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Geological context of the dinosauriform-bearing outcrops from the Triassic of Southern Brazil
Accepted Manuscript Geological context of the Dinosauriform-Bearing outcrops from The Triassic of Southern Brazil Átila A.S. Da-Rosa PII:
S0895-9811(14)00145-X
DOI:
10.1016/j.jsames.2014.10.008
Reference:
SAMES 1340
To appear in:
Journal of South American Earth Sciences
Received Date: 25 April 2014 Revised Date:
19 October 2014
Accepted Date: 24 October 2014
Please cite this article as: Da-Rosa, Á.A.S., Geological context of the Dinosauriform-Bearing outcrops from The Triassic of Southern Brazil, Journal of South American Earth Sciences (2014), doi: 10.1016/ j.jsames.2014.10.008. 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 GEOLOGICAL CONTEXT OF THE DINOSAURIFORM-BEARING OUTCROPS FROM THE TRIASSIC OF SOUTHERN BRAZIL
Átila A.S. Da-Rosa Laboratório de Estratigrafia e Paleobiologia, Departamento de Geociências, Universidade Federal de Santa Maria, Avenida Roraima, 1000, Prédio 17, Sala 1131B, 97.105-900, Santa Maria, RS, Brasil.
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[email protected]
Abstract
The Triassic of the western Gondwana (southern Brazil and northwestern Argentina) records the
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oldest dinosaurs. The Southern Brazilian Triassic fauna is subdivided into four assemblage zones (AZ’s), recorded in alluvial (channel and floodplain) deposits, split into three third-order sequences that comprise the Santa Maria Supersequence. These deposits record dinosauriforms in three of these
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AZ’s, mostly in near-channel environments (channel deposits, crevasse splays, distal floodplains) with different types of preservation (mostly partly articulated, with little carbonate deposition). There is faciological homogeneity within the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s, whereas change in fluvial style is recorded at the Riograndia AZ. So, further stratigraphic studies must include the recognition of post-depositional tectonism, in order to better understand the autogenic and allogenic mechanisms of deposition. It is suggested here that there is a lateral change on main channel
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deposition, with the areal restriction of the Santacruzodon AZ probably linked to reactivation on the Passo do Sobrado lineament and modification of the basin floor.
1. Introduction
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Keywords: Triassic, Brazil, Santa Maria Supersequence, Santa Maria Formation, Caturrita Formation.
The Triassic of southern Brazil and Argentina are the main sources of information on the
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origin and early irradiation of dinosaurs (e.g., Tucker and Benton, 1982; Langer et al., 2007, 2010; Novas, 2009; Brusatte et al., 2010; Benton et al., 2014). Although there is some dispute over where the most primitive dinosaur came from, it is mostly agreed that the group irradiated from the western Gondwana (Langer et al., 2011). Dinosaurian forms from the Brazilian Triassic include basal Saurischia, with sauropodomorph
(Unaysaurus tolentinoi Leal et al., 2004, Guaibasaurus candelariensis Bonaparte et al., 1999) and theropod (Staurikosaurus pricei Colbert, 1970, Pampadromaeus barberenai Cabreira et al., 2011) affinities, as well as non-dinosaurian dinosauriforms (Sacisaurus agudoensis Ferigolo and Langer, 2006). Fragmentary sauropodomorphs also were also recently described (Leal et al., 2005; Da Rosa et al., 2006; Dambros et al., 2007; Bittencourt et al., 2012; Temp-Müller et al., 2013a,b). The outcrops
ACCEPTED MANUSCRIPT where these fossils came from were re-studied and new information on the life/death processes are now available. In the last 20 years, the knowledge of the Triassic of southern Brazil has been greatly improved by fossiliferous discoveries and new methods of correlation and study. These new findings modified the understanding on the existing faunas and its relation to the paleoenvironments. In this
southern Brazil will be presented and discussed (Fig. 1). Insert Figure 1 here 2. Material and Methods
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paper, some of the geological information about the dinosauriform-bearing sites from the Triassic of
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The most recent faunal list of the Triassic of southern Brazil is given in Langer et al. (2007a) and reviewed by Soares et al. (2011), where four assemblage zones can be recognized for the Middle to Upper Triassic Santa Maria Supersequence (Santa Maria and Caturrita formations):
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– Dinodontosaurus Assemblage Zone (AZ), constituted by dicynodonts, cynodonts, archosaurs and procolophonoids, considered to be early Ladinian;
– Santacruzodon AZ, bearing traversodontid cynodonts and archosaurs, dated as late Ladinian – early Carnian;
– Hyperodapedon AZ, with rhynchosaurs, cynodonts, dinosaurs, archosaurs of Carnian age; – Riograndia AZ, composed by cynodonts, procolophonids, sphenodonts, dicynodonts, archosaurs nd
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dinosaurs, considered as early Norian.
Dinosauriform-bearing sites were laterally and vertically correlated by facies analysis, after the recognition of a post-depositional structural organization, as depicted by Da Rosa and Faccini (2005). The dinosauriforms discussed here are (Tab. 1):
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– Spondylosoma absconditum, recovered by Friedrich von Huene in 1929 at the district of Chiniquá, São Pedro do Sul, is a highly debated taxon (Galton, 2000), but presented here due to its biostratigraphic implications, as it comes from the same age from Argentinean dinosauromorphs.
AZ;
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Based on the faunal content, the Chiniquá sedimentary deposits were related to the Dinodontosaurus
– Staurikosaurus pricei, collected in 1935 by a joint USA-Brazil expedition (Colbert, 1970; Bittencourt and Kellner, 2009). This fossil came from von Huene’s Jazigo 1, today completely covered by the urban growth of the city of Santa Maria, but referable to the Hyperodapedon AZ; – Saturnalia tupiniquim, collected in 1998 at the Alemoa site, 100 m SE from “Jazigo 1”, and also from the Hyperodapedon AZ (Langer et al., 1999). A tiny sauropodomorph (Leal et al., 2005) and a more robust one (Da Rosa et al., 2006; Dambros et al., 2007) were also collected; – Recently dinosaurian collected material from the Linha Facão, Candelária, referable to the Hyperodapedon AZ;
ACCEPTED MANUSCRIPT – Unaysaurus tolentinoi, collected in 1998 at the limit of the São Martinho da Serra and Santa Maria municipalities (Leal et al., 2004). Although no other vertebrates were found, gnaw or scratch marks attributed to scavenging cynodonts (Leal et al., 2002) related this fossil to the Riograndia AZ. This is also true for other sauropodomorphs found in an outcrop few kilometers apart (Bittencourt et al., 2012); – Guaibasaurus candelariensis, originally collected at Candelária and later at Faxinal do Soturno
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(Bonaparte et al., 1999, 2007; Langer et al., 2011). The cynodonts on these outcrops are the biostratigraphic basis for the definition of Riograndia AZ (Soares et al., 2011);
– Pampadromaeus barberenai, collected at the Janner outcrop, southwest of the urban area of Agudo, related to the Hyperodapedon AZ, in the Exaeretodon acme zone (Langer et al., 2011);
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– Recently collected sauropodomorph material from the Linha Flores, housed at the UNIPAMPA collection (Universidade Federal do Pampa – Campus São Gabriel), in levels preliminarily related to the Riograndia AZ (Temp-Müller et al., 2013a,b);
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– Recently described sauropodomorph material from Cerro Botucaraí (or Sesmaria do Pinhal 1), at Candelária, in levels of the Riograndia AZ (Bittencourt et al., 2012);
– Sacisaurus agudoensis, collected at the urban area of Agudo, today completely covered by the urban growth of the city (Ferigolo and Langer, 2006), so it is certain to which AZ this outcrop can be related, the Hyperodapedon or Riograndia AZ.
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Insert Table 1 here
3. A brief overview of the Triassic of the Paraná Basin The first geological sketch of the Paraná Basin was erected by White (1908), in the coalfields of southern Brazil, but only at the 1930’s, the Triassic beds of the Rio Grande do Sul state were
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recognized by Huene and Stahlecker (1931). The evolution of the stratigraphic work on the Triassic of southern Brazil was summarized by Faccini (2000) and presented here as part of the figure 8. Bortoluzzi (1974) recognized the Santa Maria beds, with fossil leave imprints of the
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Dicroidium Flora, which Andreis et al. (1980) denominated as the Rosario do Sul Group and subdivided into the Sanga do Cabral Formation (Early Triassic), the Santa Maria Formation (with the Alemoa and Passo das Tropas Members, Middle to Upper Triassic) and the Caturrita Formation (Upper Triassic).
