Review of molar tooth structure research

Hong-Wei Kuang

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

1 Introduction *

Since Bauerman (1885)first introduced the term “molar tooth (MT)”, more than 300 papers have been published to discuss this special sedimentary structure that only developed in the Proterozoic carbonate rocks.Examination of 336 papers related to molar tooth structure (MTS)indicated that MTS studies represent at least three main stages.

During the first stage (before 1980), resea rchers not only focused on the morphology of MTS, including the vertical and horizontal sheet-like features and the podlike shape (O’Connor, 1972; Horod yski, 1976), but also discussed the origin of MTS, particularly of those found in the Mesoproterozoic (1.47-1.1 Ga)of the Belt Supergroup in North America.Three potential origins for MTS were recognized that (1)MTS might have resulted from cleavage of lithif i ed rock due to shear forces, and thus would be secondary in nature (Daly, 1912; Fenton and Fenton, 1937; Rezak, 1957); (2)similarly to the stromatolites, MTS was created by algae and represented an insitu biological-origin structure (Ross, 1959; Plfug, 1968;Smith, 1968; O’Connor, 1972); and (3)MTS resulted from multiple origins, such as sheet MTS formed by organisms directly, and podlike MTS originating from other activity of organisms (e.g.the decayed organic materials producing air bubbles)(Horodyski, 1976).Thus, people can imagine that these ideas marked the beginning of disputes on the origin of MTS.

During the second stage of MTS investigation (1980-1999), additional discoveries of MTS were reported,including examples from the Neoproterozoic of North America (Jameset al.,1998), Norway (Knoll, 1984),Spain (Bragaet al.,1995), West Africa (Bertrand-Sarfati and Moussine-Pouchkine, 1988; Moussine-Pouchkine and Bertrand-Sarfati, 1997), Australia (Calver and Baillie, 1990), and India (Sarkar and Bose, 1992), as well as Meso-Neoproterozoic of Siberia (Petrov, 1993; Bartleyet al.,1997)(Table1).In addition, during this period, MTS was recognized in China for the first time (Failchildet al.,1997).Moreover, with regards to the origin of MTS,several inf l uential theories emerged, including the origins related to seismic activity (Pratt, 1992, 1998, 1999; Qiaoet al.,1994), and the widespread gas expansion hypothesis that considered how air bubbles played the role to creat MTS (Frank and Lyons, 1998; Furnisset al., 1998).Because of the frequent discoveries of MTS in both Mesoand Neoproterozoic strata, the shapes of MTS became even more diverse and complicated, leading to more intricate and varied interpretations of origin.

During the third stage of MTS investigation (since 2000), many researchers turned attention to China.Professor Meng, the leader of the International Geoscience Program (IGCP)447 (2001-2005, titled of “Microsparry(Molar-Tooth)Carbonates and the Evolution of the Earth in the Proterozoic”), greatly promoted the MTS study.These investigations were built upon MTS and related lithologic discoveries during the 1980s; for example, in the northern part of Jiangsu and the Anhui provinces, where MTS was referred to as either abnormal calcite veins or the calligraphy-like structure (Jiangsu Geology and Mineral Exploration Bureau, 1984); in Jilin Province, where it was called the vermicular limestone (Jilin Geology and Mineral Exploration Bureau, 1988); and in Liaoning,Shandong, and Jiangsu Provinces, where they were called mesh-like calcite veins or calcareous veinlets (Liaoning Geology and Mineral Exploration Bureau, 1989).During this period, additional MTSs were discovered in the Meso- and Neoproterozoic across China.These Chinese MTS sites are located in Jilin, Liaoning, Jiangsu, Anhui,Shandong, Henan and Yunnan Provinces, Xinjiang and Inner Mongolia (Qiaoet al.,1994, 2001; Qiao and Gao,1999; Duet al.,2001; Meng and Ge, 2002; Liuet al.,2003;Jiaet al.,2003; Kuang, 2003; Gao and Liu, 2005; Liuet al.,2005).Subsequent to the reports of MTS discovery in the Neoproterozoic in the northeastern, northwestern and southwestern China and North China, MTS was also described from the Mesoproterozoic Gaoyuzhuang Formation in the Jixian region (Mei, 2005), Tianjing of North China and Wumishan Formation in the western Liaoning Province (Kuanget al.,2009b, 2012; Menget al.,2011).In addition to Chinese occurrences, MTS was continually recognized in the Meso- and Neoproterozoic around the world in Siberia (Bartleyet al.,2000; Petrov and Semikhatov, 2001; Popeet al.,2003), Russia and the East European Platform (Bartleyet al.,2007; Kahet al.,2007), India (Chaudhuri, 2003), Norway (Melezhiket al.,2002), Greenland (Fairchildet al.,2000), West Africa(Gilleaudeau and Kah, 2010), and South Africa (Bishop and Sumner, 2006).

During the third stage, MTS has also been discussed in broader terms, than simply its mode of origin, although the discussions of origin reached the unprecedented levels,particularly with respect to, the seismic origins (represented by Qiao)and the biological origin (represented by Meng and the expansion of understanding combined origins of MTS and its microsparry inf i ll (Bishop and Sumner,2006; Bishopet al.,2006; Pollocket al.,2006; Long,2007; Kuanget al.,2011b; Hazenet al.,2013).Additional researches have included investigation of composition,texture, geochemistry morphology, and potential origin of microsparry calcite within MTS (Crawford and Kah,2004; Crawfordet al.,2006; Bishope t al.,2006; Pollocket al.,2006; Bartley and Kah, 2007; Goodman and Kah,2007), and depositional environments (Stagneret al.,2004; Gilleaud eau and Kah, 2010).Much of this work has only been published in abstract form.

