Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap carbonate

Accepted Manuscript Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap carbonate Guilherme Raffaeli Romero, Evelyn Aparecida Mecenero Sanchez, Luana Morais, Paulo César Boggiani, Thomas Rich Fairchild PII:

S0895-9811(16)30101-8

DOI:

10.1016/j.jsames.2016.06.014

Reference:

SAMES 1581

To appear in:

Journal of South American Earth Sciences

Received Date: 23 March 2016 Revised Date:

29 June 2016

Accepted Date: 30 June 2016

Please cite this article as: Romero, G.R., Sanchez, E.A.M., Morais, L., Boggiani, P.C., Fairchild, T.R., Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap carbonate, Journal of South American Earth Sciences (2016), doi: 10.1016/j.jsames.2016.06.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Title:

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Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay

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Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap

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carbonate

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Authors: Guilherme Raffaeli Romeroa (corresponding author) a Geochemistry and Geotectonic Graduate Program, Instituto de Geociências, Universidade de São Paulo Rua do Lago, 562 - Butantã - São Paulo, SP - Brazil - CEP: 05508-080 Telephone number: +55 11 4368-5016 [email protected]

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Evelyn Aparecida Mecenero Sancheza, b a Geochemistry and Geotectonic Graduate Program, Instituto de Geociências, Universidade de São Paulo b Permanent address: Instituto de Ciência e Tecnologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri Rodovia MGT 376, Km 583, n. 5000 – Diamantina, MG – Brazil – CEP: 39100-000 [email protected]

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Luana Moraisa a Geochemistry and Geotectonic Graduate Program, Instituto de Geociências, Universidade de São Paulo Rua do Lago, 562 - Butantã - São Paulo, SP - Brazil - CEP: 05508-080 [email protected] Paulo César Boggianic c Department of Sedimentary and Environmental Geology, Instituto de Geociências, Universidade de São Paulo Rua do Lago, 562 - Butantã - São Paulo, SP - Brazil - CEP: 05508-080 [email protected]

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Thomas Rich Fairchildc c Department of Sedimentary and Environmental Geology (Senior Collaborator), Instituto de Geociências, Universidade de São Paulo Rua do Lago, 562 - Butantã - São Paulo, SP - Brazil - CEP: 05508-080 [email protected]

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Abstract

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The restriction of tubestone structures tomicrobialitic laminites in cap carbonates associated

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with the Marinoan glacial event in North and South America, Namibia, Australia and Oman

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makes them an important stratigraphic marker for the base of the Ediacaran system. This

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association has been recognized in the Mirassol D´Oeste Formation (635 Ma) adjacent to the

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northern Paraguay Fold Belt, and are reported here in isolated outcrops at Morraria do Sul and

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Forte de Coimbra in the southern Paraguay Fold Belt, west-central Brazil.. The tubestone-

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microbialite associations at all localities reveal very similar macro- and microstructures,

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mineralogy, textures and fabrics. The microbialites consist of microbial laminites made up of

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dolomicrite clustered in microclots with dolospar-filled fenestrae. Lamination is defined by

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alternation in the relative abundance of these two components, suggestive of simple

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oscillations within a relatively uniform depositional environment and paleoecological setting.

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In the two new localities the tubestone fillings consist mainly of massive dolomicrite,

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although subordinate portions with concave lamination defined by concentrated very fine

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siliciclastic grains also occur. The presence of both massive and laminated tube fillings

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indicates variation in the processes responsible for their formation. These results extend the

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occurrence of the post-Marinoan tubestone-microbialite association at least 600 km southward

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from Mirassol D’Oeste in the north and document minor variations among the localities,

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which is what one would expect over such a broad distribution of this feature. The results also

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indicate that the isolated dolostones at Morraria do Sul and Forte de Coimbra do not belong to

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the Bocaina Formation (Corumbá Group), with which they have previously been correlated.

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Keywords: Early Ediacaran - Tubestone-microbialite association – cap carbonates –

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Paraguay Fold Belt - Brazil

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1. Introduction

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The tubestone strucutures are associated with Marinoan cap carbonates that

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marks the beginning of the Ediacaran sedimentation (ca. 635 Ma). These cap carbonates

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exhibit part or all of an exclusive set of unusual sedimentary features, including megaripples,

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megapeloids, cementstones and a peculiar tubestone-microbialite facies (Hoffman and

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Scharag, 2002; Allen and Hoffman, 2005; Corsetti and Grotzinger, 2005; Romero et al., 2012;

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Bosak et al., 2013). The term tubestone as used in this paper refers to finely laminated

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dolomitic microbial laminites (= stratiform stromatolites) cut by clearly evident abundant

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vertical, cylindrical to irregular tubular structures up to a few centimeters across and, in

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exceptional cases, more than a meter in length, that cut finely laminated dolostone and are

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filled by dolomicrite and dolospar, which may or may not exhibit fine lamination, distinct

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from the surrounding rocks.

