U–Pb zircon age, sedimentary facies, and sequence stratigraphy of theDevonian–Carboniferous boundary, Daposhang Section, Guizhou, China
Yong-Qing Liu ∗, Qiang Ji, Hong-Wei Kuang, Xiao-Jun Jiang, Huan Xu, Nan PengInstitute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Received 9 November 2011; received in revised form 12 February 2012; accepted 3 March 2012Available online 14 March 2012
bstract
The Devonian–Carboniferous (D/C) boundary marks a major extinction event and an important evolutionary turnover in Paleozoic biotas, butnderstanding D/C boundary events is hampered by a lack of accurate and consistent ages. As a result, although a global stratotype section andoint (GSSP) for the D/C boundary was defined in the section La Serre trench E’ (Montagne Noire, France) and accepted and ratified by theUGS, problems arose concerning both the lithology of the La Serre section and the identification of S. sulcata. Due to these problems, in the veryecent years, the ICS decided to reconsider the GSSP. The Daposhang section in Muhua, Guizhou, China, is one of the most important referenceections for the D/C boundary: it is a conformable succession across the D/C boundary that contains conodont faunas characteristic of restrictedubtidal facies. A biostratigraphically well constrained tuff near the D/C boundary in the Daposhang section provides an excellent opportunity toate this important event in Paleozoic history. Sensitive high-mass resolution ion microprobe (SHRIMP) U–Pb analyses were conducted on singleircons extracted from the tuff (between beds 0 and 1, which may be temporally equivalent to the Hangenberg black shale) as well as an analysis ofedimentary facies and sequence stratigraphy across the D/C boundary in the Daposhang section. Bed 0 is below the D/C boundary, in the Upperraesulcata conodont zone, and bed 1 is directly above the D/C boundary, in the sulcata conodont zone. Zircon analyses yielded 20 concordantates that form a cluster with a 206Pb/238U concordia age of 359.6 ± 1.9 Ma. An abundance of bioclastic packstone and wackestone and a lackf obvious high-energy sedimentary structures or evidence of reworking in layers spanning the D/C boundary suggest conformable depositionn a quiet, restricted subtidal paleoenvironment. Fifty-nine fifth-order cycles combine to depict a shallowing-upward succession (beds 029-01;ighstand systems tract of a third-order sequence representing the terminal Devonian). The highstand systems tract is overlain by a deepening-pward succession (beds 0-29; maximum flooding surface and transgressive systems tract of a third-order sequence representing predominantlyhe basal Carboniferous) that begins at the D/C boundary. Fischer plot analysis of high-frequency cycles at the D/C boundary confirms this welleveloped sea-level pattern, which, together with the tuff and black shale depositional events, has potential for global correlation. On the basis of
his new single-zircon SHRIMP age and previously published work on the biostratigraphy and sequence stratigraphy of the D/C transition interval,he deposition of tuff took place at approximately 359.6 Ma, and the age of the D/C boundary at Daposhang, Guizhou, China, is estimated at59.58 Ma.
2012 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.
The Devonian–Carboniferous (D/C) boundary marks one ofhe major extinction events of the Phanerozoic, during which
hallow- and deep-marine organisms and terrestrial ecosystemsere severely affected (Kaiser et al., 2006). The D/C boundaryas defined in the section La Serre trench E′ (Montagne Noire,
rance) on the base of the entry of the conodont Siphonodellaulcata. The GSSP was accepted and ratified by the IUGS in990 (Paproth et al., 1991). Unfortunately, soon after the ratifi-ation, problems arose concerning both the lithology of the Laerre section and the identification of S. sulcata (Kaiser, 2009;orradini et al., 2011; Kaiser and Corradini, 2011). Due to theseroblems, in the very recent years, the ICS decided to reconsiderhe GSSP. A Task Force, composed of SDS and SCS members,
as established with the goal of finding a new criterion for theefinition of the boundary, and a new section (Corradini et al.,011). Further, the D/C boundary has been controversial because
Fig. 1. Schematic geological map of the Devonian–Carboniferous lithostrati-graphic units in Muhua, Changshun County, Guizhou Province, China. C1:unnamed Early Carboniferous; C1d: Dawuba Formation; C1w + m: Wangyouand Muhua formations; D3d: Daihua Formation; D3y: Yaosuo Formation; D3x:Xiangshuidong Formation; D2h: Huohong Formation; Q: Quaternary.
