lower permian stems as fluvial paleocurrent indicators of the parnaíba basin, northern brazil

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Journal of South American Earth Sciences 45 (2013) 69e82

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Lower Permian stems as fluvial paleocurrent indicators of the Parnaíba Basin,northern Brazil

Robson Louiz Capretz a,*, Rosemarie Rohn b

aUNESP Rio Claro, IGCE, PG Geologia Regional, Av. 24A, 1515, 13506-900 Rio Claro, São Paulo, BrazilbUNESP Rio Claro, IGCE, Departamento de Geologia Aplicada, Brazil

a r t i c l e i n f o

Article history:Received 30 March 2012Received in revised form25 August 2012Accepted 13 December 2012

Keywords:PaleofloraLate PaleozoicTocantinsTaphonomyBiostratinomyTree ferns

* Corresponding author. R. Sanito Rocha 261, CuTel.: þ55 4199129036.

E-mail address: [email protected] (R.L. Capretz

0895-9811/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jsames.2012.12.007

a b s t r a c t

A comprehensive biostratinomic study was carried out with abundant stems from the Lower PermianMotuca Formation of the intracratonic Parnaíba Basin, central-north Brazil. The fossils represent a raretropical to subtropical paleofloristic record in north Gondwana. Tree ferns dominate the assemblages(mainly Tietea, secondarily Psaronius), followed by gymnosperms, sphenophytes, other ferns and rarelycophytes. They are silica-permineralized, commonly reach 4 m length (exceptionally more than 10 m),lie loosely on the ground or are embedded in the original sandstone or siltstone matrix, and attractparticular attention because of their frequent parallel attitudes. Many tree fern stems present the originalstraight cylindrical to slightly conical forms, other are somewhat flattened, and the gymnosperm stemsare usually more irregular. Measurements of stem orientations and dimensions were made in three sitesapproximately aligned in a WeE direction in a distance of 27.3 km at the conservation unit “TocantinsFossil Trees Natural Monument”. In the eastern site, rose diagrams for 54 stems indicate a relativelynarrow azimuthal range to SE. These stems commonly present attached basal bulbous root mantles andthin cylindrical sandstone envelopes, which sometimes hold, almost adjacent to the lateral stem surface,permineralized fern pinnae and other small plant fragments. In the more central site, 82 measured stemsare preferentially oriented in the SWeNE direction, the proportion of gymnosperms is higher and cross-stratification sets of sandstones indicate paleocurrents mainly to NE and secondarily to SE. In the westernsite, most of the 42 measured stems lie in EeW positions. The predominantly sandy succession, wherethe fossil stems are best represented, evidences a braided fluvial system under semiarid conditions. Thelow plant diversity, some xeromorphic features and the supposedly almost syndepositional silicaimpregnation of the plants are coherent with marked dry seasons. Thick mudstones and some coquinitesbelow and above the sandy interval may represent lacustrine facies formed in probably more humidconditions. The taphonomic history of the preserved plants began with exceptional storms that causedfast-flowing high water in channels and far into the floodplains. In the eastern site region, many treeferns only fell, thus sometimes covering and protecting plant litter and leaves from further fragmenta-tion. Assemblages of the central and western sites suggest that the trees were uprooted and transportedin suspension (floating) parallel to the flow. Heavier ends of stems (according to their form or because ofattached basal bulbous root mantle or large apical fronds) were oriented to upstream because of inertialforces. During falling water stage, the stems were stranded on riverbanks, usually maintaining theprevious transport orientation, and were slightly buried. The perpendicular or oblique positions of somestems may have been caused by interference with other stems or shallow bars. Rare observed stems wereapparently waterlogged before the final depositional process and transported as bedload. The differencesof interpreted channel orientations between the three sites are expected in a braided fluvial system,considering the very low gradients of the basin and the work scale in the order of tens of kilometers. Themean direction of the drainage probably was to east and the flows apparently became weaker down-stream. This study seems to provide reliable data for paleocurrent interpretations, especially consideringareas with scarce preserved sedimentary structures.

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R.L. Capretz, R. Rohn / Journal of South American Earth Sciences 45 (2013) 69e8270

1. Introduction

Abundant petrified large stems exposed in southern ParnaíbaBasin, central-north Brazil, justified the implementation of a32,000 ha conservation unit by the Tocantins State Government,namely the “Tocantins Fossil Trees Natural Monument” (TFTNM).The majority of the stems are lying on the ground and few arepartially embedded in the original sediments. The fossil-bearingdeposits are attributed to fluvial beds of the Lower PermianMotucaFormation, Balsas Group.

Many stems attract additional attention by their parallel orien-tations. This particular fact and the scarce prior research in thisregion encouraged the present biostratinomic study.

Until the first half of the 20th century, studies of fossil trunkorientation in fluvial deposits were rare or inconclusive (Potter andPettjohn, 1977). Research on this subject only advanced in the lastdecades, since the publications of Froggatt et al. (1981), Macdonaldand Jefferson (1985), Mcknight et al. (1990), Keller and Hendrix(1997), Heckert and Lucas (1998), Guerra-Sommer and Scherer(2000), Roberts and Hendrix (2000), Fielding and Alexander(2001), Assine et al. (2003), Gastaldo (2004), Martín-Closas andGaltier (2005), Banerjee (2005) and Gastaldo et al. (2005).

In comparison to the mentioned examples, the studied areacertainly is one of the largest, with the highest number of longexposed fossils. Important issues about the composition andpaleogeographywere found: (1) Themost common fossilized stemsfound correspond to tree ferns (not to gymnosperms, which other-wise are the usual represented silica-permineralized Paleozoic fos-sils); and (2) The fossils record an almost unknownfloristic provincebetween the typical Early Permian Euramerican and south Gond-wanic floras, in the transition from the subtropical to the tropicalzone, in a supposedly dry climatic belt (e.g., Rees et al., 2002).

2. Background

2.1. Geology

The stratigraphicunits exposed in theTFTNMregionare thePedrade Fogo, Motuca and Sambaíba Formations of the Balsas Group, adominantly siliciclastic succession of the intracratonic Parnaíba Ba-sin (advanced information about the stratigraphy of Parnaíba Basinare available in Pinto and Sad, 1986; Dias-Brito et al., 2009).

The landscape in the studied region, especially in the northeastof the TFTNM (Fig. 1), is characterized by an almost flat plain ataltitudes of 200e250 m and plateau mountains with steep edges,reaching altitudes up to 500 m (Dias-Brito et al., 2009). Accordingto maps of Pinto and Sad (1986) and Dias-Brito et al. (2009), the

Fig. 1. Landscape in the Tocantins Fossil Trees Natural Monument in a view towards nortformations, and the plain in the foreground with outcrops of the Motuca Formation.

lower terrains mainly expose the Permian Motuca Formation andlocally (in the southwest and in the north of the TFTNM) the older(Permo-Carboniferous?) Pedra de Fogo Formation. These forma-tions represent the transition from coastal to continental environ-ments (Góes and Feijó, 1994). The plateau margins clearly showeolian beds of the Triassic Sambaíba Formation (upper part of theBalsas Group; Góes and Feijó, 1994). At least one of the plateau tops(Justino Mountain), investigated under this research, presents ba-salts of the Upper Triassic Mosquito Formation.

