Seismic Volcano-Stratigraphy in the Basaltic Complexes on the Rifted Margin of Pelotas Basin, Southeast Brazil
Published:December 01, 2015
- PDF LinkChapter PDF
Andres Cesar Gordon, Webster U. Mohriak, 2015. "Seismic Volcano-Stratigraphy in the Basaltic Complexes on the Rifted Margin of Pelotas Basin, Southeast Brazil", Petroleum Systems in “Rift” Basins, Paul J. Post, James Coleman, Jr., Norman C. Rosen, David E. Brown, Tina Roberts-Ashby, Peter Kahn, Mark Rowan
Download citation file:
The synrift and breakup stages of the Pelotas basin in southeast Brazil are characterized by scarce siliciclastic deposits and widespread volcanism in the form of seaward-dipping reflectors (SDRs). Using high-quality seismic reflection and refraction profiles integrated with gravity, magnetics, and exploratory boreholes, a volcanostratigraphic analysis has been undertaken to understand the geological processes observed during the rifting and breakup stages of this segment of the South Atlantic continental margin. Ten volcanic units have been identified and mapped within the extended continental crust and into the transitional and oceanic crusts. The magmatic cycle began during the early synrift stage, with alkaline, high TiO2 basalts produced at 125 Ma. This was followed by the formation of a series of voluminous tholeiitic, high TiO2 SDR wedges during the late synrift and breakup stages. The end of the breakup process was marked by flat-lying, late synrift/early postrift, tholeiitic, low TiO2 basalts at 118 Ma. During the Late Cretaceous and Early Paleogene, the magmatic activity continued only in the oceanic crust, forming igneous intrusions (volcanic cones or seamounts).
A comparison between the Pelotas basin and the Lüderitz and Walvis basins offshore Namibia is discussed by integrating regional geological maps, potential field methods, seismic data, and results of exploratory drilling. The SDR province in the Pelotas basin coincides geographically with the Paraná basin continental flood basalts onshore Brazil, which crop out near the coastline. This makes the Pelotas basin an ideal place to understand the relationships between the tectonic-magmatic events that preceded and continued during the Gondwana breakup, which resulted in the development of continental margin rift basins and the formation of the South Atlantic Ocean.
This contribution is a regional analysis of the volcanic rifted margin of the Pelotas basin in the southeast Brazilian Atlantic margin, focusing on the seismic stratigraphy of its voluminous volcanic flows. This analysis includes a comparison with neighboring volcanic provinces, such as the Paraná basin continental flood basalts in Brazil, and analysis of the volcanism in the Walvis and Lüderitz basins along the conjugate margin of West Africa. We also discuss the relationship between lithospheric extension and volcanism observed in the Pelotas, Paraná, Walvis, and Lüderitz basins in order to understand the possible tectonic mechanisms and magmatic episodes that operated during the formation of a passive margin regime throughout the Gondwana breakup (Fig. 1).
The Brazilian seismic reflection data set comprises approximately 15,000 km of 2D seismic lines, courtesy of the “Agência Nacional de Petróleo (ANP),” the Brazilian Navy (LEPLAC Project), and the IONGXT website showing public data. The potential field data integrates ANP data sets with publically available worldwide gravity and magnetic data sets (Sandwell and Smith, 2009; Maus et al., 2009). The well data set provided by ANP includes 28 exploratory wells drilled in the Pelotas and Paraná basins. The work has been completed by integrating refraction seismic profiles (Fig. 2) after Ewing et al. (1963) and Leyden et al. (1971). For the West African segment counterpart, the data set used includes 2D seismic lines and exploratory wells from published data.
The South American/West African and the South Atlantic large igneous provinces
Two of the world’s most extensive large igneous provinces are found in the South Atlantic, across the conjugate continental margins of South America and West Africa (Menzies et al., 2002). These are the Paraná-Etendeka continental flood basalt province (CFB in figures) and the South Atlantic seaward-dipping reflector wedges (SDR) province (Fig. 1), which are both related to Gondwana breakup and the opening of the South Atlantic Ocean (Gladczenko et al., 1997; Courtillot et al., 1999). The volcanic units of interest here are related to the northwest arm of the South Atlantic SDR province, here referred to as the Pelotas volcanic province (abbreviated VP in figures). There is an extensive array of geological literature that focuses on the Paraná-Etendeka continental flood basalt province and the South Atlantic SDR provinces. In order to discuss the linkage between these large provinces and the Pelotas volcanic province, a regional overview of the magmatic episodes geochronology is discussed below.
The Paraná and Etendeka continental flood basalt provinces
The large igneous province of Paraná, also known as the Serra Geral Formation in Brazil (White, 1908), consists of lava flows, dykes, and sills. The Paraná continental flood basalt province crops out from southeast Brazil to northern Argentina, eastern Paraguay, and northwest of Uruguay in South America, and correlates with the magmatic province of Etendeka in Namibia (Fig. 1; Erlank et al., 1984; Bellieni et al.,1984). Regional estimates suggest depositional areas in the order of 1,2000,00 km2, and 1.7 km of thickness have been drilled in the deepest parts of the Paraná basin (Bellieni et al., 1986a, b; Zalán et al., 1987; Peate, 1997; Nardy et al., 2002; Frank et al., 2009). Compositionally, the Paraná continental flood basalt province consists of basic rocks (97%) and minor acid rocks, named (sensu lato) as “basalts” and “rhyolites” respectively (Hawkesworth et al., 2000), although petrologically, the province comprises a broader range of compositions.
