Download GEOS240-W17-Lab-03-Rocks

Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
Name ___________________________________ Partner ____________________________________
LAB 3: ANCIENT ENVIRONMENTS AND LITHOLOGIES (CLASTIC ROCKS,
CONGLOMERATES & SANDS)
OBJECTIVE
This lab is based on Fichter and Poche’s Ancient Environment’s manual with general lithology
identification support from the Busch and Tasa AGI manual. You will be examining various hand
specimens withtheir respective co-coordinating rock slides for properties which help to identify
their origins and likely environmental setting.
There are three parts to this lab:

Part 1: Review of Major Rock Types and their Tectonic Environments

Part 2: Examination of Modern Coarse Clastic Sediments

Part 3: Analysis of Four Conglomerates
PART 1: LITHOLOGIES AND TECTONIC ENVIRONMENTS
(Refer to A&E III: pp 1 – 31 or II: pp 1 – 25 and selective sections on Plate Tectonics, Igneous and
Metamorphic Rocks in AGI Manual: Ch 2-7).
Clastic sediments consist of rock fragments and mineral detritus of pre-existing rocks. Therefore
analysis of their grain composition can reveal the rock types, particular minerals and geological
setting from which they werederived. Clastic (detrital) sediments derives from adjacent eroding
uplands and preserve distinctive grains to record their geological source area (provenance).
This source region is referred to as the hinterland. The sediments themselves collect in an adjacent
lowlying or basinal area that is called the foreland, as it is usually in front of a mountain belt.
While each lithology (rock type) is not necessarily unique to a particular tectonic setting and
geological environment, they all come from a unique location and usually the assemblage of rocks
from a given location or setting is characteristic to that environment. (Environments do tend to
generate particular rock types). To identify a source terrane, all we need is a sand grain sized piece
of a rock if it has a unique mineralogy or texture.
Note: A terrane is a crustal block or fragment that preserves a distinctive geologic history that is
different from the surrounding areas and that is usually bounded by faults.
For each of the following rocks, examine the hand specimens and/or thin sections provided. Make a
few short notes on special textures or minerals that each of these rocks contains. Then examine the
following charts for the various stages of the Wilson Cycle for a hypothetical region and locate each
of the rock types in its most probable setting. Go back to your descriptions and add this tectonic
setting to your description for that specimen. Examples of geological environments for source-lands
and sediments are: continental shields or cratons, mountain belts, peneplains, coastal, tropical,
marine, accretionary wedge, island arc, carbonate bank, abyssal plain
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
Most of the crust is made of igneous rocks. Basalts (rift formed) of the upper ocean crust (fine
grained, vesicular, porphyritic, amygdaloidal) overly Gabbros (coarse grained, plagioclase plus
pyroxene or hornblende) of the lower ocean crust. Basalts is the typical merl derived from
peridotite upper mantle in any setting. It also occurs in all other volcanic settings such as
Continental Margin Arcs like the Cascades or Island Arcs like the Aleutians, or within plate-hotspot
like Hawaii/Kilauea, Kilimanjaro,Reunion. Intermediate igneous rocks typify arcs such as
porphyritic or pyroclastic andesites on top of the crust with diorites and granodioites at depth.
Rhyolites occur in evolved stratovolcanoes and as the eruptive product of shallow calderas.
Granites, especially those with abundant potassium in alkali feldspar and 2 micas only form at
continent-continent collisions where clay rich sediments have partially melted.
Metamorphic rocks get exposed in and eroded from uplifted convergent margin mountain belts.
Regional metamorphic rocks with foliations and folding, porphyroblasts and coarser layering into
felsic and mafic bands like schists and gneisses are the most abundant and while they form within
the base of the upper plate along active collisional margins they are most commonly eroded an
exposed from ancient mature continental shields. Mudrocks rich in aluminous
clays are the most common protolith for these with volcanics being a subordinate starting
composition. Aluminous minerals like garnet, kyanite, staurolite and corundum persist and
make it into sediments. Local contact metamorphic fine grained or spotted hornfels, or skarns with
coarse non-oriented textures and unusual minerals (sulphides, magnetite, apatite, fluorite or calc
silicates) form and derive from the narrow contact aureoles around shallow plutons. Hudrothermal
and zeolite facies metamorphism occurs in lithic sediments and volcaniclastic rocks especially in
low geothermal gradient forearc settings. Dynamic (strain) metamorphism (breccias,
slickenedsides, mylonites) from in major crustal fault zones both for transcurrent faults and
eroded, uplifted thrusts.
