Artinskian conodonts from the Dingjiazhai Formation of the Baoshan Block, west Yunnan, southwest China

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 bisselli (Clark and Behnken), and an unidentified ramiform element. Based on the known stratigraphic distribution of 5. 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 Paleoequatorial Tethyan and cool/cold Gondwanan realms, and which became well developed during Middle Permian time.

Early Permian Conodonts from the Baoshan Block, Western Yunnan, China

“…Reimers (1991) assigned the fauna to the Bolorian (Kungurian). Ueno et al (2002) reinterpreted the (Youngouist, Hawley et Miller) by Reimers (1991, pl. 1, fig.…”

“…14 lateral platforms. Ueno et al (2002) suggested that the number of accessory nodes is a variable morphological feature, and is, therefore, not a good criterion for species discrimination. We think that Ueno et al's opinion about the variance of accessory nodes is reasonable, and therefore classified our specimens with only one accessory node to Rabeignathus bucaramangus.…”

“…In the Baoshan Block, Ueno et al (2002) illustrated specimens that have simple configuration of accessory nodes on the cup. They concluded, these morphological features could indicate that the Pa. elements of R. bucararnengus from the Baoshan Block are more primitive representatives of this species.…”

“…The finding of conodont fossils from the Dingjiazhai Formation provided another means of determining the age of this formation. Wang et al (2000) and Ueno et al (2002) described conodonts from the limestones of the top part of the Dingjiazhai Formation from the Dongshanpo and Dingjiazhai sections, and assigned the upper part of the Dingjiazhai Formation to the middle Artinskian corresponding to the upper part of the Sweetognathus whitei-Mesogondolella bisselli Zone. However, the fusulinids from the top part of the Dingjiazhai Formation indicate probably a late Sarkmarian age in the traditional view of fusulinoidean biochronology (Ueno et al, 2002).…”

Lopingian (Late Permian) foraminiferal faunal succession of a Paleo‐Tethyan mid‐oceanic carbonate buildup: Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China

“…In contrast, the Baoshan and Tengchong blocks of the western part of West Yunnan show strong Gondwanan affinities in terms of both stratigraphy and fossil faunas and floras of Carboniferous-Permian time (Jin 1994;Wang et al 2001). These two blocks are considered to form the fragments in West Yunnan of the Cimmerian continent (Ueno 1999(Ueno , 2003, which was separated from Gondwanaland at around late Early Permian time (Ueno et al 2002). The Changning-Menglian Belt between the Cathaysia-related tectonic units and Gondwana-derived blocks represents, in a broad sense, the remnant of the closed main Paleo-Tethys in this area Wu et al 1995;Metcalfe 1999;Ueno 1999;Ueno et al 2003).…”