Palaeogeographical implications of the Miocene Quendeque Formation (Bolivia) and tidally-influenced strata in southwestern Amazonia

Palaeogeography, Palaeoclimatology, Palaeoecology 243 (2007) 23 – 41 www.elsevier.com/locate/palaeo

Palaeogeographical implications of the Miocene Quendeque Formation (Bolivia) and tidally-influenced strata in southwestern Amazonia Jussi Hovikoski a,⁎, Matti Räsänen a , Murray Gingras b , Shirley Lopéz c , Lidia Romero d , Alceu Ranzi e , Janira Melo e a

b

Department of Geology, University of Turku, 20014 Turku, Finland Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Canada T6G 2E3 c Sergeomin, La Paz, Bolivia d Laboratorio de Paleontología, INGEMMET, Avenida San Borja 1470, Lima, Peru e Laboratorio de Pesquisas Paleontologicas, Departamento de Ciencias de Natureza, Universidade Federal do Acre (UFAC), 69.915-900 Rio Branco-Acre, Brazil Received 26 October 2005; received in revised form 18 April 2006; accepted 3 July 2006

Abstract The Miocene palaeogeography of the western Amazonian Foreland Basin (Venezuela, Colombia, Ecuador, Peru and western Brazil) and the Paraná Basin (mainly Argentina) are contentious. Several studies have hypothesized for and against a possible connection of these epicontinental depositional systems during Miocene. However, the lack of well defined palaeoenvironmental data, especially from the Bolivian lowlands, has hindered the delineation of the Miocene palaeogeography in these areas. The objective of this study is to provide new data concerning this problem. This paper presents sedimentological and ichnological data from the Quendeque Formation in the northern Subandean zone of Bolivia (lat. 15°S). This virtually unstudied formation relates the marine-influenced Paranan (northernmost reported occurrence lat. 18°S) and western Amazonian (southernmost reported occurrence lat. 13°S) depositional systems and is contemporaneous with them. The palaeogeographical significance of the new data and recently discovered tidal deposits from southern Peru (Madre de Dios) and western Brazil (Acre) is discussed. New palaeocurrent and rhythmite data from the Madre de Dios and Acre Sub-basins are presented. The studied deposits are interpreted as alluvial, deltaic and estuarine coastal plain. These data coupled with recently published data from south and central Bolivia indicate that thin Miocene tidally/marine influenced levels are present throughout the Bolivian forelands. The distal–proximal facies relations between the Late Miocene Chaco and Madre de Dios strata, and the south–southwest oriented palaeocurrent directions collected from the Madre de Dios Sub-basin suggest at least South Atlantic source for the marine influence in these areas. The episodically open hydrodynamic connection between the southwestern Amazonia and Paranan Sea may have provided a dispersal route, especially for fresh-water and euryhaline aquatic taxa. Thus, the results may explain the reported fossil faunal similarities between these areas and the modern biogeography of certain aquatic species. © 2006 Elsevier B.V. All rights reserved. Keywords: Sedimentology; Ichnology; Miocene; Palaeogeography; Bolivia; Western Amazonia; Paraná Basin

⁎ Corresponding author. E-mail address: [email protected] (J. Hovikoski). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.07.013

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1. Introduction Several previous studies have hypothesized for and against a possible connection between the marineinfluenced Amazonian and the Paranan epicontinental depositional systems during the Miocene (Fig. 1A) (e.g. Boltovskoy, 1991; Räsänen et al., 1995; Webb, 1995; Marshall and Lundberg, 1996; Lundberg et al., 1998; Hamilton et al., 2001; Marengo, 2000; Cione et al., 2000; Boeger and Kritsky, 2003; Brito and Deynat, 2004; Albert and Crampton, 2005; Hernández et al., 2005; Hovikoski et al., 2005; Hoorn et al., 2006; Hulka et al., 2006). The contemporaneous strata in southwestern Amazonia (Acre and Madre de Dios Sub-basins) and Paraná bear fossil faunal similarities such as croakers, dolphins, crocodiles, sirenids and stingrays (Cione et al., 2000; Hamilton et al., 2001; Boeger and Kritsky, 2003; Brito and Deynat, 2004). Furthermore, some of the foraminifera discovered in the Paranan Basin are of Caribbean origin (Boltovskoy, 1991). In concert, these data indicate that a connection existed between these basins during Miocene and/or that the faunal similarities are a result of earlier, extensive Cretaceous epicontinental embayments. Most publications dealing with the area's palaeobiogeography have explored the hypothesis that the central and northern Bolivian lowlands acted as barrier between these two systems during the Miocene and consequently no interconnection existed (e.g. Lundberg et al., 1998;

Boeger and Kritsky, 2003; Brito and Deynat, 2004; Lovejoy et al., 2006). This view is primarily based on the structural geology study of Sempere et al. (1990), which indicated the presence of a buttress in the central Bolivian lowlands since the initiation of the Andean Foreland Basin in the Late Oligocene (Chaparé Buttress) (Fig. 1A). Moreover, there have not been any known marineinfluenced deposits north of latitude 18°S in Bolivia until recently (Marshall et al., 1993). New studies have shown that the marine-influenced Yecua Formation goes on in subsurface at Chaparé (ca. lat. 17°S) in central Bolivia and that fluvio-tidal sediments outcrop in the northernmost Bolivia (lat. 12°S) (Hernández et al., 2005; Roddaz et al., 2006). The remaining part in the northern Bolivian lowlands are palaeoenvironmentally one of the least studied areas of the Andean Foreland; consequently, direct palaeoenvironmental data for or against the Miocene interconnection have been lacking to date. The key sedimentary unit regarding this issue is the Quendeque Formation in the northern Subandean zone of Bolivia. This 2000-m thick, fossil-poor, virtually unstudied formation is located between the marine-influenced Paranan (northernmost reported occurrence lat. 18°S) and western Amazonian (southernmost reported occurrence lat. 13°S) depositional systems (Marshall et al., 1993; Hovikoski et al., 2005). The Quendeque Fm. is considered an equivalent of the marine influenced Miocene Yecua Formation of southern Bolivia) (Marshall and Sempere, 1991) and the tidally influenced Solimões and Madre de

Fig. 1. A) Map of South America showing the potential Miocene sea connections (labelled A–E) and the western Amazonian and Paranan epicontinental embayments (the ruled areas) (compiled after Marshall et al., 1993; Wesselingh et al., 2002; Gingras et al., 2002a; Martinez and del Río, 2002; Hovikoski et al., 2005). MA — Marañon Sub-basin, MD — Madre de Dios Sub-basin, AC — Acre Sub-basin, BE — Beni Sub-basin, NSZ — northern Subandean zone of Bolivia, CH — Chaco Sub-basin, PA — Paranan Basin. Stratigraphic framework of each sub-basin is given in Table 1. Lined square marks the study area. B) Study locations: QU1) Quendeque River (S 14.98914°, W 67.78700°); QU2) Quendeque River (S 15.00913° W 67.76438°) and QU3) Beni River (S 14.85935° W 67.62575°).

