Geochemical study of the vertebrate assemblage zones of the Santa Maria Supersequence (Middle to Late Triassic), Paraná Basin, Brazil

The Middle-Upper Triassic section of the Paraná Basin is included in the Santa Maria Supersequence. This stratigraphic unit is classified as a second-order Supersequence, being subdivided into three third-order sequences in which four Assemblage Zones (AZ) based on tetra-pods are recognized. In this work, chemical analyses of fossils and hosting rocks of each AZ were performed in order to verify whether the geochemical patterns of the rocks could serve to distinguish them even in the absence of fossils. For this purpose, nine samples of fossils and their sedimentary matrices were analyzed, by ICP-MS. The results show that there were changes in the chemical parameters in force at the depositional time of each sequence (and corresponding AZ). In general, the lower portion of the package was deposited under more basic and dry environmental conditions, indicated by the Ca and Ba concentrations found in the rocks and bones that contains them. By its turn, the top of the section is characterized by more acid and humid conditions, evidenced by Al, Si, and Sr increase. Such a geochemical shift from dry to humid conditions occurred just at the beginning of the Late Triassic and is probably related to the Carnian Pluvial Episode.


INTRODUCTION
The Paraná Basin (PB) is filled with an 8,000 m thick sedimentary package. This package does not display a depositional continuity, but polycyclic events resulting from successive sedimentation episodes related to the tectonic events that hit the SW portion of the Gondwana (Milani 1997, Milani et al. 1998). The Middle-Upper Triassic strata (Santa Maria Supersequence -SMS sensu Zerfass et al. 2003, Horn et al. 2014, Horn et al. 2018a, 2018b) that occurs only in Rio Grande do Sul State ( Fig. 1) are worldwide recognized by their vertebrate fossil content. It includes Synapsida -dicynodonts and cynodonts (taxa containing the sister-group of mammals) -and Diapsida -rhynchosaurs and archosaurs, including the first dinosaur representatives , Langer et al. 2007.
In this context, chemical analyses of the fossils and their sedimentary matrices were carried out, in each biozone, aiming to get geochemical patterns that allow us to characterize and individualize these AZ, even without their particular fossil content. Furthermore, these analyses also sought to obtain some paleoclimatic and paleoenvironmental conditions that prevailed at the time of deposition of the rocks that encompass each of the AZ.

GEOLOGICAL SETTING
The Santa Maria Supersequence (SMS) was defined and subdivided by Zerfass et al. (2003), from base-to-top, in the third-order sequences Santa Maria I, II, and III. Within the first two, four biozones were defined based on its tetrapod occurrences, Dinodontosaurus AZ and Santacruzodon AZ, in SMS I, in addition to Hyperodapedon AZ and Riograndia AZ, in SMS II (Soares et al. 2011). Then, Horn et al. (2014 dismembered the SMS I into two sequences, so that the younger Santacruzodon AZ (Early Carnian age) became incorporated into a fourth third-order sequence, named Santa Cruz. In this context (the presence of a new sequence included between the SMS I and SMS II sequences by Zerfass et al. 2003), Horn et al. (2014) renamed all the previous sequences from base-to-top as Pinheiros-Chiniquá Sequence (including the Dinodontosaurus AZ -Ladinian), Santa Cruz Sequence (encompassing the Santacruzodon AZ -Early Carnian), Candelária Sequence (encompassing the Hyperodapedon and Riograndia AZ -Carnian to Norian), and Mata Sequence (Rhaetian). This latter corresponding to the Santa Maria III Sequence from Zerfass et al. (2003) and the only one devoid of fossil vertebrates.
The vertebrate biozones relative ordering is corroborated via biostratigraphic correlation with similar faunas found in Argentina and Africa. In addition, some absolute ages were obtained from fossiliferous outcrops of the SMS by the U-Pb method in dating detrital zircons (Langer et al. 2018, Philipp et al. 2018

