Paleoweathering features in the Sergi Formation (Jurassic-Cretaceous), northeastern Brazil, and implications for hydrocarbon exploration

Journal of South American Earth Sciences 29 (2010) 412–426

Contents lists available at ScienceDirect

Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames

Paleoweathering features in the Sergi Formation (Jurassic-Cretaceous), northeastern Brazil, and implications for hydrocarbon exploration q Cristina Pierini a,*,1, Ana M. Mizusaki a,2, Nuno Pimentel b, Ubiratan F. Faccini c, Claiton M.S. Scherer a a

Instituto de Geociências, Universidade Federal do Rio Grande do Sul (UFRGS), 91501-970 Porto Alegre, RS, Brazil Departamento e Centro de Geologia da Universidade de Lisboa, 1749-016 Lisboa, Portugal c Programa de Pós-Graduação em Geologia, Universidade do Vale do Rio dos Sinos (UNISINOS), 93022-000 São Leopoldo, RS, Brazil b

a r t i c l e

i n f o

Keywords: Paleoweathering Stratigraphy Jurassic-Cretaceous

a r t i c l e Palavras-chave: Paleoalterações Estratigrafia Juro-Cretáceo

a b s t r a c t Paleoweathering in the Sergi Formation has been classified and analyzed to ascertain its origin and relationship with stratigraphic evolution. The Sergi Formation belongs to the pre-rift sequence of the Recôncavo Basin (northeastern Brazil) and comprises a complex association of eolian and fluvial sandstones and lacustrine mudstones. This formation can be subdivided into three depositional sequences bounded by regional unconformities. Four paleoweathering types, each one related to a distinct origin, have been described in the Sergi Formation: (1) textural mottling, which is distinguished by alternating rock colors as a result of the iron oxide mobilization within mineral phases that evolved under alternating oxidation (yellowish, brownish and reddish shades) and reduction (grayish or greenish hues) conditions; (2) non-textural mottling, which displays a discoloration pattern that is independent of the original rock texture; (3) carbonate concentrations, usually related to carbonate nodule formation, which display a massive internal structure that reveals their origin through continuous growth or crystallization; and (4) banded carbonates (silicified), associated with the beginning of regular surface formation due to the chemical precipitation of carbonates within lacustrine environments. Both mottling color motifs and carbonate accumulation usually represent groundwater oscillation rather than pedogenesis. Only carbonate intraclasts and banded carbonate (silicified) have their origin ascribed to pedogenesis sensu stricto, although the carbonate intraclasts do not represent soil deposits in situ, but calcretes eroded from areas close to channels, and the banded carbonates (silicified) have strong diagenetic modifications. Therefore, it is reasonable to assume that fluvial and meteoric water have controlled paleoweathering evolution as well as deposition, yet both aspects are ruled by the same mechanisms (relief, sedimentation rate and, above all, climate). Ó 2009 Published by Elsevier Ltd.

i n f o

r e s u m o As paleoalterações da Formação Sergi foram classificadas e estudadas com o objetivo de determinar sua gênese e sua relação com a evolução estratigráfica. A Formação Sergi faz parte da seqüência pré-rift da Bacia do Recôncavo (NE Brasil) e é composta por uma complexa justaposição de arenitos eólicos e fluviais e pelitos lacustres. Essa formação pode ser subdividida em três seqüências deposicionais, limitadas por discordâncias regionais. As paleoalterações da Formação Sergi foram classificadas em quatro tipos com diferentes interpretações genéticas: (1) marmoreado textural é caracterizado por uma alternância espacial de tons na rocha, resultante da mobilização dos óxidos de Fe em fases minerais onde ocorre a alternância entre estágios de oxidação (colorações variando entre o amarelo, castanho e vermelho) e redução (colorações variando entre cinza ou verde); (2) marmoreado não-textural apresenta descolorações independentes das texturas originais da rocha; (3) concentrações carbonáticas estão relacionadas, principalmente, à formação de nódulos de carbonato, os quais apresentam estrutura interna maciça que reflete o modo como foram formados pelo crescimento ou cristalização contínuos; e (4) carbonatos bandados

q

The article has been printed as an uncorrected proof * Corresponding author. E-mail address: [email protected] (C. Pierini). 1 ANP Doctorate Scholarship. 2 CNPq Researcher.

0895-9811/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.jsames.2009.04.002

413

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

(silcificados), que estão associados ao início da formação de superfícies regulares, interpretados como resultado da precipitação química de carbonatos em ambiente palustre. O padrão de cores do marmoreado, bem como as acumulações de carbonatos não são, na sua maioria, produtos de processos pedogênicos, mas refletem predominantemente a atividade de águas subterrâneas e de variações do nível freático. Os intraclastos carbonáticos e os carbonatos bandados (silicificados) têm sua gênese relacionada a processos pedogênicos em sentido estrito. Deste modo é natural que as águas fluvial e meteórica sejam os fatores essenciais para que a evolução das paleoalterações na Formação Sergi acompanhe a tendência deposicional, já que ambos são controlados pelos mesmos fatores –clima, relevo e taxa de sedimentação. Ó 2009 Published by Elsevier Ltd.

1. Introduction Continental terrigenous successions consist commonly of deposits with heterogeneous grain sizes. This characteristic affects rock porosity and permeability, influencing the circulation of interstitial fluids and then the precipitation of several minerals, such as carbonates (nodules), oxides and clay minerals, which might produce different types of paleoweathering (e.g. Birkeland, 1999; Retallack, 2001). Paleoweathering features have a singular effect on the petrophysical characteristics of hydrocarbon reservoirs within terrigenous successions (Hanneman et al., 1994). Paleoweathering features are abundant in the Sergi Formation, which is the main and the largest continental reservoir of the Recôncavo Basin (northeastern Brazil), containing 362 million m3 of oil in place (Netto, 1978; Lanzarini and Terra, 1989). However, previous studies of this unit have not dealt with important points related to the origin of the paleoweathering (i.c., subaerial, subaqueous or groundwater-related), as well as its relation to paleogeographic settings. The main objective of this study is to describe paleoweathering features in the Sergi Formation in order to analyze their genesis, understand their depositional relation to the systems and analyze their role in the heterogeneity and compartmentation of hydrocarbon reservoirs, helping to improve the knowledge of their influence on hydrocarbon exploration and production.

2. Paleoweathering Paleoweathering is herein employed as a general term for a set of changes that took place under influence of past climate conditions

H2O

H2O vapor

CO2

CO2

due to the proximity to the depositional surface (including the vadose and groundwater zone). Either intermittent or incessant sedimentation tends to expose or bury materials, keeping them at or distant from the land surface, hence yielding distinct paleoweathering products. In fluvial systems, a non-deposition and subaerial exposure on a flood plain favor soil development. Below the pedogenetic zone, alluvial sediments are altered by groundwater action, producing phreatic weathering. Therefore, a detailed study of paleoweathering features might help to understand the evolution of alluvial systems. Weathering occurs after sediment deposition, during the thousands to millions of years, right after deposition or long after deposition, when a deposit is unburied. Based on their original depth, two types of paleoweathering can be distinguished (Fig. 1): Pedogenic paleoweathering or paleosol refers to particular type of paleoweathering that took place in the first meters below the depositional surface in association with plant colonization and consequent soil formation. It includes a superficial aerated zone and a lower vadose zone, which is just above the phreatic level. In the vadose zone either water or air can fill both sediment and sedimentary rock pores, as a result of the capillary effect that takes place just above the groundwater table. Pedogenic weathering transforms soils, rocks and sediments, hence above groundwater level, and usually presents well-developed and differentiated profiles. Frequently, pedogenic horizons might be absent due to erosion and truncation during the soil profile development (Alonso-Zarza et al., 1998). Phreatic paleoweathering refers to those alterations related to the initial burying of sediments (down to tens of meters) that are in direct contact with the atmosphere or biosphere. This type of weathering also takes place under a strong influence of the phreatic zone that might oscillate through time. Phreatic weathering often produces the

Fluvial Soil-Forming Processes

Soil Mixture Zone

AERATED

VADOSE

Gravitational Water Zone Capillary Fringe Groundwatwer

PHREATIC

Pedogenic Calcretes WATER TABLE

Descending Water

Capillary Transport

Phreatic Calcretes

Non-Available Water

Fig. 1. Illustration showing the position of the aerated, vadose and phreatic zones; the first two constitute the pedogenic zone (adapted from Carlisle, 1983).

414

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

replacement of the original sediment, including intense dissolution of detrital silicates. As recharge water reaches the surface, it is primarily free of any enrichment but, during its downward course, a progressive increase of the Mg/Ca ratio takes place and leads to calcite and subsequent dolomite precipitation (Wright, 1995). Phreatic weathering commonly produces mottling feature forming parallel bands. Mottling is characterized by the occurrence of diverse colors in the rock as a result of iron oxide mobilization within mineral phases caused by alternating oxidation (brownish and red) and reduction (gray and green) phases.

