Facies architecture in a tectonically influenced estuarine incised valley fill of Miocene age, northern Brazil

Journal of South American Earth Sciences 17 (2004) 267–284 www.elsevier.com/locate/jsames

Facies architecture in a tectonically influenced estuarine incised valley fill of Miocene age, northern Brazil Dilce F. Rossettia,*, Antoˆnio E. Santos Ju´niorb a

Instituto Nacional de Pesquisas Espaciais-INPE, Centro de Observac¸a˜o da Terra, Divisa˜o de Sensoriamento Remoto-DSR, Rua dos Astronautas 1758, Jardim da Granja-CP 515, Sa˜o Jose dos Campos, CEP 12245-970, Sa˜o Paulo, Brazil b Universidade Federal do Para´, Centro de Geocieˆncias, Campus do Guama´ S/N Bele´m-PA, Brazil Received 1 January 2003; accepted 1 August 2004

Abstract The Miocene Barreiras Formation in the Middle Rio Capim area records an incised valley system for which facies analysis and ichnology (Skolithos, Ophiomorpha, Planolites, Gyrolithes, Taenidium) suggest an estuarine character. Three stratigraphic units are recognized (from bottom to top): Unit 1 includes an inner estuarine tidal channel complex and tidal flat/salt marsh deposits; Unit 2 consists of estuarine bay/lagoon and flood tidal delta deposits related to the estuary mouth; and Unit 3 includes a tidal channel with a tidal point bar, as well as tidal flat/salt marsh deposits similar to those from Unit 1. These units and their bounding surfaces record the history of relative sea level changes in the estuary. After a sea level drop, the valley was inundated and formed an amalgamated sequence boundary and transgressive surface. Transgression (Unit 1) promoted the landward shift of flood tidal deltas and lagoon settings (Unit 2). The system then moved seaward, with the superposition of inner estuarine deposits (Unit 3) over Unit 2. Facies architecture seems to have been controlled by tectonics, as shown by: the paleovalley orientation according to the main tectonic structures of the basin; the presence of faults and fractures that displace the basal unconformity; and the abundance of soft sediment deformation. q 2004 Elsevier Ltd. All rights reserved. Keywords: Facies analysis; Incised valleys; Miocene; Northern Brazil; Sequence stratigraphy

1. Introduction Much work on incised valley fills has been presented during the past decade and provided important new insights into their facies relationships and stratigraphic organization (Dalrymple et al., 1992, 1994; Zaitlin et al., 1994). The great interest in these settings reflects their significance not only for sequence stratigraphic models but also for targeting hydrocarbon (Wood and Hopkins, 1992) and water reservoirs. Incised valleys represent narrow morphological features, but constitute important parts of the sedimentary record because of their excellent preservation potential. However, it remains difficult to identify facies and the stratigraphic expression of incised valley fills, or establish exploration models, because * Corresponding author. Tel.: C55 12 394 56440. E-mail addresses: [email protected] (D.F. Rossetti), antoniojr@ museu-goeldi.br (A.E. Santos Ju´nior). 0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.08.003

they are highly variable and architecturally complex. The introduction of a tripartite model based on the interplay of fluvial and marine inflows (Nelson and Bray, 1970; Reinson, 1977; Roy et al., 1980; Roy, 1984) has significantly increased our ability to characterize ancient estuarine systems. In addition, it led to the establishment of an estuary classification based on the dominance of either waves or tidal currents at the marine end, which has been applied to all ancient successions (Dalrymple et al., 1992). However, modeling estuarine systems, which volumetrically represent the main portion of incised valley fills, is still a challenge because many variations must be better documented to develop a model that depicts facies and stratigraphic features precisely. Furthermore, the majority of incised valley systems documented in the literature relate to relative sea level changes (Dalrymple et al., 1994), but few studies attempt to discern the controlling mechanism of such changes (Slack, 1981; Weimer et al., 1982; Jenette et al., 1991).

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In this article, we describe the sedimentological characteristics of a Miocene succession exposed in the eastern margin of the Cameta´ sub-basin in northeastern Para´ State, Brazil. The detailed analysis of facies architecture extracted from this study area provides elements to identify the sedimentary signature of relative sea level fluctuations in a coastal plain incised valley system during transgression and highstand. We suggest tectonics as the most likely control on base-level variation and valley incision.

2. Regional geological framework An elongated area in the northeast of Para´ State subsided as a result of the extension related to the opening of the Equatorial South Atlantic Ocean during the late Jurassic/early Cretaceous (Azevedo, 1991; Galva˜o, 1991; Villegas, 1994). This process led to the development of several basins along the north Equatorial Brazilian margins; the largest, the Marajo´ Graben system (up to 1.5!106 km 2), comprises four depocenters (Fig. 1), of which the asymmetric Cameta´ Sub-basin (10 km deep, 80 km wide, 140 km long) is

the easternmost one. Basin asymmetry is defined by sets of listric, NW–SE-oriented normal faults with offsets of up to 2 km (Villegas, 1994). Seismic data indicate a sediment pile up to 10 km thick at the depocenter of the Cameta´ Sub-basin. However, only three wells are available. They show a nearly 5.5 km thick, Cretaceous synrift interval, which includes the Breves (Albian–Cenomanian) and Limoeiro (mostly Upper Cretaceous) Formations (Fig. 2) and a much thinner (900 m) Tertiary post-rift interval (Galva˜o, 1991). The latter interval has been referred to as the Marajo´ (Paleocene–Middle Miocene) and Tucunare´ (Late Miocene–Pleistocene) Formations. We studied the Tertiary deposits in two quarries near the margins of the Middle Rio Capim (Fig. 1). In these localities, these deposits unconformably overlie arkosic (now kaolinitized) strata of the Upper Cretaceous? Ipixuna Formation and are overlain by another unconformity marked by erosion and reworked ferruginous concretions that laterally grade to lateritic paleosol. This paleosol correlates with he lateritic paleosol formed at the top of the Barreiras Formation throughout eastern Amazonia (Truckenbrodt et al., 1995; Rossetti, 2000, 2001). According to

Fig. 1. Location of the study area in the Cameta´ Sub-basin of the Marajo´ Graben system, northern Brazil. The main tectonic lineaments are indicated (IRCC and PPSAZstudied quarries).