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Cloud (1968) first described such features in the Noonday Dolomite in Death

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Valley, USA, likening them to trace fossils, but later coined the term “tubestone structures”

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for them (Cloud et al., 1974). Since then, other origins have been suggested for such

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structures, including fluid escape (Cloud et al., 1974; Kennedy et al., 2001), unusual

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hydrodynamic processes influencing development of microbial laminites (Bosak et al., 2013),

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and as a rare end-member microbialite resulting from the peculiar conditions of carbonate

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supersaturation at the time of cap carbonate deposition (Corsetti and Grotzinger, 2005). Other

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authors have referred this combination to as “geoplumb stromatolites” (Hoffman, 2011) and

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“tubestone stromatolites” (Bosak et al., 2013).

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The tubestone-microbialite association has been recognized in many cap

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carbonates associated with the Marinoan glaciation in the USA (Alaska and California),

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Namibia, Canada, Mongolia and Brazil (Cloud et al., 1974; Corsetti and Grotzinger, 2005;

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Hoffman et al., 2009; Hoffman, 2011; Romero et al., 2011; Bosak et al., 2013). In fact, the

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geological record of the tubestone-microbialite association appears to be temporally restricted

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to this particular post-glacial episode at circa 635 Ma. This paper discusses two occurrences

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of this association in the southern Paraguay Fold Belt. In Brazil, the tubestone-microbialite association was first observed in the

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Mirassol D´Oeste Formation, basal unit of the Araras Group, that together with the lower part

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of the Guia Formation, are considered a Marinoan cap carbonate in the cratonic cover

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succession adjacent to northern Paraguay Fold Belt in Mato Grosso State (Nogueira, 2003;

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Nogueira et al. 2003; Soares et al. 2013). The Mirassol D’Oeste Formation directly overlies

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massive glacigenic diamictites of the Puga Formation, with pebble-sized striated clasts of

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varied lithologies (e.g. sandstone, granite) in a sandy argillaceous matrix. The full cap

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overlying the Puga Formation shows of the entire set of sedimentary structures considered as

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unique to post-Marinoan caps including megaripples, megapeloids, cementstones and the

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tubestone-microbialite association (Nogueira et al., 2003; Font et al., 2010). Other evidence

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coherent with the post-Marinoan interpretation for the Mirassol D´Oeste Formation includes

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the negative δ13 C isotope profiles ranging from -3.5 to -8.9 ‰ (Nogueira et al., 2007) and

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al., 2006; Nogueira et al., 2007, Halverson et al., 2010).

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Sr/86Sr values of 0.7074 to 0.7090 similar to other post-Marinoan units worldwide (Font et

In successions traditionally attributed to the Corumbá Group of middle to late

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Ediacaran age in the southern Paraguay Fold Belt,, Boggiani et al. (2010) described a possible

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tubestone-microbialite association at Porto Morrinhos, near Corumbá city, and Morais (2013)

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recently suggested the presence at Forte de Coimbra, at the margins of Paraguay river, and at

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Morraria do Sul in the Serra da Bodoquena, , This paper describes and compares these

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occurrences and examines the paleoenvironmental and stratigraphical implications of their

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identification as part of the unique set of sedimentological structures restricted to Marinoan

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cap carbonates. The occurrence of tubestones at different localities motivated the study, once

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no strong evidence of Neoproterozoic post-glacial cap carbonate has been yet recognized at

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southern Paraguay Belt, the results would extend the paleogeographic occurrence of

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glaciation at the beginning of Ediacaran Period to the Southern portion of the Paraguay Belt

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and better constrain the time range of the Corumbá Group.

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2. Geological Setting

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The Paraguay Fold Belt was formed when the Amazonian Craton, the Congo-

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São Francisco Craton (to the east), and the Rio de La Plata Craton (to the south) collided

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during final amalgamation of the western portion of the supercontinent Gondwana at the end

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of the Brasiliano/Pan-African event in the mid-Cambrian (~520 Ma) (Almeida, 1984;

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Alvarenga et al., 2000). The Brazilian portion of this fold belt extends in an arc opening

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southeastward that is initially oriented N-S in the southern portion in the state of Mato Grosso

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do Sul (Serra da Bodoquena, Urucum Massif and Serra do Amolar), curving northeastward in

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its northern portion in the state of Mato Grosso (Figure 1a), it continues southward with

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outcrops in Paraguay (Warren et al., 2012). The sedimentary history is different in the

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northern and southern parts of this fold belt (Trompette et al., 1998; Alvarenga et al., 2000;

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Boggiani et al., 2010, Rudnitzki et al., 2016).

