Chronostratigraphy and Paleoclimatology of the Lodeve Basin, France: Evidence for a Pan-Tropical...
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Chronostratigraphy and Paleoclimatology of the Lodève Basin, France: Evidence for a pan-tropical aridification event across the Carboniferous–Permian boundary Lauren A. Michel a, ⁎, Neil J. Tabor b , Isabel P. Montañez c , Mark D. Schmitz d , Vladimir I. Davydov d,e a Perot Museum of Nature and Science, 2201 N. Field Street, Dallas, TX 75201-1704, USA b Roy M. Huffington Department of Earth Sciences, Southern Methodist University, PO Box 750395, Dallas, TX 75275-0395, USA c Department of Earth and Planetary Sciences, University of California, Davis, CA 95616, USA d Department of Geosciences, Boise State University, Isotope Geology Laboratory, Boise, ID 83725, USA e Kazan Federal University, 18 Kremlyovskaya St., Kazan 420008, Republic of Tatarstan, Russian Federation abstract article info Article history: Received 16 November 2014 Received in revised form 5 March 2015 Accepted 11 March 2015 Available online 10 April 2015 Keywords: Paleoclimate Carboniferous Permian Paleosols Lodève Basin Paleosols preserved within the Carboniferous–Permian succession of the Lodève Basin, Massif Central, France change stratigraphically from Histosols to calcic Vertisols and Calcisols to gypsic Vertisols and ultimately back to calcic Vertisols and Calcisols. New high-precision U–Pb zircon ages (CA-IDTIMS) for tuff beds within the Lodève and adjacent Graissessac basins significantly revise the chronostratigraphy of these and correlated Perm- ian terrestrial basins of eastern Euramerica. Under the newly revised chronostratigraphy presented here these stratigraphic changes in morphology indicate a substantial drying of paleoenvironments across the Carbonifer- ous–Permian boundary with a trend toward progressively more arid and seasonal climates through most of the early Permian. This newly-realized chronology provides a paleoenvironmental and paleoclimatic timeline for eastern tropical Pangea that is contemporaneous with similar records in western Pangea and suggests pan- tropical, progressive climate change toward aridity and seasonality occurred from the Late Carboniferous through early Permian. © 2015 Elsevier B.V. All rights reserved. 1. Introduction There is a first-order correlation between atmospheric pCO 2 concen- trations and global climate throughout the past half billion years (Royer et al., 2004). As present-day CO 2 increases there is a need to assess the ef- fects of climate change in a warming world. Because the Carboniferous– Permian is the last icehouse to greenhouse transition on a vegetated and metazoan-populated Earth, and this time period corresponds to an increase in atmospheric CO 2 concentrations (Montañez et al., 2007), there has been a focus on reconstructing global terrestrial paleoclimate across this transition interval (e.g., Cecil, 1990; Cecil, 2003; Tabor et al., 2004; DiMichele et al., 2006; Montañez et al., 2007; Peyser and Poulsen, 2008; Tabor and Poulsen, 2008; Tabor et al., 2013; DiMichele, 2014). While Carboniferous–Permian terrestrial paleoclimate reconstructions based on paleosol records have emerged from the western tropics (Tabor and Montañez, 2004; Rosenau et al., 2013a,b) and high latitudes (Beauchamp, 1994; Gulbranson et al., 2010; Limarino et al., 2014; Gulbranson et al., 2015), only recently have proxy records of this nature been constructed for central and eastern tropical regions of Pangaea (Schneider et al., 2006; Thomas et al., 2011; Eros et al., 2012). Existing paleoclimate reconstructions from western tropical Pangea indicate the onset of seasonality toward the close of the Carboniferous and a clear aridification trend through the early Permian (Tabor and Montañez, 2002; Tabor et al., 2008; Bishop et al., 2010). While, there are hints of this aridity trend seen in eastern Ukraine during the Carboniferous–Perm- ian boundary (Eros et al, 2012), the long-term trend of this aridity through the Permian is unknown. At this time it remains unclear if this paleoclimate trend is limited to the western tropics or if it extends throughout the tropics. The Lodève Basin in the south of France was cho- sen as an ideal location to extend Carboniferous–Permian climate trends from the western, to the central, Pangean landmass because of previously developed stratigraphic and paleoecological frameworks and the pres- ence of multiple ashes throughout the succession (Körner et al., 2003; Schneider et al., 2006; Pochat and Van Den Driessche, 2011). Presented herein are new chronostratigraphic constraints and paleosol data for the Carboniferous–Permian formations of the Lodève and adja- cent Graissessac basins, southern France. The new chronostratigraphy places the majority of the basin infill within the Ghzelian to upper Sakmarian. Changes in paleosol micro- and macromorphology indicate a change from humid everwet climates in the latest Carboniferous to sea- sonal and semiarid climates, with a progressive trend toward aridity, through the early Permian. Therefore, this study documents a similar and contemporaneous climate trend in central Pangea as seen in previous Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131 ⁎ Corresponding author. Tel.: +1 214 756 5786; fax: +1 214 756 5890. E-mail address: [email protected] (L.A. Michel). http://dx.doi.org/10.1016/j.palaeo.2015.03.020 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo
Transcript of Chronostratigraphy and Paleoclimatology of the Lodeve Basin, France: Evidence for a Pan-Tropical...
Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo
Chronostratigraphy and Paleoclimatology of the Lodève Basin,France: Evidence for a pan-tropical aridification event acrossthe Carboniferous–Permian boundary
Lauren A. Michel a,⁎, Neil J. Tabor b, Isabel P. Montañez c, Mark D. Schmitz d, Vladimir I. Davydov d,e
a Perot Museum of Nature and Science, 2201 N. Field Street, Dallas, TX 75201-1704, USAb Roy M. Huffington Department of Earth Sciences, Southern Methodist University, PO Box 750395, Dallas, TX 75275-0395, USAc Department of Earth and Planetary Sciences, University of California, Davis, CA 95616, USAd Department of Geosciences, Boise State University, Isotope Geology Laboratory, Boise, ID 83725, USAe Kazan Federal University, 18 Kremlyovskaya St., Kazan 420008, Republic of Tatarstan, Russian Federation
⁎ Corresponding author. Tel.: +1 214 756 5786; fax: +E-mail address: [email protected] (L.A
http://dx.doi.org/10.1016/j.palaeo.2015.03.0200031-0182/© 2015 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f oArticle history:Received 16 November 2014Received in revised form 5 March 2015Accepted 11 March 2015Available online 10 April 2015
Keywords:PaleoclimateCarboniferousPermianPaleosolsLodève Basin
Paleosols preserved within the Carboniferous–Permian succession of the Lodève Basin, Massif Central, Francechange stratigraphically from Histosols to calcic Vertisols and Calcisols to gypsic Vertisols and ultimately backto calcic Vertisols and Calcisols. New high-precision U–Pb zircon ages (CA-IDTIMS) for tuff beds within theLodève and adjacent Graissessac basins significantly revise the chronostratigraphy of these and correlated Perm-ian terrestrial basins of eastern Euramerica. Under the newly revised chronostratigraphy presented here thesestratigraphic changes in morphology indicate a substantial drying of paleoenvironments across the Carbonifer-ous–Permian boundary with a trend toward progressively more arid and seasonal climates through most ofthe early Permian. This newly-realized chronology provides a paleoenvironmental and paleoclimatic timelinefor eastern tropical Pangea that is contemporaneous with similar records in western Pangea and suggests pan-tropical, progressive climate change toward aridity and seasonality occurred from the Late Carboniferous throughearly Permian.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
There is a first-order correlation between atmospheric pCO2 concen-trations and global climate throughout the past half billion years (Royeret al., 2004). As present-day CO2 increases there is a need to assess the ef-fects of climate change in a warming world. Because the Carboniferous–Permian is the last icehouse to greenhouse transition on a vegetatedand metazoan-populated Earth, and this time period corresponds to anincrease in atmospheric CO2 concentrations (Montañez et al., 2007),there has been a focus on reconstructing global terrestrial paleoclimateacross this transition interval (e.g., Cecil, 1990; Cecil, 2003; Tabor et al.,2004; DiMichele et al., 2006; Montañez et al., 2007; Peyser and Poulsen,2008; Tabor and Poulsen, 2008; Tabor et al., 2013; DiMichele, 2014).While Carboniferous–Permian terrestrial paleoclimate reconstructionsbased on paleosol records have emerged from the western tropics(Tabor and Montañez, 2004; Rosenau et al., 2013a,b) and high latitudes(Beauchamp, 1994; Gulbranson et al., 2010; Limarino et al., 2014;Gulbranson et al., 2015), only recently have proxy records of this naturebeen constructed for central and eastern tropical regions of Pangaea(Schneider et al., 2006; Thomas et al., 2011; Eros et al., 2012). Existing
1 214 756 5890.. Michel).
