Devonian–Carboniferous transition containing a Hangenberg Black Shale equivalent in the Pho Han Formation on Cat Ba Island, northeastern Vietnam

Abstract

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 organic-carbon-rich black shale and minor scour-filling bioclastic, peloidal, and intraclastic packstone, and may represent a marginal basin plain environment surrounding a carbonate ramp. The alternations of organic-carbon-rich black shale and dark gray packstone (Facies 3) show no evidence of bioturbation and have high TOC contents (0.18–5.73wt.%). 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.