Diagenesis of the microbialites in the Permian-Triassic boundary section at Laolongdong,Chongqing, South China

Jiang Hongxia, Wu Yasheng

Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

1 Introduction*

Microbialites are the important component of the Permian-Triassic boundary (PTB)sections, especially those in Guizhou and Chongqing areas of China.In reported PTB sections, microbialites usually have a domed dendroid structure or a speckled structure (Wanget al., 2005).

Reinhardt (1988)studied a PTB section at Tudiya, in Chongqing.Wignall and Hallam (1996)examined another PTB section at Laolongdong, which is near the one at Tudiya, and documented the lithofacies across theboundary, but did not mention the microbialites.Reinhardt(1988)interpreted the top of the bioclastic limestone of the Uppermost Permain Changxing Formation as the result of evaporatic inter-/supratidal deposition, but Wignall and Hallam (1996)gave an opposite viewpoint that it was deposited in anoxic environments caused by the sea-level rise.The viewpoint of anoxic environments was accepted by many researchers (Kershawet al., 1999, 2002; Ezakiet al., 2003; Adachiet al., 2004; Liuet al., 2007).However,Wuet al.(2006a)demonstrated that at least one sea-level drop event occurred in this area.

Kershawet al.(1999)studied one PTB section at Laolongdong and two in the Huaying area, and found a strange“?microbialite” crust with digitate and locally thrombolitic structure, and suspected that it possibly had an organic origin, but not determined.Kershawet al.(2002)considered the crust to be the Lowest Triassic in age according to the occurrence of the conodontHindeodus parvus.

Ezakiet al.(2003)studied the same kind of rocks in Huaying area, and found spheroidal microbial fossils in earliest Triassic thrombolites.But Kershaw (2004)questioned them as the controlling factors for the formation of the “?microbialite” crust as well as the environmental conditions.Adachiet al.(2004)considered the skeletal limestone and microbialites in the PTB sections in Guizhou Province to be comprised of peloids.

Besides Chongqing, microbial carbonates were also reported in Guizhou (Lehrmannet al., 1998, 2001; Lehrmann, 1999; Adachiet al., 2004; Wanget al., 2005; Wuet al., 2007), Hubei (Wanget al., 2005; Yanget al., 2006)and Jiangxi (Wuet al., 2006b)provinces.

Jiang and Wu (2007)studied the dendroid-structuredrocks in the PTB section in Xiushui, Jiangxi Province, and proposed that the dendroid-structure was formed by diagenetic processes including dolomitization and dedolomitization.This view is different from the previous research which declared that the dendroid-structured rocks are of microbial origin.This paper studies the diagenesis of the microbialites at Laolondong, Chongqing for the first time.

Fig.1 Location and regional geology of Laolongdong, Chongqing (modified from Kershaw et al., 1999).J3-Upper Jurassic; J2-Middle Jurassic; T+J1-Triassic+Lower Jurassic; P-Permian; O+C-Ordovician+Cambrian.

2 Lithology of the PTB section at Lao?longdong

The Upper Permian Changhsingian at Laolongdong is a reef sequence including a 29.9 m-thick calcisponge bafflestone in its middle part.The PTB section begins with massive calcisponge limestone at the top of the Changhsingian reef-facies, and ends with the tabular marly lime mudstone of the bottom of the Lower Triassic Feixianguan Formation.The “microbialite” that occurs in the middle of the section shows a distinctively different appearance from the underlying and the overlying strata.The lithologic succession and main fossils are listed in ascending order as follows (Fig.2).

Bed 1: 5 m thick, is a grayish white massive wackestone yielding fossils including calcisponges,Tubiphytes,foraminifers, echinoderms, calcareous algae, ostracods,bryozoans, brachiopods, andColaniella.

Bed 2: 10.6 m thick, contains grayish white thick-bedded wackestone and packstone.The skeletal grains are dominated byColaniellaand echinoderms (mainly crinoids).The minor grains include brachiopods,Palaeofusulina, andTubiphytes.

