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Figure 1: Interactive map showing the field trip stops. Interact with it to gain a sense of where these stops are relative to each other.
Figure 2: Physiographic map of the Southeast US with stops shown by markers. Physiography is the combination of the geology and surface expression; you can see this reflected in the outcrops.
Introduction
Introduction
“We cannot take one step in geology without drawing upon the fathomless stories of by-gone time” -Adam Sedgwick
The geology of the Southeast is a hidden gem, of which most people only get a glimpse. Geologic processes have given us rocks ranging from Paleoproterozoic (1.8 Ga) to ongoing morphology that we can observe. The SE contains the tallest mountain east of the Mississippi (yea, I said it, New Hampshire), has a huge variety of mines, and has coastal areas affected by the Gulf Stream and north of the Gulf Stream. The geologic history of the Southeastern US is a dynamic one, shared by most of the east coast states. It’s been affected by continental collision, accretion, and rifting. It had tropical seas, huge deltas, and ocean basins on its front porch.
The range of geologic assets we currently have and have offered in the past is huge. In the mountains, there are countless different mines-producing everything from garnets, to emeralds, to corundum. These now highly valued gems formed from the high-grade metamorphism that took place during mountain-building episodes millions of years ago. If you go even slightly east of there, you enter into one of the first-ever gold rush sights. If you go towards central NC, just east of Greensboro, you reach coal mines that were shut down less than a hundred years ago. These were deposited in basins during the Triassic. Down around Charlotte, you have the Kings Mountain belt, where there are kyanite and other unique minerals (mined for their aluminum). Throughout the Southeast, there are granite and graphite mines that are active today. Even the coastal areas have mineral sands and evaporite deposit mines, in addition to offering a unique chance to observe modern fluvial and sedimentary processes, as well as the effects of the gulf stream, while also telling us about the effects of past sea-level rise.
The Geology of the Piedmont and Blue Ridge
The Geology of the Piedmont and Blue Ridge
We'll be spending some time on this “trip” in the Blue Ridge, and, while the piedmont is certainly a different place than the mountains, most of the rocks we see share many similarities between the two (especially compared to the coastal regions which differ greatly). Keep in mind what the Piedmont really is - it's the weathered and eroded areas of what used to be mountains. So it shouldn’t come as a surprise that they share many of the same rocks.
Most of what we see in the piedmont and blue ridge has been altered by metamorphic events or igneous intrusions. With that being said, there are several extensional basins that have formed in central NC - but those aside, most of the rocks have undergone intense alteration.
The most significant boundaries within North Carolina's Piedmont are faults that separate large pieces of crust called terranes. Collisions between tectonic plates welded the terranes to the edge of North America during Paleozoic times during several mountain-building events or orogenes.
Let's go through the orogenic history of the eastern US:
The Grenville Orogeny was a mountain-building episode that finalized the formation of the supercontinent of Rodinia. The orogeny itself took place from 1250 to 980 Ma, but rocks as early as 1800 Ma can be attributed to early Rodinia forming episodes. This formed the base for the Appalachian mountains - though few of these rocks are remaining and exposed in NC. After the formation of a supercontinent, eventually, things rift apart again, and we get the opening of an ocean basin (rift-to-drift sequence)
The Taconic Orogeny was the first orogeny to form the Appalachians as we know them and is given an age of 490-440 Ma. This happened as the previously passive margin on the east flank of North America became an active margin. The resulting collision and accretion is more visible further north- but still had an effect on the metamorphism of some of the Southeast.
The Acadian Orogeny was the second collisional episode to form the Appalachian and is given an age of 375-325 Ma. Once again, while the majority of its effects were further north, it did result in the accretion of the Carolina terrain.
The Alleghanian Orogeny was the final orogenic event to contribute to the formation of Appalachians- and is the most responsible for what we see. it lasted from roughly 325-260 Ma and resulted in Pangea forming.
While these are the events that lead to the formation of the gneiss, schists, and mountains we see, there had to be previously existing rocks to undergo this metamorphism.