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Figure 1. A) Geological map of the Paraguay Belt along the southeastern border of the

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Amazon Craton and Rio Apa Block in the states of Mato Grosso and Mato Grosso do Sul,

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Brazil (modified from Trompette et al., 1998; Babinski et al. 2013), 1- Cordani et al, 2010, 2-

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De Min et al., 2013, 3- Piacentini et al. 2013, 4- Romero et al. 2012, 5-Warren et al. 2011; 6-

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Babinski et al., 2006. B) Simplified stratigraphy of the southern Paraguay Belt – Corumbá

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Group (after Boggiani, 1998; Gaucher et al., 2003).

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Glaciogenic diamictites of Puga Formation are overlain by a Marinoan cap

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carbonate in the north, represented by dolostone and limestone of the Mirassol D´Oeste and

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basal Guia formations, respectively, within the Araras Group (Nogueira et al., 2003, Hoffman

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et al., 2010). The temporal context and stratigraphic relations of the basal portion of the

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succession in the southern part of the fold belt, however, are still not satisfactorily constrained

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(Boggiani et al., 2010). In the exposures around Corumbá, it begins with the Jacadigo Group

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which consists of continental conglomerates, diamictites, immature sandstones and arkoses

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and passes conformably upward to lacustrine or marine banded iron formations (Freitas et al.,

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influence on the deposition of the banded iron formation. Glaciogenic diamictites have long

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been known in this region several tens of kilometers south of Corumbá in the isolated Morro

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do Puga, the type locality of the Puga Formation (Maciel, 1959; Boggiani and Coimbra,

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2002). Directly above these diamictites lie reddish non-microbial calcareous laminites

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interpreted as a cap carbonate (Boggiani et al. 2003; Babinski et al. 2013), which do not,

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however, exhibit tubestone structures. Completing the section at Morro do Puga is a thick

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succession of poorly exposed limestone assigned to the Bocaina Formation of the Corumbá

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Group (Maciel, 1959; Boggiani and Coimbra, 2002; Boggiani et al. 2003; Babinski et al.

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2013). This same formation also overlies the Jacadigo Group closer to Corumbá (Freitas et

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al., 2011) and is broadly distributed in the Serra da Bodoquena. By the same token that

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extensive occurrences of diamictites in the Paraguay Fold Belt have been attributed to the

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Puga Formation (Boggiani and Coimbra, 2002), practically all dolostones in the southern part

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of this fold belt have traditionally been assigned to the Bocaina Formation, including the

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isolated outcrops described here. However, stratigraphic relationships among all these

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outcrops have yet to be clearly demonstrated (Almeida, 1965; Boggiani et al. 1993; 2010),

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which is an important point in this paper.

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The dolostones containing the tubestone-microbialite structures described here

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have previously been considered as part of the Bocaina Formation in the Corumbá Group

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(Figure 1b) (Boggiani, 1998). The depositional age of this formation is poorly constrained,

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but must be younger than the Jacadigo Group and the Puga Formation and older than the

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overlying Tamengo Formation of confirmed latest Ediacaran age, based on the presence of the

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metazoan index fossil Cloudina Germs 1972 (Beurlen and Sommer, 1957; Zaine and

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Fairchild, 1985; Grant, 1990; Zaine, 1991) and a U-Pb SHRIMP age of 543 ± 3 Ma for zircon

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crystals from volcanic tuffs intercalated within the Cloudina–bearing carbonate beds

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(Babinski et al., 2008). The Bocaina Formation is clearly younger than both the Jacadigo Group and

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the Puga Formation (Almeida, 1946; Freitas et al., 2011; Piacentini et al., 2013), and older

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than the overlying Tamengo Formation of the Corumbá Group, but its precise age is still

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unknown. 40Ar / 39Ar age dating of cryptomelane in manganese ore within the Jacadigo Group

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indicates mild metamorphism at about 590 Ma (Piacentini et al., 2013), but says nothing as to

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the age to the Bocaina Formation. Age dating of detrital zircon in Puga Formation diamictites

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and stable isotope studies of the reddish limestones overlying the Puga Formation provide

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information on the maximum age of all post-Puga rocks (Corumbá Group) in the southern

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Paraguay Fold Belt, including the Bocaina Formation (Boggiani et al., 2003; Babinski et al.,

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2013). Detrital zircon grains within the Puga Formation in the Serra da Bodoquena indicate a

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maximum age (U-PB) of glaciation of 706 ± 9 Ma (Babinski et al., 2013), which is slightly

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younger than available age determinations for Sturtian glaciation (720 Ma) (Macdonald et al.,

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2010), but is also consistent with deposition during Marinoan glaciation (635 Ma). The

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metamorphic age of the Jacadigo Group practically eliminates deposition during the Gaskiers

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event (580 Ma). δ13C values around -5 ‰ and

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consistent with a post-Marinoan age for the Puga cap carbonate (Nogueira et al., 2007;

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Halverson et al., 2010).

