- Email: [email protected]
Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: Classification, genesis and paleoclimatic implications
Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: Classification, genesis and paleoclimatic implications B.L.D. Horn, V.P. Pereira, C.L. Schultz PII: DOI: Reference:
S0031-0182(13)00089-8 doi: 10.1016/j.palaeo.2013.02.013 PALAEO 6414
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date: Revised date: Accepted date:
10 May 2012 1 February 2013 12 February 2013
Please cite this article as: Horn, B.L.D., Pereira, V.P., Schultz, C.L., Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: Classification, genesis and paleoclimatic implications, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.02.013
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.
ACCEPTED MANUSCRIPT
Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: classification, genesis and paleoclimatic implications
T
B. L. D. Horn, V. P. Pereira and C. L. Schultz
Telephone: 55 51 33 08 63 66
NU
FAX: 55 51 33 08 73 02
SC R
Corresponding author: Bruno Ludovico Dihl Horn
IP
Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre (RS), Brazil
E-mail: [email protected]
AC
CE P
TE
D
MA
Address: Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre (RS), Brazil. CEP: 91501-970
1
ACCEPTED MANUSCRIPT
2
ABSTRACT Carbonate concretions are common features in arid and semi-arid climates. They can be formed
IP
T
under pedogenic conditions, developing complex profiles with or without biogenic contribution or in groundwater conditions, forming thick massive beds of carbonate. Together with other
SC R
data, calcretes can be used as tools for paleoclimatic and paleoenvironmental interpretation. In Southern Brazil, the Triassic Santa Maria Supersequence (Sequences I and II) includes four
NU
types of calcretes with distinct modes of formation: a) precipitation of carbonate in the oscillation zone of the phreatic level, with posterior overlap of vadose and phreatic diagenetic
MA
processes; b) precipitation of pedogenic carbonate, forming crusts in the paleosol; c) precipitation of carbonate forming concentric concretions in the stable phreatic level; d) preservation of root casts due to processes of precipitation in the oscillation zone of the phreatic
TE
D
level. The presence of silica minerals and barite in the concretions is attributed to seasonal changes in water table level. Due to the high evaporation rate in semi-arid climates, the most
CE P
compatible process of formation of these calcretes is the per ascensum model. Integration of calcrete data with the sedimentological and paleontological framework allowed a refinement of paleoclimatic data, suggesting that, despite the scarcity of plant fossils, the Middle Triassic in
AC
Southern Brazil was vegetated with plants.
Keywords: calcretes, paleosol, paleoclimate, Triassic.
ACCEPTED MANUSCRIPT
3
1. Introduction Calcrete is a carbonate-cemented duricrust developed in low-humidity conditions,
IP
T
common in semi-arid and arid climates. They occur in a wide range of forms and in distinct sedimentary environments, as alluvial fans and floodplains, and are the result of introduction of
SC R
calcite and dolomite in soils and sediments (Alonso-Zarza and Wrigth, 2010). Calcretes that commonly form within soil profiles, in superficial settings above the
NU
groundwater, are pedogenic calcretes and commonly show well developed profiles. However, groundwater may induce carbonate precipitation on the capillary fringe in less superficial
MA
settings under influence of plants (Semenieuk and Meagher, 1981), or seasonal groundwater oscillation. These are termed phreatic or groundwater calcretes and indicate the presence of a
D
relatively shallow water table (Alonso-Zarza, 2003). In these calcretes, biogenic features are
TE
unusual, but plants can generate rhizoliths. Klappa (1980) classified the rhizoliths as root moulds, root tubules, root casts, rhizocretions and root petrifications, explaining their
CE P
characteristics and genesis.
In some cases, it may be difficult to distinguish calcretes that formed in groundwater
AC
conditions from those formed in pedogenic environments, due to superposition of process (Alonso-Zarza, 2003; Mack et al., 2000). Calcretes, along with other geochemical information, are important paleoenvironmental and paleoclimatic indicators, and can aid the interpretation of complex sedimentary settings. Moreover, they contain important information that helps interpret ancient ecosystems, palaeogeography and the tectonic, climatic and sedimentary regimes in which they were formed (Alonso-Zarza, 2003; Wright, 2007) . The calcretes in the Triassic Santa Maria Supersequence (Sequences I and II) (Zerfass et al., 2003) have been poorly studied so far. Silverio da Silva (1997) was the first to recognize and describe them, proposing that their fabric was alpha type, intermediate between pedogenic
ACCEPTED MANUSCRIPT
4
and phreatic, and formed by per descensum model of Goudie (1983). According to Silverio da Silva (1997), the negative δ18C and δ13O values suggested that they precipitated from
T
freshwater. Da Rosa (2005) has dealt mostly with the macroscopic features of calcretes in the
IP
Santa Maria Sequences (Santa Maria Supersequence; Zerfass et al., 2003), assigning their origin
SC R
to groundwater variation and eodiagenesis. Da Rosa (2005) also stated that phreatic calcretes tend to have larger calcite crystals than pedogenic ones and, when calcite is microcrystalline, it tends to be displacive. Da Rosa et al. (2004) worked with the paleoalterations and paleosols in
NU
rocks of the Santa Maria Supersequence, identifying mottled mudstones, carbonate veins and phreatic carbonate formation. Some aspects, as formation mode and detailed petrographic
MA
description remain open.
The present work aims to provide the detailed description of phreatic carbonate formation, using
D
petrography, chemical and X-ray diffraction analyses to propose a formation mode and give
CE P
2. Geological Setting
TE
some new climatic information about Santa Maria Sequences I and II.
The Santa Maria Supersequence (Zerfass, 2003) is a Triassic continental, tectonically
AC
controlled system located on the central portion of Rio Grande do Sul State, Brazil (fig.1). It is composed of three sedimentary sequences (fig. 2). Figure 1
The Santa Maria Sequence I (SMSI) comprises clast-supported conglomerates and cross-bedded sandstones, superposed by laminated mudstones, interpreted as a transition from high-energy river to shallow lake deposits. The Santa Maria II Sequence (SMSII) is composed of medium to fine-grained, cross-bedded sandstones and mudstones lenses at the base, gradating to thick mudstone deposits on the middle part, deposited by a fluvial system with high-sinuosity rivers and floodplains. At the top, the sequence comprises a coarsening-upward succession, composed of rhythmites intercalated with lenses of fine-grained, cross-bedded and climbingrippled sandstones that represent a lacustrine–deltaic depositional system. Santa Maria III
ACCEPTED MANUSCRIPT
5
(SMSIII) consists of cross-stratified and conglomeratic sandstones with abundant silicified logs. This lithofacies was interpreted as fluvial channels.
T
Diverse terrestrial vertebrate fossils are reported from the Santa Maria Supersequence.
IP
These include dinosaurs, rhynchosaurs, dicynodonts and cynodonts (Barberena, 1977; Scherer
SC R
et al., 1995; Schultz et al., 2000). Due to the restricted lateral exposure of outcrops and its tectonic framework, this Supersequence is biostratigraphically divided into four vertebrate assemblage zones (fig. 2).
NU
Calcretes are found in all four biozones, but this paper deals only with the lowest three, which belongs to the upper SMSI and SMSII Sequences. The samples were collected in Vera
MA
Cruz, Santa Cruz do Sul and Venâncio Aires municipalities outcrops, showed by a composited section on figure 3.
D
Figure 2
TE
Figure 3
CE P
3. Materials and Methods
Calcrete samples were collected after describing the outcrops and constructing sedimentary graphic logs. They were described macroscopically and with a binocular
AC
stereomicroscope. Thin sections were analyzed with optical and scanning electron microscope (SEM). The macroscopic description and classification of calcretes was defined on basis of outcrop occurrence mode, form, mineralogy, presence of rock fragments and fractures. The semi-quantitative analyses were made in a JEOL® JSM-5800 scanning electronic microscope with acceleration voltage of 15 kV from the Centro de Microscopia Eletrônica, Universidade Federal do Rio Grande do Sul (UFRGS). Major oxides of calcrete samples and related rocks were determined by ICP-MS analyses at Acme Labs, Canada. Mineralogy was also identified by X-ray powder diffraction analyses, using a Siemens D5000 powder diffractometer (BraggBrentano geometry, Cu Kα, 40 kV, 25 mA) from Instituto de Geociências, UFRGS. The X-ray diffractograms were performed between 2º and 72º 2θ, using a step size of 0.02º 2θ per second.
