Paleogene and Early Neogene Lacustrine Reefs in the Western Qaidam Basin, China

Typical reefs in the Paleogene and early Neogene strata of the Qaidam Basin, Tibetan Plateau, China, reveal their internal structures and sedimentation environments and consist mainly of algal reef, stromatolite reef and thrombolite reef with distinct reef structures, fore-reef, back-reef and reef-plateau. The fore-reef is characterized by a combination of pinnacle reef, thrombolite and algal reef. The back reef is composed of stromatolite reef and algal reef. The pinnacle reefs (micro-atoll), most of which are several tens of centimeters in diameter (whereas some exceptionally big ones are over 200 cm in diameter), and several tens of centimeter to 2 m in height, are situated on the far front-edge of the reef; the pinnacle reef is also often of recumbent form with a gravel-filled circular hole in the center. The algal reef is in the form of dome and irregular beds, and filled with algal detritus, ostracodes, spirorbis fossils, ooid and terrigenous debris, and worm traces; cavities and scour marks are often developed. The algal reef is gray commonly when fresh and weathers to a brown color. The lacustrine thrombolite in the Qaidam Basin is light gray or deep gray when fresh, white-gray or brown when weathered, dense and homogeneous with abundant pores filled by oil and bitumen. Observed under the microscope, the thrombolite consists mainly of brown or brown-black clots with a little algal debris, ooid, pellet, ostracodes, spirorbis fossils and terrigenous debris, in some cases, terrigenous debris, even gravel, is abundant. Many features of the thrombolite suggest that it is formed in a high-energy envkonment. The stromatolite reefs developed on the lacustrine algal reef in the Qaidam Basin are very complex whether in shape or in internal structure. The simplest ones form laminated layers and the most complex ones have intensely branching structures. The size is also variable.

Petrography and origin of the Lower Ordovician microbial carbonates in the Songzi Area of Hubei Province,middle Yangtze region,China

This study is the first systematic assessment of the Lower Ordovician microbial carbonates in Songzi,Hubei Province,China.This paper divides the microbial carbonates into two types according to growth patterns,namely nongranular and granular.The nongranular types include stromatolites,thrombolites,dendrolites,leiolites and laminites;the granular types are mainly oncolites and may include a small amount of microbiogenic oolite.According to their geometric features,the stromatolites can be divided into four types:stratiform,wavy,columnar and domal.Additionally,dipyramidal columnar stromatolites are identified for the first time and represent a new type of columnar stromatolite.The thrombolites are divided into three types:speckled,reticulated and banded.The grazing gastropod Ecculiomphalus and traces of bioturbation are observed in the speckled and reticulated thrombolites.This paper considers these two kinds of thrombolites to represent bioturbated thrombolites.These findings not only fill gaps in the field of domestic Ordovician bioturbated thrombolites but also provide new information for the study of thrombolites.Based on the analysis of the sedimentary characteristics of microbialites,the depositional environments of the various types of microbialites are described,and the distribution patterns of their depositional environments are summarized.The relationship between the development of microbialites and the evolution and radiation of metazoans during the Early to Middle Ordovician is discussed.Consistent with the correspondence between the stepwise and rapid radiation of metazoans and the abrupt reduction in the number of microbialites between the late Early Ordovician and the early Middle Ordovician,fossils of benthonic grazing gastropods(Ecculiomphalus)were found in the stromatolites and thrombolite of the study area.It is believed that the gradual reduction in microbialites was related to the rapid increase in the abundance of metazoans.Grazers not only grazed on the microorganisms that formed stromatolites,resulting in a continuous reduction in the number of stromatolites,but also disrupted the growth state of the stromatolites,resulting in the formation of unique bioturbated thrombolites in the study area.Hydrocarbon potential analysis shows that the microbialites in the Nanjinguan Formation represent better source rocks than those in the other formations.

Distribution and depositional model of microbial carbonates in the Ordovician middle assemblage,Ordos Basin,NW China