Recently, there are only a few published papers regarding the MTS study from outside of China.During the period of 2000-2014, among more than 230 papers related to MTS, there are 130 papers involving Chinese MTS research approximately (Table 1 and Figure 1).Moreover, the theme of concern is no longer the origin of MTS oftentimes, but using it as a symbol of subtidal facies or as a specif i c facies useful for geochemical analysis(Bartleyet al.,2007; Kahet al.,2007; Petrov, 2011; Boseet al.,2012; Hoffmanet al.,2012; Kahet al.,2012).Eventually, the upsurge of MTS research just comes to China in the recent decades.

ope et l.,noll and 1, 2009; P 012 iu et a errington and et al.,0 l., 2000 0 ang et al., 2 4; L ao 7; G References 3 iro, 1987; K atov, 1962 airchild et al., 200 atov, 1998, 200 sca et al., 2011 a, 1978 l., 1995 976; H airchild et al., 200 airchild et a 000; Y l., 200 al., 200 Sp airchild and aby, 1 iu et a Siedleck 2005 ett, 1990; F Braga et a 2009 To Semikh 986; F airchild et al., 199 arfati and C l., 1997, 2 984; F Petrov and oll, 1Sw Bertrand-S Fairchild, 1989; F Jackson, 1 ild et a Fairch l., 1994; F Jia, 2004; Jia et al., 2011; L Qiao et a Kn ipheanSemikh a F ormaroup up per R it arssârssuk horikh Burovaya and S tions ormation ormation roup (upper)ormation orld ikerbreen G ay G ikerbreen, P Un en groups rm Midin the W Up olarisbreen oaldtopp roup: Nation Fo Akadem Båtsfjord F mincun F Xing and R Huainan gro mincun F Xing TS Eleonore B Akadem Thule G ccurrence of M eria orway ib ib egion, S i Table 1 O cation ge, S ern N pain eria reenland Greenland hina Lo id Yenisei R rukhansk R Spitsbergen albard Sv orth Southeast S ast G Northern North C Jiangsu and Anhu iaonng and Jinlin Tu Finnmark, N Central E Southern L try Coun ssia ay ay ay Ru ssia Norw Norw Norw ain Sp Greenland China China Age range (Ma)1121China 1000 00Ru 1030-8 900 00Greenland 700-700 800-700 600 800-12 700-?; ＜1000-600 930 890-＜1000 Era Neoproterozoic 005 Liu, 2 Gao and ormation ao F Hejiay enan t. in H Song M China 4a, 2004b,uang et al., 200 Ge, 2002, K 04c, 2006a 20 Meng and ormainggouzi F Wanlong and Q tions Southern Jilin China l., 2010 iu et a l., 2007; L Zhang et a roup nyang G Ku unnan Central Y China er l., 2007; this pap Wang et a ormation angzhuang F Shiw andong Sh uthern So China g, 1982 Youn roup (lower)Tindir G cenastrdillera/ E Co tral Alaska Northern SA Canada/ U 9 77;1270-1200(8 50)800-000 Walter et al., 2 agunt Kw n anyo d C Gran A US＞742 oung, 1981 g, 1977; Y on g and L Youn oint nolds P ey: R roup niatt formations Shaler G yn and W Western Arctic Canada 1200 723 700-7 to ＞＜ 107

Table 1, continued es et al.,2 997 981 985 es et al., 1998 g, 1981; Jam 2 9 References n and Ianelli, 1 ofmann, 1 2 hields, 200 u et al., 201 ho ofmann, 1985; Jam 2 ong, 1977; Youn ertrand-Sarfati, 1 1998 1; S Baillie, 1990 hields-Z oussine-Pouchkine, 1988;Jackso Aitken, 1981; H Shields, 200 2; S Shields, 200 981; H Calver and arfati and M Miller et al., 200 chkine and B ou Aitken, 1 ung and L Southgate, 199 Shields, 200 Yo Bertrand-S Moussine-P ernits Cliffs up al G ociety up it ormation p: S ro upergrou Un ation ountains Sroup rm roup: Irby S ilt-Fo ittle D prings F ape Gstone Wonoka Bylot S Mackenzie M group: L Bitter S Rocky C roup: I-5 and I-7 u Atar G Tambien G Taoudenni basin Eastern Arctic iopia cation rdillera t.Co stralia ania anges anges th Lo Northern Mackenzie M Victoria Island Central Au Tasm Flinders R Flinders R West Africa rthern E est Africa No Northw try Coun Canada Canada Canada Australia Australia Australia Australia Mauritania Ethiopia Algeria-Mali Age range (Ma)00Canada 827 12 00781 000 550 700-10 700-3 to ＞900＜ 108-850 (850)1080-723 (850)1080 700 (750); ＞～750 750-800-1100-1＜600 to ＞Era Neoproterozoic ose, 1992 Sarkar and B Godavari rift valley Central India 00In dia 00-7 10 3 dhuri, 200 Chau upergroup Godavari S south India dia In rupenin,997 aslov and K ikhatov, 1 em humakov, 1983; M 1991 etrov and S 995; P Kah et al., 2001 Keller and C rokhov et al., 1 Go hean es ip ormation ak Middle R Miroedikha F al L Dism iberia rals South U Uplift, S Turukhansk Western Arctic ssia Ru ssia Ru Canada 680 1650/1350-723＜ 1270 to ＞Meso-terozoic Neopro Bartley et al., 2000 roup Billyakh G aya r M Uchu ssia Ru urcellPratt, 1 998, 1999 Belt-P ordillera Central C 00US A/Canada 14＜1500-glin, 2004 arshall and An 973; M urcellTaylor and S tott, 1 Belt-P ritish thern B ou to S SA Western U Columbia and Alberta A/Canada US 1600-700 (1450)Mesoproterozoic 0 i et al., 201 5, 2007; L Mei, 200 ormation uzhuang F Gao Y eijing jing, B Tian China 1560

