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

Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo

Devonian–Carboniferous transition containing a Hangenberg Black Shale
equivalent in the Pho Han Formation on Cat Ba Island,
northeastern Vietnam
Toshifumi Komatsu a,⁎, Satoru Kato a, Kento Hirata a, Reishi Takashima b, Yukari Ogata c, Masahiro Oba c,
Hajime Naruse d, Phuong H. Ta e, Phong D. Nguyen f, Huyen T. Dang f, Truong N. Doan f, Hung H. Nguyen g,
Susumu Sakata h, Kunio Kaiho c, Peter Königshof i
a

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

b

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

a b s t r a c t
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 (alternations 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 normal 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 organiccarbon-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 terrestrial environments (Algeo et al., 1995; Hallam and Wignall, 1997; Caplan
and Bustin, 1999; Streel et al., 2000; House, 2002; Brand et al., 2004).
⁎ Corresponding author.
E-mail address: (T. Komatsu).

/>0031-0182/© 2014 Elsevier B.V. All rights reserved.

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;
Becker, 1993, 1996). The Hangenberg Black Shale (sensu lato and
sensu stricto) has been reported from known Devonian lowpaleolatitudinal 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.,
2004; Buggisch and Joachimski, 2006; Kaiser et al., 2011). The


T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

Hangenberg Event records the expansion of an anoxic environment in

the low-latitude oceans (Caplan and Bustin, 1999; House, 2002;
Buggisch and Joachimski, 2006; Kaiser et al., 2006, 2007, 2011;
Königshof et al., 2012), although its ultimate trigger is unknown.

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The Devonian to Carboniferous Pho Han Formation is exposed on Cat
Ba Island, Hai Phong Province, northeastern Vietnam (Figs. 1, 2), where
it preserves a record of the Late Devonian to Early Carboniferous sequence on the continental margin of the eastern Paleotethys Sea. The

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
exposed at Locs. 01 and 02, which are about 90 m apart.


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T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

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

formation is composed mainly of ramp platform carbonates and
slope deposits, and includes the Devonian–Carboniferous (D–C) transition (Ta and Doan, 2005, 2007; Doan and Tong-Dzuy, 2006; Komatsu
et al., 2012a,b). According to Ta and Doan (2005, 2007) and Doan and
Tong-Dzuy (2006), the lowermost part of the Pho Han Formation
(Unit 1 of Doan and Tong-Dzuy, 2006) yields Famennian and early
Tournaisian conodonts and foraminifers. Middle Tournaisian foraminifers are found in the overlying sequence (Unit 2 of Doan and TongDzuy, 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 limestone, 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 by Ta 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,
and field photographs of alternating well-laminated organic-carbonrich black shales and dark gray limestone beds. Moreover, in a preliminary assessment, Komatsu et al. (2012b) identified the basal Carboniferous index fossil S. sulcata in this section and reported on the δ13C curve
for bulk carbonates.
In this study, we report on the Devonian and Carboniferous conodont assemblages and depositional environments of the Cat Co area,
and describe the D–C transition within the alternating organic-carbonrich black shales and dark gray limestone beds of the Pho Han Formation. 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 by Kaiser 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
et al., 2012a,b). The formation yields abundant brachiopods, crinoid
stems, gastropods, cephalopods, corals, conodonts, and foraminifers.
On the Cat Co peninsula and at the Cat Co 3 Beach, outcrops of the lowermost part of the Pho Han Formation consist of fossiliferous whitish
gray to gray limestone intercalated with black chert layers and alternating sequences of organic-carbon-rich black shale and dark gray limestone (Fig. 1). These beds strike WNW–ESE and dip to the NNE. The
D–C transition is within the numbered alternations of dark gray limestone 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 limestones 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 analysis in the field and thin-section analysis in the laboratory for detailed
observation of microfossils, sedimentary structures, and carbonate
petrology. The terminology used here for carbonate petrology, depositional environments, and sequence stratigraphy follows Nummedal
and Swift (1987), Van Wagoner et al. (1988), Tucker and Wright
(1990), Walker and James (1992), and Reading (1996).


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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 containing gastropods, brachiopods, and fragments of crinoid stems.

More than 20 limestone samples (1–2 kg each) were collected from
Beds 105–132 at Loc. 01 for extraction of conodont elements. The samples 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 abundant 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 carbonate 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 analyzer with combustion at 950 °C for 5 min. Hippuric acid was used as the


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T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

standard for CHN calibration. Results of duplicate analyses were confirmed 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 either 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
et al., 2006). Therefore, Bed 115c can be correlated with the Upper
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
contains abundant foraminifers.


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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.