Milani et al. (1998) studied the complete sedimentary package of the Paraná Basin,
subdividing it into six second-order supersequences: Rio Ivaí, Paraná, Gondwana I, Gondwana II, Gondwana III, and Bauru. Scherer et al. (2000) included the southern Brazilian deposits into the Permo-Carboniferous, Triassic and Neocomian sequences. Later on, Zerfass et al. (2003) named the Sanga do Cabral and Santa Maria second-order supersequences for the southern Brazilian Triassic. Barberena (1977) provided the first biostratigraphic subdivisions for the Triassic of southern Brazil, later improved by Schultz et al (2000). The most complete faunal list is given in Langer et al
ACCEPTED MANUSCRIPT (2007) and Schultz and Langer (2007), and the biostratigraphic zonation herein presented follows Soares et al. (2011). The basic geological mapping of the Triassic of southern Brazil was historically done in a regional scale, with no details on the faciological and structural compartimentation of the sedimentary deposits. For example, Carraro et al. (1974) recognized an undivided Triassic, mapping produced by CPRM-DNPM (1982) improved the boundaries, but maintaining the undivided fashion. Only recently,
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Wildner et al. (2008) provided a more precise map of the southern Brazilian Triassic sedimentary deposits, including the presence and recognition of structural limits. Zerfass (2008) was the first to
in a 1:100.000 scale.
4. Post-depositional structural organization
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present a more detailed mapping for the Triassic, also presenting a faciological scheme, although still
Da Rosa and Faccini (2005) studied part of the Triassic section in southern Brazil, recognizing
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that NW and NE lineaments, clearly visible on the Cretaceous volcanic rocks of the Serra Geral Formation on the top of the hills and uplands, are in fact the result of a post-depositional tectonism, also affecting older units. Fault vertical displacements of nearly 100 m were recorded, allowing to recognize structural blocks, such as São Pedro do Sul, Santa Maria, São João do Polêsine, Faxinal do Soturno, Agudo, Paraíso do Sul and Candelária (Fig. 2A,B). Where more detailed geological information is available, such as subsurface drill cores or logs, the integration of surface and
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subsurface geological mapping allows to recognize minor displacements, and the clear correlation along a restrict area, defining structural sub-blocks. For instance, five sub-blocks were recognized at the urban area of Santa Maria (Fig. 2C): Cabeceira do Raimundo, Tancredo Neves, Cidade, Km 3 and Camobi.
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Insert Figure 2 here
In this paper a more complete structural organization of the post-depositional tectonism on the Triassic of southern Brazil is presented (Fig. 1). Apart of the blocks already mentioned, the following
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can be recognized:
- Mata, delineated at west by the Jaguari-Mata Fault Zone, recognized by Faccini (2000) as the limit of the Middle-Upper Triassic outcrop belt; - Vale do Sol, east of Candelária, defined by the Rio Pardo and Plumbs lineaments (west and east, respectively);
- Vera Cruz, limited by the Plumbs and Rio Pardinho lineaments (west and east, respectively); - Santa Cruz do Sul, defined by the Rio Pardinho and Passo do Sobrado lineaments (west and east, respectively); - Venâncio Aires, delineated by the Passo do Sobrado and Cruzeiro do Sul lineaments (west and east, respectively);
ACCEPTED MANUSCRIPT - Bom Retiro do Sul, defined at east by the Taquari Fault Zone sensu Faccini (2000) as the eastern limit of the sedimentary package. The recognition of a post-depositional tectonism is essential in further stratigraphic studies, showing the need for a previous understanding of block and sub-block relationships. For example, the first biostratigraphic studies realized that fossil faunas were diachronic (Barberena, 1977), but no detailed mapping could help in solving the lateral and vertical correlations. Also, recently tectonic
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models for the southern Brazilian Triassic neglected the role of a NW and NE oriented faults, resulting from a post-depositional extensional tectonism in placing side by side outcrops of different ages
(Zerfass et al., 2004). On the other hand, NE features such as the Passo do Sobrado lineament, have been reactivated since the Permian (Galli and Kern, 1998; Küchle et al., 2003). Hence, pointing out
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the difference between syn- and post-depositional tectonism is only possible after the identification and mapping of the structural blocks at the southern Brazilian Triassic.
The current tectonic model should be revised at the light of the ideas presented above. Zerfass
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et al. (2004) suggested that the Santa Maria Supersequence was deposited in an extensional tectonism, related to a rift system bounded by E-W faults. Nevertheless, there is no record of proximal facies, such as alluvial fans, or a Triassic volcanism compatible to a rift phase, for example. However, the most E-W striking features present at the central region can be related to the opening of the Atlantic Ocean and the injection of igneous rocks at transforming faults at the Jurassic-Cretaceous (Fernandes
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et al., 1995a,b).
5. Lateral and vertical correlation of outcrops
The stratigraphic correlation of the Triassic of Southern Brazil should be carried out only after a post-depositional structural study, as pointed out by Da Rosa and Faccini (2005). Here I present
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some lateral and vertical correlation of dinosauriform-bearing outcrops, under the scope of postdepositional structural blocks and sub-blocks. At the urban area of Santa Maria, a set of structural sub-blocks can be recognized (Da Rosa
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and Faccini, 2005), in which a certain stratigraphic homogeneity allows to build local composite columnar sections. For each sub-block, a sedimentary log was created and laterally correlated, based on the faciological associations (Fig. 3). The sandy channels of the Passo das Tropas Member are coarser and thicker on the Cidade Sub-Block, and constitute the type section of this unit, at the surroundings of the Passo das Tropas creek. Laterally, this unit switches into finer sandy bodies. The overlying unit is the Alemoa Member, composed by massive reddish mudstones, and its intercalation with fine sandstones. The presence of carbonatic concretions is the most impressive characteristic of this unit, sometimes forming carbonatic crusts of little lateral expression, together with other pedogenetic and phreatic oscillation features (Da Rosa et al., 2004). Along the 30 – 40 m of this unit, there is a continuum of fine-grained deposits, with only rare minor channels, with a few meters of width and thickness. As expected, these channels grade laterally into floodplain deposits, with
ACCEPTED MANUSCRIPT increasing degrees of pedogenesis and phreatic oscillation, as well as changing the preservation types of the fossil vertebrates. At least three vertical sets of channel-floodplain packages can be observed (parasequences?), in which the lateral shift from the minor channels is the most striking feature. The channels laterally change their position in an avulsion-like manner. Little pedogenetic and phreatic alterations are consistent with the idea of a steady floodplain. The overlying Caturrita Formation completely changes the fluvial style, switching it into high energy, low sinuosity channels.
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Insert Figure 3 here
At the Santa Maria Block, Staurikosaurus and Saturnalia came from the Km 3 Sub-block, from the region called Alemoa or Cerro da Alemoa (Fig. 4). A N-S correlation among selected
outcrops from this sub–block led to the recognition of the high energy channels of the Caturrita
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Formation laterally shifting into the floodplain deposits of the Alemoa Member. In this correlation, proximal floodplain deposits are recorded above distal ones, suggesting the approximation of the river channels. The preservation types of the fossil vertebrates changes, according to the facies (Da Rosa,
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2012): fossil in distal floodplains are mostly poorly preserved (complete and fragmentary skeletons, highly carbonated), whereas those in proximal floodplains have a better preservation (little carbonate substitution, but fragmentary skeletons). Staurikosaurus preserved a nearly complete vertebral series (cervical, dorsal, sacral and caudal), with the articulated pelvis and hind limb, highly carbonated and somehow diagenetically deformed (Bittencourt and Kellner, 2009). On the other hand, Saturnalia is composed of three partially articulated, well preserved specimens (Langer et al., 1999, 2007; Langer,
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2003). The tiny sauropodomorphs are also well preserved (Leal et al., 2005), but the more robust ones seem that went through some post-depositional deformation (Da Rosa et al., 2006; Dambros et al., 2007). Although Staurikosaurus was not recovered from the Alemoa site, its stratigraphic position and preservation type can be correlated to levels of the basal portion of that site.
