CHANGES IN MICROFACIES AND CARBON ISOTOPIC TRENDS IN THE EARLY CAMBRIAN OF THE MALY KARATAU RANGE, SOUTHERN KAZAKHSTAN

Rapid diversification of metazoans 542 Ma ago had a decisive consequences on the evolution of bio-, hydro- and atmosphere on Earth. However, the triggers of the Cambrian bioradiation remain debated. Here we study the Ushbas section from the northern Maly Karatau mountains in southern Kazakhstan, that documents significant paleo-environmental changes across the Precambrian-Cambrian transition. The investigations base on detailed analysis of microfacies, stable isotopes (δ¹⁸O, δ¹³C), trace elements and microfossil record.

Our inverstigations show that the Pc-C transitional strata in the Maly Karatau consist exclusively of shallow marine sediments. Latest Ediacaran (Kyrshabakty Fm.) is represented by oolithic carbonate platform, that is slump folded and subsequently karsted. Overlying basal Cambrian sediments of Chulaktau Fm. consist of black cherts, inter-bedded phosphatic rud-/grainstones and Fe-Mn rich boundstones that are covered by trilobite-bearing dolo-wackestonens of the Shabakty Fm. The δ¹³C signal shows negative excursion at the Pc-C boundary from ca. +2‰ in the latest Ediacaran to ca. -2‰ in the earliest Cambrian. In the basal Cambrian phosphorites δ¹³C signal remains constant at ca. -2‰. Within the Fe-Mn-boundstones the δ¹³C signal changes into a positive trend, reaching δ¹³C ≈ +2‰ in the trilobitic carbonates of the Shabakty Fm.

The negative δ¹³C excursion at the Pc-C boundary is reported from all regions and is generally associated with a global perturbation in seawater chemistry linked to changes in bioactivity or to recycling of buried light carbon sources. Based on our research we conclude that the negative δ¹³C excursion at the Pc-C boundary is rather triggered by an extremely high primary production, that is documented on the one hand by deposition of the earliest Cambrian phosphorites and on the other hand by progressing diversification of metazoans. We assume that high primary production caused an enrichment of organics in shallow water facies, that lead to an input of significantly high amount of ¹²C due to subsequent oxidation of the organics in the shallow marine settings.