Rapid Permian exhumation of the Montagne Noire dome recorded in provenance of Upper Paleozoic clastics in the Graissessac-Lodève Basin, France

M.S--University of Oklahoma, 2014.Includes bibliographical references (leaves 38-49).The Graissessac-Lodeve Basin (southern France) preserves a thick and exceptionally complete record of continental sedimentation spanning Late Carboniferous through Late Permian time. This section records the localized tectonic and paleogeographic evolution of southern France, in the context of the low-latitude Variscan Belt of Western Europe. This study presents new detrital zircon and framework mineralogy data that address the provenance of Upper Carboniferous-Upper Permian siliciclastic strata exposed in the Graissessac-Lodeve Basin. Detrital zircons in eight samples yielded significant populations that correspond with the ages of regional tectonic events, including: 500-445 Ma (widespread granitic magmatism and volcanism caused by rifting and back-arc extension along the northern Gondwanan margin), 378-331 Ma (high pressure-low temperature metamorphism and deformation during fore-arc compression, and Variscan arc-continent collision), and 330-285 Ma (magmatism, volcanism, and migmatization, chiefly in the southern Massif Central, related to postorogenic extension of the Variscan belt). The ages and compositions of units that constitute the Montagne Noire metamorphic core complex (proximal to the west of the Graissessac-Lodeve Basin) dictate detrital zircon age populations and sandstone framework mineralogy in Permian formations. Cambrian-Archean detrital zircon populations, and metamorphic lithic-rich sandstone framework compositions are derived from recycled detritus of the Neoproterozoic-Early Cambrian metasedimentary Schistes X, which formerly covered the Montagne Noire dome. Ordovician ages and subarkosic framework modes result from erosion of orthogneiss units (deformed granitoid intrusions in the lower Schistes X) that form an "envelope" on the flanks of the dome. In the lower-middle Permian units, the youngest zircon population 330-285 Ma, together with feldspar-rich compositions, reflect derivation from Late Carboniferous-Early Permian granite units in the axial zone of the Montagne Noire. Hence, these data record exhumation, and progressive unroofing of the Montagne Noire dome. The timing of core complex exhumation was previously assumed to have occurred in the Pliocene-Miocene. Our results include 330-285 Ma zircon populations, linked to sandstone compositions of polycrystalline quartz, feldspar, and metamorphic lithic fragments, which persist through the Permian section of the Lodeve Basin (Loiras-Salagou formations). Using estimates of maximum depositional ages, this requires uplift and unroofing of the Montagne Noire core (source terrane) by ca. 295 Ma. The most recent migmatization, magmatism, and deformation occurred at 298 ± 2 Ma, at -17 km depth (based on peak metamorphic conditions in the gneissic core). Accordingly, these new provenance data demonstrate that cooling and exhumation of the core was rapid (3.4-17 mm/year, within error), and early (300-295 Ma), reflecting local paleogeographic uplift in the southern Massif Central during post-orogenic extension. Subsidence analysis of the Graissessac-Lodeve Basin includes rapid initial subsidence( 312 m/My), characteristic of extensional rift basins during formation, followed by subsidence (70 m/My) that persists throughout the Permian, and is interpreted as the crustal response to the hypothesized progressive exhumation and unroofing of the Montagne Noire core complex during the Early Permian

Mesoproterozoic Deposition, Regional Metamorphism and Deformation in North-Central New Mexico: Evidence from Metamorphic Monazite and Detrital Zircon Geochronology in the Picuris Mountains

