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Basaltic lava flows covering active aeolian dunes in the Paraná Basin in southern Brazil: Features and emplacement aspects
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Journal of Volcanology and Geothermal Research 171 (2008) 59 – 72 www.elsevier.com/locate/jvolgeores
Research paper
Basaltic lava flows covering active aeolian dunes in the Paraná Basin in southern Brazil: Features and emplacement aspects Breno L. Waichel a,⁎, Claiton M.S. Scherer b , Heinrich T. Frank b a
Universidade Estadual do Oeste do Paraná – UNIOESTE, Rua Universitária 1619, 85819-110, Cascavel, Paraná, Brazil b Universidade Federal do Rio Grande do Sul – UFRGS, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre, Brazil Received 22 February 2007; accepted 1 November 2007 Available online 17 November 2007
Abstract Burial of active aeolian dunes by lava flows can preserve the morphology of the dunes and generate diverse features related to interaction between unconsolidated sediments and lavas. In the study area, located in southern Brazil, burial of aeolian deposits by Cretaceous basaltic lava flows completely preserved dunes, and generate sand-deformation features, sand diapirs and peperite-like breccia. The preserved dunes are crescentic and linear at the main contact with basalts, and smaller crescentic where interlayered with lavas. The various feature types formed on sediment surfaces by the advance of the flows reflect the emplacement style of the lavas which are compound pahoehoe type. Four feature types can be recognized: (a) type 1 features are related to the advance of sheet flows in dune–interdune areas with slopes N 5°, (b) type 2 is formed where the lava flows advance in lobes and climb the stoss slope of crescentic dunes (slopes 8–12°), (c) type 3 is generated by toes that descend the face of linear dunes (slopes 17–23°) and (d) type 4 occurs when lava lobes descend the stoss slope of crescentic dunes (slopes 10–15°). The direction of the flows, the disposition and morphology of the dunes and the ground slope are the main factors controlling formation of the features. The injection of unconsolidated sand in lava lobes forms diapirs and peperite-like breccias. Sand diapirs occur at the basal portion of lobes where the lava was more solidified. Peperite-like breccias occur in the inner portion where lava was more plastic, favoring the mingling of the components. The generation of both features is related to a mechanical process: the weight of the lava causes the injection of sand into the lava and the warming of the air in the pores of the sand facilitates this process. The lava–sediment interaction features presented here are consistent with previous reports of basalt lavas with unconsolidated arid sediments, and additional new sand-deformation features formed by lava breakouts and sand diapir injections are presented. © 2007 Elsevier B.V. All rights reserved. Keywords: lava–sediment interaction; surface features; lava flow emplacement; pahoehoe; Paraná Basin
1. Introduction The complete preservation of aeolian dunes requires a rapid nondestructive burial process, and occurs only rarely. Examples of dunes preserved beneath flood lavas are the Proterozoic Eriksfjord Formation in Greenland (Clemmensen, 1988) and the Cretaceous Etjo Sandstone in Namibia (Mountney et al., 1998; Jerram et al., 1999a,b). Several sedimentological features generated by the interaction of basaltic flows and aeolian sediments were described for the Etjo Sandstone, particularly the preservation of the original morphology of aeolian dunes (Mountney et al., 1999; Jerram et al., 1999a,b, 2000; Mountney and Howell, 2000).
In the Paraná Basin, southern Brazil, basaltic lavas of the Serra Geral Formation flowed over active aeolian dunes represented by the Botucatu Formation. Scherer (2002) reconstructed the facies architecture of the aeolian units, characterised some sediment– lava interaction features, and develop a stratigraphic model for the accumulation and preservation of aeolian units within this volcanic setting. Due to some new anthropogenic outcrops we are now able to describe in detail features related to the interaction between lava flows and aeolian deposits, to recognize different dune types “fossilized” by the flows and to discuss emplacement of the basaltic lavas that buried active aeolian dunes in the Paraná Basin. 2. Geological setting
⁎ Corresponding author. Tel.: +55 45 3324 3068. E-mail address: [email protected] (B.L. Waichel). 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2007.11.004
The intracratonic Paraná Basin covers an area of ca. 1,500,000 km2 in central–eastern South America. The basin
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Fig. 1. Location map of Paraná Basin and simplified geological map of Rio Grande do Sul state with study area (modified from Scherer, 2002).
comprises a thick Upper Ordovician/Upper Cretaceous volcano-sedimentary succession, divided into six supersequences by Milani (1997): Rio Ivaí (Upper Ordovician–Lower Silurian), Paraná (Devonian), Gondwana I (Upper Carboniferous–Lower
Triassic), Gondwana II (Middle–Upper Triassic), Gondwana III (Upper Jurassic–Lower Cretaceous) and Bauru (Upper Cretaceous). The supersequences are separated by regional unconformities.
Fig. 2. Simplified geological map with the location of the described outcrops in the southern region of Paraná Basin (Rio Grande do Sul state).
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Fig. 3. Crescentic dune of the outcrop A covered by lava flows.
The Gondwana III supersequence comprises the Botucatu Formation and the Serra Geral Formation, covering an area of more than 1,300,000 km2 in Brazil, Paraguay, Uruguay and Argentina. The study area (Rio Grande do Sul state, southernmost Brazil) is situated in the south eastern outcrop area of this supersequence (Fig. 1).
In the study area the Botucatu Formation consist of aeolian deposits, dominantly sets and cosets of cross-strata. Locally, the base of the formation includes conglomerates and gravelly sandstones deposited by ephemeral streams and coarse-grained sandstones interpreted to represent aeolian sand sheet deposits (Scherer, 1998). The aeolian deposits of the Botucatu Formation
Fig. 4. Type 1 features in outcrop A. (A) Schematic figure showing the filling of interdune and burial of stoss side of the dune. (B) Type 1 features (dashed lines) formed by lateral advance of lava flow on dune surface (black arrow indicates the lateral advance of the flow and white arrow the flow direction to SE.(C) Detail of the striations. Pen=14 cm.
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Fig. 5. Type 2 features in the stoss slope of a crescentic dune (A) and asymmetrical wind ripple marks preserved on the dune surface (B), outcrop A.
