First report of a Meishucun-type early Cambrian (Stage 2) ichnofauna from the Malyi Karatau area (SE Kazakhstan): Palaeoichnological, palaeoecological and palaeogeographical implications

摘要

A trace fossil assemblage from the early Cambrian phosphorites of the Aktugai and Koksu sections (Chulaktau Formation) of the Lesser Karatau area (= Malyi Karatau in the following text) in SE Kazakhstan is reported for the first time. Based on a rich and biostratigraphically significant Small Shelly Fossil (SSF) record, the ichnofauna can be attributed to the basal Cambrian (Terreneuvian: ?uppermost Fortunian to Stage 2). Alongwith rare vertical burrows, the ichnoassemblage yielded abundant simpleworm-shaped epi- and infaunal burrows of various ichnotaxa being associated with branched traces and (pre-trilobitic) arthropod traces. The non-arthropod ichnofauna consists of hyporeliefs of large straight to slightly meandering traces belonging to Palaeophycus isp., and Psammichnites isp. type traces. This ichnofauna is accompanied by horizons with short inclined U-shaped burrows of the Planolites-type. The evidence of Treptichnus pedum-like simple branching (sub-) horizontal traces is proved for the Koksu section but remains problematic for the Aktugai section as they are poorly preserved and could be mixed up with Planolites-type traces. Additionally, a dark-brown coloured phosphoritic sandy dolostone band close to an iron- manganese rich stromatolite bed on the base of the Ushbas Member contains vertical funnel-shaped burrows of more than a decimetre in length which could be attributed to dwelling burrows of infaunal lingulid brachiopods (?Lingulichnus). Several short and larger (knob-shaped) vertical burrows in the surrounding phosphoritic dolostones occasionally display faintly preserved radial running and distally branching probes. They resemble star-shaped trace fossils from the Meishucun- and from the younger (Cambrian Stage 4) Guanshan sections of South China. The arthropod traces can be attributed to Rusophycus-type traces (probably R. avalonensis) as well as to Monomorphichnus-type and problematic primitive (?) Cruziana-type trackways. The arthropod traces are concentrated in the massive phosphorites of the Karatau Member (Upper Chulaktau Formation) of the Aktugai-I and the Koksu localities. Some of them are preserved as positive hyporeliefs on the subsurface of an overlaying greenish-grey coloured chert band (panel A of the fourth figure in this article). The ichnotaxonomic inventory, the facies, the lithology and the depositional regime of the hosting rocks as well as the trace preservation in themassive phosphorites resemble the situation in the some older (Fortunian Stage) Meishucun ichnofauna of southern Yunnan, South Chinese Yangtze Platform as well as in some aspects the ichnoassemblage from the much younger (Stage 4) Wulongqing Formation of Yunnan. The here reported type of early Cambrian ichnofauna could be characteristic for the abundant basal Cambrian phosphorite-dominated shelf deposits on the northwestern realm of Gondwana.