mikes thesis
TRANSCRIPT
The Geology Of The Vila Do Bispo Municipality,
Algarve, South West Portugal.
S.S. Mapping Project 2015
Mike O’Hanrahan – 12306206
Department of Geology, School of Natural Sciences, Trinity College Dublin
Word Count: 6179
Abstract
The content herein aims to summarise all data collected during a senior sophister mapping
project conducted in the Vila Do Bispo municipality of the south west Algarve. Thorough
descriptions of all discovered lithologies are presented in section 2. Using observations from
the petrographic to the geomorphological scales, palaeoenvironmental settings are proposed
in section 3 as part of an extensive, coherent geological history.
“Geological age plays the same part in our view of the
duration of the Universe as the Earth’s orbit radius
does in our view of the immensity of space.”
John Joly
TABLE OF CONTENTS
1. Introduction .....................................................................................................................1 1.1. Overview............................................................................................................................................................. 1 1.2. Project Aims ..................................................................................................................................................... 1 1.3. Geographical Setting ................................................................................................................................... 2 1.4. Geological Map ................................................................................................................................................ 4
2. Stratigraphy .....................................................................................................................5 2.1. Introduction ..................................................................................................................................................... 5 2.2. Lithostratigraphic Units ............................................................................................................................ 6
2.2.1. Greywacke-Shale Interbedded Basement ......................................................................................... 6 2.2.2. Red Siltstone .................................................................................................................................................. 9 2.2.3. Basalt ...............................................................................................................................................................11 2.2.4. Dolostone.......................................................................................................................................................14 2.2.5. Limestone Unit ............................................................................................................................................15 2.2.6. Mareta Beach Fm. ......................................................................................................................................17 2.2.7. ‘Fossiliferous Limestone’ ........................................................................................................................20 2.2.8. Late volcanic intrusions ..........................................................................................................................22
3. Geological History........................................................................................................ 24 3.1. Introduction .................................................................................................................................................. 24 3.2. Geological History ...................................................................................................................................... 24
4. Acknowledgements ..................................................................................................... 31
5. References ..................................................................................................................... 32
6. Appendix........................................................................................................................ 33 6.1. Cross section ................................................................................................................................................. 33 6.2. Thin Sections................................................................................................................................................. 34
1
1. INTRODUCTION
1.1. Overview
This thesis is a report which is the culmination of detailed geological mapping
conducted over the summer of 2015. In an attempt to best summarise the geological
data and hypotheses proposed, the thesis focuses on the quantitative and qualitative
observations made during this time in order to formulate viable geological history.
From mid June to mid July 2015 the mapping portion of this project was
undertaken. Terminating with 28 successful days logged in the field books all data was
recorded to the highest achievable standard. All lithostratigraphic observations are
detailed in section 2.2 in progression from oldest to youngest. The corresponding
depositional environments are summarised and illustrated in section 3.2 with a summary
of the overall interpreted history in section 3.
1.2. Project Aims
The overall aim of this mapping project was to produce an informed and detailed
geological map. In order to accomplish this task it was imperative to lay out and
prioritise some aims to capitalise on time spent in the field and over the following
months in the lab.
1. To establish lithostratigraphic packages across the area and record their
respective positions using GPS coordinates and printed military maps.
2. To interpret the depositional relationship between the units from
characteristic facies, compositions and structural relationships.
3. To use the above information to complete a full 1:10,000 scale map with
accompanying cross section.
4. To produce, using everything gathered thus far, a presentable clean copy
(1:10,000) map, an accompanying cross section and a well informed
geological history all to be produced as part of this final report.
2
1.3. Geographical Setting
The assigned mapping area is a long strip of 30km2 on the most south westerly section
of the Iberian peninsula. It lies on one of the furthest reaches of the European continent,
an area of dramatic coastal exposure, parched inland pastures and rolling hills.
Specifically, the area is found to be within the municipality of Vila Do Bispo in the
picturesque region of the Algarve. Encompassing the towns of Sagres, Vila Do Bispo
and Raposeira, this area presents a lengthy coastal cliff exposure from the south to the
south-east extents. Inland is sparsely populated, extremely dry, flat and dense scrub land
used for grazing herds of goats and cattle. To the north, the land is open and meadow-
like with a conspicuous red soil on undulating hills forming the topographic highs of the
location.
Deep valleys are incised by rivers that carry only a small load of water as a
consequence dry of summer climate. The cliffs that form the southern boundaries of the
area are relatively inaccessible for face to face inspection except for the three beach
sections of Sagres, Martinhal and Barranco. Inland outcrops proved to be sparse,
however, large road cuts along the motorway and smaller excavations on country roads
circumvented this issue. Exact area of the map is represented graphically overleaf in Fig
1.1.
