review of molar tooth structure research

Journal of Palaeogeography2014, 3(4): 359-383

DOI: 10.3724/SP.J.1261.2014.00062

Biopalaeogeography and palaeoecology

Review of molar tooth structure research

Hong-Wei Kuang*

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

Abstract For more than a century, molar tooth structure (MTS) has been studied. The study developed in three stages. During the first stage (before 1980), researchers described three basic morphologies of MTS, mainly from the Belt Supergroup in North America, and they provided several hypotheses for the origin of MTS. During the second stage (1980-1999), the frequent discoveries of MTS on all continents resulted in many detailed descriptions of their shape and in several hypotheses concerning the origin of MTS. Notably, hypotheses of MTS’s origin such as seismic activity and biological activity were developed. Since 2000, research has progressed into a new stage (the third stage). This is due to discoveries of MTS in the Me-so-Neoproterozoic of China and elsewhere, and the ongoing debate on the seismic or biologi-cal origin is replaced by a hypothesis that involves gas expansion and chemically-controlled carbonate precipitation (both of them possibly affected by biological activities). This latter idea has gradually been commonly recognized as the mainstream theory. Despite continued disa-greements, researchers now agree that microsparry calcite played a controlling role regarding the development and the global distribution of MTS in time and space during the Proterozoic, the morphological diversity, and the impact on the sedimentary environment. The present con-tribution analyses the three major hypotheses regarding the origin of MTS; it also discusses the shortcomings of the hypotheses regarding a seismic or biologic origin, and it details the modern hypothesis that links formation of cracks to the precipitation of sparry calcite. It is de-duced that important questions dealing with the Precambrian can be answered, among other aspects regarding the depositional palaeogeography and stratigraphic correlations.

Key words molar tooth structure, sparry calcite, Precambrian

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), researchers not only focused on the morphology of MTS, including the

* E-mail: [email protected]. Received: 2014-03-21 Accepted: 2014-07-03

vertical and horizontal sheet-like features and the pod-like shape (O’Connor, 1972; Horodyski, 1976), but also discussed the origin of MTS, particularly of those found in the Mesoproterozoic (1.47-1.1 Ga) of the Belt Super-group in North America. Three potential origins for MTS were recognized that (1) MTS might have resulted from cleavage of lithified rock due to shear forces, and thus would be secondary in nature (Daly, 1912; Fenton and Fenton, 1937; Rezak, 1957); (2) similarly to the stroma-tolites, MTS was created by algae and represented an in-situ 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

360 JOURNAL OF PALAEOGEOGRAPHY Oct. 2014

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 (James et al., 1998), Norway (Knoll, 1984), Spain (Braga et al., 1995), West Africa (Bertrand-Sarfati and Moussine-Pouchkine, 1988; Moussine-Pouchkine and Bertrand-Sarfati, 1997), Australia (Calver and Bail-lie, 1990), and India (Sarkar and Bose, 1992), as well as Meso-Neoproterozoic of Siberia (Petrov, 1993; Bartley et al., 1997) (Table1). In addition, during this period, MTS was recognized in China for the first time (Failchild et al., 1997). Moreover, with regards to the origin of MTS, several influential theories emerged, including the origins related to seismic activity (Pratt, 1992, 1998, 1999; Qiao et al., 1994), and the widespread gas expansion hypothesis that considered how air bubbles played the role to creat MTS (Frank and Lyons, 1998; Furniss et al., 1998). Because of the frequent discoveries of MTS in both Meso- and 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 (Qiao et al., 1994, 2001; Qiao and Gao, 1999; Du et al., 2001; Meng and Ge, 2002; Liu et al., 2003;

