J-1  (N-S Cadí t.s. & Ebro foreland PDF

Balanced and restored geological cross-section J-2, modified from Vergés et al. (1994) and Mató et al. (1994). The balanced section shows the internal structure of the lower Pedraforca imbricate system of thrusts. The lower Pedraforca thrust sheet is composed by 6 different imbricates constituted by Triassic to Paleocene rocks. In the southernmost external units, lower Eocene deposits show syntectonic geometries. The Mesozoic and Paleocene wedges taper towards the south recording the sedimentary infill geometry of the original Mesozoic rifted basin. The length, lateral extension and displacement of the tectonic imbricates decrease to the south where the original stratigraphic wedge is thinner.

The lower thrust of the lower Pedraforca thrust sheet (Pedraforca thrust) overthrusts lower to middle Eocene foreland basin deposits. The geometry of the hangingwall is flat whereas the footwall geometry is a gently inclined ramp.

In the balanced section, the pin point A is located in the northernmost outcrop of the turbidites of the Campdevànol Fm. (footwall of the Pedraforca thrust). Intermediate point B is located at the leading point of the lower Eocene sediments on top of the Pedraforca thrust sheet. Point C is located in the northernmost outcrop of the Jurassic rocks of the Pedraforca thrust sheet.

The reconstruction of the eroded footwall ramp of the Pedraforca thrust has an angle (~7º) similar to the restored basal angle for the Mesozoic wedge. This reconstructed Pedraforca footwall ramp allows the leading zone (front) of the Pedraforca thrust sheet to advance below sea level, with no important internal deformation at the front, and with no major uplift at the back.

Total shortening between points A-C is 39.5 km. Because of the coincidence of points A and C in the balanced section, the shortening is 100%. The internal imbricate thrust system represents 9.5 km and the southwards motion of the whole thrust sheet is 30 km, which can be divided into 17 km for the well constrained part of the ramp and 13 km for the reconstructed part.

Lower Eocene detrital limestones (Cadí Fm.) record the formation of the imbricate system of thrusts at 55 Ma. The southward motion of the Pedraforca thrust sheet over the Eocene foreland basin deposits (currently forming the Ripoll syncline) is recorded by Middle Eocene sediments (Armàncies and Campdevànol formations). The middle Lutetian (~47 Ma) transition sediments fossilise the Pedraforca thrust sheet.

Although the basal Pedraforca thrust was blocked, the Pedraforca thrust sheet was internally deformed by a break-back sequence of thrusting during the emplacement of the lower and younger Cadí thrust sheet. The rate of shortening for the imbricate system of thrusts and southward motion of the Pedraforca thrust sheet (55-47 Ma) is 4.4 mm/a.

Balanced and restored geological cross-section J-3. The balanced section shows the complete structure of the southern Pyrenean thrust sheets, deformed Ebro basin and the undeformed southern margin of the basin linked to the Catalan Coastal ranges. The section will be described from south to north:

A set of different trending folds and thrusts characterise the structure of the eastern Ebro Foreland Basin. These folds and thrusts are detached above the Cardona salt décollement level (Vergés et al., 1992). Cross-section J-3 traverses the Oló, Súria and Cardona anticlines (Link to: Cardona anticline). The Cardona anticline constitutes a gently south verging salt cored anticline ("la Muntanya de sal", Fig. 18). North of the Cardona anticline, the Puig-reig anticline is formed above a footwall ramp where the basal décollement climbs from a lower detachment level to the Cardona salt level. Alluvial and fluvial Upper Eocene-Lower Oligocene sediments show a double syntectonic wedge geometry related to the growth of the Puig-reig anticline. Seismic lines across the detached folds in the foreland show that deformation is localised above the décollement level but not below. Seismic line S-12 across the Puig-reig anticline shows a complex internal structure for this anticline and very gentle north dipping reflectors around 2.1 s (T.W.T.) on the northern side, interpreted as lower-middle Eocene shallow marine limestones. The northern seismic line S-19 shows the northern continuation of lower reflectors and a prominent north dipping reflector which crosses the complete line and is interpreted as the Vallfogona thrust. The footwall of the Vallfogona thrust is constituted by subhorizontal reflectors.

