Fluvial architecture variations linked to changes in accommodation space: Río Chico Formation (Late Paleocene), Golfo San Jorge basin, Argentina

Sedimentary Geology 294 (2013) 342–355

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Fluvial architecture variations linked to changes in accommodation space: Río Chico Formation (Late Paleocene), Golfo San Jorge basin, Argentina Nicolás Foix a,b,⁎, José M. Paredes a, Raúl E. Giacosa c,d,e a

Dpto. de Geología, Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco, Ruta Prov. No 1 S/N km 4, (9005) Comodoro Rivadavia, Chubut, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina Servicio Geológico Minero Argentino, Delegación Regional Comahue, CC 228, (8332) General Roca, Río Negro, Argentina d Universidad Nacional de Río Negro, General Roca, Provincia de Río Negro, Argentina e Universidad Nacional del Comahue, Neuquén, Provincia del Neuquén, Argentina b c

a r t i c l e

i n f o

Article history: Received 31 March 2013 Received in revised form 1 July 2013 Accepted 2 July 2013 Available online 9 July 2013 Editor: J. Knight Keywords: Fluvial architecture Accommodation space Río Chico Formation Late Paleocene Golfo San Jorge basin Argentina

a b s t r a c t The Upper Paleocene Río Chico Formation is a 50–180 m thick fluvial succession developed in a passive-margin setting, Golfo San Jorge basin, Central Patagonia, Argentina. A detailed description and interpretation of outcrops was carried out, analyzing exposures from the northern basin margin to the most complete successions at the southern depocenter. The unit is characterized by a regional fluvial system that flowed to the south-east. Five main lithofacies associations were defined: (I) active fluvial channels, with three sub-types: braided, meandering and low-sinuosity, (II) sheet-flood deposits, (III) proximal floodplain (natural levee and crevasse-splay), (IV) distal floodplain, and (V) abandoned channels. Lateral/vertical changes in fluvial architecture of the Río Chico Formation were recognized by variations in preserved thickness, fluvial styles, geometry of fluvial channels, regional paleoflow directions, and channel/floodplain ratios. Close to the northern basin margin, the fluvial succession is 50–60 m thick, composed of braided channels, sheet-flow deposits, and high channel/floodplain ratio. In a basinward direction, the alluvial succession increases to 180 m in thickness, the dominant fluvial styles change to low-sinuosity and meandering channels and channel/floodplain ratio reduces. The fluvial architecture of the Río Chico Formation shows two main depositional trends that resulted from changes in accommodation space across the basin. The interpreted break-point coincides with the underlying Cretaceous basin-boundary, thus the synsedimentary extensional reactivation of the pre-existing tectonic lineament generated differential subsidence, delimiting two different accommodation settings. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Distribution of depositional systems and the stacking patterns of alluvial sequences are controlled by a combination of autocyclic and allocyclic processes in sedimentary basins (e.g., Catuneanu et al., 2005). Numerous works have focused on the spatio-temporal variations of alluvial successions related to changes in tectonics, climate, base level and sedimentary influx at several scales (e.g., Bridge and Leeder, 1979; Miall, 1993, 1996; Ethridge et al., 1998; Arche and López-Gómez, 1999; Catuneanu et al., 2005; Catuneanu, 2006; Allen and Fielding, 2007). Some of the most well documented alluvial stratigraphic variations are: (a) changes in fluvial styles, (b) changes in sandbody geometry (width/thickness ratio), and (c) changes in channel/floodplain ratio. In this sense, the presence of isolated, narrow meandering channels in

⁎ Corresponding author at: Departamento de Geología, Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco. Ruta Prov. No 1 S/N km 4, (9005) Comodoro Rivadavia, Chubut, Argentina. Tel.: +54 297 4550339; fax: +54 297 4559616. E-mail addresses: [email protected] (N. Foix), [email protected] (J.M. Paredes), [email protected] (R.E. Giacosa). 0037-0738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2013.07.001

fluvial successions with low channel/floodplain ratios have been related to high-accommodation settings; whereas tabular, amalgamated, braided fluvial channels with high channel/floodplain ratios have suggested low-accommodation settings (Sønderholm and Tirsgaard, 1998; Arche and Lopéz-Gómez, 1999, 2005; Catuneanu and Elango, 2001; Marenssi et al., 2005; López-Gómez et al., 2010; Huerta et al., 2011). Changes in accommodation depend on base level variations (Schumm, 1993) and/or basin subsidence changes (Catuneanu and Bowker, 2001; Catuneanu and Elango, 2001), which are often difficult to decipher from the sedimentary record. The Golfo San Jorge basin is a Mesozoic extensional basin with several phases of normal fault reactivation. The Río Chico Formation is a continental succession that cover the underlying marine unit (Salamanca Formation), deposited during the first Atlantic ingression in the basin (Legarreta et al., 1990; Legarreta and Uliana, 1994); both units were deposited in a passive-margin setting (Legarreta and Uliana, 1994; Figari et al., 1999) under warm-humid to subtropical–tropical conditions (Brea et al., 2009; Raigemborn et al., 2009; Krause et al., 2010) in an extensional context (Fitzgerald et al., 1990; Figari et al., 1999; Foix et al., 2008, 2012). The aim of this study was to analyze the

