Idaho, U.S.A.","authors":"P. Link, B. Crosby, Z. Lifton, Elijah A. Eversole, T. Rittenour","doi":"10.2113/GSROCKY.49.1.17","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.17","url":null,"abstract":"Geomorphic mapping coupled with optically stimulated luminescence (OSL) dating reveal the late Pleistocene history and geomorphic development of the narrow canyon of Big Creek, a major tributary to the Middle Fork of the Salmon River in central Idaho. The most prominent feature in the region is the Soldier Bar landslide, which consists of slumped and rotated blocks of Mesoproterozoic quartzite bedrock that slid northward from an arcuate headwall, damming both the east-flowing Big Creek and Goat Creek, a southdraining tributary. Water impounded behind the dam ultimately overtopped the deposit. Overflow laterally","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"17-31"},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.17","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312188","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Paleoenvironmental records from southeastern Wyoming have been compiled to show the development of forests in the Medicine Bow Mountains. The late-glacial period had little to no tree cover and a landscape dominated by alpine tundra, or alpine steppe-like conditions based on high abundances of Artemisia pollen. Initial conifer forest development began after 13,000 calibrated years Before Present (cal yr BP) with patchy, mixed fir-spruce-pine forests forming throughout the Medicine Bow Mountains. At lower tree line, pines in these forests contained limber pine based on pollen and macrofossil data, whereas at mid- and high elevations pine trees were most likely lodgepole pine. Forest densities increased after 9000 cal yr BP, with upper elevations dominated by Engelmann spruce and subalpine fir, while lower elevations were dominated by lodgepole pine that replaced limber pine. Modern forest conditions began to form after 5000 cal yr BP. The timing of past vegetation change in southeastern Wyoming appears consistent with those observed in the Greater Yellowstone region though pollen assemblages vary regionally.
来自怀俄明州东南部的古环境记录已被汇编，以显示梅迪奇博山脉森林的发展。晚冰期几乎没有树木覆盖，景观主要是高山苔原，或基于高丰度青蒿花粉的高山草原条件。最初的针叶林发展始于距今13000年之后，在整个梅迪辛弓山脉形成了斑驳的、混合的冷杉-云杉-松林。在较低的林木线上，花粉和宏观化石资料显示，这些森林的松树主要是柔软的松树，而在中、高海拔地区，松树最可能是黑松。森林密度在9000 cal yr BP后逐渐增加，高海拔地区以恩哲曼云杉和亚高山冷杉为主，低海拔地区以黑松为主，取代了针叶松。现代森林条件在距今5000万年后开始形成。过去怀俄明州东南部植被变化的时间似乎与在大黄石地区观察到的一致，尽管花粉组合因地区而异。
{"title":"Postglacial vegetation history of southeastern Wyoming, U.S.A.","authors":"T. Minckley","doi":"10.2113/GSROCKY.49.1.61","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.61","url":null,"abstract":"Paleoenvironmental records from southeastern Wyoming have been compiled to show the development of forests in the Medicine Bow Mountains. The late-glacial period had little to no tree cover and a landscape dominated by alpine tundra, or alpine steppe-like conditions based on high abundances of Artemisia pollen. Initial conifer forest development began after 13,000 calibrated years Before Present (cal yr BP) with patchy, mixed fir-spruce-pine forests forming throughout the Medicine Bow Mountains. At lower tree line, pines in these forests contained limber pine based on pollen and macrofossil data, whereas at mid- and high elevations pine trees were most likely lodgepole pine. Forest densities increased after 9000 cal yr BP, with upper elevations dominated by Engelmann spruce and subalpine fir, while lower elevations were dominated by lodgepole pine that replaced limber pine. Modern forest conditions began to form after 5000 cal yr BP. The timing of past vegetation change in southeastern Wyoming appears consistent with those observed in the Greater Yellowstone region though pollen assemblages vary regionally.