DSpace at VNU: Devonian-Carboniferous transition containing a Hangenberg Black Shale equivalent in the Pho Han Formation on Cat Ba Island, northeastern Vietnam

DSpace at VNU: Devonian-Carboniferous transition containing a Hangenberg Black Shale equivalent in the Pho Han Formation...

Devonian –Carboniferous transition containing a Hangenberg Black Shale

equivalent in the Pho Han Formation on Cat Ba Island,

northeastern Vietnam

Toshifumi Komatsua,⁎ , Satoru Katoa, Kento Hirataa, Reishi Takashimab, Yukari Ogatac, Masahiro Obac,

a Graduate School of Science and Technology, Kumamoto University, Kumamoto 806-8555, Japan

b

The Center for Academic Resources and Archives, Tohoku University Museum, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan

c

Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan

d

Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

e

College of Sciences, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

f

Vietnam Institute of Geosciences and Mineral Resources (VIGMR), Hanoi, Viet Nam

g Vietnam National Museum of Nature (VNMN), Hanoi, Viet Nam

h

Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8567, Japan

i

Senckenberg Research Institute and Natural History Museum Frankfurt, Senckenberganlage 25, 60325 Frankfurt am Main, Germany

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 24 September 2013

Received in revised form 8 February 2014

Accepted 11 March 2014

Available online 26 March 2014

Keywords:

Devonian–Carboniferous boundary

Hangenberg Black Shale

Anoxic to dysoxic facies

Extinction

Recovery

On Cat Ba Island in northeastern Vietnam, the Devonian to Carboniferous (D–C) transition consists mainly of ramp carbonates intercalated with black shale beds (Beds 1 to 176) in the Pho Han Formation and is one of the few records of the D–C transition of the eastern Paleotethys The three main facies of the sequence are Facies 1 (alternations of whitish gray to gray limestone and marl), Facies 2 (calcirudite, Bed 115b), and Facies 3 (alterna-tions of dark gray limestone and organic-carbon-rich black shale, Beds 115c–120 and 126–129) The latest Famennian (Siphonodella praesulcata Subzone) conodont assemblage of S praesulcata, Palmatolepis gracilis, Palmatolepis sigmoidalis, and Rhodalepis polylophodontiformis was recognized in Beds 113–115c Beds 105–112 commonly contain Palmatolepis expansa, P gracilis, and P sigmoidalis Bed 119 yielded a basal Carboniferous index conodont Siphonodella sulcata In Beds 116–118, solenoporids such as Pseudochaetetes elliotti and Parachaetetes sp were characteristic species in organic-carbon-rich dark gray limestone

Facies 1 is characterized by bioclastic, peloidal, and intraclastic grainstone and packstone containing massive nor-mal grading and cross-laminations, and is interpreted to represent deep ramp carbonates above storm wave base Facies 2 is represented by typical lag deposits overlying a transgressive surface Facies 3 comprises organic-carbon-rich black shale and minor scour-filling bioclastic, peloidal, and intraclastic packstone, and may represent

a marginal basin plain environment surrounding a carbonate ramp The alternations of organic-carbon-rich black shale and dark gray packstone (Facies 3) show no evidence of bioturbation and have high TOC contents (0.18–5.73 wt.%) A minor succession within the transgressive lag deposits (from Bed 115b of Facies 2 to Beds 115c–120 in the lower part of Facies 3) is equivalent to the Hangenberg Black Shale (s l.) in the middle part of the Siphonodella praesulcata to Siphonodella sulcata zones, because Beds 115b–120 characterized by no evidence

of bioturbation and high TOC contents are interpreted to be accumulated in anoxic to dysoxic conditions

© 2014 Elsevier B.V All rights reserved

1 Introduction

During the latest Famennian, the Hangenberg Event is associated

with global faunal changes and extinction event in marine and

terrestri-al environments (Algeo et al., 1995; Hallam and Wignall, 1997; Caplan

and Bustin, 1999; Streel et al., 2000; House, 2002; Brand et al., 2004)

The event is named for the Hangenberg Black Shale beds in the Rhenish Massif, Germany that are part of the Siphonodella praesulcata conodont zone (Middle Siphonodella praesulcata Subzone) (Walliser, 1984;

sensu stricto) has been reported from known Devonian low-paleolatitudinal regions of Europe, North Africa, the United States, Canada, Russia, Thailand, and southern China (Thrasher, 1987; Richards and Higgins, 1988; Caplan and Bustin, 1999; Brand et al.,

⁎ Corresponding author.

