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Your task is to infer the geologic history of

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GEOL 1020 HW#2

Spring 2016

Homework #2: Interpreting Geologic History of Boulder, CO (45 points)
Due Date: Friday, Feb 5th 8:00 PM
Directions: Your task is to infer the geologic history of the Boulder, CO based on the
geologic cross section of Boulder, CO and descriptions of the sedimentary formations.
The activities and discussions in lecture, along with the required readings posted in D2L and listed on the
“What You Need to Know” (WYNTK) pdf will be helpful in solving the geologic problems down below.
Tip: Print out pages 2 and 3 to label and makes notes on, instead of trying to do everything in your head!

Name:
Pre homework questions (1 pt):
Below are statements that reflect some of what you should be getting out of this assignment.

1. Before you complete the assignment, rate how true each of the following statements is for you using
a scale of 1-5, where 1 is “not at all true of me” and 5 is “very true of me”. These answers for the
pre-question are graded only for being complete (1 pt all or nothing) and not what your answers are.
Your rating (1
through 5)
A. I am comfortable interpreting the potential depositional environment for a
sedimentary formation based on observations and descriptions.
B. I am comfortable applying the five relative ordering principles to determine
the order of geologic events for an outcrop or cross section of an area
C. I can distinguish between the three different types of unconformities using
field photographs or cross sections
D.

I am comfortable reconstruct the geologic history of an area by combining
my knowledge of the relative ordering principles and the rock cycle.

GEOL 1020 HW#2

Spring 2016

Geologic Cross Sections across the Front Range. Tip: Print out this page to make notes as you work
through the questions on the homework.

Down below is a cross section of Golden, CO. Note this cross section covers a much wider area from
West to East than the Boulder, CO cross section

GEOL 1020 HW#2

Spring 2016

Table of Lithology (rock characteristics) of the rock formations named above. Tip: Print this page out to
make notes on.
Formation
(alphabetically)

Lithologic information and any relevant
age information

Benton Formation

Black shale with very fine layers and
marine organisms such as clams.

Boulder Creek
Batholith
Dakota Sandstone

Fountain
Formation
Lykins Formation

Lyons Formation

Morrison
Formation
Niobrara
Formation

Pierre Shale

Table Mountain
Basalt (Golden)
Volcanic Ash
Layer in Pierre
Shale
Ice Age Gravel

Granite to Diorite in composition, dated to
have formed 1.7 billion years ago.
A sandstone with white, well sorted and
rounded quartz grains with some dinosaur
footprints and fossil marine clams in it and
bi-directional cross bedding
Reddish, sandstone and conglomerates,
contains clasts of the Boulder Creek
Batholith.
Limestone with a lot of mud and
stromatolites (preserved algal mounds)
Red, well sorted fine sands with large
cross beds typically formed by wind and
some fossil footprints. 260 million years
ago
Light green and grey mudstone,
limestone, and sandstone. Contains
dinosaur fossils dated to about 200 million
years ago
Mainly limestone beds with some
mudstone and shale beds in between,
fossils include oysters and clams.
Dark, black shale with very fine layers and
abundant fossilized remains of marine
reptiles, sharks’ teeth, and small pelagic
(floating) marine organisms like
foraminifera

Interpretation
A shallow sea environment that is
slowly rising.
?

?

A steep mountain stream.
Tidal flat
A desert with wind blown sand
dunes

Rivers and floodplains,

?

?

Dated to 63 million years ago

Basalt flow

A thin bed of volcanic ash that is dated to
about 72 million years ago.

?

Unsorted and unlayered conglomerate
material with pollen and bones dated at
20,000 years ago

Glaciers

GEOL 1020 HW#2

Spring 2016

Part 1: Filling in the missing rock interpretations (14 pts).
In the chart above three rock formations are missing interpretations on how they formed. Use the
descriptions of their lithology to provide the missing interpretations down below in one to two sentences.
2. Here is a hand sample similar to the Boulder Creek Batholith. Based on the photo and the
description in the table on page 3 answer the following questions (3 pts)
A. What type of rock is the Boulder Creek
Batholith? (Igneous, Metamorphic, or
Sedimentary).

