Tsagaan Oloom Formation Research Articles

Diversification of heterotrophic protists at the eve of Cambrian explosion

Molecular clocks suggest that at least some heterotrophic protists, such as foraminifera, testate amoebae and tintinnids, were present in ecosystems as far back as 1000 Ma. Vase-shaped microfossils of earlier testate amoebae from the supergroups Amoebozoa and Rhizaria occur globally in pre-Sturtian shallow marine deposits, and have thus been considered as a potential biostratigraphic tool, especially at a time span between 789 and 729 Ma. Organic-rich three-dimensional tests having flask-like shapes, constricted necks, distinct and often thickened collars of possible tintinnids affinity were recovered from 715 to 635 Ma carbonate deposits from the Tsagaan Oloom Formation, southwestern Mongolia. Phosphatic scaly structures of unknown affinity were reported from mid-Neoproterozoic (pre-Sturtian) strata of northwest Canada. Anyhow, the fossil records of heterotrophic protists aftermath the Cryogenian age are still shroud in mist. The three putative ciliate fossils, namely Eotintinnopsis, Wujiangella, and Yonyangella from the Neoproterozoic Doushantuo Formation in China were reassessed and likely to be taphonomically and diagenetically distorted and altered acritarchs. Here, we report a diverse assemblage of problematic from the terminal Ediacaran Gaojiashan biota in southern Shaanxi Province, China. Apart from the renowned vase-shaped Protolagena and Sicylagena, likely thought to be the earliest foraminifera, new findings include a hollow pear-shaped structure with long extending, a solid spindle-shaped structure with straight narrow central tube. Possible foraminifera from the Gaojiashan biota may throw welcome new light to the long obscured history of heterotrophic protists at the eve of the Cambrian explosion.

Neoproterozoic complexes of the shelf cover of the Dzabkhan terrane basement in the Central Asian Orogenic Belt

The formation stages of high-grade metamorphic complexes and the related granitoids of the Dzabkhan terrane basement are considered. The age data (U–Pb method, TIMS) of zircons from the trondhjemite block of the eastern part of the Dzabkhan terrane, which is directly overlain by the dolomite sequence of the Tsagaan Oloom Formation, are given. Trondhjemites yield the U–Pb zircon age of 862 ± 3 Ma. In their structural position, they are assigned to typical postmetamorphic formations that determine the formation and cratonization of rocks of the host block. The geochronological study of trondhjemites gives grounds to distinguish fragments of the continental crust in the Dzabkhan terrane basement, the formation of which occurred at different periods of time: ∼860 and ∼790 Ma. Geological–geochronological and Sm‒Nd isotope–geochemical studies indicate that the Dzabkhan terrane basement is not a single block of the Early Precambrian continental crust, but a composite terrane, comprising Neoproterozoic ensialic and island-arc structural and compositional complexes. Correlation of Sr isotopic characteristics with the 87Sr/86Sr variation curve in the Neoproterozoic and Cambrian seawater shows that carbonate deposits accumulated at the eastern margin of the Dzabkhan terrane near the end of the Neoproterozoic, 700–550 Ma, and in the central part of the terrane in the Early Cambrian, 540–530 Ma.

Late Riphean age of the crystalline basement of the carbonate cover of the Dzabkhan microcontinent

The Dzabkhan microcontinent was earlier considered as a fragment of an ancient craton in the structure of the Central Asian Orogenic Belt. Deposits of the Tsagaan Oloom Formation were included in the shelf zone, under the assumption that they were related to the regional unconformity between the Early-Late Precambrian crystal formations. The carbonate sequence of the Tsagaan Oloom Formation overlaps crystalline rocks only in the eastern part of the Dzabkhan microcontinent, where dolomites lie unconformably on high-grade metamorphic rocks intruded by granitoids of the Bogdyngol massif. The latter were included in the composition of both the Early Precambrian basement and the Middle Riphean intrusive complex. We have determined the U-Pb zircon age of these granitoids at 717 ± 5 Ma and the Nd model ages of granitoids and gneisses of the basement of the Tsagaan Oloom Formation at 2.0–1.9 Ga at eNd = −10.0...−6.6. Recent geochronological and Nd and Pb-Pb isotopic and geochemical data indicate that intrusive and high-grade metamorphic complexes are absent in the crystalline basement of the Dzabkhan microcontinent, similar to those in ancient cratons. One can assume that the Late Riphean carbonate cover (Tsagaan Oloom Formation) deposited on the Late Precambrian continental block.