Whenever carbonate precipitation takes place at shallow depths, phreatic weathering can spread laterally for hundreds of kilometers affecting depths up to 10 m (Carlisle, 1983; Arakel, 1986; Arakel et al., 1990; Maizels, 1987). The main carbonate precipitation mechanisms are evaporation, evapotranspiration and CO2 loss (Wright and Tucker, 1991). In this context, it is important to define the term ‘‘calcrete”, applied herein sensu Khadkikar et al. (1998): calcrete is a near-surface accumulation of predominantly calcium carbonate whose formation within the vadose and phreatic zone is mediated either

2. Pedogenic calcretes are formed if the sedimentation rate is low. The soil-forming, vadose and phreatic processes interfere with each other as the phreatic level rises.

1. Alluvial sedimentation. The groundwater calcretes form in the phreatic and capillary zones.

LEGEND:

carbonate mud

clastic deposit

3. The base level surpasses the depositional surface, hence forming a shallow lake. Carbonate mud deposited in the lake bottom is colonized by plants.

mottling

P.L.

groundwater calcretes

P.L.

palustrine deposits

lacustrine deposits groundwater calcretes

Phreatic Level

groundwater pedogenic calcrete calcrete

groundwater alluvial calcrete deposition

Lake Level

4. The lake level drop causes the exposure and pedogenic modification of the carbonate mud (palustrine deposits).

pedogenic carbonate nodule

groundwater carbonate nodule

Fig. 2. Model representing a complete cycle of phreatic level rise and fall within a distal alluvial or fluvial system. The superposition of phreatic imprints on pedogenic calcrete is common. Both phreatic and pedogenic features can change previous lacustrine deposits (adapted from Alonso-Zarza, 2003).

38° 30'

38° 00'

R/2

A/1 A/6

EA OC C TI AN AT L

R/1 A/2 A/4

PACIFIC OCEAN

12° 00'

N

BRAZIL

TUCANO BASIN

0

A/3

1000 Km

A/5

RECONCAVO BASIN

12° 30'

G/2

AT LA

NT I

C

OC

EA N

G/1

13° 00' 10

20 Km

Legend Basement (Precambrian) Exposure area of the Brotas Group (Sergi Formation) G/2

Cored wells Cross-section (supplied in Figure 11)

Fig. 3. Recôncavo Basin setting with its exposure area, described cores and cross sections locations.

415

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

mudstone of the Aliança Formation (Capianga Member), abruptly overlain by the lacustrine mudstones of the Itaparica Formation (Bruhn and De Ros, 1987; Lanzarini and Terra, 1989; Carrasco et al., 1996; Lanzarini, 1996). The Sergi Formation occurs along the entire Recôncavo Basin, although it crops out only in its northern and western borders (Fig. 3). According to Scherer et al. (2007), the deposits of the Sergi Formation can be subdivided into three, third-order depositional sequences, limited by regional unconformities, which record distinct depositional episodes in the basin: Sequence I, recognized in all studied wells and outcrops, consists of a complex juxtaposition of eolian and fluvial sandstones and lacustrine mudstones. Its thickness ranges from 100 to 160 m. The lower boundary cannot be precisely delineated, as it interfingers with the underlying Capianga Member (Aliança Formation). Its upper boundary consists of a regional-scale

STRATIGRAPHIC CHART RECONCAVO BASIN TIME M.a.

PERIOD

1.6

QUATERNARY

LITHOSTRATIGRAPHY (Formations) & LITHOLOGY

TECTONIC EVOLUTION

SE

NW SPA

BARREIRAS

TERTIARY

110

SABIÁ

MARIZAL 120

SÃO SEBASTIÃO

POJUCA

130

CRETACEOUS

TAQUIPE

MARFIM PIT

SALVADOR

through pedogenic (soil related) or groundwater processes, or both. Calcretes are important components of semi-arid depositional systems and characteristic of regions experiencing a mean annual rainfall of around 500 mm (Wright and Tucker, 1991). Therefore, calcretes are not only confined to soil profiles, when called pedogenic calcretes (Netterberg, 1980; Machette, 1985; Wright and Tucker, 1991; Tandon and Gibling, 1997; Williams and Krause, 1998). They can also be formed, for instance, below the soil-forming zone, as groundwater or phreatic calcretes (Wright and Tucker, 1991; Spötl and Wright, 1992; Rodas et al., 1994; Pimentel et al., 1996; Williams and Krause, 1998; Pimentel, 2002). Distinguishing pedogenic from phreatic calcretes and, amongst the latter, discriminating those formed under vadose or phreatic influence is important for paleogeographic reconstructions. Terrestrial carbonates can be formed under a large range of environments, from permanent water bodies to entirely sub-aerial settings. Paludal environments and ephemeral lakes can also be included in this large spectrum of conditions. Palustrine carbonates are typical of lake borders and wetlands. Their development depends on the existence of flat, low energy surfaces (Nickel, 1985; Platt, 1989; Platt and Wright, 1992). It is possible to describe a continuous transition from lacustrine to subaerial environments. The distinction between palustrine carbonates and groundwater or pedogenic calcretes is rather difficult due to the fact that minor changes in the phreatic level can lead to major environmental modifications (Fig. 2). This implies that only through a careful analysis of microfacies and specific textures can these various carbonate products be differentiated. Climate plays a major role on terrestrial carbonate formation just as it controls surrounding environments. Palustrine carbonate formation is favoured by strongly seasonal, semi-arid or sub-humid climates. However, it is important that enough precipitation takes place to carry carbonate-rich solutions, both as superficial runoff and groundwater (Alonso-Zarza, 2003). Pedogenic calcretes are also associated with less humid climate zones (mean annual rainfall lower than 500–600 mm). However, it is possible to quote several examples of pedogenic calcretes ascribed to more humid periods (Goodfriend and Magaritz, 1988; Sancho and Meléndez, 1992; Alonso-Zarza and Silva, 2002). Groundwater calcretes also need a certain amount of humidity, although high evaporation and/or evapotranspiration rates are also required for carbonate chemical precipitation (Mann and Horwitz, 1979).

RIFT

MARACANGALHA 140

AS

EI

D AN

3. Geological setting

C

GRANDE

AGUA

ITAPARICA SERGI

PRE-RIFT

A NÇ

JURASSIC

IA

AL

150 250 PERMIAN

AFLIGIDOS

450

PRE-CAMBRIAN

LEGEND

The Recôncavo Basin (Fig. 3) covers an area larger than 10,000 km2 in northeastern Brazil. Its evolution is related to the crustal stretching that led to Gondwana breakup and the consequent separation of Africa and South America during the opening of the South Atlantic Ocean. The basin is enclosed in the Afro-Brazilian Depression, a huge NW–SE intracontinental rift that resulted from this tectonic process. It is possible to recognize three major tectonic systems in the Recôncavo Basin: pre-rift, rift and drift. The Jurassic-Cretaceous Sergi Formation is included in the pre-rift phase (Milani, 1987; De Cesero and Ponte, 1997) (Fig. 4). The pre-rift phase represents a long-lasting subsidence and consequent formation of an intracratonic basin, in which the continental sediments of the Brotas Group accumulated. The Sergi Formation is part of the last sedimentation stage of the Brotas Group, and represents a continental depositional system characterized by alternating fluvial-eolian sandstones and red lacustrine mudstones. At the base, this unit is inter-layered with lacustrine

SANDSTONE

MUDSTONE

SALT

CONGLOMERATE

BASEMENT

UNCONFORMITY STUDY UNIT

Fig. 4. Chrono-lithostratigraphic table from the Recôncavo Basin emphasizing the Sergi Formation position (adapted from Milani et al., 1994).

416

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

erosional surface (Scherer et al., 2007). Sequence I is composed of lacustrine mudstones at its base, overlain by fine- to mediumgrained sandstones deposited by eolian dunes, eolian sand sheets and ephemeral fluvial streams (Table 1). Sequence II ranges from 0 to 80 m in thickness and its lower boundary comprises a regional scale unconformity, identified both in cores and outcrops. Its upper boundary is also defined by an erosional surface that can be correlated throughout the basin. Sequence II comprises braided channel belt deposits that show no changes in terms of depositional architecture. Sequence III is 0–30 m thick and is bounded, at its base, by a flat erosional surface that truncates the braided channel fluvial strata of Sequence II. The upper boundary of Sequence III cannot be identified within the studied interval. Sequence III is composed of fineto medium-grained sandstones ascribed to sheet-floods, eolian dunes and eolian sand sheets at the base, overlain by lacustrine

deposits. This sequence records the return of depositional conditions similar to Sequence I, although dominated by eolian processes relative to rare fluvial and lacustrine deposits (Scherer et al., 2007). 4. Methodology Samples and data were obtained from 10 cored wells, two from the central (G/1, G/2), two from the northeastern (R/1, R/2) and six from the northwestern (A/1, A/2, A/3, A/4, A/5, A/6) portions of the Recôncavo Basin (Fig. 3). During borehole description, the presence of some particular features, such as color (according to Munsell, 2000), discoloration or carbonate accumulation, was noticed. Their depth of occurrence, associated rock and the way they change the rock aspect (e.g. discoloration associated with stratification) were then recorded.