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Insert Figure 4 here
At the Agudo structural block, three dinosauriforms were recorded: Pampadromaeus, Sacisaurus and a group of sauropodomorphs under study. The first came from Janner outcrop, which
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also recorded cynodonts like Exaeretodon and Trucidocynodon, placing it into the upper part of the Hyperodapedon AZ (Cabreira et al., 2011). In this outcrop, it is very clear the upwards faciological change, from a distal to proximal floodplain, and then unconfined channels. The Sacisaurus site is at the city of Agudo, unfortunately destroyed by the urban growth (Ferigolo and Langer, 2006). Geological information available from previous field work (Da Rosa, 2005), as well as from outcrops nearby suggest that the fossils (a series of fragmentary skeletons) came from metric sandy depositional lobes, intercalated with milimetric mudstones, characteristic of crevasse splays. There are no clear signs of pedogenic or phreatic activity at this outcrop. Finally, the Linha das Flores outcrop, currently under study (Temp-Müller et al., 2013a,b), shows channel and crevasse architectural elements and at least three almost complete sauropodomorphs, well preserved (little carbonate substitution) recovered from the basal channel.
ACCEPTED MANUSCRIPT Insert Figure 5 here Guaibasaurus candelariensis was found in two outcrops, from different structural blocks: Sesmaria do Pinhal 2 outcrop, at the Candelária block (Bonaparte et al., 1999), in crevasse channels related to the Riograndia AZ (Bittencourt et al., 2012) and at the Faxinal do Soturno block, at the base of the Linha São Luiz outcrop, also related to crevasse channels of the Riograndia AZ (Bonaparte et al., 2007; Langer et al., 2011).
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Spondylosoma absconditum was found at the Sanga da Árvore outcrop (Huene, 1942), one of the many erosive gullies of the district of Chiniquá, São Pedro do Sul block. It is not clear from which level, so there is no information about its provenance or sedimentary environment. It is poorly
preserved, fragmented and carbonated. The coexistence with dicynodonts places this fossil into the
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Dinodontosaurus AZ (Langer et al., 2007).
6. Reconstruction of the southern Brazilian Triassic floodplains
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The studied dinosauriform-bearing outcrops are part of the Santa Maria Supersequence, which encompasses three third-order sequences, and a change on the fluvial style (Zerfass et al., 2003). The Santa Maria 1 (SM1) sequence is composed by the Dinodontosaurus and Santacruzodon AZ’s, preserved in a low energy fluvial system, with distal and proximal floodplain environments, as well as levees, crevasses and low to middle sinuosity channels. The Santa Maria 2 (SM2) sequence encompasses the Hyperodapedon and Riograndia AZ’s, the first one mostly composed by floodplain
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deposits (distal and proximal, levees, crevasses, minor channels), and the second composed mainly by a channel-crevasse system, in a high energy fluvial system. The Santa Maria 3 (SM3) sequence has not yielded vertebrate fossils yet, and is composed by high energy channels. The passage from the Middle to the Upper Triassic witnessed two different fluvial systems at
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Paraná Basin. This environmental change does not clearly correlate with the third-order sequences of Zerfass et al. (2003). Three AZ’s were recorded on channel-floodplain systems, corresponding to SM1 and part of the SM2. A change of fluvial style is only recorded at the top of SM2. This was mentioned
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by Holz and Scherer (2000), which correlated this faciological shift to an increase in the humidity towards the end of the Triassic, linked to Pangea breakup and formation of the equatorial Atlantic Ocean. This data is corroborated by Pierini et al. (2002) that recorded a geochemical trend in some outcrops near Santa Maria, also linking it to an increase of humidity, at the passage from Hyperodapedon AZ to the Riograndia AZ. Two faciological associations can be envisaged for the southern Brazilian Triassic (Fig. 6). The sandstones (Passo das Tropas Member) and mudstones (Alemoa Member) of the Santa Maria Formation are characteristic of a low energy fluvial system (Fig. 6A), in which main and secondary channels are recorded, as well as the aggradation of the floodplains, by the fast deposition of crevasse splays, or the slower sedimentation through subsequent inundations. The more continuous exposure of far-from-channel deposits led to pedogenesis and phreatic oscillation, both in sediments and in the
ACCEPTED MANUSCRIPT contained fossils. Following this rationale, Spondylosoma and Staurikosaurus are badly preserved (partially disarticulated, high carbonate substitution and incrustation) were recovered from distal floodplain deposits, while the well preserved Saturnalia and Pampadromaeus (less disarticulation, only carbonate substitution) seem to be closer to the channel, in proximal floodplain deposits. The medium to fine sandstones of the Caturrita Formation are related to a high energy fluvial system (Fig. 6B). Recorded facies are the mobile, braided channels, crevasse splays and very rare
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floodplain deposits. It is more common to find the floodplain deposits as mud or carbonate clasts in the intraformational conglomerates. Unaysaurus is well preserved (little disarticulation and carbonate substitution) and it was found in massive, medium sandstones just above a 50 cm thick
intraformational conglomerate, just at the limit of channel-crevassing facies. Sacisaurus, and
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Guaibasaurus were recorded in massive, fine sandstones of crevasse splays, and are well preserved fossils as well (moderate and little disarticulation, respectively, and only carbonate substitution). Insert Figure 6 here
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There is a clear link among the preservation type of fossil vertebrates, the effect of pedogenesis and phreatic oscillation and the faciological location (Da Rosa, 2005, 2011; Fig. 7). Far from the channel facies, such as the distal floodplain, lacustrine and aeolian deposits are only reached by a great, millenary inundation. Carcasses are mainly scattered and trampled before burial, which is slow, leading to a very poor preservation, due to incremental carbonate deposition. On the other hand, near channel facies, such as the channel itself, or the levee, crevasse and proximal floodplain deposits
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are frequently flooded, leading to the erosion and resedimentation of previous deposits and skeletons. Thus, near-channel fossils are frequently fragmented or disarticulated, rapidly buried, and only slightly carbonated, forming weakly developed calcretes (Horn et al., 2013). Insert Figure 7 here
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The preservational type of the Triassic dinosauriforms of southern Brazil ranges from extremes, being a good preservation the most common type. Spondylosoma is the most fragmented and carbonated, Staurikosaurus is partly articulated, but carbonated, Saturnalia, Unaysaurus,
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Pampadromaeus are little carbonated, but fragmented or disarticulated, and finally, Guaibasaurus is almost complete, articulated and well preserved. A recent sauropodomorph discovery recorded three articulated and one disarticulated specimens, including complete skulls and skeletons. After the recognition of syn- and post-depositional features over the Southern Brazilian
Triassic, it is possible to evaluate its sedimentary dynamics, and the role of autocyclic and allocyclic mechanisms, responsible for the deposition and organization of those deposits.
7. A brief discussion on autocyclic/allocyclic mechanisms Sequence stratigraphy was developed in the late 1970’s to cope with the shallow marine to deltaic interval of the sedimentary system (van Strien, s.d.), but applying shoreline sequence stratigraphic concepts to fluvial systems puts the wrong emphasis on controlling processes (Willis et
ACCEPTED MANUSCRIPT al., 2010). Moreover, extending sequence stratigraphic concepts into fluvial strata is problematic because key criteria such as geometric observations (e.g., coastal onlap, shoreline trajectory) and surfaces (e.g., marine flooding surfaces) are not present in nor easily inferred from fluvial deposits (Cleveland et al., 2012). Shanley and McCabe (1994) related the incision of valleys, terrace formation, amalgamated sand bodies deposition, and fine-grained dominance to increasing and decreasing base level in alluvial
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dynamics, in a clear correlation to retreating, rising and lowering shoreface deposits. Wright and
Marriot (1993) recognized the crescent soil maturity in alluvial soils to lowstand, transgressive and highstand system tracts. However, these models were always linked to coastal and marine
sedimentation, in passive margins or rift basins. None of these models served for extensional
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intracontinental basins, far from the oceanic margins, in which the deposition space (accommodation) is created by other mechanisms.