1wecp2w
taDabi
3p
0clftH
Y.-Q. Liu et al. / Palae
f the apparent diachroneity of the event and the lack of an accu-ate, absolute age (Kauffmann, 2006). Previously reported datesere not from material exactly at the D/C boundary. Ages of53.2 ± 4.0 Ma and 353.7 ± 4.2 Ma were provided by U–Pb sen-itive high-mass resolution ion microprobe (SHRIMP) analysisf zircons extracted from the bed 79 metabentonite, which isirectly above the D/C boundary in the Hasselbachtal auxiliaryection (Sauerland, Germany; Claoué-Long et al., 1992, 1995),fter which 354 Ma was used as a reference age for the D/Coundary (Fordham, 1992; Menning et al., 2001). Tucker et al.1998) reported an older boundary age of 362 Ma (ID-TIMS zir-on age) from the Piskahegan Group (New Brunswick, Canada).ost recently, a date of 360.7 ± 0.7 Ma was obtained by ID-
IMS for the D/C boundary in the Hasselbachtal section, whichas used as the age of the D/C boundary (Trapp et al., 2004).he Geological Society of America, however, recommended ange of 359.2 Ma for the D/C boundary.
Numerous geological events took place at approximately theame time at the end of the Devonian in addition to the massxtinction: a pronounced sea-level fall and rise, tuff deposition,nd black shale deposition (reflecting perturbation of the carbonycle; Wang et al., 1993). These events may have contributed to aass extinction in the marine and terrestrial biosphere. Although
hese events are globally recognized, their exact causes arencertain, and it remains unclear whether they were isochronousr diachronous and whether they can truly be correlated in dif-erent facies globally. The applicability of different analyticalechniques and biostratigraphic indices to the identification ofhese events is contentious, and so their utility as criteria for thedentification of the D/C boundary is in dispute.
The establishment of a reliable D/C boundary age will dependn the dating of tuffs that lie close to the D/C boundary,ogether with further exploration and correlation of sequence-tratigraphic events and black shale deposits. This study reportsHRIMP U–Pb analyses of single zircons that were extractedrom a tuff that is stratigraphically very close to the D/C bound-ry in the Daposhang section, Changshun County, Guizhou,hina. This work focused on calibrating the age of the D/Coundary, identifying sedimentary facies and cyclicity, and char-cterizing volcanic events associated with the D/C boundary inhis section.
. Devonian–Carboniferous boundary section
The Daposhang section is located in Muhua village, 25 kmouth of Changshun County in Guizhou Province, southwesternhina (Fig. 1), and contains a conformable succession across the/C boundary. The sediment appears to have been deposited in
restricted subtidal environment, as indicated by an abundancef micro- and macrofossils (conodonts, ammonoids, trilobites,stracodes, brachiopods, corals, and vertebrate microfossils)Hou et al., 1984; Ji et al., 1989). The Daposhang section con-ains a diverse and abundant conodont fauna as well (Hou et al.,
984; Ji et al., 1989).