Table 1 Summary chart of the facies associations and depositional sequences identified in the Sergi Formation (based on Scherer et al., 2007). Sequences

Facies association

Description

Interpretation

I

Lacustrine

These facies associations can be organized into two main vertical trends: (i) 1–4 m thick packages entirely composed of either reddish, massive or flat laminated mudstones with ostracodes (Caixeta et al., 1994) and plant debris; (ii) 1–4 m thick, coarsening-upward successions, composed of either massive or laminated mudstones at their base, sometimes displaying mudcracks, overlain by finegrained sandstones with ripple cross-lamination, and of fine- to medium-grained sandstones at the top, massive and/or with trough cross-bedding

Ephemeral fluvial channels

This facies association is made up of several, 1–4 m thick sandstone bodies, width/ thickness ratio >30. The bounding surfaces of basal sandstone bodies are flat to concave-up and are marked by 10 cm thick lags of intraformational conglomerates (calcrete and/or mudstone intraclasts). Internally the sand bodies are composed of fine- to medium-grained sandstones, massive, horizontal to low-angle crossbedded or with planar and trough cross-bedding (0.2–0.5 m thick sets) and less commonly ripple cross-lamination This association occurs as 3–20 m thick (4 m on average) and hundreds of meters wide tabular bodies composed of well-sorted, fine- to medium-grained sandstones arranged in 0.5–3 m thick cross-bedded sets. Internally, the sets are composed of 1–4 cm thick, massive to inversely-graded grainflow strata on the steeper portions (30°) of the foresets. Down dip, grain flow strata pinch out, intertonguing with tangential wind-ripples toesets These deposits consist of white, well-sorted, fine- to medium-grained sandstones arranged in tabular bodies up to 8 m thick and hundreds of meters wide. Internally, these units are characterized by 0.3–2 m thick sets of horizontal to lowangle cross-stratification (<5°) composed of inversely graded, up to 10 mm thick, wind-ripple laminae

Massive and laminated mudstones represent the settling of suspended matter within water bodies. The ostracode fauna in a continental setting (Caixeta et al., 1994) suggests a lacustrine environment. The presence of mudcracks indicates ephemeral shallow lakes. The coarseningupwards cycles represent lake marginal deposits formed due to the deceleration of distal streams as they enter water bodies (delta-front deposits) This facies association is interpreted as fluvial channel deposits. The presence of dominantly massive and parallel to low-angle cross-bedded sandstones suggests poorly confined sheet flood deposits

Eolian dunes

Eolian sand sheets

The presence of well-sorted and well-rounded, fine- to medium-grained sand grains, organized as large-scale sets of cross-strata composed of wind-ripples and grain flow strata allows the interpretation of facies association as residual deposits of eolian dunes The horizontal to low-angle cross-bedded sandstones are interpreted as eolian sand sheets deposits formed by the migration and subcritical climbing of eolian ripples on a dry depositional surface

II

Braided fluvial channels

This facies association is composed of sheet-like sandstone bodies, 2–10 m thick, that can be extended laterally for more than 500 m (maximum outcrop extent). The sandstone bodies are bounded by flat to concave-up erosive surface and internally show an upward fining in grain size, with basal massive to horizontal laminated conglomerates grading upward into coarse to medium-grained sandstones with trough and planar cross-bedded, 10–20 cm thick sets

The fining-upward sandstone bodies bounded by upward concave erosive surfaces can be interpreted as fluvial channel deposits. The sheet geometry of the sand bodies, the prevailing coarse-grained nature of the deposits and the dominance of sand bedforms suggest that this facies association consist of braided fluvial channels-belts deposits

III

Ephemeral fluvial channels

This facies association consists of 1–3 m thick, fine- to medium-grained, wellsorted sandstone packages, which can be either massive, display horizontal lamination or rarely trough cross bedding, with mudstone intraclasts dispersed or concentrated along stratification planes. Sandstone packages may be overlain by massive or laminated mudstones, forming fining-upward successions This up to 30 m thick facies association comprises very fine- to medium-grained sandstones with low-angle cross-lamination composed of inversely graded, 5– 10 mm thick, wind-rippled strata. Fine- to medium-grained sandstones, organized as isolated or grouped trough cross-bedded sets, are also present. Foresets dip average 20° and they are predominantly composed of wind-ripple laminae. Grain flow and grainfall strata are rare and restricted to the upper part of the foresets

This facies association is interpreted as fluvial channel deposits. The presence of dominant massive and parallel to low-angle cross-bedded sandstones suggest poorly confined sheet flood deposits

Eolian sand sheets

Lacustrine

This association is composed of thinly bedded, greenish gray to gray mudstone. The beds range in thickness from <1 to a few cm. The beds are composed of laminated mudstones with streaks and thin cross-laminated lenses of siltstone or very fine-grained sandstones, containing plant debris and ostracodes

Low angle strata composed of eolian ripples are interpreted as eolian sand sheets. Trough cross bedding composed of wind-ripple and grainflow/grainfall strata are interpreted as residual deposits of eolian dunes. The dominance of horizontal to low angle wind-ripple lamination, associated with the restricted occurrence of cross-stratification, suggests that the eolian dune bedforms were spatially isolated and migrated across extensive eolian sandsheet plains These deposits are thought to have accumulated in offshore parts of lake environments, and their sedimentary characteristics indicate suspension fallout combined with weak, transient traction currents

417

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

The proposed differentiation of the Sergi Formation paleoweathering types was initially based on macroscopic features, such as their relationships with the original rock texture, presence of carbonates, structures, and colors. The calcium carbonate content was visually estimated through hydrochloric acid reaction (diluted to 10% of 1 M of solution), providing the distinction of five stages that are proportional to the amount of calcium carbonate (Retallack, 1997, 1998). Later, this visual estimate was confirmed by geochemical analysis. Thin sections helped to refine the classification of distinct types of paleoweathering. The volumes of detrital and diagenetic components were determined by counting 300 points in 64 representative thin sections. After their formation, the paleoweathering features underwent diagenetic processes identified on thin sections, and their products were specified as ‘‘total diagenetic components”. Some samples were prepared following the procedures recommended by Mizusaki (1986), and analyzed in a JEOL scanning electron microscope (JSM-5800 model) coupled with a dispersive energy detector (EDAX), which contributed for a more precise mineralogical and morphological characterization. Chemical analyses, including major and trace elements, were performed with ICP/MS technique from Activations Laboratories Ltd. (ACTLABS, Canada). The samples were run for major oxides and selected trace elements on a combination simultaneous/ sequential Thermo Jarrell-Ash ENVIRO II ICP or a Spectro Cirros ICP. Calibration was performed using seven prepared USGS and CANMET certified reference materials. d13C and d18O isotope analyses were performed at the Laboratory of Isotopic Geology of the Pernambuco Federal University, with calcite fragments selected from fourteen paleoweathering

samples, prepared according to Al-Aasm et al. (1990). Precision was ±0.02‰ for results presented according to both PDB and SMOW standards (Craig, 1957). Eight samples of carbonate fragments were selected for Sr isotope analysis of calcite minerals to determine the 87Sr/86Sr ratio following the method proposed by Kawashita (1972). The analyses were performed in a VG Sector-64 mass spectrometer at Laboratory of Isotopic Geology of the Rio Grande do Sul Federal University.

5. Paleoweathering: description and origin The following four types of paleoweathering, with distinct characteristics, were identified in the Sergi Formation, both in outcrops and in core samples (Table 2): 5.1. Macroscopic features 5.1.1. Textural mottling This kind of paleoweathering presents discoloration in accordance with the original texture of the rock. It is possible to identify the following two examples: (1) Textural mottling with preserved lamination (Fig. 5A), characterized by alternating colors and preservation of the primary sedimentary structures of the rock, as well as textural and mineralogical features, and: (2) Textural mottling with deformed lamination (Fig. 5B), distinguished by alternating colors without preservation of the original sedimentary structures. This pattern occurs within

Table 2 Summary of the characterization, origin and colors of the distinct types of paleoweathering of the Sergi Formation (Recôncavo Basin). Paleoweathering type

Macroscopic features

Origin

Colors

T1 – Textural mottling

With preserved lamination – alternating colors and preservation of the rock original structures With deformed lamination – alternating colors with no preservation of the rock original structures

Textural heterogeneities control the percolation of the meteoric reducing fluids with some carbonate

Brown, dark brown, light grey, pale olive

T2 – Non-textural mottling

With reduced spots – alternating colors forming irregular masses isolated within the rock

Percolation of reducing, carbonate fluids took place along an irregular path through the rock promoting diffuse reduction and carbonatio