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Dios/Ipururo Formations (Brazil and Peru) (e.g. Hermoza, 2004) (Table 1). Previous sedimentological works dealing with the Quendeque Formation in Bolivia include reconnaissance work by YPFB between 1939 and 1964, in which the formation was interpreted as a fluviolacustrine unit (Suarez and Diaz, 1996). The objective of this study is to provide new palaeoenvironmental data of the Quendeque Formation. For this purpose, a 300-m thick succession was sedimentologically and ichnologically studied in the Alto Rio Beni area, Bolivia (lat. 15°S) (Fig. 1B). The data were complemented by observations from two other outcrops in the area. Thickness variations of the heterolithic strata were analyzed by statistical methods (Fourier transform) to identify possible cyclicities. The studied deposits were interpreted as alluvial and deltaic–estuarine coastal plain sediments. The palaeogeographical significance of the new data and other recently discovered tidal deposits from southern Peru (Madre de Dios/Ipururo Fms.) (Hovikoski et al., 2005) and south-western Brazil (upper Solimões Fm.) (Gingras et al., 2002a) is discussed. Previously unpublished palaeocurrent and rhythmite data from these formations are presented. 2. Study area The Quendeque Formation is a N 2000-m thick, semilithified sedimentary succession that outcrops in the central and northern Subandean zone of Bolivia and the

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southern Subandean zone of Peru. It was first described by Schlagintweit in 1939, and its type locality is located ca. 1 km downstream from the confluence of the Quendeque River with the Beni River (Suarez and Diaz, 1996). The Formation belongs to the Beni Group. The Quendeque Fm. situates conformably above the Bala Formation and has an angular discordance to the overlying Charqui Formation. The Formation represents Middle/Upper Oligocene–Upper Miocene age based on recent AFTA and 40Ar/39Ar isotope dates (Hermoza, 2004; Strub et al., 2005). The study area represents the distal wedge-top depozone of the Andean Foreland Basin system. The studied strata outcrop in a piggy-back basin delineated from the Beni Sub-basin by the current thrust front. During the Early–Middle Miocene, the area represented the very proximal foredeep depozone close to the influence of thrust front propagation (Strub et al., 2005). This study is based on a 300-m thick sedimentary succession along the Quendeque River near the eastern border of the state of La Paz, Bolivia (locality QU1 in Fig. 1B). The data are complemented by observations from two other outcrops along the Quendeque River (QU2) and Beni River (QU3). The locations can be put into a stratigraphical order based on the northwest oriented tectonic tilting direction of the strata: QU1 and QU2 represent approximately the middle part of formation; QU3 is closer to the top. Based on this rough estimation, they were possibly deposited within Early–Middle/earliest Late Miocene time frame. The limitation of the QU1

Table 1 A tentative Miocene stratigraphic chart of Amazon–Paraná Basins compiled after 1) Hoorn, 1993; Hermoza, 2004; 2) Campbell et al., 2001; Hermoza, 2004, 3) Hoorn, 1993, 4) Hermoza, 2004; Strub et al., 2005, 5) Hernández et al., 2005; Hulka et al., 2006; 6) Aceñolaza, 2000

Some of the known absolute ages are shown. Embayment units are the following: Pebas Formation — brackish–fresh-water bay margin sequences (Gingras et al., 2002b; Wesselingh et al., 2002), Nauta Formation — tidal channel complexes (Rebata et al., 2000), Ipururo–Madre de Dios formations — tidally influenced and continental strata (Hermoza, 2004; Hovikoski et al., 2005); Solimões Formation — tidally influenced channel complexes and continental strata (Räsänen et al., 1995; Gingras et al., 2002a); Quendeque Formation — alluvial and deltaic/estuarine coastal plain strata (this study); Yecua Formation — restricted marine to lacustrine deposits (e.g. Marshall et al., 1993; Hulka et al., 2006); Paraná Formation — shallow marine and coastal strata (e.g. Aceñolaza, 2000).

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and QU3 outcrops is that the facies can be followed laterally only ca. 20–30 m. Finally, the deposits are weathered thus making detailed observations difficult in places. The additional data presented in Figs. 7 and 8 are collected from the Madre de Dios/Ipururo Formations (Madre de Dios Sub-basin) and the Solimões Formation (Acre Sub-basin) (localities are shown in Fig. 8).

modified after Hovikoski et al. (2005). The data in Fig. 7I and J are previously unpublished. These data are compared with the modern, synodic, predicted tidal station data from Recife, Brazil (NOAA, 2001).

3. The methods

In the text below, the facies associations are described and interpreted. The occurrence of each facies association in relation to the relative sea level change is discussed in the following chapter.

The basic data set consists of sedimentological and ichnological field descriptions. Grain size, sedimentary structures, nature of bedding, and bedding contacts were described. The sedimentological approach also includes analysis of rhythmite-thickness data. The thickness variations of the sand–clay or silt–clay couplets (a couplet is defined as a sand or silt lamina and the bounding upper clay lamina) were measured from two vertically accreted rhythmite successions (n = 41, from QU1 and n = 125, from QU3) (Fig. 5A–G). The thickness variations of sedimentary couplets were calculated from high-resolution field photographs (QU1) and a polished rock slab by a stereomicroscope (QU3). The 84 first couplets of the QU3 rhythmite were analyzed by the spectral procedure of the SAS Enterprise Guide (finite Fourier transform) to detect possible cyclicities (SAS Institute INC., 2002). The sources of error are that in places the strata are affected by erosion and synsedimentary deformation. This was especially the case in QU3. Consequently, the true number of events in a cycle may be higher than observed. The ichnological approach included documentation of genera, trace fossil size and bioturbation intensity. On the basis of sedimentological and ichnological criteria, the deposits are divided into 8 recurring facies (Table 2). The facies are further grouped into 5 facies associations. There is a possible source of error in this approach: Due to the weathered nature of the deposits no detailed observation of the sedimentological and ichnological properties was possible in places. Consequently, the reported ichnological diversity may be lower than in reality and the facies with “massive appearance” may in fact be stratified. The additional data from the Madre de Dios and Acre Sub-basins consist of palaeocurrent and rhythmite data collected by JH, MR and MG. The palaeocurrent data comprise of previously unpublished field data coupled with the data presented in Räsänen et al. (1995) and Hovikoski et al. (2005). The palaeocurrent directions were measured from heterolithic ripple and dune cross-strata. The rhythmite thickness data presented in Fig. 7E–H are