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 common 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 overlies 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 probably 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 typical 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 foraminifers, and solenoporids. Fragments of Pseudochaetetes elliotti and
Parachaetetes sp. are common in Beds 117 and 118 (Fig. 6) in shell


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T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

Fig. 6. Thin sections from the Pho Han Formation (Loc. 01). (1) Parachaetetes sp. in dark gray organic-carbon-rich limestone (Bed 117). (2) Pseudochaetetes elliotti, in dark gray organiccarbon-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 contains poorly preserved thin-shelled brachiopods. Scale bars are 1 mm.

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.
(1987), Bai and Ning (1988), and Hallam and Wignall (1997) reported
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 marine carbonate platform facies above storm wave base and below fair
weather wave base (Yu et al., 1987). During the earliest Carboniferous,
minute solenoporids probably flourished in a shallow sea and accumulated 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.


T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

5. Depositional environments and anoxic to dysoxic facies in the
Devonian to Carboniferous transition of the Pho Han Formation

37

carbon-rich black shales and dark gray marls. The lenticular beds and
lenses contain minor scour fills of dark gray bioclastic and intraclastic
packstone.

5.1. Depositional facies
5.2. Depositional environments
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 limestones (WGL) and micritic limestones 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 and flat basal surfaces, and contain massive normal grading and cross- and parallellamination. 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 matrix of the intraclastic rudstone is organic-carbon-rich dark gray limestone, 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 brachiopod 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 organiccarbon-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-carbonrich 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 sequence, Beds 115c–120; upper sequence, Beds 126–129) and characteristically 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 microfossils (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 several meters wide and 1–20 cm thick). The LABD contains both thinbedded and massive dark gray packstones that typically contain erosional surfaces. Convolute lamination and secondary deformation are
common in the thin carbonate beds and lenticular layers. The thin limestone beds and lenses are overlain by parallel-laminated organic-

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 brachiopods. The grainstones reflect accumulation in a setting where current or wave energy was strong enough to winnow away the fine
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 carbonate 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 parallellaminated sediments are commonly found in tempestites (Aigner,
1982; Kreisa and Bambach, 1982; Walker and James, 1992). The crosslaminated 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 suspension 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 scourfill deposits in the marginal basin plain. Offshore minor channel and
scour-fill deposits are common in offshore muddy facies (Komatsu
et al., 2008). The erosive channels and scours are formed by passing
gravity flows 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 erosive waves and currents during a rapid rise of sea level in a shallow marine 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 containing 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.


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T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

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 3dimensional 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).

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.


T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

39

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.

5.3. Anoxic to dysoxic facies
In the dark gray limestone and black shale of Facies 3, the TOC content 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 to Arthur and Sageman (1994), the TOC content
of recent anoxic offshore mudstones is generally more than 1 wt.%. Furthermore, these organic-carbon-rich black shales, marls, and dark gray
limestones show no evidence of bioturbation (Figs. 8, 10), and they contain pyrite aggregations (e.g. Bed 116). Therefore, Facies 2 (Bed 115b)
and Facies 3 may have been deposited under anoxic to dysoxic conditions in a marginal basin plain surrounding a carbonate ramp. The anoxic to dysoxic 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
(Walliser, 1984; Becker, 1993, 1996; Kaiser et al., 2006, 2011). The

Hangenberg Black Shale (sensu stricto) in the northern Rhenish Massif
is intercalated within the Middle S. praesulcata Subzone (Walliser,
1984; Kaiser et al., 2006, 2011). The beginning of the Hangenberg
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, southern 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
Kaiser et al. (2006, 2011), the Hangenberg Black Shale has high TOC
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 maximum flooding.
In contrast, the Hangenberg Black Shale (sensu lato) reported in
many areas of Europe, Africa, Asia, and North America (Bai and Ning,
1988; Walliser, 1996; Brand et al., 2004; Buggisch and Joachimski,
2006; Bahrami et al., 2011) reflects the accumulation of organiccarbon-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
Bustin (1999) correlated what they called Hangenberg Black Shale sequences 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 of Caplan 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 equivalents of the Hangenberg Event layer in the Mae Sariang section, northwestern 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


40

T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

Fig. 10. Thin sections from the Pho Han Formation (Loc. 01). (1) Organic-carbon-rich laminated black shale (lower part of Bed 116). (2) Organic-carbon-rich laminated black shale (upper
part of Bed 116). (3) Peloidal and bioclastic grainstone (lower part of Bed 121). Articulated ostracods (O) are common. (4) Organic-carbon-rich bioclastic and intraclastic packstone (lower
part of Bed 119). Thin-shelled brachiopods (B) are common. (5) Organic-carbon-rich silty black shale (upper part of Bed 122). (6) Peloidal, bioclastic, and intraclastic grainstone (lower
part of Bed 132). Scale bars are 1 mm.