The frontal Vallfogona thrust limits the allochthonous south Pyrenean units (Cadí and Pedraforca thrust sheets). A number of syntectonic deposits  cover the internal structure of the thrust sheets. The geometrical disposition of middle and upper Eocene syntectonic deposits  on both the hangingwall and footwall of the Vallfogona thrust show both the emergence of the Vallfogona thrust above the footwall ramp and a break-back sequence of thrusting deforming the lower Pedraforca thrust sheet.

The upper Pedraforca thrust sheet (based on Vergés & Martínez, 1988, and Martínez et al., 1991) is constituted mainly by Lower Cretaceous rocks (from Prada Fm. limestones to Lluçà Fm. marls). These rocks form a high hanging wall cut-off angle with the basal thrust detached within Keuper evaporites. The upper Pedraforca thrust sheet represents the positive inversion along the southern margin of the Lower Cretaceous extensional basin. The basal thrust climbs up across the footwall Upper Cretaceous rocks incorporating thin slices of these rocks. Paleocene (Garumnian) alluvial (Roca Roja South) and fluvial (Coll de la Trapa) strata overlap the basal thrust recording the latest emplacement of the  Upper Pedraforca thrust sheet.

The present gentle south dipping position of the Upper Pedraforca basal thrust is due to the general southern tilting of thrust sheets during the emplacement of lower and younger basement thrust sheets (compare with westward cross-sections J-6 and J-7).

The reference horizontal lines for the restored cross-section correspond to near-horizontal strata during deposition: the top of the Cardona and Beuda evaporites constitute two excellent levels in the foreland and Cadí thrust sheet. The top of the Paleocene fluvial and lacustrine deposits and the top of Cenomanian limestones (Santa Fe sequence) are traced on the lower and upper Pedraforca thrust sheets. Cross-section J-3 was line-length balanced but not the Cardona salts which were areally balanced.

Total shortening between points A and B is 69.2 km (54%). These can be divided into 11 km for the Upper Pedraforca thrust sheet, 36.2 km for the lower Pedraforca thrust sheet, 16.4 km for the Cadí thrust sheet and frontal blind thrust below the Busa syncline, and 5.6 km for the foreland basin fold and thrust system. 

The well-preserved structural geometries and the well-dated emplacement relationships among different thrust sheets allow the construction of crustal-scale balanced and restored sections. These structural parameters together with fission track cooling ages, paleobotanical assemblages, and sedimentological data were used to reconstruct a crustal scale section during middle Lutetian times (Vergés et al., 1995). A subsidence analysis has been carried out along the J-3 section from the Ripoll syncline to the Montserrat region in Vergés et al. (1998). Geometric analysis of detached folding focused on the Súria and Balsareny anticlines has been carried out by Sans and Vergés (1995) (Link to: Cardona anticline). The tectonic internal structure of the Cardona salt décollement level is described in Sans et al. (1997). Growth folding in Sant Llorenç de Morunys associated with the frontal Vallfogona emergent thrust and fold kinematic models have been published by Ford et al. (1997) and Suppe et al. (1997). Williams et al. (1998) presented a sedimentological analysis of these growth strata at Sant Llorenç de Morunys (Link to: Sant Llorenç de Morunys growth strata).

Link to: Cardona anticline

Link to: Sant Llorenç de Morunys growth strata

Geological cross-section J-4 (the northern side of the section is from Hartevelt, 1970). The section shows the cross-cutting relations between the Pedraforca and Cadí thrust sheets. The western end of the La Cerdanya normal faults cut the Cadí thrust sheet. The relatively rapid northward tapering of the Stephanian, Permian and Buntsandstein red bed deposits is well-preserved in the hangingwall of the major normal fault. As in the previous sections (section J-2 and J-3), the northern flank of the Pedraforca thrust sheet is deformed by a set of subparallel to oblique to the bedding and north directed thrusts.

