uIntroduction

uFigure captions

uStructure

uDepositional history

uEnvironment

uReferences

uIntroduction

uFigure captions

uStructure

uDepositional history

uEnvironment

uReferences

uIntroduction

uFigure captions

uStructure

uDepositional history

uEnvironment

uReferences

uIntroduction

uFigure captions

uStructure

uDepositional history

uEnvironment

uReferences

uIntroduction

uFigure captions

uStructure

uDepositional history

uEnvironment

uReferences

Figure and Table Captions

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Structural Development 

The Ainsa Basin is located at the western oblique margin of the South-Central Unit (SCU) (Muñoz, 1992) on the Gavarine thrust sheet (Seguret, 1970) (Figure 1). The central and western part of the south Pyrenean foreland basin is represented by several N-S trending folds observed both in the Ainsa Basin and westward along the Sierra Exteriores (Figure 2). The internal folds in the Ainsa Basin (Figure 3) are interpreted to represent growth folds (Dreyer et al., 1999). The Ainsa Basin is bounded to the east by the Mediano Anticline and to the west by the Boltaña Anticline.

The Mediano Anticline is suggested to be an asymmetrical detachment fold (Poblet et al., 1998) developed at a thrust termination as the displacement is transferred into folding of the leading edge of the thrust sheet (Jamison, 1987). Poblet et al. (1998) suggested that the Mediano Anticline was still active during the deposition of the Escanilla Formation (latest Eocene). On the western side the Ainsa Basin is bounded by the Boltaña Anticline. This anticline is a regional scale asymmetric anticline located above the western oblique ramp of the Gavarine thrust sheet (Holl and Anastasio, 1995). The anticline is suggested to be a fault-propagation fold above a blind thrust (Muñoz et al., 1998). The Boltaña Anticline was active between 50-36 Ma (Anastasio and Holl, 2001), which means that it was active during the deposition of the Escanilla Formation.

Depositional History 

The Escanilla Formation was deposited between late Lutetian and late Priabonian (approximately 43-36 Ma (Bentham and Burbank, 1996)). The Escanilla Formation is mainly sourced from the Pyrenean massif through large valleys, like the Sis palaeovalley (Vincent, 2001) (Figure 1). The Formation is divided into two members, the Mondot and Olson members. The Mondot Memberis a transitional unit between the underlying deltaic Sobrarbe Formation and the alluvial Olson member. The Olson Memberconsists only of alluvial deposits and is unconformably overlain by alluvial fan deposits of the Collegats Formation. During deposition of the Escanilla Formation the transport direction changed from north to west and in the final infill stage towards the south. The Collegats Formation consists of large alluvial fan deposits covering most of the south Pyrenean foreland basin. Maximum preserved thickness of the Escanilla Formation is approximately 1000 meters within the Ainsa Basin. The Formation thins towards the flanks of the Buil Syncline (Bentham et. al., 1992). It is probable that the Escanilla Formation originally was deposited on top of the Mediano and Boltaña anticlines (Bentham and Burbank, 1996), but has later been eroded.

Environment and Controlling Factors 

In the study area the Escanilla Formation is subdivided into three main units, based on changes in the alluvial geometry and architecture. These three units are further subdivided into 7 unconformity bounded sequences (Figure 4).  

Sequence 1 (Figure 4) consists mainly of narrow, sand-dominated channel deposits (FA-7) (Table 1) interbedded with overbank deposits (FA-9 and FA-10) (Table 1), and it is thought to represent an aggrading to slightly prograding coastal plain environment. The lower boundary is transitional from the deltaic Sobrarbe Formation. The architecture and geometry are thought to be controlled by a high accommodation space and high sediment supply. During deposition the shoreline position was controlled by the rate of rise of the Boltaña Anticline, and the interplay between sediment supply and the rate of rise of the Boltaña Anticline controlled deposition in the system.  

The base of sequence 2 (Figure 4) is highly erosive, above which are low-sinuous channel deposits (FA-3) (Table 1) grading into high-sinuous channel deposits (Ss-2.1) (Figure 4). The remaining part of sequence 2 is dominated by FA-6 and FA-7 (Table 1) interbedded with overbank deposits. The system was situated close to the shoreline, and this sequence represents a transition between coastal plain and alluvial plain setting. Sandstone body Ss-2.1 is thought to represent a period of low, but fluctuating A/S-ratio, resulting in a highly amalgamated sandbody with several internal erosional surfaces. The rest of the sequence was deposited during a period of first rise and then fall of accommodation space, with a high and stable sediment supply.  

The base of sequence 3 (Figure 4) marks a large basinward shift in facies; this surface is overlain by braid-plain deposits (FA-3) (Table 1). Sandstone unit Ss-3.1 (Figure 4) can be traced through the entire study area. The sequence boundary is thought to represent a bypass surface caused by a prolonged low-accommodation space and an increased sediment supply. The upper part of sequence 3 is highly dominated by overbank sediments, and it is thought to be deposited during a period of first rising and then falling accommodation space. Sequence 3 marks a change towards a period of decreasing A/S-ratio up to Ss-6.1 and also a gradual change from temperate towards arid climate.  