3
Fig. 1.1 This figure shows an overlay of two maps to represent the exact location
and dimensions of the mapping area. Corner mapping coordinates:
4105000N; 505000E
4105000N; 510000E
4095000N; 505500E
4095000N; 510000E
4
1.4. Geological Map
Fig 1.2 The figure above is a photograph of the final draft of the geological map.
5
2. STRATIGRAPHY
2.1. Introduction
The stratigraphic succession of this area is best described chronologically, beginning with the
oldest basement unit. The line of section on the map (A-A’) (see cross section in fig. 6.1.1)
best demonstrates the stratigrapahic sequence and how the units appear to spatially relate to
eachother. While the surface area of the map is dominated by deep and shallow carbonate
formations, as detailed below, the older units tell a different story with proposed links to large
continental scale geological events. Younger history has a development of regional uplift
and faulting movement which was followed by recent igneous intrusions of an intermediate
composition. All thin section descriptions are available in the appendix.
6
2.2. Lithostratigraphic Units
2.2.1.Greywacke-Shale Interbedded
Basement
Greywacke-Shale interbeds (turbidites) are the oldest
and one of the most intriguing unit in the area. While
almost all other units prove to be conformable with
each other, the contact is clearly unconformable with
the overlying red siltstone. The orientation of the
beds within the unit striking at near-orthogonal angles
to that of the overlying red siltstone (Fig 2.2).
Occupying the northern section of the produced map
and conferring with students mapping the adjacent
area we infer from the unit’s relative age and
thickness that it is the local basement rock.
Even though the bedding is clearly quite
contorted it is assumed that these beds are turbidites.
The interbedding of the sub angular grained, more
massive beds of the greywacke (immature) rock with
the more fissile shale rocks is indicative of a high
energy deposition followed by a settling of the finer
sands and pelagic sediment. In some localities it is
possible to see scour marks on the base of some
overturned greywacke beds. So we may in this case
be able to constrain the original depositional
environment to be a deep marine shelf, most likely on
a convergent margin.
Taking a detailed look at the unit it is easy to
see a complex structural history within the fabric of
the bedding. Recumbant folds in the weathering
resistant, quartz rich, greywacke beds tell the story of
what was interpreted as at least three compressional
events and possibly more looking at cleavage in the
field. The original diagenetic cleavage of deposition
Fig. 2.1 A red line overlay is used to show the internal deformation of the bedding
surfaces. The broken yellow line indicates a small thrust fault (hanging wall to the
right/south).
Colour
Fresh: Dark tone, green
hue
Weathered: Black-
Grey / Red-Purple
Clast Information
Sorting: Poor (in
greywacke)- Moderate (in
shale)
Clast Composition:
Greywacke
Quartz
Feldspars
Lithics
Matrix
Opaques
Shale
Quartz
Clay Minerals
Feldspars
Illite
Lithics
Roundness: Sub
angular (Greywacke) –
Sub rounded (Shale)
Clast Distribution:
60% (F-M sand)
8%
5%
20%
7%
50% (Silt - Mud)
30%
13%
4%
3%
Other Information
First Field Encounter: Locality no. 702
7
(S1) is overprinted and convoluted by an identifiable S2 and S3, possibly more, within the
more friable shale beds while the greywacke beds display a less ductile deformational fabric.
It is suspected that this unit has experienced high pressure, low temperature deformation so
we observe internal brittle failure.
Detailed geochemical research defines the age of this unit as reaching the late
Moscovian (Carboniferous period) and the original source area comprised of felsic and
recycled metasedimentary rocks (Jorge et al., 2013). If this age is taken as true and the strike
distribution of the unit is plotted on a rose diagram to show frequency the data plots an
average strike of 357.3. Some publications classify late palaeozoic fold belts as being
variscan if they have an age between 380-280Ma. It might be concluded that our turbidite
beds were deposited in the Moscovian and later uplifted in the variscan orogeny in which the
continents of Eurameric (Laurussia) and Gondwana collided to form the supercontinent of
Pangaea.
Fig. 2.2 Shows a rose diagram illustrating the distribution of strikes within the bedding of the
deformed basement.
8
9
2.2.2.Red Siltstone
The red siltstone unit is a lithology that
unconformably drapes over the basement greywacke-
shale turbidites. Displaying both fluvial and eolian
facies to the north and possibly shallow marine facies
further south. This is the oldest unit in the area that
shows to conform to the regional strike and dip
(average: 071/17oSE).
Aeolian depositional setting may be inferred
by large dune structures in the Vila do Bispo and
Raposeira areas. Laminated beds of the red siltstone
show large scale cross stratification, best seen in a
road cut between the two towns most notably at
locality no. 1706. Meter scale dune foresets are
apparent with a bimodal grain size distribution
between laminae are both strong points of evidence in
favour of an eolian transport setting. Most eolian
dunes comprise of mature sediments rich in quartz
but coastal marine sediments also include some high
concentrations of heavy minerals and lithic
fragments. This gives fair reason to classify these
sands as near-coastal eolian dunes.