Fluvial strata deposited during periods of increasing accommodation are composed of stacked
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autogenic cycles showing a vertical decrease in exposure time (i.e., decreasing paleosol maturity). Conversely, fluvial strata deposited during periods of decreasing accommodation are composed of stacked autogenic cycles showing a vertical increase in exposure time (i.e., increasing paleosol maturity; Cleveland et al., 2012). In the southern Brazilian Triassic, deposition showed a vertical decrease of the exposure time, from the SM1 to the SM2 sequences of Zerfass et al. (2003), i.e., a decreasing accommodation. It is possible to allocate the vertebrate assemblage zones (AZ’s) to the
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stacked autogenic cycles of Cleveland et al. (2012), as the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s occur in recurrent facies and architectural elements. The sedimentary deposits of the Dinodontosaurus AZ are composed by fluvial channels at the São Pedro do Sul and Candelária blocks, overlain by floodplain and lacustrine deposits, with intense
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carbonate precipitation (Da Rosa, 2005). At the Santacruzodon AZ, the fluvial channel is restricted to the area of Vera Cruz, Santa Cruz do Sul and Venâncio Aires blocks, with the same channel-tofloodplain deposition. The sedimentary deposits of the Hyperodapedon AZ are mainly constituted by
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fine-grained floodplain deposits, with minor channels, widespread over the São Pedro do Sul, Santa Maria, São João do Polêsine, Agudo, Candelária and Bom Retiro do Sul blocks. The Riograndia AZ was only recorded at braided channels at the Faxinal do Soturno and Candelária blocks. It seems that the fluvial channels change laterally, from the Dinodontosaurus AZ to the Santacruzodon AZ, and to the Hyperodapedon AZ (Fig. 8), forming cycles of fluvial deposition. In each AZ, avulsion may be the main mechanism of deposition, but the lateral change over the time (from AZ to AZ) is probably due to extension and modification of the basin floor. As the present outcrop belt simulates a strike section, the difference from time to time may be related to tectonism. However, the sedimentary package is not compatible with a rift system, as idealized by Zerfass et al. (2003, 2004). Insert Figure 8 here
ACCEPTED MANUSCRIPT The basal channels of the Dinodontosaurus AZ at the São Pedro do Sul and Santa Cruz do Sul blocks may be coeval, but earlier than the ones of Santacruzodon AZ at the Venâncio Aires block. The Passo do Sobrado lineament is the western limit of the Venâncio Aires block, but probably reactivated, as the channels from this block are formed by more pointed clasts than the ones at the base of the Dinodontosaurus AZ. On the other hand, channels at the base of the Hyperodapedon AZ at the Santa Maria block show a fining upward pattern, with well-rounded conglomerates at the base, shifting to
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intraformational conglomerates and fine sandstones, suggesting that the conditions for deposition (tectonism? climate?) attenuated, generating more accommodation space at this AZ.
8. Conclusions
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The Southern Brazilian Triassic fauna is subdivided into four assemblage zones (AZ’s), recorded in alluvial (channel and floodplain) deposits, split into three third-order sequences that
comprise the Santa Maria Supersequence. These deposits record dinosauriforms in three of these
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AZ’s, mostly in near-channel environments (channel deposits, crevasse splays, distal floodplains), and so with different types of preservation (mostly partly articulated, with little carbonate deposition). There is faciological homogeneity of the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s, while the change in fluvial style is recorded only at the Riograndia AZ (contra Zerfass et al., 2003). The sequence stratigraphic scheme for these deposits must be therefore revised, including the recognition of post-depositional tectonism, in order to better understand the autogenic
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and allogenic mechanisms of deposition. It is suggested here that there is a lateral change on main channel deposition, with the areal restriction of the Santacruzodon AZ probably linked to reactivation on the Passo do Sobrado lineament and modification of the basin floor.
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Acknowledgements
This work is part of my doctorate thesis, partially funded by a CAPES- PICDT-UFSM fellowship (Programa Institucional de Capacitação de Docentes e Técnicos, from the Coordenação de
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Aperfeiçoamento de Pessoal de Nível Superior), supervised by Ubiratan F. Faccini (Universidade do Vale do Rio dos Sinos) and Nuno Valente Lamas Pimentel (Universidade de Lisboa). André Weissheimer Borba made useful comments on an earlier version of this manuscript.
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S. J., Desojo, J. B. and Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications v.379. Leal, L. A.; Azevedo, S.A.K.; Da Rosa, Á.A.S. 2002. Icnofósseis do Sítio Fossilífero de Água Negra, Formação Caturrita, Neotriássico do Sul do Brasil. Arquivos do Museu Nacional, Rio de Janeiro, RJ, Brasil, v. 60, n.3, p. 217-222.
Leal, L. A.; Da Rosa, A. A. S.; Boelter, R. A. 2005. Tiny archosauromorphs from the Late Triassic of southern Brazil. In: II Congresso Latino-Americano de Paleontologia de Vertebrados, 2005, Rio de Janeiro. Boletim de Resumos, p. 149-150. Leal, L.A.; Azevedo, S.A.K.; Kellner, A.W.A.; Da Rosa, A.A.S. 2004. A new early dinosaur (Sauropodomorpha) from the Caturrita Formation (Late Triassic), Parana Basin, Brazil. Zootaxa 690, 1-24.
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Soares, M. B.; Schultz, C. L.; Horn, B. L. D. 2011. New information on Riograndia guaibensis Bonaparte, Ferigolo and Ribeiro, 2001 (Eucynodontia, Tritheledontidae) from the Late Triassic of southern Brazil: anatomical and biostratigraphic implications. Anais da Academia Brasileira de Ciências 83(1), pp. 329-354.
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Temp-Müller R.; Aires A. S. S.; Silva, L. R.; Pereira, A. O.; Cure M. B.; Pacheco C. P.; Rangel A. O.; Oliveira, T. M.; Ziemann, D. R.; Da Rosa, A. A. S.; Dias-da-Silva, S. 2013a. New record of sauropodomorphs (Dinosauria: Saurischia) from the Upper Triassic of Southern Brazil: The most complete Brazilian dinosaurs. In: 1st Brazilian Dinosaur Symposium, 2013, Ituiutaba. Paleontologia em Destaque (Rio de Janeiro), 2013. p. 46.
Temp-Müller R.; Langer, M. C.; Aires A. S. S.; Da Rosa, A. A. S.; Dias-da-Silva, S. 2013b. Morphological differences between Unaysaurus tolentinoi and a new specimen of Saurischian Dinosaur from Caturrita
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Formation. In: XXIII Congresso Brasileiro de Paleontologia, 2013, Gramado. Paleontologia em Destaque: Boletim Informativo da Sociedade Brasileira de Paleontologia, 2013. v. 1. p. 252. Tucker, M.E.; Benton, M.J. 1982. Triassic environments, climates and reptile evolution. Palæogeography, Palæoclimatology, Palæoecology, Amsterdam, 40:361-379. Van Strien, W.J. s.d. Fluvial sequence stratigraphy. Essay posted March 24th, 2011, at
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Willis, B.; Bracken, B.; Payenberg, T. 2010. Another Look at Fluvial Sequence Stratigraphy. Search and Discovery Article #40624 (2010), posted October 29, 2010, accessed March 29th, 2013, at http://www.searchanddiscovery.com/documents/2010/40624willis/ndx_willis.pdf Zerfass, H. 2008. Geologia da Folha Agudo, SH.22-V-C-V. CPRM - Serviço Geológico do Brasil, escala 1:100.000. Zerfass, H.; Lavina, E. L.; Schultz, C. L.; Garcia, A. G. V.; Faccini, U. F.; Chemale Jr, F. 2003. Sequence stratigraphy of continental Triassic strata of southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and palaeoclimate. Sedimentary Geology 161, 85-105.
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Figure and tables captions
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Table 1. Dinosauriforms from Argentina and Brazil related to the Brazilian Triassic assemblage zones.
Figure 1. Location of the Triassic of Southern Brazil, in the context of Paraná Basin (A), a generalized geological map from the sedimentary rocks at the Rio Grande do Sul state (B), and the location of dinosauriform-bearing sites (stars) along the structural blocks from the Santa Maria Supersequence (C). Note the location of Figure 2 in C. Yellow = sandy channels from Passo das Tropas Member;
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magenta = fine-grained floodplain deposits from the Alemoa Member; light orange = sandy channels from the Caturrita Formation. Structural blocks: Mt = Mata, SPS = São Pedro do Sul, SM = Santa
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Maria, SJP = São João do Polêsine, FxS = Faxinal do Soturno, Ag = Agudo, PS = Paraíso do Sul, C = Candelária, VS = Vale do Sol, VC = Vera Cruz, SCS = Santa Cruz do Sul, VA = Venâncio Aires, BRS = Bom Retiro do Sul.
Figure 2. Recognition of post-depositional organization in part of the Triassic of Southern Brazil (modified from Da Rosa and Faccini, 2005). RADAR image, showing the fractures, faults and
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structural lineaments on the Cretaceous volcanic rocks from the Serra Geral Formation (A), the recognition of structural blocks and their relation to each other (B), and a more detailed correlation in the urban area of Santa Maria, with the recognition of structural sub-blocks (C). In B, A = up, B =
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down.
Figure 3. Structural and faciological correlation of the Km 3 sub-block at the urban area of Santa Maria. (A) Location of studied outcrops (stars for dinosauriform-bearing sites); (B) Columnar sections
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of each outcrop, and lateral correlation of facies and type of preservation (almost white circles = good preservation; almost black circles = bad preservation); (C) Architectural elements present at the (Cerro da) Alemoa outcrop, and location of dinosauriform findings.