ACCEPTED MANUSCRIPT
6
4. Results
T
4.1 Host sediment
IP
Calcretes are a common feature in massive or laminated, fossiliferous red silty
SC R
mudstones. The mineralogy consists mainly of detrital quartz and K-feldspar with authigenic calcite and hematite (Zerfass et al. 2000). These are the most common and abundant sedimentary rocks in SMS I and II Sequences. These deposits were interpreted as formed by
NU
floodplain deposition in a semi-arid climatic context (Holz and Scherer, 1998; Zerfass et
MA
al.2003).
D
4.2 Calcrete Petrography
TE
The carbonate concretions were classified in three types, according to their macroscopic features: microcrystalline nodules, crusts, rhizoliths and concentric nodules. The nodules are
CE P
rounded or elongated, sometimes with host rock fragments, and generally occur within the mudstones, at times forming large accumulations (fig.4A; G). They can be concentric, with large calcite crystals (0.2 to 2 mm) or microcrystalline, with variable sizes. When
AC
microcrystalline, they are rounded and massive with fractures. Crusts are lenticular, 30-50cm thick, with granular aspect due to considerable quantities of host rock fragments (fig.43B). Rhizoliths are cylindrical and elongated, being bifurcated or angulated; they do not have host rock fragments and are also microcrystalline (fig. 4E). Under the optical microscope, all calcretes are constituted predominantly of nonmagnesian calcite, and can contain large quantities of siliciclastic grains floating on a carbonate cement. The crusts are composed of micritic cement, a great quantity of siliciclastic grains and, sometimes, root marks filled with sparry calcite (fig. 4D). The concentric nodules have coarse mosaic calcite, with concentric growth and, often, formed by the coalescence of several
ACCEPTED MANUSCRIPT
7
concretions (fig. 4H). These nodules can have small quantities of floating quartz and feldspar grains, and silicification by fibrous chalcedony. The microcrystalline nodules are mainly
T
composed of microcrystalline calcite, and may have microspar spots. They are extremely
IP
fractured and filled by mosaic or drusy calcite; sometimes with alternating precipitation of iron
SC R
oxides. Besides these minerals, some fractures are filled by chalcedony and barite (fig. 3B). The rhizoliths are microscopically similar to microcrystalline nodules, with the same process of fracturing and filling (fig. 4F,5). No biogenic features or preserved root parts were found inside
NU
the root casts. Table 1 shows a summary of the calcrete types presented.
MA
Table 1 Figure 4
D
Silicification occurs in several degrees as chalcedony or quartz. These minerals replace
TE
calcite and form rims or subhedral crystals filling fractures. In concentric nodular calcretes only
CE P
chalcedony occurs, while in microcrystalline nodules silicification takes place mainly along fractures, as chalcedony or quartz (fig.5). Iron and manganese oxides are common. They can be widespread in concretions or
AC
restricted to specific places. In microcrystalline nodules, oxides occur mainly with finelycrystalline calcite and along fractures as irregular or circular spots, or alternately precipitated with calcite. Figure 5 4.3 X-ray Diffraction X-ray diffraction (XRD) analyses allowed to identify calcite, barite, quartz, plagioclase and k-feldspar in the calcrete paragenesis (fig. 6). Calcite was detected through XRD in four out of five samples, but calcite is present in all samples, identified by optical and electronic microscopy. The sample that did not bear calcite according to the XRD analysis has barite and
ACCEPTED MANUSCRIPT
8
quartz. Quartz and barite peaks are very broad, and maybe are masking the calcite peaks that were not identified. Quartz, plagioclase and k-feldspar are inherited from the sediments and
IP
T
dispersed in the carbonate cement; calcite, barite, but also quartz , are authigenic minerals. Figure 6
SC R
4.4 Chemical Analysis
Chemical compositions of calcrete samples from SMS I and II are presented in table 2,
NU
as well as the composition of calcretes from other localities.
MA
Table 2
Major-element chemical analysis showed that in the calcrete samples microcrystalline
D
nodules and root casts have a high quantity of calcium in relation to silicon, and high lost
TE
ignition (LOI), and crusts and concentric nodules display a lower proportion of those elements,
CE P
being silicon predominant, and less LOI.
AC
5. Interpretation
The microscopic features found in SMS I and II calcretes have an inorganic origin and could be classified as an alpha assemblage (Wright and Tucker, 1991). They alternate iron oxide and carbonate precipitation which are typical groundwater features, but they also display displacive calcite growth and rhizoliths, typical of vadose calcretes. This means that calcretes from SMS I and II have a mixed origin (groundwater and vadose). This conclusion corroborates the results presented by Da Rosa (2005) and Silverio da Silva (1997). It is important to note that the overlap of phreatic and vadose processes modify the features of the oldest process due to the groundwater oscillation, which is evidenced by the alternated precipitation of calcite and hematite.
ACCEPTED MANUSCRIPT
9
In the studied profiles, the major process involved in calcrete formation was displacive calcite cementation. The best evidence for displacive growth is the broad presence of
T
siliciclastic grains and host rock clasts floating in the carbonate cement (fig. 7A). Replacement
IP
process was less representative, but also present, as shown by some feldspar grains corroded by
SC R
calcite that crystallized on their rims. Figure 7
NU
The microcrystalline nodules show alternate precipitation of calcite and iron oxide, mosaic calcite cement and silicification, features typical of groundwater oscillation zone
MA
(Moore, 1989). These nodules were formed by precipitation of calcite within mudstone pores followed by displacive growth, until few grains remained in the carbonate cement. With the
D
lowering of phreatic level and drying of the upper portion of the profile, many fractures were
TE
produced. This process was discussed by Tandon and Friend (1989), who stated the cracks could be result of shrinkage of the host sediment at low depths. Pierini et al. (2001) identified
CE P
smectite and illite in the sediment, what corroborates this hypothesis that drying could be the cause of fissures. When a new pulse of rising water occurred the fractures were filled mainly by
AC
calcite. The concentration of nodules in the same level suggests that carbonate lenses suffered fracturing that generated the nodules. The presence of these carbonate lenses indicates a high phreatic level. Da Rosa et al. (2004) proposed that the carbonate nodules were from palustrine origin, but the absence of microscopic biogenic features suggests that their formation occurred in the subsurface, though at low depths. Moreover, Alonso-Zarza (2003) stated that palustrine carbonates necessarily form on a previous lacustrine host. These nodules were formed in mudflat deposits. Hence these carbonates were formed at a depth in which phreatic and vadose processes frequently overlapped, being close enough to the surface to generate the cracks. Another evidence of oscillating groundwater is the presence of iron and manganese oxides, which are commonly remobilized and precipitated by groundwater oscillation.
ACCEPTED MANUSCRIPT
10
The studied rhizoliths were classified as root casts (Klappa, 1980), formed by cementation inside moulds after root decomposition. As their micromorphology is very similar
T
to phreatic nodules, they were probably formed in same geochemical conditions. Despite the
SC R
large quantities, indicating a large population of plants.,.
IP
few records of fossil plants (Iannuzzi and Schultz, 1997) in SMS I and II, root casts occur in
The calcrete crusts were interpreted as a massive horizon with typical features of a well-
NU
developed pedogenic profile (Esteban and Klappa, 1993), such as micrite cement and the abundance of siliciclastic grains. Other horizons along the pedogenic profile were not found.
MA
Concentric concretions where interpreted as the product of precipitation within the phreatic zone, below the phreatic oscillation level. This is based on the larger crystals and
D
concentric growth, which needs much time and a large ion contribution to crystallize. The
TE
positions of the described calcretes on the schematic profile are showed in figure 8.
