Download Forest River – Field Trip

GLS100 Lab: Investigating the Geology of Forest River Park
GLS100 Physical Geology –Dr. Hanson
Goals: To apply the knowledge you obtained in lab to rocks in the field, by making,
documenting and interpreting your observations.
Part I Interpreting the Rocks
The purpose of this section is to define events -- the features that represent
them, and their order of occurrence. For review, the laws of relative age
dating are listed below:
1. Law of original horizontality
2. Law of superposition
3. Law of cross cutting relationships
4. Law of inclusion
5. Law of faunal succession
Stop 1: Small outcrop southeast of Parking Lot
a. Evaluate the rocks: Look for the presence of layering and foliation. Are
these rocks: (circle all that apply) clastic, crystalline, foliated, non-foliated,
stratified
b. What minerals do you see?( Circle all that apply) ferromagnesium silicates,
feldspars, quartz, calcite, halite, clay
c. Now identify the rock category: (circle one) igneous, sedimentary, or
metamorphic
d. Cite the evidence supporting your answer?
e. Is the sediment that overlies the outcrop younger or older? (circle one)
f. State the law used to relative age date the rock and the sediment.
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g. In the box below draw the outcrop and label the relative ages (1=oldest) of
the two rocks exposed in the outcrop Include a north arrow and scale.
h. State the law you used to determine their order.
______________________________
i. Note and sketch the following additional features: grooves, joints,
unconformity, and sediment.
j. In the table below summarize the geologic history interpreted from this
exposure.
Age
Feature
Event Recorded
6.
5.
4.
3.
2.
1.
K. List all the laws used to relative age date the events recorded.
____________________, _________________________, _____________________
In the space below write a brief summary of what you learned:
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Stop 2: Shore exposure (photo on next page)
a. Do these rocks look like (circle one)
i. they were deposited in layers
ii. metamorphosed: foliated and folded
iii. repeatedly fractured and intruded
b. Identify the rock category these rocks belong to (Circle one)
sedimentary, metamorphic, igneous
c. On figure 1 label the following and determine their relative ages:
i.
prophryritic basalt dike with mafic phenocrysts (b) ______
ii.
prophryritic basalt dike with feldspar phenocrysts (d)______
iii.
grey gabbro (e) ______
iv.
syenite intrusions (a) ______
v.
fault (c) ______
d. List the two laws used to determine the relative age of these features.
___________________________ and ________________________________
e. Which two laws clearly don’t apply?
___________________________ and ________________________________
f. In the table below summarize the geologic history interpreted from this
exposure.
Age
5.
4.
3.
2.
1.
Feature
Event Recorded
g. Draw a north arrow and scale on the photo. For the two dikes and the fault
draw the strike and dip symbols indicating their orientation.
h. Identify the fault: (circle one) normal fault, reverse fault, thrust fault,
strike-slip fault
i. Draw arrows on the photo indicating the relative motion along the fault.
NOTES:
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Figure 1. Outcrop at stop 1.
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Erosion and Unconformities (complete on your own)
Erosion is the removal or rock and weathered debris. Water in the form of
runoff and streams is the most effective agent of erosion. Most likely, more
than 95% of the sediment derived from the erosion of the Appalachian
Mountains was eroded and transported to the ocean by water. In New
England, glaciers also played an important role in shaping the landscape.
Erosion produces gaps in the rock record.
Assume that the rocks exposed here at Forest River Park are approximately
400 million years old and were formed 3 miles (~15,8400 feet) beneath the
surface, as indicated by the mineralogy and texture of the large intrusions.
The local sediments overlying the rock consists of glacial till, deposited
12,000 years ago, beach deposits, and landfill, deposited within the last 100
years. The contact between this sediment and the underlying rock is a buried
erosional surface, or unconformity. An unconformity represents a period of
time during which there is no preserved record of events.
a. Approximately how many years of geologic history has been removed by
erosion? ___________________ Does this vary across the landscape? (circle one
and explain) Yes/No
b. If the glaciers removed approximately 150 feet of sediment and rock, how
much rock was eroded from the landscape prior to glaciation?
___________________feet.
c. Assuming that this are was exposed to erosion over the last 400 million
years, calculate the rate of erosion? _______ in/yr.