MATERIALS AND METHODS
Nine fossil samples and their respective matrices were analyzed by inductively coupled plasma -mass spectrometry (ICP-MS) at Acme Analytical Labs Ltd., located in Vancouver, Canada.
The analyzed fossils belong to the Paleovertebrate Section of the Geosciences Institute of the Universidade Federal do Rio Grande do Sul collection (UFRGS-PV-T), as well as, to the Museu Municipal Aristides Carlos Rodrigues collection (MMACR-PV-T), from Candelária, Rio Grande do Sul. The fossils' description and their provenance are presented in Table 1.
For the correlation between the chemical elements (Pearson's Correlation), Excel Software (2010) was used. The results with positive (direct correlation) or negative values (inverse correlation), higher than 0.7, were considered significant and made it possible to group chemical elements with similar geochemical behavior.

RESULTS
Here were considered the chemical analysis results of the major and trace elements that have relatively high contents and were contained in the sedimentary matrices and the fossil bones. Despite the number of samples for each AZ not being expressive, it was possible to identify and differentiate the different biostratigraphic units according to their geochemical data. It suggests that the same result can be obtained in the outcrops that do not present the characteristic fossil content of any biozone. The distinction of the AZ sediments composition, from arithmetic means, is synthesized in Table 2.
The distinct composition of the AZ bones, from arithmetic means, is synthesized in Table 3.
The most relevant data analyses that allow to individualize each AZ are described below. The sedimentary rock results are described first, followed by fossil bones.

Hyperodapedon AZ
For this AZ, the rock matrices around a cynodont lower jaw (Exaeretodon riograndensis) and a rib and a skull of a rhynchosaur (Hyperodapedon sp.) were analyzed (see Tab. 1 for details). These samples presented a significant number of elements with high average contents (Na, Mg, Al, Si, K, Sc, Ti, Mn, Fe, Co, Ni, Cu, Ga, Rb, Sr, Zr, Nb, and Pb). The most relevant contents are Al (9.18%), Si (31.84%), Fe (2.28%), Rb (115 ppm), Sr (889 ppm), Zr (392 ppm), and Pb (46 ppm), as showed in Table 2. A relative high content of Nb (12 ppm) was detected. According to Nagarajan et al. (2007), Nb's average in sedimentary rocks is 1.90 ppm. On the other hand, the associated rocks of this biozone have the lowest contents of F (0.05%) and Ca (1.05%).

Riograndia AZ
We analyzed the host sediments of a dicynodont rib (Jachaleria candelariensis) and an undetermined archosaur post-cranial fragment (see Tab. 1 for details). The results of this AZ indicated the highest content of F (0.41%), P (2.18%), V (271 ppm), and Y (585 ppm). The Y anomaly is highly surprising when compared to the Anders and Grevesse (1989) chondrites average (1.56 ppm). The high values of Nb (7 ppm) are also anomalous both in relation to the chondrites (246 ppb) and Post-Archean Australian Average Shale (PAAS) (1.90 ppm, Nagarajan et al. 2007) standards. Despite the fact that Nb can be relatively concentrated in comparison to the PAAS pattern, this biozone has the lowest contents of this element. In addition, the Riograndia AZ has the lowest values of Fe (1.14%) and Ba (573 ppm).
The characteristic elements of each AZ, in the host rocks around the fossils, are presented in Table 4.

Santacruzodon AZ
In this biozone, the bones registered high contents of Be, Cr, Mn, Fe, Co, Ga, As, Y, Ba, W, and Pb. The most relevant values are those of Cr (0.01%), Mn (2.05%), As (364 ppm), Y (2,358 ppm), and Ba (27,466 ppm). On the other hand, this AZ has the lowest contents of K (0.1%).

Hyperodapedon AZ
The fossils of this AZ had the highest average contents of Na, Al, Sc, Ti, Ni, Rb, and Sr. The most relevant values are those of Al (0.91%) and Sr (5,939 ppm). This AZ presented the lowest average values of Be (2 ppm), Ca (31.14%), and Mn (0.34%).

