scholarly journals Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth

2020 ◽  
Vol 117 (19) ◽  
pp. 10172-10180
Author(s):  
Rebecca M. Flowers ◽  
Francis A. Macdonald ◽  
Christine S. Siddoway ◽  
Rachel Havranek
Keyword(s):  
Sedimentary Rocks ◽  
Wall Rock ◽  
Snowball Earth ◽  
Precambrian Basement ◽  
Near Surface ◽  
Surface Conditions ◽  
Extensive Region ◽  
Regional Tectonic ◽  
Tectonic Features ◽  
Erosional Event

The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.

scholarly journals North polar trough formation due to in-situ erosion as a source of young ice in mid-latitudinal mantles on Mars

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
J. Alexis P. Rodriguez ◽  
Kenneth L. Tanaka ◽  
Ali M. Bramson ◽  
Gregory J. Leonard ◽  
Victor R. Baker ◽  
...  
Keyword(s):  
Katabatic Wind ◽  
Spiral Pattern ◽  
Near Surface ◽  
Valuable Source ◽  
History Of ◽  
North Polar ◽  
Human Exploration ◽  
Erosional Event

AbstractThe clockwise spiral of troughs marking the Martian north polar plateau forms one of the planet’s youngest megastructures. One popular hypothesis posits that the spiral pattern resulted as troughs underwent poleward migration. Here, we show that the troughs are extensively segmented into enclosed depressions (or cells). Many cell interiors display concentric layers that connect pole- and equator-facing slopes, demonstrating in-situ trough erosion. The segmentation patterns indicate a history of gradual trough growth transversely to katabatic wind directions, whereby increases in trough intersections generated their spiral arrangement. The erosional event recorded in the truncated strata and trough segmentation may have supplied up to ~25% of the volume of the mid-latitude icy mantles. Topographically subtle undulations transition into troughs and have distributions that mimic and extend the troughs’ spiraling pattern, indicating that they probably represent buried trough sections. The retention of the spiral pattern in surface and subsurface troughs is consistent with the megastructure’s stabilization before its partial burial. A previously suggested warm paleoclimatic spike indicates that the erosion could have occurred as recently as ~50 Ka. Hence, if the removed ice was redeposited to form the mid-latitude mantles, they could provide a valuable source of near-surface, clean ice for future human exploration.

The Fairbanks earthquakes of June 21, 1967; aftershock distribution, focal mechanisms, and crustal parameters

1969 ◽  
Vol 59 (1) ◽  
pp. 73-100
Author(s):  
Larry Gedney ◽  
Eduard Berg
Keyword(s):  
P Wave ◽  
Fault System ◽  
Crustal Layer ◽  
Near Surface ◽  
Fault Surface ◽  
True Value ◽  
Regional Tectonic ◽  
Tectonic System ◽  
Relationship Of ◽  
The Relationship

Abstract A series of moderately severe earthquakes occurred in the vicinity of Fairbanks, Alaska, on the morning of June 21, 1967. During the following months, many thousands of aftershocks were recorded in order to outline the aftershock zone and to resolve the focal mechanism and its relation to the regional tectonic system. No fault is visible at the surface in this area. Foci were found to occupy a relatively small volume in the shape of an ablate cylinder tilted about 30° from the vertical. The center of the zone lay about 12 kilometers southeast of Fairbanks. Focal depths ranged from near-surface to 25 kilometers, although most were in the range 9-16 km. In the course of the investigation, it was found that the Jeffreys and Bullen velocity of 5.56 km/sec for the P wave in the upper crustal layer is very near the true value for this arec, and that the use of 1.69 for the Vp/Vs ratio gives good results in most cases. The proposed faulting mechanism involves nearly equal components of right-lateral strike slip, and normal faulting with northeast side downthrown on a system of sub-parallel faults striking N40°W. The fault surface appears to be curved—dipping from near vertical close to the surface to less steep northeast dips at greater depths. The relationship of this fault system with the grosser aspects of regional tectonism is not clear.

scholarly journals A nanocrystalline monoclinic CaCO3precursor of metastable aragonite