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"61-74"},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.61","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312427","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Quaternary Period encompasses the past 2.6 million years, the evolution of our species, and the emergence of the world as we know it today. Consequently, geologists have long been interested in this most recent period of time and its history of ice ages, species extinctions, volcanic eruptions, and other events. The Quaternary is not an ancient period locked away in Earth's rock record, but was the period that put the final touches on our modern landscapes. “Quaternary” implies fresh landslides, still hot volcanoes, and fossils not yet turned to minerals. Consequently, Quaternary geology reveals processes that could take place today, but that we have not witnessed simply because the historic record of scientific observation is too short. This issue of Rocky Mountain Geology draws attention to the valuable, ongoing work on Quaternary geology in the Rocky …
{"title":"Introduction: Studies in the Quaternary of the Rocky Mountains","authors":"B. Shuman","doi":"10.2113/GSROCKY.49.1.I","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.I","url":null,"abstract":"The Quaternary Period encompasses the past 2.6 million years, the evolution of our species, and the emergence of the world as we know it today. Consequently, geologists have long been interested in this most recent period of time and its history of ice ages, species extinctions, volcanic eruptions, and other events. The Quaternary is not an ancient period locked away in Earth's rock record, but was the period that put the final touches on our modern landscapes. “Quaternary” implies fresh landslides, still hot volcanoes, and fossils not yet turned to minerals. Consequently, Quaternary geology reveals processes that could take place today, but that we have not witnessed simply because the historic record of scientific observation is too short. This issue of Rocky Mountain Geology draws attention to the valuable, ongoing work on Quaternary geology in the Rocky …","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.I","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Thermopolis hydrothermal system is located in the southern portion of the Bighorn Basin, in and around the town of Thermopolis, in northwest Wyoming. It is the largest hydrothermal system in Wyoming outside of Yellowstone National Park. The system includes hot springs, travertine deposits, and thermal wells. Published models for the hydrothermal system propose the Owl Creek Mountains as the recharge zone, simple conductive heating at depth, and resurfacing of thermal waters up the Thermopolis Anticline. The geochemistry of the thermal waters of three active hot springs—Big Spring, White Sulphur Spring, and Teepee Fountain—is similar in composition and characteristic of carbonate or carbonate-bearing siliciclastic aquifers. Previous studies of the Thermopolis hydrothermal system postulate that the thermal waters are a mixture of waters from Paleozoic formations. Major element geochemical analyses available for waters from these formations are not of sufficient quality to determine whether the thermal waters are a mixture of the Paleozoic aquifers. In the time frame of this study (one year), the geochemistry of all three springs was constant through all four seasons, spanning spring snowmelt and recharge as well as late-summer and fall dryness. This relationship is consistent with a deep source not influenced by shallow, local hydrogeology. Anomalies are evident in the historic data set for the geochemistry of Big Spring. We speculate that anomalies occurring between 1906 and 1926 suggest mixing of source waters of Big Spring with waters from a siliciclastic formation, and that anomalies occurring between 1926 and 1933 suggest mixing with waters from a formation containing gypsum or anhydrite. Decreased concentrations measured in our study—relative to concentrations measured between 1933 and 1976—may reflect mixing of thermal waters with more dilute waters. Current data are not sufficient to rigorously test these suggestions, and events of sufficient scale taking place in these timeframes have not been identified.