The fossil plants in the TFTNM are found in deposits assigned tothe Motuca Formation (Pinto and Sad, 1986; Dias-Brito et al., 2009).However, it is important to mention that the permineralized fossilplants of the Parnaíba Basinwere traditionally ascribed to the Pedrade Fogo Formation. This interpretation probably resulted from thefact that the stems commonly lie loosely on the soil and are mixedwith chert pebbles, which have been considered as the mostdiagnostic characteristics of the Pedra de Fogo Formation. However,it is clear that stems still embedded in the original sandstones orsiltstones in the TFTNM belong to a 20e30m siliciclastic successionabove cherts, as well as carbonates, evaporites, shales and othertypical facies of the Pedra de Fogo Formation (Pinto and Sad, 1986;Dias-Brito et al., 2009).

Good outcrops of the formation are few in the region due to therelatively flat relief, vegetation covering or intense weathering.Normally all sediments are fine, without pebbles, except for aconglomerate at the base of the formation at Vargem Limpa Site,south from Bielândia (Dias-Brito et al. 2009). Themost typical faciesare white to reddish fine sandstones with tabular or trough cross-stratifications, which may be stacked and form up to 10 m thicksandstone intervals (Rössler, 2006, Dias-Brito et al., 2009). Silt-stones (up to 5e10 m thick) may occur as lateral equivalents ofsome sandstones (Dias-Brito et al., 2009) or succeed slightlyupward-finning sandstones (Rössler, 2006).

According to Rössler (2006), the facies are arranged as approx-imately 10 m thick fluvial cycles. The sandstones begin the cyclesand correspond to single or stacked channel bodies. They are suc-ceeded by mottled sandstones with muddy intercalations, whichwould represent floodplain deposits and, in part, paleosoils ofseasonally alternating wet and dry conditions. The siltstones,especially some beds with bivalve mollusks, were considered aslacustrine. The cycles would end with thin chert layers as result ofevaporation of the lake water after climatic change to dryer con-ditions. Rössler (2006) observed that small moderate to wellrounded stem fragments occur at the base of the channel bodiesand the abundant long stems are found in the floodplain deposits.

The upper part of the Motuca Formation in the TFTNM region,beyond the fossiliferous interval, presents a 2e3 m thick gypsum

h, showing the plateau mountains, mainly composed of the Sambaíba and Mosquito

R.L. Capretz, R. Rohn / Journal of South American Earth Sciences 45 (2013) 69e82 71

bed (Dias-Brito et al., 2009). Evaporites are also known in otherareas of the Parnaíba Basin, as well as eolian sandstones (Lima Filho,1998). These lithologies substantiated interpretations that theMotuca Formation is representative of sabkha or playa environ-ments (Faria Jr., 1984; Lima Filho, 1998). However, these conditionscannot be generalized for the whole succession or for all areas ofthe basin and no detailed stratigraphic correlations reached theTFTNM region.

2.2. Paleobotany

Psaronius brasiliensis Brongniart, 1872 is the first formally pro-posed fossil species from the Parnaíba Basin, as well as from Brazil.The genus refers to a Permo-Carboniferous tree fern stem originallyrecorded in the Euramerican Flora. During many decades afterBrongniart’s (1872) publication, little paleobotanical work wasdone in the Parnaíba Basin. In the beginning of the 20th century, asecond tree fern species was reported e Psaronius arrojadoiPelourde, 1912.

Dollianitti (1972), in a preliminary approach, commented thatsome Permian fossil plants apparently had closer affinities to taxafrom Europe and North America, than to those from the ParanáBasin (southern Brazil), which belonged to a typical GlossopterisFlora. Coimbra and Mussa (1984) and Mussa and Coimbra (1987)corroborated this hypothesis through the description and identifi-cation of some northern plant genera, especially gymnosperms andsphenophytes (Arthropitys). One important exception, first recog-nized by Herbst (1985), is Tietea singularis Solms-Laubach, 1913, atree fern from the Paraná Basin.

Further tree ferns from the Parnaíba Basin were described byHerbst (1986, 1992, 1999) and other ferns by Rössler and Galtier(2002a, b, 2003). Grammatopteris freitasi Rössler and Galtier,2002a is a fern that possibly represents the best chronostrati-graphic index of the Motuca Formation, taking into account thescarce other useful fossils for age interpretations. Grammatopteris isrestricted to the basal Permian in Germany and France (Rössler andGaltier, 2002a) and is therefore a good evidence of a probable EarlyPermian age of the fossil plant-bearing strata. Tree fern stems andleaves, new sphenophytes, gymnosperms and epiphytes were

Fig. 2. Map of the Tocantins Natural Fossil Trees Monument (T

preliminarily focused by Rössler and Noll (2002). Interesting fernpinnules with probable xeromorphic adaptations were shortly re-ported in Tavares et al. (2008). A detailed revision and descriptionof tree fern stems, fronds and petioles are provided in the thesis ofby Tavares (2011).

Rössler (2006) and Dias-Brito et al. (2009) briefly mentionedthat the stems are arranged in a preferential WeE direction, but nostrictly taphonomic study was done, nor detailed paleoecologicconsiderations were presented.

Bolzon and Silvériodasilva (2004) and Matysová et al. (2010)discussed the chemical factors in fossilization of stems, includingcomments about fossils from Parnaíba Basin. In summary, theyinterpreted that the plants were permineralized in fluvial sandsunder the influence of pronounced seasonal distribution of pre-cipitation in a relatively warm climate. In these conditions, silicaminerals as feldspars were weathered and produced silica as aprimary permineralising agent.

3. Methods

3.1. Study area

The taphonomic investigations were done in three sitesroughly aligned in a WeE direction over a distance of about27.3 km in the Tocantins Fossil Trees Natural Monument (TFTNM),between Araguaína and Filadéflia, relatively near to the TO-222road (Fig. 2).

The Andradina site is located in the central part of TFTNM, closeto Bielândia village (7�2705300 S, 47�5003700 W). The originalassemblage of this area was relatively depleted by illegal fossilcollection, but it is still rich in stems and has the best outcrops ofsandstones with cross-stratifications.

The Buritirana site is near the eastern boundary of TFTNM(7�2705800 S, 47�4200000 W). In addition of stems, fern leaves andpetioles are also found.

The Peba site is close to the western boundary of TFTNM(7�2501500 S, 47�5701800 W). It has a lower number of exposed fossilsin comparison to the other two sites, but some specimens bear verywell preserved tissues and cells.

FTNM), with the location of the three main studied sites.


3.2. Data collection

A total of 178 fossil stems were found in the three fossiliferoussites in the TFTNM. Identifications were made at genus levelor at higher taxonomic ranks according to the macroscopiccharacteristics.

As already mentioned, few exposed stems are still embedded (atleast, partially) in the original sedimentary deposits. Many stemsoccur scattered on the ground because of the resistance of thesilica-permineralized stems to present-day weathering andremoval of the less resistant sedimentary matrix by erosion. Theexposed stems show transversal fractures or they are divided intofragments commonly at intervals of 30e60 cm. The fragments arefrequently aligned and apparently maintained the original direc-tion of the stems (Fig. 3A). In many cases, the most proximal anddistal parts of the stems are no longer available because they musthave been removed by modern weathering/erosion processes.

