Estuary deposits in the Río Baguales Formation (Chattian-Aquitanean), Magallanes Province, Chile

Very little work has been done on the stratigraphy and sedimentology of the upper Oligocene-Lower Miocene Río Baguales Formation in the Magallanes Province of southern Chile since its original definition in 1957. A detailed stratigraphic section of the upper part of the formation exposed west of the Baguales River is presented, with an interpretation of depositional environments. This indicates a prograding shoreline in which estuary mouth, middle estuary (lagoon) and bay head delta facies are represented. Large-scale delta slope foresets indicate progradation towards the north and northwest, whereas southeast-directed cross-beds on the delta front probably suggest wave action. The general sequence reflects a gradual sea-level fall largely counteracted by tectonic subsidence, which provided the necessary accommodation space for thick tidal flat and subtidal deposits to accumulate. A longer period of transgression towards the top of the succession may be related to a rise in sea-level or accelerated tectonic subsidence, whereas a major regression commencing at around 23.3 Ma probably coincides with the opening of the Drake Passage and the growth of the East Antarctic Ice Sheet.

On tite 1:1,000,000 Geological Map of Chile (SERNAGEOMIN, 1982), tite deposits are simply indicared as sulrlittoral siltstones and shales correlared witlt tite Eocene-Miocene Babía Inútil Group, altltough !he succession as a whole seems to correlaú: better witlt Oligocene-Miocene marine sandstones and coquinas eas! of Puerto Natales. The latter are correlared on tite map witlt tite Guadal Formation, described by Frassinetti and Covacevich (1999) in tite Lake General Carrera district about 440 km to !he north. According to!bis nomenclature, !he Guadal Formation is overlain by tite Santa Cruz Formation (De la Cruz and Suárez, 2006), a fluvial succession witlt mammal fossils tbat seems to correspond to tite Las Flores Fonnation ofHoffsretter el al. (1957).
Consideringtlte large distance between!he type area oftlte GuadslFormation and SienaBagoales, as well as tite facttbat differen!names are in use on differen! sides oftlte border, we coDSider tite original defittition oftlte succession in !bis area as tite Río Baguales Formation to be a better alremative, al leas! uotil correlation can be proved by direct mapping or dating.
The present field campaign has yielded abundant teeth that allow at least two different ecological and chronostratigraphic assemblages to be recognized. The first is represented by several taxa previously known exclusively from the Maastrichtian of central Chile, including one cosmopolitan family (Schlerorhynchidae) that remains unreported after the Cretaceous/Paleogene boundary. The studied assemblage includes the species Ischyrhiza chilensis Philippi (Rajiformes, Schlerorhynchidae), a very frequent taxon in Maastrichtian beds of the Quiriquina Formation (Biró-Bagóczky, 1982) and equivalent units of central Chile (Suárez and Cappetta, 2004), and the genus Centrophoroides (Squaliformes, Squalidae), known in Chile only from these units. A third characteristic taxon is constituted by a still unnamed dasyatid ray (Myliobatiformes; Dasyatidae), previously known exclusively from the Quiriquina Formation (Suárez et al., 2003;Muñoz-Ramírez et al., 2007). The beds hosting this material are located in the western part of Sierra Baguales and stratigraphically underlie the measured section of this study by at least several tens of meters.
The second assemblage is characterized by a rich diversity of sharks, rays and chimeroid fishes, that includes the following identified taxa: Striatolamia macrota Agassiz (Lamniformes, Mitsukurinidae); Carcharocles aff. angustidens (Lamniformes, Otodontidae); Myliobatis sp. (Myliobatiformes, Myliobatidae) and Callorhinchus aff. regulbensis Gurr (Chimaeriformes, Callorhynchidae). Most of the host beds have been lost as a result of erosion higher up on the mountain, but hard material such as elasmobranch teeth, spines and other phosphatic bony elements have nevertheless been preserved in the soil. The scarce in situ material indicates that the host beds are located below conglomerates with bivalve casts possibly correlating with our unit 4 (see below). This assemblage shows affinities with typical Late Eocene faunas of the Tethyan coastal basins.
Plant remains are fairly rare in the Río Baguales Formation, being mostly of the genus Nothofagus (Rodríguez and Oyarzún, 2008). In the overlying Las Flores Formation, we collected 389 samples of which 70% are Nothofagus, including the species N. paleoalessandri, N. subferruginea, N. simplicidens, N. serrulata, N. densinervosa, and N. variabilis. Associated are 19 species of angiosperms.
In general, invertebrate fossils recovered from the Río Baguales Formation confirm its tentative correlation with the Chattian (28.4-23.0 Ma) Guadal and Río Turbio/Centinela Formations, whereas the second assemblage of sharks, rays and chimeroid fishes suggest a maximum Late Eocene age (33.9 Ma) for the basal part of our measured section.