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Sr/86Sr of 0.7077 in the pink limestone are

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The upshot of this discussion is that, technically, deposition of the Bocaina

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Formation is presently restricted to the interval 542 – 706 Ma based strictly on available

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radiometric data. Nevertheless, the Puga cap carbonate would appear to confine this interval

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between earliest (635 Ma) to latest (542 Ma) Ediacaran. These data indicate that it could

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possibly be as old as the cap carbonates, however, where the Bocaina Formation crops out in

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the Serra da Bodoquena and in the Corumbá region, the features typical of post-Marinoan cap

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carbonates are never present, with exception of the two isolated localities described here,

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which we claim do not belong to this formation..

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3. Methods

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Studied sites are in the state of Mato Grosso do Sul, southwest Brazil: 1)

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neraby the village Morraria do Sul (MS), 40 km West of the city of Bodoquena (Baia das

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Garças Farm, 20°32'40'' S/56°53'47'' W); 2) Forte de Coimbra (FC), on the right bank of the

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Paraguay River (19°55'14''S/57°47'32''W), 116 km south of the city of Corumbá, and

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accessible only by boat (Figure 2).

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Figure 2. Geology of the southern Paraguay Fold Belt (Corumbá Group) in the state of Mato

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Grosso do Sul and location of the outcrops at Forte de Coimbra (FC) and Morraria do Sul

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(MS) discussed in this paper.

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According to concepts presented by Hofmann (1969) and Fairchild and

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Sanchez (2015), the study of the tubestone-microbialite association in outcrop focused upon

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local stratigraphic setting of the microbialites (macrostructure) patterns of microbial

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lamination and mode of occurrence of the tubular strucutures within the microbialites

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(mesostructure); and, at the microstructural scale, features such as alternation of laminae

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within the microbialites and characteristics of the filling of the tubular structures. Further

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analysis of meso- and microstructural details of lamination in the tubular structures,

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composition of the tubular filling and microbialite and tubestone fabrics was carried out on

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cut and polished hand samples and in 18 petrographic thin sections. Thin sections were

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examined

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photomicrographs were obtained with Zeiss Axio Vision 4.8 software.

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4. Results

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Stereomicroscope

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Axionlab

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4.1. Macro and mesostructural features of the Tubestone microbialite association at

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Morraria do Sul and Forte de Coimbra

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At Morraria do Sul the microbialites consist of well-preserved pink dolomitic

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microbial laminites exposed in low sinuous outcrops with rounded crests, five to ten meters

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long and up to one meter high, scattered in a pasture sloping slightly upward to much larger,

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fully exposed blocks of microbialitic dolostone up to five meters high (Figures 3 and 4a). The

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shape of the microbial build-ups. As most of the outcrops consist of continuous, flat-

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laminated microbialites, it is inferred that they are all part of extensive biostromes rather than

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lenticular bioherms. The laminae are repetitive and well preserved (Figure 4e), 0,8 to 1 cm

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thick, with planar to occasionally wavy profiles close to the tubestone structures (Figure 4d),

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with low synoptic relief of up to 2 cm in these portions and no evidence of erosional micro

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disconformities.

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Figure 3. Schematic stratigraphic columns for the outcrops at Morraria do Sul and Forte de

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Coimbra. The outcrop at Forte de Coimbra is also a pink dolomitic microbial laminite

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with rhythmic lamination. Nonetheless, this outcrop is very small (Figure 3), only about 4 m

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long and 2 m high in the period available for visitation. It was not possible to confirm the

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suspicion that similar blocks of the same biostrome probably crop out along the hillside and

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riverbank. However, it is also true that the apparent lack of other blocks of microbialites may

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indicate that the microbialites occur here not as biostromes but as lenticular bodies

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(bioherms). The question remains open. The lamination in these microbial laminites is

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identical in thickness, form and relief to that at MS (Figure 4b).

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At both outcrops, MS and FC, tubestone structures occur throughout the entire

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microbial biostrome (Figure 4a). The reddish to purple tubular features that characterize the

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tubestone structures appear to begin and terminate at random and cut the lamination without

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deforming the laminae, without any clear evidence of a common surface of origin or end of

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their occurrence (Figures 4a and d). The structures of the tubes are perpendicular to the

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bedding, slightly sinuous, cylindrical to irregular in shape, parallel to subparallel (Figures 4a,

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c and d). In outcrop, they vary from 12 cm to 1.6 meter in length (Figure 4a). They present

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rounded transverse cross-section and diameters that may pinch and swell from 2 to 4 cm

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along their length. These structures are closely spaced, commonly 1,5 to 3 cm apart, and

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exhibit no cross-cutting relationships, contact, or coalescence. These structures have sharp,

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well-defined limits, with no evidence of laminar bridges extending from the host microbial

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laminites and crossing the tubular structures.

