GeoResources - Clastic Shelf-Basin Transition at Orcau, Tremp Basin, Aren Formation, Maastrichtian


Virtual Geology - Pyrennees Foreland Basin and Antiformal Stack Clastic Shelf-Basin Transition at Orcau, Tremp Basin, Aren Formation, Maastrichtian




Virtual Field Trip Stop 5 At Orcau, the complete succession from distal offshore to foreshore can be studied for seimic-scale vertical stacking patterns and lateral depositional geometries. In contrast to all other locations (Stops 2, 3) at the southern flank of the Sant Corneli-Anticline (Stop 4 ), the Aren Formation is strictly subdivided into four large coarsening-upward trends which are interpreted as parasequences. Each of them represents a progradation stage of the coastline and clastic shelf depositional system, bounded at the top my a major flooding surface or a deepening upward trend with several minor flooding surfaces. The farthest progradation width into the time-equivalent Vallcarga Basin to the S-SW is documented by the last Aren parasequence, which outcrops between the village and the hill crest right of the castle ruins. It features coarse-grained, intensively cross-bedded litharenites of the shoreface to foreshore with large-scale slump structures. All four Aren parasequences show internal progradation geometries (hill crest below castle ruins) often associated with thin turbidite channnels (above dirt-track road in the foreground). Well visible is the lateral interfingering between Aren shoreface and Vallcarga proximal offshore deposits. The basinal Vallcarga Formation is visible left of the castle ridge. Internal discontinuity surfaces were triggered by slumping and offshore currents. The Aren parasequences can be physically traced all along the southern margin of the Sant Corneli-Anticline from west of Orcau to Col Faidella (ridge in the background, cf. Stop 2). Aren parasequences at Orcau form excellent analogues for subsurface reservoirs. Litharenites are highly mature with very good sorting, good roundness and no matrix. Lateral contiunuity ranges in the order of 20 km. Vertical connectivity of coarsening upward trends increases towards the anticline. In addition, also connectivity increases in updip direction towards Col Faidella (Stop 2 ). There, all four parasequences are condensed to a single sandstone succession without any recognizable flooding surfaces or transgressive intercalations. The Aren Formation at Col Faideall was situated closer to the southern flank of the Sant Corneli Anticline with higher sediment input an decreased subsidence. Accommodation space was constantly filled to sea level. At Orcau, in relatively larger distance to the Sant Corneli Anticline, higher subsidence rates and decreased input of coarse clastics prevented the constant infill of accommodation space. As a result, sea-level fluctuations are clearly documented in the sediment succession. Orcau-type subsurface reservoirs include trapping by onlap against the anticline and its top erosional unconformity. Alluvial fines of the Tremp Formation act as seal. The only limiting factor in comparable subsurface reservoirs would be carbonate cementation because of increased levels of carbonate detritus in the sandstones. Additional structural-stratigraphic reservoir potential would exist withion the karstified top of the anticline. The core of the western anticline with Late Cretaceous carbonates is visible in the very left of the movie. The right part of the movie shows the center of the Tremp Basin. Fluvial to limnic sediments of the Tremp Formation reach thicknesses of up to 800 m. In the far distance, the Sierra de Montsec mountain range limits the Tremp Basin to the south against the Graus Basin. At the southern foot of the Sierra, a branch of the basal detachment of the Tremp Basin crops out. Together, Tremp and Graus Basin form the South Central Pyrenean Unit (SCPU, piggy-back basin, cf. Stop 17 for Ainsa piggy-bach basin), which was transported towards the south and thrusted on the Ebro Basin. The subsidence history and basin fill of the SCPU and its individual subbasins (Tremp/Graus, Ainsa, Jaca piggy-back basins) were controlled by (i) diachronous orogenic push due to the oblique collision of the Iberian Microplate and the Eurasian Plate); (ii) gravitational movement of the SCPU towards the foreland during times of strong orogenic uplift; (iii) xxxx. Individual piggy-back basins are separated by anticlines, which run oblique to perpendicluar to the foreland axis: (i) Mediano-Anticline, separating the Tremp/Graus and Ainsa Basins (Stop 17 ), (ii) Boltana-Anticline, separating the Ainsa and Jaca Basins (Stop 24 ). The development of these anticlines controlled the change from axial sediment transport through the complete foreland basin until the Early-Mid Eocene (directed ESE-WNW) to transversal sediment transport especially within the Ainsa Basin in the Late Eocene. The movies shows the panoramic view on the Orcau succession as seen from a small ridge, approx. 500 WNW of Orcau.

Selected Literature Ardevol, L., Klimowitz, J., Malagon, J. and Nagtegaal, P.J.C. (2000) Depositional sequence response to foreland deformation in the Upper Cretaceous of the southern Pyrenees, Spain. - Amer. Ass. Petrol. Geol. Bull., 84, 4, 566-587. Sgavetti, M. (1993) Photostratigraphic characteristics of sequence stratigraphic features and patterns: Upper Cretaceous and Eocene strata of the South-Central Pyrenees. - in: Weimer, P. and Posamentier, H.W. (eds.) Siliciclastic sequence stratigraphy, recent developments and applications. Mem. Amer. Assoc. Petrol. Geol., 58, 411-448. Sgavetti, M. (1992) Criteria for stratigraphic correlation using aerial photographs; examples from the south-central Pyrenees. - Amer. Assoc. Petrol. Geol. Bull., 76, 5, 708-730. Puigdefabregas, C. and Simo, A. (1986) Evolution and cyclicity in a tectonically controlled shelf; Aren Sandstone, Maestrichtian, Tremp, south-central Pyrenees. - in: Knight, J.R., McLean, J.R. (eds.) Shelf sands and sandstones. Memoir - Can. Soc. Petrol. Geol., 11, 332. Nagtegaal, P.J.C., van Vliet, A. and Brouwer, J. (1983) Syntectonic coastal offlap and concurrent turbidite deposition; the Upper Cretaceous Aren Sandstone in the south central Pyrenees, Spain. - in: Boersma, J.R. and Terwindt, J.H.J. (eds.) Basin analysis and sedimentary facies; sedimentology at various scales. Sediment. Geol., 34, 2-3, 185-218.


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GeoResources - Clastic Shelf-Basin Transition at Orcau, Tremp Basin, Aren Formation, Maastrichtian

Virtual Geology - Pyrennees Foreland Basin and Antiformal Stack

Clastic Shelf-Basin Transition at Orcau, Tremp Basin, Aren Formation, Maastrichtian