Bed 3: 0.35 m thick, consists of gray medium-bedded lime mudstone and dark-colored wackestone, with speckled structure and contains ostracods and tube-like microbial fossils.Cubic pyrites began to be present from this bed through the remaining overlying beds.

Bed 4: 0.2 m thick, contains the gray thin-bedded lime mudstone and dark-colored wackestone, with speckled structure, containing abundant euomphalid microgastropods, a few ostracods, some irregular spherical microbial fossils, and sporadic tube-like microbes.

Bed 5: 0.55 m thick, contains gray lime mudstone with dark-colored wackestone, thin-bedded packstone, speckled structure, and contains abundant irregular spherical microbial fossils and a few conical microgastropods.

Bed 6: 0.3 m thick, is a grayish grainstone with fibrous cements.The skeletal debris includes microgastropods and bivalves.

Bed 7: 0.85 m thick, contains gray medium-bedded lime mudstone and dark wackestone, with dendroid structure, containing irregular spherical microbal fossils, conical microgastropods, bivalves, and ostrocods.Kershawet al.(2002)recovered the conodontHindeodus parvusin a sample collected from the rock 0.3 m below the top of the“?Microbialite” and placed the unit in the Lowest Triassic.

Bed 8: 0.02 m thick, is a gray-yellowish packstone,containing abundant bivalves and some microgastropods.

Bed 9: 0.05 m thick, is a caesious tabular marly lime mudstone, containing a few (＜5%)microgastropods.

Bed 10: 0.07 m thick, also is a caesious tabular marly lime mudstone, containing some microgastropods.This bed contains rounded, internally recrystallised, small pebbles 0.3-3 mm wide.

The above beds are caesious and tabular, and were not measured.

3 Diagenesis of the speckled and den?droid microbialites in the Laolong?dong section

In Kershawet al.(1999), the dendroid rock is directly underlain by crinoidal limestone.In this study, we found a speckled interval between the dendroid rock and crinoidal limestone (Fig.2).Both the speckled and the dendroid rocks are composed of a grayish part and a brownish part(Figs.3, 4).The brownish part is referred to as the dendroid area, and interpreted to be a “?microbialite” by Kershawet al.(1999), and was assumed to be microbialite by Ezakiet al.(2003).So if the dendroid is the “microbialite”,the speckled unit should also be the same.Therefore, the“microbialite” in this study includes two kinds of rocks:the speckled and the dendroid.

3.1 Features of the speckled rocks

The speckled rocks include Bed 3, Bed 4, Bed 5 and Bed 6, and are totally 1.4 m thick.The lower 0.35 m is medium-bedded, and the upper 1.05 m is thin-bedded.The contact relation between the speckled rock and the underlying crinoidal limestone is flat, indicating no erosion.Thus, it is possible that there was no obvious depositional hiatus between them.In outcrop, the speckled rocks consist of grayish spots and brownish areas.The grayish spots are irregularly rounded in shape with irregular edges, and are less than 0.5 cm in diameter.The distance between the adjacent spots is about 0.5 cm.The brownish areas are speckled, and mixed with the grayish spots (Fig.3A).

In thin section, the brownish areas of the outcrops looklimpid or lighter in color, but the grayish areas in outcrops look relatively darker (Fig.3B).Under the microscope,large, limpid, blocky calcites are visible in the lighter areas.The large blocky calcites contain small dirty calcite rhombs and irregular yellowish residual lime mudstone,which is the same as the surrounding rock (Fig.3C).

Fig.2 Lithology and main organisms of the Permian-Triassic boundary (PTB)section at Laolongdong, Chongqing.The position of the PTB follows the one defined by Kershaw et al.(2002).Mb-Medium-bedded.