Sedimentary rocks also get uplifted and eroded to redeposit as recycled sediments. They show
bedding or other sedimentary structures, fossils and in thin section their minerals or rock
fragments can be identified. Also, porosity is evident, its shapes and connectedness in addition to
any post depositional pore-filling cements. Limestone forms mainly offshore in tropical settings
that are starved of clastic sediment input. Limestones dissolve readily in acid rain or cold ocean
water and it is too soft to withstand much sedimentary transport so fragmental brecciated
limestones are due to either solutional collapse or close to their source such as a reef face. Shales
are deposited in deep water in low current settings or at slack stages of floods on land or flood tides
offshore. They are generally compacted but not cemented rocks so they weather and abrade easily
and just contribute clay load to long system or second cycle sediments. It takes an abundant forest
and bayou environment to accumulate any cellulose, preserve it below water and burial to
transform it into coal. Detrital coal is too soft to make it very far and it takes rapid burial to
preserve even to catch a few little coalified plant fragments or charcoal from fires in sandstones.
Sandstones (granular), conglomerates (pebbly) and cherts (banded, tough, hard, fine-grained) are
all hard and withstand transport so they are common sedimentary particles in second cycle
sandstones. Cherts tend to be deposited in deep cold ocean settings and far offshore in high
latitudes. Most of these tough rocks eventually weather to individual quartz grains as do igneous
and metamorphic rocks so quartz and other silica minerals are commonly found as sand grains.
Quartz from different rock types exhibits different internal textures due to strain, cooling rates etc.
and can be distinguished. Surface abrasion of quartz also varies with fluid and environment of
transport so much can be told from common sand grains.
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
PART 1:
LITHOLOGIES AND TECTONIC ENVIRONMENTS
Read p.1-5 in Ancient Environments or Document called Wilson Cycle Rocks and Tectonics on your
course web page as support reading for this lab. A premise of geology is that each rock type derives
via a particular process in a specific environment. The biggest control on changing geological
settings is the dynamic engine of plate tectonics, rifting continents, opening new ocean basins,
building mountain belts by convergence & collision etc. J. Tuzo Wilson realized the relationship of
rock cycle to the tectonic cycle of breaking and making supercontinents with the formation and
destruction of ocean basins in between. Use the Lettered Stages of the Wilson Cycle in that reading
above to assign a letter to each rock you describe to find a plausible setting for its formation.
For all of the following rocks try and find at least 1-2 major, essential minerals and a
distinctive texture which allow you to recognize it in hand specimen and later as a rock fragment
in a clastic sediment. Mineral test kits and hand lenses are available. Look at the tectonic cross
sections and crustal evolution diagrams in the AE books in the lab or reader on the website, with
respect to the supercontinent (Wilson) cycle (p. 4 below) and locate a possible source setting and
Stage in the Wilson cycle for each rock to form. In each of those diagrams pay close attention to
not only what rock types are present and capable of eroding, but where specifically in the cycle
they are formed and first appear to be eroded and recorded in sediments. Sketch and label
textures, structures or minerals that help you recognize each rock. These same features will help
you recognize a pebble or sand grain sized piece of that very rock in detrital sediments.
Rock Notes: Label your drawings with mineral or textural names (quartz, clays, bedding, grading,
etc.) and a scale bar to show how big these are. Label your scale bars in cm not in magnification
factors. Label your drawing as a thin section sketch or a hand specimen. For thin sections use either
the brown low power objective or the yellow medium power. The diameter of the field of view is 40
microns for low power and 140 microns for medium power. Assign a stage A-F in the Wilson Cycle.
1. Andesite
(5)
2. Rhyolite (ignimbrite, pyroclast and pumice; black glass, pink and white ash layers) (5)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
3. Diorite
(5)
4. Granite Pegmatite – What textures and minerals indicate that this rock had to come from a
continental setting?
(6)
5. Peridotite – Note the predominant minerals and texture of this hand specimen. This is a
piece of the Jurassic upper mantle that crashed into the continental margin and got uplifted
near southern Oregon and Northern California near the Klamath Mountains.
(5)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
6. Gabbro – How can you tell the cooling rate was different for this rock than basalt? (6)
7. Greenstone – (Low grade metamorphosed basalt). Note secondary chlorite in hand
specimens from Vancouver Island and textures in the thin section of Keewanawan Basalt. (5)
8. Slickensides – (Low grade regional metamorphism from a fault zone on Southern Vancouver
Island). Judge from the mineral composition and structure, whether or not this specimen was
formed from a pre-existing rock or explain how it came to be formed.
(5 )
9. Muscovite bearing Pink Marble – Near Eagle Pluton West of Princeton, B.C. What is the
sedimentary protolith for this rock and where was it likely deposited?
(6)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
10. Hornfels – (High temperature-low pressure contact metamorphosed mudstone). Note the
highest grade minerals present and its textural appearance in this thin section. Explain why
you chose a different setting for this rock than specimens 8 or 9 above?
