Sea level change through geological time and other features relevant to fieldwork Flashcards
What are the main sea level change principles?
Eustacy
Isostasy
Thermal expansion
What is eustacy?
Global change in amount of water in oceans due to climate and/or changing
basin geometry/volume due to tectonics changing the volume of water oceans can hold
What is isostacy?
Changes in relative height of the Earth’s crust due to loading (buoyancy,
subsidence)
What is thermal expansion?
changes in volume in relation to temperature change; warmer water has a
larger volume
What is regression?
sea levels fall and facies migrate seawards
What is transgession?
sea level rise facies migrate inland
What can regression and transgression identify?
when sea level has changed but not absolute height
What is a geological proxy?
natural recorder of direct measurement for environmental or climate variability
What are some examples of proxy data?
tree rings, ice cores, fossil pollen and spores, ocean sediments, corals and
historical data
Is there any reliable proxy for water depth?
only generalities and relative estimates
What is faunal proxy evidence?
fossil with eyes suggest organisms lived within photic zone
What is the photic zone?
depth light penetrates the water column
What are the problems with faunal evidence?
Fossil not ubiquitous
Eyes maybe not reserved
Photic zone depth varies (up to 200m)
Dont know eyes functional or relict
Was fossil in photic zone or transported
What can cause the photic zone to vary?
salinity, sediment levels/turbidity, biological
composition
What 2 ways can depositional water depth in the rock record be split?
proximal and Distal
What will proximal sediment composition be like?
High energy
Siliciclastic minerals
Coarse grain size
Rapid deposition
Frequent sedimentary structures
What will distal sediment composition be like?
Low energy
Mud dominated
Fine grain size
Slow deposition
Sedimentary structures less common
What will normal conditions be like for wave form and wave base?
Fair weather wave base (FWWB)
What can crest to crest length tell us above the waves?
depth of water displaced
What will conditions be like above FWWB in normal conditions?
water column and sediment affected by wave action and in coastal areas tides. May
also be influenced by currents
What will conditions be like below FWWB in normal conditions?
water column and sediment unaffected by affected by waves and tides, but may be
influenced by currents
What will the depth of the FWWB be?
varies between 5 and 15m
What does FWWB define in sedimentary environments?
upper shore face
What is SWB?
Storm Wave Base
What is the affect of larger wavs in SWB?
affect water column to a deeper level
How will SWB occur in relation to FWWB?
SWB will be below FWWB but not at same time
What is the depth of SWB?
15-40m but deeper in extreme events (hurricane, typhoon)
What does SWB define in depositional environments?
lower shore face
What will conditions be like above SWB?
water column and sediment: water interface affected by storm wave action agitating and re-working the sediment.
What will conditions be like below SWB?
water column and sediments not affected by storm wave action – currents may occur
What will be the cause for sedimentation below SWB?
current activity or from muddy sediment settling from suspension
What will the duration of background sedimentation for FWWB?
significantly longer longer than SWB
How will the water column be affected FWWB?
at this height not affected by wave or tidal action
What sort of energy condition is FWWB?
low energy - no wave agitation
What is the sedimentation rate of FWWB like?
very low- sediment as mud falling from suspension
What might have a potential to introduce high energy conditions/ sediments?
perhaps current activity
How would frequency of event sedimentation be described?
infrequent
What is the duration of event sedimentation?
short duration
How will the water column be affected in event sedimentation?
affected by wave action
What will the energy conditions be like in event sedimentation?
high- mixing sediment and agitating sea floor if at this depth
What will the sediment be like in event sedimentation?
coarse grained
poorly sorted
rapidly deposited
What sediment structures are present in event sedimentation?
cross bedding
ripples
lenticular bedding
What is the sedimentation rate be like event sedimentation?
high sedimentation rate
What are the requirements with ripples?
Unconsolidated sand sized sediment particles
Current moving over the sediment (wind, water, ice)
What is key observation of ripple shape in cross section?
symmetrical or asymmetrical
What is key observation of ripple shape in plan view?
parallel
sinuous
bifurcating
What is key observation of ripple wavelength?
distance crest-crest, or trough to trough
What is key observation of ripple height/amplitude?
height from bottom of trough to top of crest
What is key observation of ripple orientation?
orientation on ripple plane (plunge)
What processes can exacerbate weathering?
Heating/cooling
Wetting/drying
Physical contact (abrasion) including wind and water transported debris
Biological weathering
Chemical weathering (often through water)
What are the major physical properties of rock that affect susceptibility to weathering?
Composition
Hardness
Planes of weakness
What are some examples of planes of weakness?
faults
fractures
joints
cleavage
What is competent rock?
not easily broken or fragmented
What are some examples of competent rock?
granite, gneiss, basalt, some sandstone, some limestone
What can competent rock be used for?
building stone or aggregate
What is incompetent rock?
easily broken or fragmented
susceptible to weathering
what are some examples of incompetent rock?
mudrocks (mudstone, siltstone, claystone), some sandstones, some limestone
What are some ways you can tell a rock has been weathered?
orange colours
honeycomb texture
joints and faults
What will orange colour in rock suggest?
iron content oxidised
What will honeycomb texture indicate with weathering?
heterogeneous calcium carbonate cement
careous weathering
What will fresh rock surfaces allow for?
identification of original features grain/crystal size, shape and colour fabrics and textures within the rock
What will weathered surfaces allow you to identify?
alteration products changes in mineralogy (shape, size, colour) additional fabrics within the rock
When will baked and chilled margins occur?
when hot igneous body intruded into or extruded onto pre-existing and cooler rock strata
What is a baked margin?
altered by contact metamorphism (hot rock affecting cold rock)
What rock often present at baked margins?
hornfels
What is a chilled margin?
when got intrusion cooled by country rock with smaller sized crystals
What is sill?
igneous rocks intruded into existing strata
What is lava flows?
igneous rocks extruded onto Earth’s surface
What are ammonites?
linage of molluscs within class Cephalopoda
What is distinctive about ammonites?
whorled shell with internal divisions into chambers
Where are ammonites extensively found in fossil record?
Jurassic and cretaceous
What are the positives of using ammonites?
Diverse and species/genera/families readily identified
Exclusively in marine rocks
Geographically widespread
Biostratigraphic applications (biozone 1Ma)
What are trace fossils?
Physical evidence of fossil organisms in the absence of body fossils
Feeding, movement and habitation traces
What is diagenesis?
Physical and chemical changes that occur in the transition of sediment to a sedimentary rock
What are some examples of diagenesis?
Dewatering
Compaction
Dissolution
Cementation
Lithification
What are diagenetic concretions (nodules) form?
Rounded mineral masses (typically calcium carbonate, iron carbonate or pyrite) that form after the host rock was deposited by diagenesis
What can form if diagenetic concretions fracture?
fracture and recrystallise forming septarian nodules)
What is taphonomy?
The science of death and decay; controls what and how organisms enter into the fossil record
What are the key elements of taphonomy?
Life cycles of organisms
Physical transportation
Preservational biases
Mineralisation and geochemistry
Diagenesis
Decay zones and processes