2018 ◽  
Vol 4 (12) ◽  
pp. eaau6178 ◽  
Author(s):  
Péter Németh ◽  
Enrico Mugnaioli ◽  
Mauro Gemmi ◽  
György Czuppon ◽  
Attila Demény ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Structure Determination ◽  
Diffuse Scattering ◽  
Diffraction Tomography ◽  
Synthetic Material ◽  
Near Surface ◽  
Surface Conditions ◽  
Terrestrial Sediments ◽  
Living Organisms ◽  
New Phase

Despite its thermodynamical metastability at near-surface conditions, aragonite is widespread in marine and terrestrial sediments. It abundantly forms in living organisms, and its abiotic formation is favored in waters of a Mg2+/Ca2+ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO3polymorph, which precipitates before aragonite in a cave. The new phase, which we term monoclinic aragonite (mAra), is crystallographically related to ordinary, orthorhombic aragonite. Electron diffraction tomography combined with structure determination demonstrates that mAra has a layered aragonite structure, in which some carbonates can be replaced by hydroxyls and up to 10 atomic % of Mg can be incorporated. The diagnostic electron diffraction features of mAra are diffuse scattering and satellite reflections along aragonite {110}. Similar features have previously been reported—although unrecognized—from biogenic aragonite formed in stromatolites, mollusks, and cyanobacteria as well as from synthetic material. We propose that mAra is a widespread crystalline CaCO3that plays a hitherto unrecognized key role in metastable aragonite formation.

scholarly journals Model-dependence of the CO<sub>2</sub> threshold for melting the hard Snowball Earth

2011 ◽  
Vol 7 (1) ◽  
pp. 17-25 ◽  
Author(s):  
Y. Hu ◽  
J. Yang ◽  
F. Ding ◽  
W. R. Peltier
Keyword(s):  
Surface Temperature ◽  
Greenhouse Effect ◽  
Surface Albedo ◽  
Snowball Earth ◽  
Pressure Broadening ◽  
Near Surface ◽  
Induced Absorption ◽  
Cloud Forcing ◽  
Clear Sky ◽  
Convective Model

Abstract. One of the critical issues of the Snowball Earth hypothesis is the CO2 threshold for triggering the deglaciation. Using Community Atmospheric Model version 3.0 (CAM3), we study the problem for the CO2 threshold. Our simulations show large differences from previous results (e.g. Pierrehumbert, 2004, 2005; Le Hir et al., 2007). At 0.2 bars of CO2, the January maximum near-surface temperature is about 268 K, about 13 K higher than that in Pierrehumbert (2004, 2005), but lower than the value of 270 K for 0.1 bar of CO2 in Le Hir et al. (2007). It is found that the difference of simulation results is mainly due to model sensitivity of greenhouse effect and longwave cloud forcing to increasing CO2. At 0.2 bars of CO2, CAM3 yields 117 Wm−2 of clear-sky greenhouse effect and 32 Wm−2 of longwave cloud forcing, versus only about 77 Wm−2 and 10.5 Wm−2 in Pierrehumbert (2004, 2005), respectively. CAM3 has comparable clear-sky greenhouse effect to that in Le Hir et al. (2007), but lower longwave cloud forcing. CAM3 also produces much stronger Hadley cells than that in Pierrehumbert (2005). Effects of pressure broadening and collision-induced absorption are also studied using a radiative-convective model and CAM3. Both effects substantially increase surface temperature and thus lower the CO2 threshold. The radiative-convective model yields a CO2 threshold of about 0.21 bars with surface albedo of 0.663. Without considering the effects of pressure broadening and collision-induced absorption, CAM3 yields an approximate CO2 threshold of about 1.0 bar for surface albedo of about 0.6. However, the threshold is lowered to 0.38 bars as both effects are considered.

scholarly journals 3D lithostratigraphic model of the Paleogene of the onshore part of the Moesian Platform (Northeast Bulgaria)

2018 ◽  
Vol 47 (1) ◽  
pp. 23-36
Author(s):  
Boris Valchev ◽  
Dimitar Sachkov ◽  
Sava Juranov
Keyword(s):  
Spatial Distribution ◽  
Sedimentary Rocks ◽  
Eastern Slope ◽  
The South ◽  
The North ◽  
Lithostratigraphic Units ◽  
Moesian Platform ◽  
Regional Tectonic ◽  
Model Database ◽  
The Individual

