Santa Bárbara Formation (Caçapava do Sul, southern Brazil): depositional sequences and evolution of an Early Paleozoic postcollisional basin

Santa Barbara Formation (Cacapava do Sul, southern Brazil): depositional

sequences and evolution of an Early Paleozoic postcollisional basin

Andre Weissheimer de Borba*, Ana Maria Pimentel Mizusaki

Instituto de Geociencias, Universidade Federal do Rio Grande do Sul (IG/UFRGS), Av. Bento Goncalves, 9500, 91501-970, Porto Alegre, RS, Brazil

Received 1 January 2003; accepted 1 April 2003

Abstract

The Santa Barbara Formation, located in southernmost Brazil, comprises red-colored conglomerates, sandstones, and siltstones. It was

deposited in the Early Paleozoic under continental conditions in a restricted, strike-slip basin (Santa Barbara basin) in response to the

postcollisional stresses of the Brasiliano/Pan-African cycle (900–500 Ma). Five facies associations are recognized: alluvial fans, fan-delta

front, lacustrine, sandy braided, and gravel bed braided river deposits. The application of sequence stratigraphy enables the recognition of

three depositional sequences. The two basal sequences (I and II) represent a coherent depositional pattern, with axial fluvial and fan-delta

systems that deposit northeastward and lateral contribution from alluvial fans. Such coarse deposits are composed mainly of metamorphic

clasts derived from the erosion of ‘Cacapava high’, the eastern steep margin of the Santa Barbara basin. Sequence III lies unconformably over

the basal subunits and reflects the inversion of the axial systems, in that the paleocurrents of the gravel bed deposits systematically point

south/southwestward. The alluvial fan deposits of Sequence III also suggest a tectonic rearrangement of the basin, with partial erosion of the

basal sequences and the presence of granitoid fragments, which reflects a deeper denudation stage for the Cacapava high and possibly a

significant hiatus at the base of Sequence III.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Continental strata; Early Paleozoic; Santa Barbara Formation; Sequence stratigraphy; Southernmost Brazil

1. Introduction

The Neoproterozoic/Paleozoic boundary in southeast-

ern South America was marked by the late to postcolli-

sional phases of the Brasiliano/Pan-African cycle

(Porada, 1979). This long-lived (900–500 Ma) orogenic

event was characterized by the amalgamation of crustal

blocks and the consequent assembly of southwestern

Gondwana. The latest phases of this event were marked

by shoshonitic to alkaline magmatism (Lima and Nardi,

1998) and the inception of several fault-bounded basins,

which occupied a depositional locus defined as the

Camaqua basin (Paim et al., 2000). In this context, the

Santa Barbara Formation (Robertson, 1966) is believed to

be the preserved portion of an individual, restricted (ca.

45 km long, 15 km wide), strike-slip basin (Paim et al.,

2000; Menegat and Fernandes, 2001; Borba, 2001) called

the Santa Barbara basin.

The Santa Barbara Formation is composed of red

conglomerates, sandstones, and siltstones with pronounced

mineralogical and textural immaturity. These strata were

deposited in alluvial fans, fan-deltas, lakes, and braided river

depositional systems. The semiarid climate and continental

character of this sedimentation were proposed in early

studies of the Santa Barbara Formation (Robertson, 1966;

Ribeiro et al., 1966), but recent contributions have suggested

transitional or near-marine conditions (Rosa, 1999;

Almeida, 2001). The present study applies the time- and

scale-independent tools of sequence stratigraphy (Vail et al.,

1977; Van Wagoner et al., 1988, 1990; Posamentier and

James, 1993) to the strata of the Santa Barbara Formation.

Although sequence stratigraphic concepts were developed in

marine and marginal-marine systems, their application to

continental depositional environments has increased in the

past decade (Schumm, 1993; Kocurek and Havholm, 1993;

Shanley and McCabe, 1994; Martinsen et al., 1999).

The aims of the present paper are to (1) describe the

recognized facies associations of the Santa Barbara

Formation and their stacking pattern, (2) build a reliable

0895-9811/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0895-9811(03)00102-0

Journal of South American Earth Sciences 16 (2003) 365–380

www.elsevier.com/locate/jsames

* Corresponding author. Fax: þ55-513316-7302.

E-mail address: [email protected] (A.W. de Borba).

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sequence stratigraphic framework based on chronostrati-

graphically significant surfaces, and (3) propose a model for

the inception and depositional evolution of the Santa

Barbara basin.

2. Geotectonic setting

The Santa Barbara Formation (Robertson, 1966) crops

out in the Cacapava do Sul region in the central portion of

Rio Grande do Sul state, southernmost Brazil, near 308S,

538W (Fig. 1). The region is located in the Sul-rio-

grandense shield, a geotectonic unit generated or deformed

during the Neoproterozoic Brasiliano/Pan-African cycle

(900–500 Ma) (Porada, 1979).