are within the Upper S. praesulcata Subzone (Walliser, 1996; Buggisch
and Joachimski, 2006).
In the Cat Co 3 section, Vietnam, the Hangenberg Black Shale equivalent consists of alternations of dark gray limestone and organiccarbon-rich black shale (Beds 115c–120) and transgressive lag deposits
(Bed 115b), and was deposited in the middle or upper parts of the
Siphonodella praesulcata to Siphonodella sulcata Zones. This sequence
was probably deposited under anoxic to dysoxic conditions in a marginal basin plain surrounding an outer carbonate platform in a TST
(transgressive lag deposits of Facies 2) and the early stage of an HST
during the latest Devonian to earliest Carboniferous. There was probably a hiatus just before deposition of Bed 115b as a result of rapid

transgression. Van Steenwinkel (1993) reported that the base of the
Hangenberg Black Shale coincides with a deep marine flooding surface at the base of a TST. Furthermore, he interpreted the accumulations of Hangenberg Black Shale to represent condensed sections
deposited during a period of maximum flooding. In the southern Tafilalt
area, Morocco, several ammonoid zones are absent just below the
Hangenberg Black Shale, indicating a hiatus influenced by rapid transgression in a deep sea environment characterized by turbidite facies
(Kaiser et al., 2011).
Modern slope and marginal basin plain environments are likely to be
depleted in dissolved oxygen as a result of past oceanographic changes,
such as increased surface ocean productivity and/or stagnation of

T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

vertical mixing (e.g. Arthur and Sageman, 1991). In addition, rapid
eustatic sea-level rise during a global climatic warming in the latest Devonian probably enhanced the expansion of anoxic to dysoxic water
masses and caused widespread deposition of the Hangenberg Black
Shale (Kaiser et al., 2006). Rapid transgressions during global warming
were important triggers of Cretaceous oceanic anoxic events (OAEs)
in the early Aptian (OAE 1a), earliest Albian (OAE 1b), and at the
Cenomanian–Turonian boundary (OAE 2), because leaching of nutrients
on coastal lowlands during a transgression leads to increased fertilization and productivity in the adjacent oceanic basin (e.g. Erbacher
et al., 1996). The formation of large igneous provinces is considered to
have been the trigger for the major OAEs of the Cretaceous Period, because they not only caused global warming and sea-level rise but also
promoted primary productivity by emitting sulfate and bio-limiting
elements (e.g. Leckie et al., 2002; Takashima et al., 2006; Adams et al.,
2010).
In contrast, it is difficult to evaluate the complex mechanisms that
led to the latest Devonian to earliest Carboniferous deposition of
the thick organic-carbon-rich black shales and dark gray limestones
(Hangenberg Black Shale, s. l.) that have been recognized in Vietnam
and many other parts of the world (Caplan and Bustin, 1999; Bahrami
et al., 2011). Caplan and Bustin (1999) proposed hypothetical scenarios
of events leading to and consequences of the Hangenberg Event. In their
reasonable scenarios (and the proposed consequences of Jewell, 1995),
equatorial upwelling currents and sea-level changes during glacial
events may have strongly influenced the expansion of anoxic to dysoxic
water masses. In fact, short-lived glacial events and rapid sea-level
fluctuations in the upper part of the Middle S. praesulcata Subzone
to Siphonodella sulcata Zone have been reported (e.g. Powell and

Veevers, 1987; Streel et al., 2000; Isaacson et al., 2008). Brand et al.

41

(2004) pointed out that the onset and duration of the latest Devonian
glacial events strongly affected sea-level fluctuations and subtropical
climatic changes.
Devonian to Carboniferous sections of the Pho Han Formation on
the South China Block were deposited in paleoequatorial to lowpaleolatitude regions of the eastern part of Paleotethys (Metcalfe,
1998, 2009). Generally, intensified ocean upwelling, enhanced primary
productivity, and deposition of organic matter are caused by the steep
latitudinal thermal gradient during glacial events (e.g. Steph et al.,
2010). In Vietnam, the accumulation of the Hangenberg Black Shale
equivalent during the latest Devonian to earliest Carboniferous appears
to have been strongly influenced by the latest Famennian glaciation in
northeastern Gondwana.
Intercalations of near-shore sediments just below the D–C boundary
provide evidence of a major regression and significant shallowing after
deposition of the Hangenberg Black Shale (s. s.). In Europe and Northwest Africa, in particular, the Hangenberg Sandstone was deposited as
intercalations in the upper part of the Middle and Upper Siphonodella
praesulcata Subzones during a short-term eustatic sea-level fall influenced by glaciation. However, this regressive phase and sea-level fall
are not recorded in the Cat Co sections. A major regression is recognized
in the Siphonodella sulcata to Siphonodella duplicata Zones of the Pho
Han Formation (Bed 121), but not in the S. praesulcata Zone. The lack
of development of the latest Devonian regressive phase in Vietnam
probably reflects local tectonic movements. Continental collisions
between the South China and Indochina blocks took place in the
Ordovician–Devonian and Permian–Triassic (Osanai et al., 2004, 2008).
Although in our preliminary assessment we did not consider extinction events in the middle and late Famennian or faunal recovery in the
early Tournaisian, the faunal composition of mega- and microfossils in