Thermopolis热液系统位于大角盆地的南部，位于怀俄明州西北部的Thermopolis镇及其周围。它是怀俄明州除黄石国家公园外最大的热液系统。该系统包括温泉、石灰华矿床和热水井。已发表的热液系统模型提出Owl Creek Mountains是补给区，深层简单的导电加热，以及热城背斜上的热水重新表面。大泉、白硫泉和圆锥形喷泉三个活跃温泉的热水地球化学成分和特征与碳酸盐或含碳酸盐的硅橡胶含水层相似。以前对热城热液系统的研究假设热液是古生代地层水的混合物。对这些地层的水进行的主要元素地球化学分析，其质量不足以确定热水是否是古生代含水层的混合物。在本研究的时间框架内(一年)，所有三个泉的地球化学在所有四个季节都是恒定的，包括春季融雪和补给以及夏末和秋季干旱。这种关系与不受浅层局部水文地质影响的深层源相一致。在大泉的地球化学历史数据集中，异常是明显的。我们推测，在1906年至1926年之间发生的异常表明，大泉的水源与来自硅质地层的水混合，而在1926年至1933年之间发生的异常表明，与来自含石膏或硬石膏地层的水混合。我们研究中测量到的浓度下降——相对于1933年至1976年之间测量的浓度——可能反映了热水与更稀的水的混合。目前的数据不足以严格检验这些建议，而且在这些时间范围内发生的足够规模的事件尚未确定。
{"title":"Aqueous geochemistry of the Thermopolis hydrothermal system, southern Bighorn Basin, Wyoming, U.S.A.","authors":"J. Kaszuba, K. Sims, Allison R. Pluda","doi":"10.2113/GSROCKY.49.1.1","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.1.1","url":null,"abstract":"The Thermopolis hydrothermal system is located in the southern portion of the Bighorn Basin, in and around the town of Thermopolis, in northwest Wyoming. It is the largest hydrothermal system in Wyoming outside of Yellowstone National Park. The system includes hot springs, travertine deposits, and thermal wells. Published models for the hydrothermal system propose the Owl Creek Mountains as the recharge zone, simple conductive heating at depth, and resurfacing of thermal waters up the Thermopolis Anticline. The geochemistry of the thermal waters of three active hot springs—Big Spring, White Sulphur Spring, and Teepee Fountain—is similar in composition and characteristic of carbonate or carbonate-bearing siliciclastic aquifers. Previous studies of the Thermopolis hydrothermal system postulate that the thermal waters are a mixture of waters from Paleozoic formations. Major element geochemical analyses available for waters from these formations are not of sufficient quality to determine whether the thermal waters are a mixture of the Paleozoic aquifers. In the time frame of this study (one year), the geochemistry of all three springs was constant through all four seasons, spanning spring snowmelt and recharge as well as late-summer and fall dryness. This relationship is consistent with a deep source not influenced by shallow, local hydrogeology. Anomalies are evident in the historic data set for the geochemistry of Big Spring. We speculate that anomalies occurring between 1906 and 1926 suggest mixing of source waters of Big Spring with waters from a siliciclastic formation, and that anomalies occurring between 1926 and 1933 suggest mixing with waters from a formation containing gypsum or anhydrite. Decreased concentrations measured in our study—relative to concentrations measured between 1933 and 1976—may reflect mixing of thermal waters with more dilute waters. Current data are not sufficient to rigorously test these suggestions, and events of sufficient scale taking place in these timeframes have not been identified.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"1-16"},"PeriodicalIF":0.0,"publicationDate":"2014-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312588","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Paleoproterozoic rocks in the Black Hills of southwestern South Dakota record deformation associated with accretion of Proterozoic terranes from the south and suturing of the Wyoming and Superior cratons (ca. 1780–1715 Ma). Folding associated with suturing of these Precambrian terranes is well documented. However, penetrative fabrics related to shearing are characterized in only a few localities. The research presented herein identifies and characterizes a ≥4-km-wide, pervasive, thick-skinned shear zone, which deforms the regional-scale folding in the Black Hills portion of the Wyoming craton. Shearing is bracketed between ca. 1740 Ma (post regional-F2 folding) and ca. 1715 Ma (intrusion of the Harney Peak Granite). This high-strain zone is characterized by narrow anastomosing shear zones (m-scale) in the Paleoproterozoic rocks in the east-central Black Hills near Rockerville, South Dakota. The shear zones converge to form a km-wide zone of deformation in Paleoproterozoic and Archean rocks to the north near Nemo, South Dakota. Kinematic indicators such as composite foliations, microfolding, and asymmetric mantled porphyroclasts from within the shear zone support left-lateral, east-side-up transpression. Furthermore, strain associated with the shearing is coupled to vertically plunging, isoclinal F3 folds, commonly identified only as hinge areas preserved between strongly sheared limbs, within and adjacent to the shear zone in the study area. Correlation of this deformational event from the Paleoproterozoic rocks in the east-central Black Hills to older Paleoproterozoic and Archean rocks to the north has implications for these structures to have formed during a basement-involved deformational event. Evidence that stresses associated with this ∼1740–1715 Ma event were transferred inboard to the Wyoming Province is present to the southwest in the Hartville Uplift and Laramie Mountains of southeastern Wyoming. In both locations, structures with similar timing and kinematics overprint Cheyenne-Belt deformation. The regional extent and thick-skinned nature of these structures are interpreted to signify the change to continent-continent collisional tectonics during the final suturing of the Wyoming and Superior Provinces.