Chemale (2000) recognizes four major segments in the

Sul-rio-grandense shield (Fig. 1): (1) the Taquarembo block

(Soliani, 1986), composed of a ca. 2.0 Ga granulite complex

intruded by granitoids of the late and postcollisional phases

of the Brasiliano/Pan-African cycle; (2) the Tijucas belt,

consisting of 780 Ma supracrustal metamorphic units and

older basement portions; (3) the Vila Nova belt (Chemale

et al., 1995), formed by juvenile accretion (760–700 Ma) in

an island arc tectonic setting (Babinski et al., 1996) and

comprised of orthogneisses, ophiolites, and supracrustal

rocks; and (4) the easternmost Dom Feliciano belt, made up

of granite–gneiss–migmatite rocks with intense Neoproter-

ozoic magmatism (650–500 Ma) and Paleoproterozoic

crust.

The evolution of the Brasiliano/Pan-African cycle in this

region included two phases of eastward subduction in island

arc (900–800 Ma) and continental arc (800–700 Ma)

settings (Chemale, 2000). Between 650 and 540 Ma, there

was apparently westward subduction, as well as the final

collision of the Rio de la Plata (west) and Kalahari (east)

continental plates. These events are believed responsible for

the assembly of southwestern Gondwana in southeastern

South America.

In response to the late and postcollisional stresses of the

Brasiliano/Pan-African cycle, several fault-bounded sedi-

mentary basins were formed in the foreland retroarc tectonic

setting, in a depositional locus called the Camaqua basin

(Paim, 1994; De Ros et al., 1994; Chemale et al., 1995;

Gresse et al., 1996). Its sedimentary and volcanic strata have

been subdivided into four allogroups (Paim et al., 1995)

bound by angular unconformities: the Marica, Bom Jardim,

Santa Barbara, and Guaritas. Recent studies have inter-

preted each major unit as a record of individual basins (Paim

et al., 2000; Menegat and Fernandes, 2001). Thus, the Santa

Barbara Formation is the geological record of the restricted

Santa Barbara basin, which developed in a transtensional

postcollisional tectonic setting in the Early Paleozoic

(Borba, 2001).

3. Regional geology

The basement of the Santa Barbara basin is composed of

mainly Neoproterozoic and Early Paleozoic units (Figs. 2

and 3). The Passo Feio metamorphic complex (PFMC,

Bitencourt, 1983) contains basic metavolcanics, metapelitic

rocks, and minor marbles, which show greenschist to

Fig. 1. Schematic geotectonic map of southernmost Brazil highlighting the Sul-rio-grandense shield, a geotectonic feature that records the Neoproterozoic

Brasiliano/Pan-African cycle (900–500 Ma), its subdivision into blocks or terranes, and the location of the deposits associated with the Camaqua basin.

A.W. de Borba, A.M. Pimentel Mizusaki / Journal of South American Earth Sciences 16 (2003) 365–380366

amphibolite metamorphic facies. The Bom Jardim and

Marica groups represent elongated NE-oriented foreland

strike-slip basins. Their shallow marine, alluvial, and

lacustrine strata are interlayered with shoshonitic inter-

mediate volcanics (Hilario Formation) of ca. 590–580 Ma

(Chemale et al., 1999; Remus et al., 1999). The Acampa-

mento Velho Formation is represented by acidic alkaline

lavas and pyroclastic flows. This volcanic event, dated at

,545 Ma (Almeida et al., 1996, 2002), displays a close

relationship with strike-slip displacement along NE-trend-

ing structures.

The study area also comprises the Lavras do Sul intrusive

complex (LSIC, Gastal and Lafon, 1998). The nucleus of

this complex has intermediate composition and shoshonitic

affinity, with a U/Pb zircon age of 592 ^ 5 Ma (Remus et al.,

1997). The acidic alkaline rim of the LSIC was dated at

580 ^ 11 Ma by U/Pb (Leite, 1995), confirming a Neopro-

terozoic age. The Cacapava do Sul granite complex (CSGC)

is composed of foliated biotite monzogranites, granodior-

ites, and leucogranitoids of high-K, calk-alkaline affinity. Its

intrusion is associated with displacement along a NE-

trending shear zone active at approximately 550 Ma (Sartori

and Kawashita, 1985; Leite, 1995).

The Santa Barbara Formation, the main focus of this

work, is composed of red-colored conglomerates, sand-

stones, and siltstones. The Santa Barbara Formation strata

dip 10–188 northeastward in the southern outcrop area,

whereas in the northern portion, the strata are tilted 26–458

Fig. 2. Schematic geological map showing the main stratigraphic and lithodemic units cropping out in the region of Cacapava do Sul, in the central portion of

Rio Grande do Sul State, Brazil.

Fig. 3. Simplified stratigraphic column of the Cacapava do Sul region showing the units presented in Fig. 2, with the available geochronological data recorded

in these lithologies.