E-mail address: komatsu@sci.kumamoto-u.ac.jp (T Komatsu).

http://dx.doi.org/10.1016/j.palaeo.2014.03.021

Contents lists available atScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / p a l a e o

Fig 1 Maps showing the location of the study area in the Cat Co area of Cat Bat town, on Cat Ba Island, Hai Phong Province, North Vietnam Geologic map of the Cat Co area on southeastern Cat Ba Island There are extensive outcrops of the Upper Devonian to Lower Carboniferous Pho Han Formation in the Cat Co area Devonian and Carboniferous boundary sections are

formation is composed mainly of ramp platform carbonates and

slope deposits, and includes the Devonian–Carboniferous (D–C)

transi-tion (Ta and Doan, 2005, 2007; Doan and Tong-Dzuy, 2006; Komatsu

(Unit 1 ofDoan and Tong-Dzuy, 2006) yields Famennian and early

Tournaisian conodonts and foraminifers Middle Tournaisian

foramini-fers are found in the overlying sequence (Unit 2 ofDoan and

Tong-Dzuy, 2006)

The section that includes the D–C transition is in the Cat Co area

(near Cat Co 3 Beach) on southeastern Cat Ba Island It consists mainly

of whitish gray to gray fossiliferous and bioturbated bedded limestones

intercalated with alternating layers of dark gray limestone and black

organic-carbon-rich shale, and black chert layers The dark gray

lime-stone, black shale, and several whitish gray to gray limestone beds in

the western part of the Cat Co area (Loc 01,Figs 1–3) were numbered

from 1 to 167 byTa and Doan (2005, 2007) They reported that the

late Famennian conodont assemblages of Beds 100–115 are composed

of Palmatolepis gracilis and Palmatolepis sigmoidalis and that Bed 122

contains the Early Carboniferous conodonts Siphonodella sulcata

and Siphonodella duplicata.Komatsu et al (2012a)illustrated the late

Famennian conodonts from Bed 115 (Bed 115a here), such as

Palmatolepis expansa, P sigmoidalis, and Rhodalepis polylophodontiformis,

andfield photographs of alternating well-laminated

organic-carbon-rich black shales and dark gray limestone beds Moreover, in a

prelimi-nary assessment,Komatsu et al (2012b)identified the basal

Carbonifer-ous index fossil S sulcata in this section and reported on theδ13C curve

for bulk carbonates

In this study, we report on the Devonian and Carboniferous

cono-dont assemblages and depositional environments of the Cat Co area,

and describe the D–C transition within the alternating

organic-carbon-rich black shales and dark gray limestone beds of the Pho Han

Forma-tion Some calcareous microfossils that are characteristic around the

D–C transition are found in the organic-carbon-rich dark gray limestone

of the Cat Co 3 area We correlate this sequence in the Cat Co 3 area with

the European Hangenberg Black Shale reported byKaiser et al (2011),

and discuss the nature of the Hangenberg anoxic event in the eastern

Paleotethys

2 Geologic setting in the Cat Co 3 area The Upper Devonian to Carboniferous Pho Han Formation in the study area is about 500 m thick and is composed of carbonate platform limestone, marl, shale, and chert (Doan and Tong-Dzuy, 2006; Komatsu

stems, gastropods, cephalopods, corals, conodonts, and foraminifers

On the Cat Co peninsula and at the Cat Co 3 Beach, outcrops of the low-ermost part of the Pho Han Formation consist of fossiliferous whitish gray to gray limestone intercalated with black chert layers and alternat-ing sequences of organic-carbon-rich black shale and dark gray lime-stone (Fig 1) These beds strike WNW–ESE and dip to the NNE The

D–C transition is within the numbered alternations of dark gray lime-stone and black organic-carbon-rich shale in the west of the Cat Co area (Loc 01,Figs 1–3) The alternations of dark gray limestone and black organic-carbon-rich shale also crop out in the eastern part of the Cat Co area (Loc 02) Beds 109 and 114–116 are well exposed in both areas