The assumption in c is easy to make, but probably not entirely correct.
Between the present and the time these rocks were emplaced mountains and
rift basins were created and destroyed when Pangea assembled and rifted
apart. Sea level also rose and fell several times, producing episodes of
deposition and erosion.
During the Pleistocene (Ice Age), glaciers advanced over the area at least
twice. South of New York, beyond the limit of glaciation, coastal beaches rest
on and are supplied by variably preserved cover of Late Mesozoic and Cenozoic
coastal plain sediments. Throughout most of New England glaciation stripped
off these younger sediments and deposited their remains on the continental
shelf. Preglacial sediments are found only in the terminal moraine exposed on
Martha’s Vineyard and Block Island. Glaciation also modified the landscape,
creating basins and valleys in weaker rock, and forming streamlined hills from
solid rock and glacial debris.
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d. Review the topographic map and describe the shape of the coast along the
North Shore. Explain how and why the New England coastline is so different
from the straight, sediment-covered coastlines of Delaware and the Carolinas.
e. What is the only event that an unconformity records? ___________________
f. What do unconformities erase?
g. From the evidence found on the outer islands and south of New England do
you that the erosion forming the unconformity was continuous or intermittent
throughout the past 400 million years? __________ Discuss.
h. If we trace this unconformity throughout the North Shore will it always
separate the same rock units and layers of sediment? (circle one and explain)
Yes / No
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Part II – Sedimentary Environments
Stop 3: Pocket Beach
a. Describe the sediment in the Beach:
i. Sorting (circle one): non-sorted, poorly sorted, well sorted
ii. Dominant grain size (circle one): clay and silt, sand, gravel
iii. Angularity (circle one): angular, moderately well rounded, extremely
well rounded
iv. List the components of the beach:
____________________________ , ____________________________
____________________________, ____________________________
____________________________, ____________________________
b. Would you classify this sediment as mature or immature? (circle one and
explain)
c. Name the rock formed if this sediment were lithified.
d. If after 10 million years the rock became exposed, what features would
indicate its age?
e. State a hypothesis explaining the source of this sediment for the beach and
cite your evidence.
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f. Explain why you would not expect to see the same sediment a half mile off
shore.
g. Sketch a map view of the beach and its headland and explain why this
beach is called a pocket beach.
NOTES:
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Part III – More relative age dating and signs of glaciation
Stop 4. Large outcrop on southern shore
a. Two outcrops are cut by a south-dipping dike. Is the valley between them a
fault? (circle one and explain) Yes/No
b. Can the dike be the same age at the basalt dikes seen at stop 1? (circle one
and explain) Yes/No
Signs of Glaciation
The Pleistocene Epoch (1.8 ma – 10 ka) of the Quaternary Period is known as
the ice age. During this time the climate was cool enough for ice sheets to
descend form Canada and cover New England. The glaciers scoured, polished,
and quarried bedrock surfaces, and locally deposited glacial till, a poorlysorted sediment containing boulders and pebbles in a matrix of sand and clay.
Glacial striations, grooves, polished pavements, and streamlined asymmetrical
bedrock ridges and knobs (fig. 2) are all signs of glacial scouring.
Figure 2. Formation of a roche moutonnee beneath a glacier. These small
asymmetrical bedrock knobs are formed by abrasion (up-ice side) and quarrying
(down-ice side). Their presence tells is that 1) the glacier was warm-based,
which means that it was riding on a thin layer of meltwater, and 2) the absolute
direction of glacial flow.
a. Looking along the base of the large outcrops, identify and describe any signs
of glaciation and describe how these features were created.
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b. Tide permitting, observe and draw a profile view of the roche moutonnee
located at the low tide level.
Roche Moutonnee at Stop 4
e. On your sketch draw an arrow indicating the direction of ice flow. Include a
north arrow and scale on your sketch map.
f. Referring to figure 2 explain how this feature was formed.
Part IV: Describing The Landscape.
Make observations of the local landscape and describe it. Is it flat? Hilly? Is the
bedrock deep or shallow? How does the topography compare to what you
observed while completing the topographic map lab? Is there a general
relationship between the topography exhibited on the scale of the outcrop with
that exhibited by the topographic map. What structures do you think control
the locations of valleys? Why do of the hills on the topographic map exhibit
steeper southerly slopes and a NW-SE orientations.