A.W. de Borba, A.M. Pimentel Mizusaki / Journal of South American Earth Sciences 16 (2003) 365–380 367

southeastward. The unit rests unconformably on the Bom

Jardim group and disconformably on the Acampamento

Velho Formation. Ichnology studies performed in the basal

deposits of the Santa Barbara Formation (Netto et al., 1992;

Rosa, 1999) present poorly developed ichnofaunas, which

suggest an Early Cambrian age and a transitional (?)

environment.

The overlying Guaritas Formation is also characterized

by continental strata, with the important component of

eolian facies. Its sedimentation is Ordovician in age; the

basaltic alkaline rocks of the Rodeio Velho member,

interlayered in the Guaritas sedimentary package, have a

U/Pb zircon age of ca. 470 Ma (Hartmann et al., 1998).

4. Facies associations of the Santa Barbara Formation

Three composite stratigraphic profiles (P1, P2, and P3 in

Fig. 4) were constructed to aid in establishing basin-scale

controls on the distribution of the facies associations and

surfaces with chronostratigraphic significance in the Santa

Barbara Formation. In each profile, texture (grain size,

sorting, roundness), framework composition, geometry and

lateral extension of beds, sedimentary structures, color, and

paleocurrents were analyzed. Qualitative aspects of prove-

nance and diagenesis were provided by analyses of 20 thin

sections of sandstones, siltstones, and conglomerates.

Five facies associations are recognized in the Santa

Barbara Formation: alluvial fans, fan-delta front, lacustrine,

sandy braided, and gravel bed braided river deposits. The

facies associations are described and interpreted in terms of

depositional systems. In common, they show a character-

istic reddish color that reflects iron and titanium oxide grain

coatings, which are widespread in the thin sections. All the

analyzed rocks are markedly immature, both texturally

(angular clasts, poor sorting, low roundness) and composi-

tionally (well-preserved feldspars, lithic volcanic grains).

4.1. Alluvial fan facies association

The western and southwestern exposures of the Santa

Barbara Formation (Fig. 4) are characterized by very coarse-

grained deposits that mark the base of the succession. The

deposits comprisemeter-scale beds (up to tens of meterswhen

amalgamated) that feature a distinctive red-brown color and a

predominance of sharp and erosional bed contacts. Lens-

shaped and tabular beds of clast- and matrix-supported

conglomerates are massive (facies Gmm and Gcm) or show

inverse grading (facies Gmg and Gci, Fig. 5a, Facies codes for

alluvial fans are after Miall, 1996). Clast-supported conglom-

erates with plane-bed stratification and crude to absent clast

imbrication (facies Gh, Fig. 5b) also occur.

Grain size varies from granules in the basal beds to

cobbles upward, with poorly sorted sandy matrix and

angular to subrounded clasts of mainly volcanic compo-

sition. Along the western border of the basin, most

fragments were eroded from acidic alkaline volcanic

rocks, whereas in the southwest, andesites and trachytes

dominate. In both cases, sedimentary clasts derived from the

Marica and Bom Jardim groups are common.

Associated with the conglomerates are meter-scale, lens-

shapedbedsofcoarse- tomedium-grainedsandstones,mainly

withplane-bedstratification(faciesSh), troughcross-bedding

(facies St), horizontal laminae, and lags of outsized pebbles.

The sandstones fine upward and terminate with sandy and

pebbly red-brown massive siltstones (facies Fsm).

Along the eastern limit of the Santa Barbara Formation

outcrops (Fig. 4), even coarser grained deposits are present.

The base of this package consists of lenticular beds of

oligomictic, matrix-supported conglomerates (facies Gmm).

Fig. 4. Schematic geological map of the Santa Barbara Formation, extracted

from Fig. 2, showing the five facies associations and the locations of the

profiles measured and presented. The location of the paleotopographic

feature termed Cacapava high (1) is also indicated.


The andesitic and metasedimentary clasts reach up to 1 m in

diameter in a sandy red-brown matrix (Fig. 5c). They are

associated with fine- to medium-grained sandstones and

sandy siltstones that show horizontal bedding and current

ripples (facies Sh, Sr, and Fsm). In thin sections, these

sandstones reveal a clear predominance of volcanic clasts

(Fig. 6a). Stratigraphically above are very poorly sorted,

disorganized, matrix-supported conglomerates (facies

Gmm) with red sandy matrix and angular clasts (up to

32 cm in diameter) composed of quartzite and metavolcanic

greenschists. The same detrital composition is shown by

associated lens-shaped beds of disorganized, clast-sup-

ported conglomerates (facies Gcm) and moderately sorted

sandstones (Fig. 6b) with normal grading and trough cross-

bedding (facies St). Inversely graded, clast-supported and

chaotic, matrix-supported conglomerates (facies Gci and

Gmm) with red sandy matrix and granitic clasts up to 60 cm

in diameter occur at the top of this succession (Fig. 5d). The

associated conglomeratic sandstones also contain granitic

granules and pebbles (Fig. 6c).

Intrabasinal, matrix-supported conglomerates (beds are

approximately 3 m thick) are also present in the Santa

Barbara Formation (Fig. 7). The angular conglomerate

clasts are of sedimentary origin and comprised of reddish

siltstones and sandstones dispersed in a yellowish sandy

matrix.

