越南東北部Cat Ba島泥盆系-石炭系過渡層的初探

The Pho Han Formation is exposed on southern Cat Ba Island, Hai Phong Province in northeastern Vietnam, and intercalates the Devonian and Carboniferous (D-C) boundary (Ta and Doan, 2007; Komatsu et al., 2012). The D-C boundary section consists mainly of limestone beds, numbered from 1 to 167, intercalated with alternating black organic-rich shales. The limestone yields abundant brachiopods, crinoid-stems and conodonts. Preliminary investigations on stratigraphy (conodont biostratigraphy andδ13C) and sedimentology of beds 113-133 were undertaken in this study.

Ta and Doan (2007) reported that the Late Devonian (late Famennian) conodont assemblage was composed ofPalmatolepis expansa,Palmatolepis gracilis,Palmatolepis sigmoidalisandRhodalepis polylophodontiformis(bed 115) and the Early Carboniferous conodontSiphonodella duplicate(bed 122).We collected the basal Carboniferous index conodontSiphonodella sulcataandPolygnathusspp. (bed 119)and abundant Late Devonian conodonts (bed 115).

Carbonates here are characterized by a typical turbidite sequence, consisting of graded turbidites and hemipelagic marls intercalating debris-flow deposits.The graded turbidites and hemipelagic marls in beds 1-115 and 129-167 frequently contain bioturbations,but the alternating black shales and dark gray limestone beds (bed numbers 116-129), each measuring approximately 90 cm, are characterized by rare bioturbation. These black shales and dark gray limestone may have been accumulated in anoxic facies, and the black shales contained pyrite grains and well-preserved laminations.

Aδ13C curve of bulk carbonate is presented for around the D-C boundary, and three positive excursions inδ13C with amplitudes of +1-1.5‰ were observed in the intervals of beds 114-116, 117-119 and 124-129. These positiveδ13C excursions are found in the accumulations within the alternating black shales and dark gray limestones.

TA H P, DOAN N T. 2007. Discussion on the Devonian/ Carboniferous boundary at the south of Cat Ba Island. Journal of Geology, Series A, 298: 12-17.

KOMATSU T, TAKASHIMA R, NARUSE H, TA H P, DANG T H,NGUYEN D P, DINH C T, HO T C, TRAN T V, KATO S,MAEKAWA T, GATOVSKY Y. 2012. Devonian-Carboniferous transition in the Pho Han Formation on Cat Ba Island, northeastern Vietnam. The Journal of Geological Society of Japan,118: 5-6.

Devonian to Carboniferous Transitional Beds on Cat Ba Island,Northeastern Vietnam–A Preliminary Assessment

Toshifumi KOMATSU1), Reishi TAKASHIMA2), TA Hoa PHUONG3), Tran Huyen DANG4),Duc Phong NGUYEN4), Huu Hung NGUYEN5), Satoru KATO1), Kento HIRATA1),Takumi MAEKAWA1)
1)Graduate School of Science and Technology, Kumamoto University, Kumamoto806-8555,Japan;
2)The Center for Academic Resources and Archives Tohoku University Museum, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai980-8578,Japan;
3)College of Sciences, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi Vietnam;
4)Vietnam Institute of Geosciences and Mineral Resources (VIGMR), Hanoi, Vietnam;
5)Vietnam National Museum of Nature (VNMN), Hanoi, Vietnam

D-C boundary; Conodont;δ13C; Cat Ba Island; Vietnam

10.3975/cagsb.2012.s1.19