mikes thesis

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.

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


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


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


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


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


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

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These

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2015

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P25

298 Marl

Maret

a

Beac

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Form

ation

Mareta

Beach,

Sagres

[050576

409551]

Gemm

a Mc

Gee &

Micha

el

O'Han

rahan

(2015

)

GMM

H105

Gemm

a Mc

Gee &

Micha

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O'Han

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Respe

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These

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2015

Mus

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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

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These

s

2015

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eum

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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

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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

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Respe

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These

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2015

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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

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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

mikes thesis

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