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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
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