Irish Stratigraphy How Plate Tectonics and Climate Shaped the Geological Record HANDOUTS

8/18/2019 Irish Stratigraphy How Plate Tectonics and Climate Shaped the Geological Record HANDOUTS

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GEOL10060

Stratigraphical successions: how plate tectonics and climate shape the geological record

Siccar Point, Scotland

angular unconformity between vertically oriented Silurian greywackes (deep marine sandstones) and gently inclined Devonian “r ed beds” –

sandstones and conglomerates deposited in a terrestrial environment; the contact between the two represents about 65 million years


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• what happens if a lithospheric plate drifts from A – B?

• how would we know what had happened 200 million years later?

550

Ordovician

Cambrian

Silurian

Devonian

Permian

Carboniferous

500

450

400

350

300250

m i l l i o n s

o f y e a r s a g o

g e o l o

g i c a l p e r i o d

Death of an ocean:

the Cambrian – Silurian geological record of Britain and Ireland

important to remember the sequence of periods

not necessary to know the dates


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from the start……

southern parts of Britain and Ireland

• part of Gondwana

• at high southerly latitude

NW Ireland and NW Scotland

• part of Laurentia

• at low southerly palaeolatitudes

micro continent of Avalonia rifts away from Gondwana…..

….Avalonia drifts north…. …. Rheic Ocean opening between Avalonia

and Gondwana

….. accretes onto margin of Laurentia

...major continental landmass…

Early Ordovician

Middle Ordovician

Early Silurian

Early Cambrian


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….followed by subduction and closure of Rheic Ocean

….. leads to formation of supercontinent (Pangaea)

Terranes

tectonostratigraphic terrane

• regional scale

• bounded by faults

• own geological history that is distinct

from adjacent terranes

• often form as island arcs, fragments of

crust + sediments are accreted to over-

riding plate margin

• note trends (NE-SW) of (most)

boundaries

Gondwanan terranes

Laurentian terranes

intermediate

accreted

terranes

Laurentia

Gondwana

e.g. Midland Valley

Southern Uplands


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Mac Niocaill (2000)

What is evidence for existence of Iapetus Ocean and the plate

tectonic history of Avalonia?

• palaeomagnetism

• faunal provincialism• sedimentology (Southern Uplands terrane)


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low latitudeshigh latitudes

Ecology of marine environments

• benthic – living on or within floor of water body

• pelagic – water not at/close to floor of water body, nor its margins (e.g. shore)

• planktonic – drift or swim weakly

• nektonic - free-swimming in water column

• epifaunal – living on surface of sediment column

• infaunal – living within sediment column• sessile versus motile (note can be different between juvenile and adult stages)

littoral zonesublittoral zone

continental shelf

continental slope

continental rise

abyssal plain

"Southeastern United States continental shelf"

National Oceanic and Atmospheric Administration

physiographic terms

shelf break

Faunal Provincialism


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2 shallow marine

sessile infaunalbenthos proximal to

landmass

1 open oceanic plankton

would environmental distribution of 1 or 2

be more restricted by physiography ?

low latitudes high latitudes

shallow marine

sessile infaunal

benthos

open oceanic plankton

other variables e.g. temperature gradients in surface waters may play a role

continent

equator

20°S

40°S

60°S

continental shelf

3 deep marine benthos

independent of position of landmass

?less variation in environmental conditions

?wider latitudinal spread

faunal provinces

Faunal Provincialism

faunal provinces for shallow marine

benthos around continental margin


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Trilobites• marine arthropods (Phylum Arthropoda)

• extinct

• Cambrian to Permian


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Graptolites

• Phylum Hemichordata

• Colonial, marine, invertebrates

• M.Cambrian – U. Carboniferous

• organic periderm

extant hemichordates

attached sessile benthos

planktonic

dendroid graptoloid

Rhabdopleura

hypothesised reconstruction of

zooids of graptoloid based on

extant relative Rhabdopleura


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strong latitudinal control

Cyclopygid trilobites:

• characterized by enlarged eyes: wide angle of view, both horizontal and vertical

• high southerly palaeolatitudes

low latitude province includes Opipeuter – note adaptations for nektonic lifestyle (visual system)

Early Ordovician pelagic trilobites (extinct type of arthropod)

Opipeuter

cyclopygid trilobite


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• weak to no latitudinal control

• west-east variation in composition of faunas

• ? controlled by dispersal of larvae by ocean currents

• Avalonia + Baltica – considerable overlap – contribution from west and east

Late Ordovician trilobites (extinct type of arthropod)

Baltica

Avalonia

Laurentia

Ampyxina

benthos: note the dorso-ventral flattening

Calyptaulax

Panderia


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Ordovician graptolites

• some types of graptolite have a cosmopolitan distribution Isograptus


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Ordovician graptolites• ‘Atlantic Province’: mid to high S palaeolatitude

• ‘Pacific Province’: low-equatorial palaeolatitudes

• Atlantic Province: Gondwana + Avalonia

• Pacific Province Laurentia

• Siberia and Baltica (mid latitudes) a mix of both

l b i


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Dictyonema

breakdown over time of

faunal provincialismfreshwater fish

benthic ostracods

General observationsComplex patterns while Iapetus existed

• e.g. (a) strong latitudinal control on distribution of certain pelagic trilobites

• (b) other distributions possible : e.g. west/east division in Late Ordovician trilobites (? ocean currents as control)

• (c) some graptoloids cosmopolitan distribution: others divided into Atlantic and Pacific Provinces

• As the Iapetus Ocean closed the level of faunal provincialism decreased

• graptoloid faunas were among the first to lose a distinctive faunal provincialism. Why would they be more likely to do so than, for

example, trilobites and brachiopods?