Lithostratigraphy
We subdivide the measured portion of the Río Baguales Formation into nine informal lithostratigraphic units (Fig. 2).

Unit 1: Trough and planar cross-laminated sandstones containing Skolithos ichnofacies
The lower part of unit 1 (sub-unit 1a) is formed by a 15 m thick, coarsening-upward succession of fine-to coarse-grained, cross-laminated sandstones with some plant remains (fragments of tree trunks and leaves), trace fossils and rare shark teeth. The sandstones are light grey where fresh and weather from buff to light and reddish brown.

Unit 4: Lenticular interbeds of pebbly sandstone, sandy, locally fossiliferous conglomerates and mudrock lenses
Overlying unit 3 with a sharp contact, this 13.8 m thick unit is characterized by sandy conglomerates, pebbly sandstones, sandstones with conglomerate and mudrock lenses and locally fossiliferous conglomerates with sandstone and mudrock lenses.
The basal sub-unit (4a) is a lenticular, massive, clast-supported conglomerate with pebbles generally less than 5 mm in diameter, but reaching up to 2 cm. It has a medium-grained sandstone matrix. Lenses of intercalated clay and very fine-grained sandstone are present in its lower portion, and towards the top it grades into massive, medium-grained sandstone with conglomerate lenses up to 20 cm thick. Sub-unit 4a has a maximum thickness of 7.6 m and is overlain with a sharp contact by sub-unit 4b, a horizontally-stratified conglomerate containing oyster fragments (possibly Ostrea torresi, considered to be synonymous to Ostrea orbignyi by Brandmayr, 1946) and other shell debris, as well as thin shale lenses, grading upward into coarsegrained sandstone intercalated with clay lenses. A total thickness of2.4 m was recorded. Sub-unit 4c, 3.8 m thick, also has a sharp basal contact and consists of well-stratified clast-to matrix-supported conglome-rates intercalated with lenticular shale up to a few cm thick, which in tum fine upward into medium-grained sandstone with clay lenses.
3.5. Unit 5: Interbedded siltstone and shale with minor sandstone Similar to unit 2, this unit is largely covered, but shows occasional outcrops offine-to medium-grained sandstone. It has a total thickness of about 17 m.
3.6. Unit 6: Horizontal-planar laminated and cross-Iaminated, bioturbated sandstone This unit, about 7 m thick, is formed byupper flow regime horizontal-planar laminated, medium-grained sandstone with conglomerate lenses, sharply overlain by fine-to medium-grained, trough and planar crosslaminated sandstone with clay and shale lenses. The latter are oxidized and partially bioturbated.
3.7. U nit 7: Lenticular interbeds of pebbly sandstone, sandy, locaUy fossiliferous conglomerates and mudrock lenses Unit 7 is lithologically similar to unit 4, but reaches a total thickness of about 32 m. At the base are about 20 m ofmassive, fossiliferous, clast-supported conglomerates interspersed with less fossiliferous, matrix-supported conglomerates and subordinate fine-to coarse-grained sandstone with rare shell fragments and conglomerate lenses (sub-unít 7a). Contacts between these different lithologies are dominantly gradual. Although the sandstones are generally massive, vague cross-Iamination can be discerned locally. The conglomeratic interval is overlain by a dominantly arenaceous zone about 6 m thick (sub-unít 7b), consisting offine-to very coarse-grained sandstone showing massive bedding, horizontal-planar lamination, as well as trough and planar cross-Iamination. This sub-unít is followed by another