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relative influence of the main forcing factors on sedimentation of the Río Chico Formation (Upper Paleocene), based on spatial–temporal changes in lithofacies, lithofacies associations, fluvial styles, geometry of fluvial channels, preservation of floodplain deposits and total thickness of the unit. Detailed correlations of well-logs were used to extend the architectural analysis to unexposed areas. 2. Geological setting 2.1. Basin evolution The Golfo San Jorge basin is a mainly extensional basin related to the fragmentation of the Gondwana paleocontinent during the Late Jurassic and Early Cretaceous (Fitzgerald et al., 1990). The basin is broadly E–W oriented and located in the southern Argentina (Patagonia) (Fig. 1A). The study area is located in the Northern Flank of the Oriental Region of the basin, where extensional tectonic processes were dominant (Fig. 1B, C). The initial Mesozoic synrift stage is represented by the Marifil Complex, composed of acid volcanic rocks and volcaniclastic flows of Jurassic age that crop out in the northern part of the study area (Fig. 2A). The main synrift stage is represented by the mainly lacustrine sedimentary rocks of the Las Heras Group (Uppermost Jurassic to Lower Cretaceous), followed by a thermal subsidence stage in the remainder of the Cretaceous period, where deposition of a thick pile of continental successions (Chubut Group) occurred (Hechem and Strelkov, 2002). Since the uppermost Cretaceous, the basin behaved as a wide tectonic depression with moderate thermal subsidence (Legarreta et al., 1990) in a passive-margin setting (Legarreta and Uliana, 1994; Figari et al., 1999), covering areas outside the boundaries of the Cretaceous extensional basin (Feruglio, 1949; Lesta et al., 1973; Andreis et al., 1975). Paleocene sedimentary rocks in the basin are represented by the marine Salamanca Formation (Early Paleocene) and the continental Río Chico Formation (Late Paleocene). The remainder part of the Cenozoic succession is completed with the Sarmiento Formation,

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Chenque Formation, Santa Cruz Formation and the glaciofluvial gravel strata known as “Rodados Patagónicos” (Fig. 2B). 2.2. Stratigraphy and sedimentology The Río Chico Formation was originally defined by Simpson (1933) and its Late Paleocene age was established from mammalian fauna and radiometric/magnetostratigraphic data (Simpson, 1935, 1948; Marshall et al., 1981, 1983). Recently, an Upper Paleocene to Lower–Middle Eocene age was assigned based on stratigraphic relationships (Krause and Bellosi, 2006; Krause et al., 2010; Raigemborn et al., 2010). The unit attains a maximum thickness of 250 m in the subsurface of the basin (depocenter), decreasing northward to ~50 m at Bahía Bustamante. Feruglio (1949) defined the unit as a continental succession composed of conglomerates, sandstones, claystones and tuffs. He also demonstrated a gradational basal contact from the marine Salamanca Formation, suggesting mainly fluvial and rarely lacustrine paleoenvironmental conditions during evolution of the unit. The paleontological record of the Río Chico Formation indicates subtropical to tropical paleoclimatic conditions (Pascual and Odreman Rivas, 1971; Brea et al., 2009); similar climatic conditions were inferred from lateritized tephric paleosols preserved in the basin boundary (Krause et al., 2010). Clay mineral composition studies and analysis of paleobotanical assemblages of the Río Chico Formation support the interpretation of a Paleocene–Eocene climatic shift from temperate warm, humid and highly seasonal precipitation conditions to subtropical–tropical, with more continuous year-round rainfall conditions throughout the unit (Raigemborn et al., 2009). The Río Chico Formation was divided in the Las Violetas Member and Visser Member by Andreis et al. (1975), each representing meandering fluvial subcycles draining to the southeast. The basal cycle represents high-energy conditions and has a predominance of volcanic/pyroclastic components in coarse-grained channel-fills; the upper cycle shows reduced sediment transport capacity and abundance of medium-grained sandstones (Andreis et al., 1975). Legarreta and Uliana (1994) defined the Río Chico Group, made up

Fig. 1. (A) Radar image with the location of main Mesozoic basins in the southern end of South America. I — Cañadón Asfalto basin and II — Austral basin and III Malvinas basin. Location of B. (B) Main structural regions of the Golfo San Jorge Basin (modified from Figari et al., 1999). Key: 1) Oriental Region: 1a) Northern Flank, 1b): Center of Basin, 1c): Southern Flank, 2) San Bernardo Fold Belt and 3) Western Flank. (C) North–South seismic section (A–A′) showing the asymmetric basin profile and distribution of principal units. Key: a) Paleozoic basement, b) Marifil Complex (Jurassic), c) Las Heras Group (Neocomian), d) Chubut Group (Cretaceous) and e) Cenozoic. (D) Satellite image of the studied area in the Northern Flank of the Golfo San Jorge basin. Dashed line (a) indicates the southernmost outcrops of Río Chico Formation. Subsurface data used corresponds to oilfield areas.

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Fig. 2. (A) Simplified geologic map of the Golfo San Jorge basin. Contour lines represent an isopach map of the Uppermost Creatceous continental unit (El Trébol Formation), modified from Fitzgerald et al. (1990). Note that the study area exceeds the Cretaceous basin margin. (B) Main lithostratigraphic units in the Golfo San Jorge basin. Cretaceous deposits comprise about 7 km of continental sedimentary rocks. The Cenozoic sedimentary record shows an alternation of transgressions and regressions in a passive-margin context, where the Río Chico Formation is the first continental cycle. Modified from Figari et al. (1999).

of three formations: Peñas Coloradas Formation (moderate to highsinuosity fluvial channels), Las Flores Formation (fluvial deposits with alluvial and lacustrine claystones) and Koluel Kaike Formation (loessic deposits with pedogenic condensation, previously named “Argiles Fissilaris” by Ameghino, 1906 or “Tobas de Koluél Kaike” by Feruglio, 1938), the latter only preserved at basin margins. Recently,

a new stratigraphic redefinition divided the Río Chico Group in four formations (Raigemborn et al., 2010): Las Violetas Formation (low-sinuosity fluvial channels), Peñas Coloradas Formation (moderate to high-sinuosity fluvial channels), Las Flores Formation (moderate to high-sinuosity fluvial channels and shallow lakes) and Koluel Kaike Formation (floodplain deposits, shallow water bodies and loess).

