Deerfield Basin Geology Field Trip: Explore, Sketch, Learn

Chapter 6 Deerfield Basin Field Trip About this lab: UMass Amherst is located in the Deerfield Basin, a sedimentary rift basin that started forming 200 million years ago. This trip will allow you to become familiar with some of the basic, local geology. This field trip will place concepts learned in class into a geologic, field-based context. Geology is predominantly a hands-on, field-based science, so looking at real rocks in their geologic setting is critical to fully understanding geology. Chapter 7 of Written In Stone discusses the rifting that happened in the Northeast during the Mesozoic. The Deerfield basin is the northern extent of a failed rift along the Connecticut River Valley. Chapter 7 will provide a more regional context for what we will see on this field trip. It is well worth reading this chapter before going on the field trip. Learning Objectives: You will be able to: • Sketch geologic features and characteristics; • identify geologic characteristics of rocks in outcrop; and • relate geologic characteristics to geologic history. An answer sheet is at the end of this lab. Please look at it now so you know what questions to answer during the field trip. Introduction As the supercontinent Pangea broke apart during the Triassic, a series of rift basins were produced along eastern North America. The Deerfield basin is one of those rift basins (Figure 6.3). Sedi- mentary rocks and volcanic basalt flows filled the basin. During the Triassic, this area may have These field trip materials were selected from the Dynamic Digital Map of the Deerfield Basin. The project was overseen by Professor Chris Condit of the University of Massachusetts Amherst. Peter Ames, Eric Helfrich, and Mary Ellen Loan made contributions to this document. The Dynamic Digital Maps includes interactive photos, animation and flythroughs. To see the DDM visit: http://ddm.geo.umass.edu/ddm-sedrxwma/index.html 1

Geology 101/131 October 20, 2022 Figure 6.1: Field trip location map. Figure 6.2: A cartoon cross-section of the Deerfield Basin. 2

Geology 101/131 October 20, 2022 Figure 6.3: Block Diagram of the Holyoke Range. looked quite similar to the present day East African Rift system. Modern-day analogs help us better imagine what was happening in similar past environments. We can match features in the rock record to present day features to better understand the processes that occurred in the past. Figure 6.2 shows the shape of the rock layers within the basin. The basin’s rock layers dip to the east, towards the normal fault that helped form the basin. Follow along in this field trip guide and then answer accompanying questions. Make sure to reference the map and geologic timescale on this page to put each stop in spatial and temporal context. At each stop , turn to the last page of this chapter of the lab manual and make a sketch of a 1 foot by 1 foot area of the outcrop (showing relative grain size, shape, etc.) in the correct box. From oldest (bottom) to youngest (top), the stratigraphy consists of five units (Hubert and Dutcher, 1999) (Figure 6.4): • Mount Toby Conglomerate (300 - 2000+ m thick) • Turners Falls Formation (2000 m thick playa, lake, and river deposits) • Deerfield basalt (0 to 100 m thick) • Fall River beds (0 - 9 m thick lake deposit) • Sugarloaf Arkose (800 - 2300 m thick river deposit) From Wikipedia, a geological unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, litho- logic or paleontologic features (facies) that characterize it. 3

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Geology 101/131 October 20, 2022 Figure 6.4: Stratigraphic column of Deerfield Basin basin-fill deposits. 4