fining-upward succession of fossiliferous conglomerates grading upward into very coarse-to medium-grained sandstones (sub-unít 7c). It commences with a matrix-supported conglomera te containing poorly to moderately sorted chert and volcanic pebbles as well as scattered clay pellets, showing medium-scale (-50 cm), planar cross-bedding dipping 28° towards the southeast. Fossil fragments include bivalves, oysters and gastropods. The conglomera te is overlain with a sharp contact by a medium-grained, clast-supported conglomerate containing chert and volcanic clasts at the base, but showing an increase in clay pellets towards the topo This unít is massive to low-angle planar cross-bedded.
In addition to scarce bivalve fragments, slightly sinuous, branching traces about 1 cm in diameter and more than 20 cm long are present, showing an incomplete lining of clay pellets (Fig. 5). In shape the traces resemble Spongeliomorpha nodosa, although the clay pellets are more typical of Ophiomorpha irregu/aire FIG.5. Trace of Spongeliomorpha nodulosa in conglomerates ofunit 7c. Frey et al., 1978. Schlirf (2000 recommended synonymization of Ophiomorpha Lundgren, 1891 with Spongeliomorpha Saporta, 1887, but Bromley and Pedersen (2008) suggested that Ophiomorpha should rather be considered as an ichnosubgenus of Spongeliomorpha. However, because Ophiomorpha irregu/aire has thus far been reported only from the Jurassic and Cretaceous and seems to have wider tunnels (Bromley and Pedersen, 2008), we consider the ichnogenus to be Spongeliomorpha.
3.8. Unit 8: Cross-laminated, very coarse to mediumgrained sandstone with clay pellets and finegrained, bioturbated, dark red sandstone lenses Unít 8 has a gradational contact with the conglomerates of sub-unít 7c and reaches a total thickness of about 13 m. The basal part fines upward into light grey to lmfl: very coarse, trough cross-Iaminated sandstone with small clay pellets and fine-grained, dark red sandstone lenses. The latter contains small (2 mm thick, 2 cm long) y-shapedPo/yk/adichnus traces, as well as calcareous concretions on average 20-30 cm in diameter. This is followed by medium-grained sandstone with numerous small clay pellets, also containing dark red, fine-grained sandstone lenses with Po/ykladichnus traces. At the top is a reddish brown, fine sandstone with scattered chert pebbles, clay pellets and plant fragments.
3.9. Unit 9: Interbedded siltstone and shale with minor sandstone and coarsening-up conglomerate lenses The basal sub-unít 9a, which has a total thickness of 69.5 m, is composed oflargely covered, buff-colored shale with occasional beds or lenses of yellowish to dark brown, fine-grained, massive sandstones containing Polykladichnus traces. This predominantly fine-grained succession is interrupted by sub-unít 9b, a 9 m thick, coarse-grained interval with a sharp basal contact and gradual topo The basal part consists of trough crosslarninated, fine-to very coarse-grained sandstone intercalated with thin (up to 10 cm) conglomerate lenses. The latter contain small, well-rounded pebbles of sandstone and chert. The sandstones grade into coarsening-up conglomerates varying from matrixsupported at the base to clast-supported at the top, where pebbles reach up to 5 cm in diameter. Lenses of medium-to coarse-grained sandstone are present within the conglomerate. The top of the conglomerate grades through very fine-grained sandstone to 6.8 m ofinterbedded siltsrone and shale (sub-unít 9c) up to the base of a tbick sill (50"44'09.2"sn2"27'44.3