CE P
Figure 8
Some of the identified morphologies are common in other groundwater calcretes, such as calcified root marks formed by phreatophytes (Semeniuk and Meagher, 1981) and massive
AC
beds (Mack et al., 2000). However, in the studied area the nodules are the product of disaggregation of massive calcrete beds due to cracking, filling of these cracks by calcite and posterior dissolution. The main source of ions for carbonate precipitation was groundwater. Considering the semi-arid climate, eolian processes must have been important, thus carbonate could have been brought into the system by dust. Probably some of the Ca that forms the calcite precipitates came from the dissolution of feldspar grains. Due to the high evaporation rate and profusion of plants, the dominant formation model should be per ascensum, in which the ions go up in the profile through evaporation and precipitate calcite, but per descensum process could have had part in the process during rainy seasons.
ACCEPTED MANUSCRIPT
11
Calcite crystals have corroded the borders of detrital grains where silicification occurs. Also, there is corroded calcite mainly in contact with silica minerals, indicating that CaCO3 was not
T
stable when SiO2 minerals precipitated (calcite corroded by chalcedony) (fig. 8B). Detrital grain
IP
dissolution increases with the rise of carbonate in solution, which increases the pH of the
SC R
system. Chalcedony and quartz crystals fill the fractures that cut microcrystalline nodules. In these sites the silicification is more prominent and diverse, reflecting relevant pH fluctuations and the carbonate/silica saturation conditions, probably resulting from seasonal changes (Arakel
NU
et al. 1989). Figure 9 presents a schematic representation of the evolution of this process that
MA
generates the calcretes. Figure 9
D
The difference between the silicon and calcium concentrations in chemical analyses are
TE
due to the variable quantity of detrital quartz grains in calcite cement. The crystallization in microcrystalline nodules seems to be more displacive than in the crusts, leaving less detrital
CE P
grains inside the nodule. All the analyzed calcrete samples have Ba, but it is anomalously high in the TCB73 sample. That suggests that some specific geochemical conditions were achieved
AC
to precipitate barite on the studied profile. The calcretes chemical analyses showed highly variable Ba contents (between 0.01 to 0.8%). Barite can occur after halite and gypsum in evaporites (Pye and Krinsley, 1986). Despite the common occurrence of barite in mesodiagenesis, Zerfass et al. (2000) stated that Santa Maria Supersequence rocks did not go thru mesodiagenesis. Besides, as Na is much depleted in Santa Maria Supersequence and Ca was utilized to form calcite, barite could have been formed in a high evaporation environment, during an anomalously hot season. Retallack and Kirby (2007) propose a correlation between high barium content in rocks and global warming periods, but we could not find any evidences for concluding that conclusion in Brazilian Triassic data. 6. Comparisons with other known calcretes
ACCEPTED MANUSCRIPT
12
The calcretes of SMS I and II have some characteristics in common with others already described in literature. Massive crusts are very similar in microscopic features and mineralogy
T
to those showed in pedogenic calcrete profiles such as the ones in Esteban and Klappa (1983)
IP
and Alonso-Zarza (2003). The groundwater calcretes showed similarity of mineralogy and
SC R
structure with those described by Semeniuk and Meagher (1981) for Australia and Mack et al. (2000) for New Mexico (USA), in which massive phreatic crusts 10-50 cm thick and root casts are common. The presence of filled cracks is not recorded for the previously cited calcretes, but
NU
it is discussed by Tandon and Friend (2000) for Arran cornstone of Scotland. The main differences between them are the host sediments and the process of nodule formation caused by
MA
the dissolution of fracture-filling calcite. In terms of depositional context, SMS I and II are similar to Unit II of Tianshui Basin, China, where calcretes are found (Alonso-Zarza et al.,
TE
have a similar genesis.
D
2010). Nodules and massive groundwater carbonate are found in both units, and they seem to
CE P
7. Climate and sedimentary dynamics The occurrence of calcretes corroborates the semi-arid, seasonal climate proposed by
AC
earlier workers (Holz and Scherer, 1998; Zerfass et al., 2003). Crusts of groundwater calcrete indicate a high phreatic level, and the precipitation of iron oxides indicates that this level oscillated between dry and wet seasons. Pedogenic calcretes are less common, maybe because floods in the wet season dissolve or erode the calcite precipitated at the surface. The presence of a developed (but not fully preserved) profile indicates that the sedimentation rates were commonly low. There is no evidence of short-lived lakes, but due to the high water table they could have existed in landscape depressions. Barite found in nodular calcretes indicates that climate was not highly regular, and abnormally dry seasons could happen. The association of calcretes with iron oxide and illite usually occurs in places where precipitation rates are between 100–500 mm/year (Alonso-Zarza, 2003), so it might be an approximated precipitation value for the studied sequences.
ACCEPTED MANUSCRIPT
13
In some conglomeratic sandstones found in SMS I and II, there are concretionary clasts and no evidences of older units reworking, indicating that processes of cementation and
T
concretion formation were syn-sedimentary and suffered later reworking by fluvial processes.
IP
The Santa Maria Supersequence deposition is highly controlled by tectonics (Zerfass et al.
SC R
2004). These conglomerates could be formed by tectonic movements or climatic pulses during the sequence deposition.
NU
The calcretes in SMS I and II are the south westernmost record of calcretes in Gondwana for Middle Triassic (Fig. 10). They lie within the zone interpreted as subtropical,
MA
under the influence of an arid climate. Figure 10
TE
D
8. Conclusions
The calcretes of Santa Maria Sequences I and II have an inorganic origin and were
CE P
classified as an alpha assemblage. They are the product of oscillation of the phreatic level and overlapping of vadose processes. These carbonate concretions form nodules, crusts and rhizoliths, which are composed by calcite and barite as authigenic minerals, and quartz,
AC
plagioclase and K-feldspar inherited from sediments, dispersed in the carbonate cement. Late silicification by fibrous chalcedony, together with iron oxides, is also present in these concretions. The model proposed for calcrete genesis is per ascensum, in which the ions go up in the profile with evaporation and precipitate calcite, based on the high evaporation rate in semi-arid climates and the large population of plants in these successions. However, per descensum processes must have had part during the rainy seasons. PH oscillations and, as consequence, the carbonate/silica saturation promoted detrital grain dissolutions. Through the integration of calcrete information described here with the sedimentological and paleontological data, this study corroborates the hypothesis of a semi-arid seasonal climate for Triassic in Southern Brazil, as previously proposed by Holz and Scherer (1998) and Zerfass et al. (2003). It
ACCEPTED MANUSCRIPT
14
also provides the first estimative of rainfall for the studied Sequences and proposes a paleoclimatic implication of these calcretes for Gondwana, being the southwesternmost record
IP
T
of calcretes, which is used for inference of the aridity belt.
SC R
Acknowledgements
The authors thank Prof. Dr. Luiz Fernando de Ros, Prof. Dr. Claiton Scherer, and Prof. Dr. Marina Soares for the constructive suggestions, Prof. Dr. Karin Goldberg for reading the
NU
manuscript and Tomaz Melo for the field support. We also thank for the reviwers for the useful
MA
suggestions about the paper. We thank CNPq for the financial support. References
D
Alonso-Zarza, A. M., 2003. Palaeoenvironmental significance of palustrine carbonates and
TE
calcretes in the geological record. Earth-Science Reviews 60, 261–298. Alonso-Zarza, A.M. and Wright, V.P., 2010. Calcretes. In: Alonso-Zarza, A.M., Tanner, L.H.
CE P
(Eds.), Carbonates in Continental Settings: Facies, Environments, and Processes. Amsterdam, Elsevier, pp.225–267.
AC
Alonso-Zarza, A.M., Zhao, Z., Song, C.H., Li, J.J., Zhang, J., Martín-Pérez, A., Martín-García, R., Wang, X.X., Zhang, Y. and Zhang, M.H., 2010. Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess. Sedimentary Geology 222, 42–51.
Arakel, A., Jacobson, G., Salehi, M. and Hill, C., 1989. Silicification of calcrete in palaeodrainage basins of the Australian arid zone. Australian Journal of Earth Sciences 36, 73–89. Barberena, M. C., 1977. Bioestratigrafia preliminar da Formação Santa Maria. Pesquisas 7, 111–129.
ACCEPTED MANUSCRIPT
15
Da Rosa, Á. A. S., Pimentel, N. L. and Faccini, U. F., 2004. Paleoalterações e carbonatos em depósitos aluviais na região de Santa Maria, Triássico Médio a Superior do sul do Brasil.