Figure 4. Structural and faciological correlation of the sub-blocks at the urban area of Santa Maria. (A) Composite columnar section for each sub-block, depicting the presence of architectural elements, fossil type, and type of preservation; (B) Cartoon showing small scale channels within the Alemoa Member, in at least three moments (parasequences).
ACCEPTED MANUSCRIPT Figure 5. Structural and faciological correlation of outcrops at the urban area of Agudo. (A) Location of studied outcrops (stars for dinosauriform-bearing sites); (B) Columnar sections of each outcrop; (C) Architectural elements present at the Janner outcrop, and location of dinosauriform findings.
Figure 6. Paleoenvironmental reconstructions for the Middle (A) and Upper Triassic (B). Fixed, low energy anastomosed channels (Passo das Tropas Member), with levees, crevasses and floodplain
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deposits (Alemoa Member) of the Santa Maria Formation (A). Mobile, braided channels and crevasse deposits of the Caturrita Formation (B). Modified from Eberth and Miall (1991).
Figure 7. Schematic model of fossil vertebrate preservation type along the floodplain deposits of the
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southern Brazilian Triassic (modified from Da Rosa, 2005). Preservation types become progressively worse with the distance from the main channels, exposure of the floodplain, and the effect of
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pedogenesis and phreatic oscillation.
Figure 8. Tentative chronostratigraphic diagram for the Triassic of southern Brazil (modified from Faccini (2000). Ages according to Gradstein et al. (2012). Structural blocks: Mt = Mata, SPS = São Pedro do Sul, SM = Santa Maria, SJP = São João do Polêsine, FxS = Faxinal do Soturno, Ag = Agudo, PS = Paraíso do Sul, C = Candelária, VS = Vale do Sol, VC = Vera Cruz, SCS = Santa Cruz do Sul,
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VA = Venâncio Aires, BRS = Bom Retiro do Sul.
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The contribution places all known Brazilian Triassic dinosaurian forms into a geological context. There is a discussion on the type of preservation, faciological
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subdivision and stratigrahic and tectonic implications.
S0895-9811(14)00145-X
DOI:
10.1016/j.jsames.2014.10.008
Reference:
SAMES 1340
To appear in:
Journal of South American Earth Sciences
Received Date: 25 April 2014 Revised Date:
19 October 2014
Accepted Date: 24 October 2014
Please cite this article as: Da-Rosa, Á.A.S., Geological context of the Dinosauriform-Bearing outcrops from The Triassic of Southern Brazil, Journal of South American Earth Sciences (2014), doi: 10.1016/ j.jsames.2014.10.008. 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 GEOLOGICAL CONTEXT OF THE DINOSAURIFORM-BEARING OUTCROPS FROM THE TRIASSIC OF SOUTHERN BRAZIL
Átila A.S. Da-Rosa Laboratório de Estratigrafia e Paleobiologia, Departamento de Geociências, Universidade Federal de Santa Maria, Avenida Roraima, 1000, Prédio 17, Sala 1131B, 97.105-900, Santa Maria, RS, Brasil.
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[email protected]
Abstract
The Triassic of the western Gondwana (southern Brazil and northwestern Argentina) records the
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oldest dinosaurs. The Southern Brazilian Triassic fauna is subdivided into four assemblage zones (AZ’s), recorded in alluvial (channel and floodplain) deposits, split into three third-order sequences that comprise the Santa Maria Supersequence. These deposits record dinosauriforms in three of these
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AZ’s, mostly in near-channel environments (channel deposits, crevasse splays, distal floodplains) with different types of preservation (mostly partly articulated, with little carbonate deposition). There is faciological homogeneity within the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s, whereas change in fluvial style is recorded at the Riograndia AZ. So, further stratigraphic studies must include the recognition of post-depositional tectonism, in order to better understand the autogenic and allogenic mechanisms of deposition. It is suggested here that there is a lateral change on main channel
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deposition, with the areal restriction of the Santacruzodon AZ probably linked to reactivation on the Passo do Sobrado lineament and modification of the basin floor.
1. Introduction
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Keywords: Triassic, Brazil, Santa Maria Supersequence, Santa Maria Formation, Caturrita Formation.
The Triassic of southern Brazil and Argentina are the main sources of information on the
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origin and early irradiation of dinosaurs (e.g., Tucker and Benton, 1982; Langer et al., 2007, 2010; Novas, 2009; Brusatte et al., 2010; Benton et al., 2014). Although there is some dispute over where the most primitive dinosaur came from, it is mostly agreed that the group irradiated from the western Gondwana (Langer et al., 2011). Dinosaurian forms from the Brazilian Triassic include basal Saurischia, with sauropodomorph
(Unaysaurus tolentinoi Leal et al., 2004, Guaibasaurus candelariensis Bonaparte et al., 1999) and theropod (Staurikosaurus pricei Colbert, 1970, Pampadromaeus barberenai Cabreira et al., 2011) affinities, as well as non-dinosaurian dinosauriforms (Sacisaurus agudoensis Ferigolo and Langer, 2006). Fragmentary sauropodomorphs also were also recently described (Leal et al., 2005; Da Rosa et al., 2006; Dambros et al., 2007; Bittencourt et al., 2012; Temp-Müller et al., 2013a,b). The outcrops
ACCEPTED MANUSCRIPT where these fossils came from were re-studied and new information on the life/death processes are now available. In the last 20 years, the knowledge of the Triassic of southern Brazil has been greatly improved by fossiliferous discoveries and new methods of correlation and study. These new findings modified the understanding on the existing faunas and its relation to the paleoenvironments. In this
southern Brazil will be presented and discussed (Fig. 1). Insert Figure 1 here 2. Material and Methods
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paper, some of the geological information about the dinosauriform-bearing sites from the Triassic of
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The most recent faunal list of the Triassic of southern Brazil is given in Langer et al. (2007a) and reviewed by Soares et al. (2011), where four assemblage zones can be recognized for the Middle to Upper Triassic Santa Maria Supersequence (Santa Maria and Caturrita formations):
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– Dinodontosaurus Assemblage Zone (AZ), constituted by dicynodonts, cynodonts, archosaurs and procolophonoids, considered to be early Ladinian;
– Santacruzodon AZ, bearing traversodontid cynodonts and archosaurs, dated as late Ladinian – early Carnian;
– Hyperodapedon AZ, with rhynchosaurs, cynodonts, dinosaurs, archosaurs of Carnian age; – Riograndia AZ, composed by cynodonts, procolophonids, sphenodonts, dicynodonts, archosaurs nd
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dinosaurs, considered as early Norian.
Dinosauriform-bearing sites were laterally and vertically correlated by facies analysis, after the recognition of a post-depositional structural organization, as depicted by Da Rosa and Faccini (2005). The dinosauriforms discussed here are (Tab. 1):
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– Spondylosoma absconditum, recovered by Friedrich von Huene in 1929 at the district of Chiniquá, São Pedro do Sul, is a highly debated taxon (Galton, 2000), but presented here due to its biostratigraphic implications, as it comes from the same age from Argentinean dinosauromorphs.
AZ;
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Based on the faunal content, the Chiniquá sedimentary deposits were related to the Dinodontosaurus
– Staurikosaurus pricei, collected in 1935 by a joint USA-Brazil expedition (Colbert, 1970; Bittencourt and Kellner, 2009). This fossil came from von Huene’s Jazigo 1, today completely covered by the urban growth of the city of Santa Maria, but referable to the Hyperodapedon AZ; – Saturnalia tupiniquim, collected in 1998 at the Alemoa site, 100 m SE from “Jazigo 1”, and also from the Hyperodapedon AZ (Langer et al., 1999). A tiny sauropodomorph (Leal et al., 2005) and a more robust one (Da Rosa et al., 2006; Dambros et al., 2007) were also collected; – Recently dinosaurian collected material from the Linha Facão, Candelária, referable to the Hyperodapedon AZ;
ACCEPTED MANUSCRIPT – Unaysaurus tolentinoi, collected in 1998 at the limit of the São Martinho da Serra and Santa Maria municipalities (Leal et al., 2004). Although no other vertebrates were found, gnaw or scratch marks attributed to scavenging cynodonts (Leal et al., 2002) related this fossil to the Riograndia AZ. This is also true for other sauropodomorphs found in an outcrop few kilometers apart (Bittencourt et al., 2012); – Guaibasaurus candelariensis, originally collected at Candelária and later at Faxinal do Soturno
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(Bonaparte et al., 1999, 2007; Langer et al., 2011). The cynodonts on these outcrops are the biostratigraphic basis for the definition of Riograndia AZ (Soares et al., 2011);
– Pampadromaeus barberenai, collected at the Janner outcrop, southwest of the urban area of Agudo, related to the Hyperodapedon AZ, in the Exaeretodon acme zone (Langer et al., 2011);
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– Recently collected sauropodomorph material from the Linha Flores, housed at the UNIPAMPA collection (Universidade Federal do Pampa – Campus São Gabriel), in levels preliminarily related to the Riograndia AZ (Temp-Müller et al., 2013a,b);
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– Recently described sauropodomorph material from Cerro Botucaraí (or Sesmaria do Pinhal 1), at Candelária, in levels of the Riograndia AZ (Bittencourt et al., 2012);
– Sacisaurus agudoensis, collected at the urban area of Agudo, today completely covered by the urban growth of the city (Ferigolo and Langer, 2006), so it is certain to which AZ this outcrop can be related, the Hyperodapedon or Riograndia AZ.