are up to 100 m thick, but are absent in some regions of the state of Rio Grande do Sul due to non-deposition. Scherer (2000) interpreted the Botucatu Formation as the record of a dry aeolian system as indicated by the accumulation of aeolian dunes without development of wet interdune facies. The Serra Geral Formation is a succession of volcanic rocks with a maximum thickness of approximately 1700 m, composed mostly of tholeiitic basalts with minor rhyolites and rhyodacites in the upper portion (Melfi et al., 1988). The basalts are divided into two groups on the basis of Ti contents, High Ti basalts–HTi (TiO2 N 2%) and Low Ti basalts–LTi (TiO2 b 2%) (Bellieni et al., 1984; Mantovani et al., 1985). Basaltic lavas covered the Botucatu palaeoerg and preserved the morphology of the aeolian dunes (Scherer, 1998, 2000). Thin (b 15 m) and discontinuous (b1 km wide) aeolian deposits are interlayered with lava flows in the lower and middle portions of the Serra Geral Formation, indicating the continuity of desertic conditions during the magmatic event. Ar40/Ar39 dating of the Serra Geral volcanic rocks in the state of Rio Grande do Sul yielded ages of about 132 Ma (Renne et al., 1992; Turner et al., 1994), supplying a reliable chronological reference for the end of the Botucatu sedimenta-
tion. The close relationship between aeolian sandstones and lava flows and the absence of supersurfaces within the aeolian succession suggest that the Botucatu Formation, at least in the southern portion of the Paraná Basin, represents a shorter interval of sedimentation, probably less than a few hundred thousand years of aeolian sedimentation prior to the beginning of volcanic activity (Scherer et al., 2000). 3. Preserved features related to lava–sediment interactions In this paper we focus on features generated in aeolian sediments overrun by basaltic flows in three outcrops in the state of Rio Grande do Sul (Fig. 2). The supersurfaces formed by lava flows covering aeolian sediments possess several features related to lava–sediment interaction indicating that the lava flows were emplaced onto actively migrating aeolian dunes (Scherer, 2002). The features imprinted on the sediments indicate local lava advance directions, and distinct patterns permit the reconstruction of flow lobe advance. The main factors which control the formation of the described features are the flow style of the lavas (pahoehoe) and the geometry of the dunes.
Fig. 6. Reconstruction of the advance of the small lobes in outcrop A.
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Fig. 7. (A) Striations parallel to flow direction (SE) on crescentic sand ridges; (B) fragments of the crusts of the lava lobes pressed into the sand, outcrop A.
3.1. Outcrop A In outcrop A a crescentic dune with a height of at least 15 m is preserved (Fig. 3). Internally, the dune is composed of
tangential cross-beds dipping 25° to 068°. The top surface of the dune dips at 10°, and exposes asymmetrical wind ripples with a migration trend of 070°, parallel to the migration direction of the dunes.
Fig. 8. Transverse section of a linear dune from outcrop B.
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Fig. 9. (A) Type 3 features on a large surface of a linear dune, arrow indicates local lava flow direction. Hammer = 26 cm.; (B) preserved lava toe (right side) and type 3 feature (left side), outcrop B. Scale = 25 cm.
Two surface features formed by lava flows can be observed on the stoss face of the dune. The first features (type 1) are of high amplitude, up to 8 m in width, low convexity, and are located on dune surfaces whose dip is less than 5° (Fig. 4). Type 1 features are formed by the lateral advance of the lava flows that fill the interdune area and progressively buried the stoss side of the dune. There are striations on the dune surface that are discontinuous, finely spaced and parallel to the features (Fig. 4B,C). Striations with a complex pattern are locally observed. The second features (type 2) are up to 1.5 m wide and 3.0 m long, and located at the stoss slope of crescentic dunes with higher dip (8–12°, Fig. 4A). These features are connected and are interpreted to have formed during advance of the lava as a series of small lobes, which caused deformation of the sandy ground forming ridges at the front of lobes. The features imprinted in this way on to the dune surface are called crescentic ridges, previously called “crescent marks” by Scherer (2002). These crescentic ridges are composed of massive sand ridges, 0.5 to 3.0 m wide and up to 10 cm high (Fig. 5A). The spacing between the crescentic ridges is of about 1 to 3 m and between adjacent ridges the sand is only slightly modified,
preserving the wind ripples present on the aeolian dune's stoss slope (Fig. 5B). The height of the sand ridges increases, and the dimensions of the lobes decrease, toward the crest of the dune. In outcrop A, the advance of small lava lobes can be reconstructed in detail (Fig. 6), and the convex sides of the sand ridges indicate a lava palaeoflow direction to SE. Striations occur within type 2 features and on crescentic ridges, are discontinuous, closely spaced and parallel to the flow direction (Fig. 7A). Locally, near the crescentic sand ridges, some fragments of the crust of the lava lobes are detached, and pressed into the sand (Fig. 7B). 3.2. Outcrop B In outcrop B a linear dune is covered by lava flows and the contact can be observed in longitudinal and transverse sections. In the transverse section it is possible to identify the symmetrical geometry of the linear dune, characterized internally by a zigzag pattern of cross-strata that dip 17 to 130° and 23° to 280° (Fig. 8). The contacts between the lava and the sediments are sharp, and mixtures of lava and sand, such as
Fig. 10. (A) Sand diapir and breccia zone near the crest of a linear dune in outcrop B. (B) Breccia zone composed of vesicular lava clasts and friable sand. Scale = 8.5 cm.
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breccia-like rocks, are generally absent. Near the contact, a 15– 20 cm thick sandstone layer has undergone silicification. Type 3 features are irregular, up to 60 cm long and 10 cm wide and occur on the faces of linear dunes (Fig. 9A). The advance of lava toes that descended the dune face is interpreted to have formed these features, and locally the lava toes are preserved (Fig. 9B). Sand diapirs and breccia-like rocks are associated with the linear dune in outcrop B, both occurring at the base of the lava flow above the crest-line of the dune. The sand diapirs are composed of friable and more reddish sandstone, have diameters of 0.5–1.0 m, and occur only near the crest-line of the dune (Fig. 10A). In general appearance these structures are somewhat similar to the spiracles described by Moler and Cabrera (1976) in the Serra Geral Formation. The structures are
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connected to breccias composed of lava clasts mixed with sand. The sediment in the breccia domain is friable sand and does not show stratification or vesicles. The juvenile clasts are vesicular, have sub-rounded to sub-angular shapes, sizes ranging from 5 to 40 cm and quench margins are absent (Fig. 10B). The contacts of lava clasts and sand are marked by thin halos, related to posterior oxidation of the basalt. In a vertical section near the crest of linear dune, the structures formed of sand diaper and breccia zones have a roughly orientated form that may indicate local lava advance directions (Fig. 10A). 3.3. Outcrop C Outcrop C is characterized by a crescentic dune (150 m long and 27 m thick) under- and overlain by lava flows (Fig. 11).
Fig. 11. Geometry of a crescentic dune sandwiched between lava flows in outcrop C.
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Internally, the dune is composed of trough cross-beds dipping 22° to 290°. The stoss slope of the dune dips 10° to 15° and exhibits surface features up to 0.5 m in width and 4.0 m in length (type 4). On the upper portion of the stoss slope the features are elongate, with lengths up to 4 m and widths up to 50 cm (Fig. 12). Locally, small semi-circular features are inferred to have been formed by lateral breakouts. Some lobes are inflated and the increase in lava weight cause the deformation of sand with formation of channels and partially preserved lava lobes are observed filling these features (Fig. 13). The thickness of the first set of lava lobes that covered the dune is up to 20 cm and subsequent lobes had thickness up to 50 cm, and are observed in a near vertical exposure. Another surface feature observed on the inner portion of stoss slope is a set of corrugations probably related to generation of frontal breakouts. The inflation and pressure increase inside the lobe cause the fracture of the crust and generation of new lobes, in proximities of the break point hot pressurized lava probably move more fast forming the corrugations. 4. Emplacement of lava flows The preservation of the morphology of the dunes, inferred flow-emplacement styles, the surface features and structures
Fig. 12. Type 4 feature, up to 4 m long, on a stoss face of a crescentic dune in outcrop C. Arrows indicate frontal and lateral breakouts.
