THE AINSA BASIN:
- 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
considered to be an analogue to the
7.-14. October 2006 the University of Oslo
Sorbrarbre Delta in the Ainsa Basin.
arranged a field course to the Pyreneans.
Sedimentological
The purpose of the field course was to
processes will be emphasized.
environments
and
study tectonic processes, basin infill
dynamics and structural framework of the
These formations will be discussed, but in
Ainsa Basin, with particular emphasis on
order to get an overview of the tectonic
the Ainsa Basin as an analogue to potential
setting at the time of deposition, the
oil prospects in similar basins.
evolution
of
the
Ainsa
Basin
from
Palaeocene to Oligocene, will be described
The work done in the Ainsa Basin covers
the San Vincent and Escanilla formations.
The Rhoda delta in the adjacent TrempGraus
Basin
is
also
discussed
and
briefly.
THE AINSA BASIN
Figur 1. Map overview.
1. Locality: San Vincent, Ainsa 1 turbudites
2. Locality: Sorbrarbe Delta
3. Locality: Escanilla Formation
4. Locality: Rhoda Delta
REGIONAL GEOLOGY - EVOLUTION
PALAEOCENE TO EARLY EOCENE
OF THE AINSA BASIN
In Palaeocene to early Eocene, before the
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).
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
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THE AINSA BASIN
Figure 2. Regional overview. Figure from Kjemperud, 2004.
Basin. The Ainsa Basin was at this time a
develop and separated the Jaca, Ainsa and
delta slope depositional environment with
Tremp-Graus
turbidite channels (San Vincent formation)
blocked
(Zühlke, 2005).
transport.
MIDDLE TO LATE EOCENE
LATE EOCENE TO OLIGOCENE
During the middle to late Eocene the
In late Eocene to Oligocene deposition of
Mediano and Boltaña Anticlines began to
the Sorbrarbre formation and the Escanilla
the
basins.
The
anticlines
ESE-WNW
sediment
Figure 3. Profile of the Ainsa Basin. Figure from Kjemperud, 2004.
3
THE AINSA BASIN
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.
4
Log 1. Ainsa Quarry
THE AINSA BASIN
The turbidite channels of the San Vincent
The conglomerate beneath this erosional
Formation were sourced from prograding
surface clearly has a different genetical
deltas in the Tremp-Graus Basin in
origin then the turbidites, so this surface
Palaeocene to early Eocene at the time
probably represents the base of the Ainsa 1
when the Ainsa Basin functioned as a
turbidite channel.
transfer basin.
The turbidite channel consists of sandstone
The destination of study, the Ainsa Quarry,
beds ranging from only a few tens of cm,
is shown on fig.1 and represents the Ainsa
or even less, in thickness up to about one
1 turbidite channel. The outcrop was
or a few meters. These sandstone beds are
studied as a whole to see the lateral trends
most likely independent turbidites. This
of the channel. A vertical section was also
interpretation is further strengthened by the
logged.
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
Figur 5. Picture. Ainsa 1 turbudites.
Logged section in the background. (The
Picture is identical to the sketch in figure 4.)
with clasts well rounded from fluvial
systems of the prograding delta in a mud
matrix. This should indicate that the
LOG AND GENERAL OBSERVATIONS
turbidite channel is quite close to the delta
As base datum for the log an erosional
slope.
surface with clear solemarks such as
groove casts and flute casts was chosen.
5
THE AINSA BASIN
If we zoom out and look at the Ainsa
A
Quarry as a whole, it can be recognized
especially in the mud layers. Here epi
that the beds are dipping slightly towards
relief structures seemed to be especially
the north (Nystuen, pers. com.). This
common. Also the sandy deposits of the
indicates that the turbidites here might be
turbidites contained some traces. These
part of huge point bars, which indicates
were typically vertical semi relief and
that
turbidite
might be escape structures (or attempted
channel probably was meandering at this
escape structures) from benthic organisms
point.
buried by the turbidites.
BIOTURBATION AND STRUCTURES
INTERPRETATION/SUMMARY
the
submarine
Ainsa1
number
of
traces
where
present
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
THE AINSA BASIN
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
muddrapes. Approaching the top of the log
sandstone beds dominate. The sandstones
at the top of the log represent the delta
Log 2. Rhoda
tidal channel with tidal bundles and
front. The Rhoda Formation therefore is
coarsening and shallowing upwards due to
the prograding Rhoda Delta. In terms of
7
THE AINSA BASIN
delta terminology, the lower, muddy parts
ESCANILLA
of the log represent the prodelta/shelf,
FLOODPLAIN DEPOSITS
FORMATION
–FLUVIAL
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
Two whole days were spent studying the
Escanilla formation at a locality close to
Olson (se figure 1).
water is abundant at the top.