Fissile claystone bedding is common within this unit showing reduction horizons.
Migration of groundwater is likely to be the cause of the reduction horizons where ferric
oxide (Fe2O3) changes in places to a more soluble ferrous oxide (FeO). The shape of these
reduction spots seems to support the theory of channelisation within these beds. It is possible
that these claystones are the result of braided channel deposition.
This unit then grades into a coarser, more dense arrangement towards the top.
Bedding towards the top of this unit proves to be thicker and coarsening upwards. Quartz
grains present are well rounded and much larger (2-4mm), this is an indication of the further
transgressive nature of the palaeoshoreline with the energy increase of the deposition
reflected in the grain size increase.
Fig. 2.1 Outcrop of the red siltstone as described at locality 701.
Colour
Fresh: Medium tone, red
hue… Pale grey horizons.
Weathered: Pale tone,
red hue
Clast Information
Sorting:
Well sorted
Clast Composition:
Quartz
Lithics
Hematite
Roundness:
Well rounded
Clast Distribution:
>60% (Silt- fine sand)
20±10%
>7% (Red hue)
Other Information
First Field Encounter: Locality no. 701
Porosity: High (10%)
Classification: Lithic Arkose
10
The literature focusing on palynostratigraphic of this region assigns the name ‘Grés
de Silves’ to this lithology. The rare occurrence of Praecirculina sp. (a pollen) and
Tiradispora (a spore) concurrently with bivalves indicates that this unit is Upper Triassic to
Hettangian in age (Doubinger, 1970).
Fig. 2.2 This QFL diagram is useful to classify this siltstone based on grain composition. The
red dot shows that this lithology plots within the lithic arkose field.
Fig. 2.3 Hand sample of the red siltone lithology with laminations as a result of fluctuating
hematite inputs.
11
2.2.3. Basalt
Named an ‘Igneous Tuff’ in the field we have reason
to revise this name in light of evidence from thin
sectioning. Appearing to be semi-consolidated
lithology with a green tint that is owing to the olivine
content, the unit is, in actuality, a thick basaltic
sequence of variable depositional features.
The oldest section of this lithology does not
show bedding features. Higher up in the stratigraphy
it can be noted to have a basaltic flow lithology of
several meters thickness. This, in turn, is underlying a
lithology of similar composition to the first,
coarsening upward as a tuff that may have been sub
acqueous in deposition a sample of which is pictured
in Fig 3.2 below.
The field classification as an igneous tuff was
based on the uncertainty surrounding the depositional
environment of the unit. The lithology was comprised
of a fissile, ash-like pale grey clay matrix with
porphyritic crystals of pyroxenes and olivines.
Crystals appeared cracked and rounded which might
suggest a reworking of the original mafic material. This material is, however, extremely
weathered and fissile leaving little to interpret with regards to bedding features.
Within parts of the unit it can be noted that there is an occurrence of marl and pale
dolomite which acted as a useful marker horizon for the top of the unit (noted at localies no.
1004, 1803 and 1602). Within this horizon it should be noted that there were slight anticlinal
folds plunging to the west indicating a complex structural relationship between this unit and
the overlying stratigraphy. A dyke within the oldest section of this unit also shows
deformation owing to a compressional setting Fig 3.3.
Mineralogically the unit can be classified as a tholeiitic basalt due to the presence of
olivine, clinopyroxene (augite) and Feldspars (An>50). Feldspars assume a basaltic texture of
being randomly orientated laths. Pyroxenes are subhedral and show exsolution lamellae of
orthopyroxene, showing that this basalt may have been quickly erupted before full growth of
Fig. 3.1 Shows the basalt unit at locality
Colour
Fresh: Dark tone, green
hue
Weathered: Lighter tone
, green hue (fig.XX.XX)
Clast Information
Texture: Porphyritic
(aphanitic ground mass)
Clast Composition:
Plagioclase (An>50)
Clinopyroxene (aug)
Olivine
Opx (exsolution)
(Clay minerals)
Classification: Tholeiitic
basalt
Clast Distribution:
40%
30%
5%
<2%
(approx. 20%)
Other Information
First Field Encounter: Locality No. 901/902
12
crystal could be established. This rock can be classified as a tholeiitic (silica oversaturated)
basalt.
Across the area this unit shows to have considerable variability in depositional
features. In most places onion skin weathering can be noted, which may suggest that this was
once a sub aqueous basaltic pillow lava. Basalts are highly susceptible to weathering which
might explain the fissility. Pillow lavas however are typically noted to contain an certain
amount of glass from the quenching nature of their emplacement which is clearly not present.