paleoclimate reconstructions from western tropical Pangea indicate theonset of seasonality toward the close of the Carboniferous and a cleararidification trend through the early Permian (Tabor and Montañez,2002; Tabor et al., 2008; Bishop et al., 2010). While, there are hints ofthis aridity trend seen in easternUkraine during the Carboniferous–Perm-ian boundary (Eros et al, 2012), the long-term trend of this ariditythrough the Permian is unknown. At this time it remains unclear if thispaleoclimate trend is limited to the western tropics or if it extendsthroughout the tropics. The Lodève Basin in the south of France was cho-sen as an ideal location to extend Carboniferous–Permian climate trendsfrom thewestern, to the central, Pangean landmass because of previouslydeveloped stratigraphic and paleoecological frameworks and the pres-ence of multiple ashes throughout the succession (Körner et al., 2003;Schneider et al., 2006; Pochat and Van Den Driessche, 2011).
Presentedherein arenewchronostratigraphic constraints andpaleosoldata for the Carboniferous–Permian formations of the Lodève and adja-cent Graissessac basins, southern France. The new chronostratigraphyplaces the majority of the basin infill within the Ghzelian to upperSakmarian. Changes in paleosol micro- and macromorphology indicate achange from humid everwet climates in the latest Carboniferous to sea-sonal and semiarid climates, with a progressive trend toward aridity,through the early Permian. Therefore, this study documents a similarand contemporaneous climate trend in central Pangea as seen in previous
119L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
climate reconstructions from western tropical Pangea, supporting apan-tropical aridification during this interval of time, which may haveoccurred in response to rising levels of atmospheric CO2 (e.g., Montañezet al., 2007).
2. Background
2.1. Geological setting
The Lodève Basin is situated to theNWofMontpellier, France, on thesoutheastern edge of the FrenchMassif Central (Fig. 1). The basin coversa total area of approximately 150 km2 (Fig. 1), and lies upon Precambri-an andCambrian rocks composed of schists, arkosic andquartzose sand-stones, limestones and volcaniclastics (Conrad and Odin, 1984). Fourbounding-faults delimit the Lodève Basin and define its characteristichalf-graben shape. In particular, vertical motions along the southern,E–W trending Les Aires fault were responsible for creation of accommo-dation and sediment accumulation during Permian time (Conrad andOdin, 1984). During the Pennsylvanian and Permian the Lodève Basinoccupied a low latitude (0 to 10°N) (Fig. 1B) position on the Pangeanlandmass approximately 400 km inland from the Tethys ocean(Schneider et al., 2006). During this time, theHercynianMountains, of un-known topography (Ziegler et al., 1997) separated the Lodève Basin fromTethys. Paleomagnetic studies of the Lodève Basin strata indicate that itwas a site of paleogeographic stability throughout the Permian, whereassurrounding contemporaneous basins underwent a component of rota-tion (Cogné et al., 1990; Diego-Orozco and Henry, 1993; Chen et al.,1997; Henry et al., 1999).
Lodève Basin fill consists of ~3000 m of Carboniferous–Permiansiliciclastics previously thought to range from Ghzelian throughChanghsingian (Zechstein) age, with unconformities that separate(1) the lower Asselian from upper Asselian strata and (2) the middleSakmarian from upper Artinskian strata (upper Lower Rotliegend to thelower Upper Rotliegend I; Schneider et al., 2006). Only Permian strataoutcrop in the Lodève Basin. However, Carboniferous outcrops occur inthe adjacent Graissessac Basin to the west of the Lodève Basin (Bruguier
Lake Salagou
Olm
et F
ault
Les Aires Faul
La Tour/Orb Fault
Orb
Fau
lt
2840 m
N
A C
3˚ 1
9’ 1
3” E
43˚ 43’ 57” Lodéve
Fig. 1.Graissessac and Lodève basins: A) Locationmap showing position of the Graissessac and LodGraissessac and Lodève basins; C) geologicmap of the Lodève Basin, with the locations of samplingmajor basin-bounding faults. The Saxonien formations include (in ascending order) the Rabajac F(For interpretation of the references to color in this figure legend, the reader is referred to the weC, modified from Schneider et al. (2006).
et al., 2003). These western outcrops include Gzhelian (StephanianC) through Lower Asselian (early Lower Rotliegend) strata.
Prior radioisotope age constraints for the Lodève and correlativePermian basins of southern France are sparse, and limited to relativelyimprecise SIMS U–Pb zircon ages, SHRIMP U–Pb zircon ages or40Ar/39Ar sanidine and biotite ages. SIMS 206Pb/238U ages on zirconsfrom five volcanic ash layers located in the southern French MassifCentral basins, including a tonstein from the Graissessac, record an av-erage regional magmatic activity from 295.5 ± 5.1 to 297.9 ± 2.1 Maand constrain the age of the Graissessac Basin to the Late Carboniferous(295.3 ± 4.8 Ma) to early Permian (Autunian to Sakmarian) (Bruguieret al., 2003). Schneider et al. (2006) developed a biostratigraphy foryounger Permian strata in the Lodève Basin to constrain the timing ofdeposition given the lack of chronostratigraphic constraints (Fig. 2).Schneider et al. (2006) also referenced unpublished U–Pb zircon agesfor the Viala Formation (289.3 ± 6.7 Ma) and Octon Member of theSalagou Formation (284 ± 4 Ma) in the Lodève Basin, but provide nodata or analytical details. Breitkreuz and Kennedy (1999) and Königeret al. (2002) presented SHRIMP U–Pb zircon ages for volcanics withinCarboniferous–Permian transition sediments of several German basins,ranging from 302 to 297 (±3) Ma. Several 40Ar/39Ar dates in the rangeof 302 to 291 Ma from Carboniferous–Permian basins of east-centralEurope have been published (Hess and Lippolt, 1986; Burger et al.,1997; Goll and Lippold, 2001); however, the accuracy of these dateshas been questioned by direct comparison to high-precision U–Pb zirconages in recent studies (Gastaldo et al., 2009; Davydov et al., 2010).
Carboniferous–Permian strata of the Lodève Basin underwent exten-sive alteration in response to deep burial and higher-than average geo-thermal heat flow in the region. Copard et al. (2000) report Tmax burialtemperatures between 600 and 610 °C for thewestern Graissessac Basinbased on vitrinite reflectance data of coals and suggested that Carbonif-erous–Permian heat flow was 180 mW/m2, a value that is 4.5 times theaverage heat flow value in typical stable cratonic basins (Blackwell,1971; Condie, 1997). Copard et al. (2000) interpreted such high heatflow values, and thus the high-temperature burial history, in theGraissessac Basin to a regional high-temperature event associatedwith the end-Permian Saalic Orogeny. In addition, Buatier et al. (2000)
Legend
fault
Cev
anne
s Fa
ult
t
study sites
Quaternary sediments
Quaternary basalts
Saxonian fms (La Lieude, Salagou, and Rabejac)
Viala Fm.