Table 1 Description and interpretation of lithofacies, Río Chico Formation. Lithofacies

Description

Interpretation

LF1: massive conglomerates

Poorly sorted, massive, not imbricate, clast supported, medium to fine gravel clasts. Volcanic and intraformational clasts are moderately rounded (Fig. 3A). Base commonly erosional. Thickness less than 1 m. Moderately-well sorted, fine gravel clasts, with fining-upward grainsize. Volcanic and intraformational clasts are moderately rounded. Individual cross-set are 0.2–0.6 m thick (Fig. 3B). Amalgamated bodies up to 2 m thick. Unimodal palaeocurrents with low variability. Base commonly erosional. Quartz and lithic components dominant, with fining-upward trend. Individual cross-sets are 0.2–0.7 m thick (Fig. 3C). Amalgamated sandbodies up to 6 m thick. Oriented wood fragments. Unimodal palaeocurrents with low variability. Quartz and lithic components, weak grading. Tangential, large-scale inclined surfaces, individual cross-set are 0.8–3 m thick (Fig. 3D). Locally grading upward to LF3. Unimodal palaeocurrents in macro and mesoforms, with low variability. Low-angle, large-scale inclined surfaces with associated mesoforms. Thickness up to 3 m (Fig. 3E). Vertical grading poorly defined. Individual cross-sets are 0.15–0.4 m thick. High variability of palaeocurrent data (N100°). Mesoforms display palaeotransport directions subparallel to the strike of the accretion surface. Tabular strata, internally ungraded, occasionally containing pebble or gravel clasts, with planar or low-angle cross-lamination (Fig. 3F). Thickness of 0.4–1.5 m, erosional base. Locally, undulated surfaces with 0.5–1.2 m of wavelength. Base frequently erosional. Sandbodies are 0.2–0.9 m thick, without tractive structures (Fig. 3G). Commonly preserved in the lowermost position of channel deposits, grading upward to LF3. Thin beds (b0.4 m), well sorted, with ripple cross-stratification. Ripple sets are 0.02–0.04 m thick (Fig. 3H). Preserved on top of LF3, LF4 and LF6 or interbedded with finer-grained lithofacies in strata 0.05–0.3 m thick. Unimodal palaeocurrents. Planar-concave base and convex upward-top (Fig. 3I). Planar and low-angle cross-bedding. Well sorted and poorly graded. Thickness up to 1 m. Commonly overlying channelized deposits. Tabular, red or brown colored, poorly consolidated, massive or with horizontal lamination (Fig. 3J). Planolites and Skolithos traces. Often interbedded with LF8. Commonly in badland-style exposures. Gray or black colors, poorly consolidated, massive mudstones (Fig. 3K), sometimes with horizontal lamination. Lack of bioturbation. Thickness range from 0.5 to 2 m. Tabular geometry, strata are massive or with horizontal lamination (Fig. 3L). Sometimes bioturbated by Planolites and Skolithos. Thickness range from 0.1 to 1.5 m

Deposition from highly concentrated sediment turbulent flow.

LF2: clast supported, trough cross-stratified conglomerates.

LF3: coarse to medium, cross-stratified sandstones.

LF4: coarse to medium, large-scale cross-stratified sandstones. LF5: coarse to fine sandstones with epsilon cross-bedding.

LF6: coarse to medium sandstones with planar or low-angle cross-lamination. LF7: massive, coarse to fine sandstones. LF8: medium to fine sandstones with asymmetrical ripples. LF9: medium to fine sandstones with lobate appearance. LF10: massive or laminated claystones and siltstones. LF11: dark colored, massive or laminated claystones. LF12: tabular, massive or l aminated tuffs.

Downstream migration of 3D dunes.

Downstream migration of 3D dunes.

Downstream migration of braid bars

Lateral migration of “point bars”. Downstream migration of 3D dunes on lateral accretion surfaces. Deposition from upper-regime turbulent flows, under unconfined conditions Sudden accumulation of bedload during short-lived events of high-discharge. Downstream migration of current ripples in lower flow regime conditions. Deposited by loss of confining and deceleration of turbulent flows. Suspension fall-out of sediments in subaerial conditions, and palaeosol development. Suspension fall-out of sediments in anoxic or poorly oxidized conditions. Pyroclastic deposits from distal ash fall-out.

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Fig. 3. Photographs of lithofacies. (A) LF1: massive conglomerates. Scale: 0.1 m. (B) LF2: clast supported, trough cross-stratified conglomerates. Hammer: 0.3 m. (C) LF3: coarse to medium, cross-stratified sandstones. Hammer: 0.3 m. (D) LF4: coarse to medium, large-scale cross-stratified sandstones. Hammer: 0.3 m. (E) LF5: coarse to fine sandstones with epsilon cross-bedding. Jacob stick is 1.5 long. (F) LF6: coarse to medium sandstones with planar or low-angle cross-lamination. Coin is 2 cm in diameter. (G) LF7: massive, coarse to fine sandstones. Coin is 2 cm in diameter. (H) LF8: medium to fine sandstones with asymmetrical ripples. Coin is 2 cm in diameter. (I) LF9: medium to fine sandstones with lobate appearance. Motorbike is 1.1 m in height. (J) LF10: massive or laminated claystones and siltstones. Pen is 0.15 m. (K) LF11: massive or laminated black claystones. Coin is 2 cm in diameter. (L) LF12: tabular, massive or laminated tuffs. Hammer: 0.3 m.