Geological cross-section J-5. From south to north the section crosses the Puig- reig anticline in the foreland basin, the Port del Comte (Cadí) thrust sheet, the Serres Marginals (Pedraforca) thrust sheet, the Bóixols (upper Pedraforca) thrust sheet and the Montsec de Tost basement unit.

The Puig-reig anticline shows a complex structure at depth. Fluvial deposits on top of the anticline display growth geometries, which attenuate the anticlinal amplitude at surface. Seismic line S-1 presents the Solsona syncline above non deformed and gently north dipping set of reflectors, interpreted as lower to middle Eocene marine sediments including the Cardona salt level at the top. To the north, the Puig-reig anticline shows a tectonic wedge geometry which is characteristic along the front of this anticline (Seismic line S-12 on cross-section J-3). The tectonic wedge geometry continues to the west at the front of the Oliana anticline (Line drawing on cross-section J-6 and cross-section J-7). Internally, the anticline is interpreted as a duplex structure with increasing shortening towards the west. Very gentle north dipping subparallel reflectors in the northern part of the line and located underneath the allochthonous South Pyrenean units account for the interpretation of this area as a south verging syncline. 

The Port del Comte thrust sheet (equivalent to the Cadí thrust sheet) constitutes a thin tectonic unit which is located at the highest topographic elevation of the studied area. The N-S geometry of this thrust sheet constrain the geometry of the Vallfogona thrust at depth, especially the location of the footwall ramp above the continental conglomerates of the northern flank of the buried syncline. 

The Serres Marginals (equivalent to the lower Pedraforca) and the Bóixols (equivalent to the upper Pedraforca) thrust sheets and the Montsec de Tost basement unit overthrusted the Cadí thrust sheet strata showing a slightly oblique footwall ramp.

The Montsec de Tost is the easternmost outcrop of the Nogueres basement unit.

Geological cross-section J-6 that helps to visualise the along strike changes in the geometry of the south Pyrenean thrust sheets (Compare with cross-sections J-5 and J-7). 

From south to north, the Oliana anticline shows a major displacement over the Cardona salt décollement level. Line drawing of seismic line L-3 and Basella-1 oil-well constrain both the geometry of the southern end of the Oliana anticline and the growth strata geometry and thickness of the continental package which is 2500 m thick in the cores of the foreland synclines.

The Port del Comte (equivalent to Cadí) thrust sheet is very reduced, ending towards the west in the footwall of the Segre oblique ramp. The change in dip of the bedding of the Montsec thrust sheet reflects the position of the footwall ramp of the Montsec (Segre thrust). This ramp cuts Eocene-Oligocene continental conglomerates of the northern flank of the buried syncline, north of the Oliana anticline. This syncline outcrops westwards between the section and the Segre river. 

The Bóixols thrust sheet displays a complex structure developed during the tectonic inversion of a set of normal faults defining the southern margin of the Mesozoic extensional basin. Sedimentological facies distribution and thickness changes indicate that the south directed Figols thrust is the major normal fault reactivated. The frontal thrust of the Bóixols thrust sheet is either a small fault or a cut off south of the main fault.

Link to: Oliana anticline

Balanced and restored geological cross-section J-7. The balanced section contains a large number of oil-wells constraining the subsurface interpretation, and shows the complete structure from the southern Pyrenean thrust sheets to the undeformed Ebro foreland basin. The section will be described from south to north:

The southernmost structure of the section is the evaporite cored Sanaüja anticline. The anticline is cut by a set of north directed thrusts. These thrusts were interpreted as developed along the southern margin of the Cardona salt basin. Northwards, the detached Vilanova anticline shows a roughly symmetrical shape. The doubly plunging Oliana anticline is the most important and northernmost structure of the foreland in this section (Link to: Oliana anticline).  Its southwestern end is overthrusted by the Serres Marginals thrust whereas the northeastern end is buried below syntectonic conglomerates. The Oliana oil-well constrains the deep geometry of the anticline, which is constituted by the superposition of two tectonic slices composed of middle Eocene marine marls. These two slices are displaced over the Cardona salt detachment. The southern margin of the anticline acts as a backthrust. On top of the northern flank of the Oliana anticline, the Serres Marginals unit is made of a set of imbricates showing a break back sequence of thrusting as evidenced by four growth strata units, upper Eocene-lower Oligocene in age. The third growth strata unit is related to the Montsec thrust sheet out-of-sequence emplacement whereas the uppermost unit overlaps the Montsec although it is slightly deformed. 

The Mesozoic and Paleocene section in the Montsec thrust sheet abruptly increases its thickness with respect the Serres Marginals one. This rapid and important change in thickness across the Montsec thrust reflects inherited Mesozoic and Paleogene structures. The dip of the bedding on the back side of the Montsec thrust sheet indicates the position of a footwall ramp, which is interpreted in this work as the footwall ramp of the Oliana duplex.

The Bóixols thrust sheet represents the tectonic inversion of the Early Cretaceous extensional basin. It is interesting to observe the lateral changes of this uppermost cover thrust along the strike, especially across the Segre oblique ramp. In section J-7 the frontal thrust is north dipping and the whole thrust sheet shows an almost untilted position. However, in eastern sections such as J- 3, the frontal thrust gently dips to the south and the thrust sheet is tilted to the south by the emplacement of a deeper basement unit. 

The basal thrust of the allochthonous units shows both the south and north directions of tectonic transport. The horizontal reference lines for the restored section correspond to: 1) the top of the Cardona salts (foreland basin), 2) the top of the Paleocene lacustrine deposits for the Serres Marginals and Montsec thrust sheets, and 3) the base of the upper Cenomanian marine limestones for the Montsec and Bóixols thrust sheets. 

The final version of the cross-section represents the final form after the completion of different restorations to account for shortening that occurred during early and middle Eocene times. This shortening is achieved by the displacement of the allochthonous units over lower to middle Eocene marine foreland marly deposits over a ramp of 8º. Total shortening between A and I points is 68 km (56%). This can be divided into 16.9 km for the deformed foreland (13.9 km for the Oliana duplex and 3 km for the southern structures above the Cardona salt layer), 22 km for displacement of allochthonous units above foreland Eocene strata, 1 km for the Serres Marginals, 3.4 km for the Montsec thrust displacement, and 7.2 km for the Bóixols thrust sheet. The Bóixols thrust sheet was fossilised during Paleocene times as evidenced to the west of this section. 

The Monstec and Serres Marginals were finally emplaced at the end of the middle Eocene as confirmed by the composition of the lower units of conglomerates outcropping in the northern flank of the Oliana anticline. The synchronous development of the Oliana anticline and the break back sequence of deformation developed within the allochthonous units during the upper Eocene and lower Oligocene times. 

The relationships between foreland evaporitic basins and the way in which they control the geometry of the foreland folding and thrusting have been studied by Vergés et al. (1992), Sans and Vergés (1995). The triangle zone propagating at the front of a triangle wedge is discussed has been documented by Sans et al. (1996). A previous version of this section has been published in Burbank et al. (1992). The break-back sequence of deformation determined by syntectonic deposits has been published in Vergés and Muñoz (1990) and in Burbank and Vergés (1994). The analysis of growth strata and kinematic of folding has been documented.

Link to: Oliana anticline

Geological cross-section J-8. The section crosses the western end of the Oliana anticline along seismic line TR-65. A north verging hangingwall anticline located in the southern flank of the Oliana anticline highlights the existence of a backthrust parallel to the Cardona salt layer. The seismic line shows complex cross-cutting relationships in the core of the Oliana anticline. As in the previous section, the integration of the Basella-1 oil-well information and field data suggests that the continental conglomerates outcropping in the southern flank of the anticline are growth units and that they developed strong angular unconformities.

Link to: Oliana anticline