The location of all logs and outcrops was done with GPS system.The standard error of the device was considered for the analysis.The altitude of each outcrop was corrected using a periodically

Fig. 3. (A) Fractured or fragmented stems lying on the ground or partially embedded in theStacked wedge shaped to tabular sandstone beds with predominantly planar cross-stratificaSite. (D) Lenticular, sigmoidal and tabular sandstones with cut-and-fill arrangement in thestratifications in the southern region of Andradina Site. (F) Mottled sandstones with irregularAndradina Site.

calibrated manual altimeter. Whithin each site, the altimetric dif-ference between the lowest and highest stem level is about 20 m.Considering that the general orientations of stems do not changenoticeably from the lowest to the highest levels, the analysis foreach site was made for the whole fossil assemblage (not discrimi-nating the vertical position). The spatial data of petrified stemswere plotted on maps using the Geographical Information System(GIS) Spring e a freeware released by the Instituto Nacional dePesquisas Espaciais e INPE (Brazilian National Institute of SpaceResearch).

The following measurements of stems were taken: length of thepreserved interval of the stem (in general the sum of the lengths ofmany fragments, here designated for practical purposes simply as“stem length”), stem diameter (measured in the most proximalpreserved part and in the most distal part) and orientation (inproximaledistal direction). In general, the shortest stems taken inconsideration for measurements were about 1 m long. Hugeamounts of smaller stems occur in the three sites, but they werediscarded in this study because they may have been displaced fromtheir original depositional orientation.

original deposits. (B) Sandstones with tabular cross-stratifications at Andradina Site. (C)tions and locally sigmoidal cross-stratifications and bedform preservation at Andradinasouthern region of Andradina Site. (E) Sandstones with predominantly trough cross-thin sandy mudstone intercalations and three stems (arrows) in the southern region of


Directions of cross-stratification sets of sandstones were alsomeasured for paleocurrent analyses. The orientation data (of stemsand cross-stratification) were plotted on rose diagrams accordingto the normal procedure for paleocurrent analyses presented byMcknight et al. (1990), Keller and Hendrix (1997), Heckert andLucas (1998) and Assine et al. (2003), amongst others. The rosediagrams were built using class intervals of 10� in the softwaresRockworks, Spring and R. Three diagrams were analyzed for eachsite: one considering all measured stems, another only using stemslonger than 2 m and a third probably less accurate diagram takinginto account the shorter stems.

4. Results

4.1. Detailed geological observations

All observed sedimentary rocks of the Motuca Formation in theTFTNM are fine grained (red mudstones to fine/medium whitish/light reddish sandstones), except w0.5 m thick conglomerates at

Fig. 4. (A and B) Sandstones and interbedded thin irregular sandy mudstones with possiblescale cross-stratifications. A tree fern stem is embedded in these deposits (in the upper partsandy mudstones in the uppermost outcrop of the Andradina Site. (D) Outcrop in the Peba Sstem at Buritirana Site. (F) Plan view of concretion in muddy fine sandstone with fern pinnmudstone above whitish sandstone, south from Bielândia.

the base (in outcrops south of Bielândia area and at the lowesttopographic positions of the Buritirana Site).

4.1.1. Andradina SiteIn the northern region of the Andradina Site (7�27034.7700 S,

47�50025.9400 W), a w7 m high section shows relatively tabularsandstone beds, 1.1 m thick in the lower part, with very regularplanar cross-stratifications oriented to NE (Fig. 3B). Close to thispoint, in a little higher position, the beds become thinner, morewedge shaped and locally show sigmoidal cross-stratification andpreservation of bedforms (Fig. 3C).

In the southern region of this site (7�27057.4100 S, 47�50035.1800

W), a section shows sandstone bodies with lenticular or sigmoidalgeometry (up tow0.6 m high andw3 mwide) and some tabular orslightly wedge shaped beds (w0.4e0.5 m thick) with planar ortrough cross-stratifications (Fig. 3D and E), locally with massiveaspect probably due to bad preservation. Cosets of small cross-stratification sets may pass laterally to one larger set (w0.5 m). Thelenticular sandstones are clearly erosionally-based and characterizea typical cut-and-fill arrangement. In planar view, the upper surface

rootmarks in Andradina Site. The sandstones are massive and mottled or present smallof the photo A). (C) Sandstones with climbing ripple cross-laminations and interlayeredite region with sandstone, mudstone and silex. (E) Mudstone and compressed tree ferna at Buritirana Site. Scale bar ¼ 1 cm. (G) Oblique view of embedded stem in reddish

Table 1Composition of the studied assemblages in the three studied sites. The numericaldata refers only to the relatively long stems which are presumed to havemaintainedtheir original sedimentary position.

fossil stems/Studied areas Peba Andradina Buritirana

Ferns Total 40 26 54Tietea Abundant Abundant AbundantPsaronius Uncommon,

fragmentedUncommon,fragmented

Uncommon,fragmented

Grammatopteris Uncommon,fragmented

0 Uncommon,fragmented

Dernbachia 0 0 RareSphenophytes Rare Rare, including

one longspecimen

Rare, includingone largerhizome

Lycophytes 0 0 Very rareGymnosperms 2 56 0 (i.e., all stems

are toofragmentary)


of some beds is characterized by asymmetrical ripples. Locally, thinmuddy bands (w3e5 cm) separate the cross-strata sets. The fore-sets point to NE, SE and rarely to SW. The respective rose diagramsare presented later with the stem orientation diagrams.

The sandstones with conspicuous cross-stratifications passlaterally (within a distance of 100m; 7�27057.5900 S, 47�50033.2300 W)into interbedded thin sandstones and sandymudstones (Figs. 3Fand4A and B). The sandstones are mottled or present small scale cross-stratifications (3e20 cm high sets). The mudstones occur as veryirregular thin brown or dark-red bands, with possible rootmarks.Cylindrical and compressed stems are embedded in these deposits.

A 1.9 m high exposition in the most northeastern and highestoutcrop of the Andradina Site (7�27029.1000 S, 47�50017.5600 W)shows slightly inclined irregularly interlayered 2e3 cm redmassivesandy mudstones and fine sandstones with conspicuous climbingripple cross-laminations (Fig. 4C). This heterolithic interval is su-perposed by sandstones with large scale trough cross-stratification.

4.1.2. Peba SiteIn the Peba Site region (7�2402400 S, 47�5602900 W), an outcrop

shows a rare clear succession of sandstone (up to 1.6 m thick),mottled sandstone (0.2 m) and mudstone (w2 m), with a silex bedintercalation (w0.1 m, about 1.5 m above the base of the mudstone)(Fig. 4D). The sandstone presents planar cross-stratification andbears a small permineralized stem fragment.

