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...

Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 Contents lists available at ScienceDirect 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 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 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 (alternations of whitish gray to gray limestone and marl), Facies (calcirudite, Bed 115b), and Facies (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 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 is represented by typical lag deposits overlying a transgressive surface Facies 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 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 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: komatsu@sci.kumamoto-u.ac.jp (T Komatsu) http://dx.doi.org/10.1016/j.palaeo.2014.03.021 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 31 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 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 32 T Komatsu et al / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 Fig Detailed columnar sections of the Devonian to Carboniferous transition See facies classifications in Fig 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 of Doan and Tong-Dzuy, 2006) yields Famennian and early Tournaisian conodonts and foraminifers Middle Tournaisian foraminifers are found in the overlying sequence (Unit of Doan and TongDzuy, 2006) The section that includes the D–C transition is in the Cat Co area (near Cat Co 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 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 area We correlate this sequence in the Cat Co 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 Geologic setting in the Cat Co 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 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) 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) T Komatsu et al / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 33 Fig 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 (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 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 N HCl was pipetted into each sample boat until carbonate was fully removed The samples were then heated to 80 °C for at least h to drive off HCl and water before analysis with the CHN analyzer with combustion at 950 °C for Hippuric acid was used as the 34 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% 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 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 T Komatsu et al / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 35 Fig Conodonts from the Pho Han Formation (Loc 01) (1) and (2) Palmatolepis sigmoidalis Ziegler; from Bed 115a, 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; from Bed 115c, 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 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 36 T Komatsu et al / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 Fig 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 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 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 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 (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 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 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 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 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 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 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 is characterized by black shales, thin lenticular carbonate beds (about to 10 cm thick), and limestone lenses (about 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 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 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 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 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 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 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 Facies classification and interpreted environment of deposition of Beds 1–167 of the Pho Han Formation 38 T Komatsu et al / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 30–43 Fig 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 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 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 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 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 (Bed 115b) and Facies 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) 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 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 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 mm are within the Upper S praesulcata Subzone (Walliser, 1996; Buggisch and Joachimski, 2006) In the Cat Co 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) References Adams, D.D., Hurtgen, M.T., Sageman, B.B., 2010 Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic Event Nat Geosci 3, 201–204 Aigner, T., 1982 Calcareous tempestites: storm-dominated stratification, Upper Muschelkalk limestones (Middle Triassic, SW Germany) In: Einsele, G., Seilacher, A (Eds.), Cyclic and Event Stratification Springer-Verlag, Berlin, pp 180–198 Algeo, T.J., Berner, R.A., Barry Maynard, J., Schecler, S.E., 1995 Late Devonian oceanic anoxic events and biotic crisis: “rooted” in the evolution of vascular land plants? 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Sea The Fig 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. .. 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... 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