The D–C transition in Beds 1–167 consists mainly of alternations of whitish gray to gray limestones, micritic limestones, and marls (Beds

1–115a, 121–125, and 130–167) and alternations of thin dark gray lime-stones and organic-carbon-rich black shales (Beds 115c–120 and

126–129) The carbonates yield abundant brachiopods, crinoid stems, conodont elements, and foraminifers The alternations of thin dark gray limestone and organic-carbon-rich black shale show no evidence

of bioturbation or pyrite aggregations Bed 115b is composed of dark gray intraclastic calcirudite (Fig 2)

3 Methods

We conducted sedimentological studies on the basis of facies analy-sis in thefield and thin-section analysis in the laboratory for detailed observation of microfossils, sedimentary structures, and carbonate petrology The terminology used here for carbonate petrology, deposi-tional environments, and sequence stratigraphy followsNummedal

Fig 2 Detailed columnar sections of the Devonian to Carboniferous transition See facies classifications in Fig 7

More than 20 limestone samples (1–2 kg each) were collected from

Beds 105–132 at Loc 01 for extraction of conodont elements The

sam-ples were processed using the conventional acetic acid technique

Almost all samples contained conodont elements, including poorly

preserved specimens and fragments Generally, whitish gray to gray

limestone and marl (e.g Beds 106, 109, 114, 115a, 122) contained

abun-dant well-preserved conodonts (more than 12 elements/kg), but

organic-carbon-rich dark gray limestone yielded only a few poorly

preserved and fragmented conodont elements, except for Bed 115c, which yielded abundant conodonts

TOC content was determined with a Yanako MT-5 CHN analyzer after removal of carbonate by acidification Samples were weighed on ceramic boats, and 1 N HCl was pipetted into each sample boat until car-bonate was fully removed The samples were then heated to 80 °C for at least 6 h to drive off HCl and water before analysis with the CHN

analyz-er with combustion at 950 °C for 5 min Hippuric acid was used as the

Fig 3 Field photographs of the Pho Han Formation in the Cat Co area (1) At Loc 01, the Pho Han Formation consists mainly of whitish gray to gray limestone (WGL) and alternations of dark gray limestone and black shale (lower alternations, LADB, Beds 115c–120; upper alternations, UADB, Beds 126–129) See facies classifications in Fig 7 (2) Beds 119–127 (Loc 01) The basal part of Bed 120 is typically an erosional surface (ES) (3) Beds 114–119 (Loc 01) Bed 115c contains Devonian (late Famennian) conodonts (black star) Palmatolepis gracilis, Palmatolepis sigmoidalis, and Rhodalepis polylophodontiformis Bed 115a contains Palmatolepis expansa and R polylophodontiformis (black star) Bed 119 contains the basal Carboniferous index conodont Siphonodella sulcata (white star) (4) Beds 115b–116 (Loc 01) Bed 115c yields abundant thin-shelled brachiopods and foraminifers (5) Beds 129–130 (Loc 01) Whitish gray to gray limestone (Bed 130) contains abundant small burrows (white arrows) (6) Carboniferous whitish gray to gray bioclastic limestone (bedding plane) on Cat Co 3 Beach con-taining gastropods, brachiopods, and fragments of crinoid stems.

standard for CHN calibration Results of duplicate analyses were

con-firmed to be identical within 5%

4 Microfossils

4.1 Conodonts

For international biostratigraphic correlation of rocks at the D–C

transition, conodonts and ammonoids are the most important fossil

groups We collected conodonts from many limestone beds (Beds

105–132,Figs 3–5) Particularly, Beds 106, 109, 112, 115a, and 115c

yielded abundant Devonian conodonts, predominantly species of

Palmatolepis Carboniferous (Tournaisian) conodont assemblages were

composed mainly of several species of Polygnathus and Siphonodella,

and were found in dark gray limestone (Bed 119) and whitish gray to gray limestone (Beds 122 and 130–132)