Jia et al., 2003; Kuang, 2003; Gao and Liu, 2005; Liu et 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 (Kuang et al., 2009b, 2012; Meng et al., 2011). In addition to Chinese occurrences, MTS was continually recognized in the Meso- and Neoproterozoic around the world in Siberia (Bartley et al., 2000; Petrov and Semikhatov, 2001; Pope et al., 2003), Russia and the East European Platform (Bartley et al., 2007; Kah et al., 2007), India (Chaudhuri, 2003), Norway (Melezhik et al., 2002), Greenland (Fairchild et 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 infill (Bishop and Sumner, 2006; Bishop et al., 2006; Pollock et al., 2006; Long, 2007; Kuang et al., 2011b; Hazen et al., 2013). Additional researches have included investigation of composition, texture, geochemistry morphology, and potential origin of microsparry calcite within MTS (Crawford and Kah, 2004; Crawford et al., 2006; Bishop et al., 2006; Pollock et al., 2006; Bartley and Kah, 2007; Goodman and Kah, 2007), and depositional environments (Stagner et al., 2004; Gilleaudeau 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 specific facies useful for geochemical analysis (Bartley et al., 2007; Kah et al., 2007; Petrov, 2011; Bose et al., 2012; Hoffman et al., 2012; Kah et al., 2012). Eventually, the upsurge of MTS research just comes to China in the recent decades.

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

361Vol. 3 No. 4 Hong-Wei Kuang: Review of molar tooth structure researchTa

ble

1 O

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362 JOURNAL OF PALAEOGEOGRAPHY Oct. 2014Ta

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363Vol. 3 No. 4 Hong-Wei Kuang: Review of molar tooth structure researchTa

ble

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ontin

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of w

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be

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d in

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(199

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(199

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hiel

ds (2

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, and

Liu

et a

l. (2

010)

.


Figure 1 The statistics of MTS papers. Pt = Proterozoic.

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 struc-ture made up of series of variously shaped voids and ptyg-matical cracks that were filled with unusual, equant micro-sparry calcites from the Precambrian (James et al., 1998; Kuang, 2003; Pollock et 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 excep-

tion 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 (Liu et al., 2010).

2.2 Single mineral composition

The molar tooth carbonate (MTC) is a particular car-

365Vol. 3 No. 4 Hong-Wei Kuang: Review of molar tooth structure research

bonate occurred throughout the Meso- to Neoproterozoic carbonates that are filled with MTS. MTS can be referred to a special sedimentary structure within MTC. Investiga-tions demonstrated that MTS within MTC is made up of microsparry calcites (5-15 μm in diameter) (Bishop and Sumner, 2006; Pollock et al., 2006; Kuang et al., 2011a). Thus, an important scientific 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 cal-cites 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 microsparry 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 (Kuang et al., 2004a, 2004b, 2004c; Pollock et 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) (Pollock et al., 2006; Kuang et al., 2011a).

2.3 Morphological diversity

MTS is often called enigmatic sedimentary structure (Frank and Lyons, 1998; Pollock et al., 2006) because of the complex and diverse shapes. Initially, MTS was a mor-phologic 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 MTS can be subdivided into ribbons (Smith, 1968; James et al., 1998; Meng and Ge, 2002; Long, 2007; Kuang et al., 2009a), the filiform, fusiform, and vermiform morphologies, and spheroidal forms (nodule and air bubble) (Ross, 1959; Frank and Lyons, 1998; Furniss et al., 1998; Pollock et 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 flow, 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; Kuang et al., 2006a, 2006b, 2008, 2009a; Peng and Kuang, 2010; Peng et 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 (Pollock et 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 Specified sedimentary environment

MTS typically occurs within dynamic sedimen-tary 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 (Kuang et al., 2004a, 2004b, 2004c, 2006a, 2006b, 2008, 2009a, 2009b, 2011a, 2011b, 2012; Liu et al., 2005, 2010; Peng and Kuang, 2010; Peng et 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