IP
T
Pesquisas em Geociências 31, 3–16. Da Rosa, Á.A.S., 2005. Paleoalterações em Depósitos Sedimentares de Planícies Aluviais do
SC R
Triássico Médio a Superior do Sul do Brasil: Caracterização, Análise Estratigráfica e Preservação Fossilífera. Unpublisehd Ph.D. thesis, Universidade do Vale do Rio dos Sinos,
NU
Brazil.
Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environments. In: Scholle, P.A.,
MA
Bebout, D.G. and Moore, C.H. (Eds), Carbonate Depositional Environments. AAPG Memoir
D
33, pp. 1 – 96.
TE
Goudie, A.S., 1983. Calcrete. In: Chemical Sediments and Geomorphology (Eds. Goudie, A.S.
CE P
and Pye, K.), pp. 93–131. Academic Press, New York.
Holz M. and Scherer C.M.S., 1998. Sedimentological and paleontological evidence of
AC
paleoclimatic change during the Southbrazilian Triassic: the register of a global trend towards a humid paleoclimate. Zentralblatfiir Geologie und Palaeontologie, Teil 1 (7–8), 1589–1611
Hay, R. L. and Reeder, R. J., 1978. Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania. Sedimentology 25, 649–673.
Iannuzzi, R. and Schultz, C. L., 1997. Primeiro registro de megafósseis vegetais no Membro Alemoa da Formação Santa Maria, RS (Triássico Médio a Superior). In: XV Congresso Brasileiro de Paleontologia, Boletim de Resumos, p. 35.
ACCEPTED MANUSCRIPT
16
Khalaf, F. I., 2007. Occurrences and genesis of calcrete and dolocrete in the Mio-Pleistocene fluviatile sequence in Kuwait, northeast Arabian Peninsula. Sedimentary Geology 199, 129–
IP
T
139.
SC R
Klappa, C., 1980. Rizoliths in terrestrial carbonates: classification, recognition, genesis and
NU
significance. Sedimentology 27, 613–629.
Moore, C. H., 1989. Meteoric diagenetic environments. In: Moore, C. H. (Ed.) Carbonate
MA
diagenesis and porosity. Elsevier, Amsterdam, pp.177–216.
D
Pye, K. and Krinsley, D.H., 1986. Diagenetic carbonate and evaporite minerals in Rotliegend
TE
aeolian sandstones of the southern North Sea: their nature and relationship to secondary
CE P
porosity development. Clay Minerals 21, 443–457.
Retallack, G. J. and Kirby, M. X. 2007. Middle Miocene global change and paleogeography of
AC
Panama. Palaios 22, 667–679. Scherer, C.M.S., Faccini, U.F., Barberena, M.C., Schultz, C.L. and Lavina, E.L., 1995. Bioestratigrafia da Formação Santa Maria: utilização de Cenozonas como horizontes de correlação. Comunicações do Museu de Ciências e Tecnologia, Série Ciências da Terra 1, 43–50.
Schultz, C.L., Scherer, C.M.S. and Barberena, M.C., 2000. Biostratigraphy of Southern Brazilian Middle/Upper Triassic. Revista Brasileira de Geociências 30, 491–494.
ACCEPTED MANUSCRIPT
17
Semeniuk, V. and Meagher, T.D., 1981. The geomorphology and surface processes of the Australind– Leschenault Inlet coastal area. Journal Royal Society of Western Australia 64,
SC R
IP
T
33–51.
Silvério da Silva, J. L., 1997. Estudo dos processos de silicificação e calcificação em rochas sedimentares mesozóicas do Rio Grande do Sul, Brasil. Unpublished Ph.D. thesis,
NU
Universidade Federal do Rio Grande do Sul, Brazil.
MA
Soares, M. B., Schultz, C.L. and Horn, B.L.H., 2011. New information on Riograndia guaibensis Bonaparte, Ferigolo and Ribeiro, 2001(Eucynodontia, Tritheledontidae) from the
D
Late Triassic of southern Brazil:anatomical and biostratigraphic implications. Annals of the
TE
Brazilian Academy of Sciences 83, 329–354.
CE P
Tandon, S. K. and Friend, P. F., 1989. Near-surface shrinkage and carbonate replacement
AC
processes, Arran Cornstone Formation, Scotland. Sedimentology 36, 1113–1126.
Wright, V.P. and Tucker, M.E., 1991. Calcretes: an introduction. In: Wright, V.P. and Tucker, M.E. (Eds), Calcretes. Blackwell Scientific Publications, Oxford, pp.1–22.
Wright, V. P., 2007. Calcrete. In: Nash, D. J. and Mclaren, S. J (Eds.) Geochemical sediments and landscapes. Blackwell Scientific Publications, Oxford, pp.10–45.
Zerfass, H., Garcia, A. J. V., Susczynsky, A. M., and Lavina, E., 2000. Análise de proveniância dos arenitos neopermianos e triássicos da Bacia do Paraná na região de São Pedro do Sul (RS): uma contribuição para o conhecimento da arquitetura estratigráfica e da evolução tectono-sedimentar. Acta Geologica Leopoldensia 51, 61–84.
ACCEPTED MANUSCRIPT
18
Zerfass, H., Lavina, E. L., Schultz, C. L., Garcia, A. J. V., Faccini, U. F. and Chemale Jr., F.,
T
2003. Sequence stratigraphy of continental Triassic strata of southernmost Brazil: a
IP
contribution to southwestern Gondwana palaeogeography and palaeoclimate. Sedimentary
SC R
Geology 161, 85–105.
Zerfass, H., Chemale Jr., F. Schultz, C. L. and Lavina, E., 2004. Tectonics and sedimentation in
AC
CE P
TE
D
MA
NU
Southern South America during Triassic. Sedimentary Geology166, 265–292.
Tables Table 1: Summary table of described calcrete types Table 2: Major-element analyses of calcretes. Oxides in weight %.
Figure captions Fig. 1: Geological setting and location map of the study area. White dots represent outcrop localization.
ACCEPTED MANUSCRIPT
19
Fig. 2: Chronostratigraphy of Southern Brazil Triassic units with vertebrate biozones (modified of Zerfass et al. 2003). Biostratigraphy after Soares et al. (2011). Fig. 3: Composite section of the studied outcrops.
SC R
IP
T
Fig. 4: Plate showing macro and microstructure of the calcretes. A and B: Macroscopic and microscopic photograph of microcrystalline nodules; C and D: Macroscopic and microscopic photograph of crusts; E and F: Macroscopic and microscopic photograph of root casts; G and H: Macroscopic and microscopic photograph of concentric nodules. Fig. 5: Photomicrographs showing the mineralogy of fracture fillings. qtz: quartz; ch: chalcedony; cal: calcite; hem: hematite; brt: barite.
NU
Fig. 6: Compilation diffractogram of analyzed samples. qtz: quartz; kf: alkaline feldspar; pl: plagioclase; brt: barite; cal: calcite. Fig. 7: Photomicrographs of corroded minerals. A: Corroded detrital quartz grains; B: Corroded calcite cement.
MA
Fig. 8: Schematic drawing showing the zones of calcrete formation proposed in this work. Fig. 9: Schematic drawing showing the proposed formation process of calcretes in the Santa Maria Supersequence.
AC
CE P
TE
D
Fig. 10: Middle Triassic paleogeographic map showing the climate boundaries and the localization of calcrete occurrences in other parts of the world (modified of www.scotese.com)
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
Fig. 1
20
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 2
21
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
Fig. 3
22
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 4
23
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Fig. 5
24
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
Fig. 6
25
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Fig. 7
26
AC
Fig. 8
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
27
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
Fig. 9
28
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
Fig. 10
29
AC
Table 1
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
30
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Table 2
31
ACCEPTED MANUSCRIPT
32
Highlights
T
IP
SC R NU MA D TE CE P
We discuss the genesis of calcretes based on calcretes of south Brazil. Calcrete climate significance is discussed for the south aridity belt of Gondwana. Presence of rootcasts indicates the presence of plants despite absence of fossils.