Based on outcrop profiles,drilling cores,cast thin sections etc.,the types,microfacies combinations and distribution pattern of microbial carbonates in the Ordovician middle assemblage of the mid-eastern Ordos Basin have been systematically analyzed.The middle assemblage of Ordovician in the mid-eastern Ordos Basin has microbial carbonates formed by the calci-fication of cyanobacteria,including microbial biostromes and microbial mounds made of stromatolites,thrombolites,and on-colites.The distribution of the carbonates shows obvious“stratum-control”and“regional”characteristics.The microbial bio-stromes 2–3 m thick each are controlled by sequence cycles and sedimentary facies changes,and were mainly formed in the tidal flat environment during the depositional stages of the Ma56 and Ma55 sub-members.The microbial biostrome in the Ma55 sub-member occurring near the carbonate-evaporite transition interface in the early stage of the transgression is distributed mainly in the Mizhi subsag in the eastern part of the basin;the microbial biostrome in the Ma56 sub-member turns up near the carbonate-evoporite transition zone in ring shape in the east of the central uplift.The ancient landform had noticeable control on the distribution of microbial mounds.The microbial mounds or mound-shoal complexes developing mainly during the de-positional stages of Ma57_Ma510 sub-members are about 15–25 m thick in single layer and distributed largely in the Wushenqi-Jingbian paleouplift.The development model of the microbial carbonate rocks shows that the carbonate-evaporite lithologic transition zone and the Wushenqi-Jingbian paleouplift are favorable exploration zones of microbial carbonates in the Ordovician middle assemblages.

Oxygenation in carbonate microbialites and associated facies after the end-Permian mass extinction:Problems and potential solutions

Carbonate sediments deposited in normally-oxygenated shallow ocean waters of the latest Permian period, immediately prior to the end-Permian mass extinction, contain well-developed diverse shelly faunas. After the extinction of these skeletal metazoans, the sediments commonly comprise microbialites(regarded by most authors as benthic) and associated facies bearing evidence interpreted by many authors to indicate reduced oxygenation of the shallow ocean waters. However, the evidence of oxygenation state is inconsistent and the sequences have gaps, indicated in the following 5 points:1) Shelly fossils occur commonly in post-extinction shallow marine limestones, and likely to have been aerated in contact with the atmosphere. Nevertheless, although the largest mass extinction in Earth history may have caused reduced body size in shelly organisms, such reduction is arguably due to environmental stress of lowered oxygenation. Discriminating between these controls remains a challenge.2) Abundant pyrite framboids in many post-extinction limestones are interpreted by several authors as indicating dysoxic contemporaneous waters, so the organisms that lived there, now shelly fossils, were dysaerobic. However, verification is problematic because pyrite framboids scattered amongst shelly fossils cannot have formed amongst living organisms, which need at least some oxygen; synsedimentary framboid formation requires anoxic conditions in the redox boundary where sulphate-reducing processes work. Thus, framboids and shelly fossils found together means taphonomic mixing of sediments, destroying original depositional relationships so that it is not possible to determine whether the shells were aerobic or dysaerobic prior to sediment mixing. Furthermore, diagenetic growth of framboids is possible, as is import of previously-formed framboids from deeper water during upwelling. Thus, there is no proof of an environmental link between framboid size and occurrence, and contemporaneous oxygenation in these post-extinction shallow water facies, so we question the validity of this model in those facies, but consider that the model is valid for deeper water facies.3) Some publications provide evidence of oxygenation, from redox-sensitive elements in post-extinction limestones, while others indicate low oxygen conditions. Redox-sensitive geochemistry requires further work to explore this issue at higher resolution of sampling than has been so far applied.4) Biomarkers recorded in some post-extinction facies contain evidence of anoxic conditions(including green sulphur bacteria) but other examples lack these, which may be indicate fluctuations of water oxygenation. However, a key issue that has not yet been resolved is determination of whether biomarkers were imported into the sites of deposition, for example by upwelling currents, or formed where they are found.Thus, there is currently no verification that biomarkers of low oxygen organisms in shallow water settings actually formed in the places where they are sampled.5) The common occurrence of small erosion surfaces and stylolites represents loss of evidence, and must be accounted for in future studies.The oxygenation state of post-end-Permian extinction shallow marine facies continues to present a challenge of interpretation, and requires high-resolution sampling and careful attention to small-scale changes, as well as loss of rock through pressure solution, as the next step to resolve the issue.

Microfabric features of microbial carbonates:experimental and natural evidence of mold holes and crusts

Results of our study based on examination of induced precipitation of carbonate by a cyanobacterium, Lyngbya in the laboratory, and the analyses of microphotographs of both modern and ancient microbial carbonates,demonstrated the importance of recognition of mold holes and carbonate crusts in understanding microbial carbonates. In the experiment, only cyanobacteria Lyngbya can induce precipitation of carbonate, forming scattered grains on the surface of Lyngbya filaments. Carbonate crusts enclosing the old parts of the filaments were formed through aggregation of these scatter grains while mold holes were formed after decay of the filaments. Mainly based on the experiment, six different ways of microbial carbonate formation were recognized:(1) trapping without mold holes,(2) trapping with mold holes,(3) particle-forming induced-precipitation of carbonate,(4) discrete crustforming induced-precipitation of carbonate,(5) induced precipitation, forming tangled crusts that build a porous construction, and(6) induced precipitation, forming a dense construction. And mold holes and crusts can form in ways(4),(5), and(6). Examination of both modern microbial carbonates from the Shark Bay of Australia, Highborne Cay of Bahamas and the atoll of Kiritimati and the microbialites from the Cambrian dolostone sequence in Tarim,Xinjiang, China all demonstrated the limitation of recognizing only mesofabric features and importance of examining microfabric features for understanding of the genesis of the microbial carbonates and their proper classification. The shape, size and arrangement of the mold holes, crusts, and the features of the minerals filling in pores between the crusts, which are referred as the microfabric features here, are keys to better understand the formation and environments of both modern and ancient microbial carbonates.