The Upper Devonian conodont assemblage in Bed 115c consists of Palmatolepis gracilis, Palmatolepis sigmoidalis, Polygnathus symmetricus, and Rhodalepis polylophodontiformis The latest Devonian index conodont Siphonodella praesulcata is found in Bed 113 R polylophodontiformis, which is also characteristically present in the uppermost Famennian

S praesulcata Zone (Wang and Yin, 1985; Gatovsky, 2009), is common

in Beds 115a and 115c Palmatolepis expansa is found in Beds 105, 106,

108, 109, 112, 114, and 115a Beds 105–112 yielding P expansa with no

S praesulcata indicates the P expansa Zone Beds 113–115b extends ei-ther to the lower part of the S praesulcata Zone (Over, 1992) or to the middle part of the S praesulcata Zone (Dreesen et al., 1986; Kaiser

S praesulcata Zone

Fig 4 Detailed columnar section (Loc 01), stratigraphic occurrences of main conodont taxa, Solenoporacea, and foraminifers, and TOC content profile The uppermost Famennian conodont assemblage is found in Bed 115c Bed 119 yields the basal Carboniferous index conodont Siphonodella sulcata Both S sulcata, and Siphonodella duplicata (black stars) were reported from Bed 122 by Ta and Doan (2005) Note Pseudochaetetes elliotti, Parachaetetes sp and Solenoporacea gen et sp indet are found only in Beds 116–118 Limestone of the Pho Han Formation

The lower part of Bed 119 yielded rare specimens of Polygnathus

dentatus and the basal Carboniferous index conodont Siphonodella

sulcata Polygnathus communis, P dentatus, and Polygnathus spp are

com-mon in the whitish gray to gray Carboniferous limestone.Ta and Doan

(2005)reported S sulcata and Siphonodella duplicata from their Bed T41

(equivalent to Bed 122 at Loc 01) Generally, the S duplicata Zone

over-lies the S sulcata Zone in the lower Tournaisian Therefore, the whitish

gray and dark gray limestones of Beds 105–115c and the alternations of

dark gray limestone and organic-carbon-rich black shales (Beds

116–129) consist of at least four conodont zones, comprising the

Palmatolepis expansa, Siphonodella praesulcata, S sulcata, and S duplicata

zones On the basis of conodont assemblages, the D–C boundary is

prob-ably within Beds 116–118, where, unfortunately, conodonts are rare

Only poorly preserved Polygnathus spp are found in Beds 117 and 118

4.2 Parachaetetes and Pseudochaetetes Solenoporids are calcareous microfossils that are systematically treated as Rhodophyta (calcareous algae) However, some species

of chaetetid discovered in recent fossil groups have characteristics typ-ical of sponges, such as tube walls and spicules (e.g.Wörheide, 1998; Riding, 2004; Higa et al., 2010) Some species of chaetetid are clearly sponges

In the Cat Co 3 section, minute solenoporids are characteristically found in Beds 116–118 (Figs 4, 6) Several thin bioclastic limestone lenses in the upper part of Bed 116 and a dark gray bioclastic limestone

in Beds 117 and 118 yield small thin brachiopod shells, simple foramin-ifers, and solenoporids Fragments of Pseudochaetetes elliotti and Parachaetetes sp are common in Beds 117 and 118 (Fig 6) in shell

Fig 5 Conodonts from the Pho Han Formation (Loc 01) (1) and (2) Palmatolepis sigmoidalis Ziegler; 1 from Bed 115a, 2 from Bed 115c (3) Palmatolepis expansa Sandberg and Ziegler from Bed 115a (4) Palmatolepis gracilis Branson and Mehl from Bed 115a (5) Pseudopolygnathus trigonicus Ziegler from Bed 115a (6) Polygnathus symmetricus Branson from Bed 115c (7) and (8) Rhodalepis polylophodontiformis Wang and Yin; 7 from Bed 115c, 8 from Bed 115a (9) Siphonodella sulcata (Huddle) from Bed 119 (10) Polygnathus sp from Bed 160 (11) Polygnathus communis (Branson and Mehl) from Bed 122 (12) Polygnathus dentatus Druce from Bed 122 a, upper view; b, lower view; c, lateral view Scale bars are 100 μm.