AC
S0031-0182(13)00089-8 doi: 10.1016/j.palaeo.2013.02.013 PALAEO 6414
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date: Revised date: Accepted date:
10 May 2012 1 February 2013 12 February 2013
Please cite this article as: Horn, B.L.D., Pereira, V.P., Schultz, C.L., Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: Classification, genesis and paleoclimatic implications, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.02.013
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.
ACCEPTED MANUSCRIPT
Calcretes of the Santa Maria Supersequence, Middle Triassic, Rio Grande do Sul, Brazil: classification, genesis and paleoclimatic implications
T
B. L. D. Horn, V. P. Pereira and C. L. Schultz
Telephone: 55 51 33 08 63 66
NU
FAX: 55 51 33 08 73 02
SC R
Corresponding author: Bruno Ludovico Dihl Horn
IP
Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre (RS), Brazil
E-mail: [email protected]
AC
CE P
TE
D
MA
Address: Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre (RS), Brazil. CEP: 91501-970
1
ACCEPTED MANUSCRIPT
2
ABSTRACT Carbonate concretions are common features in arid and semi-arid climates. They can be formed
IP
T
under pedogenic conditions, developing complex profiles with or without biogenic contribution or in groundwater conditions, forming thick massive beds of carbonate. Together with other
SC R
data, calcretes can be used as tools for paleoclimatic and paleoenvironmental interpretation. In Southern Brazil, the Triassic Santa Maria Supersequence (Sequences I and II) includes four
NU
types of calcretes with distinct modes of formation: a) precipitation of carbonate in the oscillation zone of the phreatic level, with posterior overlap of vadose and phreatic diagenetic
MA
processes; b) precipitation of pedogenic carbonate, forming crusts in the paleosol; c) precipitation of carbonate forming concentric concretions in the stable phreatic level; d) preservation of root casts due to processes of precipitation in the oscillation zone of the phreatic
TE
D
level. The presence of silica minerals and barite in the concretions is attributed to seasonal changes in water table level. Due to the high evaporation rate in semi-arid climates, the most
CE P
compatible process of formation of these calcretes is the per ascensum model. Integration of calcrete data with the sedimentological and paleontological framework allowed a refinement of paleoclimatic data, suggesting that, despite the scarcity of plant fossils, the Middle Triassic in
AC
Southern Brazil was vegetated with plants.
Keywords: calcretes, paleosol, paleoclimate, Triassic.
ACCEPTED MANUSCRIPT
3
1. Introduction Calcrete is a carbonate-cemented duricrust developed in low-humidity conditions,
IP
T
common in semi-arid and arid climates. They occur in a wide range of forms and in distinct sedimentary environments, as alluvial fans and floodplains, and are the result of introduction of
SC R
calcite and dolomite in soils and sediments (Alonso-Zarza and Wrigth, 2010). Calcretes that commonly form within soil profiles, in superficial settings above the
NU
groundwater, are pedogenic calcretes and commonly show well developed profiles. However, groundwater may induce carbonate precipitation on the capillary fringe in less superficial
MA
settings under influence of plants (Semenieuk and Meagher, 1981), or seasonal groundwater oscillation. These are termed phreatic or groundwater calcretes and indicate the presence of a
D
relatively shallow water table (Alonso-Zarza, 2003). In these calcretes, biogenic features are
TE
unusual, but plants can generate rhizoliths. Klappa (1980) classified the rhizoliths as root moulds, root tubules, root casts, rhizocretions and root petrifications, explaining their
CE P
characteristics and genesis.
In some cases, it may be difficult to distinguish calcretes that formed in groundwater
AC
conditions from those formed in pedogenic environments, due to superposition of process (Alonso-Zarza, 2003; Mack et al., 2000). Calcretes, along with other geochemical information, are important paleoenvironmental and paleoclimatic indicators, and can aid the interpretation of complex sedimentary settings. Moreover, they contain important information that helps interpret ancient ecosystems, palaeogeography and the tectonic, climatic and sedimentary regimes in which they were formed (Alonso-Zarza, 2003; Wright, 2007) . The calcretes in the Triassic Santa Maria Supersequence (Sequences I and II) (Zerfass et al., 2003) have been poorly studied so far. Silverio da Silva (1997) was the first to recognize and describe them, proposing that their fabric was alpha type, intermediate between pedogenic
ACCEPTED MANUSCRIPT
4
and phreatic, and formed by per descensum model of Goudie (1983). According to Silverio da Silva (1997), the negative δ18C and δ13O values suggested that they precipitated from
T
freshwater. Da Rosa (2005) has dealt mostly with the macroscopic features of calcretes in the
IP
Santa Maria Sequences (Santa Maria Supersequence; Zerfass et al., 2003), assigning their origin
SC R
to groundwater variation and eodiagenesis. Da Rosa (2005) also stated that phreatic calcretes tend to have larger calcite crystals than pedogenic ones and, when calcite is microcrystalline, it tends to be displacive. Da Rosa et al. (2004) worked with the paleoalterations and paleosols in
NU
rocks of the Santa Maria Supersequence, identifying mottled mudstones, carbonate veins and phreatic carbonate formation. Some aspects, as formation mode and detailed petrographic
MA
description remain open.
The present work aims to provide the detailed description of phreatic carbonate formation, using
D
petrography, chemical and X-ray diffraction analyses to propose a formation mode and give
CE P
2. Geological Setting
TE
some new climatic information about Santa Maria Sequences I and II.
The Santa Maria Supersequence (Zerfass, 2003) is a Triassic continental, tectonically
AC
controlled system located on the central portion of Rio Grande do Sul State, Brazil (fig.1). It is composed of three sedimentary sequences (fig. 2). Figure 1
The Santa Maria Sequence I (SMSI) comprises clast-supported conglomerates and cross-bedded sandstones, superposed by laminated mudstones, interpreted as a transition from high-energy river to shallow lake deposits. The Santa Maria II Sequence (SMSII) is composed of medium to fine-grained, cross-bedded sandstones and mudstones lenses at the base, gradating to thick mudstone deposits on the middle part, deposited by a fluvial system with high-sinuosity rivers and floodplains. At the top, the sequence comprises a coarsening-upward succession, composed of rhythmites intercalated with lenses of fine-grained, cross-bedded and climbingrippled sandstones that represent a lacustrine–deltaic depositional system. Santa Maria III
ACCEPTED MANUSCRIPT
5
(SMSIII) consists of cross-stratified and conglomeratic sandstones with abundant silicified logs. This lithofacies was interpreted as fluvial channels.
T
Diverse terrestrial vertebrate fossils are reported from the Santa Maria Supersequence.
IP
These include dinosaurs, rhynchosaurs, dicynodonts and cynodonts (Barberena, 1977; Scherer
SC R
et al., 1995; Schultz et al., 2000). Due to the restricted lateral exposure of outcrops and its tectonic framework, this Supersequence is biostratigraphically divided into four vertebrate assemblage zones (fig. 2).
NU
Calcretes are found in all four biozones, but this paper deals only with the lowest three, which belongs to the upper SMSI and SMSII Sequences. The samples were collected in Vera
MA
Cruz, Santa Cruz do Sul and Venâncio Aires municipalities outcrops, showed by a composited section on figure 3.
D
Figure 2
TE
Figure 3
CE P
3. Materials and Methods
Calcrete samples were collected after describing the outcrops and constructing sedimentary graphic logs. They were described macroscopically and with a binocular
AC
stereomicroscope. Thin sections were analyzed with optical and scanning electron microscope (SEM). The macroscopic description and classification of calcretes was defined on basis of outcrop occurrence mode, form, mineralogy, presence of rock fragments and fractures. The semi-quantitative analyses were made in a JEOL® JSM-5800 scanning electronic microscope with acceleration voltage of 15 kV from the Centro de Microscopia Eletrônica, Universidade Federal do Rio Grande do Sul (UFRGS). Major oxides of calcrete samples and related rocks were determined by ICP-MS analyses at Acme Labs, Canada. Mineralogy was also identified by X-ray powder diffraction analyses, using a Siemens D5000 powder diffractometer (BraggBrentano geometry, Cu Kα, 40 kV, 25 mA) from Instituto de Geociências, UFRGS. The X-ray diffractograms were performed between 2º and 72º 2θ, using a step size of 0.02º 2θ per second.