2.3. Paleocene tectonics The Eastern Sector of the Golfo San Jorge basin is characterized by the development of W–E to NW–SE striking normal faults (Lombard and Ferello, 1965) which show intermittent activity from the Jurassic to Lower Miocene (Figari et al., 1999; Giacosa et al., 2004, 2006). The major normal faults have vertical throws up to 50 m in the uppermost levels of the Río Chico Formation (Ferello, 1965). A pulse of tectonic reactivation of normal faults affected the Paleocene

succession (Fossa-Mancini, 1932; Urien et al., 1981; Zambrano, 1981; Fitzgerald et al., 1990; Chelotti, 1997; Chelotti et al., 1999; Giacosa et al., 2006; Foix et al., 2012), including evidence of paleoearthquakes affecting the upper part of the Salamanca Formation and basal strata of the Río Chico Formation (Foix et al., 2006, 2008). The trend of Lower Paleocene synsedimentary normal faults shows variation between different localities with an azimuth average of 157° (Foix et al., 2012). The presence of a basaltic effusion pulse between 64 and 62 Ma (Ferello, 1969; Marshall et al., 1981) constitutes

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Fig. 4. Interpreted lithofacies associations in Río Chico Formation, Northern Flank of the Golfo San Jorge basin. Key: LA1) active fluvial channels: LA1a) braided channels, LA1b) high-sinuosity channels and LA1c) low-sinuosity channels. LA2) Sheet-flood deposits. LA3) Proximal floodplain. LA4) Distal floodplain. LA5) Abandoned channel deposits.

indirect evidence of this extensional reactivation episode in the basin (Chelotti, 1997). 3. Materials and methods This study comprised the description and interpretation of four main stratigraphic log sections (Ea. Las Violetas, Ea. Chapital, Puerto Visser and Punta Peligro Norte localities, see Fig. 1) drawn from the outcrops and data derived from electric logs of oil wells from the subsurface of the basin. Paleocurrent data from channels and dimensions of sandbodies were also obtained from several complementary study localities (El Médano, Bosque Petrificado, Valle de la Luna and Bajo Palangana, see Fig. 1). The paleoenvironmental reconstruction was based on description and interpretation of lithofacies and lithofacies associations. The stratigraphic architecture analysis included changes in fluvial styles, width to thickness ratio (hereafter W/Th) of fluvial channels, channel/floodplain ratio and a large number of measurements of paleoflow directions. The apparent dimensions of sandbodies were measured using GPS data points at the boundaries of the channel, correcting the true channel width for each sandbody from the mean paleocurrent data. Paleocurrent directions were obtained from unidirectional sedimentary structures, such as cross-stratification. Electric well-logs (spontaneous potential and induction long depth curves) allowed estimations of the channel/floodplain ratio and channel thickness in the subsurface of the study area to be made (Fig. 1). The external shape of the fluvial sandbodies was described following the classification of Gibling (2006): broad sheets (W/Th N 100), narrow sheets (W/Th N 15), broad ribbons (W/Th N 5) and narrow ribbons (W/Th b 5). The sinuosity of channels (S) was estimated from paleocurrent measurements using the method of Ghosh (2000). 4. Results 4.1. Lithofacies and lithofacies associations Twelve lithofacies were recognized in the Río Chico Formation from field observations, summarized in Table 1 and illustrated in Fig. 3. A total of five lithofacies associations was defined from sedimentary structures, geometries of sedimentary bodies, paleocurrent data and vertical/lateral arrangement of lithofacies: (1) active fluvial channels, with three sub-types: braided, meandering and low-sinuosity, (2) sheet-flood deposits, (3) proximal floodplain (natural levee and

crevasse-splay deposits), (4) distal floodplain and (5) abandoned channel-fill deposits (Fig. 4). 4.1.1. Lithofacies association 1 (LA1) — active fluvial channels Three main sub-types of active fluvial channels were interpreted in the study area: LA1a) braided channels, LA1b) meandering channels and LA1c) low-sinuosity channels. 4.1.1.1. LA1a) Braided channels 4.1.1.1.1. Description. Tabular, sandy-gravelly bodies up to 6 m in thickness (Fig. 5A), with erosive base and well-defined fining-upward trend. A poorly organized, coarse-grained basal lag (lithofacies LF1) is common in lower reaches. The association is mainly integrated by lithofacies LF2 and LF3, lithofacies LF4 is common (Table 1). Most of the channels represent amalgamation of single-event channel-fills, forming multi-storey or multi-lateral sandbodies that show a vertical decrease in thickness and dominance of cut-and-fill structures (Fig. 5B). Convex upward, large-scale inclined surfaces display smallscale bedforms that migrated downstream. Sandbodies are broad sheets with W/Th ratios from 300 to 850, show low paleocurrent variability (R N 0.95) and estimated sinuosity values (S) lower than 1.3. 4.1.1.1.2. Interpretation. Convex upward, inclined surfaces with downstream migration of 3D macroforms are interpreted as braid bars that separate coeval and laterally adjacent channel-fill deposits. Tabular sandbodies lacking evidence of lateral migration and with low paleocurrent variability are considered to be braided channels (Bridge, 1985; Bristow and Best, 1993; Robinson and McCabe, 1998; Gibling, 2006; Allen and Fielding, 2007). The fining-upward trend with decreasing size of cross-stratified sets represents a gradual reduction in channel depth and/or flow velocity. 4.1.1.2. LA1b) Meandering channels 4.1.1.2.1. Description. Sandbodies are integrated by lithofacies LF3 at the base and lithofacies LF5, lithofacies LF1 or LF2 are rarely found (Table 1). The sandbodies mostly comprise fining-upward, large-scale cross-strata up to 4 m thick, where accretion surfaces dip 10°–15° and show superimposed 3D dunes migrating at a high-angle to the larger-scale basal surface (Fig. 5C, D). The upper part of sandbodies is dominated by medium-to coarse-grained, cross-stratified sandstones (LF3). Channelized bodies are narrow sheets with W/Th b 50, display high paleocurrent variability (R b 0.65) and estimated sinuosity values from 1.5 to 5. 4.1.1.2.2. Interpretation. The presence of inferred epsilon crossstratification (Allen, 1963) in channelized deposits represents the lateral migration of point-bars, supported from paleoflow data of

