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Geological Setting and Age

The discovery of the Chengjiang fauna was greatly aided by the excellent exposure of Cambrian rocks throughout the region. Chengjiang itself sits on the Yangtze platform where the Lower Cambrian is split into four stratigraphic units:

(* marks location of the Chengjiang fauna)

The Chengjiang fauna itself is found straddling the Eoredlicha-Wutingaspis and Pseudokunmingella-Angustacostella/Kunmingella maotianshanensis biozones. Soft-bodied remains are found in a 50 m thick section of mudstone (see stratigraphic log below) known as the Maotianshan Shale. At the time of deposition China was in an equatorial or tropical latitude. Some brachiopods are found in situ and some animals are mud-eaters (indicated by gut contents). Its therefore likely that the animals of Chengjiang were probably living in a warm, shallow sea with a muddy bottom.

The Maotianshan Shale shows a number of sedimentary features, including:

These indicate that animals were buried by sudden, storm-induced mudflows

The Yangtze platform was bounded on three sides by relatively steep mountains during this time, opening on to an ocean in the East. Various shallowing events are recorded in the strata above and below Chengjiang, indicating both eustatic sea-level change as well as localised tectonism. The fine-grained nature of the sediment indicates sporadic, but quite intense events. Deposition occurred 60-70 km East of the then shoreline, and likely came via a delta (current evidence indicates an easterly direction of flow).

The age of the Chengjiang fauna coincides with the late-Atdabanian, c. 522 Ma and is possibbly earlier than the Sirius Passet & Emu Bay Shale faunas. Consequently it is probably the most significant exceptional preservation above the Precambrian-Cambrian boundary (c. 543 Ma).

Refs: Bottjer et al, 2002; Zhang et al, 2001.

Stratigraphic Log Showing Outcrop of Chengjiang Fauna
(Image courtesy of Stefan Bengston and Jan Bergström, from Hou and Bergström, 1997)

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