Onion skin weathering may be accounted for by the warm climate expansion and contraction
of the deposit.
Because this basaltic unit is overlying triassic red siltstones we might infer that these
deposits are late triassic, early jurassic in age. To consider the regional tectonics of the time,
it should be noted that the ‘Central Atlantic Magmatic Province’ is known to have
contributed tholeiitic basalts to the regional geology around this time period. Taking this to
be true we might consider that this unit represents the first signs of rifting of the Atlantic
ocean in the jurassic.
Fig. 3.2 Hand sample from the top of this igneous unit which was extremely friable.
Shows to be very coarse compared to younger sections of the unit with larger
porphyritic pyroxene crystals. This section may indeed be a semi consolidated tuff.
Coarsening upwards within the bed would suggest that this could be a sub aqueous
deposit. It is possible that it represents a period of resedimentation of the extruded
material.
13
Fig 3.3 An dyke showing structural deformation from a north-south compressional event
within the oldest basaltic deposits.
14
2.2.4. Dolostone
Dolostone is a calcareous rock that is a direct
diagenetic descendent of a limestone. After
deposition as a limestone burial and diagenesis alters
the Mg2+/Ca+ ratio in the calcite of the limestone
resulting in a variable amount of recrystallisation of
dolomite to occur in situ. This can cause some issues
in the field as detailed below.
The dolostones of the region are a unit of
considerable surface area on the map.
Stratigraphically this dolomitic unit is calculated to
be about 30m thick but can show a huge variation in
characteristic across an outcrop. The most reliable
field procedure to diagnose a dolomite proved to be a
scratch test to determine the hardness of the lithology
and an acid test (0.5% HCL) on the scratch. Using
this test we characterised dolostones on their
susceptibility to be scratched by a stainless steel
hammer (Dolomite H= 3.5-4) and the reaction with
acid is to be very slight in a highly dolomitised
limestone (calcite would be noticeably more
vigorous).
Bedding features in the dolostones are rarely
apparent except for some cliff sections. In the
majority of outcrops bedding in the dolostones proves
to be relict which is a feature commonly associated
with dolomitisation. Within the inland sections of the
map reprecipitation of calcite from rainwater erosion
causes the rock to form a smooth, fissile, carbonate crust covering outcrop. While dolostones
have very little to tell us in the field, we can at least conclude that because they are typically
formed from the diagenesis of limestones, this unit was deposited in a marine shelf
environment. Further transgression of the palaeoshoreline would be needed to support these
findings.
Fig. 4.1 This figure shows a pronounced dextral fault scarp (locality 601)
Colour
Fresh: Tone; dark-light, Hue;
purple/red/orange/pink/brown
Weathered: Light tone,
brown to creamy hue
Clast Information
Sorting: Well sorted
Clast Composition:
Dolomite
Roundness: Sub
angular to sub rounded
Other Information
First Field Encounter: Locality no. 201
15
2.2.5. Limestone Unit
The limestone succession of this mapping area is one
of the defining units of the region. Displaying at least
three identifiable lithologies within the unit we can
observe a further deepening of the marine
palaeoenvironment. Autochthonous grains, bioclasts,
overall bedding features and lithologies are useful in
determining the depositional history of this unit.
The limestones of locality 1102 is in direct,
sharp contact with the underlying dolostones. The
lithology is oolitic and occurs in several other
locations (2003, 2404 and 2505), similarly in contact
with the dolostones. The lithology contains a high
proportion of autochthonous ooidal/ peloidal grains
described in sample no:7. A phreatic isopachous
cement is observed and secondary porosity has been
regained by dissolution of the coarse mosaic cement.
The heavily micritised ooidal packstone would be
indicative of a shallow marine shelf environment.
Bedding is restricted to localised deposits at the base
of the limestone limiting depositional morphology to
being a possible sand shoal deposit.
Stratigraphically overlying oolitic limestone
deposits is a thick succession of bioclastic
wackestones. A heavily micritised lithology that
appears to be in excess of 25m thick containing
skeletal grains listed in the right hand table. This
detrital limestone represents a sediment of deeper
deposition than the subtidal shoals of the oolitic beds.
Originally lying flat the bioclastic wackestone shows
to be quite buckled by an E-W compression showing
as southward dipping slight anticlines east of Praia
Dos Rebolinhos.
Fig. 5.1 Anticlinal buckling of locality 404
Colour
Fresh: Light tone, white
– creamy hue
Weathered: Red staining
from soil
Clast Information
Oolitic Limestone
Sorting: Very well
sorted
Cement: Calcite (5%)
Skeletal Grains:
Brachiopod
Trilobite
Foraminifera
Algae
Crinoid
Folk: Oomicrite
calcarenite
Bioclastic Limestone
Sorting: Very well
sorted
Cement: Calcite; bladed
prismatic, coarse
mosaic.
