The Experts below are selected from a list of 4704 Experts worldwide ranked by ideXlab platform
Aaron J Cavosie - One of the best experts on this subject based on the ideXlab platform.
-
nanoscale records of ancient shock deformation reidite zrsio4 in sandstone at the Ordovician Rock elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (<36 Ma) impact craters globally. Here we report a new occurrence of reidite in brecciated sandstone from the Ordovician Rock Elm impact crater in Wisconsin, United States. Electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 μm in diameter. Reidite occurs both as 200–500-nm-wide lamellar intergrowths and as nanoparticulate grains, and not only provides the first diagnostic evidence for ultrahigh-pressure shock metamorphism at Rock Elm, but is also the oldest reported occurrence of reidite. Considering its small size, and the ubiquitous presence of detrital zircon in siliciclastic Rocks, reidite may be more common in the Rock record than has been reported but has potentially gone undetected. The recognition that nanoscale reidite can be preserved over deep time within zircon in shock-metamorphosed sandstone presents new opportunities for investigating Earth9s impact record, as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed Rock Elm. Given that shocked zircons have been shown to survive sedimentary cycling, the identification of reidite within zircons in siliciclastic Rocks could facilitate investigating the impact chronology over much of the geological time scale, as the oldest terrestrial minerals known are detrital zircons.
-
Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (
Nicholas E Timms - One of the best experts on this subject based on the ideXlab platform.
-
nanoscale records of ancient shock deformation reidite zrsio4 in sandstone at the Ordovician Rock elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (<36 Ma) impact craters globally. Here we report a new occurrence of reidite in brecciated sandstone from the Ordovician Rock Elm impact crater in Wisconsin, United States. Electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 μm in diameter. Reidite occurs both as 200–500-nm-wide lamellar intergrowths and as nanoparticulate grains, and not only provides the first diagnostic evidence for ultrahigh-pressure shock metamorphism at Rock Elm, but is also the oldest reported occurrence of reidite. Considering its small size, and the ubiquitous presence of detrital zircon in siliciclastic Rocks, reidite may be more common in the Rock record than has been reported but has potentially gone undetected. The recognition that nanoscale reidite can be preserved over deep time within zircon in shock-metamorphosed sandstone presents new opportunities for investigating Earth9s impact record, as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed Rock Elm. Given that shocked zircons have been shown to survive sedimentary cycling, the identification of reidite within zircons in siliciclastic Rocks could facilitate investigating the impact chronology over much of the geological time scale, as the oldest terrestrial minerals known are detrital zircons.
-
Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (
Timmons M Erickson - One of the best experts on this subject based on the ideXlab platform.
-
nanoscale records of ancient shock deformation reidite zrsio4 in sandstone at the Ordovician Rock elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (<36 Ma) impact craters globally. Here we report a new occurrence of reidite in brecciated sandstone from the Ordovician Rock Elm impact crater in Wisconsin, United States. Electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 μm in diameter. Reidite occurs both as 200–500-nm-wide lamellar intergrowths and as nanoparticulate grains, and not only provides the first diagnostic evidence for ultrahigh-pressure shock metamorphism at Rock Elm, but is also the oldest reported occurrence of reidite. Considering its small size, and the ubiquitous presence of detrital zircon in siliciclastic Rocks, reidite may be more common in the Rock record than has been reported but has potentially gone undetected. The recognition that nanoscale reidite can be preserved over deep time within zircon in shock-metamorphosed sandstone presents new opportunities for investigating Earth9s impact record, as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed Rock Elm. Given that shocked zircons have been shown to survive sedimentary cycling, the identification of reidite within zircons in siliciclastic Rocks could facilitate investigating the impact chronology over much of the geological time scale, as the oldest terrestrial minerals known are detrital zircons.
