Upper Devonian carbonates deposited through the Frasnian/Famennian (F/F) stage boundary in the Xom Nha Formation, Central Vietnam, were studied. The section is mainly composed of fossiliferous, brecciated, and laminated limestone beds, while shale beds occur in a subordinate number. Microfacies generally suggests a hemipelagic setting on an outershelf environment with low sedimentation rates. A sediment accumulation rate of 0.217 cm/kyrs for this section is calculated. The Xom Nha section does not exhibit characteristic black shales or black limestones through the F/F boundary, but instead shows a similar lithology in comparison to other F/F sections in Southeast Asia. The carbonates yielded abundant conodonts, which represent mainly cosmopolitan species but the section differs from most sections in Europe in showing relatively high numbers of Palmatolepis linguiformis species. Deposition through the F/F boundary interval reported here for the Xom Nha section appears to have occurred during a ~1.2 Ma, and shows well-defined climate cyclicity.
Location and geological context of the studied section in Central Vietnam(Minh Hoa District, Quang Binh Province). Asteriskmarks the position of the Xom Nha section (N 17°48′26,9″, E 105°56′41,1″; ± 3m)
Field photographs of the Xom Nha Formation bracketing the F/F interval: a limestones of the Len Don mountain and entrance to the cave, b taking samples with a rock sawat the steep cliff, c monotonous marl and limestone above the F/F boundary, d details of the F/F boundary within layer 18 (boundary marked in red): sampled beds are shown with yellow numbers; white numbers are from earlier field studies that were correlated with the new sampling in 2017. The F/F boundary is placed within bed 18 (18b contains the first conodont Palmatolepis triangularis)
Important conodonts of the Xom Nha section (scale bares = 100 μm): a. Palmatolepis hassi (sample VN-XN-2); b. Ancyrodella nodosa (sample VN-XN-2); c. Ancyrodella nodosa (sample VN-XN-2); d. Ancyroides cf. leonis (broken specimen; sample VN-XN-3); e. Ancyrodella nodosa (sample VN-XN-2); f. Ancyrognathus triangularis (broken specimen, sample VN-XN-3); g. Ancyrognathus primus (broken specimen, sample VN-XN-5); h. Ancyrognathus triangularis (sample VN-XN-6); i. Ancyrognathus triangularis (lower view, sample VN-XN-6); j. Palmatolepis proversa (sample VN-XN-6); k. Ancyrognathus aff. asymmetricus (sample VN-XN-6); l. Polygnathus decorosus (sample VN-XN-8); m. Ancyrodella nodosa (sample VN-XN-8); n. Ancyrodella ioides (sample VN-XN-8); o. Palmatolepis gigas (sample VN-XN-6); Ancyrodella nodosa (sample VN-XN-8)
Important conodonts of the Xom Nha section (scale bares = 100μm): a. Ancyrodella ioides (sample VN-XN-9); b. Polygnathus decorosus (sample VN-XN-11); c. Ancyrognathus seddoni (sample VN-XN-11); d. Palmatolepis unicornis (sample VN-XN-11); e. Palmatolepis cf. Foliacea (sample VN-XN-11); f. Palmatolepis cf. hassi (sample VN-XN-11); g. Palmatolepis subrecta (sample VN-XN-13); h. Ancyrognathus seddoni (sample VN-XN-12); i. Palmatolepis rhenana rhenana (sample VNXN-12); j. Ancyrodella nodosa (sample VN-XN-13); k. Palmatolepis subrecta (sample VN-XN-14); l. Palmatolepis eureka (sample VN-XN-13); m. Palmatolepis juntianensis (sample VN-XN-13); n. Palmatolepis juntianensis (sample VN-XN-14); o. Palmatolepis subrecta (sample VNXN-15); p. Palmatolepis ederi (sample VN-XN-15)
Important conodonts of the XomNha section (scale bares = 100 μm): a. Polygnathus tenellus (sample VN-XN-15); b. Ancyrognathus seddoni (sample VN-XN-16b); c Palmatolepis linguiformis (sample VN-XN-16b); d. Palmatolepis linguiformis (sample VN-XN-17a); e. Ancyrodella curvata (sample VN-XN-17a); f. Polygnathus webbi (sample VN-XN-17a); g. Palmatolepis linguiformis (sample VN-XN-18a); h. Ancyrodella curvata (sample VN-XN-18a); i. Icriodus alternatus (sample VN-XN-18a); j. Palmatolepis triangularis (sample VN-XN-18b); k. Icriodus alternatus alternatus (sample VN-XN-18b); l. Polygnathus brevilaminus (sample VNXN-18b); m. Icriodus alternatus (sample VN-XN-19); n. Icriodus alternatus helmsi (sample VN-XN-19); o. Palmatolepis triangularis (sample VN-XN-19); p. Ancyrognathus sinelaminus (sample VN-XN-21c)
Important conodonts of the Xom Nha section (scale bares = 100μm): a Palmatolepis cf. delicatula (sample VN-XN-21c); b. Palmatolepis protorhomboidea (sample VN-XN-21c); c. Polygnathus brevilaminus (sample VN-XN-21c); d. Icriodus alternatus alternatus (sample VN-XN-21c); e. Ancyrognathus sinelaminus (sample VN-XN-21c); f. Icriodus alternatus helmsi (sample VN-XN-21c); g. Palmatolepis crepida (sample X19, reillustrated from Ta Hoa (2002), figure II.4.18-9); h. Palmatolepis quadrantinodosalobata (sample X20 reillustrated from Ta Hoa (2002), figure II.4.18-10)
Source: Hoa Phuong Ta, Peter Königshof, Brooks B. Ellwood, Thang C. Nguyen, Phuong Lan T. Luu, Dinh Hung Doan, Ariuntogos Munkhjargal. 2021. Facies, magnetic susceptibility and timing of the Late Devonian Frasnian/Famennian boundary interval (Xom Nha Formation, Central Vietnam). https://doi.org/10.1007/s12549-021-00506-y
News: Centre for Earth Sciences (CES), Vietnam National Museum of Nature, VAST