Geology 101/131 October 20, 2022 Stop 1: Mount Sugarloaf Unit 1: Sugarloaf Arkose Mount Sugarloaf is made up of the oldest unit in the Deerfield rift basin, the Sugarloaf Arkose (Figure 6.5). This unit can be as thick as 2 km in some places. This red sandstone consists of large (up to several centimeter) angular and rounded clasts that vary locally. The formation ranges from an angular conglomerate to a siltstone. The large amount of preserved feldspar and the abundance of angular clasts suggests that this material was transported only a short distance from its source rock and rapidly buried. The source of this material is the Pelham Hills to the east, which can be seen in the distance (the far hills to the east that lie beyond Mt. Toby). We use the date from the initial deposition of the Sugarloaf Arkose to determine the timing of faulting along the Eastern Border Fault. The high levels of potassium feldspar and the iron-rich cement that holds this unit together cause the distinct red color. The valley from the border fault to the east deposited the sediments of the Sugarloaf Arkose. Many former stream beds and flood events can be seen in the outcrop. The channels are wide, flattened “V” shapes that have large grains in the bottom, called channel lag. Numerous channels are exposed in this outcrop suggesting a highly dynamic depostional environment. The Sugarloaf formation at Mt. Sugarloaf is resistant to weathering and erosion compared to many other exposures of the Sugarloaf formation. Here the sands are cemented with an albite cement which almost certainly reflects hydrothermal alteration of the sandstone by hot, Na-rich fluids percolating through the sediments. This was probably the site of a hot spring in the Triassic, much like places in Death Valley today. Stop 2: Stop & Shop An outcrop spanning the upper Sugarloaf Arkose to the lower Deerfield basalt is behind the Stop & Shop grocery store along Route 2A (High Street) in Greenfield, MA (Figures 6.6). This Sugarloaf Arkose is the same unit as the one we just saw at Outcrop 1. However, it has a different color here; grayish-tan instead of the usual brownish-red. Here, hot fluids leached out the brownish-red iron-rich compounds and altered the minerals within the Sugarloaf Arkose. This area might have had a lot of hot springs. Geologists estimate that the fluids must have been as hot as 200 ◦ C in order to alter these minerals, much too hot to soak in! This outcrop of Sugarloaf Arkose is highly fractured. Many faults are present too. Find where layers are mismatched and you have found a fault. You can also see slikenlines in the outcrop. Slikenlines and offsets across fractures indicate faulting. One fault is highlighted in Figure 6.8. Figuring out how the fault slipped (which side moved in which direction) is more difficult. The rocks don’t have big clear arrows like in this photo. Geologists often want to know the dip of sedimentary deposits. The dip can help determine the geologic story of the area. Which way are the beds of the Sugarloaf Arkose dipping? Is this 5

Geology 101/131 October 20, 2022 Figure 6.5: Photograph of the Sugarloaf Arkose at Outcrop 1 with ancient river channel highlighted in white. consistent with the cross-sectional sketch in the introduction to this field trip? How might this information help you write part of the story of the Deerfield Basin? Unit 2: Fall River Beds Some geologists have noticed that the mudstone layers in this outcrop have more plants growing out of them than the coarser-grained layers. The relationship between vegetation growth and rock type is the result of groundwater moving through fractures in the arkose. When groundwater encounters a fine-grained, low permeability mudstone, the water flows along that layer to escape the outcrop as a seep. These seeps support the vegetation that you see. Geologists often correlate the vegetation on an outcrop with observed geologic features to better trace those features. But we need to be careful that the vegetation mimics the geology. That is not always the case, or the vegetation may change even though the geology stays the same. In some spots, the Fall River beds are deformed near the contact with the Deerfield basalt (Figure 6.7). Geologists have interpreted this deformation to mean that the Fall River beds must 6

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Geology 101/131 October 20, 2022 Figure 6.6: This aerial view (left) shows the outcrop to be west of the Connecticut River, approxi- mately 500 m downstream of the Turners Falls dam. Much of the outcrop is grass covered, but we can imagine the extent of the rocks. Poor exposure is a real problem for New England geology! Figure 6.7: Above is another aerial view, this one closer in. On the right are outcrops with glacial striations and armored mudballs. Behind the store are large outcrops of Sugarloaf Arkose, Fall River beds and Deerfield Basalt. The best way to see the basalt is to hike up the hill above the large outcrop. 7

Geology 101/131 October 20, 2022 Figure 6.8: Outcrop of Sugarloaf Arkose in Greenfield, Ma. 8

Geology 101/131 October 20, 2022 Figure 6.9: Typical Pillow Basalt ( Not from this Outcrop). have still been soft when the basalt was flowing on top of them. This soft sediment deformation is consistent with the observation of pillows at the base of the Deerfield basalt. From these types (and many other) of observations, we can infer that the Fall River sediments were sitting in marshes and lakes when the lava extruded and flowed into the lakes and the basalt quickly cooled. Unit 3: Deerfield Basalt You can’t see the basalt from the road behind Stop & Shop but a trail leads up the hill to the basalt. During the rifting of the valley, many volcanoes developed. The Deerfield Basalt erupted from one of these volcanoes, flowed over the land surface, and then cooled rapidly as it entered a shallow lake. How do we know this? This basalt is not square and blocky (Aa type lava), but has rounded pillow structures. The rinds on the basalt indicate that these “pillows” formed by rapidly cooling underwater. The Deerfield Basalt also has internal layers or horizons that contain abundant, small bubbles called vesicles. Vesicles form because gas bubbles are unable to escape the quickly cooling lava. Lava cools quickest where it is exposed to the air, so vesicles are most abundant near the tops of lava flows. 9