Fig. 13. Partially preserved lava lobe filling type 4 feature in outcrop C. Hammer = 26 cm.
described in above section permits the proposition of some models to explain the emplacement of the lava flows and the burying of active aeolian dunes in the study area. The structures found in the lava flows and the surface features indicate that flows that buried the dunes advanced in this area as lava toes and lobes forming compound pahoehoe lava flows. In the interdune regions the lava flows reach 40 m in thickness and show a massive aspect. The gentle advance of these flows is responsible for the preservation of the dune morphology and is reflected by characteristics the generation of the surface features here described. Infilling of an interdune area and burial of the stoss side of a large crescentic dune (outcrop A) is shown in Fig. 14. This burial model is similar to models presented by Jerram et al. (2000) and Jerram and Stollhofen (2002) in Etendeka flood basalts, NW Namíbia. The surface features indicate that lava flowed toward the SE, approximately transverse to cross-strata dip direction (NE). The lava flows first filled the interdune depression and then progressively covered the stoss slope of the dune (Fig. 14A, B). The relatively low-slope of the interdune trough allowed the pahoehoe flows to advance like unconfined sheets forming features with high amplitude (type 1). The occurrence of a complex pattern of striations can be related to the advance of multi-lobe sheet flows in these areas. On the stoss face, where the slope is higher, lava flows advanced as numerous lobes printing features type 2 (Figs. 14C and 5, 6). These lobes are inferred to have breakout from large inflated sheet lobes that filled the interdune area (Fig. 14C). Burial of linear dunes (outcrop B) in the study area is shown in Fig. 15. First the interdune area was filled by lava flows channelized among aeolian bedforms (Fig. 15A, B). At some points the lava flows reached the crest line and breakout lobes are inferred to have spilled out, forming lava toes that advanced down the dune surface and produce type 3 features (Figs. 15C and 9). Subsequent lava flows led to the complete burial of the linear dunes (Fig. 15D). The formation of breccias due to interaction between lava flows and sediments took place only near the crests of linear dunes, and is associated with sand diapirs (Fig. 10). The
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Fig. 14. Filling of an interdune area and covering of a large crescentic dune in outcrop A.
geometry of these structures is similar to spiracles, but the generation process is distinct. Spiracles are located at the base of the flows and the formation process is related to the advance of the lava over wet ground. However, the Botucatu Formation can be classified as a dry aeolian system (Scherer, 2000, 2002). It consist of cross-strata aeolian dunes bounded by first-order surfaces that do not have evidence of damp or wet interdune deposits, suggesting that the water table was located below the depositional surface during aeolian
accumulation. In this way, the breccia formation cannot be explained by lava–water interaction processes. Nevertheless something occurs in this area that led to mingling of the components. At the contact with dune the pahoehoe flows are dominated by lava toes that descend the face of linear dune. Probably the filling of the interdune by lava flows was inhomogeneous and the lava lobes did not reach the crest line at the same time. In the first stage, the lava flows are inferred to have reached the crests
Fig. 15. Emplacement of the lava flows and burial of a linear dune in outcrop B.
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Fig. 16. Emplacement of lava flows and burying of a linear dune. (A) Lava toes descending the face of the dune and (B) formation of lava lobes by coalescence of multiple toes.
and descended gently to form type 3 features (Fig. 16A). In the second stage, continued lava flow advance and inflation led to the formation of lava lobes by thickening and coalescence of the multiple toes (Fig. 16B). At some points along the crest of the dune the weight of the lava caused the injection of sand inwards into the flow lobe, forming the sand diapirs in the basal portion where the lava was more solidified (Fig. 17A). In the interior of a flow lobe, where lava is less viscous, injected sand can mingle with juvenile igneous clasts forming breccias-like rocks similar to fluidal peperite (Fig. 17B). The flow of lava within the advancing flow generates breccia domains that are elongate in the down-flow direction. The injection process is related to pressure caused by overrun lava flows coupled with the fact that unconsolidated aeolian sand has an average porosity between 47 and 49% (Atkins and McBride, 1991) with the pores filled with air. In this case, dry sand behaves as a granular fluid (Jaeger et al., 1996) and probably, the warming of the air will facilitate the injection process. The shapes of lava clasts (sub-rounded to sub-angular) are intermediate between fluidal and blocky peperite, but the generation process involves the injection of dry sand with granular fluid behavior into the lava flow. Besides the weight of lava was lowest at the crest, comparing with the dune faces, the flows along this line reach at 3 m thick indicating a considerable inflation after the sand injection.
A similar process of forming of breccia-like rocks is described in Paraná basin by Petry et al. (2007), but in this case the unconsolidated dry sand is injected as clastic dykes in the inner part of the flow forming rocks called “injection peperite” by these authors. The formation of peperite require a mingling in situ of magma and sediment and commonly, involves the presence of wet sediment in the process, with the fluidization of water facilitating the mingling of components (White et al., 2000; Skilling et al., 2002). Jerram and Stollhofen (2002) and Petry et al. (2007) argue, however, that the generation of peperites can occur without the presence of water, so that the lava fragmentation associated with unconsolidated or poorly consolidated sediment can form peperites too. In the study area the breccias are formed by the injection of dry unconsolidated sand into lava lobes. This mechanical process designated “sand injection” by Petry et al. (2007), can result in peperite-like textures. The discussion about the use of the term peperite will be avoided in this work, and the mixed rocks described here are designated “peperite-like breccias”. Outcrop C exposes a crescentic dune sandwiched between basalts. The cross-strata dip indicates that the dune was migrating to the NW, and features (type 4) indicate a lava advance direction to the SE during burial (Fig. 18). In this case, lava flows filled the interdune area and reached at the crest line
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Fig. 17. (A) Formation of a sand diapir by injection of sand inwards in the basal portion of the flow lobe; (B) breccias generated by injected sand mixed with juvenile igneous clasts at the interior part of flow lobe.
of the dune (Fig. 18A) When lava overtopped the crest it spilled out, forming lava lobes that advanced down the stoss face (Figs. 18B, 12). Features of this type have a high length/width ratio (5-8). 5. Concluding remarks The interactions between lava and sediment occurred in Paraná-Etendeka Igneous Province can be inferred in South America and Africa, and the interaction generates a variety of surface features, structures, breccias and peperite (Mountney et al., 1999; Jerram et al., 1999b; Jerram and Stollhofen, 2002; Scherer, 2002; Waichel et al., 2007; Petry et al., 2007).
In the study area, the covering of aeolian deposits by basaltic lava flows induced the complete preservation of dunes and generated sand-deformation features, sand diapirs and peperitelike breccias. The preserved dunes are crescentic and linear at their main contacts with the basalts of the Serra Geral Formation, with minor crescentic dunes where included within the sequence of lavas. Initially, the lava flows had to adjust themselves to the pre existent desert topography. The first flows filled the interdune areas and covered active migrating dunes; the still remaining mobile sands formed smaller migrating dunes and other bedforms that were engulfed and preserved between lavas in the lower part of the volcanic pile.
