Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozo... more Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozoic. This provocative finding raises a number of questions about the triggers for this transition and the geochemical and biological consequences. To better address these questions, we present a high-resolution geochemical record through the early Neoproterozoic Chuar Group. The Chuar Group is composed of more than 1500 meters of gently folded, unmetamorphosed inter-bedded fossiliferous shales and meter scale carbonate- sandstones. Focusing primarily on the shales, Fe-speciation and sulfur isotope data provide the backbone of the study. Fe-speciation data, in particular, allows for the story of Neoproterozoic ocean anoxia to be updated and extended. Interestingly, these data illustrate transitions between Fe-rich and S-rich anoxic conditions, and shed light on how these reversions behave and are initiated. Just as important as the local information gained from the Chuar sediments, these d...
Olistolith production and magmatism are processes commonly associated with extensional tectonic s... more Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U-Pb apatite ages, U-Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U-Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at
ABSTRACT The Mesoproterozoic and Neoproterozoic Pahrump Group of Death Valley, California spans c... more ABSTRACT The Mesoproterozoic and Neoproterozoic Pahrump Group of Death Valley, California spans ca. 1300-635 Ma and provides a >500 million-year record of geologic events in southwestern Laurentia. The strata analyzed include preserved sequences separated by unconformities recording syn-Rodinia basin development (Crystal Spring Formation); Rodinia stability; regional extension culminating in Neoproterozoic rifting of the Laurentian margin of Rodinia (Horse Thief Springs through Johnnie Formations); and multiple phases of glacial sedimentation and subsequent cap carbonate deposition (Kingston Peak Formation and Noonday Dolomite). U-Pb detrital zircon analyses were conducted on samples from the entire Pahrump Group and the Noonday Dolomite in the southeastern Death Valley region (20 samples, 1945 grains) to further constrain hypotheses for regional basin development during the development of the southwestern Laurentian margin. Our interpretation of provenance data expands upon and clarifies previous models defining a series of tectonostratigraphic units including: (A) the <1400 Ma basal conglomerate of the Crystal Spring Formation, comprised of metasedimentary quartzite clasts, and exhibiting a unimodal detrital zircon sample distribution at 1690 Ma with northerly source; (B) the ca. 1320-1080 Ma Crystal Spring Formation exhibiting unimodal zircon distributions derived from southerly, local Paleoproterozoic basement sources punctuated by a ca. 300 Ma duration unconformity;
The Devils Gate Limestone at Red Hill, Simpson Park Range, Eureka Co., Nevada is composed of seve... more The Devils Gate Limestone at Red Hill, Simpson Park Range, Eureka Co., Nevada is composed of several conformable transgressive sequences cropping out in a series of step-like cliffs 1--5 m high. The general sequence is as follows: (1) a 1--4 m thick in situ boundstone dominated by large, period rugosans, domical and stratiform stromatoporids and rare, 1--3 cm diameter oncolites.
... Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Format... more ... Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia. Carol M. Dehler 1 , , C. Mark Fanning 2 , Paul K. Link 3 , Esther M. Kingsbury 3 and Dan Rybczynski 1 ...
Keywords: Neoproterozoic oxygenation iron sulfur ocean chemistry paleobiology Detailed iron, sulf... more Keywords: Neoproterozoic oxygenation iron sulfur ocean chemistry paleobiology Detailed iron, sulfur and carbon chemistry through the N 742 million year old Chuar Group reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late in the basin's development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control-specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and upregulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the Chuar Group are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system.
The deepest valleys of the Andes have been cut in southern Peru by the Ríos Cotahuasi Ocoña and C... more The deepest valleys of the Andes have been cut in southern Peru by the Ríos Cotahuasi Ocoña and Colca-Majes. These canyons are Late Miocene landforms based on a new ignimbrite stratigraphy supported by 42 new 40 Ar/ 39 Ar age determinations obtained on plateau-forming and valley-filling ignimbrites. Between 19 and 13 Ma, a gently sloping surface bevelling the clastic wedge southeast of the developing mountain front was mantled by widespread ignimbrites. After 13 Ma, this paleosurface was tilted up from 2.2 km a.s.l. at the mountain front to 4.3 km a.s.l. at the base of the Pliocene and Pleistocene volcanoes that crown the southwestern edge of the Altiplano. The canyons incised this topography after 9 Ma, while the dated base of younger ignimbrite valley fills suggests that these canyons had been cut down to near their present depths as early as 3.8 Ma. By 1.4 Ma, however, the canyons had been almost completely refilled by 1.3 km-thick unwelded pyroclastic deposits, which were subsequently eroded. Valley incision since 9 Ma at an average rate of 0.2 mm yr − 1 is the response to topographic uplift after 13 Ma combined with increasing runoff due to a wetter climate recorded after 7 Ma. Although long-term aridity generated an imbalance between high long-term uplift rates and low plateau denudation rates, the combination of aridity and volcanism still promoted canyon incision because episodic volcanic fills maintained a cycle of catastrophic debris avalanches and subsequent dam breakouts.