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Insert Table 1 here
3. A brief overview of the Triassic of the Paraná Basin The first geological sketch of the Paraná Basin was erected by White (1908), in the coalfields of southern Brazil, but only at the 1930’s, the Triassic beds of the Rio Grande do Sul state were
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recognized by Huene and Stahlecker (1931). The evolution of the stratigraphic work on the Triassic of southern Brazil was summarized by Faccini (2000) and presented here as part of the figure 8. Bortoluzzi (1974) recognized the Santa Maria beds, with fossil leave imprints of the
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Dicroidium Flora, which Andreis et al. (1980) denominated as the Rosario do Sul Group and subdivided into the Sanga do Cabral Formation (Early Triassic), the Santa Maria Formation (with the Alemoa and Passo das Tropas Members, Middle to Upper Triassic) and the Caturrita Formation (Upper Triassic).
Milani et al. (1998) studied the complete sedimentary package of the Paraná Basin,
subdividing it into six second-order supersequences: Rio Ivaí, Paraná, Gondwana I, Gondwana II, Gondwana III, and Bauru. Scherer et al. (2000) included the southern Brazilian deposits into the Permo-Carboniferous, Triassic and Neocomian sequences. Later on, Zerfass et al. (2003) named the Sanga do Cabral and Santa Maria second-order supersequences for the southern Brazilian Triassic. Barberena (1977) provided the first biostratigraphic subdivisions for the Triassic of southern Brazil, later improved by Schultz et al (2000). The most complete faunal list is given in Langer et al
ACCEPTED MANUSCRIPT (2007) and Schultz and Langer (2007), and the biostratigraphic zonation herein presented follows Soares et al. (2011). The basic geological mapping of the Triassic of southern Brazil was historically done in a regional scale, with no details on the faciological and structural compartimentation of the sedimentary deposits. For example, Carraro et al. (1974) recognized an undivided Triassic, mapping produced by CPRM-DNPM (1982) improved the boundaries, but maintaining the undivided fashion. Only recently,
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Wildner et al. (2008) provided a more precise map of the southern Brazilian Triassic sedimentary deposits, including the presence and recognition of structural limits. Zerfass (2008) was the first to
in a 1:100.000 scale.
4. Post-depositional structural organization
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present a more detailed mapping for the Triassic, also presenting a faciological scheme, although still
Da Rosa and Faccini (2005) studied part of the Triassic section in southern Brazil, recognizing
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that NW and NE lineaments, clearly visible on the Cretaceous volcanic rocks of the Serra Geral Formation on the top of the hills and uplands, are in fact the result of a post-depositional tectonism, also affecting older units. Fault vertical displacements of nearly 100 m were recorded, allowing to recognize structural blocks, such as São Pedro do Sul, Santa Maria, São João do Polêsine, Faxinal do Soturno, Agudo, Paraíso do Sul and Candelária (Fig. 2A,B). Where more detailed geological information is available, such as subsurface drill cores or logs, the integration of surface and
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subsurface geological mapping allows to recognize minor displacements, and the clear correlation along a restrict area, defining structural sub-blocks. For instance, five sub-blocks were recognized at the urban area of Santa Maria (Fig. 2C): Cabeceira do Raimundo, Tancredo Neves, Cidade, Km 3 and Camobi.
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Insert Figure 2 here
In this paper a more complete structural organization of the post-depositional tectonism on the Triassic of southern Brazil is presented (Fig. 1). Apart of the blocks already mentioned, the following
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can be recognized:
- Mata, delineated at west by the Jaguari-Mata Fault Zone, recognized by Faccini (2000) as the limit of the Middle-Upper Triassic outcrop belt; - Vale do Sol, east of Candelária, defined by the Rio Pardo and Plumbs lineaments (west and east, respectively);
- Vera Cruz, limited by the Plumbs and Rio Pardinho lineaments (west and east, respectively); - Santa Cruz do Sul, defined by the Rio Pardinho and Passo do Sobrado lineaments (west and east, respectively); - Venâncio Aires, delineated by the Passo do Sobrado and Cruzeiro do Sul lineaments (west and east, respectively);
ACCEPTED MANUSCRIPT - Bom Retiro do Sul, defined at east by the Taquari Fault Zone sensu Faccini (2000) as the eastern limit of the sedimentary package. The recognition of a post-depositional tectonism is essential in further stratigraphic studies, showing the need for a previous understanding of block and sub-block relationships. For example, the first biostratigraphic studies realized that fossil faunas were diachronic (Barberena, 1977), but no detailed mapping could help in solving the lateral and vertical correlations. Also, recently tectonic
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models for the southern Brazilian Triassic neglected the role of a NW and NE oriented faults, resulting from a post-depositional extensional tectonism in placing side by side outcrops of different ages
(Zerfass et al., 2004). On the other hand, NE features such as the Passo do Sobrado lineament, have been reactivated since the Permian (Galli and Kern, 1998; Küchle et al., 2003). Hence, pointing out
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the difference between syn- and post-depositional tectonism is only possible after the identification and mapping of the structural blocks at the southern Brazilian Triassic.
The current tectonic model should be revised at the light of the ideas presented above. Zerfass
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et al. (2004) suggested that the Santa Maria Supersequence was deposited in an extensional tectonism, related to a rift system bounded by E-W faults. Nevertheless, there is no record of proximal facies, such as alluvial fans, or a Triassic volcanism compatible to a rift phase, for example. However, the most E-W striking features present at the central region can be related to the opening of the Atlantic Ocean and the injection of igneous rocks at transforming faults at the Jurassic-Cretaceous (Fernandes
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et al., 1995a,b).
5. Lateral and vertical correlation of outcrops
The stratigraphic correlation of the Triassic of Southern Brazil should be carried out only after a post-depositional structural study, as pointed out by Da Rosa and Faccini (2005). Here I present
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some lateral and vertical correlation of dinosauriform-bearing outcrops, under the scope of postdepositional structural blocks and sub-blocks. At the urban area of Santa Maria, a set of structural sub-blocks can be recognized (Da Rosa
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and Faccini, 2005), in which a certain stratigraphic homogeneity allows to build local composite columnar sections. For each sub-block, a sedimentary log was created and laterally correlated, based on the faciological associations (Fig. 3). The sandy channels of the Passo das Tropas Member are coarser and thicker on the Cidade Sub-Block, and constitute the type section of this unit, at the surroundings of the Passo das Tropas creek. Laterally, this unit switches into finer sandy bodies. The overlying unit is the Alemoa Member, composed by massive reddish mudstones, and its intercalation with fine sandstones. The presence of carbonatic concretions is the most impressive characteristic of this unit, sometimes forming carbonatic crusts of little lateral expression, together with other pedogenetic and phreatic oscillation features (Da Rosa et al., 2004). Along the 30 – 40 m of this unit, there is a continuum of fine-grained deposits, with only rare minor channels, with a few meters of width and thickness. As expected, these channels grade laterally into floodplain deposits, with
ACCEPTED MANUSCRIPT increasing degrees of pedogenesis and phreatic oscillation, as well as changing the preservation types of the fossil vertebrates. At least three vertical sets of channel-floodplain packages can be observed (parasequences?), in which the lateral shift from the minor channels is the most striking feature. The channels laterally change their position in an avulsion-like manner. Little pedogenetic and phreatic alterations are consistent with the idea of a steady floodplain. The overlying Caturrita Formation completely changes the fluvial style, switching it into high energy, low sinuosity channels.