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Altbough we consider!bis to represent the top of our section, tite succession possibly continues about 750 m towards tite WNW. Here an a1most 17 m tbick sequence of interbedded shale and siltstone witlt occasional very fine-grained sandsrones containing plan! fossi\s and thin carbon seams and lenses overlies tite sill. Showing sorne typical ebaracteristies of fluvial deposits (e.g .• sandsrones filling ehannels), tltere is a possibility tItat!bis already belongs to the base oftlte Las Flores Fonnation.

Facies description and paleoenvironmental interpretation
The presence of marine fossils in tite Río Baguales Formation, together witlt oyster and plant fragments, suggest a saltto brackish water environment witlt input from a nearby river. However, a delta can probably be ruled out, as tite suceession laeks the typical coarseoing-upward profile of a prograding delta, as well as otlter cbaracteristic features such as levee or marsb deposits. On tite otlter hand, many of tite features typical of an aggradational estuary are represented, whieh include tbiek mud-and siltstone deposits requiriog a fairly proteeted eovironmeot.

Flood ddal deltas
Within sub-unit la, a trace fossil suite of Skolilhos, Arenicoliles and Diplocrilerion belonging 337 to tite Skolithos ichnofacies, indicates tite preseoce of organisms filtering organie material kept in suspension by wave or current action or deposited within tite sedimeots during slack periods. Energy conditions of tite Skolilhos iehnofacies are generally moderate to bigh, witlt alternating periods of erosion and deposition (Buatois el al., 2002). Altltough tbis ichnofacies is typically associated witlt shoreface facies, such an eovironmeot is contradicted by the preseoce of tree tronks, otlter plant fragments, and clay pellets, wbich will rarely be found in tite surf zone along open coastlines. These materials typically eollect on tite upper beach or backshore, from where tltey may be transported by storm waves onto washover fans and flood tida! deltas within tite estuary moutlt.

Barrier spits
Barrier spits are probably represented by sub-unít le, whieh is formed by a medium-to coarse-grained sandstone witlt low-angle planar cross-Iamination and upper flow regime horizontal-planar lamination. These are typical foreshore features (Nichols, 1999).
A single, about 2 m tbiek planar cross-bedded sandstone (Fig. 6) observed at sorne distance from tite messured section bu! evidently within unít 1, might represen! tite tip of a migrating barrier spit from whieh sand avalanched into the tidal inlet channel (Johannessen and Nielsen, 1986). However, this unit was not investigated in any detail.

Middle estuary (lagoon) facies
The middle estuary can be described as that part of the system largely protected from wave action and only occasionally affected by river input. It ineludes the lagoon and surrounding tidal flats landward of the barrier spits. In the Río Baguales Formation, this facies also ineludes bar-fmger gravels that prograded into the lagoon during periods ofhigh sediment input.
In the description of tidal flat facies below, the elassification scheme of Flemming (2000) is used, where mud:flats have more than 95% mud, slightly sandy mudflats between 75 and 95% mudo sandy mudflats between 50 and 75% mud, muddy sandflats between 25 and 50% mudo slightly muddy sandflats between 5 and 25% mud, and sandflats less than 5% mudo

Subtidal zone and slightly sandy tidal mudjlats
This facies occurs in sub-units 2b, 2d, 9a and 9c. It is composed of intercalated dark grey, buff-weathering siltstone and shale forming successions between 30 m and 70 m thick, with individual beds varying between 5 and 10 cm. These deposits are interpreted as muddy subtidal (lagoon) facies and distal (with respect to the river mouth) slightly sandy tidal mudflat facies, although their generally poor exposure preeludes the proper distinction of these two sub-environments. Slightly sandy mudflats generally occur landward of slightly muddy sandflats, for example where wave action wmnows fines along the seaward edge, which are then carried by flood tides towards the supratidal zone (Reinson, 1992). However, within estuary lagooos such facies can also occur in distal areas partially protected from wave action (such as behind barrier spits) and away from the influence of strong inflowing river currents. Under such conditions they may grade into sub-tidal muds deposited in the deeper water of the lagoon.

Slightly muddy tidal sandflats
This mainly arenaceous facies occurs in units 6 and 8, where it forms a transitional zone between slightly sandy mud flats and delta front facies. The sandstones are light grey to light brown and buff-weathering, varying from very coarse-to very fine-grained. They are locally massive, but generally show upper flow regime horizontal-planar lamination, as well as trough and planar cross-Iamination. The sandstones commonly contain clay pellets and clasts, as well as lenses of conglomera te and clay, the latter ofien showing bioturbation. Shell fragments are occasionally present.
The association ofthis facies with both delta front and slightly sandy tidal mudflat facies suggests that it occurred in a more proximal environment than the latter, closer to the delta slope and river mouth (Fig. 7). The presence of trough and planar cross-Iamination indicates fairly strong currents, possibly enhanced during ebb-flow when bedforms such as lunate and straight-crested dunes migrated seaward. Horizontalplanar lamination, on the other hand, could be attributed to wave action on local beaches developing along the edge of the tidal flats. Waves passing through the estuary inlet would have broken directly on the opposite delta slope and adjacent tidal flats, whereas they would have been mitigated in the areas close to the inlet by deeper water within the estuary itself, as well as the orientation ofthe coastline with respect to the direction ofwave propagation.