Fig. 18. Emplacement of lava flows that overrun simple crescentic dunes interlayered with basalts in outcrop C.
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The various features types formed on sediment surfaces by the advance of the flows reflect the emplacement style of the lavas, which were compound pahoehoe lava flows. The direction of the flows, the disposition and morphology of the dunes and the ground slope are the main factors governing the formation of the different types of features.
Type 1 features are related to the lateral advance of sheet flows in stoss face of dunes with slope N 5°, type 2 is formed where the lava flows advance in lobes and climb the stoss slope of crescentic dunes (slope 8–12°). Type 3 is generated by toes that descend the face of linear dunes (slope 17–23°) and type 4 occurs when lava lobes descend the stoss slope of crescentic
Fig. 19. A—General view with feature types described, related dunes, general and local lava flow directions. B—Determination of local flow advance based in features type 1 and associated striations, C—Determination of local flow advance based in features type 2 and associated striations, D—Summary of the feature characteristics.
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dunes (slope 10-15°). The characteristics of the features are summarized in Fig. 19A, D. These sand-deformation features and striations on dune surfaces can be used to determinate the local lava flow directions. Type 1 features and associated striations are parallel, both indicating the local flow direction (Fig. 19B). In type 2 features the convex side of crescentic ridges and striations indicate the local flow direction (Fig. 19C). The use and efficacy of these flow indicators to determination of general lava flow direction is directly related to dimension and continuity of the exposure area. Striations are generated by the imprinting of the basal skin of lava lobes on dune surfaces. Basal skin of sheet lobes (type 1 features) and lobes on stoss face of dunes (type 2 features) have fine grooves that formed the striations. The grooves of lava skin are more finely spaced and continuous in type 2 features compared with type 1. Lava lobes descended stoss face of crescentic (type 4 features) and lava toes on linear dunes (type 3 features) have smooth basal skin. The occurrence of pahoehoe lava toes and lobes on surfaces with high slopes (10 to 23°) is associated with substrates of unconsolidated sand. Probably the soft characteristics of this substrate inhibits the increase of velocity of the lava due to the increase in slope and pahoehoe flows do not change to ‘a‘a. The injection of unconsolidated sand into lava lobes forms diapirs and peperite-like breccias. Sand diapirs penetrate at the basal portions of lobes, where the lava was more solidified, and peperite-like breccias in the inner portion where lava was more plastic, favoring the mingling of components. The generation of both features is related to a suite of mechanical processes, with the overload due to the weight of lava and the warming of the air in the pores inducing a granular fluid behavior of dry sand, leaving to the injection of the sand into lavas. The present paper adds to the model presented by the few previous workers (Jerram et al., 1999b, 2000; Scherer, 2002; Petry et al., 2007) who have analyzed the emplacement style of the first stage of the Paraná–Etendeka lavas, addressing to the burial of the Botucatu palaeoerg. The presented features and the discussion about their origin aim to improve the understanding of the emplacement style of these first lava flows. Acknowledgments We thank the students Gabriela Rizzon, Richard Kalil and Círio Simon who helped us to clean, with shovels, spades and brooms, some of the fine outcrops here presented. We also thank JDL White and D Jerram for careful revision and comments that improve the final version. References Atkins, J.E., McBride, E.F., 1991. Porosity and packing of Holocene river, dune, and beach sands. AAPG Bulletin, 75:3; Annual meeting of the American Association of Petroleum Geologists (AAPG). Dallas, TX (United States), p. 535 (7-10 Apr 1991). Bellieni, G., Comin-Chiaramonti, P., Marques, L.S., Melfi, A.J., Picirillo, E.M., Nardy, A.J.R., Roisenberg, A., 1984. High- and Low Ti flood basalts from the Paraná plateau (Brazil): petrogenetic and geochemical aspects bearing on
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their mantle origin. Neues Jahrbuch für Mineralogie Abhandlungen 150, 272–306. Clemmensen, L.B., 1988. Aeolian morphology preserved by lava cover, the Precambrian Mussartût Member, Eriksfjord Formation, South Greenland. Bulletin of the Geological Society of Denmark 37, 105–116. Jaeger, H.M., Nagel, S.R., Behringer, R.P., 1996. Granular solids, liquids and gases. Reviews of Modern Physics 68, 1259–1273. Jerram, D.A., Stollhofen, H., 2002. Lava/sediment interaction in desert settings: are all peperite-like textures the result of magma–water interaction? In: Skilling, I.P., White, J.D.L., McPhie, J. (Eds.), Peperite: Processes and products of Magma-Sediment Mingling. Journal of Volcanology and Geothermal Research, vol. 114, pp. 231–249. Jerram, D.A., Mountney, N., Holzförster, F., Stollhofen, H., 1999a. Internal stratigraphic relationships in the Etendeka Group in the Huab Basin, NW Namibia. Journal of Geodynamics 28, 393–418. Jerram, D.A., Mountney, F., Stollhofen, H., 1999b. Facies architecture of the Etjo Sandstone Formation and its interaction with the basal Etendeka Flood Basalts of northwest Namibia: implications for offshore prospectivity. In: Cameron, N.R., Bate, R.H., Clure, V.S. (Eds.), The Oil and Gas Habitats of the South Atlantic. Geological Society London, Special Publications, vol. 153, pp. 367–380. Jerram, D.A., Mountney, N., Howell, J.A., Long, D., Stollhofen, H., 2000. Death of a sand sea: an active aeolian erg systematically buried by Etendeka flood basalts of NW Namibia. Journal of the Geological Society 157, 513–516. Mantovani, M.S.M., Marques, L.S., De Sousa, M.A., Civetta, L., Atalla, L., Innocenti, F., 1985. Trace element and strontium isotope constraints on the origin and evolution of Paraná continental flood basalts of Santa Catarina State, southern Brazil. Journal of Petrology 26, 187–209. Melfi, A.J., Picirillo, E.M., Nardy, A.J.R., 1988. Geological and magmatic aspects of the Paraná Basin: an introduction. In: Piccirillo, E.M., Melfi, A.J. (Eds.), The Mesozoic Flood Volcanism of the Paraná Basin: Petrogenetic and Geophysical Aspects. IAG-USP, pp. 1–13. Milani, E.J., 1997. Evolução tectono-estratigráfica