ACCEPTED MANUSCRIPT
6
4. Results
T
4.1 Host sediment
IP
Calcretes are a common feature in massive or laminated, fossiliferous red silty
SC R
mudstones. The mineralogy consists mainly of detrital quartz and K-feldspar with authigenic calcite and hematite (Zerfass et al. 2000). These are the most common and abundant sedimentary rocks in SMS I and II Sequences. These deposits were interpreted as formed by
NU
floodplain deposition in a semi-arid climatic context (Holz and Scherer, 1998; Zerfass et
MA
al.2003).
D
4.2 Calcrete Petrography
TE
The carbonate concretions were classified in three types, according to their macroscopic features: microcrystalline nodules, crusts, rhizoliths and concentric nodules. The nodules are
CE P
rounded or elongated, sometimes with host rock fragments, and generally occur within the mudstones, at times forming large accumulations (fig.4A; G). They can be concentric, with large calcite crystals (0.2 to 2 mm) or microcrystalline, with variable sizes. When
AC
microcrystalline, they are rounded and massive with fractures. Crusts are lenticular, 30-50cm thick, with granular aspect due to considerable quantities of host rock fragments (fig.43B). Rhizoliths are cylindrical and elongated, being bifurcated or angulated; they do not have host rock fragments and are also microcrystalline (fig. 4E). Under the optical microscope, all calcretes are constituted predominantly of nonmagnesian calcite, and can contain large quantities of siliciclastic grains floating on a carbonate cement. The crusts are composed of micritic cement, a great quantity of siliciclastic grains and, sometimes, root marks filled with sparry calcite (fig. 4D). The concentric nodules have coarse mosaic calcite, with concentric growth and, often, formed by the coalescence of several
ACCEPTED MANUSCRIPT
7
concretions (fig. 4H). These nodules can have small quantities of floating quartz and feldspar grains, and silicification by fibrous chalcedony. The microcrystalline nodules are mainly
T
composed of microcrystalline calcite, and may have microspar spots. They are extremely
IP
fractured and filled by mosaic or drusy calcite; sometimes with alternating precipitation of iron
SC R
oxides. Besides these minerals, some fractures are filled by chalcedony and barite (fig. 3B). The rhizoliths are microscopically similar to microcrystalline nodules, with the same process of fracturing and filling (fig. 4F,5). No biogenic features or preserved root parts were found inside
NU
the root casts. Table 1 shows a summary of the calcrete types presented.
MA
Table 1 Figure 4
D
Silicification occurs in several degrees as chalcedony or quartz. These minerals replace
TE
calcite and form rims or subhedral crystals filling fractures. In concentric nodular calcretes only
CE P
chalcedony occurs, while in microcrystalline nodules silicification takes place mainly along fractures, as chalcedony or quartz (fig.5). Iron and manganese oxides are common. They can be widespread in concretions or
AC
restricted to specific places. In microcrystalline nodules, oxides occur mainly with finelycrystalline calcite and along fractures as irregular or circular spots, or alternately precipitated with calcite. Figure 5 4.3 X-ray Diffraction X-ray diffraction (XRD) analyses allowed to identify calcite, barite, quartz, plagioclase and k-feldspar in the calcrete paragenesis (fig. 6). Calcite was detected through XRD in four out of five samples, but calcite is present in all samples, identified by optical and electronic microscopy. The sample that did not bear calcite according to the XRD analysis has barite and
ACCEPTED MANUSCRIPT
8
quartz. Quartz and barite peaks are very broad, and maybe are masking the calcite peaks that were not identified. Quartz, plagioclase and k-feldspar are inherited from the sediments and
IP
T
dispersed in the carbonate cement; calcite, barite, but also quartz , are authigenic minerals. Figure 6
SC R
4.4 Chemical Analysis
Chemical compositions of calcrete samples from SMS I and II are presented in table 2,
NU
as well as the composition of calcretes from other localities.
MA
Table 2
Major-element chemical analysis showed that in the calcrete samples microcrystalline
D
nodules and root casts have a high quantity of calcium in relation to silicon, and high lost
TE
ignition (LOI), and crusts and concentric nodules display a lower proportion of those elements,
CE P
being silicon predominant, and less LOI.
AC
5. Interpretation
The microscopic features found in SMS I and II calcretes have an inorganic origin and could be classified as an alpha assemblage (Wright and Tucker, 1991). They alternate iron oxide and carbonate precipitation which are typical groundwater features, but they also display displacive calcite growth and rhizoliths, typical of vadose calcretes. This means that calcretes from SMS I and II have a mixed origin (groundwater and vadose). This conclusion corroborates the results presented by Da Rosa (2005) and Silverio da Silva (1997). It is important to note that the overlap of phreatic and vadose processes modify the features of the oldest process due to the groundwater oscillation, which is evidenced by the alternated precipitation of calcite and hematite.
ACCEPTED MANUSCRIPT
9
In the studied profiles, the major process involved in calcrete formation was displacive calcite cementation. The best evidence for displacive growth is the broad presence of
T
siliciclastic grains and host rock clasts floating in the carbonate cement (fig. 7A). Replacement
IP
process was less representative, but also present, as shown by some feldspar grains corroded by
SC R
calcite that crystallized on their rims. Figure 7
NU
The microcrystalline nodules show alternate precipitation of calcite and iron oxide, mosaic calcite cement and silicification, features typical of groundwater oscillation zone
MA
(Moore, 1989). These nodules were formed by precipitation of calcite within mudstone pores followed by displacive growth, until few grains remained in the carbonate cement. With the
D
lowering of phreatic level and drying of the upper portion of the profile, many fractures were
TE
produced. This process was discussed by Tandon and Friend (1989), who stated the cracks could be result of shrinkage of the host sediment at low depths. Pierini et al. (2001) identified
CE P
smectite and illite in the sediment, what corroborates this hypothesis that drying could be the cause of fissures. When a new pulse of rising water occurred the fractures were filled mainly by
AC
calcite. The concentration of nodules in the same level suggests that carbonate lenses suffered fracturing that generated the nodules. The presence of these carbonate lenses indicates a high phreatic level. Da Rosa et al. (2004) proposed that the carbonate nodules were from palustrine origin, but the absence of microscopic biogenic features suggests that their formation occurred in the subsurface, though at low depths. Moreover, Alonso-Zarza (2003) stated that palustrine carbonates necessarily form on a previous lacustrine host. These nodules were formed in mudflat deposits. Hence these carbonates were formed at a depth in which phreatic and vadose processes frequently overlapped, being close enough to the surface to generate the cracks. Another evidence of oscillating groundwater is the presence of iron and manganese oxides, which are commonly remobilized and precipitated by groundwater oscillation.
ACCEPTED MANUSCRIPT
10
The studied rhizoliths were classified as root casts (Klappa, 1980), formed by cementation inside moulds after root decomposition. As their micromorphology is very similar
T
to phreatic nodules, they were probably formed in same geochemical conditions. Despite the
SC R
large quantities, indicating a large population of plants.,.
IP
few records of fossil plants (Iannuzzi and Schultz, 1997) in SMS I and II, root casts occur in
The calcrete crusts were interpreted as a massive horizon with typical features of a well-
NU
developed pedogenic profile (Esteban and Klappa, 1993), such as micrite cement and the abundance of siliciclastic grains. Other horizons along the pedogenic profile were not found.
MA
Concentric concretions where interpreted as the product of precipitation within the phreatic zone, below the phreatic oscillation level. This is based on the larger crystals and
D
concentric growth, which needs much time and a large ion contribution to crystallize. The
TE
positions of the described calcretes on the schematic profile are showed in figure 8.
CE P
Figure 8
Some of the identified morphologies are common in other groundwater calcretes, such as calcified root marks formed by phreatophytes (Semeniuk and Meagher, 1981) and massive
AC
beds (Mack et al., 2000). However, in the studied area the nodules are the product of disaggregation of massive calcrete beds due to cracking, filling of these cracks by calcite and posterior dissolution. The main source of ions for carbonate precipitation was groundwater. Considering the semi-arid climate, eolian processes must have been important, thus carbonate could have been brought into the system by dust. Probably some of the Ca that forms the calcite precipitates came from the dissolution of feldspar grains. Due to the high evaporation rate and profusion of plants, the dominant formation model should be per ascensum, in which the ions go up in the profile through evaporation and precipitate calcite, but per descensum process could have had part in the process during rainy seasons.