We are left with a series of questions: What is MTS? And what characteristics does it have? Why is it of so frequent debates? Why researchers cannot reach a consensus after more than 100 years debate? What is the implication of MTS study? In more than a decade, the author of this paper has the opportunity to study MTS fortunately during the third stage.This paper not only summarized the study results in recent years, but it also discussed the implication and the prospect of MTS study.The most important is in a hope of drawing more attentions to this research.

2 Characteristics of MTS

Molar tooth structure (MTS)is the sedimentary structure made up of series of variously shaped voids and ptygmatical cracks that were f i lled with unusual, equant microsparry calcites from the Precambrian (Jameset al.,1998;Kuang, 2003; Pollocket al.,2006).

2.1 A limited distribution in the Proterozoic

The distributions of MTS are broadly limited in the Meso-Neoproterozoic around the world, with the exception of a few in the Paleoproterozoic to the latest Archean.Until now, no MTS from the Phanerozoic has been reported in the literatures (Table 1 and Figure 1).MTS occured in the Neoproterozoic carbonates of Greenland, Norway,Finland, Canada, United States, Siberia, India, Australia,West Africa, South Africa and China (such as the southern Jilin, eastern Liaoning, Shandong, Jiangsu and Anhui,western Henan, Xinjiang and central Yunnan )(Table 1).In the Mesoproterozoic, MTS is found in Canada, United States, West Africa, Russia, and the East European Platform(Table 1).Additionally, limestones of the Mesoproterozoic Gaoyuzhuang and Wumishan Formations in the Mt.Yanshan also contain abundant MTS.Moreover, MTS has found in the lightly metamorphosed carbonates in the Hutuo group (about 2.5 Ga)in theWutaishan region (Liuet al.,2010).

2.2 Single mineral composition

The molar tooth carbonate (MTC)is a particular car-bonate occurred throughout the Meso- to Neoproterozoic carbonates that are f i lled with MTS.MTS can be referred to a special sedimentary structure within MTC.Investigations demonstrated that MTS within MTC is made up of microsparry calcites (5-15 μm in diameter)(Bishop and Sumner, 2006; Pollocket al.,2006; Kuanget al.,2011a).Thus, an important scientif i c definition is summarized:MTS regularly occurs in the Meso- and Neoproterozoic carbonate rocks, has diverse morphologies at the outcrops,and also contains distinct microsparry carbonates made of either isogranular or irregular polygonal microsparry calcites with grain size of 5-15 μm (Meng and Ge, 2002;Kuang, 2003).Thus, MTC is made of two parts,i.e., the molar tooth structure (MTS)that consists of microsparry calcites, and the host rock (or matrix)(Figure 2A-2C, 2F-2I).

The host rock of MTS is typically made up of fine grain carbonates, such as the micrites, dolomites, or marls (Figure 2); sometimes containing small amount of terrigenous clastic materials (clays and silts).Compared with the host rock, the microspa rry calcites in the MTS are clean and bright without contamination and the grain size smaller than that in the host rock (Figure 2F-2I).The contact relationship between microsparry calcites of MTS and host rock is typically abrupt, but can show dissolving edge contact (Kuanget al.,2004a, 2004b, 2004c; Pollocket al.,2006).Under the cathodoluminescence microscope(CL), the microsparry calcites of MTS show homogeneous nuclei in the size of 3-5 μm, non-luminescent or dully luminescent, which are enveloped by the brighter irregular cement (Figure 2D-2F)(Pollocket al.,2006; Kuanget al.,2011a).

2.3 Morphological diversity

MTS is often called enigmatic sedimentary structure(Frank and Lyons, 1998; Pollocket al.,2006)because of the complex and diverse shapes.Initially, MTS was a morphologic term; afterwards, it was assigned the lithologic implication.In brief, morphological descriptions and the classification of MTS can be divided into two categories(Table 2).

The first category encompasses the origin of shape,and can be subdivided into the autochthonous MTS and the allochthonous MTS.The autochthonous M TS can be subdivided into ribbons (Smith, 1968; Jameset al.,1998;Meng and Ge, 2002; Long, 2007; Kuanget al.,2009a),the f i liform, fusiform, and vermiform morphologies, and spheroidal forms (nodule and air bubble)(Ross, 1959;Frank and Lyons, 1998; Furnisset al.,1998; Pollocket al.,2006).Furthermore, in terms of the degree of bending and brokenness, the ribbon MTS can be further divided into smooth straight ribbon, curved ribbon, and broken ribbon.The allochthonous MTS shows signs of clastic or debris fl ow, which are the secondary deposit of intraclast due to reworking of the autochthonous MTS subsequently.