Palaeogeographic variation in the Permian-Triassic boundary microbialites:A discussion of microbial and ocean process esafter the end-Permian mass extinction

Shallow marine carbonate sediments that formed after the end-Permian mass extinction are rich in a thin(maximum ca.15 m) deposit of microbialites.Microbial communities that constructed the microbialites have geographic variability of composition,broadly divisible into two groups:1) eastern Tethys sites are calcimicrobe-dominated(appearing as thrombolites in the field),with rare occurrence of sedimentconstructed microbialites and uncommon cements either within microbial structure or as inorganic precipitates,2) other Tethys sites are sediment-dominated structures forming stromatolites and thrombolites,composed of micrites and cements,with some inorganic precipitates.These other Tethys locations include western and central Tethys sites but their palaeogeographic positions depend on the accuracy of continental reconstructions,of which there are several opinions.In contrast to geographic variation of microbialites,the conodont Hindeodus parvus,which appeared after the extinction and defines the base of the Triassic,is widespread,indicating easy lateral migration throughout Tethys.Conodont animals were active nekton,although being small animals were presumably at least partly carried by water currents,implying active Tethyan surface water circulation after the extinction event.Post-extinction ammonoid taxa,presumed active swimmers,show poor evidence of a wide distribution in the Griesbachian beds immediately after the extinction,but are more cosmopolitan higher up,in the Dienerian strata in Tethys.Other shelly fossils also have poorly defined distributions after the extinction,but ostracods show some wider distribution suggesting migration was possible after the extinction.Therefore there is a contrast between the geographic differences of microbialites and some shelly fossils.Determining the cause of geographic variation of post-extinction microbialites is problematic and may include one or more of the following possibilities:1) because calcifying microbial organisms that create calcimicrobes were benthic,they may have lacked planktonic stages that would have allowed migration,2)eastern Tethyan seas were possibly more saturated with respect to calcium carbonates and microbes,so microbes there were possibly more able to calcify,3) significant reduction of Tethyan ocean circulation,perhaps by large-scale upwelling disrupting ocean surface circulation,may have limited lateral migration of benthic microbial communities but did not prevent migration of other organisms,and 4) microbes may have been subject to local environmental controls,the mechanisms of which have not yet been recognized in the facies.The difficulty of distinguishing between these possibilities(and maybe others not identified) demonstrates that there is a lot still to learn about the post-extinction microbialites and their controls.

Macro-and microfeatures of Early Cambrian dolomitic microbialites from Tarim Basin, China

The fabrics of microbialites preserved in limestones are generally better than in dolostones. What are the fabrics of the microbialites preserved in heavily dolomitized dolostones? This paper presents an example of a strongly dolomitized Cambrian microbialite profile. The Xiaoerblak Formation(Cambrian Series 2 Stage 3 and lower Stage 4)of the Sugaitblak section in Aksu, Xinjiang Uygur Autonomous Region, China is mainly composed of microbial dolostones. Due to strong alteration by diagenesis, their features, formation and environments have not been fully understood. Here, based on detailed observation on outcrops and thin sections, we show that this formation comprises four kinds of microbialites: laminite, thrombolite, thrombolitic laminite, and Renalcis framestone, in five intervals(Interval I to Interval V). We identified three main types of microbialite fabrics, i.e., clotted fabric, laminated fabric and skeletal fabric, and established a high-resolution vertical evolution sequence of the microbialites. The clotted fabric and the laminated fabric were further divided into subtypes. We found that the original fabrics were mainly affected by dolomitization, recrystallization and dissolution, and the alteration degree of the microbialite fabric is stronger in the lower part of this formation. The laminated fabric has the strongest resistance to diagenesis,followed by the clotted fabric. Based on studies of different rock types and sedimentary structures, we concluded that the sedimentary environment of Xiaoerblak Formation consists of three settings: a) Intervals I to III formed in restricted tidal flat environments, b) Interval IV and the lower part of Interval V in restricted deep subtidal environments, and c) upper part of Interval V in shallowing-up open subtidal environments.