Figure 2 Microfabric of MTS: compositions of MTS and relationship of contact. A-A MTS limestone from the Neoproterozoic Xingmingcun Formation in Dalian; the calcites within MTS are clear and either isometric or anisometric polygons; the contact with the host rock is distinct; the host rock contains clay and organic substances; B and C-MTS from the Neoproterozoic Zhangqu Formation of Anhui Province; B displays sheet calcites that contain dark argillaceous materials; C displays a SEM backscatter image showing cores of individual calcite (potentially originally vaterite; cf. Furniss et al., 1998); C is an enlarged spot of B; D-Under CL, the form of calcite grain from the MTS in the Mesoproterozoic Belt Supergroup, USA (from Pollock et al., 2006) is similar to that in E; E-CL image of calcite from MTS in the Neoproterozoic Nanguanling Formation, Dalian region; individual calcite is isometric polygon with dark color; the calcite cement fills the spaces between the crystals and they illuminate orange-yellow light; F and G-MTS from the Neoproterozoic Wanlong Formation, Jilin Province displays the ptygmatic folds; H-Calcite of MTS from the Neoproterozoic Bitter Spring Formation in Australia; the host rock is made of dolomite; I-MTS from Mesoproterozoic Wumishan Formation, western Liaon-ing; if stained with alizarin red, calcite become red, but dolomite is non-stainable under thin section in H and I. A-B and F-I were taken under polarized microscope.


Table 2 Category of MTS morphology

Origin FormContact relationship with bedding surface

Perpendicular (PE) Oblique (OB) Parallel (PA) Disordered (DI)

Autochtho-nous MTS

Ribbon

Smooth straight ribbon (SR)

PESR OBSR PASR DISR

Curved ribbon(CR)

PECR OBCR PACR DICR

Broken ribbon(BR)

PEBR OBBR PABR DIBR

Filiform (FF)

PEFF OBFF PAFF DIFF

Fusiform (FS)

PEFS OBFS PAFS DIFS

Vermiform (VF)

Note: The codes in Table 2 mean that, for example, PESR represents straight ribbon MTS is perpendicular to the bedding sur-face, followed by the same. Scale bar is 5 cm.

Spheroidal form (SF)

Allochtho-nousMTS

Clastic form


Figure 3 Morphologies of MTS around the world. A-DICR and CF MTS from Mesoproterozoic Belt Supergroup, USA (Photoed by Darrel Long); B-Mainly CR and PSFS MTS from Neoproterozoic Bitter Spring Formation, Australia; C-CR and FF MTS occurred in the Riphean stages, southern Ural (Gao et al., 2007), Russia; D-FS MTS in the limestone from the Mesoproterozoic Gaoyuzhuang For-mation, Jixian, Tianjin; E-FS, VF and SF MTS occurred in the Mesoproterozoic Wumishan Formation in Liaoning Province; F-The coarse OBCR MTS from Neoproterozoic Shiwangzhuang Formation of the Mt. Fulai in Shandong Province; G-CR, FS and VF MTS and H-mainly VF MTS from Mesoproterozoic in Siberia; I-PSER, VF and FS MTS and J-mainly PSER MTS from Neoproterozoic Zhangqu Formation, in the northern Anhui Province (I and J according to Liu et al., 2005); K-DICR MTS of Neoproterozoic Wanlong Formation in Jilin Province; L-Mainly OBCR and PEFS MTS in the Neoproterozoic Xingmincun Formation in Dalian; M-FS MTS from Neoproterozoic in the Tarim Basin, Xingjiang; N-PEFF MTS from Meso-Neoproterozoic Kunyang Group in the Yunnan Prov-ince; O-Mainly PECR and a few of DIFF MTS occurred in the Neoproterozoic in Inner Mongolia; P and Q-DISR and DIBR MTS from Mesoproterozoic Atar Group in Mauritania. Scale bar is 5 cm in A to O and 2 cm in P and Q.


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 CaCO3 and received little terrestrial matters (Kuang et al., 2004b, 2006b, 2007, 2009b, 2011a, 2011b; Peng and Kuang, 2010).

3 Origins of MTS

There are up to 10 hypotheses about the orgin of MTS since the it was initially discovered and studied (see com-

pilations in Ge et al., 2003; Kuang et 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 fissure filling and liquidation under external mechanical force; (2) Hypoth-esis of biological origin, wherein some scholars think that MTS is derived from actions of bacteria or algae directly; (3) A synthesized hypothesis, temporarily called “bio-geochemical origin”, which was proposed in recent dec-ade, and it is to analyze the origin of MTS in the terms of the origin of the cracks, the precipitation of MTC within

Figure 4 The circular MTS occurred in the Neoproterozoic Zhangqu Formation in the Suzhou, Anhui Province (D to F, provided by Dr. Zhen-Yu He).


cracks, and their relationship to the geochemical condition in paleocean and potential for biological catalysis.