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Figure 4. Tubestone–microbial laminite association at Morraria do Sul (MS) and Forte de

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Coimbra (FC). A) Typical outcrop at MS (Scale= 7cm)B) The sole block containing the

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tubestone-microbial laminate association at FC(Scale= 22cm)C) Oblique transverse section of

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the microbial laminite at MS. Arrows point to the rounded outlines of the tubular structures

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that give the tubestone structure its name, (Scale = 7 cm). D) Cut and polished longitudinal

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section of a sample of the tubestone-microbial laminite association from MS. Note the planar

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to wavy stratiform lamination of the microbial laminite and the homogeneous filling (white

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arrow) and occasional concave laminae within the tubular structures (black arrow) (Scale = 5

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cm). E) Detail of D. Note the repetitive character of the microbial laminites (Scale = 1 cm).

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Although tube fillings at both MS and FC are dolomicrite and dolospar, they

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differ with respect to their homogeneity and organization. At FC, for example, the filling is

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homogenous and massive (Figure 5e), but at MS, it is less homogeneous and, locally, clearly

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laminated (Figure 5d). Lamination is defined by alternating light and dark submillimetric

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concave laminae and, more rarely, by very thin laminar concentrations of quartz silt and very

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fine sand (Figures 5d, f), being the only evidence of siliciclastic sediments in the tubestone-

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laminite association.

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The contact between the host microbialite and the tubes is distinctly marked by

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the abrupt contact between the amalgamated peloidal clots of the microbialite and the very

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finely crystalline dolomicrite and dolosparite that fills the tubes (Figure 5c). Larger diagenetic

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euhedral dolospar crystals may occur at the contact.

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4.2.

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associations at Morraria do Sul and Forte de Coimbra

petrographic

features

of

the tubestone-microbialite

Despite the repetitive laminae of microbial laminites in MS and FC,

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and

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Microstructural

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petrographically this feature is not visible as the main textural component. At both localities,

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MS and FC, laminae are composed of a constant dark-brown microcrystalline dolomicritic

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peloids of 30 to 60 µm in diameter amalgamated into elongated patches as microclots or

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grumeaux texture (Turner et al., 2000), with no sign of clastic constituents either as intraclasts

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or terrigenous materials (Figures 5a-b). The microclots are rounded, with clear borders in MS

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(Figure 5a) and diffuse borders in FC (Figure 5b). They are aggregated and are surrounded by

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irregular fenestrae pores of a maximum size of up 520 µm, and filled by dolomicrite and

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brick-like sparry dolomite cement. Eventually euhedral quartz crystals may be seen, indicative

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of tardi silicification within the spaces. The microclots/space proportion at MS can be considerate high, with

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microclots of 500 µm long, however clots of up to 1 mm can be observed. In FC the

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proportion is lower and, on average, microclots can be 300 µm long, with a maximum size of

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up to 500 µm.

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Figure 5. Photomicrographs of petrographic thin sections of the tubestone–microbial laminite

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association at at Morraria do Sul (MS) and Forte de Coimbra (FC) as seen in plane-polarized

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light using crossed Nicols. A-B) Texture of the microbial laminites: loose, interconnected

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network of dark-brown dolomicrite peloids amalgamated into microclots at MS (A) and FC

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(B). The spaces between the microclots are filled with dolomicrite or brick-like sparry

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dolomite cement. Microclots are larger at MS (A); the proportion of cement-filled pore space

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compact dolomicrite and dolospar filling of the tubes.; D) Alternating submilimetric light and

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dark, irregular, concave laminae in the tubestone filling at MS. Observe wispy aspect; E)

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Massive dolomicrite and dolospar filling of the tubestone structures at FC; F) Rare occurrence

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of concentrated quartz silt and very fine angular sand grains defining lamination within the

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tubestone fillings in samples from MS.

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5. Discussion

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5.1. Geobiology of the tubestone–microbial laminate associations in the southern

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Paraguáy Belt

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The most prominent feature of the supposed tubestone–microbial laminite

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associations at the two studied outcrops is their stromatolitic character (microbial laminites)..

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At both localities, the microbial laminites are composed of brown to dark-brown microclots

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separated by dolospar cement.

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This association is only known in post-Marinoan cap carbonates deposited at

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the beginning of Ediacaran Period (Hoffman, 2011; Bosak et al., 2013). The carbonates at MS

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and FC here described are therefore assigned to the post-Marinoan event, once they are

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composed of rhythmically laminated stromatolitic dolostone associated with tubestone

340

structures, similar to others immediately post-Marinoan sequences worldwide (Allen and

341

Hoffman, 2005; Corsetti and Grotzinger, 2005; Bosak et al., 2013).

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342

The Marinoan cap dolostone presents evidence of rapid deposition immediately

343

after the transition from glacial to greenhouse conditions (Nogueira et al., 2003), resulting in

ACCEPTED MANUSCRIPT an initial rapid flooding of continental shelves and platforms as the ice sheets melted, leaving

345

few points above sea level that could serve as sources of siliciclastic sediments (Hoffman and

346

Schrag, 2002; Hoffman and Li, 2009). The microbialites of MS and FC are coherent with this

347

interpretation. The lateral extent, monotonous lamination and thickness of the MS and FC

348

microbial laminites indicate accumulation in a calm paleoenvironment, probably below

349

storm-wave base, where tides, currents, and storm waves would have had little effect on the

350

macroscopic form of the microbial mats. The absence of intraclasts and siliciclastic grains

351

within the microbial laminites is also explained by the relatively deep (but well-lit),

352

permanently submerged depositional setting far from siliciclastic sediment sources, coherent

353

with this model.

