THE AINSA BASIN: - A FIELD TRIP

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1 : - A FIELD TRIP Tom Erik Maast and Lars-Christian Røsberg Universitetet i Oslo, Institutt for geofag. Desember 2006 ABSTRACT The Ainsa Basin is a piggy-back basin part of the south Pyrenean Gavarine thrust sheet, and was separated from the adjacent Jaca and Tremp-Graus Basins during Eocene. The separation of the three basins was due to the development of the Mediano and Boltaña Anticlines in response to thrusting. During this deformation sediments from the Pyreneans were transported to and deposited in the Ainsa Basin and its predescendant basin. The Ainsa Basin is therefore filled in by a shallowing upwards sequence: The San Vincente formation deposited in submarine channels of the continental slope. The Sorbrarbre Delta. Prograding on the continental shelf. The Escanilla Formation. A fluvial depositional system. INTRODUCTION October 2006 the University of Oslo arranged a field course to the Pyreneans. The purpose of the field course was to study tectonic processes, basin infill dynamics and structural framework of the Ainsa Basin, with particular emphasis on the Ainsa Basin as an analogue to potential oil prospects in similar basins. The work done in the Ainsa Basin covers the San Vincent and Escanilla formations. The Rhoda delta in the adjacent Tremp- Graus Basin is also discussed and considered to be an analogue to the Sorbrarbre Delta in the Ainsa Basin. Sedimentological environments and processes will be emphasized. These formations will be discussed, but in order to get an overview of the tectonic setting at the time of deposition, the evolution of the Ainsa Basin from Palaeocene to Oligocene, will be described briefly.

2 Figur 1. Map overview. 1. Locality: San Vincent, Ainsa 1 turbudites 3. Locality: Escanilla Formation 2. Locality: Sorbrarbe Delta 4. Locality: Rhoda Delta REGIONAL GEOLOGY - EVOLUTION OF THE AINSA BASIN The Ainsa Basin represents a piggy-back basin and is part of the south Pyrenean Gavarine thrust sheet (Kjemperud, 2004). It is bounded by the Mediano Anticline to the east and the Boltaña Anticline to the west. The Mediano Anticline separates the Ainsa Basin from the Tremp-Graus Basin in the east, and the Boltaña Anticline separates the Ainsa Basin from the Jaca Basin to the west (figure 2). PALAEOCENE TO EARLY EOCENE In Palaeocene to early Eocene, before the thrusting had generated the Boltaña and Mediano Anticline, the three basins were connected. The Ainsa Basin then functioned as a transfer basin. At this time coastal and delta depositional systems were prograding in the Tremp-Graus basin towards the WNW. These depositional systems with high sedimentation rates sourced turbidites which largely bypassed the Ainsa Basin and were deposited as large turbidite fans in the western Jaca 2

3 Figure 2. Regional overview. Figure from Kjemperud, Basin. The Ainsa Basin was at this time a delta slope depositional environment with turbidite channels (San Vincent formation) (Zühlke, 2005). develop and separated the Jaca, Ainsa and Tremp-Graus basins. The anticlines blocked the ESE-WNW sediment transport. MIDDLE TO LATE EOCENE During the middle to late Eocene the Mediano and Boltaña Anticlines began to LATE EOCENE TO OLIGOCENE In late Eocene to Oligocene deposition of the Sorbrarbre formation and the Escanilla Figure 3. Profile of the Ainsa Basin. Figure from Kjemperud,

4 Formation dominated. The Sorbrarbre Formation is a delta to alluvial depositional system, sourced from the S to SE (Zühlke, 2005) and as mentioned is compared closely to the Rhoda Formation. The fluvial Escanilla Formation is separated from the Sorbrarbre Formation by an erosional unconformity (Zühlke, 2005). SAN VINCENTE FORMATION THE AINSA TURBIDITES Figure 4. Sketch. Ainsa Quarry. Log 1. Ainsa Quarry 4