3.1 Hypothesis of physical origin

Hypothesis of physical origin dates back to the origi-nation of the term “molar tooth structure”. Daly (1912) considered MTS to be similar to the structure of cleav-age. Eby (1977) was more inclined to imagine that MTS reflected an originally evaporitic structure filled 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; Qiao et al., 1994, 2001; Fairchild et al., 1997; Du et al., 2001; Jia et al., 2003, 2011). In this scenario, there are two theories: one regards liquefaction of the substrate by earthquake (Qiao et al., 2001), followed by dewatering (Qiao and Gao, 2000); another one considers the filling of seismic cracks (Pratt, 1998, 1999; Jia et al., 2011). Together, this hypothe-sis considers that syndepositional earthquakes caused par-

tially lithified sediments on the ocean floor (such as clay or marl) to dewater, shrink, and fracture; then, these cracks were filled by isogranular calcite that separated from host rocks. Finally, under pressure and shear forces, the filled MTS were deformed into folds that were overlapped with each other either horizontally or vertically, or broken. However, the hypothesis of seismic origin is difficult to explain the following questions:

1) Why does MTS only develop in the Meso-Neopro-terozoic 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 (Qiao et al., 2001), and the alternating occurrences of MTS and stromatolite (Jia et 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?

Table 3 Depositional environments of MTS

Location Geological time Depositional environment Reference

North America, Africa Meso-Neoproterozoic

Depositional system of gentle carbonate ramp on the margin of cratonic basin.

Shallow water subtidal zone Kah et al., 2012Mt. Mackenzie

northern CanadaEarly Neoproterozoic Bylot Su-pergroup, Little Dal Formation

Shallow water and mid-depth water, gentle slope of the subtidal zoneBaffin Island Late Mesoproterozoic Victor

Bay Formation James et al., 1998Kah et al., 2001

Arctic islands Early Neoproterozoic Boot inlet Formation

North America Mesoproterozoic Belt Super-group, Helena Formation

Located inside the photic zone, below the base of storm wave 50-100 m. Pratt, 1998, 1999

South Africa Archaeozoic Subtidal zone, extending to near the base of storm wave Bishop et al., 2006

Mauritania Atar Group Shallow water carbonate in subtidal zone kah et al., 2012

Ji-Liao-Xu-Huai Area, China Neoproterozoic Shallow water platform and inner to mid-ramp Meng et al., 2006

Jilin, Liaoning Provinces, the

Xuhuai region in China

Neoproterozoic

Shallow water and strong storm environment on the platform and inner to mid-ramp; on the inner ramp, MTS are rare in the coarse grain rock and stromato-lite, but plentiful in the lower intertidal zone and the

shallow water subtidal zone.

Kuang et al., 2004c

China Meso-Neoproterozoic Subtidal zone and the peritidal zone above the base of storm wave Liu et al., 2005


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6) Inside MTS, microsparry calcite show clear characteristic of early diagenesis under the CL (James et al., 1998; Frank, 1998; Furniss, 1998; Pollock et al., 2006; Liu et al., 2010; Kuang et 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 filaments in MTS was a great support to this theory (Plfug, 1968; Smith, 1968; O’Connor, 1972). James et al. (1998) thought that MTS mostly existing in the Me-so-Neoproterozoic is related to the depositional condition during this period, especially, to the development of mi-croorganic mats. The accumulation of SnO2 in carbonates was related to the activity of organisms (Bao et al, 1996). A higher abundance of SnO2 in the MTS may indicate that organic matter was involved during the forming process of MTC (Kuang, 2003; Kuang et al., 2006b). Meng and oth-ers (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 biologi-cal origin. However, the idea and interpretation of a bona fide biological origin continues to encounter challenges. The lower organic content in MTS than that in the host rocks speaks against a biological origin. Additionally, fila-mentous organism (bacteria or algae) in the MTS also can be found in the host rocks. As a result, it is very difficult to prove that the MTS was the direct product of organic activity, i.e., the biological origin can not perfectly repro-duce the forming process of MTS and the causes of vari-ous 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 (James et al., 1998) nor is a biological produced sedimentary structure. However, the creation of MTS is potentially influenced by biological activity. At the same time, the process of crystallization of MTS obviously is a typically chemical

