Wednesday, 26 December 2018

Evidence for a connection between large igneous province eruption and black shale deposition in the Mesoproterozoic.

The Mesoproterozoic Era is an arbitrarily defined period of geological history, lasting from 1600 to 1000 million years ago. Unlike more recent geological time periods the Mesoproterozoic is defined arbitrarily, without reference to geologic events, as the absence of a widespread fossil record combined with the fact that the landmasses that occurred at this time have long since broken up and reformed in different combinations, makes in hard to use the kind of events that are used to define the boundaries of more recent periods, such as mass extinctions or large igneous province emplacements. The Mesoproterozoic is divided into three Periods, again defined arbitrarily upon dates rather then geological events, with the Calymmian lasting from about 1600 to about 1400 million years ago, the Ectasian lasting from 1400 to 1200 years ago, and the Stenian, lasting from 1200 to 1000 years ago.

In a paper published in the journal Geology on 25 September 2018, Shuan-Hong Zhang of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciences, Richard Ernst of the Department of Earth Sciences at Carleton University and the Faculty of Geology and Geography at Tomsk State University, Jun-Ling Pei and Yue Zhao, also of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciences, Mei-Fu Zhou of the Department of Earth Sciences at The University of Hong Kong, and Guo-Hui Hu, again of the Institute of Geomechanics and the MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction of the Chinese Academy of Geological Sciencesn, describe the correlation of a series of igneous eruptions and black shale depositions from the Mesoproterozoic, which they suggest could be used to define the boundaries between geological periods in the way that happens with more modern geological sequences.

Zhang et al. observe that a number of Mesoproterozoic large igneous provinces have been dated to approximately 1380 million years ago, including the Mashak Large Igneous Province in on the eastern margin of the Baltica Craton (now in the southern Urals), the Hart River–Salmon River Arch Large Igneous Province on the western margin of the Laurentia Craton (the western United States), the Midsommersø–Zig-Zag Dal Large Igneous Province in northern Greenland, the Chieress Large Igneous Province in the Anabar shield of the northern Siberia Craton, the Kunene-Kibaran Large Igneous Province in the Congo Craton of Central Africa, the Pilanesberg Large Igneous Province in the Kalahari Craton of Southern Africa. Smaller igneous features in other cratons including, West Africa, Amazonia, and East Antarctica, have been dated to the same time.

Furthermore, at least two black shale deposits, the Xiamaling Formation in northern China and the Velkerri Formation in North Australia, have been dated to the same time, with several other similar deposits, while not yet directly dated, appear to be off similar age based upon their stratigraphic positions (i.e. they are found in between rocks older than 1380 million years and rocks younger than 1380 million years) including the Dzhelindukon and Vedreshev formations in Siberia, the Bijaigarh Shale and black shales from the Srisailam Formation in India, and the Serra do Garrote Formation of Brazil.

Representative stratigraphic columns of black shales in Xiamaling Formation in North China Craton (A–C) and Velkerri Formation in North Australian Craton (D–G). Zhang et al. (2018).

Black shales are typically associated with major oceanic anoxia events, such as the the one associated with the End Permian Extinction. The presence of widespread Mesoproterozoic black shales has previously been associated with generally low oxygen levels during this era, but this is no longer considered to be the case, as oxygen levels in the atmosphere are thought to have begun to rise quite quickly after the end of the deposition of banded ironstone formations about 1800 million years ago (these formations are thought to have formed as oxygen released by early Algae or photosynthetic Bactria reacted with iron in the water of the oceans, causing it to settle out as rust, with oxygen starting to build up in the atmosphere once this iron had been used up), and multicellular fossils interpreted as Eukaryotic Algae (Seaweeds) are known from about 1560 million years ago onwards.

Based upon this, Zhang et al. see evidence for a widespread igneous event about 1380 million years ago, which may have been associated with the breakup of the ancient supercontinent of Nuna, and which led to a global anoxic ocean event. Furthermore, they argue that since this event is recorded in strata around the world, that the date of 1380 million years ago should be adopted as the boundary between the Calymmian and Ectasian periods, rather than the arbitrary date of 1400 million years ago.

Distribution of ca. 1380 Ma large igneous provinces (LIPs) and black shales in paleogeographic reconstruction map of Nuna supercontinent. Zhang et al. (2018).

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