There is lack of consensus among researchers about the age, duration, magma type distribution, magmatic sources, and magmatic differentiation processes that generated the voluminous magmatism. There is a large number of radiometric ages published in the geological literature (e.g., Almeida et al., 2013) and in order to display the variability exhibited, a compilation of several 39K/40Ar, 40Ar/39Ar, 87Rb/86Sr, 238U/206Pb ages have been included in the probability density plot of Figure 3. The large age dispersion explains the different opinions in reference to the temporal-geographical evolution of this province and reflects the analytical difficulties of dating basalts, resulting from using different radiometric techniques and laboratories (Gibson et al., 2006).
In a radiometric ages reinterpretation, Gibson et al., (op cit), indicated that the initiation and finalization of the Paraná magmatic cycle, as a whole, could be marked by the alkaline complexes of Paraguay, followed by the main tholeiitic lava flows and the dyke swarms. The ages for the alkaline complexes range from 146.7 to 124.6 Ma; the main tholeiitic lava flows between 139 to 127 Ma (with a magmatic pick activity at 134/132 Ma); and the dyke swarms at 134 to 127 Ma (Gibson et al., 2006; Renne et al., 1996a; Rocha Campos et al., 1988; Turner et al., 1994; Ulbrich and Gomes, 1981; Peate, 1997).
Thiede and Vasconcelos, 2010, summarized the disputes about the age of the main tholeiitic lava flows of the Paraná continental flood basalt province generated by conflicting 40Ar/39Ar geochronology data sets (Renne et al., 1992, 1996a, 1996b, 1997; Turner et al.,1994; Stewart et al., 1996). To resolve this disagreement, Thiede and Vasconcelos (op cit), applied the laser incremental methods and re-dated three of the oldest and youngest samples of Turner et al., (1994) and Stewart et al., (1996). Their analyses resulted in statistically indistinguishable new ages that confirmed the previous 134.7 ± 1 Ma result, supporting a short time-span hypothesis for the Paraná continental flood basalt province volcanism (Thiede and Vasconcelos, 2010).
The volcano-stratigraphy of the Paraná CFB has been approached mainly by geochemical analysis of minor, trace and isotopes elements. The Paraná magma types distinguished by Peate et al., (1992) are defined solely on compositional characteristics and are not stratigraphically defined units. Peate et al. (op cit) identified several different magmatic types grouped in high and low TiO2 suites, but there is a strong agreement among the researchers that the Paraná magma types cannot be considered chronostratigraphic units (Turner et al., 1994, 1999; Garland et al., 1995; Stewart et al.,1996).
The large spread of magmatic compositions suggests variations in mantle sources, as well as differentiation mechanisms such as fractional crystallization and crustal contamination. Contrasting hypotheses have been proposed to explain the source of the large volcanic volumes, such as: (1) a lithospheric continental mantle, (2) a fertile asthenospheric mantle, (3) variable melting of a single mantle source, and (4) heterogeneous mantle sources in conjunction with magmatic differentiation mechanisms (Fodor, 1987). There is a general agreement that the origin of the Paraná continental flood basalt province is related to the passage of the Tristão da Cunha mantle plume through the South American plate during the Early Cretaceous. But in most of the petrological-based published contributions, there is a lack of a direct geochemical correlation between the Paraná rocks and the actual Tristão da Cunha and Gough islands rocks, along with the NMORB basalts, according to Peate, 1997. The role of the Tristão mantle plume appears to have been largely passive, by conductive heating facilitating mobilization of old lithospheric material (Peate, 1997).
There are also questions about the amount of partial fusion and the rate of extension needed in the process of generation of this volcanism during the rifting of the western Gondwana (Ernesto et al., 1999; Ewart et al., 1998, 2004; Kirstein et al., 2001; Hawkesworth et al., 2000). Alternative models for the origin of Paraná continental flood basalts province, as well as for the addition of heat to the crust by lithospheric stretching, can be found in Mohriak et al., 2002.
The Etendeka igneous province forms the easternmost extents of the much larger Paraná-Etendeka large igneous province that originally spanned the incipient continental margins of southern Brazil and Southwest Africa. Magmatic rocks crop out mainly in the Namib desert of northwest Namibia (Fig. 1), but remnants of the Etendeka province are scattered north and south in Nigeria, Angola, and in the Orange basin (Jerram et al., 1999; Marzoli et al., 1999; Reid et al.,1994; Gladczenko et al., 1998). This province erupted in the Early Cretaceous interval of about 122 to 139 Ma, a more narrow time span when compared with the Paraná large igneous province (Fig. 3). The areal extension is in the order of about 80,000 km2 and has a maximum preserved thickness of about 900 m (Erlank et al., 1984; Milner et al., 1995). The good exposures in three-dimensional outcrops have allowed detailing the volcanic stratigraphy of the magmatic rocks mainly in the Huab Basin (Milner et al., 1995; Jerram et al.,1999). The Etendeka lavas are divided stratigraphically into two volcanic sequences separated by a disconformity (Jerram et al., 1999). Similarly to the Paraná province, the Etendeka province igneous rocks vary in composition from basic tholeiitic basalts to alkaline rocks and dyke swarms; however, the latter are less expressively developed than in the Paraná’s dyke swarm province.