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Fig. 2. Stratigraphic chart representing the sedimentary successions in the Cameta´ Sub-basin.

the stratigraphic relationships and facies similarities, the studied deposits correlate with the Barreiras Formation of Middle Miocene age (Arai et al., 1988; Leite et al., 1997), which is included in the Marajo´ Formation defined at the subsurface.

3. Facies associations 3.1. Description The Barreiras Formation exposed in the study area consists of siliciclastic deposits that onlap against a basal, concave-up discontinuity with an erosional relief of nearly 30 m at the outcrop scale. In gamma-ray profile, the base of this succession is marked by an abrupt decrease in gammaray signals, indicating a sharp lithological contrast with the underlying Upper Cretaceous deposits. The succession typically shows an upward transition from a bell- to a funnel-shaped gamma-ray profile (Fig. 3). The main lithologies comprise quartz-sandstones, heterolithic sand/ mud interbeddings, and mudstones. The sandstones are friable and show well-sorted, subrounded to rounded, fine to medium grain sizes, though coarse grain sizes might be present. Trace fossils are abundant and mostly include Skolithos, Ophiomorpha, Thalassinoides, Planolites, Gyrolithes, Macaronichnus?, and Taenidium (Fig. 4). We have identified four main tidal-influenced depositional environments (Table 1) on the basis of the lithology, sedimentary structures, and facies relationships: channel complex (CH), tidal delta (TD), tidal flat/salt marsh (TF/SM), and estuarine bay/lagoon (EB/LG).

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Facies association CH is the thickest and most representative and consists of concave-up deposits of up to 10 m thick and 550 m long in a cross-section of the flow direction (Fig. 5A–C). The base of the deposits shows a thin (!0.15 m) lag of quartz pebbles or very coarse-grained quartz sandstone that is locally iron cemented (Fig. 5B). These basal deposits can reach 0.4 m thick. Facies association CH can be subdivided into three subfacies associations: sandy tidal channel (CH-1), tidal point bar (CH-2), and muddy tidal channel/mud plug (CH-3). Subfacies association CH-1 is better developed at the valley axis and consists of cross-stratified sandstones with tabular and trough cross-stratification of sets averaging 0.1– 0.35 m thick (Fig. 6A). Both grain and set sizes typically decrease upward, giving rise to fining and thinning upward patterns. Set boundaries are either straight or undulating and demarcated by mud drapes. Foresets form packages that average 0.1 m thick and are overlain by mud drapes or, more frequently, capped by reactivation surfaces. Mud rip-up clasts (cm scale) are very common and occur either concentrated at set boundaries or dispersed within crosssets. A characteristic feature of tidal channel cross-stratified sandstones is the presence of lateral successions of alternating thinner/thicker foreset packages that are defined by reactivation surfaces and/or mud drapes to form couplets (Fig. 6B). Some channels show compound cross-sets characterized by medium- to large-scale (0.7–1.5 m thick), very low angle dipping (!128) cross-sets with superimposed smaller scale (0.1–0.2 m thick) tabular and trough cross-sets (Fig. 6C and D). The compound sets show abundant reactivation surfaces, similar to the cross-stratified sandstones. Paleocurrent data measured from cross-sets (including compound sets) show a main NNW orientation with a subordinate SE orientation (Fig. 7A). Subfacies association CH-2 occurs at the channel margin and is represented by low angle dipping, heterolithic-bedded deposits (Fig. 5A and C). At the channel axis, the deposits reach 7 m thick. Heterolithic strata are wavy, lenticular, and pin striped, and they occur in packages up to 1 m thick, defined by concave-up to sigmoidal-shaped reactivation surfaces. Where thicker sand layers are present, they show cross lamination, which also shows abundant internal reactivation surfaces and mud drapes. Paleocurrent data indicate a preferential NE–SW orientation for the inclined heterolithic deposits of the study area (Fig. 7B), which is normal to the orientation of the cross-stratified sandstones of subfacies association CH-1. Subfacies association CH-3 consists of concave-up bodies that are entirely represented by laminated mudstones that become progressively massive and mottled upward (Fig. 5D). This subfacies association forms muddy bodies that are up to 400 m wide and 3 m thick. Facies association TD consists of mixed sandstones and heterolithic facies that form amalgamated or slightly dipping bodies, arranged as sigmoidal-shaped, prograding lobes. A succession of up to 20 sand lobes is observed in the study area

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Fig. 3. (A) Gamma-ray log and (B) generalized stratigraphic column showing the relationship of the Barreiras Formation to other sedimentary successions exposed along the Middle Rio Capim area. The arrows in A indicate upward gradation from a bell- to a funnel-shaped signal.