-
Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater
Geology, 2015Co-Authors: Aaron J Cavosie, Timmons M Erickson, Nicholas E TimmsAbstract:The terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion, burial, and tectonics. Zircon survives these processes as a shocked mineral, and above 20 GPa transforms to reidite, a high-pressure ZrSiO 4 polymorph diagnostic of impact. However, the utility of reidite has been limited by its occurrence; it has only been reported from three relatively young (
U. Linnemann - One of the best experts on this subject based on the ideXlab platform.
-
Ediacaran Rocks from the Cadomian basement of the Saxo-Thuringian Zone (NE Bohemian Massif, Germany): age constraints, geotectonic setting and basin development
Geological Society London Special Publications, 2007Co-Authors: U. LinnemannAbstract:This paper is focused on a compilation of known data from the low-grade metamorphosed Rocks of the Ediacaran period in the German part of the Saxo-Thuringian Zone at the northeastern margin of the Bohemian Massif. The geotectonic setting during the formation of Ediacaran Rock units is characterized by Cadomian orogenic processes from c. 650–540 Ma at the periphery of the West African Craton. The basin development during that time is characterized by the formation of a Cadomian backarc basin with a passive margin, and the outboard sitting Cadomian magmatic arc originated at c. 570–560 Ma. This arc-marginal basin system was formed on stretched continental crust in a strike-slip regime and reflects an active-margin setting in a style similar to the recent West Pacific. The backarc basin was closed between c. 560–540 Ma by the collision of the Cadomian magmatic arc with the cratonic hinterland: this resulted in the closure of the backarc basin and the formation of a Cadomian retroarc basin. Collision of an oceanic ridge with the Cadomian Orogenic Belt led to a slab break-off of the subducted oceanic plate resulting in an extreme heat flow, and a magmatic and anatectic event culminating at c. 540 Ma that was responsible for the intrusion of voluminous granitoid plutons. Ediacaran Rock complexes in Germany are restricted to the Saxo-Thuringian Zone in the Bohemian Massif, which represents the largest and most important basement inlier of the Central European Variscides (Fig. 1). Low-grade Ediacaran metamorphic Rocks occur mainly in the Saxo-Thuringian Zone (Germany) and the Tepla-Barrandian Unit (Czech Republic) (Fig. 2). This paper presents a review on the weakly metamorphosed Ediacaran to Early Cambrian Rocks of the Saxo-Thuringian Zone. These Rocks units are affected by the tectonometamorphic overprint of the Late Neoproterozoic–Early Cambrian Cadomian and the Late Devonian–Early Carboniferous orogenies. In some areas of the Saxo-Thuringian Zone, such as the Erzgebirge Mountains, some of the Rocks display a high-grade metamorphic overprint. These Rocks are not discussed in this paper. Cadomian orogenic processes comprise a series of complex sedimentary, magmatic, and tectonometamorphic events that spanned the period from the mid-Neoproterozoic (c. 650 Ma) to the earliest Cambrian (c. 540 Ma) (e.g. Linnemann et al. 2000; Nance et al. 2002). Rock units formed by the Cadomian orogeny are generally referred to as ‘Cadomian basement’. Due to very similar contemporaneous orogenic processes in the Avalonian microplate, the collective term ‘Avalonian-Cadomian’ orogeny has often been used in the modern literature (e.g. Dorr et al. 2004). Peri-Gondwanan terranes, microcontinents and crustal units in Europe and in North Africa are affected by the Cadomian orogeny (e.g. Murphy et al. 2004). Related orogenic events, such as the Avalonian orogeny, are known in the Appalachians (eastern US and Atlantic Canada), and from the non-Laurentian part of Ireland and the British Isles (e.g. Nance & Murphy 1996). Baltica escaped the Avalonian-Cadomian orogenic activity, although the ‘Cadomian affinity’ of the late Precambrian orogenic events in the Urals and in the Timanides on the periphery of Baltica was recognized (Roberts & Siedlecka 2002; Glasmacher et al. 2004). The Cadomian orogeny was first defined in the North Armorican Massif in France on the basis of the unconformity that separates deformed Precambrian Rock units from their Early Palaeozoic (Cambro-Ordovician) overstep sequence (Bunel 1835; Dufrenoy 1838; Barrois 1899; Kerforne 1901). ‘Cadomus’ is an old Latin term for the modern city of Caen and is the source of the name of the orogeny. The term ‘discordance cadomienne’ was first used by Bertrand (1921). The best illustration of the unconformity in Rocreux (Normandy) was published by Graindor (1957). In Central and Western Europe the unconformity is commonly referred to as the ‘Cadomian unconformity’. It is possible that the youngest metasedimentary Rocks From: VICKERS-RICH, P. & KOMAROWER, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 35–51. DOI: 10.1144/SP286.4 0305-8719/07/$15.00 # The Geological Society of London 2007. affected by the Cadomian deformation are earliest Cambrian in age. Many geologists assume that the final stages of Cadomian orogenesis were spatially