Riograndia AZ
It is possible to stand out, for this AZ, the highest average contents of Si, P, K, V, Cu, Zn, and Nb, highlighting the expressive absolute values of Si (2.79%), P (10%), and V (683 ppm). However, this AZ has the lowest average contents of Mg (0.14%), Fe (0.18%), and Ba (1,537 ppm).
The characteristic elements of each AZ in the fossil bones are presented in Table 5.

DISCUSSION
Some of the analyzed elements could be used to differentiate AZ hosting rocks, not by their high or low contents, but by their standard deviation (SD; σ). The SD represents the sample group homogeneity (Bland and Altman 1996), e.g., the F (σ = 0.0007) and the P (σ = 0.03) in the Dinodontosaurus AZ; the K (σ = 0.20) in the Hyperodapedon AZ, and Fe (σ = 0.0) in the Riograndia AZ. High standard deviation values indicate that the sample group is heterogeneous or inconstant (Bland and Altman 1996, Walker 1931), e.g., the Ba SD (σ = 1,447.45) and Sr (σ = 1,283.83) in Hyperodapedon AZ. Thus, when analyzing other outcrops in future works and encountering those high SD results, the researcher may have some notion from which AZ the samples came through. So, those SD values (high -heterogeneity and low -homogeneity) could also be used as an identification criterion for these biozones. It is applicable provided that the sample n is representative, since their values are associated to the number of samples analyzed. The SD significance increases as sampling increases and vary according to the analyses results.
Tables 6 and 7 contain, in bold, the lowest (sample homogeneity) and the highest SD values (sample heterogeneity), in red.
P and the K are relatively depleted, in relation to the average values, in both, sediments and bones of the lowermost AZ (Dinodontosaurus and Santacruzodon). P is more depleted in Dinodontosaurus AZ and K in Santacruzodon.
In the Hyperodapedon AZ, Na, Al, Sc, Ti, Ni, Rb, and Sr have high average contents, but Ca values are low. This behavior occurs in both sediments and bones. Riograndia AZ has high average contents (in both matrices and bones) of F, P, V, and Zn. However, Mg, Fe, and Ba have low values compared to the other biozones.
The high contents of Al, Si, Ti, Fe, Ni, Rb, Zr, and Nb allowed to individualize solely the Hyperodapedon AZ sediments. This AZ can also be differentiated by the lowest contents of F, P, and Ca. This particular composition from Hyperodapedon AZ can be observed in Tables 4 and 8.
The Riograndia's AZ sediments can be differentiated by the low content of Mg, meanwhile, those from Santacruzodon AZ, by the low content of As (Tabs. 4 and 8).
All the analyzed samples, regardless of the Sequence/AZ they belong to, correspond to red silt-clay sedimentary rocks. The sedimentary facies analyses made by Horn et al. (2018a) at SMS enabled these authors to identify two dominant associations across the package: • dry mudflat facies (loess deposits); • sheet delta facies (underwater deposits).
A dry mudflat association characterizes the Pinheiros-Chiniquá (Dinodontosaurus AZ) and the Santa Cruz (Santacruzodon AZ) Sequences. A sheet delta association, on the other hand, characterizes the Candelária Sequence (Hyperodapedon and Riograndia AZs). In this context, the faciological variation of the lowermost portion of the Candelária Sequence indicates a change from arid to humid conditions. Similar climatic changes were described by Zeng et al. (2019) for the Mungaroo Formation (Middle-Upper Triassic, Australia), where the climate changed from temperate and warm to humid and monsoonal. Thus, the existence of contemporary higher humidity events, in opposite portions of Gondwana, was found. Besides, the paleontological and sedimentological studies carried out by Zeng et al. (2019) detected different intensities in the megamonsoon events that hit the Northwest Shelf of Australia, during the Middle Triassic. The Mungaroo Formation records higher humidity and the Brigadier Formation, overlapping, would mark a decrease in those monsoonal effects (Zeng et al. 2019). Something similar was recorded in the passage from Hyperodapedon AZ to Table 4. Differences in the composition of the sedimentary matrices of each assemblage zone. The concentrations of elements (major and trace) of certain biozones when compared to the others are highlighted.
Riograndia AZ, in Brazil. Corroborating the sedimentological data by Horn et al. (2018aHorn et al. ( , 2018b, the high contents of Al, Si, and Fe recorded on Hyperodapedon's AZ sediments (Carnian) indicate a humidity peak. This peak, according to the dating (233.23 Ma.), could be related to the global event named "Wet Intermezzo" or Carnian Pluvial Episode (CPE), whose stipulated age is between 230 and 232 Ma. (Furin et al. 2006, Ogg 2015. In the Riograndia's AZ 6 Braz. J. Geol. (2020), 50(4): e20200014 Table 6. Standard deviation (major elements) obtained by ICP-MS analyses in sediment samples of assemblage zones from Santa Maria Supersequence.