The second category encompasses the orientation of contact relationships between the MTS and the surrounding strata, which can be horizontal, vertical, or inclined(O’Connor, 1972; Kuanget al.,2006a, 2006b, 2008, 2009a;Peng and Kuang, 2010; Penget al.,2010, 2012).It results in the four morphologies,i.e., perpendicular to bedding surface, oblique to bedding surface, parallel to bedding surface, and disordered.Combining these two categories together, various subtypes are divided (Table 2, Figure 3).In general, MTS does not appear unusual morphologies because of differences in location or time of creation,but rather these morphologies result from differences in the strength of the local substrate (Pollocket al.,2006).However, in recent years, a new MTS has been found in the Upper Neoproterozoic Zhangqu Formation in the Suzhou,Anhui Province.On this section, MTS is perpendicular to the bedding surface and has the same shapes as the other MTSs; whereas on the bedding plane, the bulky ribbons of MTSs make a circular network (Figure 4).

2.4 Specif i ed sedimentary environment

MTS typically occurs within dynamic sedimentary environments, primarily from subtidal to intertidal zones of shallow sea (Figure 5).Some scholars consider that MTS was formed in environments of occasional subaerial exposure (Knoll, 1984).Majority of researchers believe that MTS was formed in shallow water (subtidal to intertidal environments)(Table 3).According to the author's research over the past 10 years (Kuanget al.,2004a, 2004b, 2004c, 2006a, 2006b, 2008, 2009a, 2009b,2011a, 2011b, 2012; Liuet al.,2005, 2010; Peng and Kuang, 2010; Penget al.,2012), the uppermost boundary for creating MTS is the intertidal zone (with MTS rarely reaching the supratidal zone)and the lowermost boundary is near storm wave base.Most commonly, MTS occurred in the fine-grained tidal carbonate rocks, in depositional successions characterized by the rhythmic alternaton of micrite, muddy limestone and calcarenite.MTS can,alternately develop with or within stromatolite.In the latter cases, MTS occurs most commonly between the columns of stromatolites, but sometimes coexist.Over all, MTS developed primarily on the gentle carbonate slopes during the Meso-Neoproterozoic, especially on the upper subtidal zone.These environments were from weak oxidizing to reducing and the MTC appears to be precipitated from moderate salinity and warm marine waters that were supersaturated with CaCO3and received little terrestrial matters (Kuanget al.,2004b, 2006b, 2007, 2009b, 2011a,2011b; Peng and Kuang, 2010).

Table 2 Cate gory of MTS morpholog y Origin F orm Contact relationship with bedding surface Perpendicular (PE)Oblique (OB)Parallel (PA)Disordered (DI)Smooth straight ribbon(SR)PESR OBSR PASR DISR Ribbon Curved ribbon(CR)PECR OBCR PACR DICR Broken ribbon(BR)PEBR OBBR PABR DIBR Autochthonous MTS Fi liform(FF)PEFF OBFF PAFF DIFF Fu siform(FS)PEFS OBFS PAFS DIFS Ver miform(VF)Spheroidal form (SF)Note: The codes in T able 2 mean that, for example, PESR repr esents straight ribbon MTS is perpendicula r to the bedding surface, followed by the same.Scale bar is 5 cm.Allochthonous MTS Clastic form

3 Origins of MTS

There are up to 10 hypotheses about the orgin of MTS since the it was initially discovered and studied (see compilations in Geet al.,2003; Kuanget al.,2006a, 2006b,2011a, 2011b; Long, 2007).Generally, these hypotheses can be divided into three categories: (1)Hypothesis of physical origin,i.e., MTS is formed by f i ssure f i lling and liquidation under external mechanical force; (2)Hypothesis of biological origin, wherein some scholars think that MTS is derived from actions of bacteria or algae directly;(3)A synthesized hypothesis, temporarily called “biogeochemical origin”, which was proposed in recent decade, and it is to analyze the origin of MTS in the terms of the origin of the cracks, the precipitation of MTC withincracks, and their relationship to the geochemical condition in paleocean and potential for biological catalysis.

Table 3 Depositional environments of MTS

3.1 Hypothesis of physical origin

Hypothesis of physical origin dates back to the origination of the term “molar tooth structure”.Daly (1912)considered MTS to be similar to the structure of cleavage.Eby (1977)was more inclined to imagine that MTS ref l ected an originally evaporitic structure f i lled by calcite.Knoll (1984)regarded MTS to be similar to mudcrack;conversely, Calver and Baillie (1990)considered MTS to be subaqueous shrinkage crack.Cowan and James (1992)believed that MTS was formed within strata by tensional cracking resulting from wave activity.The hypothesis that best represents the physical origin is the hypothesis of seismic origin (Pratt, 1992, 1998, 1999, 2001; Qiaoet al.,1994, 2001; Fairchildet al.,1997; Duet al.,2001; Jiaet al.,2003, 2011).In this scenario, there are two theories:one regards liquefaction of the substrate by earthquake(Qiaoet al.,2001), followed by dewatering (Qiao and Gao,2000); another one considers the f i lling of seismic cracks(Pratt, 1998, 1999; Jiaet al.,2011).Together, this hypothesis considers that syndepositional earthquakes caused partially lithif i ed sediments on the ocean f l oor (such as clay or marl)to dewater, shrink, and fracture; then, these cracks were f i lled by isogranular calcite that separated from host rocks.Finally, under pressure and shear forces, the f i lled MTS were deformed into folds that were overlapped with each other either horizontally or vertically, or broken.However, the hypothesis of seismic origin is diff i cult to explain the following questions:

1)Why does MTS only develop in the Meso-Neoproterozoic fine-grained carbonates and marls?

