Pillow lavas and fluvio-lacustrine deposits in the northeast of Paraná Continental Magmatic Province, Brazil

Accepted Manuscript Pillow lavas and fluvio-lacustrine deposits in the northeast of Paraná Continental Magmatic Province, Brazil

Lucia Castanheira de Moraes, Hildor José Seer PII: DOI: Reference:

S0377-0273(17)30176-2 doi: 10.1016/j.jvolgeores.2017.03.024 VOLGEO 6054

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Revised date: Accepted date:

9 June 2016 10 February 2017 20 March 2017

Please cite this article as: Lucia Castanheira de Moraes, Hildor José Seer , Pillow lavas and fluvio-lacustrine deposits in the northeast of Paraná Continental Magmatic Province, Brazil. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Volgeo(2017), doi: 10.1016/j.jvolgeores.2017.03.024

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ACCEPTED MANUSCRIPT PILLOW LAVAS AND FLUVIO-LACUSTRINE DEPOSITS IN THE NORTHEAST OF PARANÁ CONTINENTAL MAGMATIC PROVINCE, BRAZIL Lucia Castanheira de Moraes Hildor José Seer * CEFET-MG – Centro Federal de Educação Tecnológica de Minas Gerais Av. Min. Olavo Drumond, 25, Bairro Amazonas, Araxá, Minas Gerais – Brazil * Corresponding author

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E-mail addresses: [email protected] (H. J. Seer) [email protected] (L. C. Moraes);

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Abstract

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The northeast edge of the Paraná Continental Magmatic Province was marked by a topographical barrier that gave rise to a predominantly peridesertic environment. This high allowed the recurrent of above-average rainfall in this area of the Province, in contrast with other areas where many evidences indicate that lava flows have settled in an arid climate, in a desert environment. As a result, alluvial-fan debris flows and fluvio-lacustrine systems developed occasionally. This paper presents one of these sequences, which is remarkable for the presence of sedimentary rocks and two sets of pillow lavas. Both sets of pillow lavas show interaction with the fluvio-lacustrine sediments with distinctive grain size (from pebbly sand to clay) that locally preserve the original sedimentary structures such as bedding and lamination. This interaction gives rise to peperites, which show a diversity of textures for different sedimentary granulometries. Hyaloclastites and fragments of lava also occur in the interpillow material and may be intricately mixed with sediment. These features, based mainly on detailed field observations in the northeastern region, demonstrate that the palaeorelief, sedimentary environment, stratigraphy and morphology of sedimentary units and basaltic flows are contrasting from the central and southern regions of the Paraná Continental Magmatic Province.

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1. Introduction

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Keywords: Paraná Continental Magmatic Province, fluvio-lacustrine environment, pillow lavas basalt, Triângulo Mineiro

The Paraná-Etendeka Province (PEP) occupies an estimated area of 1,200,000 km2 (Renne et al., 1992) distributed across Brazil, Argentina, Uruguay, Paraguay; in addition, it covers an approximate area of 100,000 km2 in Namibia (Peate et al, 1992) (Fig. 1). This province is known in South America as Paraná Continental Magmatic Province (PCMP) and has been cited in the literature since 1878, when Derby referred to the basalts as the Paraná Trapp (apud Mincato, 2000). In Brazil these basalts are stratigraphically known as Serra Geral Formation. The Serra Geral Formation overlaps the aeolian deposits of the Botucatu Formation and, at the basal portion of the Serra Geral Formation, lava flows are interlayered with sandstones. Both formations are part of the São Bento Group. Typically, continental flood basalts provinces (CFBPs) extrude huge volumes of basaltic lava, especially that with tholeiitic affinity, in a limited period of time and in

