Doushantuo formation

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The Doushantuo formation is from sediments of Ediacariums resulting formation in the Chinese provinces of western Hubei and northeastern Guizhou (county Weng'an ). It is known for its very old and excellently preserved fossils with very early Metazoa and possibly Bilateria .

geology

The geology in the deposit area of ​​the Doushantuo formation was described for the first time in 1924 by JS Lee and YT Chao. The formation forms part of the sediments of the End-Neoproterozoic Sinian (or Sinic system), which together with the Pre- Sinian overlay the metamorphic archaic and paleoproterozoic basement of the Yangtze Craton in a discordant manner and dip at 10 to 20 ° to the southeast. The sediments emerged in response to the breakup of Rodinia , which occurred between 830 and 750 million years ago (this is how the Yangtze Craton separated from Australia and / or India ). In the cryogenium , glacial and interglacial sediments were deposited over the period 750 to 635 million years, which document the transition from the initial rift ( Nanhua Rift ) to the drift stage.

In the area of ​​the gorges of the Yangtze River northwest of Yichang , the basement forms a dome-like bulge ( Huangling anticline ) elongated to the north-northeast, around 60 kilometers long and 30 kilometers wide , which is predominantly filled with neoproterozoic Huangling granite . The granite had penetrated the basement 819 ± 7 million years ago.

The Neoproterozoic sediment sequence of the passive continental margin of the Yangtze River craton begins with the 400-meter-thick Liantuo Formation , which is built from around 750 million year old sandstones of the cryogenium. About 100 meters of green Tillites of the Nantuo Formation , which contains a red sandstone layer in the middle section, lie above it .

stratigraphy

The Doushantuo Formation overlays the Tillites of the Nantuo Formation, which were deposited at the end of the Marino glaciation . This cryogenian the terminating glacial till ( Diamictites ) could be dated with about 654 to 635 million years ago. The Doushantuo Formation is in turn covered by black schist as well as massive dolomites and limestone of the Dengying Formation , which come from the outgoing Ediacarian.

The very strongly condensed Doushantuo Formation shows great fluctuations in thickness (between 40 and 300 meters, an average of 160 to 230 meters) and also significant changes in facies in its area of ​​distribution . At its type locality in the gorges of the Yangtze near Jiulongwan , it reaches over 200 meters in thickness and can be divided into four members:

The only 2 to 6 meters thick Member I consists of dolomitic Hutkarbonaten ( English cap carbonates ), the backfilled by numerous unusual sediment and Diagenesestrukturen as with quartz horizontal shrinkage cracks (English sheet cracks ), Makropeloide , Teepeestrukturen and Barytfächer be labeled. Three horizons can be recognized in it, a disturbed, a layered and a silty limestone-dolomite bond. The approximately 80 to 150 meters thick Member II is an alternation of thin-banked Dolomicrites, carbon-rich clayey Dolomites (with abundant pebbles) and carbon-rich black, calcareous shale clays . In the pea-sized phosphorite chert tubers contained, which are of early diagnostic origin and occur mainly in the lower half, acritics and other fossils have been found extensively . Member III is 60 to 80 meters thick and consists of massive and medium to thick banked, pebbly dolomite with thin Chert bands rich in microfossils. The dolomite is overlaid by an alternation of thin to thick banked clay limestone and medium to thick banked dolomitic claystone; Intraclasts and carbonate concretions may be present. The members IV is a 5 to 10, and sometimes up to 20 meters thick, enriched with organic carbon black shales but having (calcareous to siliceous shale or mudstone) no sedimentary structures that is very similar to the shales of the member II,; existing carbonate concretions reach the decimeter range; Also present are pyrite and barite.

A detailed description of the lithology can be found in McFadden and colleagues (2008).

Sequence stratigraphically , the Doushantuo Formation can be divided into two full cycles (transgression regression) and half of a third cycle, with the boundary between sequence 1 and sequence 2 in the middle of member II. The boundary between sequence 2 and the incomplete sequence 3 lies within member III at the transition from dolomitic to calcareous sedimentation. Furthermore, two flooding surfaces can often be observed - the lower one is identical to the limit from Member II to Member III, the upper one is slightly above the limit from Member III to Member IV.