Skeletal Grains:
Gastropoda
Foraminifera
Ostracoda
Brachiopod
Folk: Biomicrite
Roundness: Well
rounded
Micrite: 50%
% Abundance :
15-20%
8%
6%
5%
<4%
Dunham: Ooidal
packstone
Roundedness: Sub
angular- Sub rounded
Micrite: 75%
Modal grain size: 0.3mm
% Abundance:
10%
<2%
<1.5%
<1%
Dunham: Bioclastic
Wackestone
Other Information
First Field Encounter: Locality no. 404
16
Also described is a micritic limestone of containing very little skeletal grains toward
the top of the succession. Low energy, gradual, slow deposition leads to the formation of
thick deep shelf deposits that outcrop in full on the western side of the barranco beach
location (Locality no. 2708).
17
2.2.6.Mareta Beach Fm.
Mareta beach, on the south end of the town of Sagres,
consitutes a unit of three distinct lithological
groupings. The first is a an interbed of detrital
limestones with alternating layers of bioturbation.
The second is a 40m thick succession of detrital
limestones interbedded with marlstones. Crowning
this succession is a unit of coarse nodular dolomite
which constitutes the prominent Ponta da Baleeira.
The detrital limestones occur on beach level
and appear as a domed outcrop in the most south
western portion of the designated mapping area. The
rhythmic layers of bioturbation are telling of periods
of alternating depositional energy. The detrital
limestones show to be fining upwards. Bioturbation
on the surface of the layers appears as well defined
trace fossils of the ichnogenus zoophycos, commonly
associated with sediments deposited in the bathyal
zone. Zoophycos is typical of deep marine sediments,
particularly common in turbidite beds, where
polychaete worms proliferate and feed between
depositional pulses (Seilacher, 1967).
Above this deposit is a roughly 100m thick
succession of interbedded limestones and grey marls.
Dinoflagellate cysts in this succession have been used
to indicate the age of the Mareta succession as
Callovian in age (Middle Jurassic) (Borges et al., 2012). Towards the top of this succession
bedding grades into detrital limestone beds. The bedding is clearly affected by several
slumping events toward the south which show an interior fabric akin to normal faults within
the bedding which appear sigmoidal on a meter scale, no decollement sufaces further out on
the point were pinned down because of tide restricted access. Tectonic processes in the
middle to upper jurassic that seem to have resulted in the slumping of these interbeds are
likely to have been the cause of the folding of lower to mid jurassic strata. The striking
Fig.6.1 The purple bounding line shows the
limestone-marlstone package. The green lines denote interior slip surfaces of the
slumps (slumping toward the south). The red
section bounds the overlying nodular dolomite bed.
Colour
Fresh: Light grey (marl)
– pale cream (limestone)
Weathered:Dull brown to
sandy yellow (limestone)
– Pale grey to sandy
yellow (marlstone)
Clast Information
Limestone
Sorting: Moderately
well sorted
Clast Composition:
Micrite
Skeletal grains
Bivalva
Echinodermata
Roundness: Well
rounded, High sphericity
Clast Distribution:
40%
30%
Other Information
First Field Encounter: 101
18
similarity between this unit and with that at the west of ‘Praia do Barranco’, considering
slumping, lithological description and zoophycos trail concludes that these lithologies linked
in deposition and time, although the ‘barranco’ limestone marlstone interbeds do not have the
same distinct layer of dolomite capping the cliff.
Laying conformably above the slumped limestones is a heavily dolomitised limetones
with a high concentration of hematite nodules. The dolostone forms a coarse, crystalline unit
that caps the cliff at a thickness of about 30m.
Fig. 6.2 Shows a photograph of the ichnogenus zoophycos (outlined in red) taken from above
the bedding surface at locality no. 101. (Lens cap= 82mm)
19
Fig. 6.3 Illustrated the turbidity current depositional mechanism leading to the interbedding
of the detrital limestones and grey marls.
20
2.2.7.‘Fossiliferous Limestone’
One unit worthy of mention is the relatively
young ‘fossiliferous limestone’ of locality no.:
1104. Found just north of Sagres town in the
valley of Rio Da Sagres this lithology appears as
a unit of massive bedding at least 18m thick just
to the western edge of the mapping area. This
semi-consolidated rock is comprised of shell
fragments and it’s placement in the general
stratigraphy is uncertain. Shell fragments range
from a imbricated bivalve hash (3mm avg.) to
large imprints of gastropods (15mm avg.) and
mollusca (up to 55m) shells. Held together with a
calcite cement, this cement is likely
syndepositional precipitation of calcite on the
grains. The unit is normally graded, not well
bedded but the grains show a general imbrication
of grains N-S.