Elements(%)
Base Top   sediments, there is a reduction in these elements content, although they remain relatively high. It could indicate that the CPE decreased its intensity. The rock matrices of the Dinodontosaurus and Santacruzodon AZ are enriched in Ca, Sn, and Ba. According to Brookings (1988), these elements are concentrated in more basic pH conditions (≈ 7.83 and 14), indicating more arid environmental conditions. The sediments of Hyperodapedon and Riograndia, on the other hand, are enriched in Al, Si, and Sr. The first two elements are characteristic of minerals that crystallized under broad pH conditions (≈ 1 to 12.5). Meantime, also according to Brookins (1988), minerals that contain Sr are generally formed under acid to slightly basic conditions (≈ 1 to 7.32). It could indicate an acidification followed by a humidity increase across the SMS depositional evolution.

Dinodontosaurus Santacruzodon Hyperodapedon Riograndia
The highest levels of Si in studied bones are recorded in Riograndia AZ (2.79%) and not in Hyperodapedon (2.3%). This contradicts the previous observations made by Reichel et al. (2005) and Horn (2013) that detected chalcedony (with diagenetic particularity) only in Santacruzodon's sediment. These authors used thin sections to carry these observations. The relatively high levels of Si in Riograndia AZ indicate more acid conditions in weather profiles. It could be associated to many factors, including, in this case, leaching by acid rains. Acid rains at that moment would be an effect of the Large Igneous Provinces (LIPs) volcanism related to the climatic changes, as described by Benton (2018), Benton et al. (2018) and Zeng et al. (2019). Preto et al. (2010), Benton and Newell (2014), and Benton et al. (2018), specially associated to the warming and increased rainfall, to the end Triassic mass extinctions. For Benton (1986), there was more than one extinction event during the end of the Triassic period. The first one would have occurred during the Carnian and the second, in Norian (around 17 to 20 Ma. after the first). The first extinction event, although smaller, was subdivided into two sub events. One with low intensity that affects the ammonites (at the beginning of Carnian) and the second one, larger, that affects some tetrapod groups (at the end of Carnian).
A Wrangellian large igneous province (WLIP), associated with rifting at the beginning of Pangea's breaking up and to megamonsoon events, would have been responsible for triggering the CPE (Zeng et al. 2019). Initially, the CPE was interpreted as a high humidity episode, though shortlived Hinnov 2003, Rigo et al. 2007), in regional scale, restricted only to the European plate and the Tethys Ocean (Simms and Ruffell 1989, 1990, Simms et al. 1995. Meantime, recent studies (Preto et al. 2010, Benton and Newell 2014, Cheng et al. 2019, Horn et al. 2018a, 2018b, Zeng et al. 2019 have been demonstrating that its occurrence was in global scale. The CPE was detected in sediments from many different places around the world, such as, in the SW of the United States (Prochnow et al. 2006, Lucas andTanner 2018), Australia (Zeng et al. 2019), and Brazil (Horn et al. 2018a). Thus, it covers both the northern and the southern hemispheres. Benton et al. (2018) correlate a rapidly climatic changing event (dry-wet-dry) that supposedly happened in the Carnian, with the dinosaur's diversification. The analyzed sediments of Zeng et al. (2019), in Mungaroo Formation (Australia), reflects relatively rapid deposition associated to a strong hydrodynamic condition. This suggests that the megamonsoon would have happened in a relatively short, though expressive, time, at least in that region. Although these evidences point to a rapid great humidity event in the Carnian, Ogg (2015) suggests that biostratigraphic calibrations (in both marine and continental environments) and radioisotopic dating should be made to delimitate the real duration of this episode.
In relation to the variations in the Mg and Al contents, Mg is characterized by its high mobility, meanwhile Al, by its low mobility, both in weather profiles (White 2013). In sediments under Standard Ambient Temperature and Pressure (SATP), lower values of Mg (in comparison to Al) are expected. If the Al contents also quickly decrease, in comparison to the sediments average values (as the North American Shale Composite -NASC pattern of Gromet et al. (1984); 2.86% Mg and 16.90% Al), it indicates that the weathering processes were very intense (Sheldon and Tabor 2009). In the studied samples, the Mg (0.87%) and Al (9.18%) contents found in Hyperodapedon's AZ sediments are low, in comparison to the average contents of the NASC pattern. This suggests a very intense weathering process (Benton and Newell 2014) that can be associated with higher humidity episodes.
A synthetic model with the most relevant changes (faciological and geochemical) that occurs across the SMS assemblage zone depositions is illustrated in Figure 3. This model helps to identify the facies (dry mudflat -Dinodontosaurus and Santacruzodon AZ and sheet delta -Hyperodapedon and Riograndia AZ) and presents a geochemical synthesis of each particular biozone across the SMS.