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Fig. 5. Typical features of interpreted lithofacies associations. (A) Sheet-like braided channels (LA1a) in basal strata of Río Chico Formation, Ea. Las Violetas locality. (B) Well defined fining upward trend in braided channels, Ea. Las Violetas. (C) High-sinuosity channel-fill composed of thalweg and point-bar deposits (LA1c), Puerto Visser locality. Sinuosity: 1.72. (D) Detail of Fig. 5C. displaying a migration of 3D dunes on large-scale, epsilon cross-strata. Hammer: 0.3 m. (E) Low-sinuosity channels with rapid variation in thickness of sandbodies (LA1c), northeastward Punta Peligro Norte. Hammer: 0.3 m. (F) Sheet-like sandbodies with upper-flow regime structures, interpreted as sheet-flood deposits (LA2), Ea. Las Violetas. (G) Rhythmic alternation of brown-to-reddish mudstones and fine sandstones with asymmetrical ripples (heterolithic stratification) in the proximal floodplain (LA3), northward Punta Peligro Norte. Hammer: 0.3 m. (H) Detail of heterolithic stratification and well-preserved current ripples in the proximal floodplain (LA3), northeastward of Punta Peligro Norte. (I) Distal floodplain lithofacies association between channelized deposits (LA4), Bajo Palangana. Hammer: 0.3 m. (J) Preservation of a top-bar sequence in abandoned channel deposits (LA5), Bosque Petrificado northward of Punta Peligro Norte. Hammer: 0.3 m.

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Fig. 6. Correlation of stratigraphic sections of the Río Chico Formation, Northern Flank of the basin. The sketch shows southward changes in total thickness, variable preservation of floodplain deposits (channel/floodplain ratio) and lateral variations of interpreted lithofacies associations (LA).

dunes preserved on larger-scale surfaces and grain size variations (Fig. 7E). These sedimentological characteristics suggested the presence of meandering channels. Massive or cross-bedded strata deposited in the lowermost portion of the channel are interpreted as thalweg-fill units (sensu McLaurin and Steel, 2007). 4.1.1.3. LA1c) Low-sinuosity fluvial channels 4.1.1.3.1. Description. Sandbodies are integrated by lithofacies LF7 at the base and lithofacies LF3 (Table 1). Lithofacies LF4 is common and LF8 is rarely found at the top (Fig. 5F). Channelized deposits are 2–12 m thick with erosional, irregular (steps) basal surfaces. Mudstone intraclasts up to 0.8 m in diameter can be found along the channel-margins. Channel infills display a poorly-defined vertical

trend in grainsize, although commonly they show an upward decrease in thickness of the involved lithofacies. Large-scale cross-strata are up to 2.5 m thick. Most of the channels are narrow sheets with W/Th b 25, they show low paleocurrent variability (R N 0.95) and estimated sinuosity values less than 1.5. Sometimes, channel infills included silicified tree trunks up to 6 m long and 0.6 m in diameter as bedload. 4.1.1.3.2. Interpretation. Vertical gradation of lithofacies represents an upward decrease of water-discharge and channel depth. Large silicified tree trunks incorporated in the channel-fill suggest vegetated channel-margins and/or source area. Parallelism between macro/meso/microforms' paleocurrent data and low variability of paleocurrent measurements suggest a low-sinuosity flow pattern (Bridge et al., 2000). The lack of lateral migration structures implies

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that the channels were laterally stable (Bridge, 2003), probably due to the presence of stabilizing vegetation and the cohesive nature of the floodplain. 4.1.1.4. Lithofacies association 2 (LA2) — sheet-flood deposits 4.1.1.4.1. Description. Sheet-like sandbodies are 0.5–2 m thick and are integrated by interbedded lithofacies LF6 and lithofacies LF7 (Fig. 5G). These bodies have planar or erosional basal surfaces, incorporating intraclasts from underlying strata, but lacking major internal erosional surfaces. The dominant sedimentary structure is horizontal lamination. Lower-regime structures are not preserved throughout the packages, and the deposits lack vertical trends in grain size. Isolated pebble-conglomerates contain volcanic or tuffaceous clasts up to 1 cm in diameter. 4.1.1.4.2. Interpretation. The lack of lower-regime flow sedimentary structures and vertical grain size variation, absence of internal erosional surfaces and extensive sheet-like geometry, suggest unconfined, turbulent flows and the occurrence of subaerial, mono-episodic highdischarge events (Tooth, 1999; Blair, 2000; Fisher et al., 2007). Similar sheet-flood deposits have been interpreted as sandy accumulations in wide alluvial-plains during unconfined overbank flooding of major fluvial systems (Kraus, 1996; Makaske, 2001; Therrien, 2006; Fisher et al., 2007), in many cases corresponding to short-lived braided rivers (Bell and Suárez, 1995). 4.1.1.5. Lithofacies association 3 (LA3) — proximal floodplain. Two main sub-types of proximal floodplain deposits were interpreted in the study area: LA3a) Crevasse-splay deposits and LA3b) natural levees. 4.1.1.6. LA3a) Crevasse-splay deposits 4.1.1.6.1. Description. Crevasse-splay deposits are mostly constituted by sandy lithofacies (lithofacies LF8 and LF9), but they can contain thin mudstone strata (lithofacies F10). Lens-shaped or lobate sandbodies are up to 1.4 m in thickness and 20 m wide, generally these bodies are encased into fine-grained lithofacies. Occasionally, this lithofacies association is composed of a rhythmic alternation of sheet-like, fine-sandstone bodies containing asymmetrical ripples and brownto-reddish mudstones that define heterolithic stratification (Fig. 5H, I). 4.1.1.6.2. Interpretation. Lobate sandbodies are interpreted as crevasse-splays, deposited by flow expansion and loss of flow competence as discharge leaves the confines of the channel (Miall, 1996). On the other hand, rhythmic alternation of fine-sandstones and mudstones represents the repetition of traction/suspension fall-out processes during the input of tractional sedimentation in low-energy settings of the floodplain. The lobate sandbodies and the heterolithic sheets likely reflect proximal to distal gradation in crevasse-splay deposition (Smith et al., 1989; Mjos et al., 1993; Ghosh et al., 2006).