(Fig. 8A). Individual lobes average 20–50 m long and up to 5 m thick. Lobe size decreases gradually in the flow direction, which is mostly SSE oriented. In general, sandier lobes occur in the upflow direction, whereas heterolithic deposits dominate downstream. However, some lobes may be composed entirely of either lithology, with heterolithic beds dominant. The sandstones display large-scale dipping foresets that conform to clinoform shapes. Lobe bases are

either sharp or gradational with the underlying, muddier lithologies of facies association EB/LG. Slumps and soft sediment deformation structures are common in facies association TD and mostly include convolute bedding, fluidized sediment mass, ball-and-pillow, and overturned and oversteepened beds (Fig. 9A and B). Facies association TF/SM forms units that are less than 5 m thick and occurs as tabular packages laterally

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stressed laminations; wavy beddings are dominant. In the basal sandier portions, cross-laminated beds are 0.05–0.1 m thick and contain foresets with abundant reactivation surfaces and mud drapes. Mudstone layers show sharp contacts with sandstones and internally present parallel laminations disturbed by tiny feeding burrows. Thicker mudstone layers occur at the top and form massive, whitish units with abundant root marks (Fig. 10B). Facies association EB/LG is up to 3–4 m thick and consists of intergrading massive mudstones, very finegrained sandstones, and dominant heterolithic deposits. This facies association lithologically resembles facies association TF/SM, but deposits occur underlying and/or laterally intergrading with facies association TD here (Fig. 8A). Mudstones are massive and may display convolutelaminated sandstones that sink down into them through loading. 3.2. Interpretation

Fig. 4. Trace fossils typical of nearshore, environments, representative of the Barreiras Formation in the Middle Rio Capim area, illustrating (A) Skolithos (Sk) in trough cross-stratified sandstones of tidal channel deposits (facies association CH); (B) Ophiomorpha (Op), Skolithos(Sk), and Planolites (Pl) in sandstones, also attributed to tidal channel deposits (facies association CH); and (C) Gyrolithes (Gy) in subtidal cross-stratified sandstones of tidal flat deposits (facies association TF/SM). Note mud drapes along the foresets (Md) and mud clasts (Mc) dispersed within the sandstones in C.

interlayered with subfacies association CH-1 (Fig. 5A). At the base, they consist of sandier lithologies, which become progressively muddier and form fining-upward successions 1–2 m thick (Fig. 10A). Heterolithic-bedded deposits are dominant and include wavy, lenticular, and pin-stripe highly

Several lines of evidence indicate that the Barreiras Formation was formed in an incised valley system. First, the deposits occur within a large, concave-up depression cut down into underlying Cretaceous rocks, which suggests a valley configuration (Fig. 11). Second, the deposits are part of a sedimentary succession with a sharp base and an upward gradation from a bell to a funnel shape in gammaray profiles. This pattern, similar to many ancient and modern incised valley analogs (Zaitlin and Shultz, 1990; Van Wagoner et al., 1990; Jenette et al., 1991), records a transition from fining to coarsening upward as the estuary evolves from transgression to highstand. An estuarine setting also is supported in this case by the ichnological assemblage, represented by Skolithos, Ophiomorpha, Planolites, Gyrolithes, and Taenidium. These trace fossils characterize a mixed Skolithos–Cruziana ichnofacies, common in the marine-influenced sides of many estuarine settings (Pemberton and Wightman, 1992; Pemberton et al., 1992; MacEachern and Pemberton, 1994). In particular, Gyrolithes and Taenidium support a transitional setting with significant freshwater influx (Buatois et al., 1997; Gernant, 1972; Powell, 1977; Ranger and Pemberton, 1988; Strobl, 1988). Because estuaries are common sites of tidal amplification, any large volume of estuarine deposits is characterized by tidal-generated structures. The bulk of the Barreiras Formation was influenced by tidal processes, as demonstrated by the abundance of reactivation surfaces and/or mud drapes that define lateral successions of alternating thin/thick foreset pairs. These features are attributed to tidal bundling (e.g. Visser, 1980; Boersma and Terwindt, 1981; Yang and Nio, 1985; Leckie and Singh, 1991; Shanley et al., 1992). The abundance of heterolithic beds is consistent with a tidal origin because they are volumetrically significant deposits associated with, though not exclusive to, many modern and ancient tidal settings (e.g. Clifton, 1983).

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Table 1 Summary of facies association and interpreted depositional environments of the Miocene deposits exposed along the Rio Capim area Facies/subfacies association

Description

Depositional environments

CH (CH1,CH2,CH3)

Concave-up sandstone, mudstone, and heterolithic deposits displaying a sharp basal discontinuity surface mantled by a lag of quartz pebbles and/or very coarse-grained sandstone. CH1Ztabular, trough, and compound cross-stratified sandstone in sets with abundant reactivation surfaces and mud drapes, locally arranged into couplets. Fining and thinning upward cycles are common. CH2Zinclined heterolithic deposits indicating flow directions normal to the cross-stratified sandstones. CH3Zlaminated and massive mudstone bodies with concave-up shapes. Sandstone and heterolithic-bedded deposits occurring as onshore-oriented, sigmoidalshaped lobes. Soft sediment deformation structures are frequent. Tabular heterolithic deposits arranged in fining/thinning upward cycles marked at the top by massive mudstones with abundant root marks. Massive mudstones intergraded with very fine-grained sandstones and heterolithic deposits that occur overlying or laterally interfingered with tidal delta deposits.