Detrital zircon and metamorphic monazite ages from the Picuris Mountains, north central New Mexico, were used to confirm the depositional age of the Marquenas Formation, to document the depositional age of the Vadito Group, and to constrain the timing of metamorphism and deformation in the region. Detrital zircon 207Pb/206Pb ages were obtained with the LA-MC-ICPMS from quartzites collected from the type locality of the Marquenas Formation exposed at Cerro de las Marquenas, and from the lower Vadito Group in the southern and eastern Picuris Mountains. The Marquenas Formation sample yields 113 concordant ages including a Mesoproterozoic age population with four grains ca. 1470 Ga, a broad Paleoproterozoic age peak at 1695 Ma, and minor Archean age populations. Data confirm recent findings of Mesoproterozoic detrital zircons reported by Jones et al. (2011), and show that the Marquenas Formation is the youngest lithostratigraphic unit in the Picuris Mountains. Paleoproterozoic and Archean detrital grains in the Marquenas Formation are likely derived from local recycled Vadito Group rocks and ca. 1.75 Ga plutonic complexes, and ca. 1.46 detrital zircons were most likely derived from exposed Mesoproterozoic plutons south of the Picuris. Ninety-five concordant grains from each of two Vadito Group quartzites yield relatively identical unimodal Paleoproterozoic age distributions, with peaks at 1713-1707 Ma. Eastern exposures of quartzite mapped as Marquenas Formation yield detrital zircon age patterns and metamorphic mineral assemblages that are nearly identical to the Vadito Group. On this basis, I tentatively assigned the easternmost quartzite to the Vadito Group. Zircon grains in all samples show low U/Th ratios, welldeveloped concentric zoning, and no evidence of metamorphic overgrowth events, consistent with an igneous origin. North-directed paleocurrent indicators, such as tangential crossbeds (Soegaard & Eriksson, 1986) and other primary sedimentary structures, are preserved in the Marquenas Formation quartzite. Together with pebble-toboulder metaconglomerates in the Marquenas, these observations suggest that this formation was deposited in a braided alluvial plain environment in response to syntectonic uplift to the south of the Picuris Mountains. Metamorphic monazite from two Vadito Group quartzite samples were analyzed with an electron microprobe (EMP). Elemental compositional variation with respect to Th and Y define core and rim domains in monazite grains, and show lower concentrations of Th (1.46-1.52 wt%) and Y (0.67 wt%) in the cores, and higher concentrations of Th (1.98 wt%) and Y (1.06 wt%) in the rims. Results show that Mesoproterozoic core and rim ages from five grains overlap within uncertainty, ranging from 1395-1469 Ma with an average age of 1444 Ma. This 1.44 Ga average age is the dominant timing of metamorphic monazite growth in the region, and represents the timing of metamorphism experienced by the region. An older 1630 Ma core observed in sample CD10-12 may be interpreted as a result of low temperature metamorphism in lower Vadito Group rocks due to heat from ca. 1.65 Ga granitic intrusions. Core ages ca. 1.5 Ga are likely due to a mixing age of two different age domains during analyses. Confirmed sedimentation at 1.48-1.45 Ga and documented mid-crustal regional metamorphism in northern New Mexico ca. 1.44-1.40 are likely associated with a Mesoproterozoic orogenic event

Loess in eastern equatorial Pangea archives a dusty atmosphere and possible upland glaciation

Carboniferous–Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (∼1.5 km) succession of exclusively fine-grained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ∼25% of the section. In contrast, data presented here indicate that the lower-middle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and τNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ∼4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation

Rock Magnetic Cyclostratigraphy of Permian Loess in Eastern Equatorial Pangea (Salagou Formation, South-Central France)

We present the findings from analysis and modeling of a stratigraphic series of magnetic susceptibility (MS) data measured with a portable MS meter from the Permian Salagou Formation loessite (south-central France). The results reveal discernible Milankovitch-scale paleoclimatic variability throughout the Salagou Formation, recording astronomically forced climate change in deep-time loessite of eastern equatorial Pangea. Optimal sedimentation rates are estimated to have ranged between 9.4 cm/kyr (lower Salagou Formation) and 13 cm/kyr (mid-upper Salagou Formation). A persistent 10-m-thick cyclicity is present that likely represents orbital eccentricity-scale (∼100 kyr) variability through the middle to late Cisuralian (ca. 285—275 Ma). Subordinate, higher frequency cycles with thicknesses of ∼3.3–3.5 and ∼1.8 m appear to represent obliquity and precession-scale variability. If the driver of magnetic enhancement is pedogenic, then the ∼10 m thick cyclicity that is consistent over ∼1000 m of section may represent the thickness of loessite–paleosol couplets in the Salagou Formation. Laboratory rock magnetic data show generally low magnetic enhancement compared to analogous Eurasian Quaternary loess deposits. This is related to the predominance of hematite (substantially weaker signal than magnetite or maghemite) in the Salagou Formation which may be explained by different conditions of formation (e.g., syn depositional processes, more arid, and/or oxidizing climate conditions) than in present Eurasia and/or post depositional oxidation of magnetite and maghemite. © Copyright © 2020 Pfeifer, Hinnov, Zeeden, Rolf, Laag and Soreghan