4. Results 4.1. The facies

4.1.1. Facies association 1 FA1 is a common facies association occurring in all three study locations. It consists of yellowish or reddish, massive or trough cross-stratified, silty fine to medium grained sand (FA) that grades upwards into rooted mud (FG). Commonly, it sharply overlies root bearing mud (FG) or pedogenized mud (FH). Laterally, it may grade into sheets of sandy silt (FD). FA1 occurs as 1–5 m thick tabular or trough-shaped units that can be laterally followed some tens of meters. The deposits are generally non-bioturbated with the exception of occasional low-density assemblages of meniscus-bearing trace fossils of various sizes and small Planolites that descend from the top of FA1 or from the overlying FA2. The backfilled trace fossils are lined (Beaconites) or unlined (Taenidium) (cf. Keighley and Pickerill, 1994). FA1 is transitional with FA4. The differences are: FA1 generally lacks mud clasts on its lower boundary and on strata (exception at 25 m in the log), it has more massive appearance and more uniform grain size because it lacks mud drapes. Moreover, inclined heterolithic stratification (IHS of Thomas et al., 1987) (FC) are generally absent and occasionally there are ichnological differences (no clay-lined tubular burrows of regular diameter referable to Palaeophycus/Ophiomorpha isp.). 4.1.2. Interpretation of FA 1 “Fluvial point bar” The sharp-based, upward fining units are most compatible with a channel environment. The lack of mud clasts on its lower boundary and IHS facies may indicate limited lateral accretion of the meandering channel. The massive appearance is probably partly due to diagenetic changes: the almost homolithic strata are poorly visible in these cemented and weathered deposits. The occasionally visible trough cross-strata point to traction and relatively high hydrodynamic energy. Stratigraphically, FA1 is interbedded with relatively thin flood plain deposits and

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Table 2 Diagnostic criteria of the facies. Ichnological abbreviations are explained in Fig. 2 Facies

Contacts and occurrence

Facies (A) unbioturbated, stratified/massive silty sand

• Lower contact erosional

Facies (B) bioturbated, mud draped crossstratified sand

Facies (C) inclined heterolithic stratification (IHS)

Facies (D) sandy silt sheets

Facies (E) massive or stratified, Gyrolithes bearing brown mud and sand

Facies (F) massive or laminated brown mud

Facies (G) massive

Sedimentology

• Trough cross-stratified or massive • Upper contact gradational • Silty, fine- to medium grained sand • Overlie FG, FF or FH • Reddish or yellowish • Grade upwards into FG • Commonly lithified or FH by Ca • May grade laterally to FD • Thickness 1.5–ca. 10 m • Lower contact erosional • Trough cross-stratified, • Upper contact gradational planar cross-stratified, climbing ripples as thin • Transitional facies with FA intervals in the top of facies • Overlie FG, FE, FF or FH • Strata clay draped • Grade upwards into FC • Occasional cyclic or FG rhythmites • Trough-shaped units • Rare scour-and-fill • Thickness 1–5 m structures with sedimentary couplets occur in the top of the facies • Lower contact gradational • Interbedded massive or with FB or FA stratified sandy silt and • Upper contact gradational massive or stratified with FG brown mud (B2) • Laterally limited tabular or wedge shaped • Thickness of the facies • Sandy lithosome may bear 1–2 m great number of heterolithic • Sandy lithosome up to 20 lamina, and mud clasts cm thick, muddy lithosome • Individual beds have low around 5 cm thick depositional dip • Commonly interbedded • May show upward with FG or FH thickening bed • May grade laterally to FA thickness trend • Bed thickness range from • The contact between mud 1 to 20 cm and sandy silt is gradational, • Facies thickness ca. 1 m whereas the contact from (changes laterally) sandy silt to mud is sharp (upward coarsening units) • Lower contact gradational • Non-rooted or sharp • Sharp upper contact • Massive, laminated or with FB ripple cross stratified • Transitional with FF • Laterally extensive • As coset may form flaser–lenticular bedding • Thickness of facies up to 5m • Lower contact gradational • Non-rooted or abrupt • Upper contact gradational • Massive or laminated with FG or FH • Transitional with facies FE • Occasionally reworked • Laterally extensive by bioturbation • Thickness of facies up to 5 m • Lower contact gradational • Commonly rooted

Ichnology

Interpretation

• Normally unbioturbated

• Sedimentation from traction • Relatively high hydrodynamic energy and rapid sedimentation • Basal part of a fluvial point bar

• Rare Ta burrowing from above

• Sporadic bioturbation • Low diversity assemblages consisting of Pa/Op isp. (clay lined tubular burrows), Sk and Ta

• Low bioturbation intensity

• Pulsative energy, alternating traction and suspension • Possibly occasional brackish-water colonization • Tidally influenced, locally dominated • Semi-diurnal rhythmites • Basal part of a fluvio-tidal point bar • Interbedding represents longer time span than N–S

• Pa/Op isp. (1 cm), backfilled burrows (?Ta) • Associated with increased accommodation space

• Lateral accretion of a channel • Moderate bioturbation intensity

• Alternating traction and suspension sedimentation

• Meniscus-bearing burrows (few mm in diameter)

• Laterally limited, sheet like floods close to subaerial setting • Crevasse-splay

• Moderate to high bioturbation intensity

• Low energy, moderate to low depositional rate

• Low-diversity assemblages consisting of Gy, Th, Te, Pl, Pa/Op

• Subaqueous deposition • Restricted brackishwater influence • Deltaic/estuarine shoal

• Low–high bioturbation intensity • Pl, Pa, irregular burrows with thinner branches, backfilled burrows with irregular menisci, Rh, large unlined burrows of unknown affinity • Rare to abundant meniscus

• Low energy and depositional rate

• Subaqueous deposition • Fresh-watered • Lacustrine • Low energy setting (continued on next page)

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Table 2 (continued ) Facies rhizolite bearing brown mud

