Download Abby Murray Geology 11 Final Project December 20, 2007 Geologic

Abby Murray
Geology 11 Final Project
December 20, 2007
Geologic Findings on the
Chamber of Secrets Area
The results of our geologic survey of the Chamber of Secrets geologic area are as
follows: The area is predominantly flat, and has no significant topography. The potential
mining opportunities appear promising, because ore deposits most likely are included
within the Chamber of Secrets area.
The Chamber of Secrets area has six distinct sections of geologic activism: a plate
from an Old Continent, which we will call the Dursley continent and which dates to
before 1.1 Ga, a varied mixture of rock in an accretionary wedge, andesitic volcanic melt,
tilted and thrust-faulted sedimentary layers, a granitic pluton which metamorphosed a
section of those layers, and younger tilted sedimentary layers.
Originally horizontal, laterally continuous layers of mudstone, limestone, and
basalt made up the Dursley Continent. They are, from youngest to oldest, mudrock (the
protolith of both gneiss and schist), limestone, and basalt. The initial folding and
metamorphism of the sedimentary layers was caused by a collision with another continent
from the east. This collision took place before 1.1 Ga years ago, because the granite had
not yet intruded, or it too would have been folded and metamorphosed. Radiometric
dating samples from the granite tell us that the granite is 1.1 Ga old, and we know that the
sample is of the granite and not the surrounding rock because radiometric dating only
works on igneous rocks. The intrusion resulted from the orogeny of the collision. The
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formation of mountains from the collision caused isostasy, and the rocks which were
thrust down deep enough began to partially melt, and the melt rose up through the
overlying rock. There were un-metamorphosed sedimentary layers at the top, and below
there was metamorphism according to isograds. The sedimentary layers were erode away
to reveal the metamorphic core. The granite we see in the northwest corner of the
Chamber of Secrets area was a granitic dike. It was originally vertical, and it tilted
during the second collision.
After the first collision, sediments began building out in layers off the southern
coast of the Dursley Continent. They were transported by water from the mountain
ranges in the Dursley interior, and deposited in horizontal, laterally continuous layers in
the ocean. By looking at the dip of the layers in the survey, we can ascertain that calcite
rich shells secreted by ocean critters were deposited on top of mud, which was deposited
on top of sand, which was over a layer of conglomerate. These layers continued to push
farther and farther out into the ocean, forming the continental margin of the Dursley
Continent, which we will call the Dursley Shelf.
The fossils preserved in the shelf beds gave us clues to the history and deposition
environment. The trace fossil Climactichnites wilsoni , preserved in the bottom layer of
sandstone, is 500 Ma years old and comes from the Cambrian period. It was native to a
beach setting with mature sandstone. The sandstone probably was deposited in a coastal
marine area.
Both the limestone and mudrock layers contain evidence of marine organisms.
Anthropods and glass sponges and echinoderms lives in shallow seas. The younger layer
of mudrock holds eurypterid anthropod fossil Baltoeurypterus tetragonopthalmus, which
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is 420 Ma years old and comes from the Silurian period. There are glass sponges,
Hydnoceras bathense, in both layers of the limestone, both of which date from 370 Ma.
In one layer of limestone there is a fossil of an echinoderm, Platycrinus sp., from 330 Ma
and the early Carboniferous period. All these fossils are indicative of a marine deposition
environment.
The next step of geologic activity in the region was the development of
subduction zone on the southeastern side of the Dursley plate. As the old, dense plate
began to subduct under the overlying plate, which we will call the Petunia Plate,
sediments were scraped off the ocean floor of the Dursley plate. They accumulated in a
jumbled mélange of basaltic rock, immature sediments like coarse sandstone, clay and
plankton, sand, and chert to create an accretionary wedge. Mud could have arrived far
out in the ocean via airborne sediments which fell into the ocean. Basaltic rock makes up
the ocean floor, and andesite comes from the volcanic arch on the Petunia Continent,
which falls off directly into the trench from the overlying plate. Chert is made by the
precipitation of shells of plankton out on the ocean floor. The components of the
accretionary wedge are condensed and contorted by the high pressures. Some of the
wedge material gets sucked into the suture between the subducting plate and the
overriding plate. The high pressure conditions in this area are of the blueschist facies,
and yields quantities of glaucophane schist. The area in the southeast corner has
blueschist rocks with glaucophane, coarse sandstone, chert,
Glaucophane is a rare type of amphibole which can only be created under the
unique conditions of the accretionary wedge. It needs to be under high pressure and low
temperature, like the blueschist facies. Since an accretionary wedge can be as deep as 20
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kilometers, the rocks at the bottom of the wedge are deep enough to be under sufficient
pressure to create glaucophane. Since the subducted ocean lithosphere is cold, it provides
the low temperatures also needed.