In terms of sedimentary processes, the matrix-supported,

massive, inversely graded, and clast-supported conglomer-

ates are interpreted as deposited by gravitational processes,

mainly debris flow (Blair and McPherson, 1994). Matrix-

supported deposits are very common, where the source

rocks demonstrate porosity (and thus, the ability to retain

variable amounts of water and air, Blair, 1999), as

exemplified by the intrabasinal conglomerates (Fig. 7). In

contrast, plane-bed stratified conglomerates, clast imbrica-

tion, plane-bedded sandstones, and lags of outsized pebbles

are features that suggest the action of sheetflood processes

(Blair and McPherson, 1994). In addition, trough cross-

bedded sandstones are ascribed to the migration of

subaqueous bedforms (dunes).

Thus, the facies association can be interpreted as

generated by combined debris flow and sheetflood pro-

cesses, with some fluvial reworking. This interbedded

association of debris flow and sheetflood deposits, accord-

ing to McPherson et al. (1988), is diagnostic of alluvial fans.

Alluvial fans are often the proximal, subaerial portion of

fan-deltas (Nemec and Steel, 1988). The deposits represent

type-A feeder systems (Postma, 1990) that prograded into

Fig. 5. Field aspects of the alluvial fan facies association of the Santa Barbara Formation: (A) inversely-graded conglomerates, suggestive of debris flow

processes; (B) plane-bed stratification with incipient clast imbrication in clast-supported conglomerates, suggesting sheetflood processes; (C) matrix-supported

cobble-to-boulder conglomerate with andesitic and metasedimentary clasts, derived from the Cacapava high at the base of the alluvial fan system of the

northeastern border of the Santa Barbara Formation; and (D) granitic cobbles at the top of the alluvial fan system of the northeastern basin border, reflecting the

exposure of the CSGC (photographic lens cover between the three cobbles for scale).


a shallow lake, as indicated by the siltstone deposition

associated with flooding periods. Intense reworking of the

fan-deltas by fluvial processes can be inferred. The alluvial

deposits, which are coarser close to the eastern fault margin,

reflect the probable asymmetry of the Santa Barbara basin.

This asymmetrical profile is expected in basins in which

strike-slip movement occurs (Massari and Colella, 1988).

Good examples of asymmetrical alluvial deposits in strike-

slip-related troughs are the Ridge basin in California

(Crowell, 1975) and the Hornelen basin in Norway

(Gloppen and Steel, 1981; Steel, 1988).

4.2. Fan-delta front facies association

Centimeter-scale (up to 30 cm), red-colored, tabular, and

lenticular beds of fine- to medium-grained, moderately

sorted sandstones are typical of this facies association. They

occur as massive bodies (facies Sm, Postma, 1990) or show

plane-bed lamination and low-angle cross-bedding (Fig. 8)

highlighted by silty and muddy laminae. These sand bodies

grade to thin (5 cm) beds of very fine sandstones to sandy

siltstones, with small-scale (wavelength up to 10 cm)

current and wave ripples. Fine- to medium-grained,

moderately sorted sandstones with planar cross-bedding

marked by mudstone and siltstone foresets also occur, with

associated intraclasts and desiccation features. Sigmoidal

yellowish siltstones are also present.

Fig. 6. Aspects of the provenance of the Santa Barbara Formation

sandstones, analyzed in thin sections: (A) grains of volcanic intermediate

composition related to the Hilario Formation of the Bom Jardim group, (B)

detail of metamorphic clast in fluvial sandstone, and (C) clasts of granitic

composition.

Fig. 7. Intrabasinal debris flow deposits of the Santa Barbara Formation.

The clasts are composed mostly of siltstones and sandstones from the basal

units.


At the northeastern border of the Santa Barbara

Formation outcrop area, tabular beds (up to 1 m thick) of

red-colored sandy and pebbly siltstones are interlayered

with fine-grained, poorly sorted sandstones that show plane-

bed and current ripple laminations (Fig. 9a). Thin (10 cm)

tabular beds of inverse to normally graded granule

conglomerates show plane-bed stratification and rippled

tops. There are also conglomerates and coarse sandstones

with trough and planar cross-bedding (Fig. 9b) and current

ripples, as well as a considerable amount of intraclasts

(Fig. 9c). Medium-grained sandstones with subaerial

exposure features, such as desiccation cracks and raindrop

imprints, and plane-bed laminated sandstone beds with

granule-to-pebble lags are also present (Fig. 9d). Sigmoidal

stratification in sandstones and conglomerates can be

observed in some places.

Traction processes (including traction carpets) were

responsible for the deposition of conglomerates with

plane-bed stratification and trough and planar cross-bedding

(facies Gh, Gt, and Gp, facies codes for the fan-delta front

are after Postma, 1990), as well as of some sandstones

(planar-bedded and planar cross-bedded, Sh and Sp).