Sequence 4 (Figure 4) is represented by large lateral and vertical variations within the study area. The lower boundary is placed at the base of an interpreted basin-wide conglomeratic body (Ss-4.1). The sequence was deposited in an alluvial-plain setting, mainly covered by overbank deposits and a few aggrading fluvial channel deposits. The amalgamated conglomeratic channel deposits are thought to have formed by stabilization of the main distributaries by differential movements of intra-basinal folding. In this setting, intra-basinal areas with low accommodation space became barriers for lateral movement of the river systems. The high mud content also stabilized the river systems. There is a clear eastward shift of the system at the level of sandstone unit Ss-4.2 (Figure 4). This shift is thought to represent a gradual rotation of the system brought about by change in direction of the controlling folding. All the coarse-grained material in the system was either deposited in the main aggrading channels, or it was bypassing the system. The accommodation space is thought to have been quite high during deposition, with a gradual decrease upwards. The amalgamated sandstone bodies of the Ss-4.1 level are interpreted to have formed as interplay between increased sediment supply and a low-accommodation space. In the upper part of sequence 4, a marked increase in frequency of calcrete horizons may indicate a reduction in the A/S-ratio and a change towards a more arid climate.  

Deposition of sequence 5 (Figure 4) started with a thick lateral extensive conglomeratic deposit covering most of the study area (Ss- 5.1). The remaining of the sequence is dominated by FA-6 (Table 1) grading into FA-4 (Table 1), interbedded with overbank deposits with abundant, extensive calcrete horizons. Sequence 5 is thought to represent a period with low-accommodation space and low-sediment supply. The main distributary system is interpreted to have shifted out of the study area during deposition of Ss-5.1 (Figure 4), resulting in a high content of fine-grained material.  

Sequence 6 (Figure 4) represents a shift to a hinterland stepping alluvial plain setting, and is introduced by a laterally extensive conglomeratic body. Calcrete horizons are frequently observed. The coarse-grained material is interpreted to have been deposited during large flooding episodes, interbedded with overbank deposits during periods of low drainage, which implies short-lived river systems. Climate is a major factor controlling deposition in sequence 6. The decrease in coarse-grained material in the zone Z-6.3 (Figure 4), was influenced by increasing accommodation space, suggested by the lack of calcrete deposits, and an increased sand and mud content.  

 Sequence 7 (Figure 4), at its base, is a thick conglomeratic deposit interpreted to have a basin-wide extent. In this sequence the first debris-flow deposits are observed (FA-2) (Table 1). Such deposits suggest an alluvial-fan setting. No calcrete horizons are present in this sequence. The accommodation space is suggested to have been low and the rate of sediment supply on average moderate, but strongly fluctuating. Fluctuating capacity of the river system is interpreted to have controlled the abrupt changes between mud and conglomeratic deposits. The available source material may be an important factor explaining the low percentage of sand in this sequence. Creation of accommodation space was faster than the rate of sediment supply. The lack of coarse sediments to fill in the accommodation space resulted in only background sedimentation preservation (muddy intervals). The lack of lacustrine deposits is thought to reflect the dry climate and the low potential for lakes. Some of the deposits in the upper unit may have been deposited in short-lived lakes, but no firm evidence was found. Sequence 7 is unconformably overlain by the alluvial-fan deposits of the Collegats Formation (Figure 4). Sequence 7 may be interpreted as the lowermost part of the Collegats Formation, and thus the hiatus on Figure 4 would be situated at the base of this sequence.

References 

Anastasio, D.J., and Holl, J.E., 2001, Transverse fold evolution in the External Sierra, southern Pyrenees, Spain: Journal of Structural Geology, v. 23, p. 379-392.

Bentham, P., and Burbank D.W., 1996, Chronology of Eocene foreland basin evolution along the western oblique margin of South-Central Pyrenees, in Friend, P.F. and Dabrio, C.J., eds.,  Tertiary basins of Spain: the Stratigraphic Record of Crustal Kinematics: Cambridge University Press, Cambridge, p. 144-152..

Benham, P., Burbank D.W., and Puigdefàbregas, C., 1992, Temporal and spatial controls on the alluvial architecture of an axial drainage system: late Eocene Escanilla Formation, southern Pyrenean foreland basin, Spain. Basin Research 4, 335-352.

Dreyer, T., Corregidor, J., Arbues, P., and Puigdefàbregas, C., 1999, Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain: Sedimentary Geology, v. 127, p. 127-169.

Holl, J.E., and Anastasio, D.J., 1995, Kinematics around a large-scale oblique ramp, southern Pyrenees, Spain: Tectonics, v.  14, p. 1368-1379.

Jamison, W.R., 1987, Geometric analysis of fold development in overthrust terranes: Journal of Structural Geology, v. 9, 207-219.

Millan, H., 1994, Estructura de las Sierras Exteriores Aragonesas (Tesis Doctoral): Universidad de Zaragoza, 213 p.

Muñoz, J.A., 1992, Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section, in McClay, K.R., ed., Thrust Tectonics: Chapman and Hall, London, p. 235 -246.

Muñoz, J.A., Arbues, P., and Serra-Kiel, J., 1998, The Ainsa Basin and the Sobrarbe oblique thrust system: Sedimentological and tectonic processes controlling slope and platform sequences deposited synchronously with a submarine emergent thrust system, in Hevia, A.M., and Soria, A.R., eds., Field Trip Guidebook of the 15th International Sedimentological Congress, Alicante, p. 213-223.

Poblet, J., Muñoz, J. A., Travé, A., and Serra-Kiel, J., 1998, Quantifying the kinematics of detachment folds using three-dimensional geometry: Application to the Mediano anticline (Pyrenees, Spain): GSA Bulletin v. 110, no. 1, p. 111- 125.

Seguret, M., 1970, Etude tectonique des nappes et series décollées de la partie centrale du versant sud des Pyrénées. Thèse Montpellier.

Vincent, S. J., 2001, The Sis palaeovalley: a record of proximal fluvial sedimentation and drainage basin development in response to Pyrenean mountain building: Sedimentology v. 48, p. 1235-1276.  

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