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2. Geological Context

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a continental environment from fissures systems in the Earth’s crust (Hooper, 2000). According to Jerram & Widdowson (2005), the CFBPs have a variable rate of effusion throughout their lifespan, with an average duration of 10 Ma, and 70% of the products may be formed between 0.5 and 5 Ma. Their dominant morphological and structural characteristics are lava flows that are hundreds of kilometres long, with a thickness of dozens of meters. CFBPs were explained by a model – accepted and adopted here – involving inflated pahoehoe flows with low rates of eruption (Hon et al., 1994; Self et al., 1997, 1998), well documented in the CFBPs of the Deccan, the Columbia River, and the Paraná-Etendeka (Self et al., 2008; Reidel & Tolan, 1992; Waichel, 2006). Furthermore, ignimbrites, a’a flows, volcaniclastics linked to phreatomagmatic activity, peperites, and pillow lavas could be observed in CFBPs, albeit in a subordinate manner (Jerram & Widdowson, 2005). Pillow lavas were restricted to regions where magmatic eruption took place in a subaqueous environment; nonetheless, its formation depends on the rate of lava emission (Carracedo Sánchez et al., 2012). Examples of pillow lava in CFBPs are given by McClintock et al. (2008), in Karoo; Xu et al. (2004), in the Binchuan section of Emeishan; Kerr (1995), in the North Atlantic Igneous Province; Swanson & Wright (1981) and Hooper (1997), in the base of the Columbia River (all apud White et al., 2009); and Ferreira (1985), in the Serra Geral Formation (PCMP). In the northern portion of the PCMP a systematic work of understanding the spatial distribution, sediments/basalts interaction and geological structure of basaltic flows has been developed (eg, Machado et al., 2007; Fernandes et al., 2010; Pires et al, 2011; Machado et al., 2015; Pedrosa Soares, 2017). Also, the understanding of the geological structure in which volcanism was installed has advanced. In light of new data the area researched by Ferreira (1985) was revisited. The presence of pillow lavas as well as fluvio-lacustrine deposits in the Parana Continental Magmatic Province (PCMP) is discussed.

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The current level of knowledge indicates that the PCMP originated in the upper portion of the Paraná Basin, an intracratonic and poly-history basin, in the interior of a large continent dominated by an arid climate (Scherer, 2002; Jerram & Stollhofen, 2002). The PCMP was erupted at a time when the continent was disaggregating through a process that initiated the opening of the South Atlantic ocean (Moulin et al., 2010; Turner et al., 1994). The magmatism consists of approximately 90% tholeiitic and andesitic basalts (apart from dykes and sills), approximately 3% rhyolites and rhyodacites, besides andesites (~7%) (Marques & Ernesto, 2004). From the lithogeochemical point of view, Peate et al. (1992) subdivided mafic magmatism into six types (Fig. 1): Urubici, Pitanga, Paranapanema (high TiO 2) and Gramado, Esmeralda, Ribeira (low TiO2). In turn, felsic magmatism comprises the Palmas (dacites and rhyolites with low TiO2) and Chapecó types (dacites associated with high TiO2 basalts). Based on data obtained by Renne et al. (1992), Thiede & Vasconcelos (2010), and Janasi et al. (2011), the age of the province is between 134.5 and 131.5 Ma, and it is in concordance with a maximum of four magnetic polarity inversions (Ernesto & Pacca, 1988; Ernesto et al., 1999). According to Thiede & Vasconcelos (2010), the frequency with which the magnetic poles reversed polarity in the period between 140 and 120 Ma, supports extrusion in a period less than 1.2 Ma. Janasi et al. (2011)