Fig. 8. Field aspects of the fan-delta front facies association of the Santa Barbara Formation: (A) low-angle cross-lamination in thin beds of fine-grained

sandstones and (B) tabular beds of medium-grained sandstones with rippled tops (current and wave ripples).

Fig. 9. Field aspects of coarse-grained deposits of the fan-delta front facies association of the Santa Barbara Formation: (A) interlayered tabular beds of

siltstones and coarse to conglomeratic sandstones with rippled tops, (B) trough cross-stratification in granule conglomerates, indicating transport to the

northeast (to the left in the photo), (C) intraclasts at the base of a conglomerate bed, suggesting the reworking of finer grained portions by fluvial processes, and

(D) granule to pebble horizontal lags in plane-bed stratified coarse sandstones.


Rippled sandstones (Sr) are the result of reworking of the

sediments by current and oscillatory movement, whereas

siltstone deposition suggests the contribution of suspension

processes.

This facies association is suggestive of fan-delta front

sediments, partially reworked by waves in periods of low

sediment influx and exposed to desiccation and some fluvial

reworking when sediment input increased and progradation

took place. These characteristics suggest shallow water fan-

deltas (Hjulstrom type) (Postma, 1990).

4.3. Lacustrine facies association

This facies association (Fig. 4) is characterized by the

predominance of red-brown, oxidized sandy siltstones with

dispersed granules. The siltstones are sometimes associated

with thin beds of fine to very fine, moderately sorted,

massive sandstones. Millimeter-scale mudstone laminae are

also present. The facies association represents suspension-

dominated sedimentation during periods of low sediment

influx to the basin depocenters. However, quiescence was

probably never great enough to allow deposition of

considerable thicknesses of mudstone.

4.4. Sandy braided river facies association

This facies association (Fig. 4) consists mainly of tabular

to laterally extensive lenticular beds (up to 1 m thick) of

moderately sorted, medium- to coarse-grained sandstones

with normal grading, plane-bed stratification, and wide-

spread planar and trough cross-bedding (Fig. 10a). Massive

sandstones with rippled tops and intraclast lags also occur.

Granule conglomerates and conglomeratic sandstones

showing trough cross-bedding are less common. Erosive

bases are planar and marked by the presence of pebbles.

The sedimentary processes involved in the formation of

the described facies are mainly related to the migration of

cross-stratified and plane-bed (upper flow regime) sand

bars, with some liquefaction of the bars (massive sand-

stones), as suggested by Røe and Hermansen (1993) for the

Late Precambrian fluvial systems of Norway. Some

reworking of the finer grained portions is suggested by the

presence of intraclasts. This facies association can be related

to a shallow braided fluvial plain with laterally extensive

channels. The Early Paleozoic, continental, semiarid

context (lack of vegetation and heavy, short-lived rain-

storms), as well as its association with alluvial fan

sheetflood deposits, favors this interpretation (Røe and

Hermansen, 1993). It resembles the shallow, perennial, sand

bed braided river model that is exemplified by the modern

Platte River of Colorado (USA, Miall, 1996).

4.5. Gravel bed braided river facies association

These deposits (Fig. 4) comprise amalgamated, ca. 50 cm

thick, 6 m wide beds of poorly sorted conglomeratic

sandstones and conglomerates with normal grading and

very well developed trough cross-bedding (Fig. 10b). The

bodies cut into one another both laterally and vertically (Fig.

10c) and present scoop-shaped boundaries between beds.

The clasts are generally subrounded, and a large amount of

granitic cobbles constitutes its main feature. Lens-shaped

beds of massive, poorly sorted conglomerates are less

common.

The migration of gravel and sand (minor) bars in low-

sinuosity channels is the main sedimentary process involved

Fig. 10. Field features of the braided river facies associations of the Santa

Barbara Formation: (A) tabular/lenticular beds of great lateral extension of

medium-grained sandstones with horizontal and planar cross-stratification,

characteristic of the sandy braided river facies association, (B) well-

developed trough cross-stratification in pebbly conglomerates of the gravel

bed braided river facies association, and (C) scoop-shaped contacts between

beds of conglomerate that cut into one another both laterally and vertically,

a distinctive feature of gravel bed braided river facies association.


in the deposition of this facies association. Such bedload

streams are characterized, according to Collinson (1989), by

considerable channel mobility (braiding and channel

shifting), which causes the scoop-shaped boundaries

between beds. This facies association suggests shallow,

gravel bed braided rivers, as exemplified by the Scott River

in Alaska (Miall, 1996).

5. Sequence stratigraphy and depositional evolution

The early concepts of sequence stratigraphy (Vail et al.,

1977; Van Wagoner et al., 1988, 1990) were supported

mainly by seismic data from studies of marine and

marginal-marine basins. According to these early authors,

the termination patterns of seismic reflectors should be

associated with global eustatic changes in sea level. But the

adoption of eustasy as the prevailing control on sequences

and systems tracts has been criticized by, among others,

Miall (1991) and Galloway (1989). For these later authors,

climatic and tectonic factors are important controls, even in

marine basins. This criticism led Posamentier and James

(1993) to suggest that sequence stratigraphy should be

viewed as a tool rather than a template. Posamentier et al.