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The dolomicritic peloidal-microclot texture at FC and MS is a common feature

355

in stromatolitic reefs from the Early Neoproterozoic (Turner et al., 2000) to modern times

356

(Riding, 2000). The suggested origin of this texture is via micritization of degraded

357

extracellular polymeric substances (EPS) largely through synsedimentary microbially induced

358

precipitation of micrite within the microbial mats (Chafez, 1986; Riding, 1991, 2011; Turner

359

et al., 2000; Dupraz & Visscher, 2005; Harwood & Sumner, 2012). Filamentous

360

calcimicrobial cyanobacteria were first considered the main responsible for such fabric, as

361

proposed by Turner et al. (2000) for stromatolites of the Lower Neoproterozoic Little Dal

362

Group, of Canada. However, Frasier and Corsetti (2003) identified coccoidal microfossils

363

associated with this texture in the tubestone facies of the Noonday Dolomite (USA). In the

364

correlative tubestone-microbialite associations studied here, the monotonous microclot texture

365

is composed exclusively of coccoidal peloids, with no evidence of relict filamentous

366

microfossils or other microstructures.

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Hence, it is suggested that the peloidal structures and microclot texture directly

368

reflect the dominance of colonial coccoidal microorganisms in the original microbial mat

ACCEPTED MANUSCRIPT communities, with lithification occurring penecontemporaneously with growth of the

370

microbial mat (Harwood & Sumner, 2012). The spaces between the coccoidal colonies were

371

originally filled by water, metabolic waste and other cellular products, including gases

372

(Hofmann, 1976). These spaces were filled by cement also early in diagenesis prior to

373

compaction, thereby preserving much of the original laminar structure of the deposit (Turner

374

et al., 2000). An intriguing aspect of the cap carbonate microbialites at FC and MS is their

375

textural, compositional, lateral and vertical uniformity, which also occur in post-Marinoan

376

sequences worldwide (e.g. Frasier and Corsetti, 2003; Corsetti and Grotzinger, 2005; Romero,

377

2010; Bosak et al., 2013).

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What processes could possibly be responsible for this uniformity of the

379

microbial laminae? Monty (1976) demonstrated that stromatolitic lamination reflects the

380

response of the microbial community to a combination of physical, chemical and ecological

381

changes. As previously discussed, formation of the microbialites took place in well-lit setting

382

below storm wave base, protected from the action of strong current waves surf and subaerial

383

exposure, so both hydrodynamic and photic variations, can be ruled out as major controls on

384

the development of microbialite lamination.

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Also lacking is any evidences of tectonism during the deposition, such as

386

morphological variations in laminar organization, internal erosional unconformities and

387

synsedimentary reworking of the microbialites, all of which would be expected had the based

388

been subjected to local tectonic uplift. On the other hand, if significant subsidence occurred,

389

the microbialites would be drowned and microbial growth terminated, which was also not

390

observed.. Ecological changes within the mat-forming community can also be discarded, as

391

the texture and microstructure are constant throughout all microbialites.

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Because of the time scales involved, paleogeographic changes as well as orbital

393

variations in Earth´s movement (e.g. Milankovitch cycles) were probably not important

ACCEPTED MANUSCRIPT factors in the formation of laminae and microbialites at FC and MS. Font et al. (2010)

395

considered recent microbialites from Lagoa Salgada (Rio de Janeiro, Brazil) as analogues for

396

microbialites in Marinoan cap carbonates, and estimated microbial mat growth at around 0,3

397

cm/yr. At this rate, microbial laminae at FC and MS, which range from 0.8 to 1 cm in

398

thickness, would take approximately four years to form, which is too short a time for

399

paleogeographic or orbital variations to influence in their formation.

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The chemical factor may be responsible for rhythmic generation inlaminar

401

development. Here we argue that variation in defrost rates as a response to seasonality may

402

have led to short term changes in chemical parameters of sea water, during the entire

403

transgressive regime operating in the Marinoan post-glacial phase. The addition of liquid

404

water to the oceans would have led to changes in the concentration of substances in the sea

405

water, including nutrients and basic molecules for microbial (mainly cyanobacteria) growth.

406

This phenomena could generate seasonality and may be the responsible for the pauses or

407

growth phase observed as dichotomous laminae in mesoscopic scale at MS and FC

408

microbialites, and can also be applied to other Marinoan cap carbonates stromatolites

409

worldwide.