Tuilieres-Loiras Fm.
Usclas-St. Privat Fm.
B
Lodeve
Graiss.
ève basins (star) within the FrenchMassif Central (outlined in black); B) outcrop area of thesites (red stars) studied herein. Solid black lines represent faults with bold lines representingormation, Octon and Merifons Members of the Salagou Formation, and La Lieude Formation.b version of this article.)
Sal
agou
Triassic250
255
260
La Lieude
Car
boni
fero
us
Cha
nghs
ing.
Lopi
ngia
n
Wuc
hapi
ngia
n
Merifons Mbr.
Cap
itani
an
Gua
dalu
pian
Wor
dian
Roa
dian
265
Per
mia
n270
Octon Mbr.
Kun
guria
n
Rabejac
Art
insk
ian
VialaTuileres-LoirasS
akm
aria
nCis
ural
ian
Usclas St. Privat
Graiss.
Ass
elia
nG
zhel
ian
Kas
imov
ian
Pen
nsyl
vani
an
275
280
285
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295
300
305
GT
S 2
004
(Ma)
GT
S 2012 (M
a)
Triassic250
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260
Carboniferous
Chang.
Lopingian
Wuchapingian
Capitanian
Guadalupian
Wordian
Roadian
265
Perm
ian
270
Kungurian
Artinskian
Sakm
arian
Cisuralian
Asselian
Gzhelian
Kasim.
Pennsylvanian
275
280
285
290
295
300
305
Sal
agou
La Lieude
Merifons Mbr.
Octon Mbr.
Rabejac
Viala
Tuileres-LoirasUsclas St. Privat
Graiss.
Schneideret al. (2006) this study
Fig. 2. Generalized stratigraphic column for the Graissessac and Lodève basins.Chronostratigraphy and age scale on left is based on biostratigraphic correlation using fossilcockroach wings collected in the Lodève Basin (Schneider et al., 2006) compiled with thetime scale calibration of GTS 2004 (Davydov et al., 2004; Wardlaw et al., 2004). A revisedchronostratigraphy, on the right of the diagram, is based upon ID-TIMS high-precisionU–Pb ages presented here, tied to the GTS 2012 time-scale (Davydov et al., 2012; Hender-son et al., 2012). Red lines represent locations of ashes sampled in this study. (For inter-pretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)Adapted from Schneider et al. (2006) and references therein.
120 L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
identified nacrite, a high-temperature polymorph of kaolinite (Hansonet al., 1981; Shen et al., 1994), as a burial diageneticmineral that formedwithin dolomitic strata in the basement rocks of the Lodève Basin. In thebasin, the nacrite appears to form contemporaneously with hydrother-mal barite. Fluid inclusion data from these barites indicates theseminerals formed in the presence of a high-salinity brine (25 wt.%NaCl) at temperatures ranging from 80 to 100 °C (Buatier et al., 2000).
2.2. Lithostratigraphy
The lithostratigraphy of the Lodève Basin may be divided into threeascending units. Unit I consists of coal-bearing strata of the GraissessacFormation (Fig. 2). Unit II consists of fluvio-alluvial siliciclastics of theUsclas-St. Privat, Tuilieres-Loiras, and Viala formations; Unit III includesthe Rabejac, Salagou and La Lieude formations (Figs. 2, 3). The RabejacFormation is composedoffluvio-alluvial siliciclastics,whereas, the overly-ing Salagou Formation includes mixed playa siliciclastics, evaporites andzeolites (Schneider et al., 2006). The overlying La Lieude Formation iscomposed of siliciclastics deposited within fluvio-alluvial environments(Schneider et al., 2006).
2.3. Previous paleoclimate studies
Previous studies of thewhole rock chemical composition of the Perm-ian strata within the Lodève Basin were used as indicators of climate-sensitive minerals such as analcime and albite (Schneider et al., 2006).Stratigraphic variations in whole rock chemistry indicate an increase inanalcime and albite in the Octon Member of the Salagou Formation thatwas interpreted to represent a strongly evaporitic environment in asemi-arid climate (Schneider et al., 2006). Schneider et al. (2006) andQuast et al. (2006) further used the stable carbon and oxygen isotopiccompositions of carbonate cements in sandstones and calcretes in Perm-ian strata of the LodèveBasin as potential paleoclimate proxies. Both stud-ies concluded that the carbonates were diagenetically overprinted andnot appropriate for paleoenvironmental reconstruction.
Pochat and Van Den Driessche (2011) built upon previous studies inother continental rift basins (Lefournier, 1980; Lambiase, 1990; Schlische,1991; Prosser, 1993; Harris et al., 2004; Pochat et al., 2005) to develop amodel of lacustrine sedimentation in the Lodève Basin. Their model hy-pothesizes that lakewater volumes did not change substantially through-out the Carboniferous–Permian, but that lake surface areas did change.Pochat and Van den Driessche (2011) attributed the changes in the com-position of sedimentaryfill of the Lodève Basin to changes in tectonic styleassociated with typical rift basin development as opposed to changes inCarboniferous–Permian paleoclimate.
3. Methods
3.1. Field methods
Fieldwork included identification and description of paleosols usingmodern soil description techniques (Schoeneberger et al., 2012) asadvocated for paleosols (Retallack, 1988), and placement of thesepaleosols into an existing stratigraphic framework (Körner et al., 2003;Schneider et al., 2006). Outcrops were trenched back to remove surficialweathering and sampleswere collected for petrographic analysis. Layerspreserving evidence for paleopedogenesis were logged and described indetail according to previously defined methods (Tabor and Montañez,2004; Tabor et al., 2006). Paleosol profile tops were identified on thebasis of a marked change in grain size and color, as well as preservationof primary sedimentary structures. Profile bases were delineated at thelowest occurrence of unaltered parent material. Paleosol matrix wassampled at 0.1–0.2 m vertical spacings; rhizoliths and paleosol noduleswere sampled where present. Paleosol classification primarily followsthe system of Mack et al. (1993). Paleosol types, as well as the namesand stratigraphic positions of collected samples, are provided in Fig. 3.Micromorphological analysis of thin sections (n = 46) followed themethods developed for modern soils (Bullock et al., 1985; Stoops,2003; Stoops et al., 2010).
3.2. Radiometric dating
Volcanic strata (tuffs, tonsteins, and cinerites) were identified andsampled in the field utilizing the stratigraphy summarized by Schneideret al. (2006) and references therein. Abundant populations of prismaticzircon crystals were separated from hand samples of each volcanic hori-zon by conventional density and magnetic methods. Methods for U–Pbgeochronology using chemical abrasion isotope dilution thermal ioniza-tion mass spectrometry (CA-IDTIMS) follow those previously publishedby Davydov et al. (2010) and Schmitz and Davydov (2012). A fraction ofthe zircon separate from each sample, which was selected on the basisof sharply facetted morphology, clarity and lack of inclusions, was placedin quartz beakers in a muffle furnace at 900 °C for 60 h to anneal minorradiation damage; in preparation for subsequent chemical abrasion(Mattinson, 2005). Single zircon crystals were individually subjected toa modified single step, high-T (12 h at 180 °C) version of the chemicalabrasion method of Mattinson (2005). U–Pb dates and uncertainties for
Lithologym
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-P
riva
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Paleosol Lithology Paleosol
VG
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C
CV
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T1T2T3
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Legend CV
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VG
Pedogenic Carbonate Nodules
Slickensides
Calcic Vertisol
Calcisol
Vertic GypsisolAsh
Rab
ajac
Oct
on M
embe
rLa
Lie
ude
Fig. 3.Detailed stratigraphic column of the Permian siliclastic succession that crop out in the Lodève Basin. Roman numerals and T#s are tuff horizons described in previous studies (Gandet al., 1997; Körner et al., 2005). The stratigraphic occurrence of paleosolmorphologies aremarked in the right columnand correspond to themorphologiespresented in Fig. 5: C=Calcisols,CV = calcic Vertisols, and VG= vertic Gypsisols. Yellow tuffs represent ashes sampled and data presented herein. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)Adapted from Schneider et al. (2006).