3.2 Diagenesis of the speckled rocks

The lighter areas may be formed through two processes: dissolution and cementation.The large blocky calcites are some kind of pore-filling cement, and their formation needs an accommodating space, the solution pores; so theformation of the solution pores occurred first.The pores were formed from solution instead of being original sedimentary pores based on two reasons: (1)the lighter areas are relatively big, and the original pores preserved in sediments could not be so large; (2)the large blocky calcites contain dirty and irregularly distributed yellowish residual lime mudstone, similar to the surrounding rock, which is assumed to be the remains of the original lime mudstone suffering dissolution.The occurrence of a dissolution event might be related to the action of meteoric water when the deposits were uplifted into subaerial environments after a sea-level fall.According to Wuet al.(2006a), a sea-level fall event happened in the Latest Permian, and caused the limestones at Laolongdong to emerge into subaerial environments.In a subaerial environment, the fresh meteoric water migrated downward through the more porous areas of the deposits, caused the dissolution of carbonates, and formed the spongy pores.After that, in the case that the deposits went into the phreatic zone, the limpid blocky calcites could formed in the solution pores.Because of the sufficiently long duration in the phreatic zone, the solution pores can be entirely filled up by the limpid blocky calcite cements.The residual lime mudstone that survived dissolution were enclosed in the blocky calcites.

Fig.3 A-Magnified outcrop photograph of the speckled rock, which is composed of grayish and brownish spots.Spots are irregularly rounded.B-Thin section photograph of A, the grayish part corresponds to the darker area and the brownish part to the lighter.C-Microscopic features of B.The lighter area consists of large, limpid, blocky calcites which contain small dirty calcite rhombs and dirty yellowish residual lime mudstone.Cal-Calcite; Lim-Residual lime mudstone; Wal-Boundary between the lighter area and the darker area; Cem-Large calcite cement.

The small rhombs contained in the large blocky calcites are also calcites, based on staining by alizarin red (Fig.3C).Their rhombic shape, as well as the blurred darkercore structure, shows that they formed from the alteration of dolomites by dedolomitization.Wuet al.(2003)studied limpid-rim and darker-core dolomites in the PTB section in Ziyun, Guizhou Province, and concluded that they were formed by the dedolomitization process.Jianget al.(2007)described the limpid-rim and darker-core calcites in thin sections of dendroid rocks in the PTB section in Xiushui, Jiangxi Province, and interpreted them to be formed by dolomitization and dedolomitization.Theirinterpretations were supported by the residual dolomite rhombs with irregular borders formed from dissolution.As mentioned above, Wuet al.(2006a)reported that a substantial sea-level fall occurred in the Latest Permian at Laolongdong.Dedolomitization is generally related to the action of meteoric fresh water under subaerial conditions.Therefore, it is reasonable to infer that dedolomitization occurred before the dissolution, and dolomitization occurred before the dedolomitization.

Consequently, the formation of the lighter areas possibly includes four steps: (1)a dolomitization event to form the dolomite rhombs.(2)a dedolomitization event to transform the dolomite rhombs into calcite rhombs.(3)a dissolution event to make the areas altered by the dolomitization and dedolomitization processes and more pores to be dissolved,for forming the spongy pores and leaving the residual calcite rhombs and lime mudstone.Since the dissolution tends to prefer a selective fabric instead of the controlled fractures, it formed small pores instead of large vugs, and no geopetal structures was formed in this step.(4)infilling of the solution pores with large blocky calcites.

3.3 Features of the dendroid rocks

Rocks with the dendroid structure (brownish parts)were first reported by Kershawet al.(1999)from the Laolongdong section.They described the dendroid structure as “up-branching”, and proposed that this feature is similar to that of the modern spring tufa, and assigned the dendroid rock into “?microbialite”.From their description,the determination of the “dendroid” form, especially the“up-branching” form, is critical to the determination of the origin of the dendroid structure.

We washed more than 10 m2of the outcrops with acid solution to make detailed observations about the form of the dendroid structure, and found that the brownish areas do not show typical dendroid forms in most places.In addition, no typical upward-branching form was observed(Figs.4A, 4B).In outcrop, the cross-section of the brownish areas looks like irregular patches (Fig.4C).In some places, the form of the brownish areas suggests the downward migration of diagenetic fluids (Fig.4D).