The Paleogene sedimentary rocks in the north-easternmost part of the territory of Bulgaria have been penetrated by numerous boreholes. In terms of regional tectonic zonation, the study area is a part of the onshore sector of the Moesian Platform, which partly includes the South Dobrogea Unit and the easternmost part of the North Bulgarian Dome with its eastern slope. The lithostratigraphy of the Paleogene successions consists of six formal units (the Komarevo, Beloslav, Dikilitash, Aladan, Avren, and Ruslar formations) and one informal unit (glauconitic marker). For compiling an overall conception of the regional aspects (lithology, thickness, spatial distribution, and relationships) of the individual lithostratigraphic units and for illustration of their spatial distribution, a 3D lithostratigraphic model based on reinterpretation of individual borehole sections has been created. The model database was compiled by integration of the original lithological data from 338 borehole sections.

scholarly journals Uncertainty of the CO<sub>2</sub> threshold for melting a hard Snowball Earth

2010 ◽  
Vol 6 (4) ◽  
pp. 1337-1350 ◽  
Author(s):  
Y. Hu ◽  
J. Yang
Keyword(s):  
Greenhouse Effect ◽  
Atmospheric Model ◽  
Snowball Earth ◽  
Near Surface ◽  
Cloud Forcing ◽  
Clear Sky ◽  
Critical Issues ◽  
Simulation Results ◽  
High Level ◽  
Hadley Cells

Abstract. One of the critical issues of the Snowball Earth hypothesis is how high level of CO2 is required for triggering the deglaciation. Using Community Atmospheric Model version 3 (CAM3), we study the problem for the CO2 threshold. Our simulations show large differences from previous results (Pierrehumbert, 2004, 2005). At 0.2 bars of CO2, the January maximum near-surface temperature is about 268 K, about 13 K higher than that in Pierrehumbert (2004, 2005), but lower than the value of 270 K for 0.1 bar of CO2 in Le Hir et al. (2007). It is found that the diversity of simulation results is mainly due to model sensitivity of greenhouse effect and longwave cloud forcing to increasing CO2. At 0.2 bar of CO2, CAM3 yields 117 Wm −2 of clear-sky greenhouse effect and 32 Wm−2 of longwave cloud forcing, versus only about 77 Wm−2 and 10.5 Wm−2 in Pierrehumbert (2004, 2005), respectively. CAM3 has comparable clear-sky greenhouse effect to that in Le Hir et al. (2007), but lower longwave cloud forcing. CAM3 also produces much stronger Hadley cells than in Pierrehumbert (2005).

Effect of rock strength on exhumation and the thermochronologic record: The south-central Colorado example

2021 ◽  
Author(s):  
Sabrina Kainz ◽  
Lon Abbott ◽  
Rebecca Flowers ◽  
James Metcalf
Keyword(s):  
Low Temperature ◽  
Sedimentary Rock ◽  
Rock Strength ◽  
Sedimentary Rocks ◽  
Sedimentary Basins ◽  
Crystalline Basement ◽  
Precambrian Basement ◽  
The West ◽  
South Central ◽  
Sangre De Cristo