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Insert Figure 3 here
At the Santa Maria Block, Staurikosaurus and Saturnalia came from the Km 3 Sub-block, from the region called Alemoa or Cerro da Alemoa (Fig. 4). A N-S correlation among selected
outcrops from this sub–block led to the recognition of the high energy channels of the Caturrita
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Formation laterally shifting into the floodplain deposits of the Alemoa Member. In this correlation, proximal floodplain deposits are recorded above distal ones, suggesting the approximation of the river channels. The preservation types of the fossil vertebrates changes, according to the facies (Da Rosa,
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2012): fossil in distal floodplains are mostly poorly preserved (complete and fragmentary skeletons, highly carbonated), whereas those in proximal floodplains have a better preservation (little carbonate substitution, but fragmentary skeletons). Staurikosaurus preserved a nearly complete vertebral series (cervical, dorsal, sacral and caudal), with the articulated pelvis and hind limb, highly carbonated and somehow diagenetically deformed (Bittencourt and Kellner, 2009). On the other hand, Saturnalia is composed of three partially articulated, well preserved specimens (Langer et al., 1999, 2007; Langer,
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2003). The tiny sauropodomorphs are also well preserved (Leal et al., 2005), but the more robust ones seem that went through some post-depositional deformation (Da Rosa et al., 2006; Dambros et al., 2007). Although Staurikosaurus was not recovered from the Alemoa site, its stratigraphic position and preservation type can be correlated to levels of the basal portion of that site.
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Insert Figure 4 here
At the Agudo structural block, three dinosauriforms were recorded: Pampadromaeus, Sacisaurus and a group of sauropodomorphs under study. The first came from Janner outcrop, which
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also recorded cynodonts like Exaeretodon and Trucidocynodon, placing it into the upper part of the Hyperodapedon AZ (Cabreira et al., 2011). In this outcrop, it is very clear the upwards faciological change, from a distal to proximal floodplain, and then unconfined channels. The Sacisaurus site is at the city of Agudo, unfortunately destroyed by the urban growth (Ferigolo and Langer, 2006). Geological information available from previous field work (Da Rosa, 2005), as well as from outcrops nearby suggest that the fossils (a series of fragmentary skeletons) came from metric sandy depositional lobes, intercalated with milimetric mudstones, characteristic of crevasse splays. There are no clear signs of pedogenic or phreatic activity at this outcrop. Finally, the Linha das Flores outcrop, currently under study (Temp-Müller et al., 2013a,b), shows channel and crevasse architectural elements and at least three almost complete sauropodomorphs, well preserved (little carbonate substitution) recovered from the basal channel.
ACCEPTED MANUSCRIPT Insert Figure 5 here Guaibasaurus candelariensis was found in two outcrops, from different structural blocks: Sesmaria do Pinhal 2 outcrop, at the Candelária block (Bonaparte et al., 1999), in crevasse channels related to the Riograndia AZ (Bittencourt et al., 2012) and at the Faxinal do Soturno block, at the base of the Linha São Luiz outcrop, also related to crevasse channels of the Riograndia AZ (Bonaparte et al., 2007; Langer et al., 2011).
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Spondylosoma absconditum was found at the Sanga da Árvore outcrop (Huene, 1942), one of the many erosive gullies of the district of Chiniquá, São Pedro do Sul block. It is not clear from which level, so there is no information about its provenance or sedimentary environment. It is poorly
preserved, fragmented and carbonated. The coexistence with dicynodonts places this fossil into the
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Dinodontosaurus AZ (Langer et al., 2007).
6. Reconstruction of the southern Brazilian Triassic floodplains
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The studied dinosauriform-bearing outcrops are part of the Santa Maria Supersequence, which encompasses three third-order sequences, and a change on the fluvial style (Zerfass et al., 2003). The Santa Maria 1 (SM1) sequence is composed by the Dinodontosaurus and Santacruzodon AZ’s, preserved in a low energy fluvial system, with distal and proximal floodplain environments, as well as levees, crevasses and low to middle sinuosity channels. The Santa Maria 2 (SM2) sequence encompasses the Hyperodapedon and Riograndia AZ’s, the first one mostly composed by floodplain
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deposits (distal and proximal, levees, crevasses, minor channels), and the second composed mainly by a channel-crevasse system, in a high energy fluvial system. The Santa Maria 3 (SM3) sequence has not yielded vertebrate fossils yet, and is composed by high energy channels. The passage from the Middle to the Upper Triassic witnessed two different fluvial systems at
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Paraná Basin. This environmental change does not clearly correlate with the third-order sequences of Zerfass et al. (2003). Three AZ’s were recorded on channel-floodplain systems, corresponding to SM1 and part of the SM2. A change of fluvial style is only recorded at the top of SM2. This was mentioned
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by Holz and Scherer (2000), which correlated this faciological shift to an increase in the humidity towards the end of the Triassic, linked to Pangea breakup and formation of the equatorial Atlantic Ocean. This data is corroborated by Pierini et al. (2002) that recorded a geochemical trend in some outcrops near Santa Maria, also linking it to an increase of humidity, at the passage from Hyperodapedon AZ to the Riograndia AZ. Two faciological associations can be envisaged for the southern Brazilian Triassic (Fig. 6). The sandstones (Passo das Tropas Member) and mudstones (Alemoa Member) of the Santa Maria Formation are characteristic of a low energy fluvial system (Fig. 6A), in which main and secondary channels are recorded, as well as the aggradation of the floodplains, by the fast deposition of crevasse splays, or the slower sedimentation through subsequent inundations. The more continuous exposure of far-from-channel deposits led to pedogenesis and phreatic oscillation, both in sediments and in the
ACCEPTED MANUSCRIPT contained fossils. Following this rationale, Spondylosoma and Staurikosaurus are badly preserved (partially disarticulated, high carbonate substitution and incrustation) were recovered from distal floodplain deposits, while the well preserved Saturnalia and Pampadromaeus (less disarticulation, only carbonate substitution) seem to be closer to the channel, in proximal floodplain deposits. The medium to fine sandstones of the Caturrita Formation are related to a high energy fluvial system (Fig. 6B). Recorded facies are the mobile, braided channels, crevasse splays and very rare
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floodplain deposits. It is more common to find the floodplain deposits as mud or carbonate clasts in the intraformational conglomerates. Unaysaurus is well preserved (little disarticulation and carbonate substitution) and it was found in massive, medium sandstones just above a 50 cm thick
intraformational conglomerate, just at the limit of channel-crevassing facies. Sacisaurus, and
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Guaibasaurus were recorded in massive, fine sandstones of crevasse splays, and are well preserved fossils as well (moderate and little disarticulation, respectively, and only carbonate substitution). Insert Figure 6 here
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There is a clear link among the preservation type of fossil vertebrates, the effect of pedogenesis and phreatic oscillation and the faciological location (Da Rosa, 2005, 2011; Fig. 7). Far from the channel facies, such as the distal floodplain, lacustrine and aeolian deposits are only reached by a great, millenary inundation. Carcasses are mainly scattered and trampled before burial, which is slow, leading to a very poor preservation, due to incremental carbonate deposition. On the other hand, near channel facies, such as the channel itself, or the levee, crevasse and proximal floodplain deposits
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are frequently flooded, leading to the erosion and resedimentation of previous deposits and skeletons. Thus, near-channel fossils are frequently fragmented or disarticulated, rapidly buried, and only slightly carbonated, forming weakly developed calcretes (Horn et al., 2013). Insert Figure 7 here
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The preservational type of the Triassic dinosauriforms of southern Brazil ranges from extremes, being a good preservation the most common type. Spondylosoma is the most fragmented and carbonated, Staurikosaurus is partly articulated, but carbonated, Saturnalia, Unaysaurus,
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Pampadromaeus are little carbonated, but fragmented or disarticulated, and finally, Guaibasaurus is almost complete, articulated and well preserved. A recent sauropodomorph discovery recorded three articulated and one disarticulated specimens, including complete skulls and skeletons. After the recognition of syn- and post-depositional features over the Southern Brazilian
Triassic, it is possible to evaluate its sedimentary dynamics, and the role of autocyclic and allocyclic mechanisms, responsible for the deposition and organization of those deposits.