Strata of the D/C boundary section at Daposhang consist ofhe Upper Devonian Daihua Formation (beds 029-0) and theower Carboniferous Wangyou Formation (beds 1-29; Ji et al.,
n5
r
989; Figs. 2–4). Beds 029-023 are composed of gray bioclasticackestone with weak layering, beds 022-04 comprise interlay-
red gray bioclastic packstone and wackestone, and beds 03-01onsist of bioclastic packstone. Bed 0 comprises gray bioclasticackstone with cross-bedding and scour structures, and beds 1-9 consist of thin-bedded gray bioclastic packstone interbeddedith nodular lime mudstone (Figs. 3 and 4).At the D/C boundary section, four conodonts have been iden-
ified as the Middle praesulcata, Upper praesulcata, sulcatand Lower duplicate zones (Ji et al., 1989; Figs. 2–4). The/C boundary is placed at the first appearance of S. sulcata,
t the base of bed 1 (Ji et al., 1989). Approximately 5–10 cm ofrownish–yellow bentonite (tuff) directly underlies bed 0, whichs nearly 30 cm below the D/C boundary (base of bed 1; Fig. 5).
. Sample description, zircons, and analyticalrocedures
One sample of tuff was collected from the bottom of bed, 30 cm below the D/C boundary, for SHRIMP U–Pb zir-on dating. In outcrops, the material is pale yellow, weatheredaminated shale or mudstone. Zircons (Fig. 6) were separatedrom the matrix using standard density and magnetic separationechniques at the Institute of Geology and Mineral Resources,ebei, China. The crystals are generally euhedral or short colum-ar, with rare inherited cores and growth zones approximately
0–100 �m thick.
Zircons contain 79–624 ppm uranium and 71–348 ppm tho-ium, yielding Th/U ratios from 0.3 to 1.2 (Table 1). Such
102 Y.-Q. Liu et al. / Palaeoworld 21 (2012) 100–107
F angeG ackest5
r1fco
T
ig. 2. Stratigraphic column, conodont zones, and high-frequency sea-level chuizhou Province, China. Conodont zones follow Ji et al. (1989). 1: rhythmic w: wackestone; 6: tuff; 7: fifth-order cycle thickness; 8: Fischer plot.
atios are consistent with a magmatic origin (Compston et al.,984). Interpretation of the zircon U–Pb isotopic data is there-
ore straightforward, and the 206Pb/238U ages are interpreted asrystallization ages that are contemporaneous with depositionf the tuff.
wal
at the Devonian–Carboniferous boundary in Daposhang, Changshun County,one and packstone; 2: nodular limestone; 3: bioclastic packstone; 4: packstone;
Zircon grains, together with the zircon U–Pb standardEMORA (Black et al., 2003), were cast in an epoxy mount,
hich was then polished to provide crystal cross sections for
nalysis. Zircons were examined using transmitted and reflectedight microscopy and cathodoluminescence (CL) to reveal their
Y.-Q.
Liu
et al.
/ Palaeow
orld 21
(2012) 100–107
103
Table 1U–Pb SHRIMP zircon age data for tuff at the Devonian–Carboniferous boundary in the Daposhang section, China.
104 Y.-Q. Liu et al. / Palaeoworld 21 (2012) 100–107
F y (bedP t. Con
Tp
4
f9ozt(
bHTcaa3
a7afi(dpmw
cts(ia
ig. 4. Log of the stratigraphic section of the Devonian–Carboniferous boundarrovince, southwestern China. Bed numbers and conodont zones are at the righ
he zircon U–Pb age data and data reduction procedures areresented in Table 1.
. Analytical results and discussion
The 206Pb/238U ages of zircons from the tuff specimenorm a cluster. Only three dates (133.2 ± 14.1, 792 ± 1.1, and92 ± 7.1 Ma) were excluded from the cluster, probably becausef inherited older zircon (cores) or Pb loss. The other 17ircons yielded ages of 353.4–367.1 Ma and formed a tight clus-er with a 206Pb/238U concordia age of 359.6 ± 1.9 Ma (2σ)Fig. 7).