Tidal channel (mid-channel, tidal bar, mud plug)

TD TF/SM EB/LG

The concave-up shapes of facies association CH suggest flow confinement, and the upward fining/thinning successions indicate decreasing flow energy, typical of channel settings. Subfacies association CH-1 records central areas of tidal channels at or near the valley axis. At this location, flow energy was high enough to transport sand and form cross-stratified strata. The compound cross-sets record the highest volume of sand accumulation in the estuarine channels and attest to the presence of medium- and smallscale bedforms superimposed on low angle dipping, largescale bedforms. Reactivation surfaces in the channel deposits confirm bedform migration under highly unsteady flows (e.g. Mowbray and Visser, 1984; Chakraborty

Tidal delta (flood-dominated) Tidal flat/salt marsh Estuarine bay/lagoon

and Bose, 1990; Simpson and Eriksson, 1991), as occurs in tidal settings (e.g. Houbodt, 1968; Allen, 1980; Dalrymple, 1984). The bidirectional flow conforms this interpretation and indicates a main NW–SE-oriented estuarine system. These data then suggest an approximate NE–SW paleocoast with land to the south. Thus, the main northwest flow orientation is the associated deposition by ebb tidal currents. Asymmetric tides are common in modern tidal settings (Boer et al., 1989; Nio and Yang, 1991). Subfacies association CH-2 is due to lateral accretion on estuarine point bars (Thomas et al., 1987), consistent with the low angle dipping, heterolithic beds oriented normal to tidal channels. The literature provides many examples of

Fig. 5. Facies association CH, attributed to tidal channel depositional environments. (A) Broad channel filled by cross-stratified sandstone (subfacies association CH-1) and tidal point bar (subfacies association CH-2) deposits. The inside box locates B. (B) Detail of point bar deposits, characterized by low angle dipping, heterolithic beds. Note several internal reactivation surfaces (arrows). The hatched line indicates the sequence boundary at the base of the paleovalley incised into Cretaceous rocks. (C) Detail of a tidal mud channel/mud plug (subfacies association CH-3), indicated by hatched lines.

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Fig. 6. Facies association CH. (A, B) Cross-stratified sandstone from facies association CH, illustrating foresets arranged in a succession of laterally alternating thicker/thinner packages defined by reactivation surfaces and/or mud drapes (arrows). These structures are attributed to tidal processes. (C, D) Compound cross-stratified sandstones, characterized by large-scale cross strata with superimposed medium-scale tabular cross sets (arrows in B).

tidal point bars recorded by low angle dipping, heterolithic beds (e.g. Clifton, 1983; Smith, 1988; Ainsworth and Walker, 1994; Porebski, 1995). The origin of the reactivation surfaces in these deposits (Fig. 5B) may record either seasonal fluctuations in fluvial influx or erosion due to longterm (i.e. longer than monthly) tidal oscillations. Subfacies association CH-3 reflects low-energy tidal channels with high rates of suspension settling. The massive and mottled mudstones on the top of the channel fills record abandonment and vegetation growth. Although not exclusive, an abundance of tidal channel deposits is common in estuarine settings, in line with our proposed interpretation. In an estuarine context, prograding deposits similar to facies association TD could record a bayhead tidal delta entering a sufficiently deep central bay area, a barrier spit prograding alongshore, or a tidal delta at the estuary mouth. Although facies characteristics might be similar in these settings, paleocurrent data favor the tidal delta as the most plausible explanation. Therefore, the onshore orientation of the cross-sets in facies association TD suggests small flood tidal deltas. The lobes probably reflect a heterolithic

composition because they represent the landwardmost end of the flood tidal delta, where we expect interfingering of marine-derived sands and central estuarine bay mudstones. Slumps and soft sediment deformation structures, as observed here, are naturally favored in these depositional environments, though synsedimentary seismicity might

Fig. 7. Paleocurrent data from cross-stratified sandstone of facies association CH. (A) Channel. (B) Inclined heterolithic deposits from tidal point bars.

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Fig. 8. Tidal delta facies association (TD). (A) General view of a succession of 19 SE prograding lobes. (B) Detail of a lobe illustrating its internal sedimentary arrangement with inclined, heterolithic beds displaying abundant soft sediment deformation structures. (C) Sigmoidal lobe represented by heterolithic beds. (D) Detail of B, illustrating large-scale convolute beds.

have contributed to increased deformation, as we discuss subsequently. Facies association TF/SM suggests the existence of wide, flat areas surrounding the tidal channels, consistent with tidal flat settings. Frequent alternations of traction and suspended-load deposition are favored by the time–velocity asymmetry of tidal currents (e.g. Reineck and Wunderlich, 1968; Reineck and Singh, 1986; Terwindt, 1971; Terwindt and Breusers, 1972; Mowbray, 1983). The sharp contact between the mudstone and sandstone layers indicates rapid changes between periods of quiescence (mud settling) and periods with relatively increased flow strength (sand deposition), as is favored in tidal environments. The fining-upward successions record prograding tidal flats with upward transitions from subtidal to intertidal and supratidal deposits. The mottled, massive mudstones with

rootlets at the top of the tidal flat successions indicate soil development typical of supratidal mud flats and salt marshes. Heterolithic deposits similar to facies association TF/SM also relate to facies association EB/LG, an indirect interpretation that we base mainly on the spatial relationship with the flood tidal delta deposits. The presence of sigmoidal lobes requires a low energy basin (corresponding to central estuarine areas), where sedimentation was dominated by mudstones and heterolithic facies.

4. Architecture of the estuarine fill Detailed mapping of facies architecture is required to understand estuarine evolution. Three stratigraphic units were recognized in the paleovalley fill of the study area

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Fig. 9. (A, B) Tidal delta deposits illustrating styles of soft sediment deformed beds with convolute folds forming anticlines and synclines with fluidized masses.

