Outcrop photographs and representative fossils of the Ediacaran to Cambrian strata in the southern NCC [IMAGE: NIGPAS]

In many blocks on the Earth, the boundary between the Phanerozoic and Precambrian is consistently characterized by a huge sedimentary gap — the Great Unconformity. Recently, researchers from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), carried out a synthetic geochronological study of the late Ediacaran to early Cambrian strata on the southern North China craton, which provides a new perspective on the origin of the Ediacaran-Cambrian unconformity.

The relevant research was recently published in the journal Earth and Planetary Science Letters.

While this long-drawn exhumation has been argued to trigger the initiation of modern plate tectonics, the oxygenation of the ocean and atmosphere, and the Cambrian explosion, according to thermochronologic and stratigraphic studies, the Great Unconformity has been confirmed to consist of one denudation before the Cryogenian and two exhumations during the Snowball Earth and the Ediacaran-Cambrian transitional period. While the major erosional processes before and during the Cryogenian were suggested to be associated with the Rodinia supercontinent cycle and the “Snowball Earth” event, respectively, the origin of the relatively shorter Ediacaran-Cambrian unconformity is still elusive.

(A) Comparison of the Precambrian unconformities in different regions of NCC; (B) Simplified late Ediacaran to middle Cambrian paleogeographic maps of NCC [IMAGE: NIGPAS]

Multiple geological events have been proposed as associated with the Ediacaran-Cambrian unconformity, including the final break-up of the Rodinia supercontinent, plume activity associated with continental-scale rifting, tectonic response to the assembly of Gondwana, as well as other regional plate tectonics. However, it is still unclear how different tectonics could produce the widely comparable Ediacaran-Cambrian unconformity in different blocks.

The Great Unconformity occurs widely in the North China Craton (NCC), and recent studies suggest it is composed of a prolonged sedimentary break from late Paleoproterozoic/early Neoproterozoic to late Ediacaran and a short one between late Ediacaran and early Cambrian, which resembles the development pattern of the typical Great Unconformity in North America and Qaidam. In this regard, the NCC provides us with an opportunity to understand the mechanism of the widespread exhumation that occurred from late Ediacaran to early Cambrian.

The Detrital zircon ages for the late Ediacaran-early Cambrian strata in the southern, western, and southeastern margins of North China craton show great similarities, with predominant age clusters peaking at ~1800 Ma and ~2500 Ma. The geochronological study reveals an apparent absence of synsedimentary zircons in the latest Ediacaran to earliest Cambrian strata, suggesting a significant tectonic quiescence in NCC.

Hypothesized erosional history of the Great Unconformity [IMAGE: NIGPAS]

A similar situation occurs in blocks outside the Gondwanaland in the late Neoproterozoic, indicating that the contemporaneous erosion represented by the widespread Ediacaran-Cambrian unconformity may have been dominated by a global sea-level fall, rather than regional tectonics.

“However, our result does not preclude the influence of tectonism on the manifestation of the Ediacaran-Cambrian unconformity in other blocks, especially those that make up the Gondwanaland”, says Sun. “The late Neoproterozoic global sea-level fall was possibly associated with the assembly of the Gondwanaland”.

For more information, please contact:

Liu Yun (For media)

E-mail: yunliu@nigpas.ac.cn

Nanjing Institute of Geology and Palaeontology,

Chinese Academy of Sciences

Source: Nanjing Institute of Geology and Palaeontology,

Chinese Academy of Sciences