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stressed on the fact that felsic volcanic rocks are more common in the PEP than in most of the other provinces. Moreover, they occupy a key position in the stratigraphy of the lavas, in such a manner that the use of dating techniques involving highprecision U-Pb from baddeleyite/zircon combined with stepped heating of 40Ar/39Ar may provide a breakthrough in estimating the absolute age and duration of magmatism of this province. Furthermore, the understanding of the emplacement and volcanological aspects of the PCMP (Araujo, 1982; Jerram & Stollhofen, 2002; Waichel, 2006; Licht et al., 2012; Lima et al., 2012; Machado et al., 2015) and its relations with the Alkaline Magmatic provinces present in its borders is advanced (Brod et al., 2000; Brod et al., 2004; Gomes & Comin Chiaramonti, 2005; Ernesto, 2005; Marangoni & Mantovani, 2013). The regional location of the study area is shown in Figure 1. Here, in addition to the geographical position, the edges of PCMP, structural features associated and the stratigraphic log for the Triângulo Mineiro region, Minas Gerais state, in the northeast of the Province is showed in Fig. 1B. In this area the Paraná Basin overlays Neoproterozoic metasediments and metaigneous rocks of the Brasilia Belt. It is represented by the Lower Cretaceous São Bento Group, with Botucatu and Serra Geral Formations. Above these units formed the Upper Cretaceous Bauru Basin that consists of fluvio-lacustrine sedimentary rocks. The Brasilia Belt is an NW/SE structure and represents the NE edge of the Paraná Basin, forming a high relief, at least since the Lower Cretaceous, and reactivated tectonically during the Upper Cretaceous and Cenozoic, forming a regional topographic high called Alto Paranaiba Arc. The Alto Paranaiba Arc hosts a series of dykes with NW direction – potential feeders of the PCMP volcanism - and alkaline-carbonatite bodies of the Upper Cretaceous (Fig.1C). The directions of the dykes were controlled by NW lineaments, inherited from the Neoproterozoic structure of the region. It was the source area of sediments of the Botucatu Formation and sedimentary rocks of the Bauru Basin and served as a geographic barrier to the advance of the dunes and lavas of the São Bento Group.

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Fig. 1. A) Geological illustration of the Paraná Basin with its sedimentary limits and magmatic rocks distribution overlapped by the Bauru Basin. X indicates basalts and sills, square indicates sample OU09 and + indicates basaltic dikes 40Ar/39 Ar dated by stepped heating (Janasi et al., 2011). In detail, sketch of the PEP. Modified from Janasi et al. (2011) and Waichel (2006). B) Geologic map of the Triângulo Mineiro region; 1, 2, 3 and 4 are geologic sections represented in Figure 2. C) Map of Magnetic Analytic Signal with main cities of the Triângulo Mineiro region: homogeneous anomalies = basalts and sedimentary rocks; circular anomalies = carbonatite-alcaline bodies; linear anomalies = mafic dykes genetically linked to Paraná basalts. (B and C modified from Pinto & Silva, 2014).

The geological cross-sections of Figure 2 illustrate an irregular palaeotopography higher toward northeast, in direction of the Alto Paranaiba Arc. The sections also show the thickness variation of basalts and sandstones and the relationships between them. The São Bento Group begins with the Botucatu Formation (extensive dune fields in a dry wind system) filling paleo valleys, and with thicknesses varying considerably from less than two to 110 meters. Sections 1 and 2 (Sacramento region) show some dunes over the basement and several aeolian sandstone lenses interlayered with basalt. Between Sacramento and Uberaba (Section 3) occur no basal dunes, but just many aeolian sandstone lenses interlayered with lava flows. The Section 4 (Uberlândia-Araguari region) shows just

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two small dunes to the northern of Araguari city. By contrast, lenses of alluvial fan and lacustrine deposits are present. Alluvial fans occur over the basement as conglomerate levels in recurring canalized shapes and clasts size decreases upwards from 2 to 0,2 cm. Clasts of neoproterozoic lithologies (quartzite, augen gneiss and, biotite-quartz schist) are present with low roundness, together with sand-grade quartz grains typically formed by wind transport, with a high degree of roundness, frosted surface, surrounded by hydroxide iron films. These conglomerates are dominantly matrix supported and can achieve about one meter thick. They transition to different levels of sandstones and mudrocks formed in lacustrine environment. Alternatively, they are covered by basaltic flows. Geologic cross section number 4 shows four lenses interpreted like lacustrine deposits. They are limited in area and thickness – with at least 50m long and 2 meters thick – and cover alluvial fans or are interlayered with basalt flows. The lacustrine sequence is composed by mudrocks and sandstones. Especially at the base, coarse sandstone granule-rich is common. Layers of sandstone show abrupt contact with layers of the underlying mudrock and they gradually pass to finer grain rocks toward the top. Its composition is dominated by clays and quartz (grain size between silt and coarse sand) with rare basalt in grains and/or granules and detrital mica. The dominant sedimentary structures are lamination and cross lamination and the colors vary between green and pale green. Ostracod fossils were found in three of the lacustrine deposits and fossilized conifer woods were described in Botucatu Formation, near Uberlândia city (Pires et al., 2011).