concentrations from the marginal basin plain facies, which indicates

that these remains were probably transported from a carbonate

platform

In the top part of the Nanbiancun Formation, southern China, the

lower and middle parts of the Siphonodella sulcata Zone indicating that

the basal part of Carboniferous commonly contain Pseudochaetetes

elliotti and Parachaetetes sp (Yu, 1988; Mamet, 1992).Mamet (1992)

reported that fragments of Pseudochaetetes and Parachaetetes in the

Nanbiancun Formation are reworked allochthonous remains.Yu et al

that many D–C boundary sections in the Nanbiancun Formation record

a broad range of marine environments, including carbonate ramp, slope, and basin plain In the Nanbiancun Formation, slope facies was found to contain mixed deep-marine and transported shallow-marine benthic assemblages Beds 55–56 of the section of the Nanbiancun Formation that contains the D–C boundary have been interpreted as a shallow ma-rine carbonate platform facies above storm wave base and below fair weather wave base (Yu et al., 1987) During the earliest Carboniferous, minute solenoporids probablyflourished in a shallow sea and accumu-lated on a ramp platform in the Nanbiancun Basin Some species of the earliest Carboniferous Pseudochaetetes and Parachaetetes appear to be characteristic species of the eastern Tethys

Fig 6 Thin sections from the Pho Han Formation (Loc 01) (1) Parachaetetes sp in dark gray carbon-rich limestone (Bed 117) (2) Pseudochaetetes elliotti, in dark gray organic-carbon-rich limestone (Bed 117) (3) Bioclastic, intraclastic, and peloidal grainstone (lower part of Bed 112) Brachiopods (B) and foraminifers (F) are common (4) Peloidal, intraclastic and bioclastic grainstone (middle part of Bed 114) (5) Bioclastic, intraclastic and peloidal grainstone (upper part of Bed 115a) Foraminifers (F) are abundant (6) Boundary between Bed

116 (dominated by organic-carbon-rich laminated black shale) and Bed 117 (mainly organic-carbon-rich bioclastic and intraclastic packstone) The basal part of Bed 117 commonly con-tains poorly preserved thin-shelled brachiopods Scale bars are 1 mm.

The D–C transition within Beds 1–167 consists of Facies 1–3 (Figs 2,

3, 7–11) Facies 1 comprises alternations of whitish gray to gray

lime-stones (WGL) and micritic limelime-stones intercalated with marls in

Beds 1–115a, 121–125, and 130–167 The lower parts of the gray to

whitish gray limestone beds consist mainly of bioclastic, peloidal, and

intraclastic grainstone, are characterized by sharp andflat basal

sur-faces, and contain massive normal grading and cross- and

parallel-lamination Bioclasts include brachiopod, gastropod, and ostracod

shells, crinoid stems, and conodont and foraminifer elements The top

of the graded and massive grainstone sequences changes to packstone,

and is overlain by beds of wackestone and marl (lime mudstone) The

wackestone and marls are commonly bioturbated

Facies 2 (Bed 115b) is a dark gray intraclastic and bioclastic

calcirudite (rudstone) of about 10–15 cm thickness (Figs 2, 9) The

ma-trix of the intraclastic rudstone is organic-carbon-rich dark gray

lime-stone, and is quite different from the inorganic gray to whitish gray

limestone of Facies 1 The intraclasts are rounded, gray to whitish gray

limestone granules to cobbles Poorly preserved crinoid stems and

bra-chiopod shells are commonly found in the rudstones The dark gray

intraclastic rudstone beds are characterized by sharp and erosional

basal surfaces Facies 2 is abruptly overlain by alternations of

organic-carbon-rich black shales and dark gray limestones (Beds 115c and

116, respectively) of the older of two Facies 3 sequences Bed 115c is

about 1–4 cm thick and is characterized by laminated

organic-carbon-rich dark gray limestone, commonly containing several thin lenticular

concentrations of shells

Facies 3 comprises two sequences of alternations of thin dark gray

limestones and organic-carbon-rich black shales (Fig 2; lower

se-quence, Beds 115c–120; upper sequence, Beds 126–129) and

character-istically lacks bioturbation The lower Facies 3 sequence (Beds 115c–

120, LADB: lower alternations of dark gray limestones and black shales)

is about 35 cm thick at Loc 01 and about 10–15 cm thick at Loc 02 The

upper Facies 3 sequence (Beds 126–129, UADB: upper alternations of

dark gray limestones and black shales) is 25–30 cm thick at Loc 01

and 20 cm thick at Loc 02 The thin dark gray limestones of this facies

commonly contain thin-shelled brachiopods, various calcareous

micro-fossils (e.g foraminifers), and scattered granule intra-clasts (Figs 6–8)