Didymograptus bifidus

trilobite and brachiopod genera

trilobite and brachiopod species

planktonic

benthic marine

freshwater


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Sedimentology

Gondwanan terranes

Laurentian terranes

intermediate

accretedterranes

Laurentia

Gondwana

e.g. Southern Uplands

• accretionary wedge

• deep-marine sediments

• greywackes (muddy sandstones)

• deposited as event beds from

turbidity currents

• associated with black shales and cherts

(open oceanic sediments)

accretion and mountain building…..

various subduction and orogenic events during closure of Iapetus

collectively the Caledonian orogeny

e.g. the Southern Upland terrane represents deep-marine sediments obducted in an

accretionary wedge during final closure of Iapaetus


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the result…..

amalgamation of Avalonia Laurentia and Baltica

creates a major continent: Euramerica, Laurussia or Old Red Sandstone Continent

transition from marine sediments to terrestrial sedimentation at low palaeolatitudes south of equator

red beds

desert environments

alluvial fans (scree slopes and debris flows)

episodic river activity

unconfined sheet floods over floodplains

aeolian (wind-blown) deposits

lacustrine deposits

you are not expected to know every detail on each lithofacies map


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highlands shedding very coarse

detritus on slopes as alluvial fans

major river systems in low areas

terrestrial

marine

lake systems

EARLY DEVONIAN

• localised lacustrine systems in NE Scotland

• marine influence restricted to southern Britain

• prominent topography:

extensive alluvial fan sedimentation

major river systems flowing length of topographic lows

you are not expected to know every detail on each lithofacies map,

but you should be able to distinguish which time slice each

corresponds to


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Old Red Sandstone, WexfordSandeel Bay, Co. Wexford

4FeO + O2 → 2Fe2O3 (Fe2 +

→ Fe3 +

) Ferrous iron → ferric iron (rust)

OXIDATION 5


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Channel base

6

MIDDLE DEVONIAN


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terrestrial

marine

• development of major lacustrine systems in NE Scotland (e.g. Orcadian Basin)

• marine influence further N

• decrease in alluvial fan sedimentation (reduced topography)

MIDDLE DEVONIAN


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low lake stands•

playa-type environments: evaporites• deposition of fluvial sediments

• stromatolites

• desiccation-cracked horizons

Orcadian Basin• cyclical patterns of sedimentation as lake levels rise and fall

• astronomical forcing of sediment patterns

• lake geometry reflects local physiography and tectonics

hi h l k t d


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high lake stands• deep permanent lake

• fish faunas

• finely laminated quiet water sediments

• no infauna: preserved under oxygen-free conditions

• one deep lake phase resulted in laminated sediments from

Shetland Islands to Scottish mainland

• ? intensification of monsoonal system

LATE DEVONIAN


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• alluvial plains and aeolian facies important

• rivers: some meandering not braided

• locally lacustrine deposits and evaporites

• major coastal plain in south

LATE DEVONIAN

alluvial plains

and aeolian facies

alluvial plains

meandering rivers

evaporites locally

coastal plain

marine

EARLY CARBONIFEROUS


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low lying alluvial plains

draining to south

very localised highlands

• marine influence further north than before

• carbonate platforms - limestone deposition in southern

Britain and Ireland

• deeper water facies to south

• major transgression underway

deep

sea level from south floods northwards

how do we know the direction of transgression?

EARLY CARBONIFEROUS

MIDDLE CARBONIFEROUS


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MIDDLE CARBONIFEROUS

alluvial plains

• coal swamps and deltas becoming established in NE Britain with some

marine influence during short scale transgressions

• significant marine limestone deposition across much of southern and

central Ireland and Britain

• localised deeper basins e.g. Dublin Basin

coal swamps and deltas

some marine influence

during short scale transgressions

Carboniferous limestone


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Carboniferous limestone

fossiliferous carbonate muds

rugosan corals: solitary (1) and colonial (2)

LATE CARBONIFEROUS


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LATE CARBONIFEROUS

alluvial plains

coal swamps and deltas

some marine influence

during short scale transgressions

extensive emergent (terrestrial)

surfaces in north

• extensive terrestrial (emergent) surfaces in northern part of Britain and Ireland

• coal swamps

• major economic deposits e.g. coal deposits

• burial of carbon removes CO2 from atmosphere = global cooling

• causes glaciation at high latitudes

• major sea level fall as a result

•

current global warming caused by burning fossil fuels• in UK and Ireland Upper Carboniferous coals important source

Central Clare Group

• Central Clare Group western Ireland


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Central Clare Group, western Ireland• last part of the infill of a deep water basin

• series of repeated delta deposits: cyclothems

• each cyclothem starts with offshore marine sediments and

shallows upwards, ending with delta top environments

including channels and floodplains

• modern analogue: Mississippi delta

• sea level then rises and sequence starts again• 5 such cyclothems in Central Clare Group

first (oldest) cyclothem

in Central Clare Group

older lithostratigraphic units below the Central Clare Group

Summary

interaction of several different drivers


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Summary

Gondwana

Cambrian

Ordovician

Silurian

Devonian

Carboniferous

interaction of series of variables:

• plate tectonics

Laurentia

Avalonia

Gondwana

Laurentia

Avalonia

Iapetus Ocean

m a r i n e

t e r r e

s t r i a l

O R S c o

n t i n e n t

major

orogenic

episodes

rise in sea level

transgression

flooding onto land surfaces

EXTENSIVE LIMESTONE PLATFORMS

interaction of several different drivers

MAJOR PHASE OF DELTAS AND COAL SWAMPS

GLOBAL SEA-LEVEL FALL AS ICE SHEETS BUILD UP

Irish Stratigraphy How Plate Tectonics and Climate Shaped the Geological Record HANDOUTS

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