da Bacia do Paraná e seu relacionamento com a geodinâmica fanerozóica do Gondwana sul-ocidental. Unpublished PhD thesis, Universidade Federal do Rio Grande do Sul, Porto Alegre, 255 p. Moler, W.A., Cabrera, J.G., 1976. Características de fundações sobre rochas basálticas. Congresso Brasileiro de Geologia e Engenharia. ABGE, vol. 2. Anais, Rio de Janeiro, pp. 197–216 (1/4, Rio de Janeiro). Mountney, N., Howell, J., 2000. Aeolian architecture, bedform climbing and preservation space in the Cretaceous Etjo Formation, NW Namibia. Sedimentology 47, 825–849. Mountney, N., Howell, J., Flinth, S., Jerram, D.A., 1998. Aeolian and alluvial deposition within the Mesozoic Etjo Sandstone Formation, northwest Namibia. Journal of African Earth Science 27, 175–192. Mountney, N., Howell, J., Flinth, S., Jerram, D.A., 1999. Relating eolian bounding-surface geometries to the bed forms that generated them: Etjo Formation, Cretaceous, Namibia. Geology 27, 159–162. Petry, K., Jerram, D.A., Almeida, D.P.M., Zerfass, H., 2007. Volcanic– sedimentary features in the Serra Geral Fm., Paraná Basin, southern Brazil: Example of dynamic lava–sediment interactions in a arid setting. Journal of Volcanology and Geothermal Research 159, 313–325. Renne, P.R., Ernesto, M., Pacca, I.G., Coe, R.S., Glen, J.M., Prevót, T.M., Perrin, M., 1992. The age of Paraná flood volcanism, rifting of Gondwanaland and the Jurassicp–Cretaceous boundary. Science 258, 975–979. Scherer, C.M.S., 1998. Análise estratigráfica e litofaciológica da Formação Botucatu (Cretáceo Inferior da Bacia do Paraná) no Rio Grande do Sul. Unpublished PhD thesis, Universidade Federal do Rio Grande do Sul, Porto Alegre, 202 p. Scherer, C.M.S., 2000. Eolian dunes of the Botucatu Formation (Cretaceous) in Southernmost Brazil: morphology and origin. Sedimentary Geology 137, 63–84. Scherer, C.M.S., 2002. Preservation of aeolian genetic units by lava flows in the Lower Cretaceous of the Paraná Basin, southern Brazil. Sedimentology 49, 97–116. Scherer, C.M.S., Faccini, U.F., Lavina, E.L., 2000. Arcabouço estratigráfico do Mesozóico da Bacia do Paraná. In: Holz, M., De Ros, L.F. (Eds.),
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Geologia do Rio Grande do Sul. Editora da Universidade/UFRGS, Porto Alegre, pp. 335–354. Skilling, I.P., White, J.D.L., McPhie, J., 2002. Peperite: a review of magma– sediment mingling. In: Skilling, I.P., White, J.D.L., McPhie, J. (Eds.), Peperite: Processes and products of Magma-Sediment Mingling. Journal of Volcanology and Geothermal Research, vol. 114, pp. 1–17. Turner, S., Regelous, M., Kelley, S., Hawkesworth, C.J., Mantovani, M.S.M., 1994. Magmatism and continental break-up in the South Atlantic: high
precision 40Ar/39Ar geochronology. Earth and Planetary Science Letters 121, 333–348. Waichel, B.L., Lima, E.F., Sommer, C.A., Lubachesky, R., 2007. Peperite formed by lava flows over sediments: an example from the central Paraná Continental Flood basalts, Brazil. Journal of Volcanology and Geothermal Research 159, 343–354. White, J.D.L., McPhie, J., Skilling, I.P., 2000. Peperite: a useful genetic term. Bulletin of Volcanology 62, 65–66.
Journal of Volcanology and Geothermal Research 171 (2008) 59 – 72 www.elsevier.com/locate/jvolgeores
Research paper
Basaltic lava flows covering active aeolian dunes in the Paraná Basin in southern Brazil: Features and emplacement aspects Breno L. Waichel a,⁎, Claiton M.S. Scherer b , Heinrich T. Frank b a
Universidade Estadual do Oeste do Paraná – UNIOESTE, Rua Universitária 1619, 85819-110, Cascavel, Paraná, Brazil b Universidade Federal do Rio Grande do Sul – UFRGS, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre, Brazil Received 22 February 2007; accepted 1 November 2007 Available online 17 November 2007
Abstract Burial of active aeolian dunes by lava flows can preserve the morphology of the dunes and generate diverse features related to interaction between unconsolidated sediments and lavas. In the study area, located in southern Brazil, burial of aeolian deposits by Cretaceous basaltic lava flows completely preserved dunes, and generate sand-deformation features, sand diapirs and peperite-like breccia. The preserved dunes are crescentic and linear at the main contact with basalts, and smaller crescentic where interlayered with lavas. The various feature types formed on sediment surfaces by the advance of the flows reflect the emplacement style of the lavas which are compound pahoehoe type. Four feature types can be recognized: (a) type 1 features are related to the advance of sheet flows in dune–interdune areas with slopes N 5°, (b) type 2 is formed where the lava flows advance in lobes and climb the stoss slope of crescentic dunes (slopes 8–12°), (c) type 3 is generated by toes that descend the face of linear dunes (slopes 17–23°) and (d) type 4 occurs when lava lobes descend the stoss slope of crescentic dunes (slopes 10–15°). The direction of the flows, the disposition and morphology of the dunes and the ground slope are the main factors controlling formation of the features. The injection of unconsolidated sand in lava lobes forms diapirs and peperite-like breccias. Sand diapirs occur at the basal portion of lobes where the lava was more solidified. Peperite-like breccias occur in the inner portion where lava was more plastic, favoring the mingling of the components. The generation of both features is related to a mechanical process: the weight of the lava causes the injection of sand into the lava and the warming of the air in the pores of the sand facilitates this process. The lava–sediment interaction features presented here are consistent with previous reports of basalt lavas with unconsolidated arid sediments, and additional new sand-deformation features formed by lava breakouts and sand diapir injections are presented. © 2007 Elsevier B.V. All rights reserved. Keywords: lava–sediment interaction; surface features; lava flow emplacement; pahoehoe; Paraná Basin
1. Introduction The complete preservation of aeolian dunes requires a rapid nondestructive burial process, and occurs only rarely. Examples of dunes preserved beneath flood lavas are the Proterozoic Eriksfjord Formation in Greenland (Clemmensen, 1988) and the Cretaceous Etjo Sandstone in Namibia (Mountney et al., 1998; Jerram et al., 1999a,b). Several sedimentological features generated by the interaction of basaltic flows and aeolian sediments were described for the Etjo Sandstone, particularly the preservation of the original morphology of aeolian dunes (Mountney et al., 1999; Jerram et al., 1999a,b, 2000; Mountney and Howell, 2000).