(1992) also show, by performing high-resolution sequence

stratigraphy in a small fan-delta system built in a roadside

drainage ditch in a matter of days, that this tool is time- and

scale-independent and that it can be applied to both

lacustrine and marine settings (for a good, recent review

of the main concepts and applications of sequence

stratigraphy, see Catuneanu, 2002).

The fan-delta and fluvial depositional systems inferred for

the Santa Barbara Formation are considered to have been

deposited in a continental strike-slip setting (Paim et al.,

2000; Borba, 2001). Sequence stratigraphic tools may thus

be applied using changes in the stratigraphic (local) base

level (the lower limit of subaerial erosion; Schumm, 1993) as

the main cause of the stratal stacking patterns. The changes

are controlled mainly by the interplay of two factors:

sedimentary influx and the rate of creation of accommo-

dation. The latter factor is defined by Jervey (1988) as ‘the

space made available for potential sediment accumulation’.

According to Shanley and McCabe (1994), in cases in which

accommodation space is zero, sediments only bypass the

basin, whereas in situations of negative accommodation,

erosion and incision take place regardless of the rate of

sediment influx. In a positive creation of accommodation

space, three situations are possible: (1) progradation occurs,

where the sediment supply overcomes the accommodation

space, (2) the inverse situation leads to retrogradation, or (3)

equilibrium between these two variables generates aggrada-

tion. This interplay, the A/S (accommodation/supply) ratio,

is considered by Martinsen et al. (1999) as the cause of major

breaks in fluvial styles in their definition of high- and low-

accommodation systems tracts.

For the deposition of the Santa Barbara Formation,

tectonics and climate are the prevailing controls of

the observed stratigraphic (local) base level changes. The

nomenclature used here conforms with the terms applied by

Vail et al. (1977), Mitchum et al. (1977), and Van Wagoner

et al. (1988, 1990), in that sequence is a relatively

conformable succession of genetically related strata bound

at its base and top by unconformities or their correlative

conformities (Mitchum et al., 1977). The unconformities are

considered the sequence boundaries, and a sequence can be

divided internally into lowstand, transgressive, and high-

stand systems tracts, which were defined by Brown and

Fisher (1977) as associations of coeval depositional systems

in certain positions of the base level curve characterized by

their stacking patterns of parasequences, the fundamental

shoaling-upward units. For the Santa Barbara Formation,

the integrated analysis of the three measured profiles and the

recognition of the facies associations (Fig. 11) leads to the

proposition of three depositional sequences (Fig. 12).

5.1. Sequence I

Sequence I is the first subunit of the Santa Barbara

Formation. It is 800–1000 m thick (Fig. 12) and very well

preserved in all profiles, especially P3 (Fig. 4). The strata of

this sequence, in the northern outcrops of the unit, dip from

32 to 408 ESE, whereas in the south, they are tilted 10–168

NE.

The base of Sequence I is a disconformity (Fig. 12) on acid

(northwesternregion)andintermediate(southwesternregion)

volcanic rocks or metasedimentary and volcanic rocks

(northeastern region). Its lowstand systems tract is rep-

resented by strata of the alluvial fan facies association.

Volcanic fragments dominate the framework composition of

the sandstones (Fig. 6a) and conglomerates, thus reflecting

very closely the adjacent rock units. Paleocurrent measure-

ments (trough cross-stratification and clast imbrication) point

approximately to east/northeast (in the southwestern region),

east (in the western deposits), and west in the alluvial fans

recognized in the eastern border of the basin.

The fan-delta front and lacustrine deposits represent the

transgressive systems tract, though their onlap relationship

with the lowstand alluvial fans is not clear. These deposits

are characterized by wave reworking and moderate sorting

of the sandstones. The structureless, finer grained portion

(deposited below the limit of influence of the normal waves)

includes the maximum flooding surface. The highstand

systems tract, at the top of the package, comprises

northeastward fluvial reworking of the finer grained

deposits, with large amounts of intraclasts, in a clear

prograding trend.

5.2. Sequence II

The preserved package of Sequence II (Fig. 12) is

approximately 1100 m thick in the P1 profile (Fig. 4). This

subunit begins along an unincised unconformity with

inferred sandy braided fluvial deposits (Fig. 10a) that


represent the lowstand systems tract. These strata are tilted

28–308 SE.

The lowstand deposits comprise coarse-grained sandstones

and conglomerates, particularly in the southern portion of the

unit, where the more proximal deposits were established,

because the paleocurrents still indicate a northeastward

transport. In the proximal portion of the fluvial system, the

clasts that compose the framework of the sandstones are

mainly volcanic. To the north, metavolcanic clasts (Fig. 6b)

associated with the lateral contribution of alluvial fans derived

from eastern source areas are widespread.