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5.2. Insights on the genesis of the tubestone structures from the perspective of the

412

examples in the southern Paraguay Fold Belt

413 414

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As no modern or ancient analogues for tubestone structures are known, the

415

genesis of the tubestone structures remains an open question. To be satisfactory, any

416

explanation for this phenomenon must take into account the following features: a) the vertical

417

orientation of the tubular structures; b) the shape, size, abundance, spacing, and uniformity of

418

the tubes, as well as the nature of their points of origin and termination; c) the contact

ACCEPTED MANUSCRIPT 419

between the tubes and the host microbialite; d) composition, texture, structure, fabric and

420

variations of the tube filling ; and e) differences and similarities between the tube filling and

421

the host rock.

422

Corsetti and Grotzinger (2005) and Bosak et al.

(2013) argued that the

tubestone structures in the Noonday Dolomite (USA) and Rasthof Formation (Namibia),

424

respectively, represents originally open depressions within regularly microbial laminites.

425

These depressions were episodically filled by dolomicrite lacking the grumeaux or cloted

426

texture that is characteristic of the host microbialite.

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The tubestone-microbialite association at MS and FC exhibit macroscopic,

428

mesoscopic and microscopic similarities to the tubestone structures described in the Noonday

429

Dolomite, California, which have been described in more detail than all other examples

430

(Cloud et al., 1974; Corsetti and Grotzinger, 2005). However, some differences do exist, for

431

example, in the Noonday Dolomite, concave dolomicritic stromatolitic laminae, designated as

432

“bridging laminae”, cross the tubular structures, and the intergranular spaces of the tube

433

fillings that were later filled by dolospar cement (Corsetti and Grotzinger, 2005). At FC and

434

MS, on the other hand, the tube filling consists predominantly of massive dolomicrite.

435

Concave laminae are detectable only rarely where they are defined by a sparse train of very

436

fine siliciclastic grains.

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Diagenesis can alter tubestone fillings (Corsetti and Grotzinger, 2005),

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438

however, here this is not appear to be the case. That the massive dolomicrite in the tube

439

fillings at FC and MS may possibly have resulted from recrystallization of the original infill is

440

unlikely, as this would require a peculiarly selective process that would have affected some

441

portions, but not all, of the filling, nor the host rock, which still show primary textures.

442

Assuming that the tubes at FC and MS were open during sedimentation, the

443

massive dolomicrite filling may have been resulted from some sort of nearly continuous

ACCEPTED MANUSCRIPT 444

sedimentation, perhaps by constant whitings in a carbonate-saturated post-Marinoan sea.

445

However, there is no evidence of continuous decantation of very fine carbonate within the

446

laterally equivalent host microbialite nor of interrupted microbialite growth because of limited

447

luminosity associated with this process. These observations indicate that Corsetti and Grotzinger (2005) model, related

449

to microbial growth and the tubestone fillings does not adequately explain the fillings in the

450

tubestones at MS and FC. A closer comparison of tubestone fillings in other Marinoan cap

451

carbonates will probably reveal further differences in the fabrics of tubestone fillings. In this

452

case, then the common denominator in the origin of these tubestone-microbialite associations

453

worldwide, may have been a temporally restriction to the immediately post-Marinoan

454

geologic record, with variation in the tube-filling process depending upon local or regional

455

conditions.

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456

5.3. Stratigraphic and paleogeographic implications for the Paraguay fold Belt

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As previously mentioned, all occurrences of dolostones and microbialites

460

found within the southern Paraguay Belt have been traditionally attributed to the Bocaina

461

Formation (Almeida, 1945; Almeida, 1965; Boggiani, 1998; Boggiani et al., 2003), including

462

the outcrops at MS and FC. However, the stratigraphic position of these and other isolated

463

outcrops so attributed to the Bocaina Formation, is a matter of discussion (Boggiani et al.,

464

2003).

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465

The age of the Bocaina Formation is poorly constrained within the Ediacaran

466

Period, as discussed above. Thus, given that the tubestone-microbialite association is

467

evidently restricted to the very beginning of the Ediacaran (Allen and Hoffman, 2005;

468

Corsetti and Grotzinger, 2005; Romero et al., 2011; Bosak et al., 2013), then confirmation of

ACCEPTED MANUSCRIPT the presence of this association within the Bocaina Formation would solve this problem.

470

However, a punctual occurrences in fault contact with the base portion of the Corumbá Group

471

at Morraria do Sul and a punctual occurrence without contact with other units of the Corumbá

472

Group at Forte de Coimbra. Moreover, nowhere in the vast extent of continuous outcrops of

473

the Bocaina Formation in the Serra da Bodoquena and around Corumbá has the tubestone-

474

microbialite association ever been observed, despite the mistaken identification of closely

475

spaced cylindrical columnar stromatolites as such at the Porto Morrinhos locality of the

476

Bocaina Formation at the eastern edge of Neoproterozoic outcrops in the Corumbá region

477

(Boggiani 1998).