Deposit conditions

The overall regressive Doushantuo Formation was deposited on a carbonate shelf in the south and south-east of the Yangtze River craton that led into a deeper basin area. Member II was sedimented under calm conditions below the storm wave base, but Member III shows much more agitated structures and suggests subtidal conditions.

The shelf edge surrounded lower-lying lagoons , which stretched from the tidal area to the open sea. The water masses in the lagoons sometimes took on an anoxic or euxinic character with the accumulation of hydrogen sulfide . Isotope and element concentrations in the lagoon sediments act as a proxy to recognize these fluctuations, which probably contributed to the special fossil conservation in the Doushantuo Formation.

The shelf morphology had already been created during the Nanhua Rift System . It explains the variability of the Doushantuo formation. During the following Dengying Formation, the Yangtze River platform finally developed into a deep-water basin ( Liuchapo Formation ) over the period 551 to 542 million years .

The chert nodules that occur so frequently within the formation are early diagenetic formations that originated at the sediment / seawater interface.

geochemistry

Carbon isotopes

Four negative δ 13 C anomalies can be seen in the Doushantuo formation : CANCE at the base, WANCE at the border between sequence 1 and sequence 2, BAINCE within sequence 2 and DOUNCE at the border between sequence 2 and sequence 3 in the hanging wall .

In the hat dolomite of member I there is CANCE with negative δ 13 C values, as they are typical for the early Ediacarium (generally up to - 5 ‰). The WANCE anomaly is relatively weak and reaches - 2 ‰. In the upper section of Member III, BAINCE again shows a negative δ 13 C anomaly, with values ​​up to - 8 ‰. The last anomaly DOUNCE is very strong at - 10 to - 13 ‰ and is correlated with the Shuram or Wonoka δ 13 C anomaly . At the top of Member IV, the δ 13 C values ​​then rise again towards 0. For DOUNCE in Member IV, geochemical analyzes indicate a significant oxygen enrichment event in the ocean.

This last anomaly is global in nature and occurs worldwide, but it cannot be associated with any major known event such as a mass extinction. Possibly this is a link between evolutionary innovation and the chemical composition of seawater. The newly emerged multicellular cells (Metazoa) could have withdrawn the carbon required for their growth from the water, which in turn increased the oxygen concentration in an indirect way. The increased oxygen levels in turn enabled further evolutionary steps in the Metazoa.

Oxygen isotopes

With a few minor exceptions, the oxygen isotope ratio δ 18 O shows a weakly negative correlation with the δ 13 C curve. The anomalies CANCE and DOUNCE are clearly pronounced with - 15 and - 8 ‰, respectively. The rest of the curve usually ranges between -5 and 0 ‰. In the border area between Member II and Member III, positive δ 18 O values ​​are achieved. This indicates a clear cooling of the climate and probably marks the Gaskiers Ice Age .

Strontium isotopes

The strontium anomalies are very clearly pronounced in the hat dolomite. The radiogenic 87 Sr / 86 Sr ratio drops drastically from 0.712 at the base to 0.708 ‰. The latter value is then maintained with slight fluctuations in the rest of the formation. The δ 88 Sr values, which increase from a very slight - 0.42 to + 0.38 ‰, are also significant. This can be explained as follows: after the end of the Marino Ice Age, extreme greenhouse conditions had set in and the associated warming of the world climate led to a massive melting of the global marino ice masses. This continental meltwater had a high 87 Sr / 86 Sr signature, but a very low δ 88 Sr value. An increase in carbon dioxide due to volcanic outgassing and a possible solution of methane hydrate caused the precipitation of carbonates with initially a very high 87 Sr / 86 Sr and a low δ 88 Sr. The subsequent transition in the hanging wall of the Hutdolomite to normal strontium values ​​was based on this inflowing deep water that mixed visibly with the surface water (deep water has normal δ 88 Sr and low 87 Sr / 86 Sr values).

Age

Condon and colleagues (2005) radiometrically dated the Doushantuo Formation to the time interval 635 to 551 million years. This results in a relatively low sedimentation rate of 3.45 meters / million years for the formation. The hat carbonate yielded 621 ± 7 million years. Zhang and colleagues (2005) had determined 555.2 ± 6.1 million years for the top of the Doushantuo Formation. A layer of ash 5 meters above the hat dolomite resulted in 632.5 ± 0.5 million years.