Brown, dark brown, light grey, pale olive

Venules – origin associated with phreatic level oscillations and precipitation in the vadose zone Isolated nodules – the nodules clear delineation and the lack of any pedogenic or biogenic imprint point to an origin related to precipitation from phreatic fluids within restricted areas of the rock Carbonate intraclasts – palustrine calcretes removed from areas adjacent to fluvial channels and enclosed as intraclasts

White, pinkish white

The banded structure is primary, a result of the chemical precipitation of carbonates within paludal environments (sporadically exposed shallow lakes) whereas silicification is a secondary, diagenetic feature that reproduces primary textures

White, light olive grey

With diffuse carbonation – alternating colors accentuated by carbonation T3 – Carbonate concentration

Venules – accumulations of calcium carbonate (CaCO3) forming an intricate network of millimeter-scale veins Isolated nodules – small and isolated calcite accumulations as a result of infiltration of carbonated solutions through the rock Carbonate intraclasts – juxtaposition of centimeter-scale carbonate masses within a clay matrix, which can be amalgamated or coalescent

T4 – Banded carbonates (silicified)

Discrete bands – whitish, horizontal and centimeter-scale bands with irregular borders that can be discrete or amalgamated, hence producing decimeter-scale horizons

Juxtaposed bands – very compact rock with remains of horizontal banding and overall brecciated aspect caused by a network of thin, almost orthogonal cracks

418

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

Fig. 5. Composite stratigraphic column of the Sergi Formation showing the occurrence of the main types of paleoweathering according to their stratigraphic position. A = textural mottling with preserved lamination; B = textural mottling with deformed lamination; C = non-textural mottling with reduced spots; D = carbonate concentration forming isolated nodules; E = carbonate concentration forming carbonate intraclasts; and F = carbonate (silicified) with discrete bands. In the left side of the column, chemical trends of SiO2, Al2O3, Fe2O3 and CaO.

heterogeneous beds with post-depositional deformations (e.g. convolution, bioturbation or clastic dykes).

5.1.2. Non-textural mottling This type of paleoweathering consists of discoloration (involving the same colors of textural mottling) independent of the original texture. Two types were recognized: (1) Non-textural mottling with reduced spots (Fig. 5C), differentiated by alternating colors, produced by iron oxide mobilization, which form irregular and isolated spots in the rock, and: (2) Non-textural mottling with diffusive carbonate accumulations, similar to the previous example, but with an important carbonation phase that appears in the matrix of the rock. It occurs associated with both siltstone and sandstone intervals.

5.1.3. Carbonate concentration This kind of paleoweathering is usually linked to carbonate nodule formation. Nodules present light colors (white and pinkish white) and a commonly massive or laminated internal structure, hence indicating a development related to a continuous growth or crystallization (Retallack, 2001). Three kinds of carbonate concentration were differentiated: (1) Carbonate concentration forming venules, characterized by calcium carbonate (CaCO3) accumulations forming an intricate network of thin horizontal and planar veins (1–4 mm thick) that may contain pink or grey siliceous masses; (2) Carbonate concentration forming isolated nodules (Fig. 5D), distinguished by the occurrence of small isolated nodules that result from the infiltration of carbonate solutions into the rock and subsequent calcite precipitation around grains or within pores, and:

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

(3) Carbonate concentration forming intraclasts (Fig. 5E), which consist of poorly- to sub-rounded white carbonate clasts (1–10 cm long) within a sandy matrix.

5.1.4. Banded carbonate (silicified) This paleoweathering feature was only described in cores, and its observation was only possible in thin sections. Banded carbonate (silicified) shows some relict texture that resembles carbonate. This paleoweathering is recorded by: (1) Texture similar to a carbonate with discrete band (Fig. 5F), represented as whitish horizontal bands with irregular borders and centimeter-scale thickness. These bands occur within clay-rich, greenish material, being either discrete or amalgamated, forming continuous, decimeter-scale horizons, and: (2) Carbonate with juxtaposed bands, forming a very compact, whitish rock that displays only remnants of its horizontal banding (white to pale green). This paleoweathering feature usually displays a brecciated aspect formed by a network of nearly orthogonal fractures.

5.2. Microscopic features 5.2.1. Textural mottling In the Sergi Formation textural mottling occurs either in siltstones or fine-grained sandstones. The average mineralogical composition includes detrital grains of quartz (65%), feldspar (20%), micas (0.7%), tourmaline (0.3%), and 14% of diagenetic minerals. The principal evidence for early diagenesis is clays introduced by mechanical infiltration through episodic floods into coarse alluvial sediments. The observed paleoweathering features are related to the scattered coloring of the rock due to reduction and oxidation. The oxi-

419

dized horizons contain clay minerals and iron oxides filling a finegrained quartz framework. Burrows are sand filled and cemented by calcite (Fig. 6A). Textural heterogeneities are the main cause of the discoloration that typifies this paleoweathering. The alternation of millimeter- to centimeter-scale strata of porous sandstone and mudstone controls the percolation of reducing fluids containing carbonate. As a result, the fine-grained strata maintained its primary oxidized color (red), whereas the sandy, primarily porous horizons were reduced and partly cemented by carbonate. 5.2.2. Non-textural mottling The mineralogical composition comprises detrital quartz (60%), feldspar (29%), mica (1%), tourmaline (0.3%), and about 9.7% of diagenetic components identified as mechanically infiltrated clays, iron oxides and calcite as microcrystalline aggregates. The main feature of this kind of paleoweathering is the presence of iron oxides producing irregular pigments in the rock. In thin section it is possible to observe the contrast between areas with oxidized clay matrix and irregular, millimetric areas where carbonate cementation, associated with small concentrations of clay minerals, dominates (Fig. 6B). Discoloration is also present in this kind of paleoweathering although it is independent of the original rock texture. This fact indicates that percolation of both reducing and carbonate-rich fluids obeyed an irregular trend through a relatively porous rock, promoting iron reduction and carbonation along diffuse areas. 5.2.3. Carbonate concentration The above mentioned types of carbonate cementation occur associated usually with sandstone composed of grains of quartz (60%), feldspar (10%), carbonate (6%), mica (1%) and garnet (0.5%). Diagenetic components (22.5%) include microcrystalline aggregates of calcite (Fig. 6C) and poikilotopic calcite. The carbonate concentration forms intraclasts that present alveolar and nodular textures, desiccation cracks and rhizoconcretions as primary features (Fig. 6D).

Fig. 6. Samples collected from G/2 core well (Fig. 3): (A) photomicrograph under polarized light of textural mottling paleoweathering. The darker horizons ‘1’ have clay minerals and iron oxides between fine-grained quartz grains whereas the lighter horizons ‘2’ are composed of coarser-grained sand grains with carbonate between them. (B) Photomicrograph under polarized light of non-textural mottling with diffusive carbonate accumulations. The NW segment ‘1’ contains brownish clay minerals, whereas the SE portion ‘2’ presents pore-filling, light grey calcite. (C) SEM view of calcite grains related to carbonate concentration forming venules. (D) Photomicrograph under polarized light of carbonate concentration showing carbonate intraclast with primary features (arrow indicates a rhizoconcretion). (E) Photomicrograph under natural light of banded carbonates (silicified). Numerals 1 and 2 point to layered (NE–SW trend) euhedral to subhedral carbonate grains and disseminated pyrite crystals, respectively. Matrix is composed of silica with disseminated titanium oxide. (F) Organic filaments present in fine-grained lacustrine sediments.

420

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

some high SiO2 values (83.8%), ascribed to paleoweathering. This is identified by strong silicification associated with diagenetic processes. The decreasing of some SiO2 values is correlated with increments in the CaO values that identify the carbonate concentration paleoweathering (Fig. 5). Those increments can reach values up to 47.1% in fluvial sandstones of Sequence II of Sergi Formation (Scherer et al., 2007). The values from CaO show extremely variable trend along the entire section. The Fe2O3 content is low. Some relatively high values (as 6.6%) are associated with oxidation of the textural and non-textural mottling paleoweathering. The trace element content showed a relative enrichment in the upper portion of Sequence I (Scherer et al., 2007). This may be related with the evolution of the depositional system (lacustrine at the base, changing to a fluvial-eolian system at the top of the sequence) that is directly related with climatic changes. Isotope analyses were carried out for carbonate concentrations without evidence of recrystallization. These carbonate concentrations yielded values between 4.9‰ and 8.7‰ d13CPDB, and from 3.8‰ to 7.1‰ d18OPDB (Table 5). These values are typical of nonmarine carbonates and suggest precipitation under the influence of meteoric water (Williams and Krause, 1998). Nevertheless, these values also show that the associated waters were not submitted to large compositional changes during the development of these features. Only a banded carbonate sample, at the top of Sequence II, provides a more negative d13CPDB value (about –21.3‰). High negative d13CPDB values indicate a larger influence of organic carbon (12C), and this could be explained as the banded carbonate (silicified) paleoweathering is related to paludal environments normally enriched in organic matter relative to the other environments. It is