5 The turbidite channels of the San Vincent Formation were sourced from prograding deltas in the Tremp-Graus Basin in Palaeocene to early Eocene at the time when the Ainsa Basin functioned as a transfer basin. The destination of study, the Ainsa Quarry, is shown on fig.1 and represents the Ainsa 1 turbidite channel. The outcrop was studied as a whole to see the lateral trends of the channel. A vertical section was also logged. Figur 5. Picture. Ainsa 1 turbudites. Logged section in the background. (The Picture is identical to the sketch in figure 4.) LOG AND GENERAL OBSERVATIONS As base datum for the log an erosional surface with clear solemarks such as groove casts and flute casts was chosen. The conglomerate beneath this erosional surface clearly has a different genetical origin then the turbidites, so this surface probably represents the base of the Ainsa 1 turbidite channel. The turbidite channel consists of sandstone beds ranging from only a few tens of cm, or even less, in thickness up to about one or a few meters. These sandstone beds are most likely independent turbidites. This interpretation is further strengthened by the lateral extent of the sandstone beds. They clearly show thinning or thickening laterally, and in some cases a single turbidite could be followed until it died out. The turbidites were both stacked on top of each other as amalgamated beds, and sometimes separated by a layer of mud. These mud layers are most likely either pelagic sediments or the fine grained tail of the turbidite (Bouma D/E). Among the sandstone and mud layers were also beds of conglomerate. These were quite chaotic and hard to trace laterally. A possible interpretation of these deposits would be cohesive debris flows or slumps with clasts well rounded from fluvial systems of the prograding delta in a mud matrix. This should indicate that the turbidite channel is quite close to the delta slope. 5

6 If we zoom out and look at the Ainsa Quarry as a whole, it can be recognized that the beds are dipping slightly towards the north (Nystuen, pers. com.). This indicates that the turbidites here might be part of huge point bars, which indicates that the submarine Ainsa1 turbidite channel probably was meandering at this point. A number of traces where present especially in the mud layers. Here epi relief structures seemed to be especially common. Also the sandy deposits of the turbidites contained some traces. These were typically vertical semi relief and might be escape structures (or attempted escape structures) from benthic organisms buried by the turbidites. BIOTURBATION AND STRUCTURES INTERPRETATION/SUMMARY Observations and work done on the Ainsa Quarry indicate a submarine meandering channel in close proximity to the delta which sourced the turbidites. The channel consists mainly of turbidites and debris flow deposits. Figure 6.Flute casts at the base of Ainsa 1 turbudite channel Groove casts at the base of the turbidite lying immediately above the erosional surface indicate the palaeoflow direction. The direction was measured to be approximately 310 degrees, but a lot more measurements should be done for more reliable results. Despite of this the palaeoflow direction measured fits well into the model of the turbidites being sourced from a delta prograding towards the WNW. 6

7 THE RHODA FORMATION THE PROGRADING RHODA DELTA Figure 7. Overview of Rhoda Delta and Sis Palaeo valley. We have considered the Sorbrarbre Formation of the Ainsa Basin to be genetically similar to the Rhoda Delta. Both the Sorbrarbre and the Rhoda localities are seen on the map (figure 1). LOGG AND GENERAL OBSERVATIONS The Rhoda Delta in the Tremp-Graus Basin represents a shallow marine environment with the prograding Rhoda Delta sourced from the Sis Palaeovalley. The lower parts of the log show mudstone very rich in fossils indicating a low energy environment favouring an abundant and diverse fauna. Above this a sandstone bed a few meters thick, showing crossbedding appears. This might be interpreted as a tidal channel with tidal bundles and muddrapes. Approaching the top of the log sandstone beds dominate. The sandstones at the top of the log represent the delta front. The Rhoda Formation therefore is coarsening and shallowing upwards due to the prograding Rhoda Delta. In terms of Log 2. Rhoda 7

8 delta terminology, the lower, muddy parts of the log represent the prodelta/shelf, while the upper sandier parts of the log represents the delta front. No delta plain deposits were observed at the logging locality, but fossils indicating very shallow water is abundant at the top. INTERPRETATION/SUMMARY The presence of tidal channels and mud drapes tells us that the Rhoda Delta was influenced at least by tidal processes and might be a tide-dominated delta. The presence of the prodelta mud, rich in organisms, the tidal influence and the total thickness of the formation leads to the conclusion that this was probably a shelf delta (shallow water delta). ESCANILLA FORMATION FLUVIAL FLOODPLAIN DEPOSITS Two whole days were spent studying the Escanilla formation at a locality close to Olson (se figure 1). LOG AND GENERAL OBSERVATIONS The Escanilla Formation situated in the middle part of the Ainsa Basin was logged in a scale 1:100. A massive sandstone body was chosen as our datum level, and approximate 51 meters in height was logged above the datum. The whole formation is around 800 meters (pers. com. Jens Jahren) in thickness, and consists largely of alternating mudstone and sandstone layers, lying relatively horizontal, with both light and dark red- Figure 7. Escanillia Formation, Olson in the background. Dotted line shows the logged section. Photo from Nystuen. 8