Facies (H) pedogenic mud

Contacts and occurrence

Sedimentology

commonly with FA, FB, and • Brown or white FC • Upper contact gradational • May bear unrooted with FH intervals (FF) • May be intercalated with FF or FI • Fresh-watered • Thickness 1–10 m, thin • Flood plain above FA thick above FB • Transitional with FH • Gradational lower contact • Multicolored: Reddish with F3, brown, blue and white • Upper contact truncated by FA, FB • May be intercalated with • May bear caliche intervals FG • Development depends on • Slickenside the stratigraphic level

occasionally more mature (horizonted) palaeosols (FA2). In concert, these factors point to a limited accommodation space (e.g. Wright and Marriott, 1993). However, considering the general lack of mud clasts and of thin, single-story channel bodies, most likely no relative sea level drop occurred, and the deposits represent regressive–early transgressive deposits. A possible exception to this occurs at 25 m in the log profile (Fig. 2), where a tidally influenced channel is sharply overlaid by a coarse-grained, mud-clast-bearing fluvial channel. However, even this channel is not associated with a recognizable change in palaeochannel amalgamation nor thin overbank strata, suggesting continued aggradation (Marriott, 1999). 4.1.3. Facies association 2 FA2 is very common, occurring commonly throughout QU1. It consists of massive, rhizolite-bearing brown mud (FG) which is commonly intercalated with sandy silt sheets (FD) and pedogenized multicoloured mud (FH) (Fig. 3). It gradationally overlies FA1 or F4, occasionally also FA3. Its upper contact is most often truncated by FA1 or FA4 but may also grade in some instances into FA3. The relative volumetric importance of the component facies depends on the stratigraphic level: the thickness of root-bearing mud ranges from 1 m to 10 m whereas the stacked sandy sheets usually show a thickness of ca. 1–3 m. The total thickness of the facies association is also dependant of the stratigraphic level and varies from 2 m to 20 m. The pedogenic horizons are generally best developed when FA2 forms thin successions overlying FA1.

Ichnology

Interpretation

bearing burrows of various sizes (Ta, Be)

• Alternating subaerial exposure and subaqueous conditions

• Rare stratification observed

• Irregular shafts with thin branches (insect burrows or root bioturbation)

• Completely reworked by rhizolites, Ta and insect burrows

• Prolonged subaerial exposure • Palaeosol horizon

The root-bearing brown mud lacks recognizable horizons but may bear slickenside. The root boundaries are altered and bluish. The sandy sheets have often gradational lower boundaries and sharp upper boundaries with the interbedded root-bearing mud. The thickness of individual sheets changes laterally and ranges from 1–20 cm. Internally, they have a massive appearance. The pedogenized mud bears greyish or reddish horizons that may overlie caliche layers. Finally, grey, reworked ash intervals occasionally occur in the facies association. The degree of bioturbation is commonly high. The observed ichnofossils include various size classes of meniscusbearing traces, Palaeophycus-like traces that have irregular burrow and lining width (see also FA3 for description), small to large Planolites and irregular burrows with thinner branches (Fig. 4A, B, E, F). Root bioturbation is intense. 4.1.4. Interpretation of FA2 “flood plain-palaeosol” The constant occurrence of roots or pedogenic features (slickenside) indicate subaerial exposure. These root-bearing deposits are interbedded with sandy sheets that laterally turn into fluvial channels: these are easiest interpreted as crevasse-splay deposits. The rooted mud is commonly brown and lacks horizons (?entisol) whereas the pedogenized mud may bear horizons. These facies are simply referred as flood plain and palaeosol, respectively. The determination of soil types was hindered by the lack of fresh exposures: the deposits are affected by recent pedogenesis and weathering. The meniscus-bearing burrows are occasionally associated with lined burrows that resemble Palaeophycus in

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Fig. 2. Log profile of locality QU1.

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Fig. 3. Outcrop photograph from locality QU1 (interval 313–330 m in the log). The given facies are described in Table 2.

2D view. These may represent chambers (Fig. 4A, B) and are most likely related to insects such as soil bugs and beetles (Hasiotis et al., 2002). Generally, these insects dwell in alluvial and marginal-lacustrine environments. Trails may represent foraging of larvae and associated chambers pupation or pre-pupal molting; thus, they may represent an entire life cycle of a beetle (Hasiotis et al., 2002) (Fig. 4F). 4.1.5. Facies association 3 FA3 is moderately common occurring in all studied locations. It consists of massive or laminated brown mud (FF). Occasionally, it bears bigradational (both contacts are gradational) sandy intervals that contain diminutive current ripples. Its lower contact is sharp or gradational with FA2. Towards the top, it may grade into FA2 or be truncated by FA1 or FA4. Its thickness varies from 1 to 5 m. Its lateral extent is unclear at locality QU1, but it can be followed for 100 m at locality QU2. The facies association is transitional with FA5; the dividing criterion is the lack of Gyrolithes–Thalassinoides association in FA3. Bioturbation intensity varies from unbioturbated to bioturbated. The degree of bioturbation increases towards the top of the facies association. The trace fossils include irregularly backfilled, unlined burrows (Taenidium) and Planolites of various sizes. Small Taenidium and thin, irregular lined burrows (2–4 mm) reburrow other trace fossils (Fig. 4C). The thin, lined burrows have horizontal–vertical orientation, form Rhizocorallium-like spreite (Fig. 4D), occur solitarily or as clusters (Fig. 4E). In cross-section, they may resemble

Palaeophycus. However, their lining thickness and burrow width vary irregularly. Other recognized trace fossils include large (ca. 5–10 cm diameter), passively filled, branching unlined burrows. Finally, irregular burrows with thinner branches occur near the top of FA (Fig. 4F). 4.1.6. Interpretation of FA3 “lacustrine” The lack of pedogenic features or roots coupled with the presence of hydrodynamic sedimentary structures indicate a subaqueous setting. The generally fine-grained grain size and occasionally observed diminutive current ripples further point to an overall low level of hydrodynamic energy. Further considering the high lateral continuity of FA3 and the occurrence of fresh-water/ continental trace fossils, FA3 is best interpreted as a lacustrine environment. The upward-increasing bioturbation intensity, passively filled unlined traces and insect-generated trace fossils suggest that most of the trace fossils descend from the top (marginal lacustrine) or from the overlying flood plain. 4.1.7. Facies association 4 FA4 occurs in QU1 and QU3. It forms sharp-based, 2–10 m thick, upward-fining successions of silty, fineto-medium-grained sand (FB). Towards the top, the facies association turns into IHS (FC) and/or massive root-bearing mud (FG). The basal facies forms 1–8 m thick, trough-to-tabular-shaped units. Its lower part bears mud clasts that occur both on the erosional lower contact and on the cross strata. Further, the facies is commonly trough cross-stratified, planar (low-angle?)