The mylonite revealed by the drill data signifies the presence of an orogenic belt.
Mylonite is a metamorphic rock created purely under the effects of shear, with no change
in temperature or pressure. . Instead of breaking into breccia as brittle rocks would do,
the rocks in the wedge are ductile. They are stretched and sheared into mylonite, a rock
indicative of a shear zone. Shearing causes foliation but does not change the minerals in
the rock. Within the folds of an orogenic belt there would have been occurrences of
mylonite because of the shear inherent in orogeny. Our mylonite is located in our
sedimentary layers, which would have been part of an orogenic belt millions of years ago.
When the Dursley Continent subducted below the Petunia Continent to a depth of
150 kilometers, the volatile rich ocean crust began to release the water which had been
incorporated into its crystal structure during its crystallization, and caused hydration
melting. The solid mantle located between the down going plate and the overriding plate
is partially melted by the rising volatiles, because it is suddenly governed by the wet
solidus, which allows it to partially melt at lower temperatures and pressures. The
hydration melting yields basaltic melt, which rises through the cold crust overhead. The
ultramafic basaltic melt cools as it rises, and the mafic components gradually crystallize
and settle out of the melt. By the time the melt reaches the surface, it is intermediate
andesitic melt. It creates arcs of strato volcanoes on the overriding plate. As more melt
rises from the mantle, it has to pass through more and more crust because of the volcano
formations, and so the volcanoes yield andesite. This geologic event explains the
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presence of andesite in the southwest corner of the map, where such volcanoes would
have yielded andesitic flow into the wedge area.
Another factor which proves that hydration melting was going on is the presence
of serpintinite in the wedge. Serpintinite is created by the metamorphism of peridodite
catalyzed by the presence of water. When the volatiles are metamorphosed out of the
oceanic lithosphere during subduction, the peridodite is metamorphosed into serpintinite.
As the Dursley continent and shelf are pulled towards the Petunia continent, the
shelf undergoes crustal deformation. The sequence of layers visible on the surface of the
Chamber of Secrets area suggests thrust faulting occurred in order to shorten and thicken
the crust. Fossils from the two layers of Longbottom limestone in the Dursley Shelf
show that they have the same age, and therefore used to be part of the same originally
horizontal layer. The suite of sandstone, mudrock, and limestone repeats itself again,
which lead us to postulate that they were all originally part of the same layers. The
unrepeated layer of conglomerate can be explained by how thrust faults work: they cut
across beds and then between them, finding the most efficient route of faulting. The
thrust fault in the Dursley Shelf cuts across the conglomerate bed, which is the lowest one
we know of. That is why when the bed was tilted during the collision only one bed of
conglomerate was exposed.
When the thickened, shortened Dursley Shelf collides with the Petunia continent,
tilting occurs. The shelf tilts at a 30˚ angle towards the collision area in the southeast.
The continent behind it tilts at an opposite and steeper angle; 45˚ and more towards the
north. This is probably because the Dursley Continent is not strongly connected to the
shelf, and so the pressure exerted on it by the collision fractured the connection and they
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tilted up against each other at different angles. The folds in the metamorphosed continent
cause different areas to strike and dip in different directions, but as a whole the plate
strikes north. Millions of years of erosion reveal the different layers.
The next significant development in the history of the survey area was a granitic
pluton which contact metamorphosed the surrounding rock. The pluton is caused by
isostasy and partial melting. When the Dursley Shelf thrust faulted and became at least
double the thickness, isostasy sunk the area. An orogenic belt is formed, and the rock at
the bottom of the shelf would reach temperatures at which it could begin to partially melt.