Note: Condon and colleagues had determined the basal cap carbonate to be 635.2 ± 0.6 million years and the hanging wall of the Miaohe member to be 551 ± 0.7 million years. In the meantime it has turned out that the Miaohe member does not belong to the Doushantuo formation as assumed, but rather is to be seen as a lying member of the Dengying formation. An and colleagues (2015) now assume that the Doushantuo Formation must be older than 560 million years.

Fossil guide

The fossil Vernanimalcula guizhouensis from the Doushantuo Formation

The Doushantuo Formation fossils appear in three taphonomic windows:

At Jiulongwan , carbon-rich prints resembling chuaria and an algae hallus were discovered in Member II black schists. In Miaohe there are plenty of carbonaceous prints in the black shale of Member IV. Silicified microfossils are found in pebbly tubers or lenses in both Member II and Member III.

The microscopic fossils of the Doushantuo Formation are all of marine origin, their degree of preservation is very detailed. The structure of the organisms can therefore still be examined at the cellular level. This gave rise to new insights into the embryo and larval stages of many living things.

Among the fossils to find multicellular algae and Thallophytes (seaweeds), acanthomorphe acritarchs , ciliates (ciliates), silicified cyanobacteria (as coccoid and threads), silicified tubular microfossils and mature sponges and cnidarians (Cnidaria). The latter may also be early forms of tabular corals ( tetra corals ). In addition, the taxon Parapandorina - probably the embryo of a bilaterium - and eggs (genus Megasphaera ) also occur. Some of the supposed animal embryos are in the early stages of cell division and have therefore often been interpreted as spores or algae cells. The eggs and embryos should rather belong to sponges or cnidarians, especially their mature forms of growth. The taxon Vernanimalcula , which was proposed as the oldest representative of Bilateria, is still controversial, but is viewed by Bengtson and Budd (2004) as a taphonomic artifact.

According to Bailey and colleagues (2007) there is a very good possibility that the alleged embryos and eggs are actually huge sulfur bacteria similar to the taxon Thiomargarita . Thiomargarita can already be recognized without optical aids. This view would also explain the phosphatic fossilization, because in modern environments it has been observed that phosphate is deposited with the help of bacteria. It can be countered by the fact that recent studies have found little agreement between the Doushantuo fossils and decaying Thiomargarita bacterial cells or sea urchin eggs. The dark spots in the fossils are also problematic, but interpreting them as fossil cell nuclei is unlikely.

The following taxa are present in the Doushantuo Formation:

Individual evidence

  1. ↑ Ma, G., Li, H. and Zhang, Z .: An investigation of the age limits of the Sinian System in South China . In: Bulletin of the Yichang Institute of Geology and Mineral Resources (Chinese with English summary) . tape 8 , 1984, pp. 1-29 .
  2. ↑ Jiang, G., Shi, X., Zhang, S., Wang, Y. and Xiao, S .: Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (approx. 635-551 Ma) in South China . In: Gondwana Research . tape 19 (4) , 2011, pp. 831–849 , doi : 10.1016 / j.gr.2011.01.006 .
  3. ↑ a b McFadden, KA among others: Pulsed oxygenation and biological evolution in the Ediacaran Doushantuo Formation . In: Proceedings of the National Academy of Sciences, USA . tape 105 , 2008, p. 3197-3202 .
  4. ↑ a b Zhu, M. et al .: Carbon isotope chemostratigraphy and sedimentary facies evolution of the Ediacaran Doushantuo Formation in Western Hubei, South China . In: Precambrian Research . tape 225 , 2013, pp. 7–28 , doi : 10.1016 / j.precamres.2011.07.019 .
  5. ↑ a b Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A. and Jin, Y .: U-Pb Ages from the Neoproterozoic Doushantuo Formation, China . In: Science . tape 308 (5718) , 2005, pp. 95-98 , doi : 10.1126 / science.110776 .
  6. ^ Y. Sawaki et al .: The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the three Gorges area, South China . In: Precambrian Research . tape 176 , 2010, pp. 46-64 , doi : 10.1016 / j.precamres.2009.10.006 .
  7. ↑ Zhang, S. et al .: U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in South China: constraints on late Neoproterozoic glaciations . In: Geology . tape 30 , 2005, pp. 473-476 .
  8. ↑ Zhihui An et al .: Stratigraphic position of the Ediacaran Miaohe biota and its constraints on the age of the upper Doushantuo δ 13 C anomaly in the Yangtze gorges area, South China . In: Precambrian Research . tape 271 , 2015, p. 243-253 , doi : 10.1016 / j.precamres.2015.10.007 .
  9. ↑ Tang, F., C. Yin, Y. Liu, Z. Wang and L. Gao: Discovery of macroscopic carbonaceous compression fossils from the Doushantuo Formation in eastern Yangtze Gorges . In: Chinese Science Bulletin . tape 50 , 2006, p. 2632-2637 .
  10. ↑ a b Xiao, S., X. Yuan, M. Steiner and AH Knoll. 2002: Macroscopic carbonaceous compressions in a terminal Proterozoic shale: A systematic reassessment of the Miaohe biota, South China . In: Journal of Paleontology . tape 76 , 2002, p. 347-376 .
  11. ↑ a b Xiao, S .: New multicellular algal fossils and acritarchs in Doushantuo chert nodules (Neoproterozoic, Yangtze Gorges, South China) . In: Journal of Paleontology . tape 78 , 2004, p. 393-401 .
  12. ↑ Zhou, C., G. Xie, K. McFadden, S. Xiao and X. Yuan: The diversification and extinction of Doushantuo – Pertatataka acritarchs in South China: Causes and biostratigraphic significance . In: Geological Journal . tape 42 , 2007, p. 229-262 .
  13. ↑ C.-W. Li, J.-Y. Chen, JH Lipps, F. Gao, H.-M. Chi and H.-J. Wu: Ciliated protozoans from the Precambrian Doushantuo Formation, Wengan, South China . In: Vickers-Rich, Patricia and Komarower, Patricia, The Rise and Fall of the Ediacaran Biota (Eds.): Geological Society, Special publications . tape 286 . London 2007, p. 151-156 , doi : 10.1144 / SP286.11 .
  14. ↑ Zhang, Y., L. Yin, S. Xiao and A. Knoll: Permineralized fossils from the terminal Proterozoic Doushantuo Formation, South China . In: Journal of Paleontology . 72 (supplement), 1998, p. 1-52 .
  15. ↑ Liu, Pengju et al .: Silicified tubular microfossils from the Upper Doushantuo Formation (Ediacaran) in the Yangtze Gorges area, South China . In: Journal of Paleontology . tape 83 (4) , 2009, pp. 630-633 .
  16. ↑ Yin, L., M. Zhu, AH Knoll, X. Yuan, J. Zhang and J. Hu: Doushantuo embryos preserved inside diapause egg cysts . In: Nature . tape 446 , 2007, pp. 661-663 .
  17. ↑ Stefan Bengtson and Graham Budd: Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian" . In: Science . tape 306 (5700) , 2004, pp. 1291a .
  18. ↑ Bailey, Jake V., Joye, S. B, Kalanetra, KM, Flood, BE and Corsetti, FA: Evidence of giant sulfur bacteria in Neoproterozoic phosphorites . In: Nature . tape 445 (7124) , 2007, pp. 198–201 , doi : 10.1038 / nature05457 .
  19. ↑ Schiffbauer, JD, Xiao, S., Sharma, KS and Wang, G .: The origin of intracellular structures in Ediacaran metazoan embryos . In: Geology . tape 40 (3) , 2012, p. 223-226 , doi : 10.1130 / G32546.1 .
  20. ↑ Chuanming, Z. ua: The diversification and extinction of Doushantuo-Pertatataka acritarchs in South China: causes and biostratigraphic significance . In: Geological Journal . tape 42 , 2007, p. 229-262 , doi : 10.1002 / gj.1062 .
  21. ↑ S. Xiao and AH Knoll: Fossil preservation in the Neoproterozoic Doushantuo phosphorite deposit, South China . In: Lethaia . tape 32 (3) , 1999, pp. 219-240 .