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Fig. 2. Regional geologic cross-sections (see figure 1B) representing the paleorelief of the NE limit of the PCMP and the relation between basalts from the Serra Geral Formation and aeolian (+), alluvial (o) and lacustrine (*) dominant sediments of the Botucatu Formation. Note the presence of fossilized wood at the base of Botucatu Formation in section 4.

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Similarly the lavas filled topographic depressions and free spaces between dunes, either directly on Neoproterozoic lithologies, and advancing to cover higher dunes. The preserved thicknesses of lava flows are between 10 and 200 meters. The contact between the São Bento Group formations is concordant and can show interaction between lavas/sand in the shape of grooves produced by basaltic flows on the dunes surface, peperites/pseudopeperites and degassing pipes in sandstone. Sandstone interbedded with basaltic flows can vary from about 3 centimeters to a 10 meters, indicating the intermittent nature of the two processes. However, these intercalations and their recurrence indicate that aeolian processes and volcanism occurred simultaneously. 3. Results Among the authors who contributed to the understanding of the volcanic pile structure, Ferreira (1985) calls attention for describing, for the first time, the occurrence of pillow lavas in the Serra Geral Formation. The area described is situated between the cities of Uberlândia and Araguari, along the Central Atlantic Railway (FCA), in a location known as Fundão (Fig. 3). The paleorelief in the region was highly irregular at the time volcanism began, and elevated to the north and northeast, where Neoproterozoic lithologies from the Brasília Belt (Seer & Moraes, 2015) prevail in a structure known as the Alto Paranaíba Arc (Fig. 1 and Fig. 2). The

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area and its surroundings were revisited and the stratigraphic succession along the FCA is shown in Figure 4. The irregular surface of the Neoproterozoic lithologies are covered by the Botucatu Formation represented by a succession of conglomerate, gravelly sandstone, clayey sandstone with granules and mudrock, with a tendency for granule decrescence to the top, generating cycles. These cycles (Fig. 4 and Fig. 5A) have irregular thicknesses (< 1.5m thick) and tend to be lenticular; they overlap themselves vertically and laterally randomly. The conglomerate is polymictic, immature matrix supported and dominated by sub-angular to sub-rounded granules and pebbles of quartz, quartzite, gneiss and biotite-quartz schist. The matrix of the conglomerate includes fine to medium grain of well-rounded quartz, which tend to be spherical and have a film of iron hydroxide/oxide covering its surface; sub-angular quartz grains with granulometry varying from very fine to coarse sand; detrital muscovite; and feldspar. The gravelly sandstone and clayey sandstone can show trough cross stratification and the mudrock shows, locally, mud cracks on the surface. This set has colors ranging from pinkish cream to red and is friable.

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Fig. 3. Geological map of the Fundão area (regional location on figure 1B). Waypoints related to the stratigraphic sequence of figure 4 are shown by circles indicated in black. FCA is the Central Atlantic Railway and Br050 is a federal highway that links the cities of Uberlândia (to the south) and Araguari (to the north). UTM metric grid WGS84 Datum.

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Fig. 4. Stratigraphic log in the FCA Railway (waypoints 1 to 10 in Figure 3). The contact between the Botucatu/Serra Geral Formations and the Neoproterozoic lithologies is erosive and the first one occupy topographic depressions in the metamorphic rocks, where conglomerate and sandstone are deposited. The Botucatu Formation is not very thick in this region. The basalts are predominant at the top, where they interact with lake sediments.