The highstand systems tract of Sequence II is well

developed in the northeastern outcropping region, where it

is represented by coarse-grained fan-delta front deposits

(Fig. 9a–d). Lateral alluvial fans probably represent the

feeder systems, which were responsible for the transport of

sediments into very shallow lakes that are considered the

depocenters, frequently exposed to desiccation and fluvial

reworking. In the southern portion of the outcropping area

of the Santa Barbara Formation, the transgressive and

highstand systems tracts of Sequence II have been almost

totally eroded by the deep incision of the fluvial systems

associated with Sequence III (Fig. 12).

5.3. Sequence III

Only the lowstand deposits of Sequence III (Fig. 12) are

preserved in the studied area, and they are represented by a

gravel bed braided fluvial system that incised into

underlying strata. The beds dip 23–268 ENE. Trough

cross-strata (Fig. 10b and c) show that the braided streams

flowed southwestward, which indicates a total inversion of

the depocenters and source areas. This sequence also

contains fragments of the granitoids of the CSGC (Figs.

5d and 6c) and thus reflects the final stage of denudation of

the eastern basin border during the deposition of the Santa

Barbara Formation.

Matrix-supported conglomerates near the base of

Sequence III are interpreted as debris flow deposits. These

deposits are composed of fragments of the two basal

Fig. 11. Summary of the main characteristics of the five facies associations of the Santa Barbara Formation in terms of textures, sedimentary structures,

interpretation, and representation in the columnar profiles of Fig. 12.


sequences of the Santa Barbara Formation (Fig. 7). They

demonstrate that the basal unconformity of Sequence III is

clearly associated with tectonic processes and that it can be

interpreted as a total rearrangement of the basin.

5.4. Depositional evolution

Compressional stresses related to the Brasiliano/Pan-

African cycle in the study area are believed to have

weakened before the extrusion of the acidic alkaline

volcanics of the Acampamento Velho Formation. This

inference is based on the observation that the cited unit does

not evince compressive deformation. In addition, it

unconformably overlies the folded strata of the Bom Jardim

and Marica groups (Paim et al., 1995; Almeida et al., 2002).

After this (Early Cambrian) volcanic event, extensional

processes began to dominate, with variable influences of

strike-slip movements. In this context, the strike-slip Santa

Barbara basin (Paim et al., 2000; Borba, 2001) began its

evolution, controlled by NE–SW, NW–SE, and N–S

structures inherited from Brasiliano fold belts.

The initial phase of the evolution of the Santa Barbara

basin was marked by the development of alluvial fans, with

the coarser grained fans occurring near the steeper slopes of

the N–S border fault of the Cacapava high (Fig. 13a). At

this time, the Cacapava high was covered by andesitic

volcanic rocks (Bom Jardim group) and clastic metasedi-

mentary rocks (PFMC). Evidence for this configuration is

easily observed in the conglomerate framework (Fig. 5c) of

the lowstand alluvial fans of Sequence I. The alluvial fans

along the other borders of Santa Barbara basin are generally

finer grained deposits, probably because those margins were

gentler (Fig. 13a). The widespread reddish color and

diagenetic features (iron and titanium oxide coatings and

calcite cement) of the Santa Barbara Formation, as observed

by De Ros et al. (1994) for the Guaritas Formation, are

Fig. 12. Stratigraphic correlation among the three composite profiles in the Santa Barbara Formation. The profile codes (P1, P2, and P3) refer to Fig. 4. The

small rose diagrams beside each profile represent the 125 paleocurrent measurements (N ¼ number of measures) achieved in this study.


typical of near-surface diagenesis in continental semiarid

environments (Walker, 1967), and they persisted throughout

sedimentation.

The transgressive deposits of Sequence I are related to a

period of increasing subsidence and lateral expansion of the

Santa Barbara basin that produced positive accommodation

space with respect to the sediment supply (high A/S ratio),

which enabled the fan-delta front and lacustrine systems to

onlap the lowstand deposits. This trend culminated with the

accumulation of fine-grained sandstones and siltstones,

which represent the maximum flooding surface (Fig. 13b).

When sediment supply overcame the rate of subsidence/

expansion of this basin, proximal processes started to

rework the fine-grained portions in a clear prograding trend.

The deposition of the sandy braided river facies association,

which delineates the base of Sequence II (unincised

Fig. 13. Depositional evolution of the Santa Barbara basin during lowstand systems tract (A) and the maximum flooding surface (B) of Sequence I, from map

(upper) and lateral (lower) views.


sequence boundary between fluvial deposits), could be

ascribed to a slow relative fall in stratigraphic base level, as

suggested by Wright and Marriot (1993) for their type-2

unconformities.

Sequence II shows the same stacking patterns as

Sequence I, but its lowstand systems tract comprises only

fluvial deposits. The coarser deposits related to this phase

occur in the southern outcrop area, and their framework

composition is predominantly volcanic grains. With the

progression of the sedimentation northward, clasts of

metavolcanic composition were added to the sandstones.