The dominantly horizontal and planar arrangement of the venules suggests an origin connected to phreatic processes. Groundwater oscillations within the vadose zone might have led to successive carbonate precipitation generating horizons characterized by a network of millimeter-scale ribbons. The clear delineation, as well as the lack of other pedogenic or biogenic features, suggests that the carbonate precipitated from phreatic fluids. 5.2.4. Banded carbonate (silicified) The banded carbonate (silicified) has a predominance of diagenetic components (48.5%) dominantly formed by microcrystalline silica with vestiges of thin horizontal lamination and a slightly translucent aspect. Sparse pyrite, quartz and mainly euhedral to subhedral carbonate crystals can be found within this siliceous matrix (Fig. 6E). The banded structure is primary and was interpreted as chemical precipitation of carbonates within paludal environments (shallow lakes submitted to episodic subaerial exposure). Both intense silicification and euhedral carbonate grains suggest secondary features related to diagenesis, reproducing the primary depositional texture. This paleoweathering is related to the latest stages of calcretization (sensu Williams and Krause, 1998) when the formation of laminar to banded or layered surfaces takes place. 6. Chemical and isotopic analyses The whole-rock chemical compositions for major, minor and trace elements for selected samples are presented in Tables 3 and 4, respectively. Most of the samples have high SiO2 contents (between 54.1% and 74.2%), probably related to the quartzose composition of the host rocks. In any case, it is possible to observe

Table 3 Concentrations (%) of some major elements in samples of the: T1 – textural mottling; T2 – non-textural mottling; T3 – carbonate concentrations and T4 – banded carbonates (silicified) paleoweathering of the Sergi Formation (Recôncavo Basin). Samples

Sequence

SiO2

Al2O3

Fe2O3(t)

MnO

MgO

CaO

Na2O

K2O

LOI

T1 T1 T1 T2 T2 T3 T3 T3

I I I I I I I I

74.2 66.9 74.0 67.3 54.1 29.0 79.5 26.8

7.2 9.0 4.9 7.4 14.0 2.9 7.4 2.0

2.9 4.1 1.8 3.6 6.6 0.9 1.7 0.7

0.08 0.08 0.09 0.06 0.07 0.14 0.01 0.12

1.1 1.4 1.7 2.4 2.8 0.4 0.9 0.3

3.7 5.5 7.5 6.3 6.1 36.1 0.6 38.5

0.9 1.0 0.7 0.9 1.7 0.4 0.6 0.2

2.4 3.0 1.4 2.2 4.5 1.0 2.0 0.7

6.48 8.22 7.50 8.94 8.76 28.92 5.62 30.29

T3 T4 T4 T4

II II II II

13.3 19.1 83.8 79.6

0.8 1.1 3.7 7.6

0.2 0.3 1.5 2.1

0.30 0.33 0.13 0.01

0.4 0.3 0.7 1.1

47.1 43.5 2.9 0.2

0.1 0.1 0.4 0.5

0.2 0.4 1.0 2.6

37.33 33.87 5.73 5.42

Table 4 Concentration (ppm) of some trace elements in samples of the: T1 – textural mottling; T2 – non-textural mottling; T3 – carbonate concentrations and T4 – banded carbonates (silicified) paleoweathering of the Sergi Formation (Recôncavo Basin). Samples

Sequence

V

Ba

Sr

Zr

Cr

Cu

Rb

Nd

Sm

Pb

Th

U

T1 T1 T1 T2 T2 T3 T3 T3

I I I I I I I I

70 48 60 89 118 19 86 12

316 384 264 296 707 183 285 120

106 148 107 182 207 294 125 477

198 185 96 130 323 57 148 67

60 60 40 120 100 60 50 60

50 20 10 20 30 10 <10 10

77 103 38 61 230 29 57 25

16 27.4 8.1 12.8 55.1 16.9 14.6 12.2

3.2 5.2 1.5 2.6 10.3 3.1 2.6 2.5

13 17 11 12 31 6 9 6

5.7 10.1 2.8 3.5 17.9 4.9 7.4 3.6

1.1 2.1 0.6 1 2.8 0.8 3.6 1.1

T3 T4 T4 T4

II II II II

7 19 58 56

65 78 160 217

497 340 64 72

7 21 46 123

30 60 40 40

50 30 30 20

8 13 35 86

2.9 8.9 7.9 14

0.6 1.6 1.6 2.6

7 8 13 20

1.9 14.3 2.3 4.4

0.7 4.4 1.8 1.5

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426 Table 5 d13C and d18O ‰ ratios for calcite nodules taken out from paleoweathering horizons of the Sergi Formation (Recôncavo Basin). T3 – carbonate concentrations and T4 – banded carbonates (silicified). Sample

Sequence

T3 T3 T3 T3 T3 T3 T3 T3 T3 T3 T3 T3 T3 T4

I I I I I I II II II II II II II II

d13CPDB

d18OPDB

5.8 8.7 6.9 6.0 5.5 7.0 4.9 8.3 6.5 7.1 6.2 6.7 7.3 21.3

4.4 4.6 5.1 5.1 3.8 5.4 5.0 4.2 4.8 4.0 7.2 6.8 5.2 4.4

d18OSMOW 26.3 26.1 25.6 25.6 26.9 25.3 25.4 26.5 25.9 26.8 23.5 23.8 25.5 26.3

Table 6 87 Sr/86Sr isotopic ratios for carbonates removed from paleoweathering horizons of the Sergi Formation (Recôncavo Basin). T3 – carbonate concentrations and T4 – banded carbonates (silicified) paleoweathering; r (±) is the main deviation. Sample

87

Sr/86Sr

r (±)

T3 T3 T3 T3 T3 T4 T4 T4

0.7142 0.7152 0.7183 0.7146 0.7147 0.7153 0.7158 0.7239

0.000255 0.000267 0.000449 0.000233 0.000389 0.000187 0.000146 0.000198

possible to find in the lacustrine claystone evidence of organic filaments, later wrapped by thin calcite coats (Fig. 6F). These organic filaments were also discussed by Alonso-Zarza (2003) in the lacustrine mud from the Upper Miocene of the Teruel Basin. Analyzing weathering profiles, Dasch (1969) obtained increasing Rb/Sr ratios towards more altered rocks. The 87Sr/86Sr ratios, however, are better preserved in the altered rock/soil profiles, and the derived sediments reflect the isotopic character of the source area, representing a provenance tracer for modern sediments. Sr isotopes have been applied in the identification of sediment sources in recent fluvial systems (Negrel and Grosbois, 1999), marine and coastal deposits (Eisenhauer et al., 1999; Asahara et al., 1995; Asahara, 1999). The 87Sr / 86Sr isotopic results for Sergi Formation (Table 6) are high when compared to marine carbonates, suggesting a Sr radiogenic input probably related to weathering on a continental environment.

421

relief into the eolian deposits. This kind of surface can be classified as a supersurface, marking the end of an eolian accumulation event (Scherer et al., 2007). The textural and non-textural mottling comprises most of the features present at the base of Sequence I of Sergi Formation. These types of discolorations are associated with meteoric and groundwater reducing fluids. Percolation through the sediments was controlled by their original texture, hence explaining its differences. The origin of these horizontal and planar paleoweathering features is linked to high-frequency changes on the rate of the phreatic level oscillations (Fig. 7). Whenever the phreatic level rose above the depositional surface, a lake was formed. On the other hand, during its subsequent fall, lakes dried and meteoric fluid flowed through the vadose zone, hence bleaching sediments (Fig. 8A). Groundwater oscillations have also originated venules and isolated nodules. The development of small, ephemeral lakes during deposition of Sequence I has allowed the formation of palustrine calcretes (carbonate concentration forming intraclasts). Subsequent erosion of these calcretes during events of larger fluvial discharge has controlled their reworking and concentration, thus producing carbonate intraclasts. The intraclasts are attributed to fragments of palustrine calcretes, i.e. carbonates precipitated at shallow lacustrine environments and afterwards affected by short-lived emersions and pedogenic processes (e.g. Alonso-Zarza, 2003), scoured from ephemeral lakes formed on floodplains by strongly erosive, channelized streams (Fig. 8B). Among the reworked calcretes of the Sergi Formation, the preservation of some primary textures, such as rhizoconcretions and circum-granular cracks, which suggest plant colonization and episodic desiccation, allows us to interpret them as palustrine calcretes eroded from areas close to channels and included as intraclasts into the river bedload. The extensive presence of the mottling pattern and desiccation cracks point to very shallow water bodies submitted to episodic desiccation and flooding events. These processes are typical of lacustrine, palustrine or seasonal wetland environments (Pimentel, 2002). Although calcrete profiles can be easily preserved in the geological record (Watts, 1980; Warren, 1983; Sancho et al., 1992), when poorly developed they can be entirely reworked. In this case, clasts of carbonate can be included in channel or even floodplain deposits. Reworked clasts, transported usually as fluvial bedload, are composed of carbonate and, eventually, mud aggregates. However, within ancient successions, compaction might lead to the loss of the original calcrete texture, making it difficult to determine its

7. Depositional setting and its significance 7.1. Paleoweathering and relationships to the stratigraphic succession Sequence I displays a progradational facies succession. Its base is dominated by fine-grained lacustrine deposits (Capianga Member of the Aliança Formation) that are overlain by fluvial-eolian strata with scarce, interlayered lacustrine beds (base of the Sergi Formation). While the fluvial flow was to NE, eolian dune crossbedding shows a consistent southwestward trend, indicating eolian transport towards the southern border of the basin. Eolian and fluvial deposits are laterally and vertically adjacent, producing a complex depositional architecture. The upper boundaries of the eolian successions are fluvial scour surfaces with <3 m of erosional

Fig. 7. Oxidation and reduction alternating events (arrows) within fine-grained lacustrine deposits of Sequence I produced by phreatic level oscillations.