Ice-crystal traces imply ephemeral freezing in early Permian equatorial Pangea

Delicate impressions in lacustrine strata of the lower Permian (lower Cisuralian) Usclas Formation record ephemeral freezing in equatorial Pangea. These sediments accumulated in the paleoequatorial and intramontane Lodève Basin (southern Massif Central, France) during peak icehouse conditions of the Late Paleozoic Ice Age. Experimental replication of these features supports the interpretation that they are ice-crystal molds. Evidence for films of ice in marginal-lacustrine sediment at such low latitudes and inferred low to moderate altitudes (1–2 km) calls for a reevaluation of climate conditions in eastern equatorial Pangea. Ephemeral freezing implies either cold tropical temperatures (~5 °C cooler than the Last Glacial Maximum) and/or lapse rates that exceeded those of the Last Glacial Maximum. Extreme continentality of the Lodève Basin would have amplified seasonality, albeit the climatic forcing(s) necessary to have promoted cold temperatures in equatorial Pangea remain enigmatic

Loess in eastern equatorial Pangea archives a dusty atmosphere and possible upland glaciation

Carboniferous−Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (∼1.5 km) succession of exclusively fine-grained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ∼25% of the section. In contrast, data presented here indicate that the lower-middle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and τNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ∼4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation.This work was supported by the National Science Foundation (NSF) under International Research Ex-periences for Students (IRES) grant OISE-1658614 (principal investigators G.S. Soreghan, M.J. Soreghan) and also EAR-1338331 (Sedimentary Geology and Paleobiology Program; to G.S. Soreghan). Financial support for partially offsetting publication costs was provided from the Office of the Vice President for Research and the Office of the Provost, University of Oklahoma

Detecting upland glaciation in Earth’s pre-Pleistocene record

Earth has sustained continental glaciation several times in its past. Because continental glaciers ground to low elevations, sedimentary records of ice contact can be preserved from regions that were below base level, or subject to subsidence. In such regions, glaciated pavements, ice-contact deposits such as glacial till with striated clasts, and glaciolacustrine or glaciomarine strata with dropstones reveal clear signs of former glaciation. But assessing upland (mountain) glaciation poses particular challenges because elevated regions typically erode, and thus have extraordinarily poor preservation potential. Here we propose approaches for detecting the former presence of glaciation in the absence or near-absence of ice-contact indicators; we apply this specifically to the problem of detecting upland glaciation, and consider the implications for Earth’s climate system. Where even piedmont regions are eroded, pro- and periglacial phenomena will constitute the primary record of upland glaciation. Striations on large (pebble and larger) clasts survive only a few km of fluvial transport, but microtextures developed on quartz sand survive longer distances of transport, and record high-stress fractures consistent with glaciation. Proglacial fluvial systems can be difficult to distinguish from non-glacial systems, but a preponderance of facies signaling abundant water and sediment, such as hyperconcentrated flood flows, non-cohesive fine-grained debris flows, and/or large-scale and coarse-grained cross-stratification are consistent with proglacial conditions, especially in combination with evidence for cold temperatures, such as rip-up clasts composed of noncohesive sediment, indicating frozen conditions, and/or evidence for a predominance of physical over chemical weathering. Other indicators of freezing (periglacial) conditions include frozen-ground phenomena such as fossil ice wedges and ice crystals. Voluminous loess deposits and eolian-marine silt/mudstone characterized by silt modes, a significant proportion of primary silicate minerals, and a provenance from non-silt precursors can indicate the operation of glacial grinding, even though such deposits may be far removed from the site(s) of glaciation. Ultimately, in the absence of unambiguous ice-contact indicators, inferences of glaciation must be grounded on an array of observations that together record abundant meltwater, temperatures capable of sustaining glaciation, and glacial weathering (e.g., glacial grinding). If such arguments are viable, they can bolster the accuracy of past climate models, and guide climate modelers in assessing the types of forcings that could enable glaciation at elevation, as well as the extent to which (extensive) upland glaciation might have influenced global climate.Ye