Unlike the sediments currently in the Coastal Plain, the protoliths (the rocks that existed before they underwent metamorphism) of the metamorphic rocks of this region had to have originated in deeper water. We know this for several reasons; 1) The presence of mica. Mica (muscovite and biotite in particular) forms from the metamorphism of clay minerals (just as clay minerals form from the chemical erosion of micas). In coastal sedimentary deposition, lighter, smaller particles can be carried further out to sea, while heavier particles are deposited closer to land (figure 4). If the proliths have been from a shore or upland deposit, we would expect there to be a large amount of sand present- which when metamorphosed would form a quartzite. But we very rarely see quartzite in the piedmont or blue ridge. 2) Instead of clean, quartz-rich sandstone typical of shallow water, sand settling in deep water tends to have abundant clay mixed in it. This type of “dirty” rock is called a graywacke. Its metamorphosed form, called metagreywacke, is common in the piedmont.
Figure 3: A sedimentary facies diagram showing how increasing distance from land decreases particle size. Facies just refers to the environment a rock formed in. Thus, something in the sandstone facies formed close to the shoreline. You should notice that the sedimentary facies decrease in particle size as one moves away from the coast. (Pearson 2005)
Figure 4: Development of metamorphic fabrics with mica as an example. This shows how particle size increases with increasing grade.
For metamorphic rocks, the growth of new minerals and the deformation resulting from tectonic movements creates fabrics (textures, bedding, mineral orientations) which obliterate the original sedimentary and volcanic fabrics. Many metamorphic rocks exhibit a fabric called foliation represented as cleavage, schistosity, and gneissic banding (figure 4). Besides fabrics, metamorphic rocks are characterized by their grade (figure 6) based on the new mineral assemblages that may form in a rock under increasing temperature and pressure conditions.
Figure 5: Metamorphic facies (explained below)
Figure 6: Zones of common metamorphic minerals (explained below)
Beyond metamorphic rocks, the Piedmont and Blueridge also contain igneous rocks that developed from magma. Magma is less dense than solid rock lending it the ability to rise through cracks into other rocks where it can cool and crystalize as an igneous intrusion. Some of the intrusions were later affected by metamorphism. For example, a rock that is dominated by the lighter colored minerals but has relatively thin bands of dark minerals probably began as a granite.
Diagrams and How to Use Them:
Diagrams and How to Use Them:
General:
Figure 7: Here we have the geologic time scale. The ages listed are in millions of years (Ma), and the youngest times are in the top left, while the oldest are in the bottom right. Note that not every inch of this chart represents the same amount of time — make sure you read ages!
Figure 8: Mohs Hardness Scale (might find this useful for Stop 12).
Figure 9: A cheat sheet for estimating percentages (Modal Mineralogy)
Metamorphics:
Figure 10: This is a metamorphic facies diagram, which can tell us a lot about how a rock formed. Each group shown can only be formed if a certain temperature and pressure condition is reached- which can only happen in particular tectonic situations.
On the X-axis here is the temperature required, and on the Y-axis is the pressure. Each colored area shows the metamorphic grade that results from temperature/pressure conditions.
Figure 11: Take a look at this next diagram. You should notice that though it looks different than figure 11, it really is telling you the same thing- just with the pressure/depth (Y) axis inverted. You should also notice number lines drawn that tell us what tectonic conditions can lead to these grades of metamorphism. If you have additional questions about this diagram please contact me, and I will be happy to talk through it with you.
Figure 12: This diagram shows the presence of minerals and the resulting grade. If you pick up a rock, the first sign that's going to tell you "hey this might be metamorphic" is going to be a foliation (or possibly a very large metamorphic mineral). So if you pick up a rock, and notice it either has banding, or a well-defined cleavage (Not bedding! If it has bedding it is sedimentary) you're likely dealing with a metamorphic rock. You can then roughly determine the grade based on the minerals present. If you can find one mineral, you determine a single zone to look at. If you can find two, you know you're in an area where those zones overlap.
One note for my more experienced people (if you haven't had mineralogy, and are still with me this far, don't read this part! I don't want to confuse you): It's not quite this simple. You know that minerals can be unstable at certain conditions, which can cause them to decay, and if we apply pressure to a rock to form a mineral, but keep applying that pressure then that mineral may start to transition to something else. Pay attention to whats forming where. Do you see biotite? but is it present around the rims of garnet? then you're dealing with something that's likely not stable- and should figure out whats transitioning to what- this can help you determine if you're along a reaction line.