diachronous, lasting from the latest Neoproterozoic to the earliest Cambrian. From this viewpoint the term ‘Cadomian basement’ includes Neoproterozoic (Ediacaran) to Early Cambrian sedimentary, igneous and metamorphic complexes. The stratigraphic range of the involved Rock complexes changes from region to region. In some publications, the term ‘Pan-African orogeny’ is used in the same sense as Cadomian orogeny, because both events were related to the Gondwana supercontinent in the late Precambrian and occurred more or less in the same time interval. The main difference between the Cadomian and Pan-African orogenic events is their position within the configuration of the Gondwana supercontinent in Neoproterozoic time. The crustal units affected by the Pan-African orogeny are located between the cratons assembling Gondwana and reflect continent–continent collision, at least in most cases (e.g. compilation of Windley 1996). In contrast, the Cadomian orogen or, alternatively, the Avalonian-Cadomian orogenic belt is a peripheral orogen at the edge of the Gondwanan supercontinent and was characterized by orogenic processes similar to those presently observable in the Andes and the Cordilleran chains of the American continents and in the West Pacific Region (Murphy & Nance 1991; Nance & Murphy 1994; Buschmann 1995; Linnemann et al. 2000; Nance et al. 2002; Linnemann et al. 2004). Provenance studies including: (1) U/Pb-ages of detrital zircon grains from sedimentary Rocks and inherited zircons from igneous Rocks; (2) Nd–Sr– Pb isotope; and (3) palaeomagnetic and palaeobiogeographic data, suggest that the origin of the Cadomian basement of Central and Western Europe was at the periphery of the West African craton of Gondwana (e.g. Linnemann et al. 2004; Murphy et al. 2004). Remnants of the old cratonic basement derived from the cratonic source areas are represented by the Icartian basement (2.01–2.06 Ga) in the Armorican massif, and the Svetlik gneiss (2.05–2.1 Ga) and the Dobra gneiss (1.38 Ga) in the Bohemian Massif. In view of the occurrence of these Rock complexes, and many Archaean and Palaeoproterozoic detrital zircons in Neoproterozoic sedimentary Rocks, it seems that most of the Cadomian basement had developed on a stretched and thinned old cratonic crust, from which cratonic material was available for erosion and deposition into basins and also for the incorporation of large blocks from the basement into the Cadomian orogen. Nance et al. (2002) proposed a Cordilleran model for the evolution of the Neoproterozoic to Cambro-Ordovician Rock complexes in Avalonia that was a part of the ‘Avalonian-Cadomian orogenic belt’ on the periphery of the Gondwana supercontinent during the Neoproterozoic and the Early Palaeozoic. This model uses the North American plate at Baja California as a modern analogue to explain the plate-tectonic setting, including terrane accretion, subduction-related processes, ridgetrench collision and rifting. The impact of the Variscan orogeny during the closure of the Rheic Ocean in Devonian and Carboniferous times resulted in a penetrative overprinting and recrystallization of the Cadomian basement Rocks and of their structures in many areas during Pangea amalgamation. Therefore, this compilation is focused on the Saxo-Thuringian Zone at the periphery of the Bohemian Massif, which is less affected by younger orogenies. This zone contains a number of the best-preserved and most representative Ediacaran Rock complexes in the Cadomian basement of Central Europe. The Precambrian of the Bohemian Massif The Bohemian Massif is bounded to the north and the NE by the Mid-German Crystalline zone, which most likely represents the suture formed during the closure of the Rheic Ocean in the Late Devonian to Early Carboniferous period. To the south and SE the Bohemian Massif is covered by the Meso-Cenozoic orocline of the Alps and the Carpathians. Steeply dipping faults in the SW, such as the Franconian Line and the Danube Fault, divide the basement Rocks from Mesozoic platform sediments and the Alpine molasses (Fig. 1). The Bohemian Massif represents the most prominent inlier of basement Rocks in Central Europe that underwent a long geological history from Cadomian orogenic processes to the core of the Variscan Orogen in the Central European Variscides. Some of the marginal units and inliers contain very weak metamorphosed and deformed Ediacaran to Palaeozoic Rock complexes. Good examples are the western and northern parts of the Saxo-Thuringian Zone and the Tepla-Barrandian Unit (Fig. 1). The Bohemian Massif is generally interpreted as a part of Armorica (Van der Voo 1979). Tait et al. (1997, 2000) suggested that Armorica drifted across the Rheic Ocean during Ordovician to Devonian times. In contrast, the interpretation of palaeobiogeographical, geochronological, geochemical and sedimentological data led to the conclusion that Armorica never left Gondwana mainland in pre-Pangean time (Robardet 2002; Linnemann et al. 2004). The Cadomian basement of the Armorican Massif, the French Central U. LINNEMANN 36
Robert D. Hatcher - One of the best experts on this subject based on the ideXlab platform.