Rock Magnetic Cyclostratigraphy of Permian Loess in Eastern Equatorial Pangea (Salagou Formation, South-Central France)

We present the findings from analysis and modeling of a stratigraphic series of magnetic susceptibility (MS) data measured with a portable MS meter from the Permian Salagou Formation loessite (south-central France). The results reveal discernible Milankovitch-scale paleoclimatic variability throughout the Salagou Formation, recording astronomically forced climate change in deep-time loessite of eastern equatorial Pangea. Optimal sedimentation rates are estimated to have ranged between 9.4 cm/kyr (lower Salagou Formation) and 13 cm/kyr (mid-upper Salagou Formation). A persistent 10-m-thick cyclicity is present that likely represents orbital eccentricity-scale (∼100 kyr) variability through the middle to late Cisuralian (ca. 285—275 Ma). Subordinate, higher frequency cycles with thicknesses of ∼3.3–3.5 and ∼1.8 m appear to represent obliquity and precession-scale variability. If the driver of magnetic enhancement is pedogenic, then the ∼10 m thick cyclicity that is consistent over ∼1000 m of section may represent the thickness of loessite–paleosol couplets in the Salagou Formation. Laboratory rock magnetic data show generally low magnetic enhancement compared to analogous Eurasian Quaternary loess deposits. This is related to the predominance of hematite (substantially weaker signal than magnetite or maghemite) in the Salagou Formation which may be explained by different conditions of formation (e.g., syn depositional processes, more arid, and/or oxidizing climate conditions) than in present Eurasia and/or post depositional oxidation of magnetite and maghemite.This work was supported by the National Science Foundation under International Research Experiences for Students (IRES) grant OISE-1658614 (PIs GS, M. J. Soreghan) and also EAR-1338331 (Sedimentary Geology and Paleobiology Program to GS).Ye

An eolian dust origin for clastic fines of Devono-Mississippian mudrocks of the greater North American midcontinent

Upper Devonian and Lower–Middle Mississippian strata of the North American midcontinent are ubiquitously fine-grained and silt-rich, comprising both so-called shale as well as argillaceous limestone (or calcareous siltstone) that accumulated in the Laurentian epeiric sea. Although long recognized as recording marine deposition, the origin and transport of the fine-grained siliciclastic material in these units remains enigmatic because they do not connect to any proximal deltaic feeder systems. Here, we present new data on grain size, whole-rock geochemistry, mineralogy, and U-Pb detrital-zircon geochronology from units across Oklahoma; we then integrate these data with models of surface wind circulation, refined paleogeographic reconstructions, and correlations from the greater midcontinent to test the hypothesis that wind transported the siliciclastic fraction to the marine system. The exclusively very fine silt to very fine sand grain size, clear detrital origin, widespread distribution over large regions of the epeiric sea, Appalachian sources, and paleogeographic setting in the subtropical arid belt far-removed from contemporaneous deltaic feeder systems are most consistent with eolian transport of dust lofted from subaerial delta plains of the greater Appalachian orogen and incorporated into subaqueous depositional systems. Delivery of dust that was minimally chemically weathered to Devono-Mississippian epeiric seas likely provided essential nutrients that stimulated organic productivity in these commonly organic-rich units