Variations of δ88Sr and 87Sr/86Sr in Neoproterozoic sedimentary carbonates (the Tsagaan Oloom formation, West Mongolia)

Variations of δ88Sr and 87Sr/86Sr in Neoproterozoic sedimentary carbonates (the Tsagaan Oloom formation, West Mongolia)

U-Pb age and Sr isotope signature of cap limestones from the Neoproterozoic Tsagaan Oloom Formation, Dzabkhan River Basin, Western Mongolia

The U-Pb and Pb-Pb methods were used for determining age of cap limestones from the Neoproterozoic Tsagaan Oloom Formation corresponding to the lower part of the sedimentary cover in the Dzabkahn microcontinent of Central Asia. The weighted average age value appeared to be equal to 632 ± 14 Ma (MSWD = 0.11, probability 0.74). This value allows the following assumptions: (1) the lower boundary of the Tsagaan Oloom Formation corresponds to the beginning of the Ediacaran; (2) Dzabkhan tillites are correla� tive with glacial sediments of the Marinoan Epoch. The low 238 U/ 204 Pb and 232 Th/ 238 U ratios observed in ini� tial Pb sources of limestones from the Tsagaan Oloom Formation indicate that the Dzabkhan paleobasin received at its early development stages a bulk of material from eroded upper Riphean juvenile rocks. The 87 Sr/ 86 Sr ratio in fractions of Tsagaan Oloom limestones enriched with primary carbonate material and sat� isfying geochemical criteria of Rb-Sr systems retentivity (Mn/Sr < 0.20 and Fe/Sr < 1) varies from 0.70676 to 0.70691 and reflects this ratio in the World Ocean approximately 630 Ma ago.

Agglutinated tests in post-Sturtian cap carbonates of Namibia and Mongolia

Paleomagnetic data suggest that the early Cryogenian (Sturtian) glaciation extended to sea level at low latitude. The impact of this dramatic environmental change on biota, and the composition of ecosystems in the immediate aftermath of the Sturtian glaciation remain virtually unknown. Here we report the discovery of abundant agglutinated tests in organic-rich carbonates directly overlying Sturtian glacial deposits from two different paleocontinents: the Rasthof Formation of the Congo craton in northern Namibia and the Tsagaan Oloom Formation of the Dzabkhan terrane in Mongolia. The most abundant tests preserve morphological and compositional characters consistent with those found in at least two different families of modern lobose testate amoebae (Amoebozoa), a group of heterotrophic microbial eukaryotes. The presence of spatially and compositionally variable clay minerals, quartz and microcline on the test walls is a signature of widespread biological agglutination. The post-glacial fossil assemblages differ from the most common pre-Sturtian vase-shaped fossil testate amoebae, perhaps as a result of different preservational mechanisms or of the appearance of new forms after the glaciation. The apparent local abundance of eukaryotic body fossils in the post-Sturtian carbonates suggests that the Cryogenian limestones and dolostones may host a currently unexplored fossil record of modern eukaryotes.

Chapter 29 The Tsagaan Oloom Formation, southwestern Mongolia

Abstract The Tsagaan Oloom Formation (Fm.) in southwestern Mongolia contains two Neoproterozoic glacial deposits, with diamictite in the Maikhan Ul Member (Mb.) and in the Khongoryn Mb., which are separated by over 500 m of limestone. The Maikhan Ul Mb. ranges in thickness between 5 m and greater than 300 m, expanding in deeper-water sections towards the SW, where it is composed of two massive diamictites separated by over 100 m of sandstone, siltstone and shale. The basal 10 m of the overlying Tayshir Mb. of the Tsagaan Oloom Fm. consists of a fine-laminated, dark grey limestone. The Khongoryn Mb. is composed primarily of limestone clasts in a shale matrix, and is between 0 and 23 m thick. The overlying Ol Mb. contains sedimentary structures characteristic of basal Ediacaran cap carbonates including micropeloids, tubestone stromatolites, giant wave ripples and former aragonite crystal fans. U–Pb evaporation ages from zircons in the underlying Dzabkhan Volcanics constrain the Tsagaan Oloom Fm. to &lt;773 Ma, and tuffs within the Maikhan Ul and Tayshir members testify to the potential for additional geochronology. The Cryogenian organic-rich limestone of the Tayshir Mb., which lies between the two glacial deposits, is ideally suited for geochemical studies and has been the subject of several carbon, strontium and rare earth element investigations. Limited palaeomagnetic studies suggest a mid- to low-latitude position of the Dzabkhan platform during deposition of the glaciogenic strata, and additional studies are in progress.