This is because the sedimentary rocks adhere to the wet solidus. This type of melting is
called burial melting, and it yields granitic plutons. The particular pluton in our survey
area rose up through the Dursley Shelf and metamorphosed layers of limestone into
marble, sandstone into quartzite, and mudrock into hornfels. All of these rocks fall under
the hornfels facies. The pluton never broke the surface, and erosion does not reveal the
granite on the surface either. However, we know that the pluton is there because of the
distinctive halo of contact metamorphosed rock on the surface of the survey area.
Between the times of the Accio andesite flow 265 Ma years ago and the
deposition of the Cruxio conglomerate 215 Ma years ago, a continental rift became active
under the newly fused together Dursley-Petunia Continent. The rock beds and
formations are characteristic of a half basin, which is the result of normal faulting during
continental rifting. Fossil dating from shows that the Rubeus Rojo Sandstone is younger
than the Cruxio Conglomerate. The opposing angle between the Dursley Shelf
sedimentary beds and the beds in the rift strongly suggests that a rift opened up along the
Dursley Shelf, such that sediment began accumulating in the basin between them. The
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presence of hematite in the sediments concurs with the effects of groundwater, which
produces hematite. Large, immature sediments which fell off the basin walls and were
carried by water initially filled the basin. 25 Ma years later, a bed of sandstone was
formed over the conglomerate (as shown by the dates of fossils preserved in the
respective layers). Basins are a very good place for finding fossils because of the high
rate of burial. The layer of Bertie Bott’s Every Flavor basalt between the layers of
sedimentary rock is caused by upwelling asthenosphere. The fault which underlies the
basin provides an ideal crack for the upwelling basaltic flow which accompanies
continental rifting. The flow climbs up through the rift, and then continues to wedge
itself between the sedimentary beds. Therefore, the basalt is younger than either the
conglomerate or the sandstone, even though the Law of Superposition would suggest the
opposite.
The fossil information concurs with our belief about the basin environment. The
reptilian phytosaur fossil, Machaeroprospus gregorii, lived in fresh water, like the water
than would have flowed into the basin. It was preserved in the Cruxio Conglomerate bed.
It dates from 215 Ma years ago during the Triassic period. The fern fossil preserved in
the Rubeus Rojo Sandstone was characteristic of a plant that would be found preserved in
a river which came down from a mountain into a valley or floodplain, or basin like in our
Chamber of Secrets area. The fern dates from 190 Ma years ago during the Jurassic
period.
The hanging wall continues to slip down the footwall of the Dursley plate as the
plates continue rifting. Although there is a normal fault along the surface, a right lateral
transverse fault is created in the interior of the rifting crust. The sedimentary layers must
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bend as the two walls pull apart from each other, and as they bend they scrape along the
footwall. This scraping grinds up sediment from both walls, called breccia, and creates
about a meter thick of gouge. This gouge is represented in the drill data from B1 and B1,
both of which show the occurrence of gouge.
As the surface erodes, the top layers of deposited sediment erode, and reveal the
stripes created by folding layers of previously deposited sediment interlaced with layers
of basaltic upwelling.
The Godric gabbro which is found in the accretionary wedge is an
example of a vein deposit. We believe that when the plutonism occurred under the
Petunia Continent, ore deposits formed in the magma chambers underlying the volcanic
arc. They then spread out into the veins of the surrounding rock. All crust rests on
ophialite, which is made up of gabbro. The pyrite fills veins in the gabbro, creating the
Godric gabbro.
There are very good prospects for mining ore in the area. Ore comes from
deposits of ore-rich sulfide minerals. These minerals crystallize in granitic plutons, and
sink to the bottom of the pluton chamber, where they form a heavy-ore concentrate.
When rain water filters into the ground, it goes down to the mineralized areas and it
dissolves and redistributes the metals. This leads to the formation of hydrothermal
deposits. We can predict that such hydrothermal deposits of ore-heavy concentrates
would be found surrounding the granitic pluton in the Dursley Shelf area, or the intrusion
in the older Dursley plate. The deposits in the Dursley plate may be more disseminated,
because the intrusion is older and therefore has had longer to redistribute its mineralized
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area. The ore could also have crystallized in cracks in the rock, called a vein deposit. In
conclusion, the mining prospects in the Chamber of Secrets are very promising.
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