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tabular, tuff strata (lithofacies LF12). Packages are poorly consolidated, internally massive or thinly laminated. Claystone and siltstone rocks commonly develop in badland-style exposures. Mudstone strata range from 0.1 to 10 m in thickness. Strata display variable burrow activity, with common occurrence of Skolithos and Planolites isp. 4.1.1.8.2. Interpretation. Suspension fall-out of sediment from episodic overbank floods. Reddish coloration of mudstones suggests well-drained, oxidizing conditions of the floodplain (Friend, 1966; Turner, 1980; Retallack, 1997), whereas gray siltstones are produced in poorly drained, commonly waterlogged, distal floodplain conditions (Jo, 2003). Black mudstone strata would indicate standing, shallow-water bodies with organic matter preservation. Tuffaceous strata are interpreted as distal pyroclastic deposits from volcanic ash fall-out. Bioturbated strata are interpreted as omission surfaces and/or incipient paleosols developed during minimum sedimentation intervals in distal settings from active channels. 4.1.1.9. Lithofacies association 5 (LA5) — abandoned channels 4.1.1.9.1. Description. These channel-like sedimentary bodies are filled by a thin, basal massive sandstones layer up to 0.3 m thick (lithofacies LF7) covered by packages up to 2 m thick of mudstones (lithofacies LF10), sometimes infilling well-defined incisions. In several cases an abrupt contact between coarse-grained, convex upward, largescale cross-stratified sandstones (lithofacies LF4) and fine-grained mudstones (lithofacies LF10) were found (Fig. 5K). The association is mainly interbedded with channel-fill deposits assigned to meandering sandbodies (LA1b). 4.1.1.9.2. Interpretation. Mudrock-dominated ribbon-shaped channel bodies have been interpreted as abandoned channel deposits (Stewart, 1983; Muñoz et al., 1992; Kraus and Davies-Vollum, 2004). Preservation of top-bar sequences represents a sudden change from tractional sedimentation to suspension fall-out within channel-fills. Mudstones arrive in the channel depression during overbank floods, forming shallow water-bodies or oxbow lakes. 4.2. Fluvial styles and stratigraphic architecture Near the northern basin-margin (e.g., Estancia Las Violetas and Estancia Chapital sections), the Río Chico Formation is composed of a 50–60 m thick, sandstone-rich succession (up to 75%) dominated by clasts of vesiculated, basic volcanic rocks (Fig. 6). Sandbodies can

4.1.1.7. LA3b) Natural levees 4.1.1.7.1. Description. Sandbodies are up to 1 m thick, composed of a stacking of few decimeters thick, massive or rippled cross-stratified fine sandstones. The succession is mostly composed of sandy lithofacies (lithofacies LF7, LF8 and LF9). These wedge-shaped sandstone units are laterally transitional to channelized deposits forming “wings”. They are coarsening-upward, thinning away from the channel-margin. 4.1.1.7.2. Interpretation. Wedge-shaped sandbodies (“wings” sensu Friend, 1983) connected with major channels have been often interpreted as natural levee deposits (Allen et al., 1983; Kraus, 1996; Miall, 1996; Jo, 2003). Deposition of sands during floods near active channels produces the elevation of channel-margins, developing a natural barrier that confine the stream during low-stage flows. 4.1.1.8. Lithofacies association 4 (LA4) — distal floodplain 4.1.1.8.1. Description. This lithofacies association is composed of reddish-to-brown, pale-gray or black colored claystones (Fig. 5J), siltstones, tuffaceous siltstones (lithofacies LF10 and LF11), and rarely

Fig. 7. Paleodrainage directions in the study area. Key: (1) Ea. Las Violetas, (2) Ea. Chapital, (3) Puerto Visser, (4) El Médano, (5) Bosque Petrificado, (6) Valle de la Luna and (7) Bajo Palangana. The main paleocurrent vector shows a SE direction (137°) for the fluvial system. Number in circle is the number of observations.

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Fig. 8. Interpreted photomosaic of a NE–SW coastal cliff, near to Punta Peligro locality. Channel-fill bodies of various dimensions are disposed in a restricted stratigraphic interval. A) The channel (1) shows a lateral transition to overbank deposits forming “wings”. B–C–D) Two low-sinuosity, multistorey channels (channels 2 and 4) are disposed about the same stratigraphic level. Note the tuffaceous strata as a useful local stratigraphic guide.

be either sheet-like (W/Th = 300–850), sandy-gravelly braided channels (LA1a) or sheet-like, single-event deposits with upper-flow regime sedimentary structures interpreted as sheet-flood deposits (LA2). Both deposits were accumulated during high-discharge flood events in a drainage network that flowed to E–SE (Figs. 6, 7). Major changes in the stratigraphic architecture of the Río Chico Formation were recognized in the Puerto Visser locality, 27 km southsouthwest of Estancia Chapital (Fig. 6). There, the stratigraphic record of the unit consists of a 90 m thick succession with two distinctive fluvial packages. The lower package contains a fine-grained succession (~67% in vertical logs) of mudstones assigned to LA4 with isolated, high-sinuosity fluvial channels of W/Th b 30 (LA1b). The upper package is dominated by sheet-like (W/Th 300–400) braided sandbodies (LA1a) (Fig. 6). Southward of Puerto Visser locality, nearly 95% of the fluvial sandbodies are low-sinuosity channels (LA1c), with minor occurrence of meandering channels (b 10%). Sandbodies are mainly multi-storey, comprising narrow-sheets and broad ribbons, with paleocurrents

