Trilobites Across the Cambrian-Ordovician Boundary of the Transitional Region of Sandu, Southeastern Guizhou
The strata and fossils between the Cambrian and Ordovician boundany outcropping in the northwest region of the Sandu district are transitional both in biofacies and lithofacies. Two sections, about 8 km apart, were measured. One at Pu’an, here the late Up
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TRILOBITES ACROSS THE CAMBRIAN-ORDOVICIAN BOUNDARY OF THE TRANSITIONAL REGION OF SANDU, SOUTHEASTERN GUIZHOU Lu Yanhao (Nanjing Institute of Geology and Palaeontology, Academia Sinica)
Zhou Tianrong (The 8th Geologic Research and Prospecting Team, Ministry of Geology and Mineral Resources, Guiyang)
Abstract The strata and fossils between the Cambrian and Ordovician bound any outcropping in the northwest region of the Sandu district are transitional both in biofacies and lithofacies. Two sections, about 8 km apart, were measured. One at Pu'an, here the late Upper Cambrian Maotian Formation contains dolomites, limestones, argillaceous dolostones, dolomitic and sandy mudstones with about 75% non-agnostid trilobites of the Yangtze Subprovince of the North China Faunal Province, 16.7% of the Jiangnan Subprovince of the Southeast China Faunal Province and 8.3% of endemic forms; while the early Tremadocian Guotang Formation is chiefly composed of striped dolostones and argillaceous dolostones bearing 22.2% non-agnostid trilobites of the Yangtze Subprovince, 72.2% of the Jiangnan Subprovince and 5.6% of the endemic elements. In the other section at Shizilu, the late Upper Cambrian Sandu Formation consists of thin-bedded striped argillaceous dolostones, calcareous mudstones, arenaceous dolostones and limestones with about 20% nonagnostid trilobites of the Yangtze Subprovince, 70% of the Jiangnan Subprovince and 10% of endemic forms; whereas the early Tremadocian Guotang Formation is almost entirely composed of thin-bedded striped argillaceous dolomites with very few mudstones containing 12.5% non-agnostid trilobites of the Yangtze Subprovince and 87.5% of the Jiangnan Subprovince. 57 species and 33 genera and subgenera of trilobites are described, among them, 18 species, 3 genera and 2 subgenera are new.
INTRODUCTION In the vicinity of the Sandu city at a locality between the Sandu Middle School and the Electric Power Station, the Upper Cambrian and Lower Ordovician strata are well developed, and the name the Sandu Formation of the Upper Cambrain and the name the Guotang Formation of the Lower Ordovician were proposed by Lu in 1954 and by Chien, Lee and Lee in 1959 respectively. The Cambrian and Ordovician boundary is tentatively drawn on the base of the Hysterolenus Zone and above the Hedinaspis Zone by Lu and Qian (in Lu and Mu, 1980). But the contact line between the Lower Ordovician and Upper Cambrian was later found to be a fault. While in the geological mapping work from 1977 to 1980 in the Pu'an area, about 18 km northwest of the Sandu city, Messrs. Wang Pu, Zhang Shu-Yuan, Lin Cao-an, Peng Yen-wei and the junior writer discovered two sections in which the Cambrian-Ordovician boundary is conformable in contact and they are monocline in stratigraphic succession, These sections are restudied in detail and the fossils are collected bed by bed in the boundary interval by the junior writer and his colleagues in the end of 1982. As it has long been known in China, the Cambrian and Ordovician faunas are classified into t
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