121L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
each analysis were calculated using the algorithms of Schmitz andSchoene (2007) and the U decay constants of Jaffey et al. (1971). Uncer-tainties are based upon non-systematic analytical errors, includingcounting statistics, instrumental fractionation, tracer subtraction, and
blank subtraction. These error estimates should be considered whencomparing our 206Pb/238U dates with those from other laboratoriesthat used tracer solutions calibrated against the EARTHTIME gravimetricstandards. When comparing our dates with those derived from other
122 L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
decay schemes (e.g., 40Ar/39Ar, 187Re–187Os), the uncertainties in tracercalibration (0.05%; Condon et al., 2007) and U decay constants(0.108%; Jaffey et al., 1971) should be added to the internal errorin quadrature. Quoted errors for calculated weighted means arethus of the form ±X(Y)[Z], where X is solely analytical uncertainty, Yis the combined analytical and tracer uncertainty, and Z is the combinedanalytical, tracer and 238U decay constant uncertainty.
4. Results
4.1. Field
Paleosols in the Lodève Basin exhibit variations in characteristicssuch as texture, color, structure, presence and concentration of soil-forming minerals, and accumulations of organic matter. Based uponfield observations of these characteristics, there are five different typesof paleosols in the Permian–Carboniferous strata of the LodèveBasin: Histosols, Calcisols, calcic Vertisols, gypsic Vertisols and Protosols(Figs. 4, 5, 6A–F). Protosols found in the basin are not useful forpaleoclimate reconstruction and are not discussed further herein.
Histosols (n = 7) are found only in the Stephanian GraissessacFormation. Histosols occur as seven discrete coal seams, whichrange in thickness from 1.2 to 5.2 m thick (Figs. 4, 5A). Each seamis stratigraphically separated by intercalated claystones, siltstone andvolcanic ash layers. The mineral Jarosite occurs through all strata.
Calcisols (n = 2) occur in the Tuilieres-Loiras and La Lieude forma-tions (Figs. 3, 4). Calcisols of the Lodève Basin are identified based onthe presence of stage II through stage IV pedogenic carbonate accumu-lation (Machette, 1985;Mack et al., 1993). Stage II carbonate accumula-tion includesmm- to cm-sized carbonate nodules and tubules, whereasStage III and Stage IV carbonate accumulation includes continuous later-al horizons of carbonate cementation (Machette, 1985). These soilspreserve original sedimentary structure to massive structure and areoften truncated by overlying cross-bedded sandstones. Soilswith ground-water carbonate nodules were identified based on the presence ofinclusive growth of carbonate cements around siliclastic grains and
O
ABk
Bkss1
Bkss2
BCss
C
O C S F M C
Sand
O C S F M C
Sand
Oaj
C
HistosolCalcic
Vertisol C
Legend
Jarosite
Pedogenic carbonate nodule
Slickensides
Indurated Carbonate Horizon
Pedogenic Gypsum
SOIL STRUCTURE SED
Fig. 4. Pedotypes found within the strata of the Lodève Basin. Orange dendritic texture marks thecarbonate nodules, while Ts are indurated carbonate horizons. The thin fenestral black lines denotereferences to color in this figure legend, the reader is referred to the web version of this article.)
laminations from suspension settling of grains (Fig. 5B); these carbonatenodules were excluded from the study.
Calcic Vertisols (n= 4) occur in the Viala Formation, OctonMemberof the Salagou Formation, and in the La Lieude Formation (Figs. 3, 4, 5C,D). These paleosols are identified based on the presence of prominentfeatures resulting from shrink-swell processes as well as pedogenicaccumulation of subsurface calcium carbonate. Paleosols in the LodèveBasin contain features indicative of shrink-swell processes includingslickensides (Fig. 5C), clastic dikes (Fig. 5D), and wedge-shaped aggre-gates, as well as carbonate nodules and tubules. These paleosols arebroken into multiple horizons. The lower-most horizons are often mas-sive to platy, dusky red (5YR 5/4–2.5YR 3/4) mudstones that in somelocations are calcite cemented. The upper boundary varies fromabrupt and wavy to clear and smooth. Overlying the lower horizonsare between 1 to 3 horizons containing common stage II pedogeniccarbonate nodules that usually increase in abundance down profile.These horizons are dusky red (2.5YR 3/4–10YR 4/4), silty mudstones tomudstones with angular blocky structure and slickensides. The upperboundary is typically abrupt and wavy and possesses clear mukkarasubsurface expression. Where not truncated, the overlying horizonsare weak red (7.5R 4/3) silty mudstones to mudstones. Some horizonsaremassivewhile others have angular blocky structure and slickensidesand desiccation cracks infilled by sandstone that extend up to 80 cmbelow the paleosol surface. These soils are often 1 to 2 m thick and aretypically truncated by cross-bedded sandstones.
Vertic Gypsisols (n= 2) are paleosol profiles that occur in the upperMerifonsMember of the Salagou Formation and lower La Lieude forma-tions (Figs. 3, 4, 5E, F). They are composed of two horizons; the lowerhorizon is mostly massive red mudrock (7.5R 3/3) with the lower~5 cm defined by thin platy structure and satin-spar calcite cements.This satin-spar texture is not typically attributed to calcite cement, butis commonly found in gypsum (Collins, 1984; Testa and Lugli, 2000).Therefore, we interpret this texture as being originally gypsum in thesoil-forming environment. The upper boundary between horizons isabrupt andwavy to broken due to the presence of vertically oriented clas-tic dykes. Overlying thehorizon is a 2–3 cm thick greenish-gray (G16/5G)
Bk1
Bk2
C
C S F M
Bw
CBy
C
O C S F M C
Sand
C
Sand
alcisol
1 met er
VerticGypsisol
Bkm
Cross-bedding
Laminations
IMENTARY STRUCTURE
occurrence of jarosite. Arcuate lines represent the occurrence of slickensides. White ovals arethe prior occurrence of pedogenic gypsum (now satin spar calcite). (For interpretation of the
Fig. 5. Examples of pedogenic features seen in the field. (A) Intercalated coal seams, ashes, and siliciclastics characteristic of the Graissessac Formation. (B) Groundwater carbonate nodule.(C) Pedogenic carbonate nodules and slickensides in a vertic Calcisol. Scale is ~2m frombase of section to outcrop. (D) Clastic dikes infilled by sandstone in a vertic Calcisol. (E) Close up ofthe top of a vertic Gypsisol. Note desiccation cracks on top of paleosol. (F) Stacked vertic Gypsisols of the Merifons Member, Salagou Formation Green layers are the surface.
123L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
laminated coarse silt tofine sand (Fig. 5E, F) The upper boundary is abruptand smooth, and is commonly overlain by another vertic Gypsisol in theupper Salagou (MerifonsMember). Themost prominent feature is the ac-cumulation of original pedogenic gypsum, but also exhibit vertic featuressuch as clastic dikes that extend downward from the interpretedpaleosurface (Mack et al., 1993). They also have weakly developed soilstructure. The paleosol profiles are intercalated with laminated coarsesilt to fine sandstones (Fig. 5F).