directed to the SE and SSW (Fig. 7). Channelized deposits represent about ~25% of the measured stratigraphic sections, and proximal floodplain associations (LA3) are common. In addition, the unit increases up to 100 m in thickness. In Punta Peligro study locality, multi-storey, low-sinuosity channels up to 12 m in thickness are located in the same stratigraphic level as minor-scale channelized sandbodies (Fig. 8). A 25/75 channel/floodplain ratio was obtained southward of the last exposure from well-log data, at the southernmost part of the study area (Fig. 9). Near Comodoro Rivadavia locality, sandbodies reach a maximum thickness of 18 m at the El Trébol oilfield. The main north–south stratigraphic changes observed in the Río Chico Formation (total thickness, channel/floodplain ratio and width/thickness ratio) are summarized in Fig. 10. 5. Discussion Lateral/vertical changes in fluvial architecture of the Río Chico Formation were recognized by variations in fluvial styles, geometry

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Fig. 9. (A) Location map of unexposed areas examined using subsurface information, Northern Flank of the Golfo San Jorge basin, near to Comodoro Rivadavia city. (B) Location of the well-log correlation (A–A′) in the El Trebol oilfield. (C) West-East correlation of wellbores in El Trebol oilfield, using SP (left) and ILD (right) logs. Lower Paleocene stratigraphic record is represented by tabular marine strata of the Salamanca Formation, “Banco Negro Inferior Section” (BNI) and “Banco Verde Section” (BV). The Río Chico Formation is ~180 m thick in the El Trebol oilfield. Calculated channel/floodplain ratios are expressed in relative percentage at the bottom of the figure. Amalgamated, fining-upward well-log electric responses probably represent multistorey channelized deposits.

of fluvial channels, regional paleoflow directions, channel/floodplain ratios, and preserved thickness. This evidence allow us to discuss the relatively importance of sedimentary processes and forcing factors on the fluvial architecture. 5.1. Avulsion Avulsion is the abandonment of part or the whole of a channel belt by a stream in favor of a new course in a lower position (Allen, 1965; Bridge and Leeder, 1979). Recognition of avulsion in the ancient record is based on indirect evidence, such as heterolithic successions generated by successive water and sediment influx from a main channel in wide crevasse splays (Kraus and Wells, 1999), and preservation of bar-top sequences in upper reach of channel-fills (Bristow, 1996; McLaurin and Steel, 2007). Therefore, rhythmic alternation of mudstones and fine sandstones that contain current ripples (heterolithic stratification) in proximal floodplain deposits (LA3) (Fig. 5H, I) and occasional preservation of top-bar sequences (Fig. 5K) in abandoned channels (LA5) could be considered as evidence of avulsion in the Río Chico Formation. Such features have only been recognized southward of the Puerto Visser study location, probably indicating a basinward increase in avulsion frequency. The importance of this asymmetrical record of avulsion deposits and floodplain deposits in coeval fluvial successions and their influence in the geometry/style of fluvial channels is discussed in the next section.

5.2. Aggradation rates and fluvial styles Aggradation of fluvial systems can be estimated in different ways. Bridge and Leeder (1979) and Bridge and Mackey (1993) suggested a positive correlation between aggradation rate and preservation of floodplain deposits. In addition, low width/depth ratio of channels and ribbon-shaped sandbodies both suggest that vertical channel aggradation was dominant (Roberts, 2007). Sandbodies with W/Th ratio lower than 125 have been related to high aggradation rates, and in contrast those successions containing sandbodies with W/Th ratio up to 500 have been interpreted as formed in low aggradation rate conditions (Gouw and Autin, 2008). Thus, the aggradation rate of the Río Chico Formation can be comparatively estimated using these criteria. Northward of the Puerto Visser locality, the predominance of extensive multi-lateral/multi-storey braided channels (W/Th N 350), sheetflow deposits and scarce preservation of floodplain deposits (25–30%) could indicate relatively low aggradation rate conditions. Southwards from this locality, the sandbodies have W/Th b 50 and the unit preserves a high proportion of floodplain deposits (65–85%), suggesting relatively high aggradation rates. The fluvial succession preserved in the Puerto Visser locality shows two main packages with a distinctive appearance (Fig. 6): the lower package is composed of ribbon-shaped, low sinuosity fluvial channels with W/Th b 30 and abundance of floodplain fines, while the upper package mainly consists of sheet-like, braided channels with W/Th ratio from 300 to 400. In such