4.1.3. Buritirana SiteIn Buritirana Site, preserved sedimentary structures are very

scarce. One outcrop shows weathered sandstones with apparentlylarge trough cross-stratifications. Mudstones are massive, thick(>5 m) and present compressed stems (Fig. 4E) and fern pinnaeimpressions. Muddy sandstones have concretions (Fig. 4F).

An outcrop south from Bielândia (7�29045.6700 S, 47�51023.3000

W) shows compressed stems embedded in thin mudstone, above asandstone (Fig. 4G).

4.2. Composition of the fossil assemblages

In the three studied areas, 178 fossil stems were found andmeasured. Table 1 shows these results. Almost all stems correspondto the tree fern Tietea. The other stems are less abundant and toofragmentary for reliable measuring of orientation. The AndradinaSite is an exception concerning the high proportion of gymno-sperms (68%).

The Permian tree ferns as Psaronius are reconstructed likemodern Marattiales, i.e., with tall straight cylindrical to slightlyconical stems and large fronds at the apex. The stems are trans-versally divided into a central stele and a mantle of adventitiousroots extended almost to the apex. The stele bears several band-likesinuous or irregular meristeles. The basal part of the stems isentirely composed of adventitious roots and commonly has a thickbulbous form. Tietea probably had a bauplan similar to Psaronius,but in transversal section the apical stem region is square and themeristeles are more (semi-) circular (see revision in Tavares, 2011).Butirama Site exposes several basal parts of stems, i.e., thickbulbous root mantles, not in situ, but in horizontal positions(Fig. 5A). They sometimes contain small gymnosperm epiphytes.

In Buritirana Site, two small distal stem fragments have roundedends probably caused by abrasion before deposition (Fig. 5B).

One long Tietea stem in Andradina site is exceptionally sinuous,probably due to deformation (Fig. 5C). Otherwise only some longgymnospermous stems of the TFTNM probably were originallycurved to irregular.

In the Buritirana Site, several stems bear a silica cemented 2e4 cm thick sandstone envelope (Fig. 5D). A little away from the

stems, the sand probably was less cemented and prone to easierremoval by modern weathering processes.

Three-dimensionally preserved pinnae and pinnules occur looseon the ground (Fig. 4F) or are almost adhered laterally to tree fernstems in the Buritirana Site (Fig. 5E). In the second situation, theleaves usually do not touch the surface of the stems, but areconcentrated in one side of the sandstone envelope (i.e., belowstems found in the original depositional position, as an example at7�29045.6700 S, 47�51023.3000 W, south from Bielândia). No organicconnection between leaves and stems was found. Almost all pinnaeand pinnules, as well as fern petioles, were recorded at BuritiranaSite. Rare similar stems with adhered leaves were observed inAndradina Site and south from Bielândia.

Some plant fragment concentrations were observed in raresandstone blocks lying loosely on the ground. In part, they aresimilar to the envelopes around the stems (Fig. 5F).

Pinnae and pinnules found loose on the ground at Buritirana Siteprobably fell off the sandstone stem covers with the action ofweathering. The leaves are predominantly fertile and show totalcovering of synangia by incurved pinnule margins to the abaxialside (Rössler and Noll, 2002; Tavares et al., 2008; Tavares, 2011).Some pinnae impressions were found in siltstones in this site.

4.3. Stem sizes

Forest ecological studies frequently use size histograms fordiagnosing plant populations (Capretz, 2004). A normal forestusually results in a reverse J-shaped curve because the first twoclasses have more individuals and the following classes present adecreasing tendency. This pattern reflects the phenology andontogeny of the species: a high number of individuals germinatefrom seeds or gametophytes, but only part of the young plantsmanage to reach the next stages and/or reproductive age because ofcompetition, herbivory, insufficient nutrients in soil or not enoughwater supply. The lower frequency of stems in the last histogramclasses obviously demonstrates that few specimens reach greatheights (Capretz, 2004, Begon et al., 2006). In the case of the fossilassemblages, this trend is strengthened because the tallest plantsmay be underrepresented in the record as consequence of tapho-nomic factors. A shortening of the lengths of the stems happened intwo steps: 1. during the transport and burial of the plants in thedepositional environment; 2. during the recent weathering, such asdiscussed previously.

Fig. 6 shows the size histograms of the measured stems inTFTNM. The longest stem was observed in Buritirana Site and cor-responds to Tietea. It is 10.9 m long, 50 cm thick at the proximalregion and 22 cm at the distal part. Some basal bulbous root

Fig. 5. (A) Examples of tree fern stems with preserved basal parts at Buritirana Site. (B) Distal portions of Tietea stems with evidences of abrasion at Buritirana Site (a,b). The secondstem (c,d) corresponds to an original apical part of the tree fern according to its approximately square transversal section. (C) Exceptional deformed tree fern stem at Andradina Site.(D) Tree fern stem with cemented sandstone envelope at Buritirana Site. (E) Leaves and other plant fragments practically adhered to one side of the tree fern stem surface atBuritirana Site. (F) Plant fragments concentrated in fine sandstone block lying loosely on the ground in Buritirana Site.


mantles may present diameters over 1 m. At Andradina Site, thelongest gymnosperm specimen reaches almost 7 m in length andits diameters vary from 19 cm to 14 cm (respectively at the mostproximal and most distal transversal sections).

Ellipsoid transversal stem sections related to compaction duringfossilization are observed for 33 of the 178 measured stems (18%).Most of them correspond to tree ferns (31 samples), of which 12were recorded at Peba Site and 19 at Butirama Site. Two com-pressed gymnosperm stems were observed in Andradina Site. Themost flattened stems occur in mudstones (Fig. 4). However, the rootmantles are normally less flattened or deformed than the steles.

The histograms (Fig. 6) show that the preserved fern stems arelonger than the gymnosperm stems, but according to the di-ameters, some gymnosperms may have been tall plants. It isimportant to clarify that fern stem diameters greater than 40 cmindicated on the histogram (Fig. 6) refer to the basal broad bulbousroot mantles and not to exceptional large specimens.

Some differences are noted between data collected in the threesites. The shorter stems are usually found at Andradina Site

(Table 2). Analysis of variance and the Student’s t-test two by two(Krebs, 1999), show that the differences of stem lengths from Pebaand Buritirana sites are insignificant.

According to the histograms of length classes (Fig. 6) stemsshorter than 2 meters were recorded in a higher frequency at theAndradina and Buritirana sites than at Peba Site.

Table 3 shows that from 178 measured stems, 103 (58%) have alength equal to or smaller than 2 m.

4.4. Orientation

Rose diagrams of stem orientation show relatively consistentdirections within each site, but different tendencies from one site toanother (Fig. 7). In Peba Site, in the western part of the region, theorientation of the stems is practically EeW; in Andradina Site, closeto the center of the area, the preferential direction is NEeSW; inBuritirana Site, at the eastern part of the area, the stems are mainlyoriented to the SE.

Fig. 6. Histograms for lengths and diameters of tree fern and gymnosperm stems in the TFTNM, and for stem length classes (m) at the three sites.


The stems shorter than 2 m have more variable orientationsthan the longer stems. In Peba Site, the differences between rosediagrams for shorter and longer stems are not so marked. TheWeEdirection is relatively consistent (Fig. 7B).