ACCEPTED MANUSCRIPT
11
Calcite crystals have corroded the borders of detrital grains where silicification occurs. Also, there is corroded calcite mainly in contact with silica minerals, indicating that CaCO3 was not
T
stable when SiO2 minerals precipitated (calcite corroded by chalcedony) (fig. 8B). Detrital grain
IP
dissolution increases with the rise of carbonate in solution, which increases the pH of the
SC R
system. Chalcedony and quartz crystals fill the fractures that cut microcrystalline nodules. In these sites the silicification is more prominent and diverse, reflecting relevant pH fluctuations and the carbonate/silica saturation conditions, probably resulting from seasonal changes (Arakel
NU
et al. 1989). Figure 9 presents a schematic representation of the evolution of this process that
MA
generates the calcretes. Figure 9
D
The difference between the silicon and calcium concentrations in chemical analyses are
TE
due to the variable quantity of detrital quartz grains in calcite cement. The crystallization in microcrystalline nodules seems to be more displacive than in the crusts, leaving less detrital
CE P
grains inside the nodule. All the analyzed calcrete samples have Ba, but it is anomalously high in the TCB73 sample. That suggests that some specific geochemical conditions were achieved
AC
to precipitate barite on the studied profile. The calcretes chemical analyses showed highly variable Ba contents (between 0.01 to 0.8%). Barite can occur after halite and gypsum in evaporites (Pye and Krinsley, 1986). Despite the common occurrence of barite in mesodiagenesis, Zerfass et al. (2000) stated that Santa Maria Supersequence rocks did not go thru mesodiagenesis. Besides, as Na is much depleted in Santa Maria Supersequence and Ca was utilized to form calcite, barite could have been formed in a high evaporation environment, during an anomalously hot season. Retallack and Kirby (2007) propose a correlation between high barium content in rocks and global warming periods, but we could not find any evidences for concluding that conclusion in Brazilian Triassic data. 6. Comparisons with other known calcretes
ACCEPTED MANUSCRIPT
12
The calcretes of SMS I and II have some characteristics in common with others already described in literature. Massive crusts are very similar in microscopic features and mineralogy
T
to those showed in pedogenic calcrete profiles such as the ones in Esteban and Klappa (1983)
IP
and Alonso-Zarza (2003). The groundwater calcretes showed similarity of mineralogy and
SC R
structure with those described by Semeniuk and Meagher (1981) for Australia and Mack et al. (2000) for New Mexico (USA), in which massive phreatic crusts 10-50 cm thick and root casts are common. The presence of filled cracks is not recorded for the previously cited calcretes, but
NU
it is discussed by Tandon and Friend (2000) for Arran cornstone of Scotland. The main differences between them are the host sediments and the process of nodule formation caused by
MA
the dissolution of fracture-filling calcite. In terms of depositional context, SMS I and II are similar to Unit II of Tianshui Basin, China, where calcretes are found (Alonso-Zarza et al.,
TE
have a similar genesis.
D
2010). Nodules and massive groundwater carbonate are found in both units, and they seem to
CE P
7. Climate and sedimentary dynamics The occurrence of calcretes corroborates the semi-arid, seasonal climate proposed by
AC
earlier workers (Holz and Scherer, 1998; Zerfass et al., 2003). Crusts of groundwater calcrete indicate a high phreatic level, and the precipitation of iron oxides indicates that this level oscillated between dry and wet seasons. Pedogenic calcretes are less common, maybe because floods in the wet season dissolve or erode the calcite precipitated at the surface. The presence of a developed (but not fully preserved) profile indicates that the sedimentation rates were commonly low. There is no evidence of short-lived lakes, but due to the high water table they could have existed in landscape depressions. Barite found in nodular calcretes indicates that climate was not highly regular, and abnormally dry seasons could happen. The association of calcretes with iron oxide and illite usually occurs in places where precipitation rates are between 100–500 mm/year (Alonso-Zarza, 2003), so it might be an approximated precipitation value for the studied sequences.
ACCEPTED MANUSCRIPT
13
In some conglomeratic sandstones found in SMS I and II, there are concretionary clasts and no evidences of older units reworking, indicating that processes of cementation and
T
concretion formation were syn-sedimentary and suffered later reworking by fluvial processes.
IP
The Santa Maria Supersequence deposition is highly controlled by tectonics (Zerfass et al.
SC R
2004). These conglomerates could be formed by tectonic movements or climatic pulses during the sequence deposition.
NU
The calcretes in SMS I and II are the south westernmost record of calcretes in Gondwana for Middle Triassic (Fig. 10). They lie within the zone interpreted as subtropical,
MA
under the influence of an arid climate. Figure 10
TE
D
8. Conclusions
The calcretes of Santa Maria Sequences I and II have an inorganic origin and were
CE P
classified as an alpha assemblage. They are the product of oscillation of the phreatic level and overlapping of vadose processes. These carbonate concretions form nodules, crusts and rhizoliths, which are composed by calcite and barite as authigenic minerals, and quartz,
AC
plagioclase and K-feldspar inherited from sediments, dispersed in the carbonate cement. Late silicification by fibrous chalcedony, together with iron oxides, is also present in these concretions. The model proposed for calcrete genesis is per ascensum, in which the ions go up in the profile with evaporation and precipitate calcite, based on the high evaporation rate in semi-arid climates and the large population of plants in these successions. However, per descensum processes must have had part during the rainy seasons. PH oscillations and, as consequence, the carbonate/silica saturation promoted detrital grain dissolutions. Through the integration of calcrete information described here with the sedimentological and paleontological data, this study corroborates the hypothesis of a semi-arid seasonal climate for Triassic in Southern Brazil, as previously proposed by Holz and Scherer (1998) and Zerfass et al. (2003). It
ACCEPTED MANUSCRIPT
14
also provides the first estimative of rainfall for the studied Sequences and proposes a paleoclimatic implication of these calcretes for Gondwana, being the southwesternmost record
IP
T
of calcretes, which is used for inference of the aridity belt.
SC R
Acknowledgements
The authors thank Prof. Dr. Luiz Fernando de Ros, Prof. Dr. Claiton Scherer, and Prof. Dr. Marina Soares for the constructive suggestions, Prof. Dr. Karin Goldberg for reading the
NU
manuscript and Tomaz Melo for the field support. We also thank for the reviwers for the useful
MA
suggestions about the paper. We thank CNPq for the financial support. References
D
Alonso-Zarza, A. M., 2003. Palaeoenvironmental significance of palustrine carbonates and
TE
calcretes in the geological record. Earth-Science Reviews 60, 261–298. Alonso-Zarza, A.M. and Wright, V.P., 2010. Calcretes. In: Alonso-Zarza, A.M., Tanner, L.H.
CE P
(Eds.), Carbonates in Continental Settings: Facies, Environments, and Processes. Amsterdam, Elsevier, pp.225–267.
AC
Alonso-Zarza, A.M., Zhao, Z., Song, C.H., Li, J.J., Zhang, J., Martín-Pérez, A., Martín-García, R., Wang, X.X., Zhang, Y. and Zhang, M.H., 2010. Mudflat/distal fan and shallow lake sedimentation (upper Vallesian–Turolian) in the Tianshui Basin, Central China: Evidence against the late Miocene eolian loess. Sedimentary Geology 222, 42–51.
Arakel, A., Jacobson, G., Salehi, M. and Hill, C., 1989. Silicification of calcrete in palaeodrainage basins of the Australian arid zone. Australian Journal of Earth Sciences 36, 73–89. Barberena, M. C., 1977. Bioestratigrafia preliminar da Formação Santa Maria. Pesquisas 7, 111–129.
ACCEPTED MANUSCRIPT
15
Da Rosa, Á. A. S., Pimentel, N. L. and Faccini, U. F., 2004. Paleoalterações e carbonatos em depósitos aluviais na região de Santa Maria, Triássico Médio a Superior do sul do Brasil.