reaction, which is controlled by specified 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. Furniss et 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 CO2 among 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 (CO2 gas-hydrate) destabilization. Shields (1999) further suggested that the infilling of MTS was related to the changes of marine chemical composition and the disappearance of MTS was resulted from decrease of CaCO3 concentration (and/or the increase of SO4

2-

concentration) (Shields, 2002). In general, the proposal of Shields (1999) gained the

widest support (Ge et al., 2003; Kuang, 2003; Meng et al., 2006; Kuang et al., 2007) conducting in-depth analysis of chemical properties of paleocean during the Meso-Neoproterozoic (Hofmann, 1985; Kah et al., 2001; Chu et al., 2004; Bartley et al., 2007; Kuang et 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 difficulties and limitations of the former two theories, specifically whether MTS is partly controlled by the joint influence of unique chemical properties in the paleocean and potential metabolic activities of the microorganic colonies (Shields, 2002; Bishop et al., 2006; Pollock et al., 2006; Bartley et 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. Pollock and 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 fluid flow 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 filled cracks. In the view of Bishop and Sumner (2006), filled 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 Stagner et 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 fill 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 re-lated subjects (such as lithologic features, morphologies, environments, geologic periods and geographic distribu-tions 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 paleoen-vironment, and could not be simply formed by earthquakes or the “earthquake rhythms”. Some researches link the MTS to the vanishing of stromatolites under seismic ac-tivities (Jia et al., 2011), which obviously deviates from the fundamental seismic interpretation. It is a common knowledge that stromatolite is the byproduct of microbial growth and lithification (Grotzinger and Knoll, 1999). If it is true that seismicity would cause the disappearance of stromatolites and create the MTS (Jia et al., 2011), 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 earth-quake but affected by biological activity.

In recent years, our study further proves that the crea-tion of MTS is not a solitary process, but is controlled by physical properties (temperature, and solubility of CaCO3) and chemical properties (concentrations of Fe, SO4

2-, oxy-gen fugacity, salinity, and isotopic levels of C, O and Sr)

in the paleocean; also, it is closely related to the environ-ment of atmosphere (PCO2 ), paleolatitude (which controls the temperature of seawater), and the biological processes (levels of redox and the circulation between PCO2 and car-bon in the seawater could accelerate the crystallization of CaCO3 as well) (Kuang et al., 2006b, 2007, 2008, 2009b, 2011a, 2011b; Liu et al., 2010). Based on the model of Pol-lock and others (2006), we also propose a model of MTS origin (Kuang et al., 2011b) (Figure 5), which can explain the origin of most morphologies. As a result, the bio-ge-ochemical hypothesis maybe is the most parsimonious method for analyzing the origin of MTS. Even through, it is still difficult to explain the cyclic MTS that was discov-ered 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 ori-gin theories discussed above. The study of MTS origin still has a long way to go.

4 Research significance

So far, compared to the majority achievements in the depositional environments and the origins of MTS, the studies on microtexture and related geochemistry just be-gin. The creation and occurrence of MTS is not an isolated simple event, its nature of global occurrence in a limited time interval and specified 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 strati-graphical correlation of the Proterozoic which is widely distributed in stable craton in the world). These two practi-cal functions are possibly more significant than the ori-gin 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 (Pollock et al., 2006), depositional palaeogeog-raphy, and stratigraphic correlation, also it may even be useful for comprehending the supercontinent reconstruc-tion (Gao and Qiao, 2001; Melezhik et al., 2002; Vigner-esse, 2005; Qiao et al., 2007).

