Field Trip Two
Florida, MA
Florida, MA
September 19, 2013
Geologic Context:
In this field trip, we see Laurentian basement rocks once again, as well as formations from the rift-drift phase following the Grenville Orogeny. The rift phase is categorized by continental extension, while the drift phase is categorized by seafloor spreading as the Iapetus Ocean continued to open. We see rock units caused by both of these processes in this field trip, with the formations from the passive margin period limited to the continental shelf. Analogous processes occurred in the continental slope/rise and will be explored in more detail in the next post.
Mesoproterozoic:
1.1 Ga, Grenville Orogeny (Basement rocks)
Neoproterozoic to Early Cambrian:
700 Ma: Post-Grenville rifting
Continental Rift to Drift Transition
Breakup of Rodinia
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| 2.i: Lithotectonic map showing the general location of the Week 2 Field Trip in Florida, MA, annotated in purple |
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| 2.ii: Zoomed in lithotectonic mapshowing the location of the week 2 stops within the "rift clastics" |
Outcrop 1: Stamford Granite Gneiss
Mesoproterozoic
Description from USGS MA geologic map:
-Lustrous greenish-gray schist
From the geologist's notebook- field notes
-Coarse-Grained
-Augen gneiss (feldspars are elongate, have an eye-like appearance: "augen" means "eye" in German)
-Contains k-feldspar, plagioclase, quartz, and garnet, muscovite, biotite
-Uneven texture, "nubbly," caused by uneven weathering
-Visible folds
-Core of the folds are less deformed than the outer edges
-Rapakivi granite present (K-Feldspar surrounded by thin rind of plagioclase)
-Strike: 320 degrees NW
-Dip: 14 degrees
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| Figure 2.1, Stanford Granite Gneiss from stop 1. Please note "nubbly" high relief and uneven texture around the pen. The weak foliations, outlined in blue, are indistinct and spaced apart |
Interpretation:
This is likely crystalline Laurentian Basement rock that was deformed after the Grenville Orogeny, likely during the Taconic Orogeny. This is indicated by the gneissic fabric, foliations, and elongate feldspar crystals. The presence of garnet indicates a fairly high grade of metamorphism.
Outcrop 2: Stamford Granite Gneiss
Mesoproterozoic
Description from USGS MA geologic map:
-Lustrous greenish-gray schist
-Stronger, thinner foliations
-Layering is more horizontal, finer-grained
-More weathering --> more protruding, prominent quartz and elongated feldspars
-More weathering likely occurred because the rocks were closer to the surface, exposed to groundwater and other forces before Paleozoic deformation
-Crystals are more elongated than previous outcrop, evidence of further deformation, likely due to its location near the boundary
-Strike: 260
-Dip: 16
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| Figure 2.2, Stanford Granite Gneiss from Stop 2, with thinner foliations, shown in red, can be compared to the farther spaced and less distinct foliations in Figure 2.1 |
Interpretation:
Like Outcrop 1, this is Laurentian Basement crystal. It appears to have had a granite, not sedimentary, protolith, based on the strong foliations and somewhat even texture. Since this outcrop is on the edge of the formation, it likely underwent greater stress than Outcrop 1, which would have been farther from the boundary between formations. It was also likely close to the surface for longer periods of time, allowing groundwater to increase the amount of weathering that occurred. This resulted in prominent, protruding quartz and feldspar.
Outcrop 3: Dalton Conglomerate
Neoproterozoic
Description from USGS MA geologic map:
-The MA geologic map shows Outcrop 3 as a schist in the Hoosac Formaton, like Outcrops 1 and 2
-In the field, we deliberated about whether or not this outcrop showed signs of being a basement rock or a conglomerate, see interpretation for details
From the geologist's notebook- field notes
-Clasts within a matrix of quartz, feldspar and mica (Figure 2.3)
-Uneven texture- not uniform
-Matrix shows visible bedding
-Clasts of different sizes, wide variety
-Quartz clasts are rounded
-Clasts are not parallel to bedding
-Clasts are mostly flattened
-Some clasts appear granitic
-Large pieces of quartz (Figure 2.3)
-Strike: 285
-Dip: 5
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| Figure 2.3, Dalton Conglomerate seen at Stop 3: Example of a clast of quartz circled in blue, elongate and irregular. The elongation of this quartz, a very hard rock, indicates that unlike a circular clast of quartz, it was deformed at very high pressures |
What's a conglomerate, and how do they form?
A conglomerate is a type of rock that has a fine-grained matrix with larger clasts, or pieces of other rocks, that have lithified into the matrix. They can form in a variety of ways that allow for a transportation of material from one place to another-- like bodies of water and glaciers.