南达科他州西南部布莱克山的古元古代岩石记录了与南方元古代地体增生和怀俄明克拉通和苏必利尔通缝合有关的变形(约1780-1715 Ma)。与这些前寒武纪地体的缝合相关的褶皱有很好的记录。然而，与剪切有关的渗透织物仅在少数地方具有特征。本文研究确定并表征了一个≥4 km宽、普遍存在的厚皮剪切带，该剪切带变形了怀俄明州克拉通布莱克山部分的区域尺度褶皱。剪切作用介于约1740 Ma (f2后区域褶皱)和约1715 Ma(哈尼峰花岗岩侵入)之间。南达科塔州罗克维尔附近布莱克山中东部古元古代岩石的高应变带以狭窄的吻合剪切带(m尺度)为特征。这些剪切带汇聚在南达科他州尼莫附近北部的古元古代和太古代岩石中形成了一个千米宽的变形带。剪切带内的复合叶理、微褶皱和不对称壳状斑岩碎屑等运动学指标支持左侧、东侧向上的挤压作用。此外，与剪切相关的应变耦合到垂直下陷的等斜F3褶皱，通常仅被认为是在研究区剪切带内或邻近的强烈剪切肢之间保留的铰链区域。从黑山中东部的古元古代岩石到北部的古元古代和太古代岩石的变形对比表明，这些构造是在基底变形事件中形成的。与这一~ 1740-1715 Ma事件相关的应力向内转移到怀俄明州的证据出现在怀俄明州东南部的Hartville隆起和Laramie山脉的西南部。在这两个位置，具有相似时序和运动学的构造叠加夏延带变形。这些构造的区域范围和厚皮性质被解释为在怀俄明州和苏必利尔省最终缝合期间大陆-大陆碰撞构造的变化。
{"title":"Paleoproterozoic transpressional shear zone, eastern Black Hills, South Dakota: Implications for the late tectonic history of the southern Trans-Hudson Orogen","authors":"S. Allard, Douglas H. Portis","doi":"10.2113/GSROCKY.48.2.73","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.2.73","url":null,"abstract":"Paleoproterozoic rocks in the Black Hills of southwestern South Dakota record deformation associated with accretion of Proterozoic terranes from the south and suturing of the Wyoming and Superior cratons (ca. 1780–1715 Ma). Folding associated with suturing of these Precambrian terranes is well documented. However, penetrative fabrics related to shearing are characterized in only a few localities. The research presented herein identifies and characterizes a ≥4-km-wide, pervasive, thick-skinned shear zone, which deforms the regional-scale folding in the Black Hills portion of the Wyoming craton. Shearing is bracketed between ca. 1740 Ma (post regional-F2 folding) and ca. 1715 Ma (intrusion of the Harney Peak Granite). This high-strain zone is characterized by narrow anastomosing shear zones (m-scale) in the Paleoproterozoic rocks in the east-central Black Hills near Rockerville, South Dakota. The shear zones converge to form a km-wide zone of deformation in Paleoproterozoic and Archean rocks to the north near Nemo, South Dakota. Kinematic indicators such as composite foliations, microfolding, and asymmetric mantled porphyroclasts from within the shear zone support left-lateral, east-side-up transpression. Furthermore, strain associated with the shearing is coupled to vertically plunging, isoclinal F3 folds, commonly identified only as hinge areas preserved between strongly sheared limbs, within and adjacent to the shear zone in the study area. Correlation of this deformational event from the Paleoproterozoic rocks in the east-central Black Hills to older Paleoproterozoic and Archean rocks to the north has implications for these structures to have formed during a basement-involved deformational event. Evidence that stresses associated with this ∼1740–1715 Ma event were transferred inboard to the Wyoming Province is present to the southwest in the Hartville Uplift and Laramie Mountains of southeastern Wyoming. In both locations, structures with similar timing and kinematics overprint Cheyenne-Belt deformation. The regional extent and thick-skinned nature of these structures are interpreted to signify the change to continent-continent collisional tectonics during the final suturing of the Wyoming and Superior Provinces.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"73-99"},"PeriodicalIF":0.0,"publicationDate":"2013-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.2.73","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311997","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2013-09-01DOI: 10.2113/GSROCKY.48.2.169
Donald W. Tomlinson, P. Copeland, M. Murphy, T. Lapen