2)Can earthquake rhythmicity explain the characteristics of distribution and correlation of MTS worldwide?

3)Why would earthquake activity be limited to a particular paleoenvironment from the intertidal zone to the storm wave base?

4)Can earthquake hypothesis explain the frequent occurrences of MTS in strata with a thickness of a few dozen meters (Qiaoet al.,2001), and the alternating occurrences of MTS and stromatolite (Jiaet al.,2011)?

5)MTC is mainly composed of microsparry calcite<15 μm diameter.How does the earthquake hypothesis explain liquefaction and segregation of crystal from sediments with such small grain size?

6)Inside MTS, microsparry calcite show clear characteristic of early diagenesis under the CL (Jameset al.,1998; Frank, 1998; Furniss, 1998; Pollocket al.,2006; Liuet al.,2010; Kuanget al.,2011b).How does the earthquake hypothesis explain the universal characteristic of MTC?

3.2 Hypothesis of biological origin

Hypothesis of biological origin came to being since MTS was considered to be resulted from either bacterial or algal activity.Gillson (1929)initially proposed the organic(algae)origin, and Ross (1959)considered the MTS was directly or indirectly related to living process of primary organisms that lived in the original marls.The discovery of organic fi laments in MTS was a great support to this theory (Plfug, 1968; Smith, 1968; O’Connor, 1972).Jameset al.(1998)thought that MTS mostly existing in the Meso-Neoproterozoic is related to the depositional condition during this period, especially, to the development of microorganic mats.The accumulation of SnO2in carbonates was related to the activity of organisms (Baoet al, 1996).A higher abundance of SnO2in the MTS may indicate that organic matter was involved during the forming process of MTC (Kuang, 2003; Kuanget al., 2006b).Meng and others (2006)also insisted the possibility of biological origin.Mei (2005)and Mei and others (2009)believed that the origin of MTS in the carbonate rocks of the Gaoyuzhuang Formation may be related with the activity of organisms in Jixian.The waxing and waning of MTS was correlated with the failing and booming of stromatolite respectively.Chen (2009)also suggested that MTS might be biological origin.However, the idea and interpretation of a bona fi de biological origin continues to encounter challenges.The lower organic content in MTS than that in the host rocks speaks against a biological origin.Additionally, fi lamentous organism (bacteria or algae)in the MTS also can be found in the host rocks.As a result, it is very dif fi cult to prove that the MTS was the direct product of organic activity,i.e., the biological origin can not perfectly reproduce the forming process of MTS and the causes of various complex shapes that occurred.

3.3 Hypothesis of bio-geochemical origin

Hypothesis of bio-geochemical origin is based on evidence that MTS is neither an organism (Jameset al.,1998)nor is a biological produced sedimentary structure.However, the creation of MTS is potentially in fl uenced by biological activity.At the same time, the process of crystallization of MTS obviously is a typically chemical reaction, which is controlled by specif i ed geochemical features of paleoceans during Meso- to Neoproterozoic.Horodyski (1983)initially proposed that the MTS was created by the gaseous bubbles produced by the decaying organisms.Furnisset al.(1998)suggested that this could result in occurring of CO2, as well as bacterial sulfate reduction could produce H2S.The migration of these gases would not only create molar tooth (MT)cracks, but CO2among the gases could also generate calcite (CaCO3)and H2S could form pyrite, which is sometimes observed in association with MTS.Through laboratory tests, Furniss and his team obtained cracks similar to MTS.Subsequent work by Frank and Lyons (1998)and the team of Gellatly and Winston (1999)further interpreted MTC as the product of microsparry calcite rapidly replacing an initial vaterite precursor.In addition, via the study of carbon isotopic compositions, they determined that MTS was created during the early stage of diagenesis and the forming condition must be conducive to the rapid deposit of calcite(Frank and Lyons, 1998).Similarly, Marshall and Anglin(2004)presented a hypothesis that the MTS was resulted from clathrate (CO2gas-hydrate)destabilization.Shields(1999)further suggested that the inf i lling of MTS was related to the changes of marine chemical composition and the disappearance of MTS was resulted from decrease of CaCO3concentration (and/or the increase of SO42-concentration)(Shields, 2002).

