Global microfossil changes through the Precambrian-Cambrian phosphogenic event: The Shabakta Formation of the phosphorite-bearing Maly Karatau Range, South Kazakhstan

Abstract

Subtidal cherts from phosphorite-bearing basal strata of the Early Cambrian Shabakta (Dzhylandy) Formation, Tamda Group of the Maly Karatau Range of South Kazakhstan, contain a diverse assemblage of acanthomorph acritarchs and other well-preserved organic-walled microfossils. Acanthomorphs of the Shabakta lowermost layers are taxonomically essentially identical to those of many similarly aged Early Cambrian organic-walled compression-preserved and permineralized microbiotas preserved in rocks deposited in relatively open marine settings and are biostratigraphically correlative with acanthomorph-containing microfloral assemblages of the Vergale and Rausve Horizons (Regional Stages) of the East European Platform as well as with other assemblages known worldwide from the upper Atdabanian (Series 2 of the International Union of Geological Sciences [IUGS]). Although lower Atdabanian deposits of the Talsy (Lükati) horizon are missing from the Maly Karatau succession studied here, Nemakit-Daldynian through Tommotian (the IUGS Terranovian) microbiotas both of the Shabakta-underlying Berkuta Member of the Kyrshabakta Formation and phosphoritic Chulaktau Formation are comparable to those of the pre-trilobite Rovno and Lontova Horizons (Regional Stages) of the East European Platform. Thus, the phosphatized and chert-permineralized Shabakta assemblages date from of the Early Cambrian part of the Precambrian-Cambrian phosphogenic event, an interpretation supported also by associated small shelly fossils.The studies reported here of the Shabakta microfossils – based on the use of optical microscopy, confocal laser scanning microscopy (CLSM) and Raman spectroscopy – provide information in three dimensions at high spatial resolution about their organismal morphology, cellular anatomy, kerogenous composition, taphonomy, and mode and fidelity of preservation. The combined use of CLSM and Raman, techniques introduced to Precambrian paleobiology only rather recently, is shown to reveal morphological characters of taxonomic, biostratigraphic and palaeoenvironmental significance.