The oldest and youngest radiometric ages in Etendeka are associated with alkaline intrusive rocks (Allsop and Hargraves, 1985; Issa et al., 1991; Milner et al., 1995), the same behavior has been reported by Gibson et al. (2006) for the Paraná continental flood basalts. The magmatic origin of the Etendeka lava flows province is also in debate. Most basalts of the Etendeka province do not show Tristão da Cunha hot spot signatures, except for the Damaraland igneous complexes. Plume signatures have been found in gabbros of the Okenyenya complex, gabbros and syenites from the Messum complex, and in the Brandberg anorogenic granite intrusion, which is the highest mountain in Namibia (Milner et al., 1995; Trumbull et al. 2007; Schmitt et al., 2000; Fig. 1). In the Paraná-Etendeka province, the debate on the absolute dating of the igneous rocks and the magma origin is still ongoing in the geological literature.
The South American SDRs province
The South Atlantic SDRs province extends along the southern South Atlantic Ocean in both the South American and the West African continental margins (Gladczenko et al., 1997). The South American SDRs branch prolong from the Golfo San Jorge basin, south Argentina, to the southern part of Santos basin, Brazil (Fig. 1), covers an estimated area in the order of 1,186,000 km2, and has an extruded volume of 2,400,000 km3 (Gladczenko et al., 1997; Hinz et al.,1999; Eldholm et al., 2000; Schumann, 2002; Franke et al., 2002, 2007). The West African branch extends along the coast of South Africa and Namibia. A volume of approximately 420,000 km3 is estimated for this segment of the South Atlantic (Gladczenko et al., 1998), indicating an asymmetric distribution both in prerift magmatism (Serra Geral Formation in the Paraná basin, Etendeka volcanics in Namiba) as well as in the syn- to postrift magmatism across the conjugate margins of Brazil and Namibia-South Africa. The SDRs zone reaches its maximum expression, in both the South American and the West African branches, near the Rio Grande Rise and in the Abutment Plateau, respectively (Gladczenko et al., 1998).
Regionally, the South Atlantic SDRs province seems to have evolved diachronously from south to north. The province accompanies the opening of the South Atlantic Ocean and the formation of the rifted basins of the conjugate margin (Rabinowitz, 1976). The “zipper style” of opening is interrupted by the major continental lineaments, controlling the magma distribution, shifting, and modifying the SDRs architecture (Soto et al., 2011).
The continental lineaments have been interpreted as weakness zones of inherited basement fabrics, probably in connection to the modern oceanic fracture zones (Hinz et al., 1999; Franke et al., 2002, 2007). The lineaments divide the margins into elongated segments of 4000 km long and 50 to 100 km wide. The volcanic activity inside each segment appears to have developed temporally and volumetrically from south to north in a cone-shape distribution pattern (Hinz et al., 1999; Franke et al., 2002, 2007; Mohriak, 2001; Soto et al., 2011) and is associated with oceanic propagators as defined by Hey, 2004. This mechanism repeats systematically in each segment, indicating an intermittent rifting propagation (Hinz et al., 1999; Franke et al., 2002, 2007). But in the proximity to the Rio Grande Rise and the Abutment Plateau, the conic shape distribution inverts, displaying an increase in magma volume from south to north (Gladczenko et al., 1997, Stica et al., 2013). The lithospheric stretching and adiabatic decompression is considered the main source of this voluminous magmatism (Hinz et al., 1999; Franke et al., 2002, 2007), except for the area under the influence of Tristão da Cunha plume, as observed in the magmatic distribution map of Pelotas basin (Fig. 4).
In the Torres Arch area, in southern Santos basin, the South American SDRs province is almost in physical continuity with the Paraná continental flood basalts province (Figs. 1, 2, and 4). In the area of study, the Pelotas volcanic province segment covers an area of 150,000 km2 (Fig. 4). The magmatic activity in the Pelotas basin increases oceanward, as opposed to the landward increase in activity of the volcanic flows of the Paraná continental flood basalts. This change in thickness indicates a major tilting in the area of deposition of the lava flows. Although both provinces (Paraná and Pelotas) share geochemical affinities such as low and high TiO2 tholeiitic suites (Lobo, 2007), the Pelotas volcanic province is younger than the main volcanic activity of the Paraná continental flood basalts (Fig. 3). This is also corroborated by seismic interpretation of the seaward-dipping reflector wedges which feather out towards the upper part of the syn-rift troughs along the continental margin.
The Pelotas Basin
The Pelotas basin is located between 29° to 36.5° of latitude south and 43.5° to 53.5° of longitude west, extending from the southern coast of Brazil to the northern coast of Uruguay (Fig. 2). The basin is separated from the Santos basin (Brazil) by the Florianópolis Fracture Zone to the north, and from the Punta del Este basin in the south by the Cabo Polonio High in Uruguay (Figs. 1 and 2). The basin is bounded to the west by the Paleozoic Paraná basin and by the Don Feliciano Precambrian onshore belt (G2 on Fig. 2). The basin covers an estimated area of about 226,000 km2. It is characterized by a limited onshore area of sedimentary outcrop, but the majority of the basin is offshore, extending to water depths up to of 4600 meters beyond the continental-oceanic crust boundary (Fig. 2). The Pelotas basin can be sub-divided into the northern, central, and southern sub-basins based on geological variations in the width, along the strike, of the seaward dipping reflectors zone (Fig. 4); the proposed sub-basin boundaries are the Porto Alegre and the Chui fracture zones (Bassetto et al., 2000; Vital Bueno et al., 2007; Stica et al., 2013).