TuJal creeks
The third rniddle estuary subfacies is formed by dark to reddish brown and dark red (yellowish-weathering), fine-grained, massive sandstones occurring as lenses or beds within the slightly sandy tidal mudflats of unit 9a and the slightly muddy tidal sandflats of unit 8. Distinguishing characteristics ofthese sandstones include the presence of minute, y-shaped Polykladichnus traces, abundant clay pellets, occasional plant fragments, and calcareous concretions.
Polykladichnus is presently formed by polychaetes, e.g., Nepthys and Nereis, on modero tidal flats (http://graduate.eas.ua1berta.caJ1zabcic/web-contentl Ichnology.html) and in the muddy point bar deposits of tida! creeks (pearson and Gingras, 2006). Such an environment is supported by the fining-upward nature of these sandstones, which is typical of point bars in meandering river systems. The scarcity of chert and volcanic c1asts indicates that these creeks didnot extend far into the hinterland, whereas the abundance of clay pellets can be attributed to bank co11apse of partially dried tida! tlat muds during meander undercutting. The fact this subfacies is fmer-grained than the slightly muddy sandflat facies in unit 8 may be because these creeks drained slightly sandy mudflats landward of the latter, or because winnowing of fines by waves would have been abated by the deeper water and seaward current flow within the channels.

Bar-ftngers
Fine-to very coarse-grained, massive to trough cross-laminated, lenticular sandstones with sharp basal contacts, grading upward into matrix-and then c1ast-supported conglomerate with well-rounded chert and volcanic clasts are attributed to this facies, which occurs in units 2c and 9b. The sandstones contain thin conglomerate lenses with similarpebbles, whereas the conglomera tes enc10se thin, medium-to coarse-grained sandstone lenses and fine upward into the siltstoneshale sub-facies. The erosive basal contacts, together with the presence oflithologically different lenses and cross-larnination, indicate shifting torbulent currents, so that these lenses are interpreted as har-finger gravels fonned by distributary mouth bars prograding over the delta slope and into the sub-tida! environment. Such har-finger deposits typica1ly show inverse to normal grading (Fisk et al., 1954;Leeder, 1999). However, that debris flow events may also have contributed to these deposits cannot be excluded. Kleinspehn el al. (1984) and Surlyk (1984) interpreted inverse to norma1ly graded gravels associated with fan-delta sequences as having formed from high-density torbidity currents transitional to liquefied flows. In the &io Bagua1es Formation, such hyperconcentrated flows would probably have exploited existing channels formed by distributary mouth progradation, or may have originated as slump scars on the steep, Gilbert-type delta slope (Postma el al., 1983;Massari, 1984).

Delta front (subaquatic platform)
This facies occurs in uoit 7 and is characterized by lenticular uoits offossiliferous conglomerates with sharp basal contacts grading upward into sandstones. The conglomerates are matrix-to clast-supported with numerous small, very fine-grained sandstone, siltstone and clay lenses showing biotorbation. They contain oyster, gastropod and bivalve fragments as well as occasional traces of Spangeliomorpha nodosa. The presence of oysters suggests a brackish water environment closer to the river outle!.
The conglomerates are generally massive to horizontally stratitied, but also show low-angle planar cross-bedding as well as planar cross-beds dipping 28° towards the southeast. This dip is opposite to the northerly and northwesterly dip of large-scale cross-beds on the delta slope, which either suggests that flood tides strong enough to transport pebbles and form straight-crested megaripples affected this part ofthe delta front (which we consider unlikely in the present selting), or !hat wave aetion caused the landward migration of shore-parallel gravel bars. Wave action on local beaches formed by partially submerged gravel bars may also be responsible for the horizonta!-planar stratification to low-angle planar cross-bedding (Nemec and Steel, 1984).
Spongeliomorpha nodosa has been reported from medium-to very coarse-grained sandstone containing shell debris, deposited in storm and wave-influenced upper shorefaee to foreshore environments in Jurassic deposits in France (Schlirf, 2000), and would thus tit into a delta front environment. This trace is probably made by cruslaceans (Schlirf, 2000) and commonly belongs to the GlossifUngiles ichnofacies, which reflects fmngrounds somewhat resistant to erosion, typically consisting of surfaees formed by estuarine incision, tida! channel migration, coasta! erosion, or shoreface retreat (Pemberton el al., 1992;MaeEachern el al., 1992).
The sandstones are generally pebbly, coarse to medium-grained and contain conglomerate and mudrock lenses. The presence oftrough and planar eross-lamination indicates current action, so lbat they are interpreted as distributary channel and mouth bar deposits on the delta front. Rojas and Le Roux (2005), in their study ofthe underwater morphology and sedimeotology of a Gilbert-type delta in Lake Llanquihue, southem Chile, reported distributary channels tilled with matrix-supported gravel and gravelly sand traversing pebble and cobble bars on the inner delta front. The latter are affected by wave action !hat winnows much ofthe sand fraction, whereas the somewhat deeper distributary channels are more protected and contain finer-grained sediments. The distributary mouth bar and outer delta front are also formed by pebbly sand faeies. The distribution of algae on the Lake L\anquihue delta coincides with the cobble bars of the inner delta front, which also attract gastropods. A similar situation might explain the relative abundance of oyster and other shell fragmeots within the delta front gravel bar facies of the preseot study area, in comparison with their relative scarcity within the distributary channel and mouth bar faeies, as well as in the delta slope facies.