No universally accepted age exists for the D/C boundary. Theest known D/C boundary age was obtained from bed 79 of theasselbachtal auxiliary stratotype section (Trapp et al., 2004).he bed 79 bentonite is 43 cm above the D/C boundary, in the sul-
ata zone, and yielded some discordant U–Pb SHRIMP zirconges (353.2 Ma and 353.7 Ma; Claoué-Long et al., 1992, 1995)nd U–Pb ID-TIMS zircon ages (346.6 Ma, Kramm et al., 1991;60.5 Ma, Trapp et al., 2004). Furthermore, Trapp et al. (2004)
fttr
s 01-14) and conodont zones (left) at Daposhang, Changshun County, Guizhoutinued from adjacent column; conodont zones follow Ji et al. (1989).
nalyzed a younger bentonite in the Lower duplicata zone (bed0, 57 cm above bed 79) and obtained a 206Pb/238U concordiage of 360.2 ± 0.7 Ma, based on 17 single-zircon analyses. Therst reference age for the D/C boundary was therefore 354 MaFordham, 1992; Young, 1995; Sandberg and Ziegler, 1996), aate that was revised to 360.7 Ma when the ID-TIMS data wereublished (Trapp et al., 2004). The D/C boundary age recom-ended in the “2009 Geologic Time Scale” (Azmy et al., 2009)as 359.0 Ma.Sedimentary rocks in the D/C boundary section at Daposhang
onsist mainly of bioclastic wackestone and packstone inhe lower and middle part of the section, and nodular lime-tone interbedded with wackestone and shale in the upper partFigs. 3 and 4). High-energy sedimentary structures are rare, butn the uppermost part of the section, parallel bedding and scour-nd-fill structures are common, and cephalopod and brachiopodossils are abundant. Stylolites are common throughout the sec-ion. The stratigraphic succession across the D/C boundary in
he Daposhang section is interpreted to represent a quiescent,estricted subtidal environment.
Y.-Q. Liu et al. / Palaeoworl
Fig. 5. Detailed lithology and outcrops of the dated tuff at theD
Gflte
dE
t(dCtTtiiTGfiu3cbdae
tasttcdbfdthat these deposits were geologically isochronous and that they
FSt
evonian–Carboniferous boundary in Daposhang section, China.
Fischer plot analysis of stratigraphic cyclicity (Fischer, 1969;oldhammer et al., 1990) indicates high-frequency sea-leveluctuation superimposed on third-order sea-level fall in the
erminal Devonian and on sea-level rise in the earliest Carbonif-rous. These eustatic events, together with tuff and black shale
ra
ig. 6. Cathodoluminescence (CL) images of zircons from the tuff below the D/C
HRIMP U–Pb dating, which was performed at the Beijing SHRIMP II Center, Chinhat all analyzed zircons were magmatic in origin.
d 21 (2012) 100–107 105
eposition, are also known from stratigraphic successions inurope and North America.
The 59 fifth-order cycles recognized in the Daposhang sec-ion define a cyclic, shallowing-upward depositional intervalbeds 029-01, mainly terminal Devonian) and an ensuing,eepening-upward cyclic interval (beds 0-29, mainly earliestarboniferous). A maximum-flooding interval is identified with
he tuff layer, which may be equivalent to the Hangenberg shale.he shallowing-upward interval (beds 029-01) belongs to the
erminal Devonian HST, and the overlying deepening-upwardnterval (beds 0-29) represents a TST plus condensed sectionnterval in the earliest Carboniferous (Vail and Mitchum, 1977).he theory of sequence stratigraphy (Vail and Mitchum, 1977;oldhammer et al., 1990) indicates that a third-order sequence
orms over a time span of 2–5 m.y., whereas a fifth-order cycles only 20–40 k.y. in duration. Because the 359.6 Ma tuff is sit-ated immediately below bed 0, and only one 5th-order cycle,0 cm thick, separates the tuff from the D/C boundary, the tuffan be inferred to be 20–40 k.y. older than the age of the D/Coundary, which at Daposhang is 359.58 (∼359.6) Ma. The tuffate must be accurate, not only for dating of the D/C bound-ry, but because it may be correlated with the Hangenberg shalevent.