The Pelotas basin exhibits distinctive geological features that allow its differentiation from the neighboring basins:
The presence of a volumetrically significant synrift and postrift volcanism, including seaward dipping reflectors (SDRs);
A subordinate presence of terrigenous Early Cretaceous synrift deposits, which infill narrow lacustrine troughs along the proximal continental margin;
Scarcity of evaporite deposits, except for anhydrite in the northern region (i.e., at the Florianópolis platform, Fig. 2);
Important marine sediment accumulation, reaching maximum thickness in the Rio Grande Cone area, in the central to southern segments of the basin (Fig. 2), where a thick depocenter exceeding 4000 m in thickness formed in the Miocene (Saunders and Bowman, 2014).
The crystalline basement in the offshore Pelotas basin likely consists of the Precambrian Don Feliciano mobile belt from the Mantiqueira Province (Heilbron et al., 2004). This basement crops out along the southeast Brazilian coast, from Santos to Pelotas basins, extending to the west and probably occurring underneath the eastern edge of the Paleozoic Paraná basin (G2 on Fig. 2). The Don Feliciano mobile belt comprises igneous and high grade metamorphic poly-orogenic rocks, developed from Neoproterozoic to Ordovician times (Heilbron et al., 2004).
The South America and African breakup magnetic anomalies (EMAG2; Maus et al, 2009) are well delineated in the satellite magnetic map offshore southeast Brazil (Fig. 5). In this map, it is possible to define four linear magnetic anomalies that may correspond to anomalies LMA, M4, M2, and M0, with ages estimated around 133 Ma, 130 Ma, 127 Ma, and 126.7 Ma, respectively (Moulin et al., 2010). The eastern boundary of the LMA anomaly coincides with the ‘G anomaly’ of Rabinowitz and Labrecque (1979), initially interpreted as the boundary between continental and oceanic crust. The LMA magnetic anomaly has more recently been interpreted to be related to the prebreakup basalts of the Paraná continental flood basalts (Moulin et al., 2010; Stica et al., 2013), and the M4 to M0 anomalies have been used to demarcate the ocean crust boundary (Moulin et al., 2010; Blaich et al., 2011).
In the sedimentary section of the Brazilian Atlantic rifted margin, four main tectonic-stratigraphic units are recognized (Chang et al., 1992; Mohriak, 2003). They are known as prerift, rift, transitional, and postrift megasequences. The prerift megasequence is composed of Paleozoic and Mesozoic sedimentary and volcanic rocks from the Paraná basin. Outcrops from this megasequence are recognized in the Torres syncline area (Fig. 2). In this area, the Paleozoic section of the Paraná basin is covered by the Cenozoic sediments from the Pelotas basin (Dias et al., 1994). The Neocomian-Bar-remian synrift megasequence is characterized by antithetic faults and half graben filled by siliciclastic deposits and lavas (Dias et al., 1994). Towards the eastern margin of the basin, several wedges of seaward dipping reflectors occur from the platform to the ultradeep water province (Stica et al., 2013). The transitional (evaporitic) megasequence exhibits a very restricted distribution, and is only found in the Florianópolis platform region (Fig. 2). This unit consists of anhydrite and it has been penetrated by a few exploratory wells that drilled trachyandesite layers underneath the evaporites (Dias et al., 1994). The postrift megasequence consists of marine strata deposited from the Albian onwards.
The seismic volcano-stratigraphy of Pelotas basin
This study of the Pelotas basin volcanism is based on regional interpretation of seismic data and well records. Today, 20 boreholes have been drilled in the basin and some of them have sampled sills and lava flows (Figs. 2 and 8). The good quality deep crustal seismic sections available in the area allow the interpretation of ten volcanic units in the transition from the continental to the oceanic crust. The volcanic units identified are named as units “A,” ”B” to “F,” “G1,” “G2,” “H,” and “I” (Table 1), and additionally, three deep seismic facies are recognized and interpreted as the reflection Moho, the continental crust and the typical oceanic crust. The identification of the different volcanic units is based on the differences in the dips of the reflectors and in the onlap-offlap relationships among the reflectors packages.
The volcanic activity in the area of study has a geographical and temporal evolution that indicates a physical linkage between the Paraná continental flood basalts and the Pelotas volcanic province near the Torres syncline (Fig. 4). Volcanism in the Paraná basin (Paraná continental flood basalts) began during a prerift stage (Late Jurassic–Early Cretaceous). The deposits increase in thickness landwards (towards the intracratonic basin depocenter). Later, a major tilting changed the distribution of these volcanic deposits, probably due to the initiation of the Pelotas basin rifting process in the Early Cretaceous. The Paraná basin lavas (Serra Geral Formation) are covered by the first volcanic units of the Pelotas volcanic province (volcanic unit “A”), indicating a shifting of the depocenters from land to ocean. Subsequently, very pronounced wedges of SDRs have developed from shallow to deep waters as a consequence of the evolution of the rifting process (volcanic units “B” to “F”). Finally, the magmatic activity decreased with the deposition of the volcanic unit “H,” which sealed the previous volcanic wedges, marking the late rift/post rift tectonic control of the volcanic distribution (Table 1, Fig. 4).