Because it is inconceivable that marine organisms
would be able to cope with strong river flow and gravel transport, their preseoce in lhis setting probably indicates strongly seasona! or episodic flow, with organisms being able to settle and grow during lower river stages. Rigsby (1994) described a very similar delta front facies assemblage in the Santa Ynez Mountains of California as consisting of non-graded to normally graded, sandy, cobble to pebble conglomerates and conglomeratic sandstones forming both channelized and non-channelized uoits, with abundant oysters, pecten and shallow water gastropods, burrowed bed tops, and small-scale leoticular bedding.

Gübert-type delta .lope
Occurring in uoit 3a, lhis faeies is composed of apparently unfossiliferous, coarse to very coarse sandstones and conglomerates with chert and lava pebbles geoerally less than 5 mm in diameter, as well clay pellets and clasts. The faeies is characterized by large-scale planar cross-bedding with tangential bases dipping betweeo 20 and 27° towards the north andnortbwest. Cross-bedsets are 3-3.5 m lhick, with individual eross-beds reaching 30 cm in lhickness and showing distinctly hipartite, fining-upward trends from conglomerate to coarse-grained sandstone. However, contacts between the foresets are gradual. Similar foresets have beeo described on Gilbert-type delta slopes in Lake Llanquihue (Rojas and Le Roux, 2005) and on the shores ofLake General Carrera in southern Chile (Bell, 2009).
The superposition of cross-bed sets indicates periods of rapid marine transgression followed by delta progradation. Although tbe apparent absence of fossils could be attributed lo a rapid sedimentation rate and unstable slope setting relatively hostile lo organisms, Rojas and Le Roux (2005) did record tbe presence of bivalves on tbe Lake Llanquihue delta slope. On tbe otber hand, Bell (2009) found no fossils on similar foresets at Lake General Carrera.

General paIeoenvironmentaI interpretation
Estuaries are normally classified into salt-wedge, well-mixed, partially mixed and fjord types (Pritchard, 1955;Duxbury and Duxbury, 2001). The first type forms where tbere is strong river inflow and relatively weak tides, tbe second where intertida! volumes are strong and river inflow is weak, tbe third where botb strong tida! flows and relatively high rates of freshwater discharge occur, and tbe fourth in fjords experiencing weak tida! currents, except at tbe entrance. The stodied succession apparently reflects a hydrodynamically variable estuary, witb evidence for tidal effects (presence of tidal flats witb tida! creeks) offset by strong river discharge periods. It is conceivable !hat tbe lalter episodes coincided witb relative marine regression, during which seaward progradation oftbe Gilbert-delta took place. However, seasonal discharge allowed marine organisms lo settling during low river stages. The tens of meters of slightly sandy tida! mudflat and subtida! facies, on the otber hand, were probably deposited during marine highstand periods, when tbis par! of tbe estuary ehanged from a river-and wave-dominated lo tide-dominated environment.
A generalized paleoenvironmental interpretation oftbe Rlo Baguales Formation is presented in figore 7. This shows a bay head delta prograding from tbe soutb-soutbwest into an estuary partially protected from tbe open sea by sand spits at tbe entrance. Bay head deltas are commonly associated witb wavedominated estuaries (Reinson, 1992;Roy, 1994) and !bis sub...,vironment is here attributed lo a high rate of sedimentation because of a surrounding mountainous terrain and possibly high, albeit sporadie rainfall. Separation of tbe tida! flats into slightly sandy mudflats and slightly muddy sandflats suggests wave 341 action, which would have had a stronger winoowing effect near tbe delta itself where tbe waves would have suffered little refraction. Behind tbe sand spits at tbe entrance, an environment relatively protected from wave action probably existed so tbat muddy sediments accumulated on tbe tlats.