Interpretation:
In the field, we discussed whether this outcrop was likely Laurentian Basement or the Dalton Conglomerate, which unconformably overlies the basement rocks. The clasts are flattened, but they are not parallel to bedding. If this were highly deformed granite, we would likely see flattened clasts that followed bedding. Instead, in a conglomerate, clasts are pushed around (usually by water) before becoming lithified. The lack of the uniformity of fabric, coupled with visible bedding (indicating a sedimentary protolith), tells us that this outcrop marks the beginning of the Dalton Conglomerate.
Outcrop 4: Dalton Conglomerate
Neoproterozic
Description from USGS MA geologic map:
-See notes for Outcrop 3
From the geologist's notebook- field notes
-Similar to outcrop 3, conglomerate
-Clasts within a matrix of quarts, feldspar, mica
-More deformation-- quartz crystals deformed
-Variety of deformation of clasts
-Clasts are not parallel to bedding
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| Figure 2.4: Dalton Conglomerate from Outcrop 4, shows more deformation than the similar conglomerate in 2.3. Note that the quartz, circled in red, is not parallel to the direction of the foliated grains, outlined in blue |
This stop is similar geologically to Outcrop 3. The higher degree of deformation is likely due to the location within the outcrop, showing that different areas respond differently to similar stresses, such as collisions and shearing.
Outcrop 5: Conglomerate/meta-arkose (?)
Description from USGS MA geologic map:
-Gray conglomerate
-Contains pebbles of albite and blue quartz
-Boulders of gneiss
From the geologist's notebook- field notes:
-Likely the beginning of the arksose on this hike
-Grain size smaller than previous outcrops-More quartz in the matrix, some small quartz pebbles
-Thinner beds
Interpretation:
The graded bed seen here was likely the result of changing energy levels as rifting transitioned to drifting, the ocean opened, and a passive margin was formed along the Laurentian margin, allowing thinner material to settle, undisturbed.Arkose is a sedimentary rock--a type of sandstone--that is rich in feldspar and quartz. It often forms as the result of weathering of granites or other igneous or metamorphic rocks.
Outcrop 6: Finer-grained conglomerate in a quartz-rich matrix
-Strike: 305
-Dip: 16
Outcrop 7: Arkose
Outcrop 8: Interbedded Arkose and Conglomerate
Descriptions and interpretations: See Outcrop 5
These outcrops demonstrate that these layers unconformably overlie the basement rocks. As we hiked, we saw changing amounts of deformation and differing compositions.
Outcrop 9: Garnet Schist
Lower Cambrian
From the geologist's notebook- field notes:
-Hoosac schist
-Contains garnets, small and red, unlike previous outcrops
-South-facing cliff caused by glaciation
-Strike: 260
-Dip 16
Interpretation:
The Hoosac Schist, with its fine grain size, overlies the sediments deposited during rifting. The Hoosac Schist, therefore, was likely deposited during the passive margin phase of Laurentia, when small clasts had time to settle and lithify. It was likely also formed in deeper water than the conglomerate, which was formed in a high energy environment, but nonetheless, the Hoosac Schist appears to have continental origins. Garnets indicate a high degree of metamorphism, likely from the Taconic Orogeny. The cliff we saw at this site was sheared by glaciation.
Conclusions
In the last blog post, we looked at this image from the Allen et al. paper on the Northeastern Laurentian margin:
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| 2.5: Diagram adapted from Allen et al, 2010, shows sedimentation of the Laurentian rift zone |
It is important to look at the additional information we can gain from this week's field trip, which adds in slope/rise deposits to the picture. In a similar fashion to both the field trip from week one and Allen et al's paper, we first saw crystalline basement rocks, this time in the form of the Stamford Granite Gneiss. The basement rocks show differing degrees of deformation depending on their location within the rift zone. Overlying the basement rocks are the synrift deposits, formed on the continental slope/rise, shown as "iv" in Figure 2.5 above. We see a large variation in sedimentation within the formations comprising this zone, which further supports the claim made by Allen et al. that a detachment model of rifting results in different sedimentation within different parts of the rift zone. We see a variation between the outcrops of Dalton Conglomerate with varying degrees of deformation, meta-arkoses, and nearly pure arkose.
Unlike the shelf deposits, which are formed in shallow, usually warmer waters and produce meta-carbonates and quartzites, the slope/rise deposits are formed in the deeper water of the slope/rise. In these deeper water and lower energy environments, rocks like shales are formed. It is in this environment that we see the arkose that overlies the conglomerate. The garnet schist seen in outcrop 9 was once a mudstone formed in the deepest water that we see on this field trip. The propensity towards darker, heavier rocks continues in next week's lab, when we see sediments that were once attributed to the deep ocean.
To see the Google map of these sites, go here!
"Earth, an Introduction to Physical Geology, tenth addition," by Tarbuck, Lutgens, and Tasa is used as a reference
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