B. How did this type of rock form? Limit your
answer to one to two sentences.

3. Here is a sample of the Dakota Sandstone, using the photo and the description of the facies in
the table on page 3, answer the following questions (3 pts)

A. What is the most likely depositional environment?

B. What observations did you use to determine that
environment? (Explain your choice in one to two
sentences).

4. Here is a sample of the Niobrara Formation showing one of the large dinner plate sized clams
that is it famous for. Using the photo and the description of the facies in the table on page 3, what
is the most likely depositional environment? (3pts)

GEOL 1020 HW#2

Spring 2016

A. What is the most likely depositional
environment?

B. What observations did you use to determine that
environment? (Explain your choice in one to two
sentences).

5. Using the description of the facies in the table on page 3, what is the most likely depositional
environment of the Pierre Shale? (3 pts)
A. What is the most likely depositional environment?

B. What observations did you use to determine that environment? (Explain your choice in one to
two sentences).

6. What is the most probable cause for the ash layer seen within the Pierre Shale in Boulder, CO (2
pts)

GEOL 1020 HW#2

Spring 2016

Part 3: Interpretation of Geologic History of Boulder (28 points)
Use the cross sections, lithologic information chart, and your answers to the above questions to interpret
the geologic History of Boulder.
7. Fill in the chart below for the three types of erosion surfaces/unconformities that can be identified
in the Boulder, CO cross section by describing the unconformity’s location (i.e. between what
sedimentary or rock formations) (2 points)

Type of erosion surface/unconformity
1

Non conformity

2

Location

Angular unconformity

8. Use the answers from the questions, the cross sections, and relative ordering principles above to
order the events listed below in the proper order to create a simplified geologic history of Boulder,
CO.
1 indicates the oldest age/first event (20 points)
Number order of
events (1 being
the oldest)

Description of events

A. Uplift and exposure of granite to the surface where it experiences weathering and
erosion. This most likely occurs over a time span of 1.3-1.4 billion years.
B. There is a small decrease in relative sea level, exposing a beach. Dinosaurs are
often seen walking along the shoreline
C. There is a volcanic explosion in the area, and the volcanic dusts settles in the deep
water of the interior sea.
D. The area becomes wet again, with more tidal flats close to the coastline and then
followed by a rise in relative sea level.
E. Relative sea level begins to rise again, initially the area is a shallow sea, but
eventually is submerged under deep water (over 700 meters deep)
F. There is tilting of the region, most likely to some tectonics event
G. Cooling of felsic magma
H. Around 250 million years ago, the climate becomes arid and the region is now
covered with deserts with large sands dunes.
I.

Ice age glaciers scour the landscape.

J.

Steep mountain streams transport material weathered from high mountains and in
some location deposit conglomerates and sandstones

GEOL 1020 HW#2

Spring 2016

9. Two geologic features, the two faults, were not specifically mentioned in the previous question.
Based only on the Boulder Cross Section and information in the descriptive chart above, which
formations must they have occurred after based on the Principle of Cross Cutting Relationships.
(2 points) (e.x the fault must of occurred after the deposition of the Pierre Formation).
A. Maxwell Fault :

B. Boulder Fault:

10. Based on the information provided by the cross sections of Boulder, CO and Golden, CO and the
interpretations of the geologic history of these area from the previous questions; Select which of
the following tectonic events would be the most reasonable explanation for the Boulder Fault (2
points) Select only one.
Description of tectonic events in Colorado history

Place an X for
your choice

A subduction boundary 1700 million years ago affecting rocks in what is
now Wyoming and NW Colorado
Mountain building events, known as orogenies, about 1400 million years
ago that affect most of what is now Colorado
Large, widespread continental rifting from about 1100 to 600 million ago
The uplift of the Ancestral Rockies, most likely due to major continental
collision around 300-325 million years ago
Major mountain building event referred to as the Laramide orogeny that
occurs between 72 million to 40 million years ago.
11. In one to two sentences explain why you choose the event that you did for the timing fo the
Boulder Fault. (2 pts)

GEOL 1020 HW#2

Spring 2016

Post-homeworks questions (2 points)
After you completed the assignment, rate how true each of the following statements is for you using a
scale of 1-5, where 1 is “not at all true of me” and 5 is “very true of me. These answers for the prequestion are graded only for being complete (1 pt all or nothing) and not what your answers are.
12. Post homework questions (1 pt):
Your rating (1
through 5)
A. I can place depositional and other geologic events as to when they occurred
in the geologic time scale based on information provided such as geologic
period or millions of years ago
B. I am comfortable applying the five relative ordering principles to determine
the order of geologic events for an outcrop or cross section of an area
C. I can distinguish between the three different types of unconformities using
field photographs or cross sections
D.

