Using the following list of events, table of information and diagram :i)
Place the events in chronological order starting with the oldest event.
events may be used more than once.
Note:
ii)
Write a brief geological history of the area. How are the rocks in the sequence
formed and what environmental conditions led to their formation? Start with the
oldest event and state why you have placed the events in the order you have.
Events
A time gap in deposition
Deposition of shale
Intrusion of granite pluton
Folding of shale
Metamorphism of shale
Deposition of limestone
Uplift, weathering and erosion
Rock
Grain
Size
(mm)
Texture
Minerals
Igneous,
sedimentary or
metamorphic?
Name
Environment and process
a.
10
Coarse
Abundant
alkali
feldspar
Igneous
Granite
Formed from cooling pockets of magma
trapped beneath the earth’s surface.
b.
0.010
Fine
Abundant
clay
minerals
Metamorphic
Slate
Formed at earth’s crust from layers of
sediment being compressed together.
c.
5
Fine
Calcium
carbonate
Sedimentary
Limestone
Formed from the breakdown of calcites
located on seabed, lakebed and sea
shells.
d.
0.003
Fine
Abundant
clay
minerals
Sedimentary
Shale or
mudstone
Formed from compacted layers of clay.
youngest
Deposition of shale
D
Deposition of limestone
C
A time gap in deposition (the slate and the intrusion are cut by the time gap)
Uplift, weathering and erosion
Intrusion of granite pluton (likely causing upward doming – uplift. Also would cause
contact metamorphism – see high-light on diagram)
Metamorphism of shale
A
Folding of shale
B
Metamorphism of shale (regional metamorphism of pre-existing shale creating slate
unit B is kind of what happen during folding so folding forces are what causes
metamorphism )
oldest
Geologic History:
At oldest extent this area was a quiet nearshore environment and likely close to a river
mouth. (Due to the clay mineral flux in the area – consider the Louisiana delta as a
similar example in present time – the quiet accumulation of muds or terrestrial origin
in a marine environment) Over time the area suffers a sea level change (most likely an
increase in sea height) assisting in burial of the layered muds. These muds will slowly
lithify (turn to rock) into shales first and if underging sufficient pressure will
metamorphose into slates. (So metamorphosis into slate could occur before during and
after folding since pressure is the major formative condition – it would require a
fabric analysis of the slate to ascertain when in the geologic sequence metamorphosis
actually occurred.) Sometime after burial and lithification (and the potential
metamorphosis already mentioned) the area undergoes tectonic pressures - plate
collision for example - that causes regional flexure, folding the slate (metamorphosed
shale and perhaps causing additional metamorphosis). Subsequently volcanic activity
ensues – likely following planes of weakness due to fractures associated with the prior
tectonic events – allowing intrusion of magma. This intrusive event causes doming – a
rounded upward flexure of the region causing uplift. Intrusion also causes a contact
metamorphic aureole along its boundary (baking the rocks around its edges.) Uplift
causes the area to be exposed to the environment leading to erosion and the
subsequent unconformity (time gap) in the rock sequence. Over geologic time this
area is “flattened out” and eventually experiences changes in sea level yet again. The
fact that the area has shale underlain by limestone is indicative of sea level regression
– or a shallowing of water meaning that this area likely experienced a rapid rise in sea
level as we are not seeing a transgressive facies(or rising sequence of rock layers) or
this anomaly may be due to the uplift in the region and is lost in the “time gap”. At
any rate, during this regression the deeper deposited limestones become buried by
mud-forming shales as the water gradually shallows towards the present.
Supplementary info:
Granite: this is an igneous intrusive (plutonic) rock of felsic to intermediate character.
That is to say, it occurs via the intrusion of magma at depth and cools in that
environment forming “largish” (phaneritic - meaning visually discernible) crystals due
to slow cooling. Its chemistry - being felsic - would refer to it occurring in a
continental zone where the intruding mafic magma would interact with the thicker
continental crust during ascension (refer in your text to Bowen’s reaction series) or
see: http://maps.unomaha.edu/Maher/geo117/bowens.html
Slate: Slate is a low grade metamorphic rock derived from clay-sized particles (and
generally clay minerals - hydrated phyllosilicates mainly see:
http://mineral.galleries.com/minerals/silicate/clays.htm for details) along with other
secondary minerals/elements that generally add color and/or other minor
characteristics.
* slates form from shales (layered sedimentary rocks of clay muds origin) in
predominantly pressure driven, low temperature metamorphic regimes (meaning
burial and often folding of the parent shale.) While the parent shales are
predominantly marine, there are lucustrine and even riverine shales.
Limestone: See http://www.mineralstech.com/limestone.html – the limestones are a
large group of sedimentary rocks that have in common the minerals calcite and
aragonite (both forms of calcium carbonate - CaCO3) and the fact that are almost
uniquely marine in origin – and temperate to warm marine at that (since calcite’s
solubility is inversely proportional to temperature.)
Different limestone occur in different areas from different origins. For instance, in
near shore environments coral rubble and shells may form a fossilferous limestone,
while in shallow lagoons small ovoid beads may form in the swash zone to form a
oolitic limestone. Still other forms occur such as the precipitation of carbonates in
open ocean and the sedimentation of limey oozes that form massive crystalline
limestones or the accumulation over geologic time of calcareous microskeletons of
coccolithophores to form deposits of chalk.
Limestone occurs due to the sedimentation of calcium carbonate containing matter in
temperate to tropical marine environments and its subsequent burial and lithification.
Shale / mudstone: (See:
http://csmres.jmu.edu/geollab/Fichter/SedRx/Rocks/shale.html) Both of these rocks
are sedimentary in origin and occur due to the accumulation of clay muds in quiet
water environments and their subsequent burial and lithification. Cements may be any
common cement mineral such as silica, calcite or iron oxides (or combinations
thereof). The difference between shale and mudstone is that mudstones are “massive”
(either large single depositional events or continuous and even deposition) while shale
has fissile laminations (layers) that are due to rhythmic variation (often seasonal) in
deposition.
For info on transgressive/regressive depositional facies see:
http://gpc.edu/~pgore/geology/geo102/facies.htm