Stratigraphic and tectonic implications of a newly discovered glacial diamictite–cap carbonate couplet in southwestern Mongolia

Research Article| February 01, 2009 Stratigraphic and tectonic implications of a newly discovered glacial diamictite–cap carbonate couplet in southwestern Mongolia Francis A. Macdonald; Francis A. Macdonald * 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA *E-mail: fmacdon@fas.harvard.edu. Search for other works by this author on: GSW Google Scholar David S. Jones; David S. Jones 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Daniel P. Schrag Daniel P. Schrag 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Francis A. Macdonald * 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA David S. Jones 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Daniel P. Schrag 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA *E-mail: fmacdon@fas.harvard.edu. Publisher: Geological Society of America Received: 21 Jan 2008 Revision Received: 23 Sep 2008 Accepted: 06 Oct 2008 First Online: 12 Oct 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2009 Geological Society of America Geology (2009) 37 (2): 123–126. https://doi.org/10.1130/G24797A.1 Article history Received: 21 Jan 2008 Revision Received: 23 Sep 2008 Accepted: 06 Oct 2008 First Online: 12 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Francis A. Macdonald, David S. Jones, Daniel P. Schrag; Stratigraphic and tectonic implications of a newly discovered glacial diamictite–cap carbonate couplet in southwestern Mongolia. Geology 2009;; 37 (2): 123–126. doi: https://doi.org/10.1130/G24797A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report here the discovery of a new end-Cryogenian glacial diamictite and an overlying basal Ediacaran cap carbonate within the Tsagaan Oloom Formation of southwestern Mongolia. The identification of the Cryogenian-Ediacaran boundary, coupled with new δ13C chemostratigraphic profiles, facilitates the integration of the Neoproterozoic stratigraphy of Mongolia with records elsewhere. These correlations indicate that a previously unrecognized, −16‰ Cryogenian δ13C anomaly is present in the newly defined Tayshir member (informal) of the Tsagaan Oloom Formation. Furthermore, chemostratigraphic and lithostratigraphic relationships suggest an ~35 m.y. depositional hiatus below the phosphorite-bearing Zunne Arts member (informal) of the upper Tsagaan Oloom Formation and that subsidence renewed in the latest Ediacaran–Early Cambrian. We propose that the lower ~1500 m of the Tsagaan Oloom Formation was deposited on a thermally subsiding passive margin after Rodiniaage rifting, whereas the Zunne Arts member and the overlying ~1600 m of Early Cambrian strata were deposited in a foredeep basin that formed as the southern margin of the Dzabkhan terrane was subducted beneath the Khantayshir-Dariv arc. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stratigraphical and biological significance of negative carbon isotopic anomalies in the basal Cambrian series of Guizhou Province

The early Early Cambrian strata within the bounds of Guizhou Province are almost predominated by black shale deposition. Recently, however, the authors have found a section consisting of a set of mudstones interbedded with limestones at the basement of the Cambrian at Yingping, Fuquan County, Guizhou Province, which provides favorable conditions for the study of marine geochemical characteristics of the early Early Cambrian. The characteristics of intense negative carbon isotopic anomalies near the Precambrian/Cambrian boundary at Yingping, Fuquan County, Guizhou Province, can be correlated with those of global carbon isotopic anomalies at the same time on a global scale, corresponding to the intense negative carbon isotopic anomalies near the Neoproterozoic/Cambrian boundaries at the bottom of the Xiaowaitoushan Member, Huize, Yunnan Province, at the top of the Tsagaan Oloom Formation of Mongolia and at the top of the Salarmy Gol Formation of Siberia, as well as those observed in southwestern United States, Iran, Amman, Poland, Newfoundland, the Great Britain, Canada, etc. Negative carbon isotopic anomalies can be used as the important basis for the division of the Precambrian/Cambrian boundaries. There have been found 8 m-thick black shales and cherts below the intense negative carbon isotopic anomalies in this profile. In the black shales there have been found large amounts of fossils such asBradorida, gernusArchotuba, indicating that during the period of sedimentation of black shales under anaerobic conditions there would be large quantities of organic species living in the sea. But in the section of grayish-green mudstones interbedded with limestones with intense negative carbon isotopic anomalies almost no organic fossil has been found. This may imply that the anaerobic event seems to have little bearing on the mass extinction near the Precambrian/Cambrian boundary, whereas the intense negative carbon isotopic anomaly event appears to be more closely related to the mass extinction occurring near the Precambrian/Cambrian boundary.