4.2. Micromorphology
Micromorphology of Calisols, calcic Vertisols, and vertic Gypsisolsreveals evidence of biological processes including orabated mite fecalmaterial (Fig. 6A), bacterial films, primary boxwork fabric originatingfrom bacterial processes (Von Der Borch et al., 1977), which now havebeen altered to microspar and dolomite (Fig. 6B) and rooting (Fig. 6C).Calcic Vertisols also show clotted micrite textures (Fig. 6D) and wedge-shaped aggregates with Fe-oxide-stripping and concentrations (Fig. 6F).Additionally, the calcic Vertisols preserve hematite after primary clayfabric while the vertic Gypsisols have both hematite coatings after prima-ry clay as well as illuvated clay coatings along roots (Fig. 6E). The Vertic
Gypsisols have been diagenetically altered and the gypsum has beenreplaced by calcite spar cement (Fig. 6E).
4.3. U–Pb geochronology
4.3.1. Graissessac FormationEleven single zircon grains were analyzed from a tonstein collected
from coal bed 4 (Monte Sénégra 3; Fig. 7). Four anomalously older oryounger zircons are interpreted as either bearing an inherited core com-ponent, or containing residual Pb-loss domains. Seven other grainsyielded a cluster of equivalent U–Pb isotope ratios, with a calculatedweighted mean 206Pb/238U date of 304.07 ± 0.07(0.17)[0.36] Ma(MSWD= 0.71), which is interpreted as the eruption and depositionalage of the tonstein.
Nine single zircon grains were analyzed from a tonstein collectedfrom coal bed 5 (Monte Sénégre 2; Fig. 7), which was approximately20 m above Monte Sénégra 3. One slightly older discordant analysis isinterpreted as containing an inherited core, while two anomalouslyyounger grains may have residual Pb-loss. Six single grains yielded acluster of dates that are equivalent with a weighted mean 206Pb/238Udate of 303.95 ± 0.08(0.17)[0.36] Ma (MSWD = 1.4). This date isinterpreted as the eruption and depositional age of the tonstein.
A B
C D
E F
2mm 2mm
2mm2mm
2mm 2mm
Fig. 6. Examples of pedogenic features in thin section. All imageswere taken under 1.25×magnification. (A)Orabatedmite fecalmaterial. (B) Boxwork fabricwas originallymicritic calciteand is now altered to microspar and dolomite. (C) Brown areas are roots seen both in axial and longitudinal view. Note the longitudinal root has been dissected probably as a result ofshrink–swell processes. Also seen is a root halo (white area encompassing part of the root). (D) Clotted micrite texture of a rhizolith encircling Fe–Mn infilled root chamber.(E) Hematite replacing primary clay. (F) Wedge-shaped peds.
124 L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
While slightly younger than the underlying tonstein from coal bed 4,the two tonstein ages are within analytical uncertainty thus establish-ing rapid depositional rates through this interval of the GraissessacFormation.
4.3.2. Tuilières–Loiras Formation (Loiras Member)Eleven single zircon grains were analyzed from tuff bed V, which is
in the upper portion of the middle Loiras Member (Figs. 3, 7). Fiveolder analyzed grains are interpreted as reworked crystals from previ-ous eruptions or containing inherited core components. Six single grainsyielded a cluster of youngest dates that are equivalent with a weightedmean 206Pb/238Udate of 293.94±0.08(0.16)[0.35]Ma (MSWD=0.81).This date is interpreted as the eruption and depositional age of the tuff.
Thirteen single grains from the overlying tuff bedVI (Figs. 3, 7) yieldeda cluster of ten concordant and equivalent zircon grains, with a weightedmean 206Pb/238U date of 293.85 ± 0.10(0.17)[0.36] Ma (MSWD= 1.49).This date is interpreted as the eruption and depositional age of the tuff.
Three variably younger analyzed grains are interpreted as containingresidual Pb-loss domains.
4.3.3. Viala FormationTwenty single zircon grainswere analyzed froma sample taken from
tuff bed III, which is approximately 60 m above the base of the VialaFormation (Fig. 3). Ten of those analyses spread along the concordiacurve with ages from 292.2 to 294.4 Ma and are interpreted as contain-ing variable inherited components. Three anomalously younger grainsmay have residual Pb-loss. The remaining seven single grains yieldeda cluster of dates that are equivalent with a weighted mean 206Pb/238Udate of 290.96 ± 0.19(0.24)[0.39] Ma (MSWD = 4.28). This date isinterpreted as the eruption and depositional age of the tuff bed.
4.3.4. Salagou Formation (Octon Member)Zircons were analyzed from four tuff beds spanning over 650 m of
the lower two-thirds of the Octon Member of the Salagou Formation
306
305
304
303
302
301
0.0477
0.0479
0.0481
0.0483
0.0485
0.0487
0.345 0.347 0.349 0.351 0.353
0404044
206 P
b/23
8 U
207Pb/235U
Monte Senigre 2303.95 ± 0.08 Ma
n = 6; MSWD = 0.80
306
305
304
303
302
0.0477
0.0479
0.0481
0.0483
0.0485
0.0487
0.345 0.347 0.349 0.351 0.353
040404304304044444
206 P
b/23
8 U
207Pb/235U
Monte Senigre 3304.07 ± 0.07 Ma
n = 7; MSWD = 0.71
296
294
292
290
0.0456
0.0460
0.0464
0.0468
0.0472
0.330 0.334 0.338 0.342
949442942944444444444444444444
206 P
b/23
8 U
207Pb/235U
V293.94 ± 0.08 Ma
n = 6; MSWD = 0.81
296
294
292
0.0456
0.0460
0.0464
0.0468
0.0472
0.330 0.334 0.338 0.342
44444
206 P
b/23
8 U
207Pb/235U
VI293.85 ± 0.10 Ma
n = 10; MSWD = 1.49
296
294
292
290
288
0.0456
0.0460
0.0464
0.0468
0.0472
0.326 0.330 0.334 0.338
206 P
b/23
8 U
207Pb/235U
III290.96 ± 0.19 Ma
n = 8; MSWD = 2.78
285
284
283
0.0446
0.0448
0.0450
0.0452
0.0454
0.320 0.322 0.324
3833
285
284
283
282
0.0446
0.0448
0.0450
0.0452
0.0454
0.320 0.322 0.324
838333883838383
286
285
284
0.0446
0.0448
0.0450
0.0452
0.0454
0.322 0.324 0.326
2828888484848484884888848844444444444444444444444444444444444444444444444
206 P
b/23
8 U
207Pb/235U
T12282.86 ± 0.13 Ma
n = 5; MSWD = 1.22206 P
b/23
8 U
207Pb/235U
T9283.53 ± 0.10 Ma
n = 7; MSWD = 1.15206 P
b/23
8 U
207Pb/235U
T1284.40 ± 0.13 Ma
n = 8; MSWD = 0.79
286
285
284
0.0446
0.0448
0.0450
0.0452
0.0454
0.322 0.324 0.326
206 P
b/23
8 U207Pb/235U
T2284.46 ± 0.10 Ma
n = 6; MSWD = 1.06
Fig. 7. U–Pb concordia diagrams illustrating single zircon isotopic analyses for dated tonstein and tuff beds of the Graissessac and Lodève basins. All error ellipses are plotted at the 2σconfidence interval.Weightedmean 206Pb/238U dates and uncertainties (95% confidence interval) are indicated for the clusters of analyses (shaded) interpreted to represent themagmaticpopulations; analyses with open symbols were excluded from the weighted mean calculations (see Results section for discussion).