4.1 Global change in the Precambrian

During different historical periods of the Earth, the li-thology or mineral composition and diversity are different. For example, the carbonate minerals include many types,


such as the seafloor crusts of fibrous crystals, micrites, aragonite fans, and stromatolites (Kah and Knoll, 1996; James et al., 1998; Bartley et 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 organ-isms and for evolution of multicellular organisms.

During this special period, the major occurrence of MTS globally is correlated to the occurrence of banded magnet-ite, high value of isotopic carbon, low value of isotopic ox-

ygen (Kuang et al., 2011b), and warm sea water. Over all, the thermal field of the Earth inclined to a relatively high end, which might be influenced by relatively higher partial pressure of carbon dioxide (PCO2 ) of the Precambrian (Bar-tley and Kah, 2007; Kah and Riding, 2007) or be affected by volcanic events. Furthermore, from the view of the pal-aeogeographic location of tectonic plates, the developmen-tal region of MTS locates around the mid-low latitude area (Zhang et al., 2000; Zhao et al., 2004; Jaffrés et al., 2007; Li et 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

Figure 6 Origin model for MTS. A-Location of the contact zone (the yellow rectangle) discussed in B-E; B-Gases are produced by decayed organic matters inside sediments at the contact zone with pore fluids, and cracks are created in the matrix; C-Due to the lower rate of diagenesis in matrix along with the increase of gaseous concentration, the pressure increases as well; finally, in a well-sealed system, the continual gas supply resulted in the development of cracks of various sizes, diverse magnitudes, and mixed patterns (Pollock et al., 2006); the activities of exterior forces, such as the storm wave or the sliding on the slope, also could create cracks and further enlarge or deform them; D-Mineralogically pure calcites within cracks precipitated from the interstitial water of matrix, which helps the rapid crystallization and diagenesis; the organic or argillaceous catalysis accelerates the nucleate formation of microsparry calcite in the MTS (Naka and Chujo, 2001); E-MTS from the cracks produced by the gaseous activity is often in a banded or spotty shape; alternatively, MTS from the cracks deformed or expanded by the exterior forces are not only in divergent shapes, but also are mixed with the matrix (the microsparry calcites of MTS as the cement are scattered in the matrix).


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-Zhou et al., 2012).

The origin of MTS thereby appears to have been closely related to the Proterozoic palaeoenvironment, paleocean, and palaeoclimate. After searching microtexture and geo-chemical characteristic of MTS, it is proved that the crea-tion and vanishing of MTS not only was controlled by the alteration of chemical properties in paleocean, but also jointly influenced by atmosphere, climate, and organic activity. The visible characteristic of MTS is the organic assemblage of internal composition and the external con-figuration. Therefore, based on the aspects above, the es-tablished origin model can favorably explain the source of materials for the microsparry calcite, the process of crea-tion, and the control factors (Figure 6, and Kuang et 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 (James et 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 cor-relation of the Proterozoic sedimentary strata is a chal-lenge due to the lack of suitable organisms for detailed bi-ostratigraphy and the shortage of geochronological data, in addition to the complicated litho-stratigraphy (Kah et 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 co-ordinated 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 environ-ment and lithologic facies. The characteristics of global distribution of MTS will be a good index to solve the prob-lems 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 specified 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 of ca. 1700-750 Ma (may be earlier than 720 Ma, see Mac-donald et al., 2010) before the Sturtian glaciation. MTS completely disappeared before the Marinoan glaciation (630 Ma) (Meng et al., 2006; Shields et al., 2012). Clearly, before the glaciation, during the Late Mesoproterozoic to the Early Neoproterozoic convergence of Rodinia super-continent, the warm oceanic environment was beneficial 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 (Zhao et al., 2004), while the convergence of the Rodinia supercontinent took place around 1.2-1.0 Ga (Sun et al., 2012) or 1.2-0.9 Ga (Qiao et al., 2007; Li et 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 connec-tions are clearly to supercontinents, or perhaps to sea level and the distribution of environments that result from su-percontinent formation, needs more evidence to be estab-lished (Figure 7) in the future.

5 Advanced research for the future

MTS has important implications for researches in paleocean and palaeogeography. It will be helpful for ex-ploring 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 confirm the possibility whether MTS can be used as a marker for stratigraphic correlation and for sedimentary facies identi-fication or not. As a result, it can be applied to the research-es on paleocean and palaeogeographic reconstruction and will make contribution to solve some significant scientific problems.

