THE AINSA BASIN: - A FIELD TRIP

Tom Erik Maast and Lars-Christian Røsberg Universitetet i Oslo, Institutt for geofag.

Desember 2006

INTRODUCTION 7.-14. 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.

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.

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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

Figur 1. Map overview. 1. Locality: San Vincent, Ainsa 1 turbudites 3. Locality: Escanilla Formation 2. Locality: Sorbrarbe Delta 4. Locality: Rhoda Delta

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Basin. The Ainsa Basin was at this time a

delta slope depositional environment with

turbidite channels (San Vincent formation)

(Zühlke, 2005).

MIDDLE TO LATE EOCENE

During the middle to late Eocene the

Mediano and Boltaña Anticlines began to

develop and separated the Jaca, Ainsa and

Tremp-Graus basins. The anticlines

blocked the ESE-WNW sediment

transport.

LATE EOCENE TO OLIGOCENE

In late Eocene to Oligocene deposition of

the Sorbrarbre formation and the Escanilla

Figure 2. Regional overview. Figure from Kjemperud, 2004.

Figure 3. Profile of the Ainsa Basin. Figure from Kjemperud, 2004.

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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

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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.

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.

Figur 5. Picture. Ainsa 1 turbudites. Logged section in the background. (The Picture is identical to the sketch in figure 4.)

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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.

BIOTURBATION AND STRUCTURES

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.

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.

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

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THE RHODA FORMATION –THE

PROGRADING RHODA DELTA

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

Figure 7. Overview of Rhoda Delta and Sis Palaeo valley.

Log

2.

Rho

da

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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.

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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.

Esc

anill

a

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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.

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 10-13 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

mixed load (meandering) river. The water

depth is likely to have been close to 3

meters.

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

Figure 8. Type 1 channel deposits with ”mud blocks” and cross bedding.

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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.

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REFERENCES

Zühlke, R.(2005): Virtual Fieldtrip

Southern Pyrenees Foreland Basin,

available at: http://www.uni-

heidelberg.de/institute/fak12/geol/sediment

/zuehlke/virttrip/pyr/stop14/ (Accessed:

12.11.06)

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:

http://www.searchanddiscovery.net/docum

ents/2004/kjemperud/index.htm (Accessed:

12.11.06)

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.