125L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
(Fig. 3). Eleven grainswere selected from tuff T1 for CA-IDTIMS analysisbased on their uniform nature. Of these, eight yielded a concordant andequivalent cluster of isotopic ratios with a weighted mean 206Pb/238Udate of 284.40 ± 0.07(0.16)[0.34] Ma (MSWD = 0.79), which isinterpreted as the eruption and deposition of this tuff. Three grainsyielded significantly older ages of 285.2 to 290.2 Ma. Tuff bed T2,approximately 40 m higher in the section, produced a number of olderinherited grains ranging from 284.8 to 285.4 Ma, as well as a clusterof six equivalent analyses with a weighted mean 206Pb/238U date of284.46 ± 0.10(0.17)[0.35] Ma (MSWD = 1.06), interpreted as the ageof eruption and deposition.
Eight single crystals were dated from tuff bed T9, which was sam-pled ~300 m above T2. These yielded one significantly older inheritedzircon (305 Ma) and a cluster of seven equivalent analyses with aweighted mean 206Pb/238U date of 283.53 ± 0.10(0.17)[0.34] Ma(MSWD = 1.15), and are interpreted as dating the eruption and depo-sition of this tuff. Among the fourteen single zircon analyses from tuffbed T12, six yielded Neoproterozoic to Ordovician dates attesting tocommon inheritance in some zircons from this tuff. Another two analy-ses yielded dates of 289.8 Ma and are also interpreted as inherited orrecycled from earlier volcanics. Of the remaining six analyses, fiveyielded equivalent isotope ratios and a weighted mean 206Pb/238U
126 L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
date of 282.86 ± 0.13(0.19)[0.35] Ma (MSWD = 1.22) interpreted asthe eruption and depositional age of the tuff, while one slightly youngercrystal appeared to suffer from a small degree of residual Pb-loss.
5. Discussion
5.1 . Revised chronostratigraphy
The U–Pb zircon data reported in this study represent the first high-precision CA-IDTIMS ages for interstratified volcanics within theCarboniferous–Permian rift basins of Europe. The resulting precise andaccurate ages obtained from the Lodève and Graissessac basins thushave direct consequences for the numerical age of correlated continen-tal sequences throughout eastern Euramerica. These numerical agesalso allow the first accurate terrestrial-marine correlation to the lateCarboniferous–early Permian global time scale, which has been signifi-cantly revised through recent high-precision CA-IDTIMS geochronology(Davydov et al., 2010; Schmitz and Davydov, 2012).
The coal-bearing strata of the basal Graissessac Formation of thewestern Lodève and eastern Graissessac basins have been previouslyassigned a biostratigraphic age of Late Stephanian (B + C), or latestCarboniferous, on the basis of abundant and well-preserved paleoflora(Gand et al., 2013 and references therein). The reproducible deposition-al ages for coal 4 (304.07 ± 0.08 Ma) and coal 5 (303.95 ± 0.07 Ma)tonsteins at Monte Sénégra refine this intercomparison between thecontinental European and global marine time scales, placing depositionof these strata in the uppermost Kasimovian to lower Gzhelian stages.By comparison the SIMS U–Pb zircon date for the same coal 4 tonsteinreported by Bruguier et al. (2003) is significantly younger at 295.5 ±5.1 Ma. Unlike chemical abrasion ID-TIMS, the ion probe protocol lacksany grain pre-treatment to remove andmitigate open-system behavior,which can produce anomalously younger ages; thus we interpret thesystematics of these ion probe data to be biased by Pb-loss. Other ionprobe U–Pb zircon ages of 289 ± 7 Ma and 284 ± 4 Ma for the VialaFormation and Octon Member of the Salagou Formation, respectively(Schneider et al., 2006) are more accurate by comparison with theprecise CA-IDTIMS ages obtained in this study for the same units(Table 1). However with a 40 to 80-fold improvement in precision, ourages place muchmore restrictive constraints on the timing of deposition.
A revised chronostratigraphy for the Permian Lodève Basin is pre-sented in Fig. 2, which also illustrates the CA-IDTIMS zircon ages thatpin each formation to the current global time scale (Davydov et al.,
Table 1CA-TIMS U-Pb age summary.
Formation Sample Profile1 Latitude Longitu
Salagou Formation (Octon Member)T12 LM 26 43.65035 3.30138T9 R VIII 43.66550 3.33481T2 R V 43.67664 3.35763T1 R V 43.67736 3.35698
Viala FormationIII S 43.69246 3.34706
Tuilières–Loiras Formation (Loiras Member)VI LDCI A 43.69910 3.17872V LDCI 25 43.69988 3.17893
Graissessac FormationMS-2 (coal 5) Monte Sénégre 43.68720 3.14499MS-3 (coal 4) Monte Sénégre 43.68769 3.14536
Notes: 1Stratigraphic profiles illustrated by Schneider et al. (2006). 2All weighted mean ages asquare root of the MSWD and the Student’s T multiplier for n-1 degrees of freedom. Uncertain3mean squared weighted deviations.
2012). When compared with the age chart of Schneider et al. (2006),significant compression of the Lodève record is apparent. Within thefirst phase of basin development, the shortening of the numerical dura-tions of the Asselian and Sakmarian global stages combined with twovolcanic tuff dates restrict significant rock accumulation in the Viala,Tuileres-Loiras, and Usclas-St. Privat formations to late Asselian andSakmarian time, between ca 296 and 289 Ma. The hiatus at the top ofthe Viala Formation is constrained to probably less than two millionyears, before initiation of the second phase of deposition recorded inthe overlying Rabejac, Salagou, and La Lieude formations.
Four dated tuff beds in the lower Octon Member of the SalagouFormation (Fig. 3) span over 650 m of section (the lower third of themeasured succession). The calculated rock accumulation rates for thismember are ~0.43m/ka; extrapolating this rate indicates that the entire1150 m of the Octon Member could have been deposited over approxi-mately 2.65 Ma, a much shorter duration than previously interpreted(Schneider et al., 2006; Pochat and Van Den Driessche, 2011). Similarlythe entire Salagou Formation could have been deposited in approxi-mately 4 Ma under these accumulation rates. However, given the sedi-mentological distinctions between the Octon and Merifons members,there is the potential for more frequent hiatuses in the latter formation.Therefore, we have been more conservative in our compression of theSalagou Formation, and illustrate it as extending across the Cisuralian–Guadalupian transition, which according to the current geologicaltime scale is constrained numerically at 272 Ma. This stands in starkcontrast to earlier interpretations which extended the upper MerifonsMember into the Wuchapingian stage.
Since the upper Salagou and La Lieude formations lack ashes toproduce radioisotopic ages, the only other information available toconstrain the minimum age of accumulation in the Lodève Basin is themagnetostratigraphic interpretation of the uppermost sediments ofthe La Lieude Formation (Maillol and Evans, 1993; Evans, 2012). Assummarized by Evans (2012), the entire redbed succession of theLodève Basin is characterized by reverse polarity corresponding to theKiaman superzone, with the exception of the uppermost sampled bedof the La Lieude Formation, which yielded normal polarity. These datasuggest a proximity to the base of the Illawarra mixed polarity interval,which has been constrained in West Texas to near the base of theCapitanian Stage at approximately 265 Ma (Peterson and Nairn, 1971).This constraint is consistent with our previous interpretations for theduration of sediment deposition in the upper Lodève Basin based uponrock accumulation rates. Nonetheless, we might anticipate additional
de 206Pb/238U age Ma2 MSWD3 N
282.86 ± 0.13(0.19)[0.35] 1.22 5 of 14283.53 ± 0.10(0.17)[0.34] 1.15 7 of 8284.46 ± 0.10(0.17)[0.35] 1.06 6 of 11284.40 ± 0.07(0.16)[0.34] 0.79 8 of 11
290.96 ± 0.19(0.24)[0.39] 4.28 7 of 20
293.85 ± 0.10(0.17)[0.36] 1.49 10 of 13293.94 ± 0.08(0.16)[0.35] 0.81 6 of 11
303.95 ± 0.08(0.17)[0.36] 0.80 6 of 9304.07 ± 0.07(0.17)[0.36] 0.71 7 of 11
t the 95% confidence interval, as calculated from the internal 2σ errors expanded by theties are quoted as analytical (analytical + tracer) [analytical + tracer + decay constant].