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Fig. 4. A) Meniscus-bearing burrows of FA2. Dashed circle — lined chambers associated with meniscate burrows. B) Meniscus-bearing burrows and lined irregular burrows from FA2. C) Bioturbated, laminated sandy mud (FA3). Black arrow — Taenidium. White arrow — Planolites reburrowed by Taenidium and lined irregular burrows (grey arrow). D) Rhizocorallium-like trace in FA3. Spreite is formed by a lateral movement of a narrow lined burrow. White arrow — Palaeophycus. E) Bioturbated, massive brown mud at the boundary of FA3 and FA2 (subhorizontal view). White arrow — Planolites. Black arrows — clustering lined burrows. F) Bioturbated, brown mud at FA3–FA2 boundary. Black arrows — chambers in irregular burrows. White arrows — irregular burrows.

stratified or massive. Rarely, ca. 20-cm-thick scour and fill structures with sand–mud couplets occur near the top of this facies. The cross-stratified strata are claydraped. Locally, double mud drapes are observed (Fig. 5A, B). The heterolithic strata form cyclic rhythmites: the white noise test indicated non-random origin for the sand–clay couplet succession at location QU3

(n = 84, P = 0.0251). Peaks occur at 2.2, 10.5 and 21 couplets in the spectral density plot (Fig. 5G). The IHS facies is laterally restricted and the thickness and lithology of its members changes over short distances. The fine-grained member of the IHS facies contains heterolithic lamina that grade into massive mud over a few meter distance. Rip-up-mud pebbles occur in

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Fig. 5. (A) Mud-draped strata at locality QU1. (B) Close-up of same facies. Arrows — double mud drapes. (C) Heterolithic stratification from locality QU3. (D) Close-up of same facies. (E) Bar chart displaying thickness variation of heterolithic strata shown in (A). N, S-interpreted occurrence of neap and spring tides, respectively. (F) Bar chart displaying thickness variation of heterolithic strata shown in (C) and (D). P, A — interpreted occurrence of perigean and apogean spring tides, respectively. (G) Fourier transformation data from the same series (84 first couplets analyzed). Peaks occur at 2.2, 10.5 and 21 couplets.

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both members. In the coarse-grained member, the clasts can reach up to 20 cm width and form breccia-like structures in the otherwise massive appearing sand. The apparent dip of IHS is low and decreases towards the margin of the facies association. The bioturbation intensity is low in FA4. The genera observed include clay-lined, vertically oriented tubular burrows referable as Palaeophycus or Ophiomorpha isp. (moderately common, 5–12 mm). No branching was observed in these burrows. Also, occasional clay-lined, vertically oriented Skolithos (2–4 mm diameter) occur. Finally, meniscus-bearing trace fossils may burrow the top of FA. 4.1.8. Interpretation of FA4 “tidally-influenced point bar” The irregular, erosional lower contact with mud-clasts and the upward fining succession is best explained by a channel environment. The mainly trough cross-stratified sand suggests relatively high hydrodynamic energy and deposition from traction. The strata are commonly claydraped, however, pointing to frequent presence of quiescent conditions and deposition from suspension. These regularly occurring pulses in the hydrodynamic energy are interpreted to be a result of tidal processes. Some of the clay drapes can be up to 0.5 cm thick, especially near the base of the facies association: this may attest to flocculation and/or presence of fluid muds and deposition from traction. Alternatively, they may represent amal-

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gamated clay lamina of several tidal cycles. However, the pronounced asymmetry of adjacent couplets and the number of couplets in a cycle does not bolster the amalgamation hypothesis. The cyclic rhythmites, double mud drapes and thick–thin alternation in successive sand–clay couplets (De Boer et al., 1989) support the tidal interpretation (Fig 5). The cycles of 2.2, 10.5 and 21 couplets are interpreted to correspond to semi-diurnal and truncated neap-spring cycles. The rhythmite succession observed at the top of QU1 is rather short, but the pronounced inequality in thickness of successive couplets and ca. 28 couplets in a cycle as indicated by the trend line in Fig. 5E also suggest semidiurnal regime. The sand–mud interbedding in the IHS-facies likely represents a longer cycle than neap-spring or anomalistic month cyclicity. This interpretation is based on the high number of heterolithic strata in individual beds. Furthermore, some of the IHS may occur stratigraphically close to crevasse-splay deposits, probably indicating fluvial dominance in the channel dynamics (Barwish, 1978). Consequently, the IHS-facies probably represents seasonal variations in the channel hydrology. The low bioturbation intensity is likely due to a high depositional rate and high hydrodynamic energy as indicated by the physical sedimentary structures (3D dunes). Stratigraphically, FA4 is commonly interbedded with thick (up to 10 m) flood plain deposits. This indicates

Fig. 6. Ichnofossils from facies associations 5. A) Gyrolithes (yellow arrows) — Thalassinoides (white arrows) association. B) Close-up of a well-developed Gyrolithes. C) Close-up of Thalassinoides burrow complex. D) Close-up of Teichichnus. White arrow indicates spreite.

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high accommodation space and transgression or early regression. Based on the criteria discussed above, FA4 is interpreted to be tidally-influenced point bar.

ther as an estuarine or deltaic interdistributary bay deposits based on its stratigraphical occurrence. 5. Discussion

4.1.9. Facies association 5 FA5 is moderately common in the lower part of section QU1 and in QU2. FA5 consists of massive or stratified brownish mud that bears occasional sandy intervals (FE). The sandy lithosome is locally flaser bedded. The change from mud to sand is often gradational, whereas the change from silty sand to mud is abrupt. The thickness of a sandy interval ranges from 10 to 40 cm. FA5 can reach up to 3 m thickness. The bioturbation intensity ranges from moderate to high. The low-diversity assemblages consist of moderately sized Gyrolithes, Thalassinoides and Teichichnus (Fig. 6A–D). The geometry of Gyrolithes ranges from regularly developed, 1 cm wide corkscrew spirals to irregularly occurring, narrow and closely spaced helicoidal patterns. The shafts have constant width are generally ca. 5 mm wide. These burrows are closely associated with Thalassinoides and are possibly connected with them (Fig. 6A). Thalassinoides form tubular, branching (ca. 90–130°), vertical–horizontal, up to 2-cm-thick burrows. The burrow cross-section is oval. The unlined burrow walls are occasionally uneven, bearing possible bioglyphs. Both genera are passively filled with marly matrix. 4.1.10. Interpretation of FA5 “estuarine–deltaic interdistributary bay” The non-rooted character, hydrodynamic sedimentary structures and the ichnological properties of FA5 indicate a subaqueous setting. The moderate–high bioturbation intensity caused by dwelling and feeding burrows is best explained by a low deposition rate. The strata bear current ripples indicating low hydrodynamic energy. The ichnogenera observed comprise compound forms of Gyrolithes–Thalassinoides and Teichichnus which are typical elements of Cruziana ichnofacies (e.g. Ekdale et al., 1984). The low diversity, the present genera and the absence of specialised feeding traces are best explained by a restricted, brackish-water setting (Pemberton and Wightman, 1992). The common trace makers for the above-mentioned genera are various types of decapod crustaceans that can tolerate very low salinities (Bromley and Frey, 1974). Furthermore, the spiralled geometry of Gyrolithes is likely aimed to protect the trace maker from salinity fluctuations. In fact, it is only common in marginal marine settings (Gernant, 1972). The passive infill and the unlined, striated burrow walls indicate that the substrate was compacted at the time of burrowing. FA5 is therefore interpreted ei-