This observation reveals a second stage of denudation of

the Cacapava high when this border of the basin was

covered by metavolcanic rocks of the PFMC (Fig. 14a).

Transgressive deposits and the maximum flooding surface

of Sequence II are poorly exposed. In contrast,

Fig. 14. Depositional evolution of the Santa Barbara basin during the deposition of Sequence II (A) and Sequence III (B), from map (upper) and lateral (lower)

views.


the highstand systems tract was well developed and

consisted of coarse-grained fan-delta front systems, which

deposited their sedimentary load in a shallow lake (or

lakes) situated near the steeper border fault of Cacapava

high (Fig. 14a). This fan-delta front was frequently

exposed to desiccation and reworking, which resulted in

features such as mudcracks, raindrop imprints, and large

amounts of intraclasts.

At the base of Sequence III, the most pronounced

shift in tectonic and depositional patterns in the Santa

Barbara Formation is recorded. Debris flow deposits with

angular boulders and cobbles of siltstones and sandstones

derived from the basal sequences (I and II) reveal that

former depositional areas were exposed to faulting and

erosion. The identification of such deposits confirms the

deep incision at the base of Sequence III, which led to

the complete erosion of the upper parts (transgressive

and highstand systems tracts) of Sequence II in the

southern outcrops of the Santa Barbara Formation.

In association, alluvial fans derived from the erosion

of Cacapava high contain boulders of granitic compo-

sition, which suggests the exposure of the upper portions

of the CSGC (Fig. 14b). The fluvial style of the coarse

conglomerates and sandstones of Sequence III is also a

result of the shift in the depositional conditions and

indicates important relief creation in the northern region

and paleotransport to the south/southwest.

These features support an important hiatus for the basal

unconformity of Sequence III. The probable Early Cam-

brian age of the ichnofauna described by Netto et al. (1992)

for Sequences I and II indicates that this deposition was

contemporaneous with the final phases of intrusion of the

CSGC. However, Sequence III contains boulders of these

granitoids, which were later exposed and incorporated into

the sedimentary cycle.

6. Conclusions

The Santa Barbara Formation is interpreted as the

geological record of an Early Paleozoic, strike-slip basin

(Santa Barbara basin). The basin developed in the latest

(transtensional) stages of the Brasiliano/Pan-African cycle

in the Sul-Rio-Grandense shield (Paim et al., 2000; Menegat

and Fernandes, 2001; Borba, 2001). Basin inception was

controlled by NE–SW, NW–SE, and N–S structures

inherited from Brasiliano fold belts after the end of the

volcanic event of the Acampamento Velho Formation

(Early Cambrian, Almeida et al., 2002).

Five facies associations are recognized in the Santa

Barbara Formation: alluvial fan, fan-delta front, lacustrine,

sandy braided, and gravel bed braided river deposits. The

associated deposits (conglomerates, sandstones, and silt-

stones) show a pronounced textural and compositional

immaturity, as well as a characteristic reddish diagenetic

color that results from oxide coatings. The climate is

interpreted as semiarid, with deposition occurring in a

continental setting during heavy but short-lived rainstorms.

Three depositional sequences were identified on the basis

of chronostratigraphically significant surfaces. Two basal

sequences display a coherent depositional evolution, with

axial systems (fluvial and deltaic) prograding northeastward

and a lateral contribution of alluvial fans. The base of

Sequence III represents a tectonic rearrangement of the

basin, with partial erosion of the basal units and axial

transport to southwest.

Coarse-grained deposits are located mainly along the

eastern margin of the basin (Cacapava high). These

deposits and correlated sandstones deposited basinward

reflect three main stages of denudation of this topo-

graphic high. Andesitic and metasedimentary fragments

first dominated the framework of the conglomerates

(Sequence I). Later, during the deposition of Sequence II,

the Cacapava high contributed metavolcanic clasts. Due

to the tectonic inversion of depocenters, which took place

at the onset of deposition of Sequence III, the main

contribution from the Cacapava high were granitic

fragments of the CSGC, which reflect the final denuda-

tion stage of the eastern margin during the evolution of

the Santa Barbara basin.

Acknowledgements

The first author thanks CAPES (the research fund of

Brazilian Ministry of Education) for the concession of a

grant to his MSc degree. The authors are also indebted to Dr

A.J.V. Garcia (UNISINOS, Brazil) for providing the thin

sections analyzed in this work. Thanks are due to Dr O.J.

Martinsen (Norsk Hydro, Norway) for a revision of a

previous version of the manuscript. Comments and sugges-

tions by the referees, Dr P. Eriksson (University of Pretoria,

South Africa), Dr M. Martins-Neto (Universidade Federal

de Ouro Preto, Brazil), and Dr P.S.G. Paim (UNISINOS,

Brazil), were very helpful in improving the quality of this

article.

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Santa Bárbara Formation (Caçapava do Sul, southern Brazil): depositional sequences and evolution of an Early Paleozoic postcollisional basin

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