Volcanic unit “A”
Volcanic Unit “A,” also known as the Imbituba Formation in the formal stratigraphic column of the Pelotas basin (Vital Bueno et al., 2007), has been sampled and cored by the exploratory well 1-RSS-3RS, which drilled 810 meters of amygdaloidal basalt (Table 1). The unit is located in the western part of the basin and is believed to correspond to subaerial lava flows, according to cutting and well core descriptions (Figs. 2 and 6 to 9). This unit exhibits a maximum seismic thickness of 345 msec (approximately 600 m using the well log velocity) and a map view distribution highlighted by the blue polygon in Figure 4, extending from the southern Santos basin to the boundary with the Punta del Este basin offshore Uruguay. The depositional style of the volcanic unit “A” is characterized by sheet drape external form having parallel to sub-parallel reflector patterns. But locally, this unit displays wedges with divergent seismic patterns in half-grabens, which are bounded by a counter-regional normal fault system, indicating the initiation of the rift phase by extensional faults that mainly dipped landwards (Figs. 6 and 7).
The gamma ray and sonic logs of well 1-RSS-3RS display important alternations of high and low log values that may be interpreted as compositional changes or differential weathering affecting the basalt layers. For example, the sonic velocity log exhibits interval changes that range from 3900 to 5900 m/s (Fig. 9 and Table 1).
Geochemical and geochronological analysis performed in the 1-RSS-3RS lower cored section (at 3800-3907 m) indicates ages of 125.3± 0.7 Ma (40Ar/39Ar) and 124± 8.6 Ma (39K/40Ar) (Mizusaki, 1990; and Lobo, 2000). Compositionally, this unit has an alkaline affinity and high TiO2 content (Lobo, 2000 and 2007). Recently, this unit has been considered an equivalent of the Paraná continental flood basalts (Stica et al., 2013), but the geochronological record (Fig. 3), and in addition, the shift in the distribution of the lava flow depocenters, do not allow this correlation (Misuzaki and Saracchine, 1990; Lobo, 2007).
Volcanic units “B”-“F”
The volcanic units “B” to “F” comprise sets of volcanic wedges in the Pelotas basin which have an internal divergent reflection patterns that dip seawards (Figs. 6, 7, and 9). The wedges progressively become younger to the east and have been interpreted as seaward dipping reflectors (SDRs) by Fontana (1996, Abreu (1998), and Talwani and Abreu (2000). The differences in structural dips and reflection terminations, among the wedges, allowed the subdivision of the main SDRs package into Units “B” to “F” (Fig. 6 and Table 1). The Units “B” to “F” are not affected by the initial rift tectonism, but the change in the reflector dips and the onlap relationship reflects different structural stages during the deposition of the flows. These units are likely to have been deposited in subaerial to submarine environments during the last rifting phase.
Approximately 300 meters of Unit “C” have been drilled by the exploratory borehole 2-BPS-6BP, corresponding to amygdaloidal (at the top of the layer) and microcrystalline basalts (Fig. 9). Geochemically, the tholeiitic basalts have high TiO2 contents (Lobo, 2007).
The radiometric age reported by Lobo (2007) does not agree with the seismic stratigraphy analysis. The averages of 16 fractions of 40Ar/39Ar of well 2-BPS-6-BP (sampled at 6161 m) indicate an age of 75 ± 0.8 Ma, much younger than expected. The last two fraction analyses indicate an age of 138±5 Ma, and Lobo (2007) believed this calculated age is representative of this volcanism, considering it equivalent with the Paraná continental flood basalts main tholeiitic flows of the Serra Geral Formation (139 to 127 Ma; Gibson et al., 2006). In our opinion, this age is not consistent with the seismic stratigraphy relationship observed in the area. The clear onlap relationship of the SDRs wedges of volcanic units “B” to “F” onto the older Unit “A” (125.3± 0.7 and 124± 8.6 Ma) observed in the seismic record does not allow this temporal assignment. These inconsistencies, in addition to the analytical problems, lead us not to consider the radiometric age reported by Lobo (2007) as representative of the SDR wedge drilled by the well 2-BPS-6-BP, which we interpreted bottomed at volcanic unit “C” (Fig. 9).
The sonic log velocity of unit “C” recorded alternation of low/high velocities layers (3900 to 5900 m/sec, Fig. 9 and Table 1). The seismic reflection analysis indicates a variation in thickness of the individual units from about 500 to 1400 msec (approximately from 2 to 3.5 km considering refractions velocities). The SDRs display a lateral-shift stacking depositional pattern and an increase in the magmatic volume from south to north (Figs. 4, 6, and 7; Stica et al., 2013), which differs from the general behavior observed in the southern South Atlantic SDRs province. The SDRs wedges mapped along the coast of Argentina and Uruguay decrease in magmatic volume from south to north, in an opposite pattern to that found in the Pelotas basin (Hinz et al.,1999; Schumann, 2002; Franke et al., 2002, 2007). The same pattern can be observed in the Abutment Plateau, Namibian coast (Gladczenko et al., 1998).
The stack with lateral shift of units “B” to “F,” observed in the seismic profiles, caused an overlap of the different volcanic units. This superposition does not allow the direct relationship between the volcanic units and the magnetic responses (Figs. 5 and 6) as proposed by Moulin et al., 2010.