In the Paraná Basin, southern Brazil, basaltic lavas of the Serra Geral Formation flowed over active aeolian dunes represented by the Botucatu Formation. Scherer (2002) reconstructed the facies architecture of the aeolian units, characterised some sediment– lava interaction features, and develop a stratigraphic model for the accumulation and preservation of aeolian units within this volcanic setting. Due to some new anthropogenic outcrops we are now able to describe in detail features related to the interaction between lava flows and aeolian deposits, to recognize different dune types “fossilized” by the flows and to discuss emplacement of the basaltic lavas that buried active aeolian dunes in the Paraná Basin. 2. Geological setting
⁎ Corresponding author. Tel.: +55 45 3324 3068. E-mail address: [email protected] (B.L. Waichel). 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2007.11.004
The intracratonic Paraná Basin covers an area of ca. 1,500,000 km2 in central–eastern South America. The basin
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Fig. 1. Location map of Paraná Basin and simplified geological map of Rio Grande do Sul state with study area (modified from Scherer, 2002).
comprises a thick Upper Ordovician/Upper Cretaceous volcano-sedimentary succession, divided into six supersequences by Milani (1997): Rio Ivaí (Upper Ordovician–Lower Silurian), Paraná (Devonian), Gondwana I (Upper Carboniferous–Lower
Triassic), Gondwana II (Middle–Upper Triassic), Gondwana III (Upper Jurassic–Lower Cretaceous) and Bauru (Upper Cretaceous). The supersequences are separated by regional unconformities.
Fig. 2. Simplified geological map with the location of the described outcrops in the southern region of Paraná Basin (Rio Grande do Sul state).
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Fig. 3. Crescentic dune of the outcrop A covered by lava flows.
The Gondwana III supersequence comprises the Botucatu Formation and the Serra Geral Formation, covering an area of more than 1,300,000 km2 in Brazil, Paraguay, Uruguay and Argentina. The study area (Rio Grande do Sul state, southernmost Brazil) is situated in the south eastern outcrop area of this supersequence (Fig. 1).
In the study area the Botucatu Formation consist of aeolian deposits, dominantly sets and cosets of cross-strata. Locally, the base of the formation includes conglomerates and gravelly sandstones deposited by ephemeral streams and coarse-grained sandstones interpreted to represent aeolian sand sheet deposits (Scherer, 1998). The aeolian deposits of the Botucatu Formation
Fig. 4. Type 1 features in outcrop A. (A) Schematic figure showing the filling of interdune and burial of stoss side of the dune. (B) Type 1 features (dashed lines) formed by lateral advance of lava flow on dune surface (black arrow indicates the lateral advance of the flow and white arrow the flow direction to SE.(C) Detail of the striations. Pen=14 cm.
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Fig. 5. Type 2 features in the stoss slope of a crescentic dune (A) and asymmetrical wind ripple marks preserved on the dune surface (B), outcrop A.
are up to 100 m thick, but are absent in some regions of the state of Rio Grande do Sul due to non-deposition. Scherer (2000) interpreted the Botucatu Formation as the record of a dry aeolian system as indicated by the accumulation of aeolian dunes without development of wet interdune facies. The Serra Geral Formation is a succession of volcanic rocks with a maximum thickness of approximately 1700 m, composed mostly of tholeiitic basalts with minor rhyolites and rhyodacites in the upper portion (Melfi et al., 1988). The basalts are divided into two groups on the basis of Ti contents, High Ti basalts–HTi (TiO2 N 2%) and Low Ti basalts–LTi (TiO2 b 2%) (Bellieni et al., 1984; Mantovani et al., 1985). Basaltic lavas covered the Botucatu palaeoerg and preserved the morphology of the aeolian dunes (Scherer, 1998, 2000). Thin (b 15 m) and discontinuous (b1 km wide) aeolian deposits are interlayered with lava flows in the lower and middle portions of the Serra Geral Formation, indicating the continuity of desertic conditions during the magmatic event. Ar40/Ar39 dating of the Serra Geral volcanic rocks in the state of Rio Grande do Sul yielded ages of about 132 Ma (Renne et al., 1992; Turner et al., 1994), supplying a reliable chronological reference for the end of the Botucatu sedimenta-
tion. The close relationship between aeolian sandstones and lava flows and the absence of supersurfaces within the aeolian succession suggest that the Botucatu Formation, at least in the southern portion of the Paraná Basin, represents a shorter interval of sedimentation, probably less than a few hundred thousand years of aeolian sedimentation prior to the beginning of volcanic activity (Scherer et al., 2000). 3. Preserved features related to lava–sediment interactions In this paper we focus on features generated in aeolian sediments overrun by basaltic flows in three outcrops in the state of Rio Grande do Sul (Fig. 2). The supersurfaces formed by lava flows covering aeolian sediments possess several features related to lava–sediment interaction indicating that the lava flows were emplaced onto actively migrating aeolian dunes (Scherer, 2002). The features imprinted on the sediments indicate local lava advance directions, and distinct patterns permit the reconstruction of flow lobe advance. The main factors which control the formation of the described features are the flow style of the lavas (pahoehoe) and the geometry of the dunes.
Fig. 6. Reconstruction of the advance of the small lobes in outcrop A.
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Fig. 7. (A) Striations parallel to flow direction (SE) on crescentic sand ridges; (B) fragments of the crusts of the lava lobes pressed into the sand, outcrop A.
3.1. Outcrop A In outcrop A a crescentic dune with a height of at least 15 m is preserved (Fig. 3). Internally, the dune is composed of
tangential cross-beds dipping 25° to 068°. The top surface of the dune dips at 10°, and exposes asymmetrical wind ripples with a migration trend of 070°, parallel to the migration direction of the dunes.
Fig. 8. Transverse section of a linear dune from outcrop B.
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Fig. 9. (A) Type 3 features on a large surface of a linear dune, arrow indicates local lava flow direction. Hammer = 26 cm.; (B) preserved lava toe (right side) and type 3 feature (left side), outcrop B. Scale = 25 cm.
Two surface features formed by lava flows can be observed on the stoss face of the dune. The first features (type 1) are of high amplitude, up to 8 m in width, low convexity, and are located on dune surfaces whose dip is less than 5° (Fig. 4). Type 1 features are formed by the lateral advance of the lava flows that fill the interdune area and progressively buried the stoss side of the dune. There are striations on the dune surface that are discontinuous, finely spaced and parallel to the features (Fig. 4B,C). Striations with a complex pattern are locally observed. The second features (type 2) are up to 1.5 m wide and 3.0 m long, and located at the stoss slope of crescentic dunes with higher dip (8–12°, Fig. 4A). These features are connected and are interpreted to have formed during advance of the lava as a series of small lobes, which caused deformation of the sandy ground forming ridges at the front of lobes. The features imprinted in this way on to the dune surface are called crescentic ridges, previously called “crescent marks” by Scherer (2002). These crescentic ridges are composed of massive sand ridges, 0.5 to 3.0 m wide and up to 10 cm high (Fig. 5A). The spacing between the crescentic ridges is of about 1 to 3 m and between adjacent ridges the sand is only slightly modified,
preserving the wind ripples present on the aeolian dune's stoss slope (Fig. 5B). The height of the sand ridges increases, and the dimensions of the lobes decrease, toward the crest of the dune. In outcrop A, the advance of small lava lobes can be reconstructed in detail (Fig. 6), and the convex sides of the sand ridges indicate a lava palaeoflow direction to SE. Striations occur within type 2 features and on crescentic ridges, are discontinuous, closely spaced and parallel to the flow direction (Fig. 7A). Locally, near the crescentic sand ridges, some fragments of the crust of the lava lobes are detached, and pressed into the sand (Fig. 7B). 3.2. Outcrop B In outcrop B a linear dune is covered by lava flows and the contact can be observed in longitudinal and transverse sections. In the transverse section it is possible to identify the symmetrical geometry of the linear dune, characterized internally by a zigzag pattern of cross-strata that dip 17 to 130° and 23° to 280° (Fig. 8). The contacts between the lava and the sediments are sharp, and mixtures of lava and sand, such as
Fig. 10. (A) Sand diapir and breccia zone near the crest of a linear dune in outcrop B. (B) Breccia zone composed of vesicular lava clasts and friable sand. Scale = 8.5 cm.