The rose diagrams constructed for shorter and longer stems inAndradina Site are relatively distinct, but the NEeSW directionindicated by the longer stems agrees fairly well with the mostfrequent paleocurrent directions (towards NE) determined ac-cording to the sandstone cross-stratifications (Fig. 7C).

In Butirama Site (Fig. 7D), the majority of stems longer than 2 mhave their distal side pointed to SE. The shorter stems than 2 m alsopoint to SE, but a secondary NEeSW tendency is evident.

5. Discussion

5.1. Sedimentological data

It is necessary to emphasize that all presented interpretationsare related to a small area in relation to the great distribution of theMotuca Formation in the basin. According to the literature, depositsfrom more northern regions seem to represent dryer conditions.

Table 2Mean length (L), mean basal diameter (Dbasal), and mean distal diameter (Ddistal)of measured stems in each studied locality. Standard deviation informed in brackets,calculated with the statistical computer program R.

Locality L (m) Dbasal (cm) Ddistal (cm)

Peba 3.08 (2.15) 22.70 (8.79) 15.91 (7.08)Andradina 1.70 (1.14)a 16.58 (7.35) 11.75 (5.78)Buritirana 2.74 (1.98) 23.35 (11.88) 14.90 (5.65)

a Significant difference for confidence level of 95%.

The Parnaíba Basin presented typical intracratonic conditions,such as slow subsidence and respectively limited accommodationspace for sediments, what probably caused significant reworkingprior their final deposition. According to the fine-grained sedi-ments of the Motuca Formation (not coarser than medium sand inthe studied area), the tectonic conditions apparently were verystable and the sediment source areas of the basin were relativelyflat. The gradient of the basin certainly was very low and constant,particularly due to the fact that the continental depositional system(Motuca Formation) was established on shallow marine and lowrelief coastal deposits (Pedra de Fogo Formation). The continentalenvironments themselves possibly were located not far from thelast marine waters of an epicontinental sea, which was extendedfrom Panthalassa across Solimões and Amazonas basins towardsthe Parnaíba Basin. The final regression of this sea occurred duringthe Early Permian in parallel with the establishment or dryer cli-mates, what caused accumulation of evaporites in the three basins.

The climatic modifications occurred in global scale and beganwith the glaciation in southern Gondwana during the Carbonif-erous. In tropical regions, as in the Euramerican Floristic Province,humid conditions were replaced by climates with marked dryseasons. The decline of lycophytes and sphenophytes in the LateCarboniferous and the relative increase of recorded gymnospermsor tree ferns are ascribed to these changes (e.g., Falcon-Lang, 2003;Falcon-Lang and DiMichele, 2010). Tree ferns dominated the forests

Table 3Number of stems shorter and longer than 2 m in the 3 sites.

Peba Andradina Buritirana

Stems �2 m 19 59 25Stems >2 m 23 23 29

Fig. 7. (A) Rose diagrams of stem orientations in Peba, Andradina and Buritirana sites in TFTNM considering all measured stems of each area (n ¼ number of stems). (B) Rosediagrams of stem orientations in Peba Site. (C) Rose diagrams of stem orientations in Andradina Site and rose diagram of cross-stratifications found close to the fossils. (D) Rosediagrams of stem orientation in Buritirana Site.



in several lowlands during the Early Permian and probably wereopportunistic in the plant communities (Pfefferkon and Thomson,1982; DiMichele and Phillips, 2002; Rössler, 2006).

According to paleogeographic reconstructions for the LatePaleozoic (e.g., Rees et al., 2002), the studied area of the ParnaíbaBasin was located approximately at the southern boundary of thetropical zone and in a relatively dry belt, between the typicalEuramerican and South Gondwanan floras. Almost no data isavailable from other basins in similar paleolatitudes or paleocli-matic contexts. A rare record of the paleoequatorial region corre-sponds to the Lower Permian of Morocco, presenting evidences ofseasonally wet and dry phases (Hmich et al., 2006).

The evaporites and eolian dune deposits of the Motuca Forma-tion in central and northern locations of the Parnaíba Basin (LimaFilho, 1998) may have accumulated contemporaneously with thefluvial sediments of the studied area, but detailed stratigraphiccorrelations are lacking. The predominantly reddish colours of thesediments and the absence of coaly deposits rule out anoxicdepositional environments, such as swamps, which usually arebetter represented in humid contexts. The fern pinnules bearspectacular synangia protection, what must represent a xeromor-phic adaption (Tavares et al., 2008; Tavares, 2011). As in some othertropical/subtropical regions, the climate in the southern ParnaíbaBasin may have been warm semiarid with marked seasonality.

The permineralization of plants by silica and the silica layers inmudstones are themselves evidences of dry seasons, consideringthe lack of a coeval volcanic silica source and that silica is onlysoluble in high alkaline solutions (pH >10) favored under highevaporation rates of groundwater or pans (e.g., Tavares and Rohn,2009; Matysová et al., 2010). The precipitation of silica in plantcells certainly occurred because of very local pH drops related toCO2 releases by initial decomposition (e.g., Tavares and Rohn, 2009;Matysová et al., 2010; Tavares, 2011). The almost syndepositionalpermineralizations of the stems must have controlled the silica-cementation of the sands around the stems, forming the compactsandstone envelopes (Fig. 4). In the case of more flattened tree fernstems (usually observed in muddier deposits), the relatively wellpreserved root mantles in comparison to the deformed stelessuggest that the permineralization began in the external parts,whereas themost central parts were only silica-impregnated after asignificant burial, perhaps under less influence of the climate.

Taking into account the evidences of drought, it is necessary toemphasize, however, that the water was not too scarce, as impliedby the great height of the trees and their relative abundance.Intense storms certainly hit the region, considering the modernapparently similar climatic examples (e.g., comments in Guptaet al., 2002). Another possibility is that the tree ferns and theother plants regionally flourished in wetlands within an extensivedryland (e.g., revision of wetlands in drylands provided by Toothand McCarthy, 2004). This possible wetland must have been sus-tained solely by local rainfall, assuming as correct the interpretedwide-ranging dryness in western Pangea (e.g., Rees et al., 2002),where river inflows from more humid regions were almost out ofprospect. Obviously, broad scale paleoclimatic interpretations arealways subject of refinement.

The thick mudstone packages, locally with some minor inter-bedded sandstones, and the occurrence of bivalve coquinites indi-cate that permanent water bodies, such as lakes, existed duringcertain time intervals. They may represent higher base-levels, i.e.,more humid periods. According to investigations in Andradina Sitewithin the scope of this study, it was not possible to confirm alateral relationship between thick mudstones and sandstonesindicated in a columnar section of Dias-Brito et al. (2009). Thew20 m thick sandstone intervals rather seem to present greatlateral extension. Regarding the three sites, the sandstones may

belong to an equivalent stratigraphic range according to theirtopographic positions and their approximate alignment in a strikedirection of the basin.