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Geology 101/131 October 20, 2022 Stop 3: Geologic Contact These basalts are much different from the ones we just saw even though they are the same formation. Here the basalts are blocky and fractured (Figure 6.9). The lava beds are dipping towards the east and the majority of fractures are perpendicular to bedding. These cracks are known as columnar joints, an indicator that the lava cooled (relatively) slowly, shrinking as it cooled. This cooling history differs from the last stop where the pillows indicated that the lava cooled instantly. Let’s think about a couple of things here. First, slowly is a relative term. The lava here cooled more slowly than at the previous outcrop, but this lava still cooled much more rapidly than a magma cools at depth. We need to keep context in mind when we use relative terms. Lava’s slow is much different than magma’s slow. Secondly, these lavas are erupting at the same time, but at different locations. So even though this is the same lava from the same eruption event, the lava has different character at each place because of the different conditions locally. Without the lake, we would not have pillow structures in the lava bed. This example shows the importance of spatial thinking in geology . The same thing is happening at different places, but the character of the rocks, in this case a lava flow, is different. Another feature found in the lava here looks like finger holes in a bowling ball. These are places where rock samples were taken for paleomagnetic studies. Mafic igneous rocks, like basalt, contain iron-rich minerals that align with the Earth’s magnetic field as they cool. We can use their declination and inclination (the angle of these minerals and their dip) to determine their latitude and longitude at the time they cooled . These rocks were around 20 degrees North latitude, placing them somewhere in the Caribbean today (if only it stayed that way). Unit 4: Turner’s Falls “Sandstone” We can also see the contact between the Deerfield Basalt and the Turner’s Falls sandstone, which is a lacustrine (lake) unit that grades from shales to siltstones. (How do we know it is a lake or a quiet-water environment?) Even though not much of the Turner’s Falls sandstone is visible, the abundance of vegetation on this formation indicates its presence, as we discussed before. We will be seeing this formation closer up at our next stop. Here is a question to ponder: Why are the rocks of the Turner’s Falls formation called a sandstone if it contains shales and silts? Let’s use this as an opportunity to discuss the “vagueness” of geologic terms. It isn’t necessarily wrong to refer to this rock as a sandstone. But the naming of a rock is often more complicated than easy descriptions allow. Stop 4: Barton’s Cove We see a number of finely-bedded mudstones at this site.These black shales and siltstones are lake deposits, that is, the basin must have been underwater again. The dark color suggests that the depositional environment was anoxic, or low in oxygen, like the bottom of a large lake whose water does not mix a lot. Think of that black, mucky ooze on the bottom of lakes. 10

Geology 101/131 October 20, 2022 Figure 6.10: Ripple marks from Barton’s Cove. 11

Geology 101/131 October 20, 2022 These polygonal cracks form because of desiccation or drying. Common in tidal settings, the sediment becomes saturated at high tide and, as the water moves away, then dries, forming these cracks. The same thing can happen during rainstorms and sea level changes. Can you think of a modern analog? Ripples marks are also seen at this stop. These features are evidence of flowing water. Symmet- rical ripple marks indicate back-and-forth water flow, such as waves flowing in and out, whereas asymmetrical ripples indicate unidirectional motion, such as in a stream or river. The boulder that we are looking at fell from the overlying outcrop, so we can see the ripples that exist in the outcrop a little bit closer. If you are lucky you might be able to find some dinosaur footprints at this stop! These chevron folds have variable interlimb angles but the limbs are mostly straight and the hinges are kinked, not rounded. The elaborate folding of the strata here is yet to be explained in great detail. Suggestions range from soft-sediment deformation (a landslide) to tectonic distur- bances (local faulting). References: Hubert, J. and Dutcher, J. 1999. Sedimentation, Volcanism, Stratigraphy, and Tectonism at the Triassic-Jurassic Boundary in the Deerfield Basin, Massachusetts. Northeastern Geology and Environmental Sciences, v. 21, no. 3., p. 188-201. Nichols, Gary. 1999. Sedimentology and Stratigraphy. Blackwell Publishing Co., Malden, MA. 12