In general, the proposal of Shields (1999)gained the widest support (Geet al.,2003; Kuang, 2003; Menget al.,2006; Kuanget al.,2007)conducting in-depth analysis of chemical properties of paleocean during the Meso-Neoproterozoic (Hofmann, 1985; Kahet al.,2001; Chuet al.,2004; Bartleyet al.,2007; Kuanget al.,2008,2009b, 2011a).At the same time, through analyzing the compositional and geochemical features of MTS, the hypothesis of bio-geochemical origin overcomes the diff i culties and limitations of the former two theories,specif i cally whether MTS is partly controlled by the joint inf l uence of unique chemical properties in the paleocean and potential metabolic activities of the microorganic colonies (Shields, 2002; Bishopet al.,2006; Pollocket al.,2006; Bartleyet al.,2007).In addition, the studies that carried out by Bishop and others (2006)and Pollock and others(2006)advanced understanding of origin of MTS to a new level.Pollockand others(2006)detailed how deposition of microsparry calcite could directly related with the gaseous bubbles caused by decomposition of organisms in the host rocks, and showed how the grain size and strength of lithological substrates could control crack morphology in the host rocks.By contrast, Bishop and Sumner (2006)utilized a wave-induced f l uid f l ow model to explain the formation of MTS.In this scenario,sediments cracks, forming an interconnected network of MT cracks; then, storm waves pump sea water into open MT crack networks, causing rapid microcrystalline carbonate nucleation, ripening of nuclei, and growth of granular microsparry calcites (cores)that f i lled cracks.In the view of Bishop and Sumner (2006), f i lled MTS ribbons deform plastically as host sediments are compacted and dewatered, and MTS deforms brittlely under additional compaction, so as to create various complicated shapes.Post-depositional deformation of cracks is also possible in the theory of Pollock and others (2006)(see also Stagneret al.,2004), although their work shows the effects of substrate strength as a critical factor as well.In both cases,deposition of microsparry calcite is independent with the creation of MTS cracks in the host rocks.Microcrystals grow in the nuclei of MT in these cracks and quickly f i ll the cracks.Obviously, the hypothesis of bio-geochemical origin is widely accepted, because it better explains the origin of materials and creation process of MTS.

3.4 Summary of origin research

The mechanism of MTS origin has not yet come to a complete understanding.However, the ranges of MTS related subjects (such as lithologic features, morphologies,environments, geologic periods and geographic distributions of MTS)have been recognized.The origin of MTS is basically undertood that MTS is the collective product of evolution of global paleocean, paleoclimate, and paleoenvironment, and could not be simply formed by earthquakes or the “earthquake rhythms”.Some researches link the MTS to the vanishing of stromatolites under seismic activities (Jiaet al.,2011), which obviously deviates from the fundamental seismic interpretation.It is a common knowledge that stromatolite is the byproduct of microbial growth and lithif i cation (Grotzinger and Knoll, 1999).If it is true that seismicity would cause the disappearance of stromatolites and create the MTS (Jiaet al.,201 1), then,the implication would be that earthquake activity is one of external factors affecting the biological activity; however,MTS is not created by liquefaction stimulated by earthquake but affected by biological activity.

In recent years, our study further proves that the creation of MTS is not a solitary process, but is controlled by physical properties (temperature, and solubility of CaCO3)and chemical properties (concentrations of Fe, SO42-, oxygen fugacity, salinity, and isotopic levels of C, O and Sr)in the paleocean; also, it is closely related to the environment of atmospherepaleolatitude (which controls the temperature of seawater), and the biological processes(levels of redox and the circulation betweenand carbon in the seawater could accelerate the crystallization of CaCO3as well)(Kuanget al.,2006b, 2007, 2008, 2009b,2011a, 2011b; Liuet al.,2010).Based on the model of Pollock and others (2006), we also propose a model of MTS origin (Kuanget al.,2011b)(Figure 5), which can explain the origin of most morphologies.As a result, the bio-geochemical hypothesis maybe is the most parsimonious method for analyzing the origin of MTS.Even through, it is still dif fi cult to explain the cyclic MTS that was discovered in the Zhangqu Formation in Suzhou, Anhui Province recently (Figure 4).Noticeably, a variety of complicated shapes of MTS can not be reasonably explained by the origin theories discussed above.The study of MTS origin still has a long way to go.

4 Research signi fi cance

So far, compared to the majority achievements in the depositional environments and the origins of MTS, the studies on microtexture and related geochemistry just begin.The creation and occurrence of MTS is not an isolated simple event, its nature of global occurrence in a limited time interval and specif i ed depositional environment leads us to recognize that the continual study of MTS should not only focus on the issue of origin.Although some aspects of the MTS origin are still controversial, most researchers agree with the two practical functions of MTS,i.e., as a facies indicator (indicating a shallow water subtidal zone)and as a mark of stratigraphic correlation (used for stratigraphical correlation of the Proterozoic which is widely distributed in stable craton in the world).These two practical functions are possibly more signif i cant than the origin in the Precambrian study.The MTS study provides a new approach to resolve various important Precambrian questions,i.e., it provides a better understanding on ocean chemistry (Pollocket al.,2006), depositional palaeogeography, and stratigraphic correlation, also it may even be useful for comprehending the supercontinent reconstruction (Gao and Qiao, 2001; Melezhiket al.,2002; Vigneresse, 2005; Qiaoet al.,2007).

4.1 Global change in the Precambrian

During different historical periods of the Earth, the lithology or mineral composition and diversity are different.For example, the carbonate minerals include many types,such as the seaf l oor crusts of f i brous crystals, micrites,aragonite fans, and stromatolites (Kah and Knoll, 1996;Jameset al.,1998; Bartleyet al.,2000).The global change of palaeogeography, paleocean and palaeoclimate controls the formation, evolution, distribution and disappearance of MTS.The time interval from the earliest record of MTS(the Paleoproterozoic, 2.6 Ga, in South Africa)to the latest occurrence (750 Ma, in Australia), is a crucial period for the prokaryotic organisms evolving into eukaryotic organisms and for evolution of multicellular organisms.