S. Relative sea-level and climate changes
One of tbe key differenees between deltas and estuaries is !hat tbe former are normally progradationa! sediment bodies showing a coarsening-upward suceession, whereas estuaries are mainly aggrada-tionaI, building up within a drowned river channel (Nichols, 1999). The base ofthe measured profile, unit 1, refleets an estuary moutb environment witb barrier spits succeeding tida! inlet and flood tida! delta deposits, so tbat tbis par! of the succession reflects marine transgression, probably accompanied by lateral migration of tbe inlet channel. Vnit 2 is represented by distal, slightly sandy tida! mudflat and sub-tida! deposits, whieh occur landward oftbe barrier spits and so represent seaward progradation of tbe estuary complexo Nevertheless, tbe relatively great tbickness oftbese deposits indicates tbe creation of new accommodation space, which could have been provided by tectonic subsidence and an increase in tbe relative water deptb as tbe estuary itself migrated over and eroded its distal mudflats. The delta slope deposits of unit 3 must have prograded over sub-tida! sediments on tbe estuary floor in a water deptb of at least 3 m. The superposition of 4 1arge-seale crossbed sets indicates episodes ofrelative sea-level rise interropted by pulses of delta progradation. The delta front deposits of unit 4, on tbe otber hand, retlect seaward progradation accompanied by lateral migration of distributary moutb bars and ehannels tbat eroded gravel bars subject lo wave action on tbe inner delta front. Vnit 5 marks a retom lo slightly sandy tidal mudflats, but tbe lalter in tbis case probably formed in a more proximal environment landward of tbe slightly muddy tidal sandflats flanking the delta, implying further seaward progradation. This is supported by tbe presence of tida! creek sandstones and the absence ofbar-finger gravels in unit 5. The slightly sandy tida! mudflats were replaced by tbe slightly muddy tida! sandflats of unit 6, which suggests a renewed sea-level rise due to tectonie subsidence. Lateral migration of tbe delta, possibly accompanied by a contioued rise in sea level, tben temporarily replaced lhe slightly muddy tidal sandflats wilh delta front sedimentation as reflected in unit 7, before returning once again lo slightly muddy sandflat deposition (unit 8). Unit 9 indicates initial slightly sandy tida! mudflat sedimentation followed by marine transgression lo sub-tida! depths, as suggested by lhe presence of well-developed bar-linger gravels in lhe uppermost par! of lhe suceession. However, from lhis point in time lhe entire estuarine-delta system commenced lo prograde seaward again, wilh sub-tida! deposits grading upward inlo slightly sandy tida! mudflats and finally lhe fluvial deposits of lhe Las Flores Formation.
Deep-sea 1'00 ice-volume records indicate !hat glacial-interglacial eustatic sea-Ievel fluctuations of 10-40 m associated wilh orbital cycles occurred during lhe late Oligocene (pekar el al., 2006;Naish el al., 2008), which probably account for lhe sealevel oscillations observed within lhe measured par! oflhe Río Baguales Formation. Figure 3  Ma followed by a eustatic transgressive cycle from 23.5 Ma unti123.3 Ma, and lhen a marked sea-Ievel drop commencing at 23.3 Ma. We correlate lhe fmt regression cycle wilh lhe more or less sustained fall in sea-Ievel shown by units 2-8, whereas lhe marine transgression reflected by unit 9 possibly coincides wilh lhe sea-Ievel rise between 23.5 and 23.3 Ma. The marked sea-Ievel fall during lhe fmal stages of deposition oflhe Río Baguales Formation is consistent wilh evidence for a global marine regression straddling lhe Oligocene-Miocene boundary and commencing at 23.3 Ma. Known as the Mi-I glaciation (Miller el al., 1991) lhe growth of lhe East Antarctic Ice sheet caused a relative drop of about 50 ID in sea-Ievel at lhis time (Naish el al., 2008). Various aulhors (e.g., Shackleton and Kennett, 1975;Barker and Burrell, 1977) have linked this event to lhe opening of lhe Drake Passage between Antarctica and Soulh America at lhe end of lhe Oligocene, which initiated lhe circum-Antarctic deepwater ocean current and caused cooling of lhe Antarctic landmass to its actual condition.