422

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

Fig. 8. (A) Non-textural mottling with reduced spots within Sequence I fluvial sandstone. (B) Detail of a palustrine carbonate intraclast displaying millimeter-scale internal lamination (primary features) in Sequence II fluvial sandstone.

origin (e.g. Ékes, 1993; Marriott and Wright, 1993; Khadkikar et al., 1998). The intervals of reworked palustrine calcretes of Sequence I are comparable to the reworked calcretes of types 1 and 3 described by Gómez-Gras and Alonso-Zarza (2003) in the Permian and Triassic of Minorca. According to these authors, one kind of reworked calcretes comprises lenses that fill erosional depressions related to

small channels crosscutting the floodplains. These channels are linked to ephemeral streams formed after sporadic, but heavy rainfalls that drain specific areas of the floodplain, removing important amounts of sediments (Marriott and Wright, 1993). These reworked palustrine calcretes are analogous to carbonate intraclasts present in the base of Sequence I. In contrast, another type of palustrine calcrete (Gómez-Gras and Alonso-Zarza, 2003) is linked LEGEND

SEQUENCE II

- site of paleoweathering formation

Meteoric water percolation

Paleoweathering types: 1 - Textural Mottling 2 - Non-Textural Mottling 3a - Carbonate Concentration forming Venules 3b - Carbonate Concentration forming Isolated Nodules

Paleoweathering type 4

PL

- carbonate intraclast

PL - Phreatic level

Braided fluvial channel

Paleoweathering types 1, 2, 3a and 3b

3c - Carbonate Concentration forming Carbonate Intraclasts 4 - Banded Carbonates (silicified)

SEQUENCE I TOP

Ephemeral fluvial channel

New channel Former channel

PL

Meteoric water percolation

PL

PL

Paleoweathering types 1, 2, 3a and 3b

Paleoweathering type 3c

Palustrine carbonate with primary features BASE

Eolian dunes Fluvial channel

Lacustrine body PL

Lacustrine delta

Meteoric water percolation PL

Paleoweathering types 1, 2, 3a and 3b

Fig. 9. Sketches representing the depositional evolution and paleoweathering development in the Sergi Formation. Models for the base and top of Sequence I (lower), and for Sequence II (upper).

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

to channels, rather than floodplains, and considered as channel lag deposits. Channel lateral migration or avulsion constitute the main causes for reworking of these palustrine calcretes. In the Sergi Formation, this type of palustrine calcrete is similar to the carbonate intraclasts that occur at the top of Sequence I. Sequence II consists of vertical and lateral juxtaposition of several sandstone bodies produced by successive avulsion episodes, similar to the fluvial accumulation models described elsewhere for braided fluvial systems (Jones and Schumm, 1999). This unit was deposited within an alluvial plain characterized by braided channel belts with regional transport direction towards northwest. The minor quantity of floodplain deposits indicates that overbank sedimentation was either restricted, or that the resulting deposits were reworked by fluvial channels. The dynamic regime produces a

423

depositional architecture characterized by multi-storey and multilateral, amalgamated, sheet sandstone bodies with rare preservation of fine-grained overbank deposits. In that way, Sequence II accumulation was associated with a rise in the stratigraphic base level (Scherer et al., 2007). Sequence II paleoweathering formed within an entirely distinct depositional setting and climate relative to Sequence I. The seasonal fluctuation on fluvial discharge controlled phreatic level oscillation in the channel surroundings. Therefore, phreatic level changes originated textural and non-textural mottling, as well as venules and isolated nodules. Although rare, it is possible that the phreatic level could eventually surpass the depositional surface, hence forming small temporary lakes on the floodplains, with associated calcite soils classified as banded carbonates (silicified).

Fig. 10. Paleogeographical reconstructions of the Sergi Formation. Jurassic pre-rift sedimentation. (A) At the time of maximum extension of the lacustrine systems and beginning of the fluvial sedimentation of the Sergi Formation; (B) at the time of maximum expansion of the Sergi fluvial system. The diverse paleoweathering features and the general location in the basin are shown in the squares: (1) textural mottling and (2) non-textural mottling, controlled by the phreatic level oscillations and closer to the channel rivers, (3) carbonate concentrations, some related with phreatic level oscillations and others deposited to clasts forming bed deposits of channel rivers and (4) banded carbonates (silicified), resulting in carbonate chemical precipitation within paludal environments (adapted from Garcia et al., 1998). West-south-westward migrating eolian dunes indicate east-northeasterly winds (Scherer et al., 2007).

424

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

These silicified carbonates are primarily banded as a result of carbonate chemical precipitation within paludal environments. Therefore, they represent in situ palustrine carbonates that suffered a strong diagenetic overprint. A decline on the rate of accommodation space during deposition of Sequence II induced lateral migration of channels and consequent reworking of the floodplain and marginal lake deposits, as well as soils. Although well-developed soil profiles could be formed in this stage (Plint et al., 2001), not one was found in the Sergi Formation, probably due to their low preservation potential (Fig. 9). A regional unconformity separates Sequences II and III. Its flat character at the outcrop scale and the fact that Sequence III records eolian deposits, suggests an origin related to eolian deflation associated with a stratigraphic base level fall. A change in the depositional style, from braided channel belt fluvial sandbodies to sheet-flood, dune and eolian sand sheet deposits, indicates a significant climate shift, from wetter (Sequence II) to drier (Sequence III) conditions (Scherer et al., 2007). Amalgamation of dune, eolian sand sheet and sheet flood deposits points to low rates of accommodation space. The abrupt change from fluvial-eolian deposits to lacustrine deposits (Itaparica Formation) suggests association with a fast water table rise (stratigraphic base level) and consequent basinwide flooding. This rise might have been absolute, due to climate change, or relative, produced by tectonism (Kocurek and Havholm, 1993; Rossetti, 2004; Limarino et al., 2006; Holz et al., 2006; Scherer et al., 2007). 7.2. Paleogeography, paleoclimate and the role of paleoweathering on the development of the heterogeneities in the Sergi Formation During the Late Jurassic, a large area (500,000 km2) of the Gondwana palaeocontinent, currently northeastern Brazil and

western Africa, was covered by a vast depression (Afro-Brazilian Depression) (De Cesero and Ponte, 1997). The sedimentary sequences deposited in this depression are now preserved in several small basins, which remained after fragmentation, uplift and erosion, including the Recôncavo Basin. During the Jurassic, most of this depression was occupied by a shallow lacustrine system, which resulted in deposition of mudstones from the Aliança Formation (Fig. 10A). The Middle Jurassic period was characterized by the beginning of a fluvial-eolian deposition that interfingered with lacustrine deposits. It was the beginning of the Sergi Formation deposition (Fig. 10A). In this time, the northern portion of the studied area was characterized by widespread, shallow lakes replaced towards the SW by fluvial and eolian depositional systems. The progradation was not a steady process, and produced lake expansion and contraction. These high frequency oscillations of the water table are associated with climate-related or tectonically-induced causes (Howell and Mountney, 1997; Scherer et al., 2007). The fluvial-eolian deposits of the Sergi Formation (depositional Sequence I) are marked by the presence of discolorations. These are associated with meteoric and groundwater fluids by the presence of palustrine calcretes eroded from small, ephemeral lakes and carbonate concentrations originated by groundwater oscillations. In the Late Jurassic, an endorrheic drainage system partially filled the depression (Afro-Brazilian Depression). In the northern part, extensive braided fluvial systems, which flowed from the N–NW margins to a shallow lacustrine complex in the center of the depression, deposited the braided fluvial, eolian and ephemeral lacustrine deposits of the Sergi Formation (Fig. 10B) (Garcia et al., 1998). A basinwide unconformity delineates a change in depositional style, from a lacustrine-fluvial-aeolian system (Sequence I) to entirely fluvial sedimentation (Sequence II). This change goes along

Fig. 11. Regional stratigraphic section presenting the position and correlation of facies associations and paleoweathering of sequences I and II of the Sergi Formation.