During this special period, the major occurrence of MTS globally is correlated to the occurrence of banded magnetite, high value of isotopic carbon, low value of isotopic oxygen (Kuanget al.,2011b), and warm sea water.Over all,the thermal fi eld of the Earth inclined to a relatively high end, which might be in fl uenced by relatively higher partial pressure of carbon dioxideof the Precambrian (Bartley and Kah, 2007; Kah and Riding, 2007)or be affected by volcanic events.Furthermore, from the view of the palaeogeographic location of tectonic plates, the developmental region of MTS locates around the mid-low latitude area(Zhanget al.,2000; Zhaoet al., 2004; Jaffréset al.,2007;Liet al., 2008)in the continental margins (Figure 7)and the salinity of seawater was in the normal range all the time.However, before major appearance and after disappearance of MTS, the characteristic of marine carbonate geochemistry displayed relatively high salinity, especially the content of sulfate was increased (Shields, 2002).The disappearance of MTS has been used as one of the markers concerning with the beginning of the Cryogenian in the pre-glacial period(Shields-Zhouet al., 2012).

The origin of MTS thereby appears to have been closely related to the Proterozoic palaeoenvironment, paleocean,and palaeoclimate.After searching microtexture and geochemical characteristic of MTS, it is proved that the creation and vanishing of MTS not only was controlled by the alteration of chemical properties in paleocean, but also jointly in fl uenced by atmosphere, climate, and organic activity.The visible characteristic of MTS is the organic assemblage of internal composition and the external confi guration.Therefore, based on the aspects above, the established origin model can favorably explain the source of materials for the microsparry calcite, the process of creation, and the control factors (Figure 6, and Kuanget al.,2011b, 2012).

As a special type of carbonates with an enigmatic feature worldwide, MTS only occurs in the paleocean in the transitional period between the lifeless Archean and the Phanerozoic represented by the Cambrian bio-radiation and the shell fragment carbonates (Jameset al.,1998).Thus, the MTC is the responsive deposit and the record of the Precambrian alteration around the world.In return, we can learn the global alteration of the Precambrian in term of studying the characteristics of MTS.

4.2 Stratigraphic correlation of the Precambrian

The study of either intra-basinal or inter-basinal correlation of the Proterozoic sedimentary strata is a challenge due to the lack of suitable organisms for detailed biostratigraphy and the shortage of geochronological data, in addition to the complicated litho-stratigraphy (Kahet al.,2012).On the other hand, the occurrence and vanishing of MTC was not only related with depositional environment and marine chemical conditions of that time, but also coordinated with the biological evolution on the Earth.MTC is a particular type of the Meso-Neoproterozoic carbonate with special texture and structure and it is controlled by the conditions of marine physics and chemistry.Furthermore,MTC is also precisely limited by the depositional environment and lithologic facies.The characteristics of global distribution of MTS will be a good index to solve the problems of mute stratigraphic correlation in the Precambrian.

4.3 Reconstruction on global palaeogeography

All of the MTSs around the world not only have similar characteristics in the lithology, morphology, depositional environment, microtexture, and geochemical feature, but they also are limited within speci fi ed time and space.The MTS in the Neoproterozoic carbonates in the North China can be broadly correlated with the one from other continents.The geological age of MTS is in the range ofca.1700-750 Ma (may be earlier than 720 Ma, see Macdonaldet al.,2010)before the Sturtian glaciation.MTS completely disappeared before the Marinoan glaciation(630 Ma)(Menget al.,2006; Shieldset al.,2012).Clearly,before the glaciation, during the Late Mesoproterozoic to the Early Neoproterozoic convergence of Rodinia supercontinent, the warm oceanic environment was bene fi cial to the creation of MTS.Their reliable global distribution and excellent correlation indicates that MTS was widely created on the entire Rodinia supercontinent (Figure 7),which further implies that the North China Plate and the Rodinia supercontinent may be associated in some degree.

It requires more researches to clarify whether the MTS and the supercontinent cycle have a kind of relationship or not.However, MTS occurred in the geological records by means of episodic deposit is an indisputable fact.There are at least three episodic deposits (Table 1): the first is during the Neoarchean (South Africa, 2.6 Ga)to the Early Paleoproterozoic (Canada, 1.9-1.75 Ga), and then respectively, about 1.6-1.4 Ga (represented by North America, North China and Siberia), and 1.0-0.8 Ga (North China, Europe and Australia).According to the new testing data, the final convergence of the Colombia supercontinent occurred in 1.4 Ga approximately (Zhaoet al.,2004),while the convergence of the Rodinia supercontinent took place around 1.2-1.0 Ga (Sunet al., 2012)or 1.2-0.9 Ga (Qiaoet al.,2007; Liet al.,2008).The coupling relationship between peaks of MTS occurrence and the final convergence time of the supercontinent imply that there may be some connections.Whether these connections are clearly to supercontinents, or perhaps to sea level and the distribution of environments that result from supercontinent formation, needs more evidence to be established (Figure 7)in the future.

5 Advanced research for the future

MTS has important implications for researches in paleocean and palae ogeography.It will be helpful for exploring the relationship between paleocean environment,sedimentary geochemical condition, palaeogeography and microbial activity, and to systematically study and discuss the origin mechanism of MTS.Therefore, the advanced research in the future, on the one hand is to continuously discuss the relationship among paleocean environment,geochemical condition and MTS types to improve the theory of origin; on the other hand, is to further con fi rm the possibility whether MTS can be used as a marker for stratigraphic correlation and for sedimentary facies identifi cation or not.As a result, it can be applied to the researches on paleocean and palaeogeographic reconstruction and will make contribution to solve some signi fi cant scienti fi c problems.