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The sediments were covered by lavas from the Serra Geral Formation, which shows a chilled margin (Fig. 5B) and weak interaction between them. The first flow has a thickness of 25 m, in discontinuous exposure, and is fractured and weathered. Locally, there are irregular columns that are slightly inclined and narrow (<30 cm), as well as intervals with intense horizontal fracturing in which the weathering is more intense (Fig. 5C). The basalt is fine to very fine grained, with a greenish dark gray colour, and consists of microphenocrysts of plagioclase and pyroxene, and, less commonly, olivine. The mesostasis is dominated by crystals of plagioclase, pyroxene, ilmenomagnetite and magnetite, with interstitial glass and irregular microvesicles (<2 mm), defining dickitaxitic texture. Near the top the vesicular amygdaloidal level is irregular, with a maximum thickness of 20 cm. The vesicles and amygdales have a diameter between 0.5 and 1 cm and can be spherical or flattened with their filling varying between silica, calcite, zeolites, and celadonite, which is predominant (Fig. 5D). When hidrothermalized, the colour is a striking greenish gray with a powdery appearance. Fig. 5. A) Alluvial fan matrix supported conglomerate and sandstone from the Botucatu Formation (waypoint 2 in Figure 3). B) Irregular contact between lava flow (Serra Geral Formation) and conglomeratic sandstone (Botucatu Formation); note the light green layer at the base of the lava flow that represents rapid quenching of the lava; arrow show sandstone fragment involved by the lava (scale is 8cm longer). C) Horizontal jointing in weathered basalt near waypoint 4 in Figure 3. D) Amygdaloidal basalt at the top of a lava flow at waypoint 4 in Figure 3 filled with celadonite and calcite.

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The irregular surface of the first basalt flow is covered by a sedimentary sequence that begins with a discontinuous layer of conglomeratic sandstone with basalt pebbles and up to 30 cm thick. It transitions to a layer (up to 50 cm) of medium-sized sandstone, locally with granules, rich in quartz and detrital muscovite, greenish gray, and with cross lamination. This layer is followed by a fine, rosy, laminated or friable sandstone layer whose thickness is less than 15 cm (Fig. 6A and Fig. 6B). The set is laterally persistent for at least 200 m and covered locally by a decimetric and weathered lava flow. It is superimposed by a thin and discontinuous lenticular layer of green, massive, fossiliferous (ostracodes) mudrock with quartz druses (Fig. 6C). Laterally, in the northeast direction of the railroad an exposition of pillow lavas with a thickness that varies between 5.7 and 7.4 m deforms the sediments at the base (Fig. 6D). The pillows are well preserved and sized between 30 to over 150 cm and present chilled margin rich in volcanic glass (Fig. 6E). The center may be hollow or filled, and internal radial fractures (Fig. 6F) and in triple junction on the external extremity are common. The presence of multiple crusts was identified representing flow finalization caused by the interruption of magma supply (Fig. 6G and Fig. 6H). The development of four bulbs in a pillow lobe can be observed indicating a lava flow to the northeast (Fig. 7A). The flow direction in this case is 70º. Other measurements of pillow lavas flow direction, for example Figure 7B, indicate flow to NE.

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Fig. 6. A) Lacustrine sediment layer interbedded with lava flows at waypoint 4 in Figure 3. B) Sandstone facies of lacustrine origin with pervasive cross-stratification below the pencil at waypoint 3 in Figure 3. C) Argillaceous facies of lacustrine origin with vesicles filled with quartz at waypoint 4 in figure 3. D) Contact between lava and sediment at waypoint 5 in figure 3; note the intense deformation of the sedimentary layers over the hammer with injection feature. E) Pillow lobes with chilled margin rich in volcanic glass (green) and interpillow material (clayey sediment plus volcanic glass and volcanic fragments) at waypoint 7 in figure 3. F) Pillow lobe with radial joints on the border and massive centre; interpillow material is volcanic glass and sediment, light gray, near waypoint 5 in Figure 3. G) Pillow lobe that exposes multiple crusts H) Sketch of figure 7G: a - oldest crust, b intermediate crust and c - youngest crust; the outer surface of the front display ropy wrinkles (r); arrow shows magma flow direction to northeast.