Igneous units “G1” and “G2”
The units “G1” and “G2” comprise two types of seismic mounds: the deeper mounds (unit “G1”) and the shallow mounds (unit “G2”). Both units “G1” and “G2” exhibit elongated distribution in plan-view with mounded shape in cross section (Figs. 4, 6, and 7). The mounded features are present at different crustal levels and they exhibit different relationships with the main SDRs flows. Three belts of the deeper, intracrustal unit “G1” can be recognized along a dip direction (Fig. 4), forming the belts G1’, G1”, and G1’”. The belts are progressively younger and shallower from west to east, and they are downlapped by the volcanic Units “B” to “F” (or the main SDRs packages), as can be noted in Figures 6 and 7. In our interpretation, the deep-mounded features of unit “G1” are likely to be related to the feeder system of the main SDRs flows, representing the remnants of aborted spreading centers (Mohriak, 2001; Mohriak et al., 2008).
A 2D forward gravity modeling has been performed for the deep seismic line A, in the northern Pelotas basin (Blaich et al., 2013). This modeling required the presence of a high density crust below the SDRs wedges to fit the observed gravity values. This may be indicative of a heavily intruded lower crust and can be interpreted as the region of a feeder system related to the emplacement of the SDRs wedges (Blaich et al., 2013).
The unit “G2” (shallow mounded features covered by sediments) are interpreted as volcanic centers (Fig. 10). Equivalent seismic geometries have been described as “outer highs” by Planke et al, (2000) and they have been drilled by the Deep Sea Drilling Project (DSDP) site 554 in Greenland (Planke and Alvestad, 1999). The published results of this drill site indicate hyaloclastic and volcanoclastic flows developed in a shallow marine environment, and they have been interpreted as the volcanic feeder system of the SDRs units (Planke et al., 2000).
Volcanic unit “H”
Unit “H” has been drilled by several wells in the Pelotas and in Santos basins (i.e. 1-RSS-3RS, 2-BPS-6BP, 2-SCS-2RS and 1-SCS-1RS), as observed in the well cross section of Figure 9 (well location in Fig. 2).
The analysis performed in well 1-RSS-3RS indicates that unit “H” corresponds to a gray to brown basaltic suite having amygdaloidal texture, displays a tholeiitic low TiO2 affinity, and has an 39Ar/40Ar age of 118.3± 1.9 Ma (Lobo, 2007).
Boreholes 2-BPS-BP and 2-SCS-2SC (Figs. 2 and 9) have neither geochemical nor radiometric analyses, but cutting descriptions refer to amygdaloidal trachyandesite, basalt, and trachyandesites having amygdaloidal textures. Intercalations of the volcanic units with calcarenite sediments and the presence of gas release textures are frequently reported in the mud log analysis of those wells. These descriptions are indicative of the extrusive character of this unit, which probably formed in shallow waters and even in subaerial depositional environments (ANP, well file reports).
Well 1-SCS-1-SC, located at the southern portion of the Santos basin (Fig. 2), drilled gray to brown porphyritic trachyandesite and basalts, having reported ages of 114±3 Ma (39K/40Ar) and 113.2±0.1 Ma (39Ar/40Ar; ANP well file report and Misusaki, 1990). We assigned this volcanic layer to unit “H”, which probably overlies the older volcanic wedges “D” to “F” in the Florianópolis Platform (Fig. 4). The well log of unit “H” reveals numerous intercalations of pairs of high/low density and velocities values, probably indicative of the volcanic flows interacting with unconsolidated sediments (Fig. 9 and Table 1).
This unit is an extensive and relatively thin, flat lying volcanic layer, which postdates and overlays the SDRs main packages in deep water (Figs. 6, 7, 9, and 10). Unit “H” shows parallel to sub-parallel reflection configuration, sheet drape external form, and a low structural relief. Unit “H” represents the final episode of the postrift volcanic flows.
Volcanic unit I
Unit “I” represents the latest magmatic episode of the Pelotas basin and consists of several volcanic constructions or build-ups that are only present on the oceanic crust (Figs. 11A to C, seismic lines locations in Fig. 2). Unit “I” increases in magnitude from south to north, in the direction of the São Paulo Plateau in the Santos basin (Fig. 1). They can vary from isolated volcanic cones up to large volcanic structures (Figs. 11A to C). Until now, none of these constructions have been drilled in the Pelotas basin, consequently, no radiometric ages from these postrift events are available; however, geological correlation with oceanic volcanic islands in neighboring basins and known magmatic cycles onshore allowed several authors to estimate ages for these cycles, which probably range from Late Cretaceous to Early Paleogene (Asmus, 1984; Cainelli and Mohriak, 1998; Mohriak et al., 2002 and references therein).
The South Atlantic Conjugated Margins of Pelotas Basin and Walvis—Lüderitz Basins in West Africa
The physical continuity of the volcano-stratigraphy described in the Pelotas basin has been investigated in the Namibia conjugate margin in West Africa using two regional selected seismic profiles, key well correlations, and a palinspastic reconstruction map at 125 Ma using GPLATES software (Seton et al., 2012). The location of the exploratory wells and the seismic lines reconstructed at prebreakup times is indicated in Figure 12. The regional seismic profiles, named as the “northern profile (northern Pelotas and Walvis basins), and the central profile (central Pelotas and Lüderitz basins), exhibit a quite conspicuous volcanic symmetry in both margins by displaying similar packages of SDRs (Figs. 13 and 14).