127L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
compression of the absolute duration of Lodève Basin developmentwith additional radioisotope constraints on the poorly calibratedGuadalupian global time scale (Davydov et al., 2012).
The significant changes to the chronostratigraphy of the LodèveBasin, resulting in large part from our new high-precision geochronolo-gy, highlight the difficulties in constraining the time scales of terrestrialbasin development and associated terrestrial–marine correlations. Sev-eral studies have taken the position that lithostratigraphic, biostrati-graphic, and proxy climate indicators in assumed contemporaneousEuropean basin records can be correlated into global climate signals(Roscher and Schneider, 2006; Schneider et al., 2006; Tabor andPoulsen, 2008). The extent to which these changes will call the modelsinto question will depend upon further integrative stratigraphic analysisand radioisotope calibration. The correlation of Carboniferous–Permianterrestrial basin records within eastern Euramerica and across Pangaearemains tentative; however further high-precision CA-IDTIMS U–Pb zir-con geochronology for other abundant volcanics holds considerablepromise for refining the composite stratigraphic record and associatedpaleoclimate proxy data.
5.2. Paleoclimate reconstruction
Given the evidence for extensive diagenetic overprinting of Permianstrata and paleosols in the Lodève Basin, the typical suite of geochemicalproxies (e.g., Sheldon and Tabor, 2009) cannot be applied in a straight-forward manner to develop a quantitative mineral- and geochemical-based paleoclimate reconstruction. Nevertheless, many macroscopicpaleosol features are notably resistant to diagenetic alteration(Retallack, 1991). Furthermore, the range of analogous featuresamongmodern soil profiles can help to constrain the range of environ-mental and climatic conditionswhen paleosol profileswere undergoingsoil formation in the Carboniferous–Permian Lodève Basin.
Histosols inmodern soil-forming environments are limited to nearlyeverwet conditions where precipitation exceeds 1000 mm/year andprecipitation exceeds evapotranspiration for at least 10 months eachyear (Cecil, 2003). Furthermore, slow rates of clastic sedimentationare prerequisite for formation of thick accumulations of organic matterin these types of soils. Therefore, the presence of Histosols inKasimovian-Ghzelian outcrops of the Graissessac Basin and the subsur-face of the adjacent Lodève Basin represent relatively humid conditionsand intermittently low clastic sedimentation rates during latest Carbon-iferous time. Furthermore, the presence of contemporaneous Histosolformation in separate basins around the Massif Central suggests a re-gional climate that was conducive to the accumulation of surficial or-ganic matter and peat accumulation rather than a more local tectonicdriver (cf. Pochat and Van Den Driessche, 2011). The absence ofHistosols in younger Permian strata in the Lodève Basin suggests thatclimatic conditions dried to the extent that such conditionswere no lon-ger conducive to peat formation or that sedimentary accumulation ratesincreased substantially.
The dominant pedogenic process in the formation of Calcisols is theaccumulation of carbonate in subsurface soil horizons (Mack et al.,1993). Modern soil profiles that accumulate calcium carbonate insubsurface horizons are most commonly found in climates whereevapotranspiration exceeds precipitation for most months of the year(Buol et al., 1997). Specifically, in well-drained soils that receiveN760 mm/year, carbonate will not usually be retained within the sub-surface horizons of the soil (Royer, 1999). Therefore, Calcisols in theTuilieres-Loiras and the La Lieude formations are interpreted to representrelatively dry climatic conditions compared to those associated with peataccumulation in the underlying Grasseisac Formation. The new206Pb/235U ages presented here from zircon crystals in the midTuilleries-Loiras Formation suggests that climatic conditions changedfrom nearly humid-everwet during the Ghzelian to substantiallydrier climate by the–earliest Sakmarian in the Lodève Basin.
The dominant pedogenic process in calcic Vertisols is episodicshrink–swell that results from wet–dry cycles as well as accumulationof carbonate (Mack et al., 1993). Shrink–swell processes in Vertisolsare commonly a result of the presence of expandable 2:1 clay mineralswithin the profile that swell during wet periods and contract duringdry periods (Buol et al., 1997; Schaetzl and Anderson, 2005; Southardet al., 2012). The expansion and contraction of the profiles leads tocharacteristic morphological features such as slickensides, pressurefaces, clastic dikes, and gilgai surface and mukkara subsurface expres-sion. Additionally, the clotting and presence of many coalescing nucle-ation sites seen in thin section (Esteban and Klappa, 1983; Fig. 6D)and the presence of wedge-shaped peds with Fe-oxide-stripping andconcentration (FitzPatrick, 1993; Fig. 6F) are attributed to the sameshrink–swell processes that created the morphological features de-scribed above. These soil types commonly develop in climates character-ized by strongly seasonal rainfall such as monsoonal or Mediterraneanrainfall patterns (Southard et al., 2012). Not all Vertisols form from sea-sonal rainfall patterns; some result from episodic fluctuation of thewater table (Mintz et al., 2011). However, these groundwater-affectedVertisols also have a characteristic pattern of carbonate formation thattransmitted-light and cathodoluminescence petrographic analysis canidentify (Mintz et al., 2011). An Fe/Mn ratio in calcite that favors Mn iscommon in vadose zone carbonates and will produce luminescenceunder cathodoluminence petrography (Hemming et al., 1989). Addi-tionally, presence of dog-tooth spar, microspar and sparry low-Mg cal-cite has been recorded as phreatic environment cements (Esteban andKlappa, 1983; Scholle and Ulmer-Scholle, 2003). Petrographic analysisof carbonate nodules in the calcic Vertisols from the Lodève Basindo not preserve patterns suggestive of groundwater fluctuation.Therefore, calcic Vertisols in the Viala, Rabejac, and parts of the LaLieude formations are interpreted to represent relatively dry cli-matic conditions during the early Permian characterized by highlyseasonal precipitation patterns over the Lodève Basin. CalcicVertisols in the Sakmarian - Artinskian Viala–Rabejac formations indicatedry climate, perhaps comparable to those associated with Calcisols in theAsselian–Sakmarian Tuilieres-Loiras Formation, but with a distinct sea-sonal pattern of rainfall.
The dominant process involved in the creation of vertic Gypsisolsis the accumulation of subsurface gypsum in these profiles which indi-cates substantially drier conditions than during formation of the calcite.These soils are also identified based on the presence of shrink–swellfeatures as described prior. Gypsum is commonly present in subsurfacehorizons of modern soils that are characterized by evapotranspirationfar in excess of precipitation (Eswaran and Zi-Tong, 1991; Watson,1992). Specifically, gypsum is leached from the soil in regions thatreceive precipitation in excess of 250 mm/year (Watson, 1992). There-fore, vertic Gypsisols in the Merifons Member of the Salagou Formationand the basal La Lieude Formation represent extremely dry conditionsin the Kungurian - Roadian characterized by a seasonal distribution ofrainfall and high evapotranspiration rates and relatively low precipita-tion. Vertic Gypsisols in the Merifons Member represent continued sea-sonal precipitation as was present in the Viala–Rabejac based on calcicVertisols, but with substantially less precipitation or higher rates ofevapotranspiration. These conditions are further supported by the pres-ence of illuvial clay coatings and a lack of indicators for pedogenic reduc-ing conditions seen in transmitted light microscopy, whichcollectively indicate these paleosols formed under exclusively well-drained conditions. Thus, shrink–swell processes did not occur in re-sponse to groundwaterfluctuations but rather formed in response to re-gional precipitation patterns. Collectively, paleosol morphologies in theLodève Basin define a progressive trend from humid-everwet in latestCarboniferous time to increasingly seasonal and drier climates throughthe early Permian. The peak in aridity is seen in the Kungurian - RoadianMerifons Member; however, the presence of Calcicsols and calcicVertisols in the overlying La Lieude still indicates continued seasonaland dry climates but less arid conditions during the Guadalupian.