5.1. Relative sea level change The sequence-stratigraphic interpretation discussed below is based on the QU1 locality only and therefore should be considered as tentative. The deposits are interpreted to represent a nearequilibrium, transgressive–regressive coastal plain succession under a constant relative sea level rise. The meandering fluvial channels form complete singlestorey channel units that are most often interbedded with relatively thick (N5 m) overbank successions attesting to rapid aggradation (Wright and Marriott, 1993; Marriott, 1999). Probably the slowest relative sea level rise occurs at 170–185 m in the log profile (Fig. 2) where unburrowed, thin channel bodies are associated with relatively mature, caliche bearing soil horizons and thin floodplain deposits. Although these channel units lack IHS deposits, their fine grain size coupled with their lack of clast-bearing base and their single-story nature suggest that the gradient remained low and imply that there was no noticeable relative sea level fall. Alternatively, this pedogenized interval can be explained by climatic fluctuation (a dry period). The early retrogradation is demonstrated by a gradual increase in the volumetric overbank/channel ratio (at 185–205 m in the log), appearance of lacustrine facies (locality QU2), evidence of tidal influence, and lateral accretion deposits (IHS) (at 270–300 m in the log). The presence of nonerosional standing water bodies also indicates that accommodation space was generated faster than it was filled (Plint et al., 2001). These strata grade upwards into thicker estuarine channels that are interbedded with impoverished, brackish-water ichnofossils (Gyrolithes– Thalassinoides association)-bearing interdistributary bay deposits in the zones of maximum flooding (at 210–240 m in the log and the QU2 locality). These estuarine strata show also evidence of occasional tidal dominance in deposition (cyclic rhythmites at 310 m in the log). The increasing channel depth and grain size despite of the supposed lowered gradient during retrogradation may probably be due to downriverincreasing channel size coupled with increasing tidal energy. During the subsequent highstand progradation the deposits grade into deltaic fluvial channels and thick overbank muds that are intercalated with well developed crevasse-splay deposits (at 242–269 m and especially 315–333 m in the log) (Fig. 3).

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5.2. Insight to Miocene palaeogeography The dominantly aggradational alluvial coastal plain nature of the Quendeque Formation coupled with other palaeoenvironmental data from southwestern Amazonia and Chaco constrain the possibility of a long lasting seaway connection between the Amazonian and Paranan depositional systems during the Miocene (Hernández et al., 2005; Uba et al., 2005; Hulka et al., 2006; Roddaz et al., 2006). However, the new data indicate that thin Miocene tidally/marine influenced levels are present throughout the Bolivian forelands (Hernández et al., 2005; Uba et al., 2005; Hulka et al., 2006). Our data come from outcrops that situate only ca. 300 km north from Cochabamba were Yecua Formation has been detected in subsurface by YPFB (Hernández et al., 2005). Consequently, these data strongly suggest that no permanent drainage divider existed between Amazonian and Paranan depositional systems in Bolivia during Miocene, as also suggested by Hernández et al. (2005) and Hulka et al. (2006). Hulka et al. (2006) considered three scenarios that could explain the marine influence in the Yecua Formation: 1) Yecua represents the northernmost part of the Paranan Sea; 2) Yecua is part of the Amazonian ingressions; and 3) Yecua is part of a major seaway that connected the Paranan and Amazonian embayments. They conclude that in the light of current data all three scenarios seem possible. We favor the scenario that the marine influence shaping south and north Bolivia and parts of the southwestern Amazonia (Acre and Madre de Dios Sub-basins) has primarily a Paranan origin because of 1) distal–proximal facies relations between southwestern Amazonian and south Bolivian/north Argentinean Miocene strata and 2) tidal characteristics of southwestern Amazonia. Firstly, although the poor age constrains of the studied interval of the Quendeque Formation does not allow exact time-correlation with the Chacoan deposits, the tidally influenced strata at the Pastora locality, Puerto Maldonado, south Peru, are dated as 8.8 ± 3.2 Ma (Hermoza, 2004; see also Campbell et al., 2001; Fig. 8) and thus most likely contemporaneous with the Yecua Formation (Hernández et al., 2005; Hulka et al., 2006). These and other outcropping strata in the Madre de Dios Sub-basin are mainly interpreted as inner–middle estuarine and continental sediments, with occasional brackish water influence (e.g. Hovikoski et al., 2005). The southwestern Amazonian strata bear only a few euryhaline fossil finds such as bull shark teeth (Räsänen et al., 1995). The Yecua sediments, however, contain abundant fossil evidence of elevated salinity such as foraminifera, cirripeds, marine gastropods and brachio-

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pods in certain levels (Marshall et al., 1993; Hernández et al., 2005; Hulka et al., 2006). Thus, the southwestern Amazonian strata form a more proximal landward equivalent to the contemporaneous Yecua Formation deposits. In addition, 300 km north from the study location of this study the Quendeque Formation has already a continental character (Hermoza, 2004). Secondly, this scenario is worth considering in the light of tidal data in southwestern Amazonia: there is evidence of an increased tidal range (?mesotidal) in the Madre de Dios and Acre Sub-basins (e.g. Hovikoski et al., 2005). This evidence includes wide spread tidal features such as well-developed tidal rhythmites (Fig. 7), regular thick-thin alternation in adjacent couplets, bipolar palaeocurrents, abundant reactivation surfaces and indications of tidal creeks. Moreover, the Acrean strata are dominated by IHS channel complexes which is a common feature in mesotidal settings (Räsänen et al., 1995; Gingras et al., 2002a). In the modern environments, the formation of cyclic tidal rhythmites appears to require a complex set of depositional parameters including at least mesotidal range (Archer, 1998). The few reported microtidal rhythmites are poorly cyclic and incomplete due to the lack of constant tidal dominance in deposition (cf. Shi, 1991; Brettle et al., 2002). In contrast, some of the southwestern Amazonian rhythmites are complete and show oceanic tidal resonance also during the apogean neap tides (e.g. Fig. 7E). Generally, tidal amplification occurs when tidal energy is transported from an open ocean to shallower and narrower coastal embayments and associated terrestrial systems (Archer and Hubbard, 2003). As a result, tidal range progressively builds up towards the proximal environments until its abrupt diminishing by the increasing bottom friction. An extreme modern example of tidal amplification is the Amazon River where the tidal currents propagate 800 km inland from the river mouth today (Archer, 2005). Even though tides can occur in such inland fresh-water systems, tides are essentially developed in oceans, and no closed water body of today bear considerable astronomic tides of their own (Eisma et al., 1998; Pugh, 2004). This is due to the relative weakness of the present day tide generating geophysical forces. Tidal ranges even in large, inland seas with a restricted oceanic connection (Baltic Sea, Black Sea, Caspian Sea) are typically some centimeters only (Eisma et al., 1998). These ranges can be amplified or masked by wind action and changes in the atmospheric pressure. A fundamental difference, however, is that the change in sea level in such systems is less regular than in the systems of higher tidal range due to the relative importance of the non-lunar