The northern profile across the northern Pelotas and Walvis basins (Figure 13A) shows a voluminous development of SDRs. At least five sequences of SDRs can be recognized and tentatively correlated in both margins before the development of the oceanic crust. The correlation has been tested in the conjugate margins by exploratory boreholes that have drilled volcanic rocks younger than the Serra Geral–Etendeka flood basalts. When reconstructed to their predrift position, wells 2-BPS-6-BP (Brazilian margin) and 1911/15-1 (Namibian margin) indicate that they sampled two equivalent volcanic units (Fig. 13B) below the Albian carbonates. The well 2-BPS-6-BP penetrated distinct volcanic units (units “H” to “C”). The radiometric age obtained for volcanic unit “C” (wedges of tholeiitic/high TiO2 SDRs basalts) is not reliable, but these SDR units are stratigraphically positioned above the lower unit “A” (ca. 124/125 Ma) and below the upper unit “H” (ca. 113/118 Ma).
The borehole 1911/15-1, in the Walvis Basin, drilled two volcanic units named as Groups W1 and W2-1 (Holtar and Forsberg, 2000). The lower group W1 comprises 563 m of a series of basaltic lava flows that display a typical SDR configuration and are separated by tuffaceous layers and occasionally very thin beds of siliciclastic strata (Figs. 12, 13A and 13B). Petrographically, the lavas display textural variations from massive, glassy, and non-amygdaloidal layers to reddened and amygdaloidal textures, suggesting cycles of subaerial lavas flows intercalated with sediments. Chronologically this unit has been assigned an age not younger than Barremian (ca. 130-125 Ma) by Holtar and Forsberg, 2000. We suggest that the volcanic layer W1 correlates with the volcanic unit “C” in the Pelotas basin (Figs. 9 and 13).
The upper group W2-1 consists of 76 m of muddy limestones, marls, and glauconitic claystones at the base, and two major flows of basalts at the top. Structurally this unit displays sub-horizontal seismic reflections (Fig. 13A). Unfortunately, there are no published radiometric ages for the volcanic units of well 1911/15-1, but the fossils in the carbonates rocks of W2-2 group above the W2-1 basalts indicate an age from late Aptian to middle Albian (Holtar and Forsberg, 2000). This suggests that the W2-1 basalts are possibly correlated with the younger basalts in the northern Pelotas explorary well (volcanic unit “H”).
The central profile across the Central Pelotas and Lüderitz basins (Fig. 14A) displays a lesser degree of magmatic activity during the Gondwana breakup compared to the northern profile, and as a consequence, only two sequences of SDRs (volcanic units “B” and “C”) have been identified in the Pelotas profile before the formation of the oceanic crust. The diminishing of the magmatic volume from the northern to the central segments of the rift basin, in both the Brazilian and Namibian margins, has also been reported by Stica et al (2013), Koopman et al (2014), and Holtar and Forsberg (2000). The well 1-RSS-3RS, located in the shallow water portion of the Pelotas basin (Fig. 14B), has drilled two volcanic units below a stratigraphic succession interpreted as a Late Aptian rift deposits. The upper unit, assigned to the volcanic unit “H,” consists of approximately 180 m of a tholeiitic/low TiO2 basalt, and the lower unit, here assigned as unit “A” (Fig. 9), consists of 520 m of alkaline/high TiO2 basalt, dated as 118.3 ± 1.9 Ma and 125 ± 0.8 Ma respectively (Table 1, Lobo, 2007).
The well 2513/8-1 in the Lüderitz basin has drilled basalts assumed to be older than 120 Ma by its stratigraphic position below Lower Albian sediments (Green et al., 2009). Structurally these volcanic rocks are located at the top of a wedge of rift deposits, probably of Barremian age (Green et al., 2009). Unfortunately the scarcity of published information about this well does not allow the precise correlation of this basaltic unit with the volcanic units “H” or “A” from the central Pelotas basin. However, the presence of synrift deposits below these volcanic rocks suggest a younger volcanic episode (Early Albian/Late Aptian), at the transition from W3 to W2 stratigraphic units of Holtar and Forsberg (2000), thus indicating a probable correlation with the flat layers interpreted as volcanic unit “H” in the Brazilian margin borehole. It is important to note that the “H” volcanic layer in the Pelotas basin well 1-RSS-3-RS is located below a rift unit interpreted as Late Aptian, whereas the equivalent layer in the well 2513/8-1 offshore Namibia is placed above the Barremian-Aptian rift sequence.
A contrasting difference observed in this cross section in the central Pelotas–Lüderitz basins is the remarkable asymmetry of the postrift sedimentary successions. The Tertiary section of the Pelotas basin is approximately three times thicker than the Tertiary section of the Lüderitz basin (Fig. 14A). This large difference seems to be controlled by the deposits of the Rio Grande Cone in the Brazilian Margin, which is characterized by an extremely thick depocenter of Tertiary sediments in the central to southern segment of the Pelotas basin (Fig. 2).
Summary and Conclusions
The conjugate margins of the Pelotas basin, in southern Brazil, and the Namibia basin (Walvis and Lüderitz basins), in southern West Africa, can be classified as volcanic rifted margins in which magmatic activity displays a temporal and geographical evolution from continental breakup to oceanic crust formation (Garland et al., 1996; Gladczenko et al., 1998). The magmatic cycle that starts prior and along with the Gondwana breakup displays two major magmatic subcycles: the prerift Paraná-Etendeka continental flood basalts and the syn-/postrift volcanic successions of Pelotas, Walvis, and Lüderitz basins, which are characterized by thick wedges of SDRs (Courtillot et al.,1999; Gladczenko et al., 1997).