(6)
11. Blueschist – (High pressure-low temperature metamorphosed seafloor basalt from
“grandmother rock” at Bandon on the southern Oregon Coast). What are the characteristic minerals
here and how are they recognized? What does the mineralogy, rock texture and density tell you
about the metamorphic setting.
(6)
12. Eclogite – (Super high pressure and moderate temperature metamorphosed seafloor
basalt). Note the two predominant minerals and their colours and textures in this thin section and
hand specimen pair. If you found pebbles of this rock or grains of these minerals how would it
confirm the conditions of formation and stage in the Wilson Cycle?
(6)
13. Turbidite – Lower Paleozoic, Sicker Formation, Nanoose Bay. Note the density and
hardness of this rock. Use this to account for how it got to be where it was found as well as where
it likely formed.
(5)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
PART 2:
EXAMINATION OF MODERN COARSE CLASTIC SEDIMENTS
For this part of the exercise, use a binocular microscope or hand lens to examine each of the
following sands. Describe its range of particle sizes, grain shape and sorting. As best you can,
identify the types of rock or mineral fragments that comprise the sand or gravel grains for each one
of these recent sediments. Don’t forget that you may use the mineral test kits any time you examine
minerals or rocks. Comment on its maturity (mature = all rounded and mostly quartz or
chert (long system, re-deposited, retransported) versus immature = angular, random minerals and
rocks, random sizes (one dump). From the compositions, the location name and the tectonic
diagram in AE, and the Geological Map of North America, pick a likely source terrane (bedrock map
unit and location for the derivation of each sand.
14. Baymouth Bar Sediment – Northern Oregon Coast
13. Overbank Sand – from high gradient stream in flood stage east of Portland
14. Dune Sand – Exposed Pacific coast near Florence, Oregon
(5)
(5)
(5)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
PART 3:
CLASSIFICATION OF COARSE CLASTIC SEDIMENTS
The following diagrams will help you plot your visual estimates of the proportions of grain sizes
present and the mineral or lithic make up of of the grains in each of your 4 conglomerate
specimens. Triangular diagrams take 3 end member components and allow you to plot their actual
proportions in a rock to assign a rock name. In a plane figure there are only 2 independent
components because the third is fixed by closure. Every rock’s components sum to 100 % total. An
all gravel rock with no finer grains plots at the gravel corner 100%. A rock with equal amounts of
Gravel, Sand and Mud plots in the middle of the triangle exactly on the vertical line between gM
(gravelly Mud) and (gravelly muddy Sand). A rock with only 2 components plots along the side
between those 2 end members so that a rock with 70% Feldspar (usually K-feldspar) and 30% lithic
fragments is classified as a Lithic Arkose.
PART 3: ANALYSIS OF FOUR CONGLOMERATES
Clastic sediments are classified by their grain size distributions (how much % of Gravel, sand, mud
for conglomerates or, of Sand, Silt, Clay for sandstones and mudstones). This is totalled and
normalized to 100% to get the 3 component percentages. For example if you counted an area in a
thin section or on a slab of rock and got 10 gravel grains, 5 sand grains and 5 mud areas for the
points you counted on a square grid or rock face: Gravel % = 100 x 10/20 = 50% and Sand and Mud
are 25% each. The classification of this rock would be msG or a muddy sandy gravel. For
composition we classify coarse clastic detrital (siliciclastic) sediments (Conglomerates and
Sandstones) by their proportions of Quartz, Feldspar and Lithics (sand or gravel grains made of
rock fragments like granite schist or chert). Generally we point count a thin section on a grid and
tally the results for the sum of all grid points (100 or so). The percentage calculation works the
same. For example a rock with: 100 quartz grains, 30 feldspar grains and 70 lithic grains wouls
have Q = 50%, F = 15% and L = 30% so its final classification compositionally would be a
feldspathic litharenite conglomerate. See examples on the 2 triangles below.
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
For each of your conglomerates, estimate and calculate the percentage of gravel, sand and mud in
each conglomerate. To do this try counting a grid using a ruler across the rock face in 3 transects.
Plot this on the texture plots provided on the following page. Describe the grain size distribution
(well sorted, moderate, poorly sorted) and classify the conglomerate. Next do the count again but
estimate whether the grains are Quartz, Feldspar or Lithics (rock pebbles).
15. Sharpstone Conglomerate, Upper Triassic, Hwy 3, Southern B.C.
(8)
16. Fraser-Straight Creek Conglomerate, Lower Cretaceous, Okanagon, Washington
(8)
17. Nanaimo Group, Comox Formation, Upper Cretaceous, North Saanich, B.C.
(8)
18. Sooke Formation Conglomerate, Oligocene, Shirley, B.C. (8)
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Geoscience 240 – W-2017 Lab 3: Petrology of Source Rocks, Conglomerates and Sandstones
Geoscience 240 – Lab 2: Ancient Environments and Lithologies W2015 – T.S. Hamilton
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