In thin sections, the compositional pattern of brownish areas (dendroid areas)is similar to that of the speckled rock.The brownish areas of the outcrops looks lighter, and the grayish areas are relatively darker in thin sections (Fig.4E).Under the microscope, the lighter areas are composed of large, limpid, blocky calcites (Fig.4F).The large blocky calcites contain many small dirty rhombs and small patches of residual lime mudstone, the same as the surrounding rock.The small dirty rhombs are calcite, as indicated by alizarin red.

3.4 Diagenesis of the dendroid rocks

Since the large calcites, small rhombs, and residual lime mudstone in the dendroid structure are similar to those of the speckled structure, we assume that the dendroid structure,i.e., the lighter areas in thin section, have suffered the same diagenetic processes.

The rhombs in the blocky calcite are assumed to be transformed from dolomite rhombs by dedolomitization.Thus, a dolomitization event could occur in the early diagenetic stage before the dedolomitization event.However,the dolomitized areas in the carbonate deposits have a specious dendroid form due to several factors.The main factor is the migration pathway of the diagenetic fluids,which might be controlled by the fabric of sediments, such as the distribution of fossils.After the deposits emerged into subaerial environments, the action of meteoric fresh water led to the dedolomitiztion, and altered the dolomite rhombs into the calcite rhombs.This change is assumed to occur in the phreatic zone, where the water is saturated with carbonate and the solution process is weak.As the deposits went into the vadose zone, the water would be unsaturated with carbonate, and the solution process would occur within the dendroid areas.This process continued and resulted in making the dolomitized parts to be spongy porous, but not all the calcite rhombs were dissolved.The solution also affected the lime mudstone around the dendroid patches, and left the small patches of residual lime mudstone behind.Afterwards, the deposits passed into the phreatic zone again, and the pores in the spongy dendroid parts of the rock were filled with the large blocky calcites.

Examination of thin sections found small calcite cements on the original pore walls, which are assumed to be formed in submarine environments in the earliest diagenetic stage.

4 The diagenetic patterns for the speckled and dendroid microbialites

The formation mechanism of the lighter areas in thin sections of the speckled and dendroid rocks is as follows:after the carbonate sediments were deposited, a sea-level fall happened, and the sediments passed into the superatidal environment, where the strong evaporation or the mixing of sea water with meteoric fresh water occurred,and the sea water appeared the high salinity.Downward migration of the sea water or the meteoric water both could

make the carbonate deposits change into dolomites, and its migration occurred in a roughly vertical but irregularly downward direction which caused the dolomitized parts to be irregularly dendroid in shape.The dolomitization is assumed to have preferred to occur in the original pores and fossil cavities.The unique limpid-rim and darker-core structure of the small dolomite rhombs in the large blocky calcites could imply that the dolomitization was related to the mixing of sea water and meteoric water.The change from calcite to dolomite increased the porosity of the deposits.

Fig.4 A-Outcrop photograph of dendroid rock (Layers 1-9).It is composed of grayish and brownish strips, and the grayish part is thinner than the brownish.B-Outcrop photograph of dendroid rock (Sub-beds 1-7).It is also composed of grayish and brownish strips,and the grayish part is thinner than the brownish.The arrow “a” points to the brownish part which shows no up-branching form, “b”shows the grayish part with little up-branching form, and “c” shows the brownish part with little up-braching from.C-Cross section of B.The brownish and grayish parts are irregularly rounded.The arrow “a” points to the grayish part which shows polygonal rounded form, and the arrow “b” points to the brownish part which shows irregularly speckled shape.D-Outcrop photograph of dendroid rock,the brownish part provides evidence of downward flowing of diagenetic fluids.E-Thin section photograph of A, the grayish part corresponds to the darker area (Da)and the brownish part to the lighter (La).F-Microscopic feature of the thin section of E.The lighter area consists of large, limpid, blocky calcites which contain small dirty calcite rhombs and dirty yellowish residual lime mudstone that is the same as the surrounding rock.