... to a close as more sophisticated interactive technologies find their way into everyday distan... more ... to a close as more sophisticated interactive technologies find their way into everyday distance education practice (Dede, 1996, Ullmer, 1994; Kozma, 1994). ... is an emerging paradigm that has been suggested as a basis for re-conceptualizing distance education (eg, Tam, 2000). ...
Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary... more Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from~770 to 740 Ma; (2) early rifting and volcanism along a N-S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from~720 to 660 Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from~660 to 580 Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from~570 to 520 Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from~700 to 670 Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental rifting. The lower Brigham and McCoy Creek groups contain mostly mature quartz arenite deposited in shallow marine environments, with DZ patterns recording distal Laurentian sources. The base of the Prospect Mountain and correlative formations is marked by an influx of feldspathic, coarsegrained sediment derived from local basement sources and~570-540 Ma basalt volcanism, which was followed by deposition of subfeldspathic strata with dominant 1.7-1.8 Ga DZ grains, recording sources from the SE rift margin and a marked decrease in distal sources during uplift of the Transcontinental Arch. Overlying carbonate-rich strata were deposited in shallow marine settings, with episodic influx of siliciclastic sediment derived from basement exposed during regressions. Stratigraphic thickness-age relations of Neoproterozoic to early Paleozoic strata are consistent with two episodes of rifting concentrated at ca. 700-670 Ma and 570-540 Ma along western Laurentia, leading to final development of a passive margin. Early rifting was incomplete with an estimated 25-40% extension of initially thick lithosphere that was weakened by igneous activity. Final rifting of previously thinned lithosphere involved an estimated 20-35% additional extension, renewed igneous activity, and thermal thinning of mantle lithosphere, with localized extension culminating in final separation along the continental margin. Stratigraphic, geochronologic, and available paleomagnetic data Earth-Science Reviews 136 (2014) 59-95 ⁎ Corresponding author.
Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozo... more Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozoic. This provocative finding raises a number of questions about the triggers for this transition and the geochemical and biological consequences. To better address these questions, we present a high-resolution geochemical record through the early Neoproterozoic Chuar Group. The Chuar Group is composed of more than 1500 meters of gently folded, unmetamorphosed inter-bedded fossiliferous shales and meter scale carbonate- sandstones. Focusing primarily on the shales, Fe-speciation and sulfur isotope data provide the backbone of the study. Fe-speciation data, in particular, allows for the story of Neoproterozoic ocean anoxia to be updated and extended. Interestingly, these data illustrate transitions between Fe-rich and S-rich anoxic conditions, and shed light on how these reversions behave and are initiated. Just as important as the local information gained from the Chuar sediments, these d...
Olistolith production and magmatism are processes commonly associated with extensional tectonic s... more Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U-Pb apatite ages, U-Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U-Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at
ABSTRACT The Mesoproterozoic and Neoproterozoic Pahrump Group of Death Valley, California spans c... more ABSTRACT The Mesoproterozoic and Neoproterozoic Pahrump Group of Death Valley, California spans ca. 1300-635 Ma and provides a >500 million-year record of geologic events in southwestern Laurentia. The strata analyzed include preserved sequences separated by unconformities recording syn-Rodinia basin development (Crystal Spring Formation); Rodinia stability; regional extension culminating in Neoproterozoic rifting of the Laurentian margin of Rodinia (Horse Thief Springs through Johnnie Formations); and multiple phases of glacial sedimentation and subsequent cap carbonate deposition (Kingston Peak Formation and Noonday Dolomite). U-Pb detrital zircon analyses were conducted on samples from the entire Pahrump Group and the Noonday Dolomite in the southeastern Death Valley region (20 samples, 1945 grains) to further constrain hypotheses for regional basin development during the development of the southwestern Laurentian margin. Our interpretation of provenance data expands upon and clarifies previous models defining a series of tectonostratigraphic units including: (A) the <1400 Ma basal conglomerate of the Crystal Spring Formation, comprised of metasedimentary quartzite clasts, and exhibiting a unimodal detrital zircon sample distribution at 1690 Ma with northerly source; (B) the ca. 1320-1080 Ma Crystal Spring Formation exhibiting unimodal zircon distributions derived from southerly, local Paleoproterozoic basement sources punctuated by a ca. 300 Ma duration unconformity;
The Devils Gate Limestone at Red Hill, Simpson Park Range, Eureka Co., Nevada is composed of seve... more The Devils Gate Limestone at Red Hill, Simpson Park Range, Eureka Co., Nevada is composed of several conformable transgressive sequences cropping out in a series of step-like cliffs 1--5 m high. The general sequence is as follows: (1) a 1--4 m thick in situ boundstone dominated by large, period rugosans, domical and stratiform stromatoporids and rare, 1--3 cm diameter oncolites.
... Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Format... more ... Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia. Carol M. Dehler 1 , , C. Mark Fanning 2 , Paul K. Link 3 , Esther M. Kingsbury 3 and Dan Rybczynski 1 ...
Keywords: Neoproterozoic oxygenation iron sulfur ocean chemistry paleobiology Detailed iron, sulf... more Keywords: Neoproterozoic oxygenation iron sulfur ocean chemistry paleobiology Detailed iron, sulfur and carbon chemistry through the N 742 million year old Chuar Group reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late in the basin's development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control-specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and upregulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the Chuar Group are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system.
The deepest valleys of the Andes have been cut in southern Peru by the Ríos Cotahuasi Ocoña and C... more The deepest valleys of the Andes have been cut in southern Peru by the Ríos Cotahuasi Ocoña and Colca-Majes. These canyons are Late Miocene landforms based on a new ignimbrite stratigraphy supported by 42 new 40 Ar/ 39 Ar age determinations obtained on plateau-forming and valley-filling ignimbrites. Between 19 and 13 Ma, a gently sloping surface bevelling the clastic wedge southeast of the developing mountain front was mantled by widespread ignimbrites. After 13 Ma, this paleosurface was tilted up from 2.2 km a.s.l. at the mountain front to 4.3 km a.s.l. at the base of the Pliocene and Pleistocene volcanoes that crown the southwestern edge of the Altiplano. The canyons incised this topography after 9 Ma, while the dated base of younger ignimbrite valley fills suggests that these canyons had been cut down to near their present depths as early as 3.8 Ma. By 1.4 Ma, however, the canyons had been almost completely refilled by 1.3 km-thick unwelded pyroclastic deposits, which were subsequently eroded. Valley incision since 9 Ma at an average rate of 0.2 mm yr − 1 is the response to topographic uplift after 13 Ma combined with increasing runoff due to a wetter climate recorded after 7 Ma. Although long-term aridity generated an imbalance between high long-term uplift rates and low plateau denudation rates, the combination of aridity and volcanism still promoted canyon incision because episodic volcanic fills maintained a cycle of catastrophic debris avalanches and subsequent dam breakouts.
... to a close as more sophisticated interactive technologies find their way into everyday distan... more ... to a close as more sophisticated interactive technologies find their way into everyday distance education practice (Dede, 1996, Ullmer, 1994; Kozma, 1994). ... is an emerging paradigm that has been suggested as a basis for re-conceptualizing distance education (eg, Tam, 2000). ...
Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary... more Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from~770 to 740 Ma; (2) early rifting and volcanism along a N-S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from~720 to 660 Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from~660 to 580 Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from~570 to 520 Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from~700 to 670 Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental rifting. The lower Brigham and McCoy Creek groups contain mostly mature quartz arenite deposited in shallow marine environments, with DZ patterns recording distal Laurentian sources. The base of the Prospect Mountain and correlative formations is marked by an influx of feldspathic, coarsegrained sediment derived from local basement sources and~570-540 Ma basalt volcanism, which was followed by deposition of subfeldspathic strata with dominant 1.7-1.8 Ga DZ grains, recording sources from the SE rift margin and a marked decrease in distal sources during uplift of the Transcontinental Arch. Overlying carbonate-rich strata were deposited in shallow marine settings, with episodic influx of siliciclastic sediment derived from basement exposed during regressions. Stratigraphic thickness-age relations of Neoproterozoic to early Paleozoic strata are consistent with two episodes of rifting concentrated at ca. 700-670 Ma and 570-540 Ma along western Laurentia, leading to final development of a passive margin. Early rifting was incomplete with an estimated 25-40% extension of initially thick lithosphere that was weakened by igneous activity. Final rifting of previously thinned lithosphere involved an estimated 20-35% additional extension, renewed igneous activity, and thermal thinning of mantle lithosphere, with localized extension culminating in final separation along the continental margin. Stratigraphic, geochronologic, and available paleomagnetic data Earth-Science Reviews 136 (2014) 59-95 ⁎ Corresponding author.
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