LOG AND GENERAL OBSERVATIONS
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).
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
sandstone
alternating
layers,
mudstone
lying
and
relatively
horizontal, with both light and dark redFigure 7.
Escanillia
Formation,
Olson in the
background.
Dotted line
shows the
logged section.
Photo from
Nystuen.
8
THE AINSA BASIN
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
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
Log 3. Escanilla
more easily eroded and weathered and
be pointed out that the sand-shale ratio is
9
THE AINSA BASIN
quite low (<1) in the logged section. The
mixed load (meandering) river. The water
connectivity of the sandstone bodies may
depth is likely to have been close to 3
therefore be poor and the reservoir
meters.
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.
Figure 8. Type 1 channel deposits with ”mud
blocks” and cross bedding.
Some features in common for all the
channels were: erosive base with paleoflow
Type 2: Represented by the channel at
indicators, thickness of about 2-4 meters
16,5 – 19 meters on the log. Here several
and they were all laterally extensive
normal graded units are found within a
(probably hundreds of meters). We have
single channel with a gravel lag at its base.
decided to divide this unit into three types
This is interpreted as bars deposited in a
that might classify the channels according
sandy bedload dominated river (braided).
to type of load (bed- or suspended load)
This type of channel has a higher width to
depth ratio than type 1. The water depth in
Type 1: Represented by channel deposits
this channel was probably around one
at 10-13 meters and 30-32,5 meters on the
meter.
log. Type one is a single, normal graded
channel with a gravel lag at its base. “Mud
Type 3: Represented by the coarse grained
blocks” that are likely to have been eroded
deposits at the top of the log. These
from the thalweg banks of a meandering
conglomerates were likely to have been
river were found several places. These
deposited in a gravel dominated bedload
were rounded from the flow of the river
river (braided). It would have been
and probably represent the deepest point of
interesting to study the deposits above to
the palaeo channel. Cross-bedding was
see if the change from mixed load/sandy
common in this unit which has been
bedload rivers to gravel bedload rivers
interpreted as a migrating point bars in a
continues above the log. Unfortunately
10
THE AINSA BASIN
further studies where impossible because
filled in by a shallowing upwards sequence
of the topography and schedule, but this
of sediments, ranging from:
change should reflect maybe tectonic uplift
or maybe even a change in base level.
ƒ
The turbidite channels of the San
Poblet et al. (1998) suggested that the
Vincente Formation. Representing
Mediano Anticline was still active during
the shelf slope to deep marine
the deposition of the Escanilla Formation.
environment.
This could also be taken into consideration.
ƒ
The
Sorbrarbre
Delta
or
its
INTERPRETATION/SUMMARY
analogue which we studied in more
A section consisting of floodplain deposits
detail,
(unit 1), crevasse splay deposits (unit 2)
prograding
the
Rhoda
shelf
Delta.
A
delta.
and fluvial channel deposits (unit 3) have
been studied. The lower most 43 meters of
ƒ
The fluvial Escanilla Formation
the log is probably dominated by mixed
which in a way represents the final
load to sandy bedload rivers (meandering
infill of the Ainsa Basin.
and
braided
morphology).
in
terms
Above
of
the
channel
erosional
During the deposition of these formations
unconformity at 44 meters it seems to be
thrusting has been active and separated the
an abrupt increase in grain size to a fluvial
Jaca, Ainsa and Tremp-Graus basins which
system dominated by gravel dominated
were initially one basin. We have seen
bedload rivers.
evidence
of
this
synsedimentary
deformation as the Mediano and Boltaña
Anticlines.
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
11
THE AINSA BASIN
REFERENCES
Zühlke,
R.(2005):
Southern
Pyrenees
available
at:
Virtual
Fieldtrip
Foreland
Basin,
http://www.uni-
heidelberg.de/institute/fak12/geol/sediment
/zuehlke/virttrip/pyr/stop14/
(Accessed:
12.11.06)
Kjemperud,
Brendsdal, A.,
A.,
Schomacker,
Fält, L.,
E.,
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. 111125.
12