Geoscience 240 – Lab 2: Ancient Environments and Lithologies
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LAB 2: ANCIENT ENVIRONMENTS AND LITHOLOGIES (ROCKS,
CONGLOMERATES & SANDS)
OBJECTIVE
This lab is based on Fichter and Poche’s Ancient Environment’s manual with general lithology
support from the Busch and Tasa AGI manual. You will be examining various hand specimens with
their respective co-coordinating rock slides for properties which help to identify their origins and
most probable environmental setting.
There are three parts to this lab:
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Part 1: Lithologies and Tectonic Environments
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Part 2: Examination of Modern Coarse Clastic Sediments
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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 an
internal look at their composition can reveal the rock types and geological setting they were
derived from. The geological source area (provenance) derives from adjacent eroding uplands.
This region is referred to as the hinterland. The sediments themselves collect in an adjacent low
lying or basinal area that is called the foreland.
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.
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Geoscience 240 – Lab 2: Ancient Environments and Lithologies
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1. Andesite
2. Rhyolite (ignimbrite, pyroclast and pumice)
3. Diorite
4. Granite Pegmatite – What textures and minerals indicate that this rock had to come from a
continental setting?
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 of southern
Oregon and Northern California near the Klamath Mountains.
6. Greenstone – (Low grade metamorphosed basalt). Note secondary chlorite in hand
specimens and textures in the thin section of Keewanawan Basalt.
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Geoscience 240 – Lab 2: Ancient Environments and Lithologies
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7. Slickensides – (Low grade regional metamorphism cut by a fault zone). Explain whether or
not this specimen was formed from a pre-existing rock or how it came to be formed.
8. Muscovite bearing Pink Marble – What is the sedimentary protolith for this rock and
where was it likely deposited?
9. 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 the marble or the chlorite vein above.
10. Blueschist – (High pressure-low temperature metamorphosed seafloor basalt from
“grandmother rock” on the southern Oregon Coast). What are the characteristic minerals
here and how are they recognized?
11. Eclogite – (Super high pressure and moderate temperature metamorphosed seafloor
basalt). Note the two predominant minerals and their colours and textures in this thin
section.
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Geoscience 240 – Lab 2: Ancient Environments and Lithologies
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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. Comment on its maturity and source terrane.
12. Baymouth Bar Sediment – Northern Oregon Coast
13. Overbank Sand – from river flood stage east of Portland
14. Dune Sand – Exposed west coast near Florence, Oregon
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Geoscience 240 – Lab 2: Ancient Environments and Lithologies
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PART 3: ANALYSIS OF FOUR CONGLOMERATES
Describe the percentage of mud and gravel in each conglomerate. Describe the grain size
distribution, particle shapes and sorting (i.e. well sorted, poorly sorted etc.). Note the lithology of
the grains present for each specimen and comment on its maturity and likely source terrane. Plot a
point for each specimen on a gravel-sand-mud ternary plot and also on a Quart-Feldspar-Lithics
plot. Fill out the sedimentary description forms and make notes below.
15. Eocene Challis Formation – near Wenatchee, Washington
16. Upper Cretaceous Nanaimo Group Conglomerate – Eastern Vancouver Island
17. Lower Cretaceous Conglomerate – near Straight Creek Fault, Winslow, Washington
18. Triassic Sharpstone Conglomerate – from Highway 3, Southern British Columbia
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