&lt;p&gt;Past work has used the Southern Rocky Mountains (SRM) in the U.S. state of Colorado to illustrate the important role that rock strength plays in the histories recorded by the apatite fission track (AFT) and apatite (U-Th)/He (AHe) low-temperature thermochronometers (Flowers &amp; Ehlers, 2018). The SRM were initially raised during the Laramide Orogeny, ca. 70-45 Ma, but consensus exists that the region also experienced a later, post-Laramide exhumation event. Flowers &amp; Ehlers (2018) pointed to the low erosion potential of the Precambrian crystalline basement rocks that crop out in most SRM ranges as a primary reason for the abundance of 55-70 Ma &amp;#8220;Laramide&amp;#8221; AFT and AHe dates in the region, compared to a paucity of younger dates that would presumably be produced through erosion triggered by the post-Laramide exhumation event. South-central Colorado offers a test of this hypothesis, due to lateral variations in rock erodibility provided by the presence here of both sedimentary and crystalline Laramide ranges and adjacent sedimentary basins. The combination of our ongoing AHe study with previous south-central Colorado AFT and AHe work reveals kilometer-scale post-Laramide (Oligo-Miocene) exhumation has occurred in areas that possess thick sedimentary rock sequences whereas exhumation has been negligible where crystalline basement comprises the land surface.&amp;#160;&lt;/p&gt;&lt;p&gt;South-central Colorado&amp;#8217;s Sangre de Cristo Mountains consist of an imbricate stack of thrust sheets composed of Permian sedimentary rock. About 30 km farther east stand the Wet Mountains, another Laramide range &amp;#8211; but one composed of Precambrian basement rock. The Raton Basin, a SRM foreland basin filled with 2 km of synorogenic fill underlain by a thick sequence of marine shale, lies south and east of the two ranges. The Wet Mountains thus form a peninsula of strong crystalline rock surrounded by more erodible sedimentary rocks to the west, south, and east.&amp;#160;&lt;/p&gt;&lt;p&gt;Our study and that of Landman (2018) records at least 2 km of erosion in the Raton Basin east and south of the Wet Mountains since 25 Ma. Lindsey et al (1986) obtained 24-15 Ma AFT dates from the Paleozoic sedimentary rocks of the Sangre de Cristo Mountains, demonstrating that kilometer-scale Oligo-Miocene exhumation occurred just west of the Wet Mountains. By contrast, Kelley and Chapin (2004) obtained only pre-Laramide AFT ages between 228-110 Ma for 17 samples of Precambrian basement from the crest of the Wet Mountains. A 32 Ma ash flow tuff unconformably overlies Precambrian basement on Greenhorn Mountain, the Wet Mountains&amp;#8217; highest and southernmost peak. Its presence reinforces the conclusion, based on the AFT dates, that Oligo-Miocene erosion of the Wet Mountain massif has been minimal simultaneous with kilometer-scale exhumation to the west, south, and east. These results illustrate the important role that rock strength plays in determining the dates recorded in low-temperature thermochronologic studies.&lt;/p&gt;

An Overview of the ULU Gold Deposit, High Lake Volcanic Belt, Nunavut, Canada

2004 ◽  
Vol 13 (1-4) ◽  
pp. 15-23
Author(s):  
E. FLOOD ◽  
P. KLEESPIES ◽  
M. TANSEY ◽  
H. MUNTANION ◽  
R. CARPENTER
Keyword(s):  
Mineral Assemblage ◽  
Spatial Association ◽  
Sedimentary Rocks ◽  
Volcanic Belt ◽  
Wall Rock ◽  
Potassium Feldspar ◽  
Lode Gold ◽  
The Core ◽  
Flood Zone ◽  
Alteration Minerals

Abstract BHP Minerals discovered the Archean lode gold ULU deposit on the western margin of the High Lake Volcanic Belt in 1989. The greenschist to amphibolite facies mafic volcanic and sedimentary rocks on the property are folded into a 5 km-long anticline. There is a close spatial association of Au-As zones to the trace of this F2 ULU anticline. The largest of these auriferous zones, the Flood zone, is localized at the core of the fold. The southeast-trending Flood zone consists of several anastomosing lenses that have been traced for 435 m on surface. Overall, it dips steeply (70° to 80°) to the southwest and has been intersected to depths below 600 m. The hosting high-iron tholeitic basalt displays a lower amphibolite mineral assemblage of ferrohornblende + plagioclase + ilmenite with accessory quartz and epidote. Alteration minerals include biotite, chlorite, hornblende, actinolite-tremolite, and potassium feldspar (microcline) with minor calcite, epidote, tourmaline, and titanite. The highest gold values occur where brecciated basaltic wall-rock clasts are replaced by acicular arsenopyrite + quartz + K-feldspar. BHP Minerals completed 54,783 m of both exploration and resource development drilling before selling the ULU property to Echo Bay Mines in 1995. Development of ULU as a satellite deposit to Echo Bay’s Lupin mine began in 1996. A -15% ramp was extended to the 155-meter level, and 16,011 m of underground drilling were completed before the decline in gold prices forced a suspension of activities at ULU in August 1997. The current owners, Wolfden Resources Inc., drilled 18,569 m in 2004 to increase confidence in the resource blocks. Following this drill program, a revised resource calculated by an independent consultant determined that the ULU deposit contains an inferred/indicated mineral resource of 1,130,000 t grading 11.34 g/t gold (373,748 oz) to the 360-m level. The portal was re-opened in 2005 and a prefeasibility study is underway.

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