1)Geochemistry and microfabric studies

Geologists have conducted substantial researches on the morphology and development of MTS previously.However, studies regarding characteristics of microtexture and geochemistry of MTC are still insuf fi cient.Up to now,technological measures including polarizing microscope(PM), scanning electron microscope (SEM), cathodoluminescence microscope (CL), backscattered-electron diffraction (BSED)and electron probe microanalysis (EPMA)have been used to reveal the shape of MT microsparry calcites and the relationship between them.Geologists,whereas can not clearly display the interior structure and the distribution relationship of cements between microsparry calcites, which are important to reveal the creation process, material resources and factors affecting crystallization.The crystallization process of microsparry calcite is described or analyzed very occationally on the microscopic level.In addition, the scientif i c information of temperature, pressure and external conditions and the crystallization process of MTS formation are hard to obtain,even if some simulation experiments were accomplished(Goodman and Kah, 2007; Hazenet al.,2013).The above mentioned researches should be promoted in the future.

2)Further studies of the relationships among the morphology of MTS, depositional environment and the geochemical composition

Although the complicated morphology led to the enigma of MTS origin, the relationship between morphology of MTS and depositional environment is evident.With the further study and more morphologies being discovered,the microtexture of different morphologies and geochemistry characteristics of MTS will reveal the origin of MTS on a new level.Although the relationship between morphology of MTS and sedimentary environment revealed by geochemical characteristics were emphasized and discussed previously, comparative study on the geochemical properties and sedimentary facies of different MTS are still necessary.Not only to compare the geochemical properties of different MTS in the same sedimentary environment,but also to compare the same types of MTS in different sedimentary environments, will be helpful to indicate the relationship among morphology of MTS, sedimentary environments and geochemical properties of paleocean.

3)Origin of dolomitic MTS

The researches of the past one hundred years indicated that MTS of different geologic periods around the world consists primarily of microsparry calcite.From the Mesoproterozoic Belt Supergroup in North America, Riphean stage in the southern Ural, Russia to the Gaoyuzhuang and the Wumishan Formations in the Yanshan region of China, MTS consist of microsparry calcite.Among the Neoproterozoic, although host rocks of MTS consist of dolomite, MTS is still composed of microsparry calcite,e.g.,MTS from the Bitter Spring Formation in Australia (Figure 2H)and from the Wangshan Formation in northern Anhui Province of China.In the meantime, a new problem appears: a large quantity of dolomitic MTS developed in the dolomitic strata in the lower part of the Yingchengzi Formation on the Zhaokanzi section in eastern Liaoning Province.These MTS-like structures developed inside the primary dolomite consist of dolosiltite with indistinct trait of f i lling and are lack of cementing lineage.The testing results of isotopic carbon and isotopic oxygen indicate the features of syngenesis or penecontemporaneous dolomite (unpublished data).In the previous studies, MTS is consisted of microsparry calcite.Is the dolomitic MTS-like mineral originally MTC?How is it formed? We need to understand the creation of dolomitic MTS- like, structures, especially, the diagenesis process.

4)Practical value to the Precambrian study

MTC is a type of carbonates with special origin and commonly develops during the Meso-Neoproterozoic around the world.MTC appears to have initiated in the Neoarchean-Early Paleoproterozoic, peaked its development during the 1000-900 Ma (Table1 and Figure 7)on the continents around the world, and disappeared before the Sturtian glaciation (750 Ma or 720 Ma)(Shields,2002; Macd onaldet al.,2010).As we know, the regionalscale magmatic events at 1460 Ma, 1380 Ma, and 1280 Ma can be associated with the breakup of the proposed Late Paleoproterozoic supercontinent, Nuna (Columbia);the events at 1300-900 Ma overlapped with the assembly of Rodinia; and the events at 825 Ma, 800 Ma, 780 Ma,755 Ma, and possibly 720 Ma (Liet al.,2008), were associated with the breakup of Rodinia.MTC possibly had a potential connect with the formation and evolution of supercontinent.It is also of global correlation signif i cance and can be used as a marker of global change.As a special product existed only in the Precambrian, MTS certainly is related to geochemical conditions of the unique Precambrian Ocean and the sedimentary background.So it surely will reveal the geochemical conditions and is useful in reconstructing paleocean environment in the Precambrian.As above mentioned, these are the urgent tasks of MTS studies, in order to demonstrate the viability of using MTS as an index for the Precambrian stratigraphic correlation and sedimentary environment identif i cation, and to prove its scientif i c values as early as possible.

Acknowledgements

This paper is sponsored by the National Natural Science Foundation of China (40772078, 41472082).During the writing process of this paper, I would like to express our sincere gratitude to Prof.Yong-Qing Liu, who offered useful comments and suggestions and some fantastic photos; Ms.Dechin Wang is so kind to translate the manuscript into English; thanks to the graduate student Ming-Wei Wang, Dr.Nan Peng and Qi-Chao Zhang, who made contributions to organizing references, f i gures and tables of this paper.I also express my thanks to Prof.Zeng-Zhao Feng and the editing team ofJournal of Palaeogeographyfor their much effort to polish this paper.At meantime, I give my respectably thanks to four reviewers: Dr.Linda Kah, Dr.Graham Shields-Zhou, Dr.Liu-Qin Chen and Prof.Lin-Zhi Gao for their appropriated opinions and benef i ted suggestions, and especially thanks to Dr.Linda Kah,she is so kind to streamline the whole paper.

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