Figure 13: If you've not taken petrology, this diagram might seem really complicated, but I promise you it doesn't have to be! This is a simplified version of a KFMASH diagram.
On the x-axis are temperature conditions, and on the y-axis are pressure conditions. It's important to keep in mind that neither of these stop where the diagram ends- temperatures can and do get hotter and cooler than this, and pressure gets much greater.
Shown on this diagram are the different conditions that certain minerals can exist at: which means if we know the minerals present in a rock, we can determine what conditions those minerals formed under.
This diagram uses a lot of abbreviations, and you don't need to know them all for this trip. Mineral abbreviations are standard (I didn’t make them up), so a simple google search should help you find any that you might need (They’re also in your petro book!), or I’d be happy to give you a pdf of them (just ask).
These are the ones on this diagram: ms=muscovite, chl=chlorite, qtz=quartz, grt=garnet, bt=biotite, st=staurolite, and=andalusite, sil=sillimanite, crd=cordierite, kfs=k-feldspar, spl=spinel
Every red line is showing a chemical reaction. meaning if you start on the left of the line, and increase temperature until you cross the line, the minerals listed on the left of the line will turn into the minerals listed on the right of the line (or you could do this in reverse, or it could be pressure driving it, or it could be a combination of temperature and pressure). The black lines show a mineral transition. Kyanite, sillimanite, and andalusite all have the same chemical formula (they’re polymorphs), but depending on the conditions form different minerals.
So, if you know the minerals in a rock, you can use them to determine an area on this chart where your rock formed. The area will include all the minerals present between the reaction lines that bound it.
If this still doesn't make sense, do not fret. We'll do an example with it in stop 1!
Figure 14: At low grades, we form metamorphic rocks known as slates. Slates have a weak foliation that you likely can't see- but can see them break on. What you don't see is those tiny clays in that rock have all turned so that they're facing the same directions. Think about if you have a bunch of playing cards (the clays) at random angles inside a peice of playdoh. If you squish the playdoh from two sides, the cards are going to rotate to be perpendicular to the squishing force. This creates a weak foliation and cleavage plane within the rock.
As we go to higher metamorphic grades,you start to see rocks known as Phyllites. Now, not only are the grains alligned, but they're starting to grow larger- particularly the micas, giving the rock that sheen that you will learn to recognize.
At a higher grade we see Schists. In these rocks the grains are alligned on a very plannar surface, and the minerals have grown very large- large enought that you can see them without a microscope or handlens.
At even higher grades we transition to gniesses. At this point minerals are separating to bands of like-minerals, and we get compositional banding.
Sedimentary:
Figure 15: A diagram for determining the roundness, and sphericity of a grain. Roundness refers to how smooth it is. Sphericity refers to how spherical it is. The rounder a grain is, the more erosion it's been through. This could reflect the distance it's travelled, or the environmental conditions it was under (wet and humid vs. cold and arid).
Figure 16: A diagram to sort sediment size. We use the words in the right column when describing sedimentary rocks, as they have a specific size range they refer to. Meaning if you say "pebble" your referring to a grain/rock that between 4 and 64 millimeters in diameter.
Figure 17: A diagram showing textural maturity. Similar to what's described back in figure 3, but in a more quantifiable way. The more mature a rock is, the more physical/chemical weathering it's undergone. Mature rocks, in general, have little to no clay, are well sorted (meaning all the grains are about the same size), and are well rounded (meaning not angular).
Figure 18: Goldich Stability series, shows ease of weathering (it's Bowens turned upside down...). The minerals at the top are the most stable at Earth’s surface (as they crystalize at the lowest temperatures). While the minerals at the bottom are the least stable, and are easier to weather as they are the furthest from their crystallization temperature. This is why we see quartz sand at a beach, and not olivine.
Igneous:
Figure 19: Shows the order and temperatures that minerals crystalize at (idealized). The minerals at the top crystalize and melt at the highest temperatures. The minerals at the bottom are the last to crystalize/first to melt.
Figure 20: Instusive QAP diagram
Figure 21: Ternary diagrams for describing volcaniclastics. Please see stop 11, for how to read a ternary diagram.
(To go back to where you were if you jumped to read through this)
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