-
Geologic Controls of Hydrocarbon Occurrence in the Southern Appalachian Basin in Eastern Tennessee, Southwestern Virginia, Eastern Kentucky, and Southern West Virginia
2004Co-Authors: Robert D. HatcherAbstract:This report summarizes the first-year accomplishments of a three-year program to investigate the geologic controls of hydrocarbon occurrence in the southern Appalachian basin in eastern Tennessee, southwestern Virginia, eastern Kentucky, and southern West Virginia. The project: (1) employs the petroleum system approach to understand the geologic controls of hydrocarbons; (2) attempts to characterize the T-P parameters driving petroleum evolution; (3) attempts to obtain more quantitative definitions of reservoir architecture and identify new traps; (4) is working with USGS and industry partners to develop new play concepts and geophysical log standards for subsurface correlation; and (5) is geochemically characterizing the hydrocarbons (cooperatively with USGS). First-year results include: (1) meeting specific milestones (determination of thrust movement vectors, fracture analysis, and communicating results at professional meetings and through publication). All milestones were met. Movement vectors for Valley and Ridge thrusts were confirmed to be west-directed and derived from pushing by the Blue Ridge thrust sheet, and fan about the Tennessee salient. Fracture systems developed during Paleozoic, Mesozoic, and Cenozoic to Holocene compressional and extensional tectonic events, and are more intense near faults. Presentations of first-year results were made at the Tennessee Oil and Gas Association meeting (invited) in June, 2003, at a workshopmore » in August 2003 on geophysical logs in Ordovician Rocks, and at the Eastern Section AAPG meeting in September 2003. Papers on thrust tectonics and a major prospect discovered during the first year are in press in an AAPG Memoir and published in the July 28, 2003, issue of the Oil and Gas Journal. (2) collaboration with industry and USGS partners. Several Middle Ordovician black shale samples were sent to USGS for organic carbon analysis. Mississippian and Middle Ordovician Rock samples were collected by John Repetski (USGS) and RDH for conodont alteration index determination to better define regional P-T conditions. Efforts are being made to calibrate and standardize geophysical log correlation, seismic reflection data, and Ordovician lithologic signatures to better resolve subsurface stratigraphy and structure beneath the poorly explored Plateau in Tennessee and southern Kentucky. We held a successful workshop on Ordovician Rocks geophysical log correlation August 7, 2003 that was cosponsored by the Appalachian PTTC, the Kentucky and Tennessee geological surveys, the Tennessee Oil and Gas Association, and small independents. Detailed field structural and stratigraphic mapping of a transect across part of the Ordovician clastic wedge in Tennessee was begun in January 2003 to assist in 3-D reconstruction of part of the southern Appalachian basin and better assess the nature of a major potential source Rock assemblage. (3) Laying the groundwork through (1) and (2) to understand reservoir architecture, the petroleum systems, ancient fluid migration, and conduct 3-D analysis of the southern Appalachian basin.« less