(Table 2, Fig. 11). The erosive surface at the valley floor laterally correlates with the surface of a deep weathered paleosol formed sometime between the Late Cretaceous and the Late Oligocene/Miocene (Fig. 12; see also SB1 in Figs. 11 and 13A). Short (!2 cm), horizontal to subvertical and sometimes tear-like trace fossils with diameters averaging 3 mm, attributed to the Glossifungites ichnofacies, are locally abundant along this surface (Fig. 13B and C). This discontinuity surface correlates with a surface with lateritic/bauxitic paleosol that occurs between Upper Cretaceous and Miocene deposits in other areas located at several hundreds of kilometers (e.g. Wijmstra, 1971; Prasad, 1983; Truckenbrodt et al., 1995; Rossetti, 1998, 2000, 2001), beyond the limits of the Cameta´ Sub-basin (Fig. 12). In those areas, the discontinuity surface has been disrupted by normal faults that end a few meters below ground (Rossetti, 1998, 2000). Faults are associated with this discontinuity surface in the study area, as revealed by steeply dipping (up to 608) segments, where fault planes can be traced upward into the Barreiras Formation, resulting in offsets of only few centimeters (Fig. 14A and B). Unit 1 is the thickest (up to 10 m), but it occurs only in depressed areas, located at or near the paleovalley axis, where it forms a succession of strata that downlap or onlap

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Fig. 10. (A) Tidal flat/salt marsh facies association (TF/SM), characterized by heterolithic beds that grade upward into mudstones forming fining upward cycles 1–2 m thick (arrows). (B) Mudstones from the top of fining upward tidal flat/salt marsh deposits. Note the massive, mottled aspect of these deposits and the rootlet marks (arrows), attributed to the paleosol.

against the paleovalley basal discontinuity surface that cuts into the Cretaceous deposits (Fig. 11). This unit consists of subfacies associations CH-1 and CH-2 and facies association TF/SM, which are intergraded and attributed to midtidal channel, tidal point bar, and tidal flat/salt marsh deposits. Channel deposits are volumetrically the most important and constitute approximately 70% of the deposits in this interval. The tidal flat/salt marsh deposits also may include estuarine bay/lagoon deposits; their differentiation in the field is not conclusive. The top of Unit 1 is a discontinuity surface with an erosional relief of !5 m. This surface is associated with a soil horizon, characterized by massive lithologies and root marks, which grades downward into well-structured deposits (surface FS and SB2 in Fig. 11). Fine- to mediumgrained, sand-filled burrows are superimposed on the soil horizon. Single vertical or branched burrows, including Skolithos, Thalassinoides, and Planolites with well-defined, firm walls, are present and suggest a Glossifungites ichnofacies (Fig. 13D and E). Another interesting feature related to this discontinuity surface is the presence of wedge-shaped, vertical to subvertical fractures that end with depth after a few centimeters (Fig. 15A–D). Larger fractures

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Fig. 11. Geologic section drawn from photo mosaics and field sketch recording facies architecture and stratigraphic framework of estuarine deposits in the Middle Rio Capim area (SB1–SB3, sequences boundaries; TS, transgressive surface; MF, maximum flooding).

are filled by sands from overlying strata, which may display disperse mud chips. The profile view shows that these fractures die out at depth, usually after a few tens of centimeters (Fig. 15A). In plan view, they suggest a network of straight, lensoidal parallel fractures up to 0.4 m long and 0.02–0.03 m wide (Fig. 15B), which might interconnect with one another at high angles (Fig. 15C and D). Unit 2 consists of an up to 7 m thick succession, mainly comprising facies associations EB/LG and TD (Figs. 8A and 11). Small heterolithic tidal channel deposits, cutting down into the tidal delta deposits, also might occur. Similar deposits have been recorded in the Barreiras Formation of the Sa˜o Luı´s basin (Fig. 12; uppermost portion of Unit 1, above surface RS). The degree of soft sediment deformation structures associated with deposits of Unit 2 increases significantly near a listric normal fault, which cuts down into the lobe system. The fault is not restricted to the tidal delta deposits but was reactivated through time, influencing sedimentation patterns of overlying strata even

after the Miocene. Unit 2 is bound at the top by a discontinuity surface with an erosional relief of !2 m and locally displays broad, concave-up geometries (surface MF in Fig. 11). Unit 3, deposited unconformably on top of Unit 2, reaches 9 m thick and contains a tidal channel (70% of the total) laterally intergraded with tidal flat/salt marsh deposits. Sandy channels dominate in this stratigraphic interval and intercut one another. A complete channel forms a concaveup, 5 m thick, more than 400 m long deposit (see central part of Fig. 11). In this channel, cross-stratified sandstones (subfacies association CH-1) with abundant Skolithos grade laterally into low angle dipping, heterolithic tidal point bars (subfacies association CH-2). Entirely mud-filled channels (subfacies association CH-3) also are present (Fig. 5C) and form features up to 300 m long and 2 m thick. Unit 3 grows upward into another discontinuity surface marked by an erosional relief up to few meters on the outcrop scale and a deep lateritic paleosol with a characteristic concretionary

Table 2 Stratigraphic units recognized in the Miocene deposits exposed in the Rio Capim area, with a summary of the main depositional environments and characterization of bounding surfaces Stratigraphic unit

Interpreted depositional settings

Boundaries

Unit 3

Tidal channel: mid-channel (CH1) and tidal point bar (CH2); tidal flat/salt marsh (TF/SM)

Unit 2

Tidal delta (TD); estuarine bay/lagoon (EB/LG)

Unit 1

Tidal channel: mid channel (CH1) and tidal point bar (CH2); tidal flat/salt marsh (TF/SM)

Upper: discontinuity surface with an erosional relief up to a few meters at the outcrop scale and marked by a deep lateritic soil horizon and/or a lag of concretions and quartz pebbles. The surface is correlated with the sequence boundary that ended the Miocene sedimentation in northern Brazil. Upper: discontinuity surface with an erosional relief !2 m with broad concave-up scours, attributed to tidal ravinement during maximum flooding. Lower: sharp discontinuity surface with an erosional relief up to 30 m at the outcrop scale, marked by a deep soil horizon and Glossifungites ichnofacies, attributed to a composite sequence boundary and transgressive surface. Upper: discontinuity surface with an erosional relief of up to 5 m at the outcrop scale, marked by a soil horizon with abundant fractures and cracks and Glossifungites ichnofacies. The surface is attributed to tidal ravinement on a high-frequency sequence boundary.