9 brownish colour, indicating a subaerial environment (oxidation). Some small parts of the log that are marked as mudstone was covered by recent deposited mudstone erosive, and some small uncertainties should be taken into consideration. From the log we can distinguish three main depositional units: Unit 1: Mudstone often several meters up to ten or even more. Unit 2: Fine sand to silty deposits. These deposits were thin, typically around one meter. Unit 3: Medium to coarse sand and gravel deposits. These units were the ones that where studied in most detail. They ranged in thickness from approximately 2-4 meters in the logging area. DISCUSSION OF THE DEPOSITIONAL UNITS LOGGED IN THE ESCANILLA FORMATION Unit 1: This unit was not emphasized due to our strict schedule and because it is more easily eroded and weathered and hence not as well preserved. The reddish colour is due to oxidation and traces of roots might have been observed, in other words this is likely a palaeosol. It should be pointed out that the sand-shale ratio is Log 3. Escanilla 9

10 quite low (<1) in the logged section. The connectivity of the sandstone bodies may therefore be poor and the reservoir qualities of the logged section is not great. mixed load (meandering) river. The water depth is likely to have been close to 3 meters. Unit 2: This unit was often partially covered by erosive material from unit 1 and not easy to study. More work on these units could tell about lateral continuity, grading etc. Some of the units showed a tendency towards reverse grading. Unit 3: These deposits are channel infills. Some features in common for all the channels were: erosive base with paleoflow indicators, thickness of about 2-4 meters and they were all laterally extensive (probably hundreds of meters). We have decided to divide this unit into three types that might classify the channels according to type of load (bed- or suspended load) Type 1: Represented by channel deposits at meters and 30-32,5 meters on the log. Type one is a single, normal graded channel with a gravel lag at its base. Mud blocks that are likely to have been eroded from the thalweg banks of a meandering river were found several places. These were rounded from the flow of the river and probably represent the deepest point of the palaeo channel. Cross-bedding was common in this unit which has been interpreted as a migrating point bars in a Figure 8. Type 1 channel deposits with mud blocks and cross bedding. Type 2: Represented by the channel at 16,5 19 meters on the log. Here several normal graded units are found within a single channel with a gravel lag at its base. This is interpreted as bars deposited in a sandy bedload dominated river (braided). This type of channel has a higher width to depth ratio than type 1. The water depth in this channel was probably around one meter. Type 3: Represented by the coarse grained deposits at the top of the log. These conglomerates were likely to have been deposited in a gravel dominated bedload river (braided). It would have been interesting to study the deposits above to see if the change from mixed load/sandy bedload rivers to gravel bedload rivers continues above the log. Unfortunately 10

11 further studies where impossible because of the topography and schedule, but this change should reflect maybe tectonic uplift or maybe even a change in base level. Poblet et al. (1998) suggested that the Mediano Anticline was still active during the deposition of the Escanilla Formation. This could also be taken into consideration. INTERPRETATION/SUMMARY A section consisting of floodplain deposits (unit 1), crevasse splay deposits (unit 2) and fluvial channel deposits (unit 3) have been studied. The lower most 43 meters of the log is probably dominated by mixed load to sandy bedload rivers (meandering and braided in terms of channel morphology). Above the erosional unconformity at 44 meters it seems to be an abrupt increase in grain size to a fluvial system dominated by gravel dominated bedload rivers. SUMMARY From the fieldtrip we have gained knowledge about the formation and infill mechanisms of thrust generated basins. Perhaps the most rewarding events have been studying the sedimentological successions of the Ainsa Basin and interpreting our observations. Through the localities we have been working on we have seen how the Ainsa Basin has been filled in by a shallowing upwards sequence of sediments, ranging from: The turbidite channels of the San Vincente Formation. Representing the shelf slope to deep marine environment. The Sorbrarbre Delta or its analogue which we studied in more detail, the Rhoda Delta. A prograding shelf delta. The fluvial Escanilla Formation which in a way represents the final infill of the Ainsa Basin. During the deposition of these formations thrusting has been active and separated the Jaca, Ainsa and Tremp-Graus basins which were initially one basin. We have seen evidence of this synsedimentary deformation as the Mediano and Boltaña Anticlines. 11

12 REFERENCES Zühlke, R.(2005): Virtual Fieldtrip Southern Pyrenees Foreland Basin, available at: /zuehlke/virttrip/pyr/stop14/ (Accessed: ) Kjemperud, A., Schomacker, E., Brendsdal, A., Fält, L., Jahren, J., Nystuen, J.P. and Puigdefàbregas, C. (2004): The Fluvial Analogue Escanilla Formation, Ainsa Basin, Spanish Pyrenees: Revisited*, available at: ents/2004/kjemperud/index.htm (Accessed: ) 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