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Geology 101/131 October 20, 2022 Name: TA: Answer Sheet: Deerfield Basin Field Trip Outline of the Connecticut River Valley’s 4-phase History • Mountain building and metamorphism (450 – 300 MYA) • Bulging and rifting of Pangaea (250 – 150 MYA) • The rift basin filling with sediments as Eastern Border Fault dropped down (250 MYA – 180 MYA) • Erosion, uplift, and recent glaciation (65 MYA to present) Answer these questions during the field trip. Outcrop 1: Jurassic Sediments 1. What is the grain size at this outcrop (circle as many as necessary): cobble coarse gravel fine gravel sand silt clay What does the grain size imply about the energy of the system? highest energy (near source) high energy (further down- stream) mid-level energy (gen- tly sloping stream) low energy (slow moving stream) lowest energy (quiet water) 2. What is the grain shape at this outcrop (circle as many as necessary): What does this imply about how close the source of the sediments is? near-source in-between distant 3. This outcrop is a reddish-pinkish color. Knowing this, what do you think the dominant mineral in this sedimentary rock is? 13

Geology 101/131 October 20, 2022 4. Because this is a fluvial channel deposit, the source of this rock is relatively nearby. What kind of igneous rock could be the source of this deposit? 5. What is the name of the unit seen at this outcrop? Outcrop 2: Stop & Shop 6. What is the grain size at this outcrop (circle as many as necessary): cobble coarse gravel fine gravel sand silt clay 7. What is the grain shape at this outcrop (circle as many as necessary): 8. Are the grains here larger or smaller than at the last stop? (circle one): larger smaller 9. What is the name of the unit at this outcrop? 10. What type of fault can you see? (circle one): normal reverse/thrust strike-slip 11. What type of tectonic setting does this imply? (circle one): lateral displacement compressional extensional Outcrop 3: Stop & Shop (up the hill, in the woods) 12. Look at and describe the “grains” seen here. ( Hint : Are we looking at a sedimentary rock?) How do they differ from the grains you saw at the bottom of the hill? 14

Geology 101/131 October 20, 2022 13. What is the name of the unit at this outcrop? 14. What do the pillow-shaped structures say about the setting in which this rock was formed? Outcrop 4: Route 2 Pull-off near Turner’s Falls 15. You have driven east, across the CT River valley. Knowing that the regional dip direction is east, are you passing through younger or older rocks? (circle one): younger older If we were looking at the stratigraphic column, would you be going up the column or down the column? (circle one): up down 16. You are looking at the contact between two units. What are the full names of these units? Outcrop 5: Barton’s Cove 17. We can recognize a few sedimentary features at Barton’s Cove. Mudcracks are exposed in many places. Make a simple sketch of these mudcracks. 18. The folding of layers is complex in some areas here. Make a simple sketch of these folds. 15

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Geology 101/131 October 20, 2022 19. What is the grain size at this outcrop? (circle as many as necessary): cobble coarse gravel fine gravel sand silt clay 20. What is the grain shape at this outcrop? (circle as many as necessary): 21. What is the name of the unit at this outcrop? 22. We saw a couple examples of deformation at this stop. Draw a quick sketch and explain, to the best of your abilities, why and how it formed in this area. 23. Well-preserved ripples are a strong indicator of the depositional setting of these sediments. What kind of depositional environment do these ripples indicate and why? Notice that as a geologist, we look at things like clast shape, size and composition, when trying to understand clastic sedimentary processes. 16

Geology 101/131 October 20, 2022 Conclusion: Fill in the “sketch” column with your sketches from each outcrop. Then write a brief description next to each one that includes the type of rock (sedimentary, metamorphic, igneous), the name of the rock, a physical description of the outcrop (including any obvious minerals or large characteristics you saw), and the depositional setting (ocean, desert, lake, mountain top, etc.). Sketch (1 ft. x 1 ft.) Type (circle one) Name & Description Depositional Setting Outcrop 5 Sedimentary Metamorphic Igneous Name: Description: Outcrop 4 Sedimentary Metamorphic Igneous Name: Description: Outcrop 3 Sedimentary Metamorphic Igneous Name: Description: Outcrop 2 Sedimentary Metamorphic Igneous Name: Description: Outcrop 1 Sedimentary Metamorphic Igneous Name: Description: 17

Lab 6 - Deerfield Basin Field Trip

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