Fig. 11. Sequence stratigraphic interpretation of Beds 115–116. Organic-carbon-rich black shale and intraclastic calcirudite (rudstone) were deposited at and near the maximum flooding
surface (MFS). This sequence separates a transgressive systems tract (TST) from a highstand systems tract (HST) in a marginal basin plain environment surrounding a carbonate platform.
Thin shell concentrations composed of epifaunal thin-shelled brachiopods and microfossils (Bed 115c) may represent a condensed section (CS) associated with sediment starvation. Sharp
erosive basal surfaces ,transgressive surface, TS) of Bed 115b probably formed as a result of rapid transgression, and are overlain by transgressive lag deposits consisting of intraformational
pebble and cobble limestones, poorly preserved shell remains, fragments of crinoid stems, and microfossils.


42

T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43

Fig. 12. Reconstruction of depositional environments of the Pho Han Formation. The marginal basin plain environment is characterized by anoxic conditions around the D–C boundary.
Pseudochaetetes and Parachaetetes were probably transported from the shallow ramp to the anoxic marginal basin by gravity flow.

the Pho Han Formation changed markedly around the D–C transition. In
particular, fossils and bioturbation are absent or rare in the black shales
of Beds 116–120, indicating that the activity of benthic organisms had
decreased around the D–C transition, at least in the marginal basin
plain environment. The thin shell concentrations intercalated within
the organic-carbon-rich dark gray limestone contain poorly preserved
thin-shelled brachiopods and calcareous microfossils, most of which
were probably transported from the shallow sea to the marginal basin
plain environment (Fig. 12). The faunal composition of these shell
beds and the numbers of microfossils such as conodonts and allochthonous solenoporids clearly changed at the D–C transition. Although the
taxonomy of minute solenoporids, including Pseudochaetetes and
Parachaetetes, is problematic, they appear to be calcareous algae
(Rhodophyta) or sponges (Riding, 2004; Higa et al., 2010). Calcareous
algae and sponges are important members of reef and calcareous
mound-building communities in shallow Paleozoic seas. In particular,

binding calcareous algae took over the role of reef builders after the
Frasnian and Famennian reef collapse (Chuvashov and Riding, 1984;
Fagerstrom, 1994; Webb, 1998). The Devonian reef ecosystems collapsed during the Frasnian–Famennian (F–F) extinction event. Furthermore, the aftermath of the F–F event and Famennian anoxic events
possibly impeded the recovery of reef ecosystems in the earliest
Carboniferous (Hallam and Wignall, 1997). However, Pseudochaetetes
and Parachaetetes may have been pioneer reef builders during the
early recovery stage. These genera are found only in the Siphonodella
sulcata Zone of the Nanbiancun section of South China (Yu, 1988;
Mamet, 1992). In the eastern part of Paleotethys during the earliest
Carboniferous, several species of minute solenoporids possibly formed
minor reefs and calcareous mounds on the shallow carbonate platform
surrounding anoxic offshore facies. In Vietnam, studies of the role of
minute solenoporids are the key to understanding the recovery of reef
ecosystems and shallow marine fauna in the earliest Carboniferous.
The Upper Devonian to Lower Carboniferous shallow marine carbonates
are known to yield exceptional reef builders (Nguyen, 2003; Doan and
Tong-Dzuy, 2006). We need to undertake more research to evaluate
the relationship between extinction and recovery events around the
Middle to Upper Famennian to understand the complex patterns of climate change in the eastern part of Paleotethys.
Acknowledgments
We thank the staff of both the Vietnam Institute of Geosciences and
Mineral Resources (VIGMR) and the Vietnam National Museum of Nature (VNMN) for their cooperation both in the field and in the laboratory. We gratefully thank Hisayoshi Igo (Institute of Natural History) for
the useful comments about conodont taxonomy. Our special thanks go
to anonymous referees. This study was supported by the JSPS-VAST
Joint Research Program, Grants-in-Aid for Encouragement of Young
Scientists (No. 20740300) from the Japanese Ministry of Education,
Culture, Sports, Science, and Technology, and Grants-in-Aid for Scientific Research (No. 25400500) from the Japan Society for Promotion of

Science. This paper is a contribution to IGCP 596 (Climate change and
biodiversity patterns in the mid-Paleozoic).

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