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Fig. 12. Correlation of Cenozoic depositional units and stratigraphic surfaces exposed along several onshore basins.

horizon represented by vertical columns that may be locally reworked as a lag of lateritic clasts and quartz pebbles (surface SB3 in Fig. 11).

5. Estuary evolution The discontinuity surface at the base of the paleovalley defined in the Middle Rio Capim area is clearly a sequence boundary, because the valley fill has a quartzose composition and Miocene age and the underlying deposits consist of arkosic (now kaolinized) Upper Cretaceous deposits of the Ipixuna Formation. This sequence boundary records a prolonged (!50 Ma) period when the area underwent substantial subaerial exposure, which favored soil development and erosion. The low base level is directly related to a drop in relative sea level. (We discuss possible causes of this fall in a subsequent section.) After incision, the study area was invaded by marineinfluenced waters, as recorded by estuarine deposits (i.e. Unit 1) that directly overlie the basal sequence boundary (Fig. 16A). Fluvial strata, prominent in Dalrymple et al.’s (1992) models, are not present in this paleovalley, probably

due to high tidal energy, as expected in funnel-shaped estuaries. The preponderance of tidal flux over fluvial discharge may have led to the entire reworking of any eventual lowstand fluvial strata and, ultimately, to the coincidence of the sequence boundary with a transgressive surface. The presence of Glossifungites ichnofacies along the discontinuity surface at the base of the paleovalley is consistent with its composite nature; this suite may occur where a transgressive surface and a sequence boundary amalgamate (MacEachern and Pemberton, 1994). Similar composite surfaces at the bases of incised valley deposits throughout the world have been documented (e.g. Boyd and Honing, 1992; Dalrymple et al., 1992; Allen and Posamentier, 1993; Zaitlin et al., 1994). The deposits in Unit 1 are typical of proximal estuarine areas, where channel meandering produces tidal channels with well-developed tidal point bars (Rahmani, 1988; Smith, 1988; Zaitlin et al., 1994). Widespread muddy tidal flat/salt marsh deposits in this unit are consistent with a high tidal regime, which may have promoted a strong imprint even in the headwardmost portions of the estuary. Transgression became more pronounced as the valley evolved (Fig. 16B), as shown by deposition of Unit 2, which

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Fig. 13. (A, B) Detailed view of the sequence boundary that forms the base of the Barreiras incised valley in the Middle Rio Capim area, illustrating the mottled, massive horizon attributed to paleosols (A) and trace fossils attributed to the Glossifungites ichnofacies in plan view (B). (C, D) Detailed view of the Glossifungites ichnofacies associated with the discontinuity surface at the top of Unit 1 in profile (C) and plan (D) view. (Sk, Skolithos; Th, Thalassinoides).

overlies Unit 1 and spreads out to the valley margins to directly mantle the basal composite sequence boundary directly. Increased transgression during this time is also indicated by the shift of relatively more seaward-influenced facies associations landward over Unit 1. This shift is revealed by the estuarine bay/lagoon deposits at the base of Unit 2, which include muddier lithologies, typical of turbidity zones. In addition, the majority of Unit 2 consists of flood tidal delta deposits, which suggest a barred estuary mouth developed by flow confinement through tidal inlets. The tidal delta deposits record progradation onto lagoonlike, back barrier areas. Despite the absence of wavegenerated structures, these deposits imply a wave-influenced barred coastline, with onshore flows transporting sufficient sands to form the prograding lobes. These characteristics conform to microtidal to mesotidal estuarine models (Dalrymple et al., 1992). However, the estuarine system does not show any evidence of bayhead deltas in the inner estuary (represented by Unit 1), as dictated by wavegenerated models (Dalrymple et al., 1992; Zaitlin et al., 1994). In addition, a drowned river mouth estuary with a seaward-opening funnel shape and abrupt narrowing at the coastline would be expected during increased transgression (e.g. Siringan and Anderson, 1993; Dalrymple and Zaitlin, 1994; Oost, 1995). Therefore, the estuary in the study area might not conform to end-members proposed in estuarine models but instead represent a mixed tidal and waveinfluenced type. A modern analogue is the Gironde estuary, which is a mixed tide and wave-dominated incised valley

system with a seaward barrier complex (Allen and Posamantier, 1994). An alternative explanation for these tidal delta deposits is that they may result from differential bottom topography. Studies of modern macrotidal estuaries, such as the Avon River estuary in the Bay of Fundy (Lambiase, 1980), claim that tidal deltas at the estuary mouth result from flow confinement caused by bedrock morphology. The paleosol associated with the discontinuity surface at the top of Unit 1 supports a drop in relative sea level with subaerial exposure and the formation of a sequence boundary. Together with the overlying unit and the dominance of tidal delta deposits, this drop may indicate a phase of estuary abandonment and delta progradation. However, the origin of the discontinuity surface appears to be more complex. The Glossifungites ichnofacies superimposed on the paleosol points to its composite nature and reveals the coincidence of a sequence boundary and a flooding (ravinement?) surface. This interpretation more easily justifies the landward orientation of the overlying tidal delta deposits. Therefore, the best hypothesis is that it is a composite estuarine fill, in which the initial transgression (Unit 1) was followed by a period of subaerial exposure with soil formation and subsequent erosion (SB2). This was in turn was followed by another transgressive pulse (Unit 2), during which more flood tidal deltas associated with barriers at the estuary mouth shifted landward over inner estuarine settings. The latter transgression took place during the development of a higher frequency sequence in the estuary.