Black shale deposition, eustatic sea-level change, perturba-ions of the carbon cycle (Wang et al., 1993), and tuff eventsre globally recognized at the D/C boundary; they are by neces-ity a function of complex causes that must have affected bothhe marine and the continental biosphere, and were related tohe major extinction at the D/C boundary. Although the ultimateause(s) of these end-Devonian events, whether isochronous oriachronous, is unknown; a new date of ∼359.6 Ma calibratesoth an inflection point in the eustatic sea-level curve (shift fromalling to rising at the D/C boundary), and tuff and black shaleepositional events. The new tuff date supports the hypothesis
epresent a practical criterion for identifying the D/C bound-ry. The events associated with the D/C boundary may prove
boundary in the Daposhang section, China. Circles indicate the locations ofese Academy of Geological Sciences. The Th/U ratio (below zircons) indicates
106 Y.-Q. Liu et al. / Palaeoworld 21 (2012) 100–107
F evonia s corra
tam
avatc
5
tbesAiDtTtortsoita∼t
tsc
A
vWttcCi
R
A
B
C
C
ig. 7. Concordia plot of zircon U–Pb SHRIMP date of the tuff beneath the Dges are 2-� analytical errors. Box heights in the inset are 2-�. Common Pb wage variation of analyzed spots within individual spots.
o be correlatable into very different facies and may be identifi-ble using a range of analytical techniques and biostratigraphicarkers.The age of the D/C boundary, however, remains in dispute. An
ccurate age for the D/C boundary may depend on dating otherolcanic ashes that lie close to the D/C boundary. The 359.6 Mage reported here for the D/C boundary in the Daposhang sec-ion, China, represents an important advance in determining thehronology of D/C boundary events.
. Conclusions
The Daposhang section in Muhua, Guizhou, China, is a con-inuous, conformable stratigraphic succession across the D/Coundary. The strata were deposited in a restricted subtidalnvironment during the Middle middle praesulcata, Upper prae-ulcata, sulcata, Lower duplicata and Upper duplicata zones.
biostratigraphically well controlled volcanic ash (bentonite)s present at the bottom of bed 0, which is 30 cm below the/C boundary. U–Pb SHRIMP analyses of zircons from the
uff yielded a 206Pb/238U concordia age of 359.6 ± 1.9 Ma.he predominance of bioclastic packstone and wackestone and
he absence of high-energy sedimentary structures or evidencef reworking across the D/C boundary indicate a quiescent,estricted, subtidal facies. A Fischer plot analysis indicateshat higher-order cyclicity is superimposed on a third-orderea-level fall and rise that brackets the D/C boundary. The rec-mmended age of the D/C boundary in the Daposhang sections 359.58 (∼359.6) Ma. The fundamental cause(s) for deposi-
ion of the Hangenberg black shale, eustatic sea-level change,nd tuff deposition at the end of the Devonian is unknown. The359.6 Ma date for the D/C boundary calibrates both an inflec-
ion point in the third-order sea-level curve (falling to rising at
C
an–Carboniferous boundary in the Daposhang section, China. Weighted meanected using measured 204Pb. The inset shows the error range for each spot and
he D/C boundary) and tuff and black shale deposition, whichupports the hypothesis that these event horizons may be usefulriteria for identifying the D/C boundary worldwide.
cknowledgments
Financial support was provided by the China Geology Sur-ey (1212010911068). We thank Prof. Si-Tao Wang, Xiang-Heu, and Dr. Xu-Ri Wang for stimulating discussions and par-
icipation in fieldwork, and Hui-Yi Sun for her help duringhe laboratory work and in processing the SHRIMP U–Pb zir-on data. Constructive comments by three reviewers (Carloorradini, Claudia Spalletta and Chang-Min Yu) substantially
mproved the manuscript.
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