Monday, July 30, 2018

Granite “Pavement” Exposure – Mitchell Mill State Natural Area - Rolesville, NC


Rock Type:  Granite

Geologic terrane or major geologic element:  Rolesville batholith

Age:  Late Paleozoic – approximately 300 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Rolesville

Site Access:  Heading south on Highway NC 96, there is a space to pull off the road on the right.  Park here and follow the path by the big blocks of rock.  You will see an extensive exposure of granite.  From this exposure, follow the trail from the back corner, near more blocks, and you will come to an even larger expanse of granite.  This site is Mitchell's Mill State Natural Area, part of the NC State Parks system.

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Technical Information:  Speer, J. A., 1994, Nature of the Rolesville batholith, North Carolina (pages 57-62 in Carolina Geological Society Field Trip Guide, 1994); See also description for Stop 11 on pages 105-107.
Speer, J.A., McSween, H.Y., and Gates. A.E., 1994, Generation, segregation, ascent and emplacement of Alleghanian granitoid plutons in the Southern Appalachians:  Journal of Geology, v. 102, p. 249-267.
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Introduction
       The Rolesville batholith is the largest body of granite in the southern Appalachian region.  It measures about 15 x 50 miles, and occupies the eastern third of Wake County, and about two-thirds of Franklin County.  At this site there is a huge flat exposure of granite that is a part of the batholith.

The rock body
       A pluton is a three-dimensional body of igneous rock.  Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which by definition covers an area greater than 100 square kilometers.
       Granite is an igneous rock that occurs in three-dimensional bodies called plutons.  Granite plutons range in size from a vein-like dike just a few inches wide up to the size of a batholith, which (by definition) covers an area greater than 100 square kilometers.  In three dimensions, plutons may be shaped like mushrooms or inverted teardrops, so they may extend to considerable depth beneath the surface. In the eastern Piedmont, almost all granitic plutons are between 280 and 320 million years old – Late Paleozoic in age (Figure 1). The largest such body is the Rolesville batholith, which is (in reality) ten or twenty separate plutons that intruded the same region over a period of geologic time, cutting across each other and coalescing to form the batholith (a mega-pluton). 

The rock itself
       Granite is composed of two types of feldspar – orthoclase and Na-plagioclase, plus quartz and biotite mica. Sometimes it may contain hornblende or muscovite mica or garnet. It always also contains some tiny minerals in small amounts. Granite is the most common type of rock that is quarried for crushed stone, used in making concrete and asphalt, among other things.  The granite here is a medium-grained biotite granite.

       You may notice certain features in the granite at Mitchell’s Millpond (Figure 2).  Cracks or fractures in the granite are of two types.  Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock.  Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses.  Below the surface, groundwater moves along these cracks.  You may also see thin bands of slightly different rock cutting across the granite (Figure 3).  These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite).  In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).


Origin
       The granite magmas that intruded this region during the late Paleozoic were generated in the lower crust of the earth, and moved upward until they stopped, cooled, and crystallized. The magmas formed as a result of the tremendous collision between continental plates that raised the Appalachian Mountains. Many of these magmas ascended along fault zones that were active at that time, such as the Nutbush Creek fault that runs through central Wake County. There is no evidence that any of these granitic magmas reached the surface – if they had, we would expect to see some remnants of volcanic rock of the same composition and age.
       You may notice certain features in the granite at Mitchell’s Millpond (Figure 2).  Cracks or fractures in the granite are of two types.  Exfoliation fractures are parallel to the earth’s surface; they form as the result of slow erosion that removes the weight of the overlying rock.  Joints are sets of parallel, generally steeply dipping cracks that are related to horizontal stresses.  Below the ground surface, groundwater moves along joints; if there are numerous joints and they intersect, then wells can produce ample water.  In eastern Wake County, a majority of homeowners rely on wells drilled into the granite.  You may also see thin bands of slightly different rock cutting across the granite (Figure 3).  These may be quartz veins, consisting of only gray, glassy quartz, or they may be aplite (fine-grained white granite) or pegmatite (coarse-grained granite).  In addition, you may see biotite schlieren, which are dark patches of biotite mica that formed during intrusion and crystallization of the magma (Figure 4).

Surface features
       This 93-acre site is a Registered Heritage Area.  It contains some of the best examples of native plant communities that grow in such a “granitic flatrock” environment.  You can see that erosion by running water proceeds very slowly, due to the resistance of this hard rock.  Note the numerous potholes (Figure 5).  You may notice that some of them line up along a fracture in the granite.  These potholes form over time as the result of small pebbles that get caught along a crack, and then drill away at the bedrock when the stream water is high and fast.


Figure 1.  Late Paleozoic granite plutons in the southern Appalachian Piedmont (RV=Rolesville).  From Speer and others (1994).

Figure 2.  Granite “flatrock” at Mitchell’s Millpond State Natural Area; large pothole in foreground.

Figure 3.  Dikes and veins cutting through the granite.

Figure 4.  Biotite schlieren in the granite.

Figure 5.  Pothole at Mitchell’s Millpond.  In the bottom of this hole, you would find rounded pebbles that act as a “drill bit” when they are swirled by fast-moving water.