~4°S - 4°N
~10°S - 0°N
~3°N - 10°N
EuropeanBasins
255
260
265
270
275
280
285
290
295
300
305
310
Chan.
Wuj.
Capt.
Word.
Road.
Kun
guria
n
Assel.
Gzhel.
Kas.
Mosk.
Triassic
Four CornersU.S.A.
New MexicoNort h
Neww MexicoSout h
2-4°N
0°N
1°S
5°S
~8°S
~2°S
~5°N
Nort hTexas
Marit imesBasin Canada
2-4°N5°N6°N8-10°N
Appalachianbasin, U.S.A.
Lodeve Basin,France
Low
erM
iddl
eU
pper
Art
insk
ian
Per
mia
nP
enns
ylva
nian
Sak
250
~12°S
~6°S
HU
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UM
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Humid = Dystric Argillisols, HistosolsSubhumid = eutric Argillisols, argillic subgroupsSemi-Arid = Calcisols, calcic subgroupsArid = Gypsisols, gypsic subgroups, Calcisols, calcic subgroupsExtreme Seasonal Precipitation = Vertisols
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Fig. 8. “Wet” indicates the presence of climate-sensitive facies corresponding to ever-wet or humid conditions (N1000 mm/year) and PPT N ETV for at least 9 months, including coals,laterites. “Dry” indicates the presence of climate-sensitive facies corresponding to semi-arid and arid conditions where precipitation is less that ~760 mm/year and fewer than5 months/year with precipitation in excess of evapotranspiration (Cecil et al., 2003). Orange shading denotes intervals with evidence of distinct seasonality including vertic paleosolmorphology (e.g. Tabor and Montañez, 2004; Kahmann and Driese, 2008) and fusain (Falcon-Lang, 2000; Falcon-Lang et al., 2006). The schematic climate surveys are constructedfrom high-resolution, intrabasinal studies of paleoclimate indicators cited throughout the text and references cited therein. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)Paleoclimate reconstruction adapted from Tabor and Poulsen (2008) for western and central equatorial Pangea including Lodève Basin. International geological time-scale according toGradstein et al. (2004), while basin trends have been updated using the dates fromMontañez et al. (2007) and those presented herein. Climate trends (with paleo-latitudes indicated) arebased on observed climate-sensitive facies as presented in Tabor and Poulsen (2008).
128 L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
6. Implications and conclusions
The climatic trend observed herein is similar to that observedin other Carboniferous–Permian paratropical basins across Pangaea(Fig. 8). This suggests a regional to global climatic change because allof the basins saw different tectonic controls, and likely different deposi-tional rates, yet all preserve similar stratigraphic trends in climate prox-ies, which are indicative of the onset of seasonal and relatively aridconditions that persisted from latest Carboniferous through early Perm-ian time. It is unclear if younger Salagou or La Lieude formationspreserve middle or upper Permian strata (Fig. 2), but the climatetrend preserved in the lower Permian succession documented here isremarkably similar to that documented in contemporaneous paratropicalbasins in western Euramerica (e.g., Tabor and Montañez, 2002; Cecil,2003; Tabor and Montañez, 2004; Eros et al., 2012).
Pochat and Van Den Driessche (2011) argued that the depositionalhistory of the Lodève Basin was controlled largely by tectonics, andthe resulting changes in sedimentary accommodation under a uniformset of climatic conditions. That model was based on estimated watervolume changes of a paleolake that extended across the entire Lodève
Basin as the basin infilled and deposition prograded from the southernboundary faults (e.g., Fig. 1). In light of the new chronostratigraphicconstraints, and paleosol morphological data presented herein, themodel of Pochat and Van Den Driessche (2011) is not supported forthe following reasons:
(1) The new chronostratigraphic dates limit the majority of theLodève Basin Permian strata to the lower Permian. Prior to thiswork much of the Salagou Formation was thought to be deposit-ed from Kungurian to Wuchiapingian time. It is now known thatmost of the Salagou Formation is limited to the Sakmarian. There-fore, the temporal evolution of lake volumes and sedimentaryrates in the Lodève Basin discussed by Pochat and Van DenDriessche (2011) are no longer valid.
(2) The presence of paleosols throughout the Lodève Basin sedimenta-ry record is definitive evidence that the entire basin was notalways filled by a large lake. The presence of Histosols in theCarboniferous succession indicate poorly-drained conditions andhigh levels of a regional water table. However, pedogenic carbon-ate, illuviated clays, and gypsum in subsurface horizons of Permian
129L.A. Michel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 430 (2015) 118–131
paleosols indicate a relative fall in the regional water table andwell-drained conditions that were distal from any lake body, thatmight have been present in the Lodève Basin. As a result, thewater volume changes in the Lodève Basin discussed by Pochatand Van Den Driessche (2011) were overestimated by that model.
(3) While Pochat and Van Den Driessche (2011) are correct that thisbasin fits the model of a quadripartite continental-rift sequencecontrolled by tectonics, this basin preserves a record of climateindicators and paleoclimatic change that agrees with otherparatropical basins that did not evolve under the same tectonicregimes (e.g., Tabor and Montañez, 2004; Tabor et al., 2008;Bishop et al., 2010). Therefore, while tectonics may have affect-ed depositional style in the Lodève Basin, there are clearlyextrabasinal mechanisms such as long-term aridification andincreasing seasonality that affected the long-term history ofsedimentation in the basin. A megafan sequence stratigraphicmodel for continental sedimentary basins suggests a commonevolution of sedimentary infill that evolves from initially poorly-drained conditions to progressively better drained conditionsthrough time (Weissmann et al., 2013). At first glance, the LodèveBasin fits this model in that the paleosols change from poorlydrained Histosols in the Ghezlian to well-drained vertic Gypisols,calcic Vertisols, and Calcisols in younger Permian strata. However,considering thismodel under a uniform set of climatic conditions,the poorly-drained Carboniferous Histosols that required ever-wet conditions, would likely have given way to well-drainedargillisols/Oxisols in Permian strata under a similarly humidclimate.Moreover, thewell-drained paleosols seen in the Permianwould likely have corresponding poorly-drained gypsic Gleysolsand calcic Gleysols in the Carboniferous strata under a uniformsemi-arid climate. Note that neither one of these conditionsaccommodate the evidence for seasonal precipitation that is aprerequisite for Vertisol formation. Additionally, the presenceof similar trends in penecontemporaneous low-latitudeCarboniferous–Permian basins under different types of tec-tonic controls, suggests a large-scale regional to global mech-anism that is explained most parsimoniously by climatechange. Therefore, we conclude that long-term pan-tropicalclimate change was a dominant factor in the evolution ofLodève Basin sedimentary infill.
Acknowledgments
We would like to thank Prof. Finn Surlyk, Greg Ludvigson and ananonymous reviewer for their revisions and suggestions on this manu-script. We thank Erik Gulbranson, Mike Eros, Jörg W. Schneider, andFrank Körner for the help and assistance in the field. This research wasfunded by the National Science Foundation (NSF EAR0545654 to NJTand IPM), the Geological Society of America, the University of California,Davis, and Southern Methodist University. VID was partially fundedthrough the Russian Government Program of Competitive Growth ofKazan Federal University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2015.03.020.
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