A revision of the classification as a
‘fossiliferous limestone’ in favour of the term
coquina on account of grain size and constituting
organisms is necessary. A fossiliferous limestone
generally contains organisms and skeletal fragments that contributed to the creation of the
limestone. This coquina is indicative of a higher energy marine depositional environment,
this is possibly a raised beach scenario at the top of the stratigraphy, however the unit is not
laterally continuous across the map. If there is in fact deposition occurring in a higher energy
marine environment then it can be taken that the last step in our depositional history is telling
of a regression in sea level.
Fig.7.1 This figure shows one medium sized (3cm wide) shell fragment of a cardita
(bivalve mollusc) (Lens cap= 82mm)
Colour
Fresh: Sandy yellow
with a pink streak in
places
Weathered: Smooth and
dark from lichen
Clast Information
Sorting: Poorly sorted
Shell fragments
Clast Composition:
Bivalva (hash)
Gastropoda
Mollusca (Broad ribbed
cardita)
Roundness: Angular to
sub rounded
Clast Distribution:
85%
12%
3%
Other Information
First Field Encounter: 1104
21
Fig. 7.2 Shows the recovered sample of the coquina limestone. Identifiable in the
sample are some imbricated, closely compacted bivalva and an imprint of a gastropod shell.
22
2.2.8.Late volcanic intrusions
In this mapping area the dykes of the area are
genetically linked despite alteration appearance in
outcrops. Outcrops of this kind can appear
differently depending on alteration, colour is the
main difference that causes confusion and results
in the thinking of having different emplacement
events.
The mineralogy shows that the lithology
originates from an intermediate magma source.
High concentrations of hornblende and pyroxenes
would put the composition of this episode in the
region of an andesite. Observing extinction angles
between feldspar twin lamellae the albite/
anorthite content of the feldspars can be
established. The angle proves to be <30O,
diagnosing the feldspars as being less than or
equal to 50% An in composition. Whatever is
feeding the dykes of this region is originating
from a melt semi-depleted in CaO, MgO and FeO.
The porphyritic texture is indicative that it has
spent some time residing in the lithosphere
allowing the hornblendes and clinopyroxenes to
grow before being emplaced and cooled by the
country rock.
The dykes observed in this mapping area
are clearly occurring as one of the youngest
events in the geological history as they are
abserved to have cross cut all of the units listed
above. Dykes, by definition, signal an extensional
phase in the history of the region. Taking the path
of least resistance through the stratigraphy the
dykes are observed to have advantageously intruded through some of the pre existing fault
Fig. 8.1 A photograph of one of the dykes on Praia do Baleeira. Foreground 2.2m wide.
Colour
Fresh: Dark grey- Black
crystalline
Weathered: Purple/
Sandy cream/ Black
Clast Information
Minerals:
Hornblende
Feldspar (An<50%)
Biotite
Opaques
Clinopyroxene
Texture: Porphyritic with
clinopyroxenes (up to
3mm in diameter) and
elongate hornblendes (up
to 12mm) within an
aphanitic ground mass.
Clast Distribution:
35%
25%
20%
8%
<5%
Other Information
First Field Encounter: 103
23
planes in the area. Seen to cross cut the strata of all lithologies listed above it is possible to
use the strikes of each dyke to make some assumptions about the regional tectonics activity.
The rose diagram below in Fig. 8.2 shows that the strike distribution is bimodal with trends
towards towards 006 and 040 respectively.
The bimodal fracturing of the lithologies of such angles could be argued to be
diagnostic of an area of strike slip faulting. In the case of the dykes detailed above, they are
observed to have taken advantage of older fault planes in the rock. The angles of intersection
could in fact, according to the andersonian model of faulting, diagnose an area where the
secondary principle stress (Õ2) is oriented vertically.
Fig. 8.2 Shows a bimodal distribution in the strike of dykes that have intruded
through fault planes throughout the mapping area.
24
3. GEOLOGICAL HISTORY
3.1. Introduction
The geological history of the Vila do Bispo municipality spans two major periods. The oldest
exposed rocks in this area are from the Carboniferous period are clearly unconformable to the
younger units which span from the Triassic up into the Middle Jurassic. In this section it is
attempted to condense all observations into a cohesive history incorporating depositional
environments as well as any significant structural events which may have led to the current
observable fabrics and features.
3.2. Geological History
The oldest unit in this area is the greywacke-shale interbedded basement. As detailed above
this unit is comprised of turbidites that have been heavily folded. Originally the turbidites
were, by definition, layed down by turbidity currents in a deep marine shelf environment on a
convergent boundary. The original deposition is considered to have occurred during the late
Moscovian (Jorge et al., 2013).
Fig. 3.1 An illustration of the original depositional environment of the greywacke-shale
interbedded basement.
