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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 Feature Event Recorded 5. 4. 3. 2. 1.

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 poorly-sorted 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 Sirluro-Devonian 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.