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components. Consequently, such a system may be capable of producing sporadically appearing, non-cyclic (non-lunar) or, at best, poorly cyclic rhythmites into the sedimentary record (cf. Wells et al., 2005). The Madre de Dios, Acre, northern Bolivia and Chaco deposits are located more than 2500 km from recognized main sea connections (Fig. 1A). In order to get tidal amplification this far inland, sea connection(s) needed to be profound and the basin sufficiently deep to allow oscillation and resonance (Archer, 1998). This scenario is unlikely for western Amazonia because all reported Early–Late Miocene deposits in that basin represent shallow and low-energy (excluding fluvial facies) settings (e.g. Gingras et al., 2002b). For instance, the late Early–early Late Miocene Pebas Fm. in north Peru is characterized by low energy bay-margin sequences that show commonly evidence of dysoxic conditions (Gingras et al., 2002b; Wesselingh et al., 2002): the presence of strong tidal currents would have broken the water column stratification. Fossil evidence also points to predominately restricted sea connections (e.g. Frailey, 1986; Lundberg et al., 1998; Wesselingh et al., 2002). These northern sea connections appear to be further constrained by the Late Miocene: uplift of northeastern Andes cut off the sea connection through Magdalena Valley and constrained the Caribbean connection around 10 Ma ago (Hoorn et al., 1995). Possibly, the Orinoco and the restricted Quayaquil Bay connections were limited during this time as well (G) (Diaz de Gamero, 1996; Steinmann et al., 1999). The absolute age of 7.72 ± 0.31 Ma from the Yecua Formation suggests that at least part of the Yecua Fm. probably post-dates these connections (Hernández et al., 2005). Consequently, if the southern Peruvian, Acrean, the northern Bolivian and Chacoan deposits were only part of the western Amazonian depositional system, there probably must have been an additional, hitherto unrecognized sea connection closer to these areas (through the Andes or Amazon Valley). The Amazon Valley connection towards the east cannot be discounted to have contributed to the Peruvian, Brazilian and northern Bolivian tidal facies (see palaeocurrents

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below), but the easternmost reported marine-influenced deposits crop out more than 1500 km from the current Amazon River mouth. Therefore, based on current palaeogeographical knowledge, a Paranan Sea origin for these rhythmites appears most plausible. Overall, strata associated with this embayment demonstrate a considerably more marine character than those of the western Amazonian Basin because they bear significantly more marine fossils such as echinoderms, oysters, a variety of sharks and of foraminifera (Cione et al., 2000). The maximum estimated depth of the basin was mid-shelfal and there is evidence of strong tidal currents such as exhumed gravel beds (del Rio et al., 2001). This facies forms in the outer parts of modern tide-dominated macrotidal estuaries such as Mont Saint Michel (France) and Bay of Fundy (Canada) (e.g. Larsonneur, 1975). The passive continental margin along the Atlantic coast of Argentina forms very plausible conditions for the formation of strong tidal currents (Archer and Hubbard, 2003). The Paraná embayment´s mouth was more than 1000 km wide and the basin remained several hundred kilometres width even in northern Argentina (cf. Hernández et al., 2005). In Bolivia, the depositional system was confined by the Andean thrust front to the west and the Brazilian shield to the east narrowing the basin in its upper reaches. Its palaeogeography thus resembles a funnel-shaped estuary, providing ample conditions for tidal amplification. Moreover, in this proximal part of the depositional system, the passive Paranan Basin grade into an actively subsiding Andean Foreland Basin, which may have further facilitated tidal propagation due to lowered gradient. The palaeocurrents of the cross-stratified tidal facies (mainly measured from IHS channels) in Acre and Madre de Dios Sub-basins suggest more than one source for the tidal currents (compiled unpublished field data and data published in Räsänen et al. (1995) and Hovikoski et al. (2005); Fig. 8). Generally, the inland parts of tidal systems are mostly ebb-dominated due to the temporal asymmetry of ebb and flood currents coupled with the effects of riverine flow (e.g. Pugh, 2004). The regional palaeocurrents of the tidal facies

Fig. 7. Comparison between modern Atlantic tidal station data and Miocene rhythmites from the Madre de Dios Sub-basin. (A) Predicted synodic high and low-water tidal station data from Recife, Brazil (NOAA, 2001). Both ebb and flood currents are included. Vertical axis indicate the height of tides, horizontal axis the event number. A — apogean spring tide, P — perigean spring tide. (B) Fourier transformation of the same data. Roman numerals I–III indicate approximate frequency intervals for flood–ebb, semi-diurnal and neap-spring cycles, respectively. (C) Simulated tidal range data (high-water height minus low-water height) of the data presented in (A). (D) Fourier transformation of the same data. (E) Rhythmite thickness data from the Madre de Dios Fm. Vertical axis shows the relative thickness of sand–mud lamina couplets, horizontal axis the couplet number. A — interpreted occurrence of apogean spring tide, P — interpreted occurrence of perigean spring tide. (F) Fourier transformation of the same data. (G) Rhythmite thickness data from the Madre de Dios Fm. (H) Fourier transformation of the same data. (I) Rhythmite thickness data from the Madre de Dios Fm. (J) Fourier transformation of the same data. The data of (E)–(H) are modified after Hovikoski et al., 2005. The data of (I) and (J) are previously unpublished. The data presented come from localities m) and n) in Fig. 8B.

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Fig. 8. (A) A map of southwestern Amazonia and the Chaco Sub-basin. 1) The study area, 2) Puerto Ramos drillhole of YPFB (Hernández et al., 2005), 3) the Yecua Fm. locations studied by Uba et al. (2005), Hernández et al. (2005) and Hulka et al. (2006), 4) oolitic limestones of the Middle Miocene Anta Fm. (Hernández et al., 2005), 5–8) the study areas of Roddaz et al. (2006), Hovikoski et al. (2005), Gingras et al. (2002a), and Räsänen et al. (1995), respectively. The roman numerals I–III indicate the three regions from which the palaeocurrent data (below) were collected. The rectangle demarcates the location of (B). (B) The paleocurrent data locations. a) and b) Community Katukina, c) Tarauacá, d) Feijó, e) Purus (several locations), f) Oriente, g) Seringal Amapa, h) Boca de San Pedro, i) Assis, j) km 96.5, k) Pastora, l) Belem, (several outcrops), m) Cerro Colorado/ ACCA, n) Cocha Cashu.