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Fragmented sedimentary rocks layers occur scattered between pillow lavas. They consist of fine sandstone, clayey sandstone, sandy mudrock, and mudrock. Substitution and cementation by chalcedony or calcite occur locally. The interaction between lava and sediment result in peperite and is a conspicuous feature (Fig. 7C). Fragments of lava with amoeboid forms and glassy borders are common in the sediment. Moreover, amygdales filled with sediment in the fragments of basalt can be observed. Volcanic glass fragments (hyaloclastite) that are predominantly millimetric in size, angular, and greenish and pillow fragments, also make up the interpillow material. The hyaloclastite is easily weathered and show a powdery appearance. Celadonite is common especially in the contact between lava and sediment. Towards the top of the stratigraphy the pillow lavas are covered by two pahoehoe lobes. Those are massive and amygdaloidal, with sparse horizontal fracturing and a total thickness of approximately 11 m (figure 7D). Near the top, lobes alternate with laminated sandstone lenses with up to 40 cm thick. The sandstone is yellowish, fine to medium grained, and consists of quartz, feldspar and basaltic lithoclasts and, together, the lenses have a thickness of approximately 1 meter. The pahoehoe lobes are overlayed by a new packet of pillow lavas interlayered with lenses (but discontinuous and ruptured) of sandstone, sandy

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mudrock and mudrock. The upper package of pillows varies from 6 to 8 m in thickness and is intensely weathered. In addition to abundant fragments of volcanic glass, the pillows are wrapped in fragments of sandstone, sandy mudrock, and mudrock (figure 7E). The pillow lavas are densely packed and in a position which makes it difficult to make flow measurements. A metric package of pillow breccia and abundant fragments of volcanic glass can be observed in the midst of this set of pillows. The pillow lavas fragments range from a few millimeters to 10 centimeters in diameter and are irregular in shape - from rounded to angular and with sharp edges. They occur mixed with fragments of volcanic glass and, subordinately, of the sedimentary rocks already mentioned. They are in reddish pink and green, usually in an advanced state of weathering. The second pillow lavas set is covered by a tabular pahoehoe flow (~8 m thick). The basalt is columnar, fine to very fine grained, greenish black in colour, and is well preserved from weathering (Fig. 7F). The columns are thin – not greater than 30 cm in diameter – and are curved and slightly twisted. On the ground, there are sparse blocks of basalt rich in amygdales, some with geodes of up to 25 cm, filled with quartz, chalcedony, and celadonite.

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Fig. 7. A) Pillow lobe with four bulbs. B) Packed pillow lavas inclined to the NE at waypoint 6 in Figure 3. C) Peperite with fragments of greenish or purplish (strongly oxidized) hypohyaline basalt (b) with microamygdales in the midst of mudrock (m). D) Contact between the irregular surface of the set of highly compacted pillow lavas and pahoehoe lobes. E) Weathered pillow lavas (p) at waypoint 9 in Figure 3 with volcanic glass fragments (black arrows) and sediment (white arrows) as interpillow material. F) Irregular/curved columnar jointing in basalt at waypoint 10 in Figure 3.

4. Discussion

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The interaction between the sandstones from the Botucatu Formation and the basaltic lavas from the Serra Geral Formation, which succeeded them, is well documented in the works of Jerram & Stollhofen (2002), Jerram & Widdowson (2005) – both from the Namibian portion of the PEP – and Scherer (2002), Waichel (2006), Waichel et al. (2007), Fernandes et al. (2010), Machado et al. (2015) all of which were in Brazil. The study area is particular because during the Lower Cretaceous period the north and northeast boundary of the basin was delimited by topographically elevated terrains from the Brasília Belt; the area shows a paleorelief that is highly irregular in a peridesertic region, where occasional aqueous flows generated alluvial fan deposits and/or fluvio-lacustrine systems. These deductions were possible as: The presence in the area (section 4 of Figure 2) of predominantly pebbly sand deposits, poorly sorted and with low maturity, that change laterally and vertically to sandy, even clayey, indicating torrential, flood-type flows. The sandy portion present in this material includes a large amount of aeolian quartz; To the top of the basal sedimentary layer the contribution of lithologies of the Neoproterozoic basement becomes less dominant. It gives rise to volcanic contributions and quartz grains with characteristics of wind environment, suggesting the permanence of arid conditions in the region; The characteristics of lenses - more rarely layers – decimeter, of sandstone, clayey sandstone, sandy mudrocks and mudrock found in the stratigraphic succession of Figure 4 and shown in Figure 6 suggest a lacustrine environment. Centimeter-to-decimeter lenses of immature, granule-rich sandstone and frequently

