Advances in Earth Science ›› 2011, Vol. 26 ›› Issue (5): 465-476. doi: 10.11867/j.issn.1001-8166.2011.05.0465

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Status and Prospect of Studies on Oncoid

Yang Renchao 1,2, Fan Aiping 2, Han Zuozhen 2, Chi Naijie 2   

  1. 1.Key Laboratory of Depositional Mineralization & Sedimentary Minerals of Shandong Province,Qingdao266510, China; 2.College of Geological Science and Engineering, Shandong University of Science and Technology, Qingdao266510, China
  • Received:2010-08-31 Revised:2011-03-22 Online:2011-05-10 Published:2011-05-10

Yang Renchao, Fan Aiping, Han Zuozhen, Chi Naijie. Status and Prospect of Studies on Oncoid[J]. Advances in Earth Science, 2011, 26(5): 465-476.

As an important granular structure type of microbialites, oncoid has been given extensive concern by geologists for a long time. Seven aspects of research progress on oncoid, such as definition, classification, forming environment, microecosystem, material and texture composition, growth mechanism and controlling factors, have been summarized in this paper by way of field and laboratory and literature study. Various sedimentary environments are sutible for deposition of oncoid, such as offshore intertidal zone, subtidal zone, organic bank, interbeach sea, delta, estuarine, lagoon, river, lake, tufa. etc. The growth mechanism of oncoid could be summarized by adherenceentrapment, calcification (calcification of microorganisms, biofilm and extracellular polymeric substance) and organic matter mineralization. It is significant for the forming of oncoid that benthonic cyanobacteria capture subaqueous sediment grains by adherence or entrapment. Deposition of fine-grained calcite particles involving abiotic organic polymer instead of biological activity is another important process of sedimentation.Sequence stratigraphy is a deciding factor for the forming of oncoid that it is controlled by fluctuation of sea level. Water energy, water depth, oxygen content, salinity, acidity, temperature, and transportation of sediments change by fluctuation of sea level. Oncoids output adjacent to sequence boundary, in low stand system tract, in transgressive stand system tract, near to maximum flooding surface, and in high stand system tract. What is more, distinct differences of structural features and morphology of oncoids exist in defferent status of sequence stratigraphy. Cortex texture and morphology of oncoids have important denotative meaning of hydrodynamic environment because of additional information of water energy and wter depth indicated by biotic component, entrapped sediments and their changes. Factors of fluctuation of sea level, water depth, water temeprature, trophic level, and input of clay minerals all play important roals in the forming of oncoids. And these factors bear relations with ancient climate. The surface morphology of oncoid indicates the intensity of rolling on the sea floor and thus water energy, and water depth indirectly. The point sources of terrigenous input and platform morphology increase the spatial heterogeneity of nutrient distribution and thus trophic level. Nutrient element of nitrogen and phosphorus are essential for the growth of oncoid. The high ratio of nitrogen to phosphorus play decisive role in the composition of extracellular polymeric substance. H gele D.(2006) proposed that Alz oncoids fulfilled the definition of an ecosystem because all necessary trophic levels (e.g. primary producers, consumer, and decomposers) are present in complex communities of microbial mats and biofilms. Oncoids were formed in low-lying areas or depression with ralatively low depositional velocity. As a result, changes of platform morphology are partly responsible for the ptchy distribution of onciod-rich facies and oncoid types. Some oncoid-rich limestone strata are capped by sandstone and grainstone (such as ooid bars and calcarenite), which indicate that migration of terrigenous classtic deposits, intraclast and ooid may change the environment, and thus expire the growth and depostion of oncoid. A new type of oncoid, charactered by coreless, non-lamina, amorphism, rich of algae, clear algae, which is named as phycomycetes-clotted oncoid, was discovered in Mantou formation of Cambrian system in Weishan county, Shandong province, by means of field measurement and synthetical indoor analysis. It is considered that phycomycetes-clotted oncoid is a transitional type between oncoids and clots. Difination of oncoid will be developed with increasing studies all over the world. A new defination of oncoid has been proposed in this paper combining with personal research and previous research achievements. Oncoid is a kind of concretion forms carbonate grain that it is formed in microbially dominated ecosystem through adherence-entrapment of debris and carbonate mineral during growth of muciparous cyanobacteria or other microorganisms in-situ growth of free-lying or overturned on the sedimentary floor. Usually, Oncoids were classified by morphological feature of core (nuclei)and cortex(coatings), characteristics of main components of oncoid, combining of morphology and forming environment.Classification of various types of oncoid is gradually reaches perfection with study progresses and newfounds. Oncoids were divided into micrite envelope oncoid, debris envelope oncoid, algae envelope oncoid, multiple envelope oncoid, radiated envelope oncoid and phycomycetes-clotted oncoid by texture component and texture features.Environment and growth mechanism of oncoid are becoming hot spots of carbonatite research. Scope of oncoid studies is becoming more and more extensively both on geologic history and geography. Synthetic interdisciplinarity study trend of paleogeography, sequence stratigraphy, micropaleontology, biochemistry,and geochemistry has been forming in recent years. Significance of oncoid study on multidisciplinary of earth science will be concerned in the near future.

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