indicate dominantly south-eastern flow direction in Cruzeiro do Sul area, Acre (n = 141) and in Puerto Maldonado area, Madre de Dios (n = 164): these are easiest explained by the ebb-dominated tidal flow towards the Paranan Sea. The dominantly south and north oriented palaeocurrent data presented by Uba et al. (2005) from the Yecua Formation is in line with this interpretation. In the Rio Branco area, eastern part of the Acre Sub-Basin (also in some facies of the Madre de Dios Sub-basin), the dominant palaeocurrents are towards northeast (n = 135). This may indicate a tidal influence also through the Amazon Valley from east. However, the data used in the rose diagrams compiled in Fig. 7 may represent different stratigraphic levels and therefore may also demonstrate temporal variability in the palaeogeographical configuration. The Pastora locality that bears the aforementioned absolute age of 8.8 ± 3.2 Ma (Hermoza, 2004) presents south-southwest palaeocurrent directions suggesting a Late Miocene connection. The studied intervals of the Quendeque Formation probably predates those deposits since the study area was already deformed by the thrust

front propagation in Late Miocene (Strub et al., 2005). Consequently, they may correlate with the Pebas Formation, in northeast Peru, the lower Yecua Formation, the Anta Formation and the Río Sali Formation in north Argentina (cf. Hernández et al., 2005; Hulka et al., 2006). 5.3. Biogeographical implications As discussed above, the Miocene epicontinental embayments most likely formed periodically a continuum from the Paraná Basin (Argentina) to southwestern Amazonia (Peru, Brazil) in Miocene. The northern Subandean zone of Bolivia may have represented a proximal part of this system, situated more than 3000 km inland from the possible Orinoco and Caribbean connections (Venezuela) to the open sea and about 2400 km from the Rio de la Plata (Argentina) sea connection (Fig. 1A). As a result, northern Bolivia was dominantly fresh-watered and experienced periodic physical tidal energy that propagated beyond the salt-water wedge during the maximal transgressions. The occasional presence of brackish-water ichnofossil assemblages in

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the Madre de Dios, Acre and subandean zone of Bolivia indicate that at times the connection was saline (Gingras et al., 2002a; Hovikoski et al., 2005). The alluvial and deltaic–estuarine coastal plain environments of Quendeque Formation were deposited constantly close to sea level and thus may have provided a shallow, episodically open dispersal route particularly for mobile fresh-water and euryhaline aquatic taxa between the embayments that experienced rapid temporal and spatial changes in salinity during the Miocene (e.g. Gingras et al., 2002b). The Miocene embayment strata in the southern part of western Amazonia (Madre de Dios, Acre Sub-basins) bear considerable vertebrate fossil similarities with the Paraná embayment deposits. These similarities include croakers (Sciaenidae), dolphins (Saurocetes), stingrays (?Potamotrygonidae), and manatees (Ribodon) (Hamilton et al., 2001; Cione et al., 2000; Boeger and Kritsky, 2003; Brito and Deynat, 2004). Based on the data discussed above and in recent papers (e.g. Hernández et al., 2005; Hovikoski et al., 2005; Uba et al., 2005; Hulka et al., 2006; Roddaz et al., 2006), it seems probable that the Miocene hydrographic connection enabled the faunal interchange of mobile aquatic groups between these areas. The results support a recent study that has explained the modern biogeography of the river-dolphins to be a result of Miocene dispersal and subsequent geographical isolation (Hamilton et al., 2001). The result are also nearly in line with the data of Montoya-Burgos (2003) who estimated that the splitting between Amazonian and Paranan catfish populations occurred around 10 Ma ago based on mitochondrial data. Moreover, the Miocene connection could explain e.g. the current biogeography of stingrays and drums in the both basins (cf. Lovejoy et al., 2006). Despite of these similarities, many of the marine-derived fish in the Amazonian Basin do not occur in the modern Paraná (Lovejoy et al., 2006). Moreover, the Miocene invertebrate fossil faunas of Amazonian and Paranan strata do not show much in common. For instance, the northwestern Amazonian Miocene Pebas Formation contains diverse mollusc and ostracod fossil faunas that are endemic to this region (Whatley et al., 1998; Wesselingh et al., 2002; see also Hoorn et al., 2006). This could indicate that 1) a geographical barrier situated further north from the southwestern Amazonia (Acre and Madre de Dios); 2) the connection post-dates Pebas Formation; 3) the connection(s) were brief and/or 4) that there were ecological barriers. The latter would explain the observation of Hulka et al. (2006) that Yecua invertebrate fossil faunas do not show much similarities neither with the Amazonian nor with the Paranan fossils (exception ostracod Cyprideis Amazonica that occur also in the

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Pebas Fm.) (Whatley et al., 1998; Hulka et al., 2006). Demarcation the importance of these possible factors requires improvement in chronostratigraphical understanding of the western Amazonian formations, more (micro)palaeontological sampling (e.g. in the Madre de Dios Sub-basin) and regional mapping (e.g. in the Ucayali area, Peru). Finally, delineating the precise spatial and temporal nature of the Paraná–Amazonia connection will require core data from the Bolivian forelands before the palaeogeographical development in the area can be adequately understood. 6. Conclusions 1) Strata of the studied interval of the Quendeque Formation are interpreted as alluvial, deltaic and estuarine coastal plain facies. Five recurring facies associations are described. These are: 1) fluvial point bar; 2) flood plain-palaeosol; 3) lacustrine; 4) tidally-influenced point bar; and 5) estuarine/deltaic interdistributary bay. 2) The succession is interpreted to represent a nearequilibrium, transgressive–regressive deposition under a constant relative sea level rise. During slow relative sea level rise, thin fluvial channel bodies were interbedded with soil horizons. During subsequent retrogradation, the volumetric overbank/channel ratio gradually increased and evidence of tidal influence, lateral accretion deposits (IHS) and sporadic trace fossils of probable brackish-water affinity appeared. The zone of maximum flooding is represented by cyclic rhythmites bearing inner estuarine channels and restricted interdistributary bay deposits. The fluvial channels formed complete single-storey channel units throughout the succession: this attest that no forced regression occurred. Considering that no evidence of relative sea level drop was observed, and tidally influenced strata occur on several stratigraphic levels, the deposits most likely were deposited constantly at/or close to sea level. This contradicts the earlier hypothesis that the Quendeque Formation formed a permanent drainage divider between the Paraná and western Amazonian basins. 3) In the light of recently discovered tidal data from south and north Bolivia, southern Peru and western Brazil, the Miocene epicontinental embayments most likely formed temporally a continuum from the Paraná Basin (Argentina) to the south-western Amazonian Basin (Peru and Brazil). The Bolivian lowlands interconnected the embayments and temporally formed a shallow dispersal route particularly for mobile fresh-water and euryhaline aquatic taxa and can explain the observed fossil faunal similarities between these basins and

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