Another data that could reinforce the environmental changes discussed here came from the fossil plants of the Cuyo basin, in Argentina. The Potrerillos Formation was deposited between 239 and 230.3 Ma. It is basically composed by an interleaving of peat and pyroclastic sediments reworked in conglomerates, sandstones and shales (Spalletti et al. 2008). This formation had its paleoflora analyzed by Artabe et al. (2007). They subdivided it into two Triassic biozones, the Yabeiella mereyesiaca -Scytophyllum bonettiae -Protophyllocladoxylon cortaderitaensis assemblage biozone (MBC) (Middle Triassic) and the Yabeiella brackebuschiana -Scytophyllum neuburgianum -Rhexoxylon piatnitzkyi assemblage biozone (BNP) (Upper Triassic). According to Artabe et al. (2007), the paleofloristic associations of Potrerillos (MBC and BNP) would be inserted in a dry subtropical zone of the southeast extratropical area of the Gondwana. This area should be exposed to megamonsoon regimes. This is corroborated by the presence of plant groups with seasonality adaptations and xeromorphic characteristics. According to Spalletti et al. (2005), the MBC and BNP paleofloras share species of Yabeiella and Scytophyllum genus. However, they differ in the occurrence of the genera Prtophyllocladoxylon (MBC) and Rhexoxylon (BNP), being the latter attributed to conifers, a group not closely related to the Corystosperms (Bodnar 2008). Shi et al. (2016) suggest that there is a correlation between Corystosperms and Ginkgo, a genus that continues until today. This similarity with living species allows to presume that, as in the actual Ginkgos, the Corystosperms (R. cortaderitaense) should be adapted to well-drained and high-recharging environments, like streams and river dikes (Royer et al. 2003). Fu et al. (1999 registered some Ginkgo occurrences in deciduous forests and loess deposits (well drained), whose soil's pH is around 5.0 to 5.5. Such favorable characteristics that allow the R. cortaderitaense's occurrence (acidity and welldrained soils) allow associating the BNP biozone (Cuyo's basin, Argentina) climate to the uppermost AZ from SMS, particularly the Hyperodapedon AZ. These enlarge the influence zone of the biggest humidity event of the Carnian period (CPE) until the Argentines soils. This paleoclimatic condition of megamonsoon regimes is similar to that observed in the SMS region. Horn et al. (2018b) identified some climatic changes between the SMS biozones, from driest to wetter, in particular between the lowermost (Dinodontosaurus and Santacruzodon) and the uppermost (Hyperodapedon and Riograndia). Guerra-Sommer and Klepzig (2000) and Barboni and Dutra (2015) reported occurrences of leaves, Cuticule fragments and ex situ fossil woods, with a predominance of the genus Dicroidium, in the Middle-Upper Triassic package (Passo das Tropas Member) paleoflora. This genus includes the Division Ginkgophyta. Such occurrences were first assigned to the Anisian-Ladinian age (Guerra-Sommer and Klepzig 2000), but recent studies comparing these ginkgophytes with those from different Gondwanan assemblages attributed a Carnian age for these Brazilian deposits (Barboni and Dutra 2015). This age is correlated to the Hyperodapedon's AZ dating. The occurrences of ex situ fossil woods can be interpreted as episodic flooding events, which corroborate with our hypothesis of a high humidity episode during the Hyperodapedon's AZ deposition. This work uses geochemical data obtained from vertebrate fossils and their sedimentary matrices to verify the climatic changes, unlike the Potrerillos Formation observations that used the conglomerate deposition as a criterion for the biozones (MBC and BNP) delimitation (indicating high energy flows, maybe related to a humidity peak). Subtle climatic changes are also recorded in SMS from faciological studies (gradation from loess deposits to underwater deposits) carried out by Horn et al. (2018aHorn et al. ( , 2018b. Another data source that indicates a rise in humidity at the beginning of the Upper Triassic came from the particle size analysis made by the authors with rock samples from the analyzed biozones. It was observed an increase in the clay and sand percentages toward the top of the SMS package. This reinforces the idea that the uppermost AZ of this sequence were wetter than the basal ones. Besides the clay and sand content increase, the permanence of high silt percentages (above 80%) are highlighted, especially in the thicker fractions, even in the wetter biozones. Thus, is suggests that the leading sediment supplier, in all biozones, was the wind, in a peridesertic context. In the lowermost AZ (Dinodontosaurus and Santacruzodon), the silt was deposited under massive layers (loess). On the other hand, the uppermost's (Hyperodapedon and Riograndia), the dust brought by the wind was reworked by the water, generating laminated deposits.