IP
T
Pesquisas em Geociências 31, 3–16. Da Rosa, Á.A.S., 2005. Paleoalterações em Depósitos Sedimentares de Planícies Aluviais do
SC R
Triássico Médio a Superior do Sul do Brasil: Caracterização, Análise Estratigráfica e Preservação Fossilífera. Unpublisehd Ph.D. thesis, Universidade do Vale do Rio dos Sinos,
NU
Brazil.
Esteban, M. and Klappa, C.F., 1983. Subaerial exposure environments. In: Scholle, P.A.,
MA
Bebout, D.G. and Moore, C.H. (Eds), Carbonate Depositional Environments. AAPG Memoir
D
33, pp. 1 – 96.
TE
Goudie, A.S., 1983. Calcrete. In: Chemical Sediments and Geomorphology (Eds. Goudie, A.S.
CE P
and Pye, K.), pp. 93–131. Academic Press, New York.
Holz M. and Scherer C.M.S., 1998. Sedimentological and paleontological evidence of
AC
paleoclimatic change during the Southbrazilian Triassic: the register of a global trend towards a humid paleoclimate. Zentralblatfiir Geologie und Palaeontologie, Teil 1 (7–8), 1589–1611
Hay, R. L. and Reeder, R. J., 1978. Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania. Sedimentology 25, 649–673.
Iannuzzi, R. and Schultz, C. L., 1997. Primeiro registro de megafósseis vegetais no Membro Alemoa da Formação Santa Maria, RS (Triássico Médio a Superior). In: XV Congresso Brasileiro de Paleontologia, Boletim de Resumos, p. 35.
ACCEPTED MANUSCRIPT
16
Khalaf, F. I., 2007. Occurrences and genesis of calcrete and dolocrete in the Mio-Pleistocene fluviatile sequence in Kuwait, northeast Arabian Peninsula. Sedimentary Geology 199, 129–
IP
T
139.
SC R
Klappa, C., 1980. Rizoliths in terrestrial carbonates: classification, recognition, genesis and
NU
significance. Sedimentology 27, 613–629.
Moore, C. H., 1989. Meteoric diagenetic environments. In: Moore, C. H. (Ed.) Carbonate
MA
diagenesis and porosity. Elsevier, Amsterdam, pp.177–216.
D
Pye, K. and Krinsley, D.H., 1986. Diagenetic carbonate and evaporite minerals in Rotliegend
TE
aeolian sandstones of the southern North Sea: their nature and relationship to secondary
CE P
porosity development. Clay Minerals 21, 443–457.
Retallack, G. J. and Kirby, M. X. 2007. Middle Miocene global change and paleogeography of
AC
Panama. Palaios 22, 667–679. Scherer, C.M.S., Faccini, U.F., Barberena, M.C., Schultz, C.L. and Lavina, E.L., 1995. Bioestratigrafia da Formação Santa Maria: utilização de Cenozonas como horizontes de correlação. Comunicações do Museu de Ciências e Tecnologia, Série Ciências da Terra 1, 43–50.
Schultz, C.L., Scherer, C.M.S. and Barberena, M.C., 2000. Biostratigraphy of Southern Brazilian Middle/Upper Triassic. Revista Brasileira de Geociências 30, 491–494.
ACCEPTED MANUSCRIPT
17
Semeniuk, V. and Meagher, T.D., 1981. The geomorphology and surface processes of the Australind– Leschenault Inlet coastal area. Journal Royal Society of Western Australia 64,
SC R
IP
T
33–51.
Silvério da Silva, J. L., 1997. Estudo dos processos de silicificação e calcificação em rochas sedimentares mesozóicas do Rio Grande do Sul, Brasil. Unpublished Ph.D. thesis,
NU
Universidade Federal do Rio Grande do Sul, Brazil.
MA
Soares, M. B., Schultz, C.L. and Horn, B.L.H., 2011. New information on Riograndia guaibensis Bonaparte, Ferigolo and Ribeiro, 2001(Eucynodontia, Tritheledontidae) from the
D
Late Triassic of southern Brazil:anatomical and biostratigraphic implications. Annals of the
TE
Brazilian Academy of Sciences 83, 329–354.
CE P
Tandon, S. K. and Friend, P. F., 1989. Near-surface shrinkage and carbonate replacement
AC
processes, Arran Cornstone Formation, Scotland. Sedimentology 36, 1113–1126.
Wright, V.P. and Tucker, M.E., 1991. Calcretes: an introduction. In: Wright, V.P. and Tucker, M.E. (Eds), Calcretes. Blackwell Scientific Publications, Oxford, pp.1–22.
Wright, V. P., 2007. Calcrete. In: Nash, D. J. and Mclaren, S. J (Eds.) Geochemical sediments and landscapes. Blackwell Scientific Publications, Oxford, pp.10–45.
Zerfass, H., Garcia, A. J. V., Susczynsky, A. M., and Lavina, E., 2000. Análise de proveniância dos arenitos neopermianos e triássicos da Bacia do Paraná na região de São Pedro do Sul (RS): uma contribuição para o conhecimento da arquitetura estratigráfica e da evolução tectono-sedimentar. Acta Geologica Leopoldensia 51, 61–84.
ACCEPTED MANUSCRIPT
18
Zerfass, H., Lavina, E. L., Schultz, C. L., Garcia, A. J. V., Faccini, U. F. and Chemale Jr., F.,
T
2003. Sequence stratigraphy of continental Triassic strata of southernmost Brazil: a
IP
contribution to southwestern Gondwana palaeogeography and palaeoclimate. Sedimentary
SC R
Geology 161, 85–105.
Zerfass, H., Chemale Jr., F. Schultz, C. L. and Lavina, E., 2004. Tectonics and sedimentation in
AC
CE P
TE
D
MA
NU
Southern South America during Triassic. Sedimentary Geology166, 265–292.
Tables Table 1: Summary table of described calcrete types Table 2: Major-element analyses of calcretes. Oxides in weight %.
Figure captions Fig. 1: Geological setting and location map of the study area. White dots represent outcrop localization.
ACCEPTED MANUSCRIPT
19
Fig. 2: Chronostratigraphy of Southern Brazil Triassic units with vertebrate biozones (modified of Zerfass et al. 2003). Biostratigraphy after Soares et al. (2011). Fig. 3: Composite section of the studied outcrops.
SC R
IP
T
Fig. 4: Plate showing macro and microstructure of the calcretes. A and B: Macroscopic and microscopic photograph of microcrystalline nodules; C and D: Macroscopic and microscopic photograph of crusts; E and F: Macroscopic and microscopic photograph of root casts; G and H: Macroscopic and microscopic photograph of concentric nodules. Fig. 5: Photomicrographs showing the mineralogy of fracture fillings. qtz: quartz; ch: chalcedony; cal: calcite; hem: hematite; brt: barite.
NU
Fig. 6: Compilation diffractogram of analyzed samples. qtz: quartz; kf: alkaline feldspar; pl: plagioclase; brt: barite; cal: calcite. Fig. 7: Photomicrographs of corroded minerals. A: Corroded detrital quartz grains; B: Corroded calcite cement.
MA
Fig. 8: Schematic drawing showing the zones of calcrete formation proposed in this work. Fig. 9: Schematic drawing showing the proposed formation process of calcretes in the Santa Maria Supersequence.
AC
CE P
TE
D
Fig. 10: Middle Triassic paleogeographic map showing the climate boundaries and the localization of calcrete occurrences in other parts of the world (modified of www.scotese.com)
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
Fig. 1
20
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 2
21
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
Fig. 3
22
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 4
23
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Fig. 5
24
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
Fig. 6
25
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Fig. 7
26
AC
Fig. 8
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
27
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
Fig. 9
28
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
Fig. 10
29
AC
Table 1
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
30
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
NU
Table 2
31
ACCEPTED MANUSCRIPT
32
Highlights
T
IP
SC R NU MA D TE CE P
We discuss the genesis of calcretes based on calcretes of south Brazil. Calcrete climate significance is discussed for the south aridity belt of Gondwana. Presence of rootcasts indicates the presence of plants despite absence of fossils.
AC