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Part V: The Geology of Salem – Summary Assignment
Write a 1.5-2 page typed report on the geology of Forest River Park, based on your
field observations. Submit as a word.doc through WebCt.
Include:
a) a description of the landscape (3 points) and
b) the sequence of events recognized in the rocks and sediments of the park.
Discuss in order, with the oldest event first. (3 points)
c) Include in your discussion the following terms: intrusion(s), dike, fault,
unconformity, striations, roche moutonnee, erosion, topography, hills, valleys,
coast, headlands, pocket beaches, fill, till, etc. (6 points)
d) Include a discussion explaining how you were able to determine the relative
ages of features in the park, and the events they record. (3 points)
Your essay must be well structured and written and will also be graded on clarity.
NOTES:
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Brief Geologic History of the North Shore
The rocks exposed at Forest River Park are just a piece of the puzzle. Rarely is the
geologic history of an area interpreted from a single location, but from data
obtained regionally. The senario below describes the prevailing tectonic
hypothesis interpreted from studies of the rocks in and around the North Shore
and beyond.
Paleozoic and Mesozoic History
Most rocks underlying Salem, MA are over 300 million years old, and reflect a long
history of volcanism and faulting related to both collision and rifting. The ancient
Precambrian rocks that underlie portions of Boston and the North Shore are part
of the Avalonian Terrane, a micro-continent that collided with North America
during the Paleozoic. These ancient rocks include the 600+ ma Dedham
granodiorite and Lynn Volcanics exposed locally on Marblehead Neck and in Lynn.
Avalon (figs. 1 and 2) was a micro-continent that geologists believe was a volcanic
arc either attached to or located near Proto-Africa (Gondwana). Avalon drifted
towards Proto-North America (Laurentia) while the ocean basin, the Iapetus
Ocean, subducted between the two continental masses. Collisions with North
America occurred during the Ordovician Taconic orogeny and again during SirluroDevonian Acadian orogeny. Avalon is split by a Siluro-Devonian bimodal intrusive
belt suggesting that backarc rifting was contemporaneous with closure of the
Iapetus to the west (figs. 1 and 2).
Figure 1. Tectonic zones of the
North Shore. Avalon is the
microcontinent that collided
with North America during the
Acadian Orogeny. The Paleozoic
Arc Complex is the remains of
the subduction zone. The
backarc rift zone includes a
broad belt of bimodal intrusive
rocks.
The sediments of the Iapetus Ocean were deformed, metamorphosed and folded
into a huge mountainous belt known as the Appalachian Mountains. The belt of
metamorphic schists and gneisses that comprises central Massachusetts are the
deeply eroded remains of the deformed oceanic sediments that were thrust
westward and accreted to the North American Margin.
During the Permian Period Gondwana collided with southeastern North America
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and then moved slightly northward along strike-slip faults. Gondwana underthrust
the Laurentia further uplifting the Appalachians. This third event, known as the
Alleghenian orogeny produced the fold belts and mountains of the southern
Appalachians and metamorphosed rocks in southern New England. These three
orogenic events were part of a series of worldwide collisions that culminated in
the formation of the Pangea supercontinent during the Permian Period. Pangea
remained intact for approximately 100 million years, when it started to rift apart.
Figure 2. Tectonic cross section of Acadian subduction and backarc spreading
during the Acadian Orogeny.
The last tectonic pulse to shape the area occurred during the Late Jurassic and
Cretaceous periods (about 150-80 ma) when Pangea started to rift apart and the
modern passive margin of the Atlantic Ocean began to evolve. Erosion continued,
interrupted periodically by brief periods of deposition related sea level
fluctuations and glaciation.
The Cenozoic and the Present Landscape
The landscape of the North Shore is the product of differential weathering and
erosion of a brittly deformed igneous terrain followed by glacial scouring and
deposition. Irregular basins and straight valleys, developed along faults and
fracture zones. Knobby highlands and ridges are of rock less fractured. Forest
River Park is characterized by this knock-and-lochan topography, a galic term
referring to a glacially scoured region of irregular knobs and basins. The visible
outcrops are bedrock highs poking through an irregular blanket of glacial sediment
(and man-made fill) covering the bedrock surface.
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