In Andradina Site, the stacked sandstone bodies (Fig. 3BeE) arerelatively similar to fluvial architectures commonly referred to lowbase-levels in sequence stratigraphy models (for example, seerecent approaches in McLaurin and Steel, 2007; Huerta et al., 2011;Gibling et al., 2011). The consistent unidirectional orientation ofmedium-scale planar cross-stratification foresets in several stackedbeds (Fig. 3B and C) evidences relatively straight channels with(mid-channel) bedload composed of transverse bars (2-dimensional dunes). The close association between planar andsigmoidal cross-stratifications, their similar orientations, as well asthe reactivation surfaces, indicate variable flow strengths.

The trough cross-stratification sets with the deeply scouredbounding surfaces (Fig. 3E) represent sinuous- to lunate-cresteddunes (3-dimensional dunes), which replaced the straight-cresteddunes under relatively higher flow duration or strength (e.g.,Cheel, 2005). The foresets of trough cross-stratifications obviouslyconfer a little wider array of orientations (NEeSE) than the planarcross-stratifications (NE). The lenticular sandstone bodies (Fig. 3D)represent, in part, bars in transversal or oblique views, with troughcross-stratification and cut-and-fill arrangements. Bars apparentlycut into one another, but channel migration also occurred.

The thinning upwards tendencies of the sandstone bodiesindicate that each depositional episode occurred in shallowerconditions (i.e., less depositional space was available for depositionof bars or more erosion of their upper parts occurred). These suc-cessions were usually achieved when several depositional eventsgradually filled a channel.

Although estimates of fluvial channel water depths according tothe cross-set thicknesses are relatively inaccurate, it is interestingto mention that 60 cm high sets could represent dunes formed in10e17 m deep channels (see the method in Leclair and Bridge,2001). This range may be considered relatively normal in thecontext of floods recorded in the last decades or centuries in severalparts of the World. Extreme thick sets of the studied area (w1.1 m)would imply in depths of 25e42 m, which apparently are stillacceptable values considering heavy thunderstorm rains and verycatastrophic floods, as in some modern semiarid regions(Alexander et al., 1999; Gupta et al., 2002). The most importantprocesses in fluvial systems, as channel modifications and bar mi-grations, are related to storms. During normal “good” weathertimes, the channels in semiarid regions may be very shallow oreven disappear (e.g., Tooth and McCarthy, 2004; McLaurin andSteel, 2007; Hartley et al., 2010).

The thin mudstone beds or bands between sandstones obvi-ously represent sedimentation of suspended load in low energyconditions. The alternation of mudstones and mottled sandstonesare ascribed to flood events and subsequent subaerial exposuresand initial pedogenesis. The relatively low volume of mudstones inthe sandstone packages would be an evidence of reworking, butmudclasts or mudbreccias should be more abundantly recorded inbasal portions of sandstone bodies. It seems more reasonable tointerpret that the rivers had a relatively little mud in suspension,considering the assumption of relatively dry climate and conse-quently relatively low proportion of available muds. In drylands,some floodplain deposits may be indistinguishable from channelbars because of the very similar sandy grain size (North et al., 2007).

The combination of all characteristics, mainly the constantorientation of cross-strata of several stacked sets, is consistent withrecords ascribed to high discharge intervals in braided rivers (e.g.,revisions in Mount, 1995, McLaurin and Steel, 2007; Hartley et al.,2010). Thus, multiple channels repeatedly diverged, joined,migrated and locally or periodically disappeared, with relatively

Fig. 8. Schematic drawing og Permian plant communities with tree ferns.


low sediment transport capacity and low proportion of depositedmud. The vegetation in the river region was not enough dense orresistant to avoid lateral expansion of channels during events ofmajor discharge (Mount, 1995). Flooding was also caused directlyby the part of rain water that was not infiltrated into the substrate(see North and Davidson, in press). Thunderstom rains may havelead to severe flash floods in large areas in a few hours (modernexample described in Gupta et al., 2002). Unconfined strong flows,perhaps some meters deep, may have developed parallel to themain channels, especially in areas without significant roughnesselements as vegetation, and possibly caused expressive sandaccumulation when the water began to recede (see Tanner andLucas, 2007; North and Davidson, in press). After major flowevents, especially during dry seasons, the last waters may havespilled across floodouts oriented approximately inline with thechannels (as examples in Tanner and Lucas, 2007; Tooth andMcCarthy, 2004; Hartley et al., 2010; North and Davidson, in press).

Hartley et al. (2010) commented that most of the fluvial fill incontinental sedimentary basins is related to distributive fluvialsystems (including large scale fluvial megafans), which typicallypresent confined proximal flows and expand laterally to largerareas with large volume of unconfined flows. Several parameters asgradients, climate and tectonic setting control the planform types(e.g., from simple braided channels to multiple sinuous channels),the apexetoe length (up to 700 km) and the termination type(rivers, coast, dune fields, playa lakes, permanent lakes or wet-lands). The gradient may be very low, such as in cratonic settings(Hartley et al., 2010). Up to now, the data of the studied area areinsufficient to admit a megafan control of the sedimentation, butthis possibility cannot be completely discarded.

The great abundance of tree fern fossils indicates that theseplants obviously lived in areas prone to floods, relatively near tofluvial channels, and that they were the most successful elementsof a possible riparian community. The sphenophytes and very rarelycophytes probably occupied moist areas adjacent to channels aswell, but they were secondary elements of the vegetation. Differ-ently from modern tree ferns, the possibly 15 m high Permianplants of the Parbaíba Basin dominated the forest canopy andafforded some shade for other ferns, sphenophytes, lycophytes andpresumed small gymnosperms. However, the canopy may havebeen relatively open. Gymnosperm stems with large diameters arevery rare in the fossil assemblages because they probably corre-spond to mesophylous plants, which lived further away from therivers (Fig. 8).

The tree ferns had no effective anchoring roots and their rootmantle probablywas not deeply fixed to the substrate. No plant wasfound in the original growing position. They certainly were notadapted to survive periodic inundations by fast-flowing water suchas some modern plants living in channels of sub-humid regions inAustralia (Fielding and Alexander, 2001). Even relatively weakflows may have been sufficient to fell the tree ferns. In addition,these plants grew slowly and probably took decades to reachheights of 10e15 m, according to modern examples (Mehltreteret al., 2010). Therefore, large floods were catastrophic for themand not a predictable stress factor.

5.1.1. Assemblages and sitesBuritirana Site has the most abundant tree fern stems with

preserved proximal bulbous root mantles (sometimes includingepiphytes), the most frequent pinnae adhered to stems (or insandstone envelopes around stems) and apparently the highestpreserved plant diversity. From the measurements resulted themost consistent rose diagramwith apical parts of stems pointing toSE. According to these taphonomic attributes, several tree fernsmust have only fallen, without effective transport, when they were

affected by fast-flowing water during high discharge events. Theirapical parts were turned to downstream. An additional probableevidence of this hypothesis is the mode of occurrence of the asso-ciated leaves: some stems probably fell on plant litter or on frondsstripped off during the storm, what promoted protection of theseplant fragments against transport and further breaking.

The fallen stems in Buritirana Site were buried by sands, but notdeeply, because they were not flattened or only very little com-pressed, as discussed previously. The sediment covering avoidedtheir quick destruction by herbivorous and saprophagous organ-isms. The probable very alkaline “toxic” conditions establishedduring the drier periods, which caused the almost syndepositionalfossilization of the plants, probably contributed to keep the or-ganisms away.