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Fig. 14. (A–C) Details of a faulted segment along the sequence boundary at the base of the Barreiras paleovalley. Note that the sequence boundary displays a sharp, steeply dipping segment formed by displacement along a normal fault plane. Fault reactivation affected the Miocene sedimentary pile, as illustrated by dragging of the strata (C).

Despite the distance, close facies and stratigraphic similarities indicate that Units 1 and 2 correlate with the Units 1 and 2 documented in the Barreiras Formation in the northern Sa˜o Luı´s basin (Fig. 12). In both areas, the deposits overlie Upper Cretaceous rocks topped by a sequence boundary with a lateritic paleosol. In addition, lagoon and tidal delta deposits characterize Unit 2 in the Sa˜o Luı´s Basin. However, in that basin, the strata are bound at the base by an erosional discontinuity surface interpreted as a ravinement surface (Rossetti, 2000). In the Cameta´ subbasin, this surface is associated with a paleosol, which leads to our suggestion that it is a composite surface that matches a sequence boundary and flooding surface. However, evidence of subaerial exposure, if present, may have been destroyed during

the transgressive event in the Sa˜o Luı´s Basin, whereas in the Cameta´ Sub-basin, located in a more continental position, they may have been more developed or less destroyed during the ensuing transgression. The overlying Unit 3 shows deposits similar to those of Unit 1 and represents tidal channel, tidal point bar, and tidal flat/salt marsh settings, which suggest inner estuarine areas. The occurrence of these deposits over flood tidal delta deposits evinces progradation when the headwardmost facies shifted seaward (Fig. 16C). The stratigraphic significance of the discontinuity surface between Units 2 and 3 is open to debate. Considering only the study area, we might suggest it is the maximum flooding surface between a transgressive systems tract, represented by Units 1 and 2,

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Fig. 15. Sand-filled fractures and unfilled cracks that occur at the discontinuity surface that defines the top of Unit 1. (A) Profile view of the wedge-shaped, sand-filled fractures. (B) Plan view of a set of straight, parallel, sand-filled fractures. (C, D) Plan view of sand-filled fractures and small cracks interconnected at high angles.

and the highstand prograding deposits, represented by Unit 3. However, correlation with previously studied areas from other basins in northern Brazil suggests that this surface may be correlated with a sequence boundary (Fig. 12). Therefore, before the estuary started to prograde after maximum flooding, some areas were exposed to additional subaerial exposure and formed a high-frequency sequence boundary superposed on the maximum flooding surface. The estuarine deposition was terminated by base level fall and subaerial exposure. The process culminated with development of a discontinuity surface at the top of Unit 3 (i.e. surface SB4), which is demarcated by a deep lateritic paleosol. As we mentioned previously, this paleosol is distinguished by vertically aligned lateritic columns; this characteristic, together with the stratigraphic relationships, enables us to correlate it across a distance of up to 1000 km east and north of the study area with the adjacent Sa˜o Luı´sGrajau´ basin and Bragantina platform, where it corresponds to discontinuity surfaces SB3 and DS3 of Rossetti (2000, 2001), respectively.

6. Tectonic control on valley evolution Several factors promote valley incision (Schumm and Ethridge, 1994**), such as eustasy, tectonic uplift (either of coastal zone or an inland area), and an increase in fluvial discharge due to climate change. Most recorded incised valleys are related to eustasy and/or tectonic uplift (Dalrymple et al., 1994), but deciphering which of these factors was more important to the origin of the Barreiras

paleovalley in the Middle Rio Capim is difficult. However, indirect evidence supports a tectonic influence on valley evolution. The genesis of the sequence boundary at the base of the estuarine succession seems to have been related to regional tectonic uplift. The Middle Rio Capim area was unfavorable to sediment accumulation from the Upper Cretaceous to the Miocene, despite the several eustatic changes during this time interval, as revealed by the estuarine nature of the Ipixuna Formation below the Barreiras paleovalley (Santos, 2000). To date, there is no record of Paleogene deposits along marginal areas of the northern Brazilian basins. The Rio Capim area may have raised to a position above the base for sediment accumulation, even with a high sea level; there is evidence that the Brazilian Equatorial basins experienced an inversion during the Upper Cretaceous (e.g. Azevedo, 1991). Thus, regional tectonics likely originated the sequence boundary at the base of the Miocene paleovalley. The presence of steeply dipping segments caused by faults along the basal sequence boundary indicates that tectonics also had an important role in valley development, as proposed elsewhere (e.g. Rossetti, 2000). Further support of this hypothesis comes from the paleovalley’s orientation according to the direction of the Cameta´ graben system, which is chiefly defined by NW–SE normal faults (Galva˜o, 1991). The return to sedimentation following valley incision also might have been due to tectonics. The Miocene experienced a worldwide high sea level, which could have returned deposition along the Rio Capim area and resulted in the origin of the Barreiras estuarine succession. However,