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a scenario, the Puerto Visser locality would represent a break-point in the stratigraphic architecture of the unit, separating two settings of variable aggradation rates. The fluvial and architectural characteristics attributed to relatively high aggradation rate conditions (overbank deposits N 60%, avulsion deposits, frequent crevasse splays and coexistence of channel-fill bodies of varying dimensions in a restricted stratigraphic interval) are not incompatible with anastomosing fluvial systems, because they represent a hierarchically higher order of channel patterns (Schumm, 1968). The north-to-south increase in the preservation of cohesive floodplain deposits suggests that channels were laterally stabilized by cohesive mud banks (Ghosh et al., 2006; Gibling, 2006), favoring the occurrence of multi-storey, low-sinuosity fluvial channels with relatively low W/Th ratio encased in mudstones. 5.3. Forcing factors To discriminate the relative influence of forcing factors on the fluvial architecture of the Río Chico Formation we appeal to some valid simplifications. First, the sediment supply is considered only a function of climate and tectonic effects (Ethridge et al., 1998); secondly, equivalent layer-cake stratigraphic intervals that extend over a few tens of kilometers are considered as roughly coeval. Several sedimentary basins of Argentina's Atlantic passive margin contain an Upper Paleocene, widely distributed continental sedimentary record deposited during a regional regressive stage (Malumián, 1999). The Río Chico Formation constitutes the fluvial record of this low sea-level stage in the Golfo San Jorge basin. The Upper Paleocene climatic conditions in the basin determining the development of mixed temperate-subtropical forest to mixed subtropical-tropical, humid forest (Raigemborn et al., 2009), probably generated little variation in the river discharge and therefore an increase in channel stability. For this reason, lateral changes in the macro-architecture of the unit cannot be explained by climatic fluctuations or sea-level changes, because coeval fluvial successions were developed under similar conditions. Thus, the positive correlation between fluvial styles/floodplain preservation and accommodation rates in ancient continental successions (e.g., Cant, 1998; Robinson and McCabe, 1998; Sønderholm and Tirsgaard, 1998; Arche and López-Gómez, 1999; Catuneanu and Eriksson, 1999; Catuneanu et al., 2006, 2009; Leckie, 2006) allows us to infer major spatial variations in accommodation rate during the Upper Paleocene Río Chico Formation. In addition, these stratigraphic changes are accompanied by an increase of the total thickness from 50 to 250 m over a distance of around 100 km. The break-point in the fluvial architecture of the Río Chico Formation was recognized close to Puerto Visser locality (Figs. 6, 11). This sector of the basin coincides approximately with a main Paleozoic threshold defined from well-log correlation (Lesta, 1968) and also with the Cretaceous basin margin inferred from isopach maps (Lesta, 1968; Fitzgerald et al., 1990). The Paleocene reactivation of Cretaceous normal faults (Fossa-Mancini, 1932; Urien et al., 1981; Zambrano, 1981; Chelotti, 1997; Chelotti et al., 1999; Foix et al., 2006, 2008; Giacosa et al., 2006) therefore generated differential subsidence from this pre-existing tectonic lineament, delimiting two different subsidence settings across the boundary (Fig. 11). This interpretation is supported by changes in thickness of the unit, fluvial styles, avulsion frequency, variations in W/Th ratio and channel/floodplain ratio across the boundary. The main paleoflow direction of fluvial sandbodies in the study area (137°) (Fig. 7) is roughly parallel with the NW–SE (157°) orientation of Lower Paleocene synsedimenatry normal faults described by Foix et al. (2012). The observed change in paleodrainage direction from E–ESE to SSE–SSW southward of Puerto Visser can be considered as a response of the fluvial system to local or regional change in geomorphic conditions. The abundance of coarse-grained, basaltic

clasts in channel-fill deposits at the northern part of the study area suggests derivation from a local source area, probably related to the reworking of the Lower Paleocene Pre-Salamanquense (sensu Ferello, 1969) basaltic effusions that crop out westward the study area. 6. Conclusions • The Upper Paleocene Río Chico Formation (Oriental Region of the Golfo San Jorge basin) is a 50–250 m thick fluvial succession composed of braided, meandering and low-sinuosity channels with variable preservation of floodplain environments across the basin. • The northern part of the study area is characterized by braided channels with 350–850 W/Th ratio, tabular sheet-flood deposits and a channel/floodplain ratio of about ~ 75/25. The fluvial succession is ~ 50 m in thickness. • The southern part of the study area is characterized by low-sinuosity and meandering channels with W/Th ratio b 50, channel/floodplain ratio of about ~25/75 and common evidence of avulsion (anastomosing fluvial system?). The fluvial succession is up to 250 m in thickness. • The main Upper Paleocene paleodrainage vector (137°) is consistent with the regional paleoslope of the Northern Flank of the basin and is in line with a regional base level located south-eastward the study area (Atlantic Sea). Also, the main paleocurrent direction is nearly parallel to the Lower Paleocene synsedimenatry normal faults (tectonic control?). • The changes in the fluvial architecture of the Río Chico Formation permits interpretation of two main sedimentary scenarios during the Late Paleocene in the Northern Flank of the Golfo San Jorge: 1) a northern, low-accommodation setting, and 2) a southern, basinward high-accommodation setting. • The break-point in the macro-architecture of the Río Chico Formation coincides with the location of a pre-Cenozoic tectonic lineament. The Paleocene reactivation of previous normal faults controlled the aggradation/accommodation/subsidence rate of the fluvial system and the downstream variation of fluvial styles across this boundary.

Fig. 10. Changes in the architectural parameters across the study area. (A) Increasing of total thickness with decreasing of channel/floodplain ratio. Key: Th: thickness of the unit; Ch/Fl.: channel/floodplain ratios. (B) Southward decreasing of the width/thickness ratio of fluvial channels.

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Fig. 11. (A) Schematic stratigraphic architecture of the Río Chico Formation in the Northern Flank of the basin, built-up from outcrop and subsurface data (no scale is implied); the pre-Paleocene tectonic configuration is based on Lombard and Ferello (1965) and Fitzgerald et al. (1990). Note the onlap of Paleocene units on the Jurassic basement in the northern localities. The break-point in the alluvial architecture is nearly coincident with the Cretaceous basin-margin. (B) North–South variations in the stratigraphic parameters and interpreted low and high-accommodation settings.

Acknowledgments This research was supported by a Ph.D. grant to the first author from the CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas). We are grateful to YPF S.A. for access to well-log data. The Departamento de Geología of the UNPSJB is acknowledged for logistic support. The authors greatly appreciate corrections made by Patrick G. Eriksson, an anonymous reviewer and Jasper Knight (Editor in Chief, Sedimentary Geology) which improved the quality of the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.sedgeo.2013.07. 001. These data include Google maps of the most important areas described in this article. References Allen, J.R.L., 1963. The classification of cross-stratified units, with notes on their origin. Sedimentology 2, 93–114.

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