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The conclusion we draw from these observations is that the outcrop outcrops at

479

MS and FC do not belong to the Bocaina Formation. Rather than representing a lateral

480

variation of the Bocaina Formation, a possibly more likely candidate for correlation with this

481

two outcrops in the southern Paraguay Fold Belt is the pink limestone succession that overlies

482

the diamictites in the type-section of the Puga Formation at Morraria do Puga (Maciel, 1959).

483

This limestone has values of δ13C -5 ‰ and

484

other post-Marinoan cap carbonates (Boggiani et al., 2003; Babinski et al., 2013) and thus

485

reinforce this possibility.

87

Sr/86Sr of 0,7077 that allow correlation with

EP

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478

It is now evident that the geographic distribution of post-Marinoan cap

487

carbonates in Brazil can be extended within the Paraguay Fold Belt at least 600 km southward

488

from the Mirassol D’Oeste locality (Nogu eira et al., 2003; Hoffman and Li, 2009; Romero

489

et al., 2011) to the outcrops at Morraria do Sul and Forte de Coimbra as well as to Morro do

490

Puga. Given that other outcrops in the northern Paraguay Fold Belt are now known 320 km

491

northeast of the type-locality of the Mirassol D’Oeste cap carbonate (Soares et al., 2013).

492

Given the vastness of this area it is remarkable that the textures of the tubestone-microbialites

493

association in such widely spaced outcrops are so similar, and, by the same token, it is equally

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ACCEPTED MANUSCRIPT 494

to be expected that lateral variations in these cap carbonates are to be expected and will be

495

further document in future research’s.

496 497

6. Conclusions

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The description and interpretation of the tubestone-microbialite association

500

brings new insights on the genesis of these structures, their stratigraphic relations and

501

paleoenvironmental context within the southern Paraguay Fold Belt. The microbial laminites

502

associated with tubestone structures at Morraria do Sul and Forte de Coimbra were developed

503

in a calm, well lit, protected environment below storm wave base level essentially free of fine

504

siliciclastic material in suspension, and evidently without nearby terrigenous sources.

505

Petrographically, the microbial laminites are composed of dolomicrite peloids clustered in

506

microclots, the result of micritization of colonies of coccoidal cyanobacteria during early

507

diagenesis.

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499

The tubestone-microbialite associations occurrences are very similar to each

509

other, but not identical. Differences in the presence or absence of lamination and siliciclastic

510

material within the tubes, the thickness of laminae, the relative proportion of microclots

511

within the laminae, and perhaps even the calcitic composition of the putatively correlatable

512

Puga cap are all features that one might expect to vary in very similar carbonate environments

513

scattered over such great distances. This may also indicate that the process for the formation

514

of these temporally restricted structures was the same, but the tube-filling process would have

515

varied.

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516

Similar tubestone-microbialite associations at Mirassol D’Oeste and elsewhere

517

in the world are only known in post-Marinoan cap carbonates. Hence, we claim here that the

518

occurrences at Morraria do Sul and Forte de Coimbra represent the same phenomenon and are

ACCEPTED MANUSCRIPT 519

also earliest Ediacaran in age. Furthermore, as they occur in isolated outcrops and are unlike

520

any other sedimentary features known in the Bocaina Formation, we concluded that they

521

should no longer be included in the Bocaina Formation as formerly thought. The tubestone-microbialite association described here reinforces the idea that

523

the Marinoan glaciation affected a broad area in South America, considering that these

524

typically Marinoan deposits are broadly distributed in the northern portion of the Paraguay

525

Fold Belt, as represented by the Puga and Mirassol D’Oeste formations, and are known at the

526

isolated outcrops Morraria do Sul and Forte de Coimbra in the southern portion of this fold

527

belt. The areal distribution of this association now extends more than 600 km in a north-south

528

direction.

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529 530 531

Acknowledgements

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532

The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São

534

Paulo - FAPESP – for the PhD grant to Guilherme Raffaeli Romero and financial support for

535

this research (process 2012/01331-8), the Geochemical and Geotectonic Graduate Program of

536

the Institutode Geociências of the Universidade de São Paulo. Dr. Lucas Verissímo Warren

537

(UNESP, Rio Claro, State of São Paulo) for samples and useful discussions, Gustavo

538

Evangelista Prado for discussions and reviewing this manuscript, Brazilian Army, especially

539

the 3ª Companhia de Fronteira, for permitting to our research area in Forte de Coimbra

540

outcrop and the two anonymous reviewers for the constructive criticism of the manuscript.

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541 542 543

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ACCEPTED MANUSCRIPT Research highlights -

Very similar tubestone-microbialite associations occur in two isolated dolostone outcrops in the southern Paraguay Fold Belt;

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Constancy of the microbialite fabric suggests a constant paleoenvironmental

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condition; -

The tubestone fillings do not support the idea that the tubes were open spaces;

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The association suggests a Marinoan cap carbonate to the Southern Paraguay

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Fold Belt.