The Neoproterozoic–lower Cambrian in northern Govi-Altay, western Mongolia: regional setting, lithostratigraphy and biostratigraphy

AbstractThis paper provides a framework for the stratigraphy and correlation of key sections through the Neoproterozoic–lower Cambrian of the Govi-Altay area, western Mongolia, studied by members of IGCP Project 303. Dzabkhan Formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the Baydrik–Tarbagatay microcontinent in the Baykalian interval (c. 850–650 Ma). Pre-Vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back-arc basin, in which accumulated relatively uninterrupted Neoproterozoic and earliest Palaeozoic sediments. Lithostratigraphy for the latter is defined, with detailed maps, at five sections (Bayan Gol northern and southern blocks, Tsagaan Gol, Salaany Gol and Tayshir). The Tsagaan Oloom Formation lower member contains two diamictites of possible Sturtian age. Middle and upper carbonates are compared with the Yudomian (Vendian) of Siberia, the former ending with a unit ofBoxonia grumulosastromatolites. In the upper member, phosphorite-shale is followed mainly by limestones, with the earliest skeletal fauna. The Bayan Gol Formation comprises siliciclastic–limestone intercalations with increasingly diverse invertebrate biota. Correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied. TheAnabarites trisulcatusZone of the Nemakit-Daldynian Stage begins near the base of the Tsagaan Oloom Formation upper member (unit 11). ThePurella antiquaZone begins near the base of the Bayan Gol Formation (unit 18). Assemblages typical of theNochoroicyathus sunnaginicusZone (base of the Tommotian Stage) range through units 19 to 23. Elements typical of theDokidocyathus regularisZone (middle Tommotian) appear in unit 24 near the top of the Bayan Gol Formation.

Early Cambrian bioconstructions in the Zavkhan Basin of western Mongolia

AbstractThe Neoproterozoic–Cambrian succession in the Zavkhan Basin of western Mongolia preserves early Cambrian bioconstructions of Nemakit-Daldynian to Botomian age. As elsewhere (Siberia, Morocco), the Nemakit-Daldynian bioconstructions in the upper Tsagaan Oloom Formation to lower Bayan Gol Formation interval were purely calcimicrobial. Spectacular calcimicrobial ‘patch reefs’ of presumed Tommotian age are present in the lower Bayan Gol Formation. In contrast, late Atdabanian–early Botomian bioconstructions are varied and well developed in the upper Salaany Gol Formation including, in addition to calcimicrobial bioherms, Gordonophyton–Razumovskia crusts, radiocyathan—archaeocyathan bioherms and thickets of the ramose archaeocyath Cambrocyathellus.These latter Zavkhan Basin buildups show some ecological and sedimentological features in common with coeval bioconstructions in South Australia, which also developed under a tectonically active regime.

Glacial facies associations in a Neoproterozoic back-arc setting, Zavkhan Basin, western Mongolia

AbstractDiamictites, many of glacial origin, are globally distributed in the Neoproterozoic. Recently, two relatively thin diamictites in the Maikhan Uul Member at the base of the Neoproterozoic Tsagaan Oloom Formation from the Zavkhan Basin of western Mongolia have been identified as being of glacial origin. The Mongolian diamictites form a series of backstepping units within the transgressive systems tract of two major depositional sequences associated with sea-level changes. In each case the diamictites of the transgressive systems tract are abruptly overlain by deeper water, upward shoaling highstand systems tracts consisting of thinly bedded sandstones and shales in sequence 1 and thinly bedded, dark carbonates in sequence 3. The fact that the sequences conform closely to depositional models established at other localities suggests that all are related to major ice ages and that the depositional sequences they have generated provide a valuable tool for global correlation in this part of the stratigraphic column. Available stratigraphic and isotope geochemical information presented by Brasier et al. (1996, this issue) suggests that both diamictites are likely to be of Sturtian age. A riftogenic setting and Sturtian age for the diamictites provide a link with eastern Australia and western America. It is possible, therefore, that these diamictites formed during the breakup of a supercontinental assembly including Siberia, Australia and Laurentia c. 750–725 Ma BP.