25
Since the 1960s fold belts formed between 380 to 280 Ma the name Variscan is
typically used to refer to the major orogenic period occurring around this time. When the
continents of Laurussia and Gondwana collided to form the supercontinent of Pangaea. It
may be inferred that the deformation observed is a direct result of this major tectonic event.
Folding within the beds is difficult to quantify because of the multiple deformational events
but the overall axial planes are striking northwest – southeast as in Fig. 3.2.
Fig 3.2 Shows the average axial plane of the folding (and the poles of the bedding) within the
greywacke-shale interbedded unit.
Following the uplift and some erosion of what is now the basement the red siltstone
was deposited unconformably on top in the Hettangian (Doubinger, 1970). Large cross
bedded foresets observed in the Raposeira road cuts (locality no. 701) along with a high ferric
(Fe3+) iron content implys an arid aeolian coastal depositional environment. The resulting
unconformity is illustrated in Fig. 3.2 and Fig. 3.3 the latter details the mechanism of grain
transport leading to cross beddding in an aeolian setting. This arid environment changes
however as a result of the intracontinental break up of Pangaea.
26
Fig. 3.3 Geological setting that led to the siltstone unconformity over the .
Fig. 3.4 Mechanism of grain transport that leads to cross bedding in an Eolian setting.
Basalt is deposited on top of this red siltstone in an enigmatic change to the overall
sedimentary setting. The mineralogy of this basalt is of tholeiitic affinity which, considering
modern analogues, is typical of a mid ocean ridge basalt (MORB). Deposits such as that in
Fig. 3.1 which might be a pillow lava deposit or simply onion skin weathering as a result of
the warmer climate. Considering the bed is extruded conformably on top of the siltstones
constraining the volcanism to be at least the jurassic. Contemporaneous to this extrusion of
volcanic material the supercontinent Pangaea was undergoing an extreme intracontinental
rifting stage resulting in ‘The Central Atlantic Magmatic Province’ or CAMP. Relative
27
timing and mineralogy might encourage the conclusion that this deposit is linked to the break
up of pangaea (see Fig. 3.5 below).
Fig. 3.5 Illustrates the extent of the CAMP basalts within the supercontinent of Pangea
(Green). Image adapted from: (Blackburn et al., 2013).
Stratigraphically above the Jurassic basalts begins a thick succession of carbonate
rocks beginning with the dolostone unit. The dolostones are a result of diagenetic alteration
of sub marine limestone deposits prior to lithifaction causing most bedding features and
facies to be lost. Subsidence of the region is inferred, however, turning the progression of this
28
geological history into the story of regional transgression of the palaeoshoreline. Regional
extension was occurring around this time relating to alpine orogenic tectonics where the
African plate subducted underneath the Eurasian plate and this may have caused the
transgressive system that is presented here.
The units are typically younging towards the south with the youngest limestones
showing facies changes typical of a further deepening of the marine environment. From
shallow ooidal shoals to deeper carbonate shelf micrites these Jurassic rocks are a further
testament to the transgressive nature of this region with fauna in younger stratigraphy
becoming significantly more sparse and the lithology becoming increasingly micritic.
(see Fig. 3.6).
Fig. 3.6 Illustrates a possible depositional setting to account for the lateral facies changes
observed across the area within the limestone unit.
The maretta beach formation, linked to the barranco beach slumped limestones, are
the deepest marine deposits in the mapping area. The turbidite deposits are deep marine
detrital limestones and the zoophycos ichnogenus is further testament to this setting
interpretation. The deep marine deposit is likely one of the deepest deposits before a
regressive event interpreted through the youngest lithologies. Within the bedding of this unit
southward slumping features are commonplace, it is believed that this is linked to the
regional tectonic uplift. Such uplift is likely to have caused regression in this basin while the
29
compressive nature of tectonic uplift may also be resposible for the bucking of the jurassic
limestone units detailed above (See Fig. 5.1). The unit is dated to be callovian in age (middle
Jurassic) (Borges et al., 2012) and is overlain by a nodular dolostone which could mark the
beginning of a regressive cylce in the middle Jurassic. The youngest unit, the coquina
limestone, is certainly a tidal shallow marine deposit that confirms a late regression in the
history of this area although it’s thickness, extent and exact stratigraphic position is unknown
as the locality exists outside the mapping area.
Fig. 3.7 Is an illustration to explain what may have occurrred to give the contemporary
morphology of the maretta beach formation.