ACCEPTED MANUSCRIPT detrital muscovite, with trough cross stratification are indicative of the formation of Gilbert deltas. These lakes may be created by the entrapment of rain water by lava flows or by outpourings of material from seisms linked to volcanic activity. Similar deposits are also present in sections 1 and 4 of figure 2. It is worth clarifying here that it is not rare to find fractures generating graben and horst structures in the more regional surroundings.

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Working with lava/sediment interaction, Waichel et al. (2007) argue that the climatic conditions changed from desertic in the basal portion of PCMP to humid at the top, and that volcanic quiescence intervals allowed sediments to be deposited in lakes located over lava flows. New volcanic pulses produced pahoehoe lava flows over the sediments of fine, wet, unconsolidated to poorly consolidated grains, which allowed their interaction, generating peperites. The intervals of volcanism, as argued by Waichel et al. (2007), is not a sine qua non condition for the development of fluviolacustrine environment. The profiles described indicate a paleorelief that is irregular enough to generate depressions where aqueous bodies may have accumulated. On the contrary, if a natural barrier created a lake, it may likewise function as a barrier for later effusions, forcing them to seek another route. The new situation would persist until the basaltic flow reached the top of the barrier and could enter the lake, plunging into the deposited sediments. This possible alternative depends on the dynamics of both processes. However, we agree with Waichel et al. (2007) concerning the occurrence of volcanic quiescence periods, which are necessary for the accumulation of sediments in water bodies. The flow directions of the pillow lavas indicate movement to the north and northeast, which only has local significance as the measurements taken are few. However, this bit of information is significant, as, in a more widespread mapping of the area, it may facilitate the definition of the source area localization. The map in figure 3 shows three quarries, and various other quarries are present in the surrounding area. All show features of imprinted flows on their floors. In addition, the levels of flattened amygdales are frequent at the top of basaltic flows and cylinders and vesicle pipes, some of which have a curved upper portion. These features, with well defined parameters, may be used to determine the flow direction (Walters, 1960). At the time, this little information suggests the dykes positioned to the northeast of the area –with a NW/SE orientation – as potential feeders of basaltic volcanism. The massive basalt between the two sets of pillow lava is tentatively explained in the intermittence of the lake, which was definitely shallow and in an arid environment, with sparse and torrential rains. Thus, the lake might be filled up, covered by a pahoehoe flow and later, a new water body could form. The presence of other similar deposits in section 4 of figure 2 suggests that it is a recurrent phenomenon. Alternatively, the presence of this massive flow between different sets of pillows can be explained by an increase in the effusion rate or in the slope of the surface. It is currently well established that the differences present between pillow lavas, lobulated forms, and sheet flows are due to the rate of magma supply, topographical conditions, and flow conditions. According to Batiza & White (2000, p369) “The experiments show that pillows form at the lowest effusion rates and gentlest slopes; gradually increasing the effusion rate or surface slope results in a progression to lobate flows, then lineated, folded, and jumbled sheet flows”. Walker