Facies 3 is characterized by black shales, thin lenticular carbonate

beds (about 2 to 10 cm thick), and limestone lenses (about 1 m to

sev-eral meters wide and 1–20 cm thick) The LABD contains both

thin-bedded and massive dark gray packstones that typically contain

ero-sional surfaces Convolute lamination and secondary deformation are

common in the thin carbonate beds and lenticular layers The thin

lime-stone beds and lenses are overlain by parallel-laminated

organic-5.2 Depositional environments The bioclastic, peloidal, and intraclastic grainstones and packstones

of Facies 1 typically contain shell concentrations composed of abundant shallow marine fossils such as crinoids, corals, and many taxa of bra-chiopods The grainstones reflect accumulation in a setting where cur-rent or wave energy was strong enough to winnow away thefine matrix (Wilson, 1975; Tucker and Wright, 1990) The bioclastic, peloidal, and intraclastic grainstones suggest no accumulation of suspended mud, which indicates that Facies 1 was deposited on a car-bonate ramp above storm wave base (Figs 11, 12) The abundant shell concentrations may represent repetitions of storm events; the bioclastic and intraclastic limestones of Facies 1 likely represent tempestites Normal-graded and massive shell concentrations covered by parallel-laminated sediments are commonly found in tempestites (Aigner,

cross-laminated limestones containing bioclasts and intraclasts are possibly formed by migration of a shelly calcareous sand bar onto the carbonate platform Overlying wackestone and lime mudstone may represent sus-pension deposits after storm events on the carbonate platform above and below storm wave base, respectively

The thin bioclastic and intraclastic lenticular limestones of Facies 3 characterized by erosive basal surfaces are interpreted as minor scour-fill deposits in the marginal basin plain Offshore minor channel and scour-fill deposits are common in offshore muddy facies (Komatsu

gravityflows and probably storm-driven offshore currents, and are later filled by lag deposits of accumulated shell fragments and intraclasts The overlying organic-carbon-rich black shales and marls

of Facies 3 are mostly suspension deposits from storm-driven offshore currents

Facies 2, consisting of intraclastic rudstone (Bed 115b), underlies marginal basin plain deposits surrounding a carbonate ramp (Facies 3), and overlies deep carbonate platform deposits (Facies 1) containing multiple erosive surfaces Abundant poorly preserved shell remains and intraclastic limestone pebbles and cobbles in Facies 2 represent typical transgressive lag deposits The basal erosional surface of Bed 115b is interpreted as a transgressive surface at the base of a transgressive systems tract (TST) Transgressive surfaces are formed by strongly ero-sive waves and currents during a rapid rise of sea level in a shallow ma-rine environment (Nummedal and Swift, 1987; Van Wagoner et al.,

1988;Walker and James, 1992) The basal part of Bed 115c consists of irregularly laminated organic-carbon-rich dark gray limestone contain-ing thin lenticular shell concentrations of poorly preserved shell remains and may represent sediment starvation at the maximum

Fig 7 Facies classification and interpreted environment of deposition of Beds 1–167 of the Pho Han Formation.

flooding surface A typical transgressive sequence and maximum

flooding surface are recorded in the middle to upper parts of the

Siphonodella praesulcata Zone The LADB (Beds 115c–120) consists of

marginal basin plain deposits and the overlying Facies 1 sediments

(Beds 121–125) represent a highstand systems tract (HST) The HST lies mainly within the Upper S praesulcata Subzone and Siphonodella duplicata Zone The basal surface of the UADB (Beds 126–129) probably represents a minor transgression in the Early Tournaisian

Fig 8 Vertical sections and interpretive sketches (1) Parallel-laminated organic-carbon-rich black shale (Facies 3, lower part of Bed 116) Bioturbation is absent (2) Bed 130, small 3-dimensional burrows are common in whitish gray to gray bioclastic limestone (3) Alternations of dark gray limestone and organic-carbon-rich black shale (Beds 116–117) Bioclastic and intraclastic dark gray limestone is overlain by laminated marls and black shales (Bed 117).