7. A brief discussion on autocyclic/allocyclic mechanisms Sequence stratigraphy was developed in the late 1970’s to cope with the shallow marine to deltaic interval of the sedimentary system (van Strien, s.d.), but applying shoreline sequence stratigraphic concepts to fluvial systems puts the wrong emphasis on controlling processes (Willis et
ACCEPTED MANUSCRIPT al., 2010). Moreover, extending sequence stratigraphic concepts into fluvial strata is problematic because key criteria such as geometric observations (e.g., coastal onlap, shoreline trajectory) and surfaces (e.g., marine flooding surfaces) are not present in nor easily inferred from fluvial deposits (Cleveland et al., 2012). Shanley and McCabe (1994) related the incision of valleys, terrace formation, amalgamated sand bodies deposition, and fine-grained dominance to increasing and decreasing base level in alluvial
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dynamics, in a clear correlation to retreating, rising and lowering shoreface deposits. Wright and
Marriot (1993) recognized the crescent soil maturity in alluvial soils to lowstand, transgressive and highstand system tracts. However, these models were always linked to coastal and marine
sedimentation, in passive margins or rift basins. None of these models served for extensional
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intracontinental basins, far from the oceanic margins, in which the deposition space (accommodation) is created by other mechanisms.
Fluvial strata deposited during periods of increasing accommodation are composed of stacked
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autogenic cycles showing a vertical decrease in exposure time (i.e., decreasing paleosol maturity). Conversely, fluvial strata deposited during periods of decreasing accommodation are composed of stacked autogenic cycles showing a vertical increase in exposure time (i.e., increasing paleosol maturity; Cleveland et al., 2012). In the southern Brazilian Triassic, deposition showed a vertical decrease of the exposure time, from the SM1 to the SM2 sequences of Zerfass et al. (2003), i.e., a decreasing accommodation. It is possible to allocate the vertebrate assemblage zones (AZ’s) to the
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stacked autogenic cycles of Cleveland et al. (2012), as the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s occur in recurrent facies and architectural elements. The sedimentary deposits of the Dinodontosaurus AZ are composed by fluvial channels at the São Pedro do Sul and Candelária blocks, overlain by floodplain and lacustrine deposits, with intense
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carbonate precipitation (Da Rosa, 2005). At the Santacruzodon AZ, the fluvial channel is restricted to the area of Vera Cruz, Santa Cruz do Sul and Venâncio Aires blocks, with the same channel-tofloodplain deposition. The sedimentary deposits of the Hyperodapedon AZ are mainly constituted by
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fine-grained floodplain deposits, with minor channels, widespread over the São Pedro do Sul, Santa Maria, São João do Polêsine, Agudo, Candelária and Bom Retiro do Sul blocks. The Riograndia AZ was only recorded at braided channels at the Faxinal do Soturno and Candelária blocks. It seems that the fluvial channels change laterally, from the Dinodontosaurus AZ to the Santacruzodon AZ, and to the Hyperodapedon AZ (Fig. 8), forming cycles of fluvial deposition. In each AZ, avulsion may be the main mechanism of deposition, but the lateral change over the time (from AZ to AZ) is probably due to extension and modification of the basin floor. As the present outcrop belt simulates a strike section, the difference from time to time may be related to tectonism. However, the sedimentary package is not compatible with a rift system, as idealized by Zerfass et al. (2003, 2004). Insert Figure 8 here
ACCEPTED MANUSCRIPT The basal channels of the Dinodontosaurus AZ at the São Pedro do Sul and Santa Cruz do Sul blocks may be coeval, but earlier than the ones of Santacruzodon AZ at the Venâncio Aires block. The Passo do Sobrado lineament is the western limit of the Venâncio Aires block, but probably reactivated, as the channels from this block are formed by more pointed clasts than the ones at the base of the Dinodontosaurus AZ. On the other hand, channels at the base of the Hyperodapedon AZ at the Santa Maria block show a fining upward pattern, with well-rounded conglomerates at the base, shifting to
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intraformational conglomerates and fine sandstones, suggesting that the conditions for deposition (tectonism? climate?) attenuated, generating more accommodation space at this AZ.
8. Conclusions
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The Southern Brazilian Triassic fauna is subdivided into four assemblage zones (AZ’s), recorded in alluvial (channel and floodplain) deposits, split into three third-order sequences that
comprise the Santa Maria Supersequence. These deposits record dinosauriforms in three of these
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AZ’s, mostly in near-channel environments (channel deposits, crevasse splays, distal floodplains), and so with different types of preservation (mostly partly articulated, with little carbonate deposition). There is faciological homogeneity of the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ’s, while the change in fluvial style is recorded only at the Riograndia AZ (contra Zerfass et al., 2003). The sequence stratigraphic scheme for these deposits must be therefore revised, including the recognition of post-depositional tectonism, in order to better understand the autogenic
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and allogenic mechanisms of deposition. It is suggested here that there is a lateral change on main channel deposition, with the areal restriction of the Santacruzodon AZ probably linked to reactivation on the Passo do Sobrado lineament and modification of the basin floor.
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Acknowledgements
This work is part of my doctorate thesis, partially funded by a CAPES- PICDT-UFSM fellowship (Programa Institucional de Capacitação de Docentes e Técnicos, from the Coordenação de
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Aperfeiçoamento de Pessoal de Nível Superior), supervised by Ubiratan F. Faccini (Universidade do Vale do Rio dos Sinos) and Nuno Valente Lamas Pimentel (Universidade de Lisboa). André Weissheimer Borba made useful comments on an earlier version of this manuscript.
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Figure and tables captions
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Table 1. Dinosauriforms from Argentina and Brazil related to the Brazilian Triassic assemblage zones.
Figure 1. Location of the Triassic of Southern Brazil, in the context of Paraná Basin (A), a generalized geological map from the sedimentary rocks at the Rio Grande do Sul state (B), and the location of dinosauriform-bearing sites (stars) along the structural blocks from the Santa Maria Supersequence (C). Note the location of Figure 2 in C. Yellow = sandy channels from Passo das Tropas Member;
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magenta = fine-grained floodplain deposits from the Alemoa Member; light orange = sandy channels from the Caturrita Formation. Structural blocks: Mt = Mata, SPS = São Pedro do Sul, SM = Santa
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Maria, SJP = São João do Polêsine, FxS = Faxinal do Soturno, Ag = Agudo, PS = Paraíso do Sul, C = Candelária, VS = Vale do Sol, VC = Vera Cruz, SCS = Santa Cruz do Sul, VA = Venâncio Aires, BRS = Bom Retiro do Sul.
Figure 2. Recognition of post-depositional organization in part of the Triassic of Southern Brazil (modified from Da Rosa and Faccini, 2005). RADAR image, showing the fractures, faults and
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structural lineaments on the Cretaceous volcanic rocks from the Serra Geral Formation (A), the recognition of structural blocks and their relation to each other (B), and a more detailed correlation in the urban area of Santa Maria, with the recognition of structural sub-blocks (C). In B, A = up, B =
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Figure 3. Structural and faciological correlation of the Km 3 sub-block at the urban area of Santa Maria. (A) Location of studied outcrops (stars for dinosauriform-bearing sites); (B) Columnar sections
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of each outcrop, and lateral correlation of facies and type of preservation (almost white circles = good preservation; almost black circles = bad preservation); (C) Architectural elements present at the (Cerro da) Alemoa outcrop, and location of dinosauriform findings.
Figure 4. Structural and faciological correlation of the sub-blocks at the urban area of Santa Maria. (A) Composite columnar section for each sub-block, depicting the presence of architectural elements, fossil type, and type of preservation; (B) Cartoon showing small scale channels within the Alemoa Member, in at least three moments (parasequences).
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Figure 6. Paleoenvironmental reconstructions for the Middle (A) and Upper Triassic (B). Fixed, low energy anastomosed channels (Passo das Tropas Member), with levees, crevasses and floodplain
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deposits (Alemoa Member) of the Santa Maria Formation (A). Mobile, braided channels and crevasse deposits of the Caturrita Formation (B). Modified from Eberth and Miall (1991).
Figure 7. Schematic model of fossil vertebrate preservation type along the floodplain deposits of the
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southern Brazilian Triassic (modified from Da Rosa, 2005). Preservation types become progressively worse with the distance from the main channels, exposure of the floodplain, and the effect of
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pedogenesis and phreatic oscillation.
Figure 8. Tentative chronostratigraphic diagram for the Triassic of southern Brazil (modified from Faccini (2000). Ages according to Gradstein et al. (2012). Structural blocks: Mt = Mata, SPS = São Pedro do Sul, SM = Santa Maria, SJP = São João do Polêsine, FxS = Faxinal do Soturno, Ag = Agudo, PS = Paraíso do Sul, C = Candelária, VS = Vale do Sol, VC = Vera Cruz, SCS = Santa Cruz do Sul,
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The contribution places all known Brazilian Triassic dinosaurian forms into a geological context. There is a discussion on the type of preservation, faciological
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