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breccia-like rocks, are generally absent. Near the contact, a 15– 20 cm thick sandstone layer has undergone silicification. Type 3 features are irregular, up to 60 cm long and 10 cm wide and occur on the faces of linear dunes (Fig. 9A). The advance of lava toes that descended the dune face is interpreted to have formed these features, and locally the lava toes are preserved (Fig. 9B). Sand diapirs and breccia-like rocks are associated with the linear dune in outcrop B, both occurring at the base of the lava flow above the crest-line of the dune. The sand diapirs are composed of friable and more reddish sandstone, have diameters of 0.5–1.0 m, and occur only near the crest-line of the dune (Fig. 10A). In general appearance these structures are somewhat similar to the spiracles described by Moler and Cabrera (1976) in the Serra Geral Formation. The structures are
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connected to breccias composed of lava clasts mixed with sand. The sediment in the breccia domain is friable sand and does not show stratification or vesicles. The juvenile clasts are vesicular, have sub-rounded to sub-angular shapes, sizes ranging from 5 to 40 cm and quench margins are absent (Fig. 10B). The contacts of lava clasts and sand are marked by thin halos, related to posterior oxidation of the basalt. In a vertical section near the crest of linear dune, the structures formed of sand diaper and breccia zones have a roughly orientated form that may indicate local lava advance directions (Fig. 10A). 3.3. Outcrop C Outcrop C is characterized by a crescentic dune (150 m long and 27 m thick) under- and overlain by lava flows (Fig. 11).
Fig. 11. Geometry of a crescentic dune sandwiched between lava flows in outcrop C.
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Internally, the dune is composed of trough cross-beds dipping 22° to 290°. The stoss slope of the dune dips 10° to 15° and exhibits surface features up to 0.5 m in width and 4.0 m in length (type 4). On the upper portion of the stoss slope the features are elongate, with lengths up to 4 m and widths up to 50 cm (Fig. 12). Locally, small semi-circular features are inferred to have been formed by lateral breakouts. Some lobes are inflated and the increase in lava weight cause the deformation of sand with formation of channels and partially preserved lava lobes are observed filling these features (Fig. 13). The thickness of the first set of lava lobes that covered the dune is up to 20 cm and subsequent lobes had thickness up to 50 cm, and are observed in a near vertical exposure. Another surface feature observed on the inner portion of stoss slope is a set of corrugations probably related to generation of frontal breakouts. The inflation and pressure increase inside the lobe cause the fracture of the crust and generation of new lobes, in proximities of the break point hot pressurized lava probably move more fast forming the corrugations. 4. Emplacement of lava flows The preservation of the morphology of the dunes, inferred flow-emplacement styles, the surface features and structures
Fig. 12. Type 4 feature, up to 4 m long, on a stoss face of a crescentic dune in outcrop C. Arrows indicate frontal and lateral breakouts.
Fig. 13. Partially preserved lava lobe filling type 4 feature in outcrop C. Hammer = 26 cm.
described in above section permits the proposition of some models to explain the emplacement of the lava flows and the burying of active aeolian dunes in the study area. The structures found in the lava flows and the surface features indicate that flows that buried the dunes advanced in this area as lava toes and lobes forming compound pahoehoe lava flows. In the interdune regions the lava flows reach 40 m in thickness and show a massive aspect. The gentle advance of these flows is responsible for the preservation of the dune morphology and is reflected by characteristics the generation of the surface features here described. Infilling of an interdune area and burial of the stoss side of a large crescentic dune (outcrop A) is shown in Fig. 14. This burial model is similar to models presented by Jerram et al. (2000) and Jerram and Stollhofen (2002) in Etendeka flood basalts, NW Namíbia. The surface features indicate that lava flowed toward the SE, approximately transverse to cross-strata dip direction (NE). The lava flows first filled the interdune depression and then progressively covered the stoss slope of the dune (Fig. 14A, B). The relatively low-slope of the interdune trough allowed the pahoehoe flows to advance like unconfined sheets forming features with high amplitude (type 1). The occurrence of a complex pattern of striations can be related to the advance of multi-lobe sheet flows in these areas. On the stoss face, where the slope is higher, lava flows advanced as numerous lobes printing features type 2 (Figs. 14C and 5, 6). These lobes are inferred to have breakout from large inflated sheet lobes that filled the interdune area (Fig. 14C). Burial of linear dunes (outcrop B) in the study area is shown in Fig. 15. First the interdune area was filled by lava flows channelized among aeolian bedforms (Fig. 15A, B). At some points the lava flows reached the crest line and breakout lobes are inferred to have spilled out, forming lava toes that advanced down the dune surface and produce type 3 features (Figs. 15C and 9). Subsequent lava flows led to the complete burial of the linear dunes (Fig. 15D). The formation of breccias due to interaction between lava flows and sediments took place only near the crests of linear dunes, and is associated with sand diapirs (Fig. 10). The
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Fig. 14. Filling of an interdune area and covering of a large crescentic dune in outcrop A.
geometry of these structures is similar to spiracles, but the generation process is distinct. Spiracles are located at the base of the flows and the formation process is related to the advance of the lava over wet ground. However, the Botucatu Formation can be classified as a dry aeolian system (Scherer, 2000, 2002). It consist of cross-strata aeolian dunes bounded by first-order surfaces that do not have evidence of damp or wet interdune deposits, suggesting that the water table was located below the depositional surface during aeolian
accumulation. In this way, the breccia formation cannot be explained by lava–water interaction processes. Nevertheless something occurs in this area that led to mingling of the components. At the contact with dune the pahoehoe flows are dominated by lava toes that descend the face of linear dune. Probably the filling of the interdune by lava flows was inhomogeneous and the lava lobes did not reach the crest line at the same time. In the first stage, the lava flows are inferred to have reached the crests
Fig. 15. Emplacement of the lava flows and burial of a linear dune in outcrop B.
