Artinskian conodonts from the Dingjiazhai formation of the Baoshan block, west Yunnan, southwest China

Journal of Paleontology, Jul 2002 by Ueno, Katsumi, Mizuno, Yoshihiro, Wang, Xiangdong, Mei, Shilong

ABSTRACT-Permian conodonts were recovered for the first time from the Dingjiazhai Formation, a well-known diamictite-bearing stratigraphic unit in the Gondwana-derived Baoshan Block in West Yunnan, Southwest China. The conodont fauna occurs in limestone units within the upper part of the formation and consists of Sweetognathus bucaramangus (Rabe), S. whitei (Rhodes), Mesogondolella bis.selli (Clark and Behnken), and an unidentified ramiform element. Based on the known stratigraphic distribution of S. bucaramangus (Rabe), the fauna is referable to the upper Sweetognathus whitei-Mesogondolella bisselli Zone, and thus is dated as middle Artinskian according to the current definition of the stage. The Dingjiazhai Formation is overlain paraconformably by the Woniusi Formation, which is represented mostly by basalts and basaltic volcaniclastics related to rifting volcanism during the separation of the Baoshan Block from Gondwanaland. The present discovery of conodonts from the upper part of the Dingjiazhai Formation reveals that the glaciogene diamictites in the Dingjiazhai Formation are older than middle Artinskian, and the inception of rifting volcanism of the Baoshan Block is later than middle Artinskian.

Occurrence of an essentially warm water element, Sweetognathus bucaramangus (Rabe), in the Dingjiazhai conodont assemblage notwithstanding, the entire fossil faunas including brachiopods and fusulinoideans from the limestone units of the formation can be best interpreted as a middle latitudinal, non-tropical, and still substantially Gondwana-influenced assemblage developed at the northern margin of Gondwanaland just after deglaciation in the southern hemisphere during Early Permian time. This time could be regarded as the beginning of the Cimmerian Region, which had mixed or transitional paleobiogeographic characteristics between the Paleo-- equatorial Tethyan and cool/cold Gondwanan realms, and which became well developed during Middle Permian time.

INTRODUCTION

THE BAOSHAN Block of West Yunnan, Southwest China, is well known as a Gondwana-derived continental fragment (e.g., Jin, 1994; Wang et al., 2001). It constituted part of the Cimmerian continent, along with Sibumasu of Southeast Asia, Tengchong of West Yunnan, Qiangtang (and probably Lhasa) of Tibet, the South Pamir, South Afghanistan, Iran between the Alborz and Zagros Mountains, Transcaucasia, and some parts of Iraq and Turkey (Sengor, 1984; Metcalfe, 1996, 1999). In the Baoshan Block, Late Paleozoic strata are widely distributed, and show a typical periGondwanan (or Cimmerian) type of sequence, which comprises a large late Carboniferous hiatus, subsequent Lower Permian siliciclastic-dominant strata yielding cool-water faunas and possibly glaciogene diamictites, and overlying thick basaltic lavas and volcaniclastics related to rifting volcanism. The diamictite-bearing siliciclastic sequence of the block is called the Dingjiazhai Formation, and has been studied by many students with respect to sedimentology, stratigraphy, and paleontology (e.g., Zhou and Fang, 1990; Fang and Fan, 1994; Jin, 1994; Shi et al., 1996; Yang, 1999; Zhou et al., 1999; Fang et al., 2000; Shen et al., 2000).

Wang (1983) first reported the existence of possibly glaciogene pebbly siliciclastics yielding brachiopod, fusutinoidean, and bryozoan remains from the Dingjiazhai Formation in the Baoshan Block. He dated the formation as Late Carboniferous (Mississippian) mainly because of the occurrence of Triticites. This Triticites-based assignment was followed in many subsequent studies (Yang, 1983; Chen, 1984; Cao, 1986; Fang and Fan, 1994; Jin, 1994). However, Nie et al. (1993) interpreted the Triticites to have been reworked from older strata and consequently dated the formation as Early Permian on the basis of the brachiopod Stereochia litostyla Grant. The recent, more detailed study by Shi et al. (1996) and Shen et al. (2000) of brachiopods from the formation confirmed the Early Permian (probably late Sakmarian to Artinskian) age for limestone units in its upper part. Sugiyama and Ueno (1998), Wang et al. (1999, 2000, 2001), and Ueno (2000), based on re-examination of fusulinoideans from the same limestone, also proposed an Early Permian age for the upper part of the Dingjiazhai Formation. From these recent works, an Early Permian age for the formation would appear to be more likely, but at the same time, more accurate dating on the basis of fossils with high biostratigraphic value, such as conodonts, is very desirable.

Recently, we found an Artinskian conodont fauna in limestone units in the upper part of the Dingjiazhai Formation, where brachiopods and fusulinoideans including "Triticites" had been reported. This Artinskian fauna provides strong chronologic constraints on the timing of the deposition of glaciogene diamictites in the Dingjiazhai Formation and the eruption of rifting-related basalts found in the overlying Woniusi Formation. In this paper, we describe and illustrate the conodont fauna, and discuss its age and significance for the Permian history of the Baoshan Block.