5.3 Anoxic to dysoxic facies

In the dark gray limestone and black shale of Facies 3, the TOC

con-tent of the LADB (Beds 115c–120) is about 0.36 to 5.73 wt.% (Bed 116

is 5.73 wt.%) The TOC content of the UADB (Beds 126–129) is about

0.18 to 1.05 wt.% In contrast, the TOC content of Facies 1 is about 0.06

to 0.16 wt.% According toArthur and Sageman (1994), the TOC content

of recent anoxic offshore mudstones is generally more than 1 wt.%

Fur-thermore, these organic-carbon-rich black shales, marls, and dark gray

limestones show no evidence of bioturbation (Figs 8, 10), and they

con-tain pyrite aggregations (e.g Bed 116) Therefore, Facies 2 (Bed 115b)

and Facies 3 may have been deposited under anoxic to dysoxic

condi-tions in a marginal basin plain surrounding a carbonate ramp The

anox-ic to dysoxanox-ic facies of the LADB is clearly within the Upper Siphonodella

praesulcata Subzone and Siphonodella sulcata Zone Although the age of

the UADB is not precisely known, it appears to be of early Tournaisian

age, because the lower part of the Pho Han Formation yields middle

Tournaisian foraminifers (Doan and Tong-Dzuy, 2006)

6 Discussion and concluding remarks

The Hangenberg Event was defined in the Rhenish Massif, Germany,

and occurred during the period from the middle of the Siphonodella

praesulcata Zone to the middle of the Siphonodella sulcata Zone

Hangenberg Black Shale (sensu stricto) in the northern Rhenish Massif

is intercalated within the Middle S praesulcata Subzone (Walliser,

Event corresponds to deposition of the Hangenberg Black Shale (s s.),

which is equivalent to Bed 69 (calcareous oolitic shale) at La Serre,

France, the Global Stratotype Section and Point for the base of the Carboniferous (Brand et al., 2004)

At representative D–C boundary sections (Hasselbachtal area, Rhenish Massif, Kronhofgraben; Carnic Alps, Austria; M'Fis area, south-ern Tafilalt Basin, Anti-Atlas, Morocco), the Hangenberg Black Shale was deposited in a deep sea environment in the form of basin plain, pelagic ramp, or distal turbidite facies (Kaiser et al., 2006, 2011) According to

content (2.10% in Bed 115 at the Hasselbachtal section and 1.31% in Bed 11 at the Kronhofgraben section) and was deposited under anoxic conditions during a phase of maximumflooding

In contrast, the Hangenberg Black Shale (sensu lato) reported in many areas of Europe, Africa, Asia, and North America (Bai and Ning,

organic-carbon-rich dark gray and black shales and black limestone during the latest Devonian (Palmatolepis expansa and Siphonodella praesulcata Zones) to the earliest Carboniferous (early Tournaisian).Caplan and

se-quences in Canada, the United States, Germany, Poland, Russia, and China, which they described as organic-carbon-rich black mudrocks within the P expansa to S praesulcata Zones (Fig 3 ofCaplan and Bustin, 1999).Buggisch and Joachimski (2006)reported on black shales from the P expansa to S sulcata Zones in Laurentia, and from the S praesulcata to S sulcata Zones in Europe and North Africa (Fig 7 in

Buggisch and Joachimski, 2006).Königshof et al (2012)reported equiv-alents of the Hangenberg Event layer in the Mae Sariang section, north-western Thailand from anoxic gray limestones, though anoxic shales are not present in the entire section In the Oberrödinghausen and Drever sections of the Rhenish Massif, Europe, organic-carbon-rich black shales

Fig 9 Vertical section and interpretive sketch of Beds 115a and 115b (Loc 01) Bed 115b consisting of intraclastic pebble to cobble calcirudite characterized by a sharp erosional basal surface.