1) Geochemistry and microfabric studiesGeologists have conducted substantial researches on

the morphology and development of MTS previously. However, studies regarding characteristics of microtexture and geochemistry of MTC are still insufficient. Up to now, technological measures including polarizing microscope (PM), scanning electron microscope (SEM), cathodolumi-nescence microscope (CL), backscattered-electron diffrac-tion (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 micro-

sparry calcites, which are important to reveal the creation process, material resources and factors affecting crystal-lization. The crystallization process of microsparry calcite is described or analyzed very occationally on the micro-scopic level. In addition, the scientific information of tem-perature, pressure and external conditions and the crys-tallization process of MTS formation are hard to obtain, even if some simulation experiments were accomplished (Goodman and Kah, 2007; Hazen et al., 2013). The above mentioned researches should be promoted in the future.

2) Further studies of the relationships among the mor-phology of MTS, depositional environment and the geo-chemical composition

Although the complicated morphology led to the enig-ma 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 geochem-istry characteristics of MTS will reveal the origin of MTS on a new level. Although the relationship between mor-phology of MTS and sedimentary environment revealed by geochemical characteristics were emphasized and dis-cussed previously, comparative study on the geochemical

Figure 7 Occurrence of MTS in the Rodinia Supercontinent. MTS usually developed in the margin of Supercontinent. The base map of assembly and breakup and their chronology of Rodinia are cited from Li et al. (2008); and locations and time outline of MTS are according to Table 1.

OrogenContinental rift Spreading ridge Active margin SuperplumeMTS

Tarim

Australia

EastAnt.

GreaterIndia

Siberia

Laurentia

BalticaAmazonia

WestAfrica

Kalahari

Rio de la Plata

Congo-San Francisco

North China

YangtzeCathaysia

Tarim

Australia

EastAnt.Greater

India

Siberia

Laurentia

BalticaAmazoniaWestAfrica

Kalahari

Rio de la Plata

Congo-San Francisco

North China

South China

ArabiaNubia

Sahara

Rodinia

Tarim

Australia

EastAnt. Greater

India

Siberia

Baltica

Amazonia

West Africa

Rio de la Plata

Congo-San Francisco

North China

South ChinaSahara

KalahariArabiaNubia

LaurentiaTarim

Australia

EastAnt.

GreaterIndia

Siberia

Laurentia

BalticaAmazonia

WestAfrica

Kalahari

Rio de la Plata

Congo-San Francisco

North China

South China

SeychellesE. Madagascar

Sahara

Arabia

Nubia

–720 Ma, Rodina rifting finished

–1100 Ma, Assembling Rodina–900 Ma, Rodina assembling finished

–825 Ma, Rodina rifting started


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 MTSThe 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 filling 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 cal-cite. 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 studyMTC 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 devel-opment during the 1000-900 Ma (Table1 and Figure 7) on the continents around the world, and disappeared be-fore the Sturtian glaciation (750 Ma or 720 Ma) (Shields, 2002; Macdonald et al., 2010). As we know, the regional-scale 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 (Li et al., 2008), were as-

sociated with the breakup of Rodinia. MTC possibly had a potential connect with the formation and evolution of supercontinent. It is also of global correlation significance 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 Precam-brian Ocean and the sedimentary background. So it surely will reveal the geochemical conditions and is useful in re-constructing 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 identification, and to prove its scientific values as early as possible.

Acknowledgements

This paper is sponsored by the National Natural Sci-ence 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 pho-tos; Ms. Dechin Wang is so kind to translate the manu-script into English; thanks to the graduate student Ming-Wei Wang, Dr. Nan Peng and Qi-Chao Zhang, who made contributions to organizing references, figures and tables of this paper. I also express my thanks to Prof. Zeng-Zhao Feng and the editing team of Journal of Palaeogeography for 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 ben-efited suggestions, and especially thanks to Dr. Linda Kah, she is so kind to streamline the whole paper.

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