The almost autochthonous to para-autochthonous petrifiedstems of Buritirana Site seem to give reliable evidence about theoriginal community (paleobiocenosis) and the living environment(Martín-Closas and Gomez 2004). Although very few leaves werepreserved and no other plant organs were found (as seeds), it isrelatively safe to reaffirm that the arboreous plants of the originalhygrophylous community were largely dominated by T. singularisand that this disequilibrium of composition is not an artifact oftaphonomic bias. Thus it is also restated that the community musthave lived under hydric stress (Tavares, 2011).

In comparison to the Buritirana Site, the Andradina and Pebasites present less abundant basal bulbous root mantles, shorterstems, much rarer leaves and even lower species diversity, althoughgymnospermous stems are more abundant in the Andradina Site. Ingeneral, the transport of the stems of these two sites was appar-ently longer/more distant. The plants may have entrained in waterby undercutting and collapse of riverbanks, where they lived, or inconsequence of uprooting by fast-flowing water in floodplains (seeAlexander et al., 1999). The entire tree ferns were toppled andprobably the same have occurred with other plants. Some gym-nosperm branches probably broke and were delivered to the waterseparately from the stems. The plants floated in water and showeddifferent behavior according to their shape, size, buoyancy (den-sity), presence of branches, roots and/or basal root mantles, as wellas the size and sinuosity of channels, the water depth and velocity,and other parameters (Macdonald and Jefferson, 1985; Alexanderet al., 1999; Baudrick and Grant, 2001; Gurnell et al., 2002; Abbeand Montgomery, 2003; Saldi-Caromile et al., 2004).

In analyses about transport of floating stems in fluvial channels,Baudrick and Grant (2001) classified stems larger than 1 m lengthor 10 cm diameter as “large”, independently from the size of thechannel. They emphasized the sizes of stems because larger/heav-ier wood pieces have higher momentum, being less disturbed byfrictional resistance of shallow banks and other local slowing speedfactors. Using this criterion, all measured stems in the study areamay be considered as “large”. Another important factor in thetransport of floating stems is channel size (Baudrick and Grant,2001; Gurnell et al., 2002; Abbe and Montgomery, 2003). Chan-nels should be considered “large” if they are wider than the lengthof the stems and interaction between each other is low (Gurnell


et al., 2002). In small channels, thewoodmobility or rotation can bevery limited. Braided rivers, characterized by complex smallchannels separated by sandbars may become large channels duringfloods (Gurnell et al., 2002). The transport of stems usually tend tostop when thewater depth is approximately half the stem diameter(Baudrick and Grant, 2001), but this factor also depends upon theirshape and density (Gurnell et al., 2002).

Straight stems stripped of leaves, branches and roots, floating inrelatively large channels, normally are aligned parallel to flow,particularly parallel to the thalweg, where the respective superficialwater reaches themaximum speed (Macdonald and Jefferson,1985;Wnuk and Pfefferkorn, 1987; Alexander et al., 1999; Baudrick andGrant, 2001; Gurnell et al., 2002; Saldi-Caromile et al., 2004). Sucha transport is presumed for the tree fern stems of the studied area. Ifthe root is still attached to the stem, inertial forces cause positioningof theheavierend to theback, i.e., to upstream. In shallowconditions,the rootmayanchor to the bedload. The basal bulbous rootmantle ofthe studied tree ferns probably conditioned similar situations.

Irregularly shaped stems or the presence of branches imply in amore chaotic transport, increased probability of frictional dragexerted on the riverbanks or possible trapping in the bedload(Gurnell et al., 2002; Saldi-Caromile et al., 2004). Anchored trees orbranches may represent obstacles for other floating stems andpromote gradual increase of trapped plant material (namely log-jams), including leaves, seeds and other small fragments(Alexander et al., 1999; Gurnell et al., 2002). Up to now, in thestudied area, no great accumulations of chaotic stems of differentsizes were recorded.

Depending on the plant and the flow conditions, breakage ofbranches or even of the stems may occur (Gurnell et al., 2002).Older plants break more easily than young ones during transport(Baudrick and Grant 2001). Differently from several stems of Bur-itirana Site that would have only fallen, two rounded distal ends oftree ferns (Fig. 5B) probably represent abrasion caused by collisionsagainst shallow bars during transport. In this case, the frondscertainly were stripped off. One of them (Fig. 5Ba), according of thecircular and not square transversal section, evidences that the mostapical part must have been broken up. But stems deposited withattached fronds are not ruled out (see the final part of this dis-cussion). Lateral abrasion of stems was not observed.

5.1.2. Paleocurrents and orientation of logsSeveral studies about modern rivers (e.g., Alexander et al., 1999)

demonstrated that peak flow conditions last short time (less thanone day). During the falling stage, many stems are stranded on thebar tops or channel margins (on point bars in the case ofmeandering rivers) and on floodplains. The previous orientation ofstems parallel to the flow is usually maintained when they arestranded (Alexander et al., 1999; Baudrick and Grant, 2001; Gurnellet al., 2002). Attached roots to stems usually point upstream(Gurnell et al., 2002). Partial burial of the stems by sand occurs atthe time of receding floodwaters or soon after, but later some banksmay become unstable and small plant material can be reworkedand reoriented (Alexander et al., 1999).

The rose diagrams for long stems (>2 m) of Andradina and Pebasites demonstrate a strong alignment tendency, what certainly isrelated to orientation parallel to flow during peak flow recession,after deposition from suspension, probably in a braided channelsystem. If the channels were meandering, greater array of stemdirections would be expected (Gurnell et al., 2002). Some rare longstems in the studied area lying perpendicular to many parallelstems, probably represent trapping because of obstacles (the ownparallel stems may have constituted obstacles).

Rose diagrams constructed for both long (>2m) and short stems(�2 m) show greater azimuthal range, what can be related to

rotation of the lighter stems lying loosely on the ground or tooriginal not parallel orientation to flow because of their lowermomentum and consequent greater influence of local water tur-bulence, small obstacles and other obstructions (Baudrick andGrant, 2001). An additional factor in the case of the AndradinaSite is the higher proportion of gymnosperm stems, which presentamore irregular forms and lower tendency of orientation parallel tothe flow.

The rose diagrams for stems of the Andradina and Peba sitesreveal an interesting fact: approximately half of the stems (>2 m)presents the apical part pointed to the opposite side than the otherhalf. In Andradina Site, the stems with distal parts oriented towardsthe same direction than the cross-stratification sets (NE) obviouslypoint downstream. The tree stems oriented to the opposite side(SW) must have presented heavier distal parts during transport(considering the inertial forces controlled by the relativeweight). Inthis case, one interpretation is that the floating stems still pre-sented attached large fronds, independently from attached or notbasal bulbous root mantle. Another alternative is that the plantslost all fronds and the root mantle as well, in order that the almostcylindrical stem fragments had the center of mass positionedapproximately in the middle, enabling orientation by chance todownstream or upstream. These interpretations are extended tothe bipolar stem orientations in Peba Site, where the main flowprobably was towards east. If the fronds were attached to the stemsduring transport and deposition, theymust have been decomposedshort time after accumulation. The sand cover probably was notsufficient or too coarse for protection against decomposers. Thestems certainly were more resistant than the fronds in respect todecay. However, at least petioles should be found, as some exam-ples of Buritirana Site.