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Fig. 16. Emplacement of lava flows and burying of a linear dune. (A) Lava toes descending the face of the dune and (B) formation of lava lobes by coalescence of multiple toes.
and descended gently to form type 3 features (Fig. 16A). In the second stage, continued lava flow advance and inflation led to the formation of lava lobes by thickening and coalescence of the multiple toes (Fig. 16B). At some points along the crest of the dune the weight of the lava caused the injection of sand inwards into the flow lobe, forming the sand diapirs in the basal portion where the lava was more solidified (Fig. 17A). In the interior of a flow lobe, where lava is less viscous, injected sand can mingle with juvenile igneous clasts forming breccias-like rocks similar to fluidal peperite (Fig. 17B). The flow of lava within the advancing flow generates breccia domains that are elongate in the down-flow direction. The injection process is related to pressure caused by overrun lava flows coupled with the fact that unconsolidated aeolian sand has an average porosity between 47 and 49% (Atkins and McBride, 1991) with the pores filled with air. In this case, dry sand behaves as a granular fluid (Jaeger et al., 1996) and probably, the warming of the air will facilitate the injection process. The shapes of lava clasts (sub-rounded to sub-angular) are intermediate between fluidal and blocky peperite, but the generation process involves the injection of dry sand with granular fluid behavior into the lava flow. Besides the weight of lava was lowest at the crest, comparing with the dune faces, the flows along this line reach at 3 m thick indicating a considerable inflation after the sand injection.
A similar process of forming of breccia-like rocks is described in Paraná basin by Petry et al. (2007), but in this case the unconsolidated dry sand is injected as clastic dykes in the inner part of the flow forming rocks called “injection peperite” by these authors. The formation of peperite require a mingling in situ of magma and sediment and commonly, involves the presence of wet sediment in the process, with the fluidization of water facilitating the mingling of components (White et al., 2000; Skilling et al., 2002). Jerram and Stollhofen (2002) and Petry et al. (2007) argue, however, that the generation of peperites can occur without the presence of water, so that the lava fragmentation associated with unconsolidated or poorly consolidated sediment can form peperites too. In the study area the breccias are formed by the injection of dry unconsolidated sand into lava lobes. This mechanical process designated “sand injection” by Petry et al. (2007), can result in peperite-like textures. The discussion about the use of the term peperite will be avoided in this work, and the mixed rocks described here are designated “peperite-like breccias”. Outcrop C exposes a crescentic dune sandwiched between basalts. The cross-strata dip indicates that the dune was migrating to the NW, and features (type 4) indicate a lava advance direction to the SE during burial (Fig. 18). In this case, lava flows filled the interdune area and reached at the crest line
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Fig. 17. (A) Formation of a sand diapir by injection of sand inwards in the basal portion of the flow lobe; (B) breccias generated by injected sand mixed with juvenile igneous clasts at the interior part of flow lobe.
of the dune (Fig. 18A) When lava overtopped the crest it spilled out, forming lava lobes that advanced down the stoss face (Figs. 18B, 12). Features of this type have a high length/width ratio (5-8). 5. Concluding remarks The interactions between lava and sediment occurred in Paraná-Etendeka Igneous Province can be inferred in South America and Africa, and the interaction generates a variety of surface features, structures, breccias and peperite (Mountney et al., 1999; Jerram et al., 1999b; Jerram and Stollhofen, 2002; Scherer, 2002; Waichel et al., 2007; Petry et al., 2007).
In the study area, the covering of aeolian deposits by basaltic lava flows induced the complete preservation of dunes and generated sand-deformation features, sand diapirs and peperitelike breccias. The preserved dunes are crescentic and linear at their main contacts with the basalts of the Serra Geral Formation, with minor crescentic dunes where included within the sequence of lavas. Initially, the lava flows had to adjust themselves to the pre existent desert topography. The first flows filled the interdune areas and covered active migrating dunes; the still remaining mobile sands formed smaller migrating dunes and other bedforms that were engulfed and preserved between lavas in the lower part of the volcanic pile.
Fig. 18. Emplacement of lava flows that overrun simple crescentic dunes interlayered with basalts in outcrop C.
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The various features types formed on sediment surfaces by the advance of the flows reflect the emplacement style of the lavas, which were compound pahoehoe lava flows. The direction of the flows, the disposition and morphology of the dunes and the ground slope are the main factors governing the formation of the different types of features.
Type 1 features are related to the lateral advance of sheet flows in stoss face of dunes with slope N 5°, type 2 is formed where the lava flows advance in lobes and climb the stoss slope of crescentic dunes (slope 8–12°). Type 3 is generated by toes that descend the face of linear dunes (slope 17–23°) and type 4 occurs when lava lobes descend the stoss slope of crescentic
Fig. 19. A—General view with feature types described, related dunes, general and local lava flow directions. B—Determination of local flow advance based in features type 1 and associated striations, C—Determination of local flow advance based in features type 2 and associated striations, D—Summary of the feature characteristics.
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dunes (slope 10-15°). The characteristics of the features are summarized in Fig. 19A, D. These sand-deformation features and striations on dune surfaces can be used to determinate the local lava flow directions. Type 1 features and associated striations are parallel, both indicating the local flow direction (Fig. 19B). In type 2 features the convex side of crescentic ridges and striations indicate the local flow direction (Fig. 19C). The use and efficacy of these flow indicators to determination of general lava flow direction is directly related to dimension and continuity of the exposure area. Striations are generated by the imprinting of the basal skin of lava lobes on dune surfaces. Basal skin of sheet lobes (type 1 features) and lobes on stoss face of dunes (type 2 features) have fine grooves that formed the striations. The grooves of lava skin are more finely spaced and continuous in type 2 features compared with type 1. Lava lobes descended stoss face of crescentic (type 4 features) and lava toes on linear dunes (type 3 features) have smooth basal skin. The occurrence of pahoehoe lava toes and lobes on surfaces with high slopes (10 to 23°) is associated with substrates of unconsolidated sand. Probably the soft characteristics of this substrate inhibits the increase of velocity of the lava due to the increase in slope and pahoehoe flows do not change to ‘a‘a. The injection of unconsolidated sand into lava lobes forms diapirs and peperite-like breccias. Sand diapirs penetrate at the basal portions of lobes, where the lava was more solidified, and peperite-like breccias in the inner portion where lava was more plastic, favoring the mingling of components. The generation of both features is related to a suite of mechanical processes, with the overload due to the weight of lava and the warming of the air in the pores inducing a granular fluid behavior of dry sand, leaving to the injection of the sand into lavas. The present paper adds to the model presented by the few previous workers (Jerram et al., 1999b, 2000; Scherer, 2002; Petry et al., 2007) who have analyzed the emplacement style of the first stage of the Paraná–Etendeka lavas, addressing to the burial of the Botucatu palaeoerg. The presented features and the discussion about their origin aim to improve the understanding of the emplacement style of these first lava flows. Acknowledgments We thank the students Gabriela Rizzon, Richard Kalil and Círio Simon who helped us to clean, with shovels, spades and brooms, some of the fine outcrops here presented. We also thank JDL White and D Jerram for careful revision and comments that improve the final version. References Atkins, J.E., McBride, E.F., 1991. Porosity and packing of Holocene river, dune, and beach sands. AAPG Bulletin, 75:3; Annual meeting of the American Association of Petroleum Geologists (AAPG). Dallas, TX (United States), p. 535 (7-10 Apr 1991). Bellieni, G., Comin-Chiaramonti, P., Marques, L.S., Melfi, A.J., Picirillo, E.M., Nardy, A.J.R., Roisenberg, A., 1984. High- and Low Ti flood basalts from the Paraná plateau (Brazil): petrogenetic and geochemical aspects bearing on
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