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

with a grain-size increase and a change in fluvial palaeocurrent direction (northeastward in Sequence I and northwestward in Sequence II) that can be attributed to rearrangement of the drainage system related to tectonic activity. In addition, a change in fluvial discharge regime probably took place owing to wetter climatic conditions (Scherer et al., 2007). Both the wetter climate conditions and tectonic activity had an influence on the establishment of a fluvial system (depositional Sequence II), characterized by multi-episodic and multi-lateral amalgamated, sheet sandstone bodies with rare preservation of finegrained, floodplain deposits. These conditions were accompanied by the formation of textural and non-textural mottling paleoweathering, as well as venules and isolated nodules, controlled by phreatic level oscillations. The return to a more arid climate in the south led to the development of widespread eolian deposits, typical of depositional Sequence III. More extensive fluvial systems spread from the northern rather than from the southern region into the depression, leading to higher rainfall (Garcia, 1991). These deposits are probably associated with palustrine carbonates formed due to a rise in the phreatic level, forming restricted lakes. The Sergi Formation presents a group of post-depositional features essentially related to color changes and carbonate cementation that occurs as isolated or amalgamated nodules. These features, related to the depositional dynamics, are frequently restricted to fluvial channel base or fine-grained lacustrine deposits. As a result, paleoweathering horizons are not usually continuous at basin- or even oilfield-scale (Fig. 11). This statement implies that the Sergi Formation paleoweathering does not constitute important heterogeneities in terms of sandstone reservoir compartmentalization.

8. Conclusions The changes in depositional systems observed from one sequence to another in the Sergi Formation were caused by tectonic and/or climate factors. The different paleocurrent trends in sequences I and II may be attributed to rearrangement of the drainage system in response to tectonic activity. The change from eolian and ephemeral stream deposits in Sequence I to braided fluvial channel belt deposits in Sequence II suggests a climate shift from relatively arid to more humid conditions. Sequence III is characterized by a return to drier conditions in the basin, marked by a resumption of eolian sedimentation (Scherer et al., 2007). Mottling colors, as well as carbonate accumulation, do not represent strictly pedogenic products. Although pedogenic processes have probably played a role, their products have not been preserved due to the dynamics of the depositional system. The Sergi Formation paleoweathering mostly reflects groundwater activity and phreatic level changes, similar to studies elsewhere (e.g. Pimentel et al., 1996; Pimentel, 2002). Only carbonate concentrations and banded carbonates (silicified) owe their origin to pedogenic processes strictu sensu, although well-developed soils with well-defined horizons were not formed. Therefore, fluvial and meteoric waters constitute the essential factors controlling weathering development and depositional trend in the Sergi Formation, as both are controlled by the same causes – chiefly climate, relief and sedimentation rate. Despite its advanced stage of exploration, the Recôncavo Basin still features good prospectives for continued exploration (Santos and Braga, 1990). For that reason, it is important to improve the hydrocarbon migration/accumulation models through the study of heterogeneities. The Sergi Formation reservoirs display depositional and diagenetic heterogeneities that control the distribution of the permeability and porosity space at distinct hierarchical

425

scales. Paleoweathering features did not result in heterogeneities at oil field- or basin-scale. The presence of carbonates in the sequences of the Sergi Formation implies that porosity loss can occur rapidly in some settings and could create local permeability barriers in some potential reservoir units. Acknowledgements The authors acknowledge the improvement of this work by the very appropriate reviewer’s suggestions. The authors also acknowledge the data provided by the research project entitled ‘‘Caracterização Estratigráfica - Petrológica Integrada dos Reservatórios da Formação Sergi” (FINEP/PETROBRAS) and its coordinator, for his collaboration and assistance. They also acknowledge Luiz Fernando De Ros for petrological assistance, Karin Goldberg for the final revision, and the National Petroleum Agency (ANP) for the Doctorate Scholarship of the first author. CNPq and ICCTI, through the UNISINOS – Universidade de Lisboa Agreement, supported Nuno L. V. Pimentel’s participation. References Al-Aasm, I.S., Taylor, B.E., South, B., 1990. Stable isotope analysis of multiple carbonate samples using selective acid extraction. Chemical Geology 80, 119– 125. Alonso-Zarza, A.M., 2003. Paleoenvironmental significance of palustrine carbonates and calcretes in the geological record. Earth-Science Reviews 60, 261–298. Alonso-Zarza, A.M., Silva, P.G., 2002. Quaternary laminar calcretes with bee nests: evidences of small scale climatic fluctuations, Eastern Canary Islands, Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 178, 119–135. Alonso-Zarza, A.M., Silva, P., Goy, J.L., Zazo, C., 1998. Fan-surface dynamics and biogenic calcrete development: interactions during ultimate phases of fan evolution in the semiarid SE Spain (Murcia). Geomorphology 24, 147–167. Arakel, A.V., 1986. Evolution of calcrete in paleodrainages of the Lake Napperby area, central Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 54, 283–303. Arakel, A.A., Jacobson, G., Lyons, W.B., 1990. Sediment-water interaction as a control on geochemical evolution of playa lake systems in the Australian arid interior. Hydrobiology 197, 1–12. Asahara, Y., 1999. 87Sr/86Sr variation in north Pacific sediments: a record of the Milankovitch cycle in the past 3 million years. Earth and Planetary Science Letters 171, 453–464. Asahara, Y., Tanaka, T., Kamioka, H., Nishimura, A., 1995. Asian continental nature of 87 Sr/86Sr ratios in north central Pacific sediments. Earth and Planetary Science Letters 133, 105–116. Birkeland, P.W., 1999. Soils and Geomorphology, thirrd ed. Oxford University Press. 448p. Bruhn, C.H.L., De Ros, L.F., 1987. Formação Sergi: evolução dos conceitos e tendências na geologia dos reservatórios. Boletim de Geociências da Petrobras 1 (1), 25–40. Caixeta, J.M., Bueno, G.V., Magnavita, L.P., Feijó, J.F., 1994. Bacias do Recôncavo, Tucano e Jatobá. Boletim de Geociências da Petrobras 1 (8), 163–172. Carlisle, D., 1983. Concentration of uranium and vanadium in calcretes and gypcretes. In: Wilson, R.C.L. (Ed.), Residual Deposits, vol. 11. Geological Society of London Special Publication, pp. 185–195. Carrasco, B.N., Fonseca, L.E.N., Durães, E.M., 1996. Fotointerpretação de facies e elementos arquiteturais eólicos no afloramento do Canyon do Sergi, Bacia do Recôncavo, Brasil. Congresso Brasileiro de Geologia, 39, Salvador, vol. 1, pp. 141–144. Craig, H., 1957. Isotopic standards for carbon and oxygen correction factors for mass spectrometric analysis of carbon dioxide. Geochimica et Cosmochimica Acta 12, 133–149. Dasch, E.J., 1969. Strontium isotopes in weathering profiles, deep-sea sediments and sedimentary rocks. Geochimica et Cosmochimica Acta 33, 1521–1552. De Cesero, P., Ponte, F.C., 1997. Análise comparativa da paleogeografia dos litorais atlânticos brasileiro e africano. Boletim de Geociências da Petrobras 11 (1), 1– 18. Eisenhauer, A., Meyer, H., Rachold, V., Tutken, T., Wiegand, B., Hansen, B.T., Spielhagen, R.F., Kassens, H., 1999. Grain size separation and sediment mixing in Artic Ocean sediments: evidence from the strontium isotope systematic. Chemical Geology 158, 173–188. Ékes, C., 1993. Bedload-transported pedogenic mud aggregates in the lower old red sandstone in southwest Wales. Journal of the Geological Society (London) 150, 469–471. Garcia, A.J.V., 1991. Paleogeografia do nordeste brasileiro no jurássico superiorcretáceo inferior. Geociências 10, 37–56. Garcia, A.J.V., Morad, S., De Ros, L.F., Al-Aasm, I.S., 1998. Palaeogeographical, palaeoclimatic and burial history controls on the diagenetic evolution of