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Fig. 16. Schematic representation of the Miocene estuarine evolution in the Cameta´ Sub-basin. (A) Following a prolonged period of nondeposition, subaerial exposure, and valley incision, transgression took place, depositing the first estuarine deposits at the valley floor. During this phase of the valley evolution, tidal channel and tidal point bar deposits (Unit 1) were abundant in the proximal areas of the estuary. Transgression led to reworking of eventual alluvial deposits formed at the valley floor and resulted in the amalgamation of the basal sequence boundary (SB1) with a transgressive surface. (B) A high frequency fall in relative sea level favored a renewed period of nondeposition, subaereal exposure, and soil development. This process resulted in an unconformity (SB2), represented by the discontinuity surface at the top of Unit 1. As transgression proceeded, the surface was amalgamated with a higher frequency transgressive surface. (C) A pronounced transgressive phase led to the landward migration of the estuarine facies, favoring widespread deposition of flood tidal deltas in estuarine bay areas (Unit 2). The discontinuity surface at the top is attributed to maximum flooding (MF), after which the estuarine facies shifted seaward to form the deposits in Unit 3. (D) The prograding process culminated with the closure of the Miocene deposition in the study area and a return to erosion and widespread soil development, forming sequence boundary SB3.

given the faults associated with the basal sequence boundary, we might argue that, in addition to high sea level, preservation of the Miocene transgressive deposits occurred because of the increased accommodation space, which was promoted by regional tectonics. Furthermore, the wedge-shaped fractures associated with the tidal ravinement surface between Units 1 and 2 seem to have been controlled by an external cause. These features cannot be related to desiccation cracks, because they are too straight, deep, and regular and occur in sets interconnected at high angles. Instead, they resemble other synsedimentary cracks associated with seismic shocks (e.g. Minoura and Nakaya, 1990; Shiki and Yamazaki, 1996). During a seismic shock caused by fault reactivation, for example, open fractures and cracks often develop at the surface (Matsuda, 2000). Considering the proposed tectonic genesis for this paleovalley, this interpretation seems the most plausible to explain the presence of these intriguing fractures in the estuarine succession. Further evidence for a seismic influence during the valley evolution is that Unit 2 contains a high volume of soft sediment deformed strata. Although deformation is expected because of the progradation of tidal deltas on

estuarine bay/lagoon deposits, the significantly increased degree of deformation near a listric normal fault that cuts into the lobe system further evinces seismic activity that influenced sedimentation patterns in the study area. This fault was reactivated over time and also controlled sedimentation patterns of younger deposits. Correlation of the discontinuity surface at the top of Unit 3 across a distance of up to 1000 km outside the study area shows that the end of the Miocene deposition in the Middle Rio Capim area is a result of a regional event along the north Equatorial Brazilian margin.

7. Conclusions Detailed facies analysis indicates that the Barreiras Formation in the Middle Rio Capim area records an incised valley system. The nature of the valley fill, dominated by tidal-generated depositional settings (mostly including tidal channels with tidal point bars, tidal flats/salt marshes, and tidal deltas), strongly suggests an estuarine origin. With a facies architecture and mapping of discontinuity surfaces, we conclude that the estuary did not fill continuously but

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was punctuated by periods of deposition and tidal erosion during high-frequency transgression and highstand. The prevalence of tidal channels with well-developed, inclined, heterolithic tidal point bars indicates important deposition by channel meandering, typical in inner estuarine areas. The estuary experienced an increased transgression, as recorded by the onshore shift of estuarine bay/lagoon and flood tidal delta depositional settings. The presence of open marine facies indicates maximum flooding surfaces in estuarine successions (e.g. Zaitlin et al., 1994). However, if transgression took place at low rates or if the estuary shows a high length/width ratio, the transgressive open marine facies will not necessarily occur in association with the estuarine facies. In this case, the recognition of maximum marine flooding in middle and proximal estuarine areas requires detailed mapping of the facies architecture. For estuaries with a barred mouth (i.e. wave-dominated), transgression might be indicated by increasingly developed flood tidal deltas, resulting in coarsening upward successions that overlie deposits formed in central and inner estuarine areas. In this situation, the surface at the top of the flood tidal delta deposits marks the change from transgression to highstand and corresponds to the maximum flooding surface. Because the flood tidal delta deposits are related to a time of maximum transgression, their association with a wavegenerated estuary is not favored, because transgression would have produced a drowned, funnel-shaped estuary. Thus, a barrier complex is not exclusive of micro- to mesotidal estuarine systems, but it occurs along coasts with mixed tide and wave influence or in estuaries with irregular bedrock morphology. In general, deciphering the causes for valley incision is difficult. In this case, however, the paleovalley orientation, according to the main tectonic structures of the Cameta´ graben system; the presence of faults that displace the unconformity at the base of the paleovalley and create steeply dipping segments; the fractures along the discontinuity surface between Units 1 and 2; and the soft sediment deformation structures that increase in abundance toward a listric normal fault in Unit 2 suggest that tectonics had a strong influence on the origin and evolution of this paleovalley.

Acknowledgements The authors had financial support from CNPq (Grant #474978/2001-0). Logistical support was provided by the Goeldi Museum. The Imery-Rio Caulim Capim-IRCC and Para´-Pigmentos S/A-PPSA provided permission to access the quarries. Geologists Carlos Henrique L. Bastos, Sa´ Pereira, Ana Maria Go´es, and Marivaldo Nascimento provided companionship and discussions in the field. The authors are also very grateful to Dr Gustavo Gonzalez

Bonorino and Dr Matti Ra¨sa¨nen for suggestions that contributed to the final version of this publication.

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