A large dextral fault is clearly seen to run north – south through the eastern side of the
mapping area shuffling contacts accordingly along the river valley deeply incised by the river
‘Benaçoitão’. This river has most likely taken advantage of the weakened fault plane within
the rock and hence created a valley that defines this large fault. We observed that the large
amounts of slip were accommodated by many other smaller faults in the field. In some places
the slip is accommodated by transtensional mounds (locality no. 1303) which geometrically
characterise an area of strike slip faulting. Dykes in the area advantageously intrude through
older fault planes geometrically defining, in accordance with the andersonian model of
faulting, an area of strike slip tectonics (see Fig.8.2). The most obvious evidence of the
30
dextral slip can be seen on the coast in the form of straight, vertical fault scarps on the scale
of 100s of meters as photographed in the dolomite beds of Fig. 3.1 and the flower structures
of locality no. 2604.
The last necessary phase in the evolution of this area to wrap up and explain all the
findings herein is an extensional phase. To allow for the dykes to intrude there must have
been some extensional phase occurring late in the geological history. Normal faulting is
relatively common across the area shuffling the stratigraphy and, like the dykes, affecting all
units. Geomorphological features such as beaches and valleys also attest to a late stage
extension in the evolution of this area.
31
4. ACKNOWLEDGEMENTS
I would like to extend a warm thank you to my mapping supervisor Sean McCleneghan for
selflessly giving up his time to help my colleagues and I. It is with his guidance that this
project was able to come to it’s full fruition.
32
5. REFERENCES
Seilacher, A., 1967, Bathymetry of trace fossils: Marine Geology, v. 5, no. 5-6, p. 413-428.
Doubinger, J., 1970, New details of the stratigraphic series of basic Mesozoic Portuguese: Academy of Sciences
of Paris, p. 270.
Borges, M., Riding, J., Fernandes, P., Matos, V., and Pereira, Z., 2012, Callovian (Middle Jurassic)
dinoflagellate cysts from the Algarve Basin, southern Portugal: Review of Palaeobotany and Palynology, v. 170,
p. 40-56.
Jorge, R., Fernandes, P., Rodrigues, B., Pereira, Z., and Oliveira, J., 2013, Geochemistry and provenance of the
Carboniferous Baixo Alentejo Flysch Group, South Portuguese Zone: Sedimentary Geology, v. 284-285, p. 133-
148.
Blackburn, T., Olsen, P., Bowring, S., McLean, N., Kent, D., Puffer, J., McHone, G., Rasbury, E., and Et -
Touhami, M., 2013, Zircon U-Pb Geochronology Links the End-Triassic Extinction with the Central Atlantic
Magmatic Province: Science, v. 340, no. 6135, p. 941-945.
33
6. APPENDIX
6.1. Cross section
Fig
. 6.1
.1 C
ross
sec
tion
from
lin
e A
-A’.
Corr
espond
ing
colo
urs:
G
ray=
Gre
ywac
ke-
Sha
le i
nter
bed
ded
bas
emen
t; B
lue=
Red
silt
stone
; T
eal=
Bas
alt; B
row
n= D
olo
stone
;
Yel
low
= L
imes
tone
; P
urple
= M
aret
ta B
each
Fm
.
34
6.2. Thin Sections
TCD
nu
mbe
r
Status
(Figur
ed in
public
ation)
Rock
Identifi
cation
Geolo
gical
horiz
on
Geograp
hical
location
and Grid
Referenc
e
Collect
or and
date
Colle
ctor's
field
numb
er
Deter
mined
by
Public
ation
No
tes
Stor
age
loca
tion
P25
294
Bioclas
tic
Wacke
stone
limes
tone
Praia dos
Rebolinh
os
(050797
409822)
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H101
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
295
Biomic
rite
limes
tone
Praia do
Barranco
(050876
410085)
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H102
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
296
Calcite
growth
limes
tone
Praia do
Barranco
(050838
409765)
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H103
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
297
Tholeiit
ic
Basalt
Thole
iitic
Basal
t
monte da
ribeira
abaixo
[050947
410253]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H104
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
298 Marl
Maret
a
Beac
h
Form
ation
Mareta
Beach,
Sagres
[050576
409551]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H105
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
35
P25
299
Dolomi
te
Dolo
mite
Ponta da
Baleeira
[050608
409510]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H106
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
300
Ooidal
Limest
one
Ooida
l
Limes
tone
vale da
Torre de
Cima
[050800
410135]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H107
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
301
Volcani
c Dyke
late
volca
nic
intrus
ions
Monte da
Ribeira
Abaixo
[050965
410274]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H108
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
302
Altered
Volcani
c Dyke
late
volca
nic
intrus
ions
Raposeir
a
[050907
410398]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H109
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
P25
303
Siltsto
ne
baked
margin
late
volca
nic
intrus
ions
Raposeir
a
[050907
410398]
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
(2015
)
GMM
H110
Gemm
a Mc
Gee &
Micha
el
O'Han
rahan
Respe
ctive
These
s
2015
Mus
eum
Buil
ding
36
37
38
39
40
41
42
43
44
45
46
47