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Ecological and environmental change recorded through stratigraphy and sedimentology at selected Ediacaran-Cambrian boundary sections in East and Central Asia

This joint sedimentological-paleobiological project focuses on  Precambrian geology, palichnology and (bio)stratigraphy. We support the paleobiological and chemostratigraphic/geochronological projects of the Forschergruppe by robust stratigraphic frameworks and sedimentyra expertise. This includes correlation, the continuity of the stratigraphic record and the diagenetic overprint of the sedimentary rocks. In doing so , paleobiology and sedimentary geology  work hand in hand to better constrain the the sedimentary environment of several spectacular Pc-C body and trace-fossil locations.

   

Two of the seven Ediacaran-Cambrian outcrop sections in the Malyi Karatau Range, southern Kazakhstan, which we investigated in 2010 and 2011. Left: Aktugai section; right: Ushbass section. Orange truck at the base of the outcrops for scale.

Summary

We are currently investigating three aspects of sedimentary-stratigraphic change at the Precambrian-Cambrian boundary (PCB; ca. 543 Ma): (1) the effects of the agronomic revolution, recorded by paleoichnology (B.Weber); (2) the significance of karsting at the PCB on the Yangtze Platform (X. Sun); and (3) the stratigraphic architecture of Ediacaran-Cambrian sections from southern Kazakhstan (C. Heubeck).

These are explained in a bit more detail below:

(1) The fossil record of Ediacaran to Lower Cambrian shelf deposits demonstrates how large volumes of fine-grained sediment found their way into the global chemical-oceanographic cycles through the demise of microbially-sealed surfaces during the agronomic revolution. To date, the timescale, facies dependency and degree of this enlargement of the Earth system cannot yet be incorporated in geochemical modeling calculations nor can it be convincingly linked to the expansion of the metazoa. In this subproject, we will link the bioturbation index from key sections across the PCB to geochemical proxies and relate those in turn to the chemostratigraphic record from profiles in West China (Tarim and Yili microcontinents) and Kazakhstan (Karatau Range).

(2) Because the contact between Ediacaran and Cambrian deposits in the shallow-water facies of the Yangtze Platform is virtually everywhere unconformable and karsted, resulting from one or several periods of relative sea level fall and subaerial exposure of Dengying Fm. carbonates, strata representing a critical time span of unknown duration (approx. 6-10 Ma ?) at the PCB are apparently missing. This can be studied in numerous well-exposed localities of Sichuan, Guizhou and Hubei Provinces. We are assessing this event to infer duration and intensity of chemical weathering and compare our results to other well-exposed PCB reference sections worldwide. Our results, complemented by a concurrent biostratigraphic study (subproject Steiner), will fill in a neglected gap in our understanding of the base-Cambrian radiation in South China.

   

Two of the nine Ediacaran-Cambrian sections investigated in southern China in 2011 (left: Meishucun mine; right: Lishuping mine). At both localities, subaerially exposed and karsted Ediacaran carbonates are unconformably overlain by basal Cambrian phosphorites.

(3) Thick and lithologically diverse Ediacaran strata in central Asia, best exposed in the Karatau Range of southern Kazakhstan, offer a rich and poorly studied record of paleoecological change. We are documenting this change through its facies and stratigraphic architecture, with a focus on tracing the Ediacaran sedimentary evolution from suspected basal tillites through shallow-water and volcaniclastic environments to the thick base-Cambrian phosphorites. Our stratigraphic measured sections show exposure surfaces, karst horizons, high-energy coastlines, and seismites. These contribute to the selection and characterization of geochemical and paleobiological samples and also assist in regional and global paleogeographic reconstructions.

The Malyi Karatau Range in southern Kazakhstan exposes large anticlines made primarily of Lower Paleozoic carbonates in a semiarid continental climate. Neoproterozoic, Ediacaran, and Cambrian strata are exposed in imbricated thrust sheets within a fault-and-fold belt.