Laramide-style shortening structures in southwest New Mexico chiefly affect Cretaceous and older rocks, making the estimation of the cessation of shortening in the Paleogene problematic. The cessation of Laramide shortening is generally thought to be about 40 Ma in southwest New Mexico. However, in the Silver City Range, Grant County, New Mexico, shortening younger than 34.6 Ma was documented by Copeland et al. (2011). The Little Burro Mountains, 10–20 km south of the Silver City Range, contain a monocline trending northwest–southeast with beds dipping ∼12° in the backlimb and up to 30° in the forelimb. The youngest folded unit in the monocline is the tuff of Wind Mountain (Twt), which yielded a 206 Pb/ 238 U zircon age of 30.9 ± 0.5 Ma. Normal faults strike orthogonal to the axial trace of the fold with low displacement (10s of meters), which we interpret to have formed synchronous with the monocline to accommodate variations in shortening along strike. Field observations, trishear fault-propagationfold modeling, and the structural style and trends of the region are consistent with the development of the monocline related to a deeply rooted blind thrust or reverse fault. These results indicate that Laramide-style shortening in southwest New Mexico was active into the late Oligocene.
{"title":"Oligocene shortening in the Little Burro Mountains of southwest New Mexico","authors":"Donald W. Tomlinson, P. Copeland, M. Murphy, T. Lapen","doi":"10.2113/GSROCKY.48.2.169","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.2.169","url":null,"abstract":"Laramide-style shortening structures in southwest New Mexico chiefly affect Cretaceous and older rocks, making the estimation of the cessation of shortening in the Paleogene problematic. The cessation of Laramide shortening is generally thought to be about 40 Ma in southwest New Mexico. However, in the Silver City Range, Grant County, New Mexico, shortening younger than 34.6 Ma was documented by Copeland et al. (2011). The Little Burro Mountains, 10–20 km south of the Silver City Range, contain a monocline trending northwest–southeast with beds dipping ∼12° in the backlimb and up to 30° in the forelimb. The youngest folded unit in the monocline is the tuff of Wind Mountain (Twt), which yielded a 206 Pb/ 238 U zircon age of 30.9 ± 0.5 Ma. Normal faults strike orthogonal to the axial trace of the fold with low displacement (10s of meters), which we interpret to have formed synchronous with the monocline to accommodate variations in shortening along strike. Field observations, trishear fault-propagationfold modeling, and the structural style and trends of the region are consistent with the development of the monocline related to a deeply rooted blind thrust or reverse fault. These results indicate that Laramide-style shortening in southwest New Mexico was active into the late Oligocene.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"169-183"},"PeriodicalIF":0.0,"publicationDate":"2013-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.2.169","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311798","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2013-09-01DOI: 10.2113/GSROCKY.48.2.185
K. I. Singer, R. V. Fodor
The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indi
{"title":"Petrology of Stewart Mountain basalt field in central Arizona, U.S.A.: A lithospheric source with small-scale trace element and isotopic heterogeneities","authors":"K. I. Singer, R. V. Fodor","doi":"10.2113/GSROCKY.48.2.185","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.2.185","url":null,"abstract":"The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indi","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"76 1","pages":"185-210"},"PeriodicalIF":0.0,"publicationDate":"2013-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.2.185","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311928","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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