CAMBRIAN - EARLY ORDOVICIAN BRACHIOPODS FROM MALYI KARATAU, THE WESTERN BALKHASH REGION, AND TIEN SHAN, CENTRAL ASIA

Abstract

Carbonate platform deposits of the Shabakty Group in the Malyi Karatau Range of southern Kazakhstan contain rich brachiopod faunas of early Cambrian (Botomian)-early Ordovician (Arenig) age. Seven biostratigraphically successive assemblages are defined across this interval. Four further discrete microbrachiopod assemblages occur in Middle-Upper Cambrian sequences of various adjacent Central Asian regions, including the Suukadyr Mountains of the southern Betpak-Dala Desert (Darbaza Formation), the western side of Lake Balkhash (Zhalgyz Formation), north-west Balkhash (Sarykumy Formation), the Kostek Range of North Tien Shan (Karagajly Formation), and the Moldotau Range in Central Tien Shan (Karadzhorga Formation). Linguliformean (organophosphatic-shelled) stocks are dominant, comprising 74 of the 88 described species. In a total of 55 identified genera, 47 are linguliformeans and eight are rhynchonelliformeans. Diencobolus is defined as a new linguliformean genus, and there are 11 new species of linguliformeans, referred to the lingulides Notiobolus indefinitus, Canalilatus? major, Mirilingula postuma, Diencobolus simplex, Canthylotreta atasuica, Dactylotreta septata, Picnotreta karakichiensis, Stilpnotreta minuta, Pomeraniotreta obtusa, Kleithriatreta kostekensis, and Eoscaphelasma? delicata. Twenty-four taxa are described under open nomenclature. Brachiopods from different segments of the Central Asian tectonic collage have different biogeographical signatures. In the early Cambrian, Malyi Karatau was probably part of the Yangtse Plate (South China). Middle Cambrian affinities were with East Gondwana (eastern Australia), but by the end of the mid Cambrian westward drift introduced links with Baltica and Laurentia. Balkhash and Tien Shan terranes retained East Gondwanan relationships throughout the Cambrian.