Differently from many authors, Gastaldo (2004) criticized theuse of woody phytoclasts as indicators of paleocurrent trends influvial systems. This mistrust arose when he observed Paleogenestems in Germany in oblique and perpendicular positions to thepresumed paleocurrents. However, a significant fact is that thesestems are preserved within trough cross-beds. Therefore, an alter-native hypothesis is that they did not floated, but werewaterloggedand belonged to the bedload, lying in troughs between sinuouscrested channel dunes.

In the studied area, some exceptions concerning floating wereapparently recognized. One example is the previously mentionedsmall fragmented gymnospermous stem at the base of sandstonecross-stratification sets in Peba Site. It was probably waterloggedand transported as bedload. Another example is the sinuouslydeformed tree fern in Andradina Site (Fig. 5C). An enhanced flexi-bility of the stem fibers probably resulted from waterlogging. Thedeformation may have occurred during transport of the stem asbedload. An informal “home-experiment” was done during thepresent study with a small, initially dry, adventicious root fragmentof a modern tree fern: it floated during eight days in a water boxbefore it began to sink. Assuming that the plants in Parnaíba Basinwere removed abruptly from the living place by storms, they musthave had enough time to drift before they were waterlogged.Gurnell et al. (2002) commented that “streams that experiencelarge floods would be expected to have little decayed wood”. Thegeneral similar preservation of cells and other structures of severaltree fern stems of the studied area (Tavares, 2011) suggests thatthey were fossilized in equivalent conditions and discards timeaveraging or distinct decay stages before permineralization. Afterthe silica impregnation, but still in the depositional environment,the large stems were too heavy to have been reworked.

Rare discrete concentrations of small stem, rachis and otherfragments in loose thin sandstone blocks on the ground (Fig. 5F)may correspond to brokenparts of envelopes around stems (such as


those discussed above) or possibly are strandline deposits formedduring falling stage water level on emergent bars (see detailedcharacterization of strandline deposits in Alexander et al., 1999).Better records of these fragmented assemblages are required formore accurate interpretations.

The cross-stratifications and the large stem orientations in thestudied area indicate different paleocurrents from one site toanother: mainly to east in Peba Site, northeast in Andradina Siteand southeast in Buritirana Site. Considering the three aligned sitesin westeeast direction, the general paleocurrent trend is towardseast, i.e., from the Peba to the Buritirana sites. Variations of channeldirections are expected even in braided fluvial systems in a scale oftens of kilometers (McLaurin and Steel, 2007; Hartley et al., 2010).Usually, low gradient areas present sinuous channels, but a highsediment supply, such as in relatively dry regions, determines theoccurrence of braided planforms (Hartley et al., 2010).

The general sedimentary and fossil records of the most distalarea (Buritirana Site) apparently represent deposition related toweaker flow conditions than in the other areas. The paleoenviron-mentmayhave corresponded tofloodplains or to laterallyexpandedchannels with respectively decreased discharge. In addition, loss ofenergy of the flowing water may have occurred downstream if thestorms affected preferentially areas located upstream. In contrast, inmore proximal areas (best recorded in Andradina Site), the abun-dant sandstones with cross-stratifications, as well as the relativelyhigh proportion of gymnosperm stems, are respectively compatiblewith higher energy channel environments and very wide floodsaffecting the mesophylous vegetation.

6. Conclusions

Few previous works about stem orientation could be carried outwith so a high number of specimens and so a large areal distribu-tion as in the studied area of Parnaíba Basin. Most of the approachesrelated to this topic refer to modern fluvial environments. No otherLate Paleozoic assemblage of permineralized stems contains so ahigh proportion of tree ferns (especially Tietea), additionally withso long lengths. Particularly in relation to the Permian of the Par-naíba Basin, no previous detailed work has addressed the bio-stratinomy of fossil plants and only very preliminary sedimentarystudies of the fossil-bearing deposits were formerly presented.

The data set, including information from the literature, reinforcethe interpretation that the studied region had semiarid climatewithmarked seasonality, low relief (including the adjacent areas of thebasin) and a braided fluvial system mostly controlled by rain waterdischarges. The tree ferns belonged to a low density riparianhygrophylous vegetation and the majority of the gymnospermsprobably were mesophylous and lived a little further away from theriver. The largefloods thatpromoted theplantpreservationprobablywere exceptional and not periodic (e.g., not once a year) because thetree ferns probably took long time to reach great heights.

In the present study, it was possible to recognize stems that onlyfell, sometimes covering leaves and other plant fragments (mainlyin the most downstream studied area), and those stems thateffectively were uprooted, drifted in floating condition parallel tothe flow and deposited during falling water stages. The upstreamareas of the studied region (at west) presented the most intenseflows, taking into account the higher proportion of allochtonousstems and of gymnosperms, the latter derived in part from themore distant floodplain settings. Some rare stems probably werewaterlogged and transported as bedload.

Among the stems deposited after transport in suspension, somepresent the distal part pointed to downstream and others in theopposite position. This orientation probably depended on thepresence of attached heavier organs at one or both ends of the

stems, such as the bulbous basal root mantle of the tree ferns(commonly still connected to stems) or the very large fronds (notfound in organic connection to the stems, but possibly decomposedafter deposition). Another alternative is that the almost cylindricaltree fern stems, already devoid of heavier organs attached to theirends, drifted parallel to the flow without any preferentialorientation.

In conclusion, the predominantly parallel orientations of thestems give reliable indication of the river direction during highdischarges in the studied area. The mean direction of the drainageprobably was to east. The amount of collected data permitted tounderstand and provide important findings about the weatherpatterns and the environment of that region in Lower Permian.

Acknowledgements

Thanks are expressed to CNPq, FAPESP and PRH-05 (ANP-UNESP) for financial support, as well as PhD grants for the firstauthor (CNPq and FAPESP), and to Naturatins (“Instituto daNatureza do Tocantins” of the Tocantins State Governant) andDNPM (“Departamento Nacional de Pesquisa Mineral”) for allowingaccess to the “Tocantins Fossil Trees Natural Monument”. Natur-atins functionaries generously provided accommodation in Bie-lândia during one field study and helped in the form of logisticsupport. SEPLAN (“Secretaria do Planejamento” of the TocantinsState Governant) contributed cordially loaning a car during the firstfield-trip. The authors particularly thank Tatiane Marinho VieiraTavares for the outstanding field assistance and constructive sug-gestions during the whole research at UNESP-Rio Claro. RonnyRössler, Robert Noll, Dimas Dias-Brito and Joel C. de Castro arethankfully acknowledged for the fruitful discussions and for someimages kindly made available. The authors appreciated the addi-tional literature provided by Mario Assine and the partial trans-lation of the Portuguese text to English by Howard-Peter KombrinkDavies. Additional thanks to the anonymous reviewers for thesuggestions made to the draft.

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