CONCLUSIONS
Throughout depositional evolution of the SMS, there was a change in the facies association, from dry mudflat facies (Pinheiros-Chiniquá and Santa Cruz Sequences) to sheet delta facies (Candelária Sequence). This facies change followed by geochemical variations record the passage from more basic conditions (characterized by the highest values of Ca and Ba of the Dinodontosaurus and Santacruzodon AZ) to a more acid environment (characterized by the highest percentages of Al, Si, and Sr of the Hyperodapedon and Riograndia AZ). This set of evidence points to a record of the CPE also for the SW portion of the Gondwana. The sedimentary matrices of the Santacruzodon AZ can be diagnosed by the low contents of As. The sediments of the Hyperodapedon AZ, in their turn, are differentiated by the high contents of Al, Si, Ti, Fe, Ni, Rb, Zr, and Nb and the low values of F, P, and Ca. The Riograndia AZ's sediments can be individualized by their low content of Mg. The low contents of Mg and Al observed in Riograndia AZ attest a wetter period in this SMS biozone. However, if compared with the high contents of Al, Si, and Fe recorded in the Hyperodapedon AZ's sediments, it is possible to conclude that the highest humidity peak of the SMS is associated to this AZ. This hypothesis is more plausible, since sediments that are relatively enriched in Si, Al, and Fe used to be more mature (sediments exposed for very long periods of intense weathering, could be their soil profile leaching) than those with Mg and Al. Therefore, the studied section records a high humidity episode, in the uppermost AZ of the SMS (Carnian) that could be related to the global event named CPE.
In addition, the geochemical study of the assemblage zones of the SMS shows that these biozones could be used as a stratigraphic element of correlation due to their characteristic geochemical compositions. However, due to the chemical heterogeneity observed in the studied samples, it is considered important to continue the studies in this area. More samples need to be analyzed in order to precisely determine the content variations of these elements across the AZ.
More detailed studies of geochemical variations of the fossiliferous of the studied AZ are needed. A paleofloristic study, similar to those made in Argentina, can also be performed. The particle size studies also need a refinement, in order to verify how the global paleoclimatic variations can influence the facies and AZ genesis in the Santa Maria Supersequence.