I am comfortable reconstruct the geologic history of an area by combining
my knowledge of the relative ordering principles and the rock cycle.

13. Finish only one of the following sentences regarding skills and topics covered in this
homework (1 pt)
a. I am feeling much more confident about:

b. Even after this homework, I have questions about

GEOL 1020 HW#2

Spring 2016

Geologic Time Scale including Geologic Periods and dates
Use this information to answer and assist in putting the events in order for the geologic history
of Boulder

23 million years ago
65 million years ago
145 million years ago

Jurassic

200 million years ago
251 million years ago

Permian

299 million years ago

Pennsylvanian

318 million years ago

Mississippian

359 million years ago

Devonian

416 million years ago

Silurian

443 million years ago

Ordovician

448 million years ago

Cambrian
Proterozoic/Precambrian

Neogene

Triassic

Paleozoic

11,500 years ago

Cretaceous
Mesozoic

Starting point for Geologic
Period

Paleogene

Cenozoic

Geologic Period
Quaternary (Holocene)

Geologic Era

542 million years ago
4.54 billion years ago
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Tutorial Preview …facies xx the xxxxx on page xx what is xxx most xxxxxx xxxxxxxxxxxxxxxxxxxxxxx of xxx Pierre Shale? xx pts)A What xx the xxxx xxxxxx depositional xxxxxxxxxxxxx What observations xxx you use xx determine xxxx xxxxxxxxxxxx (Explain xxxx choice in xxx totwo sentences) x What xx xxx most xxxxxxxx cause for xxx ash layer xxxx within xxx xxxxxx Shale xx Boulder, CO xxxxxx GEOL 1020 xxxxxxxxxx 2016Part xx xxxxxxxxxxxxxx of xxxxxxxx History of xxxxxxx (28 points)Use xxx cross xxxxxxxxx xxxxxxxxxx information xxxxxx and your xxxxxxx to the xxxxx questions xx xxxxxxxxxxxx geologic xxxxxxx of Boulder x Fill in xxx chart xxxxx xxx the xxxxx types of xxxxxxx surfaces/unconformities that xxx be xxxxxxxxxxxx xxx Boulder, xx cross section xx describing the xxxxxxxxxxxxxxxx location xx x between xxxxxxxxxxxxxxx or rock xxxxxxxxxxx (2 points)Type xx erosion xxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx unconformity8 xxx the answers xxxx the questions, xxx cross xxxxxxxxx xxx relative xxxxxxxx principles above xxxxxxx the events xxxxxx below xx xxx proper xxxxx to create x simplified geologic xxxxxxx of xxxxxxxxxx x indicates xxx oldest age/first xxxxx (20 points)Number xxxxx ofevents xx xxxxxxxx oldest)Description xx eventsA Uplift xxx exposure of xxxxxxx to xxx xxxxxxx where xx experiences weathering xxxxxxxxxx This most xxxxxx occurs xxxx x time xxxx of 1 xxx 4 billion xxxxx B xxxxx xx a xxxxx decrease in xxxxxxxx sea level, xxxxxxxx a xxxxx xxxxxxxxx areoften xxxx walking along xxx shorelineC There xx a xxxxxxxx xxxxxxxxx in xxx area, and xxx volcanic dusts xxxxxxx in xxx xxxxxxxxx of xxx interior sea x The area xxxxxxx wet xxxxxx xxxx more xxxxx flats close xx the coastline xxx thenfollowed xx x rise xx relative sea xxxxx E Relative xxx level xxxxxx xx rise xxxxxx initially the xxxx is a xxxxxxx sea, xxxxxxxxxxxxx xx submerged xxxxx deep water xxxxx 700 meters xxxxxx There xx xxxxxxx of xxx region, most xxxxxx to some xxxxxxxxx eventG xxxxxxx xx felsic xxxxxx Around 250 xxxxxxx years ago, xxx climate xxxxxxx xxxx…
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