426

C. Pierini et al. / Journal of South American Earth Sciences 29 (2010) 412–426

reservoir sandstones: evidence from lower cretaceous Serraria sandstones in the Sergipe-Alagoas Basin, NE Brazil. Special Publication of the International Association of Sedimentologists 26, 107–140. Gómez-Gras, D., Alonso-Zarza, A.M., 2003. Reworked calcretes: their significance in the reconstruction of alluvial sequences (Permian and Triassic, Minorca, Balearic Islands, Spain). Sedimentary Geology 158, 299–319. Goodfriend, G.A., Magaritz, M., 1988. Paleosols and late Pleistocene rainfall fluctuations in the Negev Desert. Nature 332, 144–146. Hanneman, D.L., Wideman, C.J., Halvorson, J.W., 1994. Calcic paleosols: their use in subsurface stratigraphy. American Association of Petroleum Geology Bulletin 78 (9), 1360–1371. Holz, M., Küchle, J., Philipp, R.P., Bischoff, A.P., Arima, N., 2006. Hierarchy of tectonic control on stratigraphic signatures: base-level changes during the early Permian in the Paraná Basin, southernmost Brazil. Journal of South American Earth Sciences 22, 185–204. Howell, J.A., Mountney, N.P., 1997. Climatic cyclicity and accommodation space in arid to semi-arid depositional systems: an example from the Rotliegend group of the Southern North Sea. In: Ziegler, K., Turner, P., Daines, S.R. (Eds.), Petroleum Geology of the Southern North Sea: Future Potential, vol. 123. Geological Society of London, Special Publication, pp. 63–86. Jones, L.S., Schumm, S.A., 1999. Causes of avulsion: an overview. In: Smith, N.D., Rogers, J. (Eds.), Fluvial Sedimentology IV, vol. 28. International Association of Sedimentologists Special Publication, pp. 171–178. Kawashita, K., 1972. O Método Rb/Sr em Rochas Sedimentares: Aplicação Para as Bacias do Paraná e do Amazonas. Ph.D. Thesis. São Paulo University, 111p. Khadkikar, A.S., Merh, S.S., Malik, J.N., Chamyal, L.S., 1998. Calcretes in semi-arid alluvial systems: formative pathways and sinks. Sedimentary Geology 116, 251–260. Kocurek, G., Havholm, K.G., 1993. Aeolian sequence stratigraphy – a conceptual framework. In: Weimer, P., Posamentier, H.W. (Eds.), Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, vol. 52. Society of Economic Paleontologists and Mineralogists, Special Publication, pp. 393–409. Lanzarini, W.L., 1996. Geometria das Unidades Genéticaqs Fluviais e Eólicas das Formações Aliança e Sergi na Borda Oeste da Bacia do Recôncavo. Congresso Brasileiro de Geologia 39, Salvador, Anais 1, pp. 328–331. Lanzarini, W.L., Terra, G.J.S., 1989. Fácies sedimentares, evolução da porosidade e qualidade de reservatório da Formação Sergi, Campo de Fazenda Boa Esperança, Bacia do Recôncavo. Boletim de Geociências da Petrobras Rio de Janeiro 3 (4), 365–375. Limarino, C., Tripaldi, A., Marenssi, S., Fauqué, L., 2006. Tectonic, sea-level, and climatic controls on late Paleozoic sedimentation in the western basins of Argentina. Journal of South American Earth Sciences 22, 205–226. Machette, M.N., 1985. Calcic soils of the southwestern United States. Geological Society America Special Paper 203, 1–21. Maizels, J.K., 1987. Plio-Pleistocene raised river channel systems of the western Shariya (Wahiba), Oman. In: Frostick, L.E., Reid, I. (Eds.), Desert Sediments: Ancient and Modern, vol. 35. Geological Society of London Special Publication, pp. 31–50 Mann, A.W., Horwitz, R.C., 1979. Groundwater calcrete deposits in Australia: some observations from Western Australia. Journal of the Geological Society of Australia 26, 293–303. Marriott, S.B., Wright, V.P., 1993. Palaeosols as indicators of geomorphic stability in two Old Red Sandstone alluvial suites, South Wales. Journal of the Geological Society (London) 150, 1109–1120. Milani, E.J., 1987. Aspectos da evolução tectônica das bacias do Recôncavo e Tucano Sul, Bahia, Brasil. MSc Thesis. Ouro Preto University, Brazil, p. 139. Mizusaki, A.M.P., 1986. Uso do Microscópio eletrônico de varredura no estudo de rochas reservatório de hidrocarbonetos, 2° Seminário de Geologia de Desenvolvimento e Reservatório. Petrobras, Rio de Janeiro, vol. 1, pp. 322–330. Munsell Soil Color Chart. Year 2000, Revision. Negrel, P., Grosbois, C., 1999. Changes in chemical and 87Sr/86Sr signature distribution patterns of suspended matter and bed sediments in the upper Loire river basin (France). Chemical Geology 156, 231–249. Netterberg, F., 1980. Geology of southern African calcretes. 1. Terminology, description, macrofeatures and classification. Transactions of the Geological Society of South Africa 83, 255–283.

Netto, A.S.T., 1978. A implantação da fase pré-rifte na Bacia do Recôncavo. Congresso Brasileiro de Geologia, 30, Recife, 1, pp. 596–517. Nickel, E., 1985. Carbonates in alluvial systems, an approach to physiography, sedimentology and diagenesis. Sedimentary Geology 42, 83–104. Pimentel, N.L.V., 2002. Pedogenic and early diagenetic processes in Palaeogene alluvial fan and lacustrine deposits from the Sado Basin (S Portugal). Sedimentary Geology 148, 123–138. Pimentel, N.L.V., Wright, V.P., Azevedo, T.M., 1996. Distinguishing early groundwater alteration effects from pedogenesis in ancient alluvial basins: examples from the Palaeogene of southern Portugal. Sedimentary Geology 105, 1–10. Platt, N.H., 1989. Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W Cameros Basin, N Spain. Sedimentology 36, 665–684. Platt, N.H., Wright, V.P., 1992. Palustrine carbonates at the Florida Everglades: towards an exposure index for the fresh-water environment. Journal of Sedimentary Geology 62, 1058–1071. Plint, A.G., Mccarthy, P.J., Faccini, F., 2001. Nonmarine sequence stratigraphy: updip expression of sequence boundaries and systems tracts in a high-resolution framework, Cenomanian Dunvegan Formation, Alberta foreland basin, Canada. American Association of Petroleum Geology Bulletin 85 (11), 1967–2001. Retallack, G.J., 1997. A Color Guide to Paleosols. Wiley, Chichester. 175p. Retallack, G.J., 1998. Fossil soils and completeness of the rock and fossil record. In: Donovan, S.K., Paul, C.R.C. (Eds.), The Adequacy of the Fossil Record, Wiley, Chichester, pp. 131–162. Retallack, G.J., 2001. Soils of the Past: An Introduction to Paleopedology. Blackwell Science. 404p. Rodas, M., Luque, F.J., Mas, R., Garzon, M.G., 1994. Calcretes, palycretes and silcretes in the Paleogene detrital sediments of the Duero and Tajo basins, Central Spain. Clay Mineralogy 29, 273–285. Rossetti, D.F., 2004. Paleosurfaces from northeastern Amazonia as a key for reconstructing paleolandscapes and understanding weathering products. Sedimentary Geology 169, 151–174. Sancho, C., Meléndez, A., 1992. Gênesis y significado ambiental de los caliches Pleistocenos de la región del Cinca (Depresión del Ebro). Revista de la Sociedad Geológica de España 5, 81–93. Sancho, C., Meléndez, M., Signes, M., Bastida, J., 1992. Chemical and mineralogical characteristics of Pleistocene caliche deposits from the central Ebro Basin, NE Spain. Clay Minerals 27, 293–308. Santos, C.F., Braga, J.A.E., 1990. O ‘‘estado da arte” da Bacia do Recôncavo. Boletim de Geociências da Petrobras. Rio de Janeiro 4 (1), 35–43. Scherer, C.M.S., Lavina, E.L.C., Filho, D.C.D., Oliveira, F.M., Bongiolo, D.E., Aguiar, E.S., 2007. Stratigraphy and facies architecture of the fluvial-aeolian-lacustrine Sergi Formation (Upper Jurassic), Recôncavo Basin, Brazil. Sedimentary Geology 194, 169–193. Spötl, C., Wright, V.P., 1992. Groundwater dolocretes from the Upper Triassic of the Paris Basin, France. A case study of an arid, continental diagenetic facies. Sedimentology 39, 1119–1136. Tandon, S.K., Gibling, M.R., 1997. Calcretes at sequence boundaries in Upper Carboniferous cyclothems of the Sydney Basin, Atlantic Canada. Sedimentary Geology 112, 43–67. Warren, J.K., 1983. Pedogenic calcrete as it occurs in quaternary calcareous dunes in coastal South Australia. Journal of Sedimentary Petrology 53, 787–796. Watts, N.L., 1980. Quaternary pedogenetic calcretes from the Kalahari, mineralogy, genesis and diagenesis. Sedimentology 27, 661–687. Williams, C.A., Krause, F.F., 1998. Pedogenic-phreatic carbonates on a Middle Devonian (Givetian) terrigenous alluvial-deltaic plain, Gilwood Member (Watt Mountain Formation), northcentral Alberta, Canada. Sedimentology 45, 1105– 1124. Wright, V.P., 1995. Losses and gains in weathering profiles and duripans. In: Parker, A., Sellwood, B.W. (Eds.), Quantitative Diagenesis: Recent Developments and Applications to Reservoir Geology. Kluwer Academic Publishers, Dordrecht, pp. 95–123. Wright, V.P., Tucker, M.E., 1991. Calcretes: an introduction. In: Wright, V.P., Tucker, M.E. (Eds.), International Association of Sedimentologists Reprint Series, vol. 2, pp. 1–22.