As the sediments were raised farther above sea level,only the meteoric water could affect the deposits.The water migrated downward along inter-crystal pores within the dendroid dolomitic patches, and led the dolomite rhombs change into calcite rhombs.

After the deposits went into the vadose zone, the carbonate-unsaturated meteoric water migrated downward into the dendroid patches and dissolved the calcites to form pores and make the dendroid areas spongy.But some calcite rhombs and limestone patches were left.

In a later diagenetic stage, large blocky calcite cements fully filled the spongy pores.The rhombic calcite crystals and residual limestone patches included in the blocky calcites were randomly distributed.All of them comprise the lighter area in thin sections.

Therefore, the diagenetic processes are: (1)carbonate sediments were exposed to the subaerial environment because of a sea-level fall; (2)dolomitization formed the dendroid dolomitic areas; (3)dedolomitization formed the dendroid calcite areas; (4)dissolution of carbonate in the dendroid areas formed the spongy pores (5)filling of the pores by the large blocky calcites.

5 Discussion

Ezakiet al.(2003, 2008)and Liuet al.(2007)proposed that the dendroid areas are composed of clots formed by calcified coccoidal cyanobacteria.In the microphotographs of these papers, the microbial fossils are surrounded by micrites, which suggests that the microbial fossils and micrites are possibly the residual limestone patches.The current study shows that the dendroid areas are composed of large blocky calcite cements, which contains small calcite rhombs and irregular patches of the residual original limestone.The residual limestone patches in the large blocky calcites have a dot-like or conglomerate outline and can be easily regarded as microbial clots.

Cyanobacterial fossils are common in the Paleozoic carbonate deposits, and researchers have found a great deal of cyanobacterial fossils in Pre-Cambrian carbonate deposits (e.g., Riding, 1991, 2000; Schubert and Bottjer,1992), which promoted the study of microbial fossils.Riding (2000)proposed that microbialites formed by calcified cyanobacteria include stromatolite, thrombolite and dendrolite.This study, however, shows that even though cyanobacteria played an important role in the formation of the dendroid and speckled structures in the Permian-Triassic boundary sections at Laolongdong, the diagensis also contributed a lot to the formation of these two structures.

Kershawet al.(1999, 2002)proposed that the dendroid areas in Laolongdong section are “?microbialite” according to the similarity of their shape to that of recent tufa(Guo and Riding, 1994), and the “upward-branching” features.However, detailed examination of the outcrops did not support their observation of the up-branching form.Based on our observation, the “dendroid” areas are not always in the dendroid form; in most cases, they do not have a typical dendroid form.

Some researchers thought the “?microbialite” at Laolongdong look like stromatolites (Personal communication with Robert Riding).In thin section, however, the dendroid areas do not have the laminated structure of stromatolites,so no evidence supports classifying them as stromatolites.

6 Conclusions

Speckled-structured limestone has been found in the PTB section at Laolongdong, Chongqing.It underlies the famous dendroid-structured limestone reported by previous researchers (e.g., Kershawet al., 1999, 2002).Both the two kinds of rock consist of lighter-colored areas and darker-colored areas.The darker areas in outcrop, corresponds to the lighter areas in thin section, which are more pervious than micrites.In thin section, the lighter speckled areas and the dendroid areas both consist of large blocky calcites with small calcite rhombs and residual lime mudstone, similar to the surrounding rock.Their formation mechanism is summarized as follows: (1)dolomitization formed the dendroid or speckled dolomitic areas, (2)dedolomitization of dendroid or speckled dolomite areas formed the small calcite rhombs, (3)dissolution of the calcite rhombs formed the spongy pores, and (4)large blocky calcites filled the spongy pores.Therefore, even though the microbes played a vital role in the formation of the microbialites, diagenesis was also important, especially for the formation of the dendroid and speckled structures.

Acknowledgements

We are grateful to Liu Jianbo at Peking University and Robert Riding at Cardiff University for their helpful advice.This study was supported by the National Natural Science Foundation of China (40802001)and the China Postdoctoral Science Foundation (20070420523).

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