The age of the Paraná-Etendeka province ranges from Valanginian to Barremian, with a peack of magmatic activity during the Hauterivian. In both the Paraná and Etendeka provinces, the magmatic activity seems to start and finish with alkaline rocks, but the main volcanic activity consists of tholeiitic basalt flows. The lava flows of the Paraná continental flood basalts reach their maximum thickness in the central part of the Paraná basin and pinch-out towards the continental margin of the Santos and Pelotas basins, where Precambrian rocks crop out near the coastline.
There are several magma types in the Paraná continental flood basalts and they have been grouped into high and low TiO2 suites, which are related to melting of different mantle masses. In a general sense, the high TiO2 suites are more abundant in the northwest and the low TiO2 are more important in the southeast of the province (Garland et al., 1996; Peate, 1997).
During Barremian to Aptian times, the Pelotas basin developed relatively narrow rift troughs during the main extensional phase. This is expressed by small rift blocks controlled by antithetic faults affecting prerift sediments (Paleozoic sediments from the Paraná Basin) and Serra Geral basalts, and subsequently the magmatism shifted the main locus from inland to oceanward. The first magmatic phase of the Pelotas volcanism started with the volcanic unit “A,” in a synrift tectonic domain, at the Barremian/Aptian boundary. Compositionally this volcanism initiated with alkaline/high TiO2 basalts. Subsequently, the volcanism evolved into a series of SDR wedges (units “B” to “F”) of tholeiitic/high TiO2 basalts during the Aptian. The SDR wedges display an oceanward evolution, becoming younger basinward and developing unconformities between the different wedges, thus reflecting the changes in the structural configuration of the basin during the late rift/early postrift transition. Additionally, deep and shallow mounded features that can be identified in the seismic records (units “G1” and “G2”) may represent magma conduits (magma feeder systems) in the crust. The highly intruded and rather dense lower crust is supported by 2D forward gravity modeling.
Regionally, the SDR wedges (Units “B” to “F”), decrease in number of wedges and in the volumes of magmas involved from north to south. This behavior has also been observed in the conjugate West African margin. The north to south SDRs evolution, observed in the vicinity of the Rio Grande/Walvis chain, behaves in an opposite way to the SDR wedges that occur in the Southern South Atlantic margin along the Argentinean coast (Hinz et al., 1999; Schumann, 2002; Franke et al. 2002, 2007). The magmatic activity in the transitional crust of the Pelotas basin finishes with the flat volcanic layers of unit “H,” in a postrift stage around late Aptian times. Compositionally this unit “H” is made of tholeiitic/low TiO2 basalts.
The Pelotas volcanic province exhibits a geochemical evolution from alkaline to tholeiitic affinities and from high to low TiO2 basalts. Experimentally, the partial melting of less than 25% of an enriched lherzolite mantle source at adiabatic decompression can explain the evolution from alkali basalt to olivine tholeiitic basalt, tholeiitic basalt, and quartz tholeiitic basalt (Jaques and Green, 1980). Alkaline rock implies lower degree of partial melting and very high pressure regimes (Wilson, 1989). Later, during the evolution of the rifting, the increment of the lithospheric thinning and the higher degree of partial melts might explain the shift of this volcanism into tholeiitic affinities (Wilson, 1989). The high TiO2 suites are interpreted to be related to a high pressure (or depth) of magma generation, a lower adiabatic decompression with a lesser degree of partial melts, and minor lithospheric thinning of a garnet-rich lithospheric mantle source (Lobo, 2007). The low TiO2 suite, on the contrary, reflects: a lower pressure regime, a higher amount of adiabatic decompression associated with a greater degree of partial melts, and a major lithospheric thinning of a garnetfree lithospheric mantle source (Lobo, 2007). The evolution of the volcanism the Pelotas volcanic province is in agreement with the increase of the lithospheric thinning during the rifting process.
In both magmatic provinces across the southern South Atlantic (Paraná-Etendeka and Pelotas/Walvis-Lüderitz) there is a clear impact of the Tristão da Cunha plume, but according to the numerous published geochemical studies, the effect of the plume appears to be, in general, more thermal than compositional, with the exception of volumetrically minor lavas in both margins. The effect of the plume is also reflected in the high velocity/density lower crust (underplate) extending below the rift zone and the of the Paraná basin. The underplate effect has been recorded and modeled in several gravity, magneto-telluric, and refraction studies in both conjugate margins (Gladczenko et al., 1997, 1998; Molina et al., 1988), and is expressed in the deep seismic profiles available in the southeastern Brazilian and southwestern African continental margins (Bauer et al., 2000).
The authors wish to thank the Agência Nacional de Petróleo and the Brazilian Navy for providing the seismic data used in this contribution. We also thank the GCSSEPM referees for several comments and suggestions that helped improve the final text of this manuscript.
Figures & Tables
Petroleum Systems in “Rift” Basins
- Atlantic Ocean
- basin analysis
- Cape Basin
- continental margin
- flood basalts
- geophysical methods
- geophysical profiles
- geophysical surveys
- gravity methods
- igneous rocks
- large igneous provinces
- Lower Cretaceous
- magnetic methods
- Pelotas Basin
- petroleum exploration
- plate tectonics
- seismic methods
- seismic profiles
- South America
- South Atlantic
- Southern Africa
- Southwest Atlantic
- volcanic rocks
- southeastern Brazil
- Luderitz Basin