Mesozoic of the British Isles Flashcards
What systems was the Triassic initially dominated by?
Non-marine fluvial and aeolian systems (global warm period)
What did the increasing influence of marine conditions lead to in the Triassic?
Evaporites
What makes correlation difficult for the Permo-Trias boundary?
Lack of significant marine deposits (non-marine dominance till end Triassic)
What occurred in the Carnian (230 Ma)?
Ongoing breakup of Pangea
Why is the Carnian important?
Important rifting - with deposition of sherwood sandstone and sucession of Mercia mudstone
What conditions were sherwood sandstone deposited under?
Crustal extension
What conditions was the Mercia Mudstone deposited under?
Post-rift thermal subsidence
Where is the base of the Triassic taken from?
First pebble bed in the Triassic Sherwood sandstone
What are the rock groups of the Triassic? (old to young)
Sherwood sandstone
Mercia Mudstone
Penarth group
What does palynology prove?
Facies are diachronous
What is a suggestion of the mid-Triassic English midlands?
Marine influence
What does the Penarth group mark?
the beginning of the significant marine influence
What us early Triassic sedimentation like in the UK? (deposition of Sherwood sandstone)
Aeolian and fluvial
Where is a major source of the Sherwood sandstone?
Variscan Highlands transporting many well-rounded Quartzite clasts northwards
What are some examples of rock formations in the Sherwood sandstone?
Otter sandstone
Budleigh salterton pebble sands
What are the mudstones like in the mercia mudstones?
Red and green mudstones
What environment was the mercia mudstone produced in?
Restricted basins (salt assumulation)
Where is 90% of the UKs Halite located?
Cheshire basin.
Worth £¼ Billion p.a. to the U.K.
Currently producing about 6 Million tonnes p.a
What was the depositional environment of the Mercia Mudstone?
Less fluvial input.
Carbonate soils and marginal halites
What was the depositional environment of the Sherwood sandstone?
Fluvial input, braided rivers.
Coarse alluvial fans with anhydrite or carbonate rich soils.
What provides a biostratigraphic framework through the Jurassic?
Ammonites
What are ammonites good biomarkers?
Evolve rapidly
Planktonic (widespread distribution)
What marks the early Jurassic?
Global transgression
What caused significant changes to the sedimentary system throughout the Jurassic?
Fluctuating eustatic sea-level change, plate tectonics and local structural factors
What marks the end Jurassic?
significant global lowstand reflects global cooling
What was the early Jurassic palaeogeography?
Central Atlantic Magmatic Province
UK shallow shelf seas on northern Tethys
What is the basin structure of the Jurassic outcrop in the UK?
Basin and swell
What are the swell (block) features of the UK Jurassic?
Mendips
Moreton
Market Weighton
What might the highs and basins of the Jurassic swells in the UK have been like?
High - potentially emergent, Thinner and condensed
Basins - Thicker
What was the early Jurassic lithostratigraphy like?
Almost exclusively marine (mud and shales)
What is there evidence of near the top of the Early Jurassic?
Shallowing (sandstone and ironstone)
What is found within the Blue liass formation and what does it show?
Ammonites, bivalves, crinoids, brachiopods, vertebrates, trace fossils
(well oxygenated environment)
What is the jet rock of whitby mudstone an example of?
Ocean anoxic event
What evidence is shown in the jet rock?
Perturbation of carbon cycle and ocean system
What was the result of the oceanic anoxic event on the jet rock?
No benthos
Laminated sediments
Exceptional preservation of nekton/ plankton
Marked carbon isotope excursion (CIE)
What is an example of the Toarcian and Aalenian in the UK?
Bridport Sands
What are the Bridport sands?
Shallow marine, offshore shoal sands, on
top of which are the earliest beds of the Inferior Oolite
What rock formation marks the Aalenian to the Bathonian?
Inferior Oolite of S England
What is the Oxford clay?
Marine clay
Callovian to lower Oxfordian
What are the characteristics of the Oxford clay?
Fine-grained, fissile, dark-grey shale, minor
mudstones/limestones
Rich in invertebrates (benthics)
What is the economic importance of the Kimmeridge clay?
The source rock for much of our North Sea Oil
What is displayed in the Kimmeridge clay?
Milnakovitch scale cyclicity
How was the Kimmeridge clay deposited?
Under a stratified water column
What marks the late Jurassic to earliest cretaceous?
Tithonian Portland Gp and Purbeck Gp
What are the Tithonian Portland Gp and Purbeck Gp like?
Lagoonal limestone and shale with rich invertebrate and vertebrate and trace fossils
Gypsum and palaeosols
What does the gypsum and palaeosols of the Tithonian Portland Gp and Purbeck Gp an indicator of?
Indicate climatic and environmental
conditions
What was late cretaceous palaeogeography like?
Widening N. Atlantic & opening S. Atlantic
Tethys - Atlantic - Pacific connections
High sea levels - flooded continental margins
Where can UK outcrops of the cretaceous be found?
Wessex and Hampshire basins, S England to East Anglia.
Weald Basin of SE England.
Cleveland Basin of the Yorkshire- Lincolnshire Coast.
East Antrim, NE Ireland
What is the Berriasian like?
‘Purbeck’ marls & Lmst in the south
(alternating FW, brackish, marginal
marine); sands and marine clays to
the north
What is Hauterivian like?
‘Wealden’ silts and clays in the
south (non-marine); marine marls,
clays & sands in north
What is Aptian like?
Marine conditions return to the south
as Mn-Fe-rich sandstones (Greensands) & clays
What is Albian like?
Marine clays and sands in the south
& east, iron-rich limestones in the
N Sea
What occurred in the mid to upper Albian in the wessex basin?
Marine transgression (Gault clay and upper green sandstone)
What happened in the weald basin between Lower Hauterivian to the Barremian?
Non marine - weald clay formation
What is found in the weald clay formation?
Fluvial and mudflat/overbank
sediments; Main UK source of dinosaur
remains (Iguanodon); FW-Brackish
invertebrate fauna/flora.
What are the characteristics of the Speeton clay?
Mid- to dark grey clay with horizons of concretions, glauconite and fossil
concentrations.
What is the age of the speeton clay?
Earlt cretaceous
How thick is the speeton clay at coastal exposure?
About 100 m
What beds are present in the Speeton clay?
Bssal nodular beds (phosphatic nodules)
What do organic rich shales in the Early Barremian indicate?
widespread anoxic event
How will rocks be aligned in the Gault clay formation?
Grey calcareous clay, generally mid-late Albian, sandwiched between Lr & Ur
Greensands
What is the late cretaceous dominated by?
Chalk
What is the chalk of the late cretaceous like?
Pure limestone
Entirely planktonic/ biogenic
What is the result of planktonic forms having evolved on the chalk?
fine-grained, pure low-magnesium calcite limestone (a biomicrite)
What distinctive feature can be found in rhythmic black nodular layers?
Flint
What are the characteristics of flint?
Hard, brittle, siliceous; early- diagenetic remobilization of biogenic silica (mainly sponge spicules, some radiolaria)