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(1992, Figure 9) described a similar example in Strumble Head, Wales, where massive lava developed above a set of pillow lavas. Fragments of preserved volcanic material embedded in the sediment or as highly weathered interpillow material (Fig. 6E and Fig. 7C) indicate that fragments of pillow lavas and of volcanic glass contributed significantly to the construction of this profile. The presence of hyaloclastites is expected in this environment, and their formation has been explained by three processes: fragmentation by thermal shock; fragmentation in the external margins of the pillows due to continuation of flow in the internal portion, generating pressure on the already-solid portion; and/or through impact between pillows when one of these detaches from the set and falls to the base of the pile (Batiza & White, 2000). That is, in this case hyaloclastites and fragments of pillow lavas are being interpreted as generated in non-explosive process. However, a part of the interpillow material preserved is peperite, which is believed to have formed from an active mixture of still-fluid lava and moist and poorly lithified sediment, essentially in situ (White et al., 2000), as shown in Figure 7C. The interactions are complex and highly diversified, demonstrating the presence of water and sediment lenses with distinct granulometry. The sediment lenses are not very thick, and are ruptured and destroyed on coming in contact with the basalt flow. In the process, it loses a part of its identity, as it breaks into countless fragments often with amoeboid edges. Finally, Figure 7F shows narrow and irregular columns in the basalt that covers the upper occurrence of pillow lavas, typical of entablature columns. This basalt shows a fine to very fine texture without resembling the texture of hypocrystalline or hypohyaline, as would be expected according to Long & Wood (1986), Lyle (2000), and Forbes et al. (2014), for example. Nevertheless, the morphological characteristics of the basalt corroborate a humid environment for the region. The presence of ostracod fossils in three of the lacustrine deposits and fossilized conifer woods described in Botucatu Formation, near Uberlândia city reinforces this idea. 5. Conclusions

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The northeast portion of the PCMP is settled in an irregular palaeorelief in a typically peridesertic climate, with occasional aqueous flows giving origin to alluvial fan and/or localized and ephemeral fluvio-lacustrine deposits together with dunes that occupy topographic depressions, generating discontinuous deposits. The possible dimensions to be evaluated in the fluvio-lacustrine deposit indicate a length of 1,300 meters and a minimum thickness of 27 meters. This lake, as well as at least two other known lakes in the region, were deposited over, and were overlain by basaltic flows. What distinguishes it from the others is the presence of two sets of pillow lavas, respectively, 6 and 8 meters in thickness. The first one is very well preserved but the upper one is weathered. The pillows vary between 30 and 150 cm in length, and show internal radial fractures with a zone rich in vesicle/amygdale below the glassy crust. Both pillow lavas sets show interaction with the fluviolacustrine sediments that locally preserve the original sedimentary structures, such as bedding and lamination. The interaction gives rise to peperites, which show diverse textures for sediments with distinctive grain size (pebbly sand, sand, clayey sand and clay); glassy borders in the fragmented lava are more common in the contact with the clayey sediment, and alteration to celadonite is disseminated.

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Hyaloclastite also occurs in the interpillow material, which may be intricately mixed with sediment and fragments of lava and pillow breccia. The two sets of pillow lavas are separated by 11 meters of lava, which has pahoehoe lobes characteristics at the base but becomes a highly weathered package, apparently massive and interlayered with ruptured layers of sandstone. The upper set, in turn, is covered by at least 8 meters of well-preserved basalt with thin columns – not surpassing 30 cm in diameter – that are curved and slightly twisted. Performed measurements indicate flow directions to NE, consistent with the NW/SE direction of the dikes found to the northeast. All these data indicate that the morphology of the sedimentary deposits and basaltic flows of the São Bento Group, in the northeast boundary of the PCMP, was influenced by the existence of the geographical barrier of the Brasilia Belt. This geographical barrier provided a more rugged palaeorelief and a more humid climate than those described for the central and southern portions of the Province.

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Acknowledgements

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The authors are grateful for the financial support of FAPESP - Project 2011/10508-6. To Prof. Dr. Evandro F. de Lima from the Universidade Federal do Rio Grande do Sul for valuable discussions in the field. To Prof. Dr. Edgardo Cañon Tapia of the Centro de Investigación Científica y de Educación Superior de Ensenada, Mexico for comments. To Prof. Dr. Breno Leitão Waichel of the Universidade Federal de Santa Catarina and an anonymous reviewer for the detailed review and suggestions. References

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Pillow lavas are described in the Paraná continental magmatic province. Pillow lavas interacted with fluvio-lacustrine sediments in a peridesertic environment. Lava/sediment interaction produced peperites