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 NC96, 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 Mill State Natural Area, part of the NC State Parks system.


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.
The Rolesville batholith is the largest body of granite in the southern Appalachian region.  It measures about 25 x 70 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 occurs in bodies called plutons. Plutons usually look like pink blobs on geological maps (pink is the most common color used to denote granite). 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 of the granitic plutons are between 280 and 320 million years old – Late Paleozoic in age. The largest such body is the Rolesville batholith, which is in reality 10 or 20 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 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 across Western Boulevard just down the road from Jordan Hall. 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.

Surface features
First, we will drive to the northeast corner of the county and visit Mitchell’s Millpond State Natural Area, located where the Little River crosses Highway 96.  This 93-acre site is a Registered Heritage Area, and contains some of the best examples of native plant communities that grow in such a “granitic flatrock” environment.  It is also an excellent spot to examine a very large exposure of granite belonging to the Rolesville batholith.  Here we will see medium-grained biotite granite, with a variety of special features, including pegmatite dikes and quartz veins that cut the granite.  We will see how such rock weathers by fracturing along joints and exfoliation surfaces.  We will also see the effects of erosion by running water, including numerous potholes.  While there, we can talk about ground-water resources in the area, and we may touch upon the idea of burying high-level radioactive waste in such a body of granite, as was proposed by the U.S. Department of Energy some years back.
Medium-to-coarse grained, massive to weakly foliated biotite monzogranite: the Rolesville batholith is the largest single pluton in the southern Appalachians, covering over 772 miles² in outcrop scale (a batholith is a continuous area of plutonic rocks larger than 40 miles²). This igneous intrusive rock (pluton) has a high percentage of quartz, feldspar, and biotite - different combinations of these minerals, and others, classifies this large exposure as granite with many distinct granitic facies varying relatively in texture and mineralogy. These differences in compositions are believed to be caused by multiple sequential magma pulses that differed slightly in composition. (pp. 5, 57-61)

Spheroidally Weathered Diabase Dike – Garner, NC

Rock Type:  Diabase

Geologic terrane, element, or event:  Opening of the Atlantic Ocean

Age:  Jurassic – 200 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Garner

Site Access:  This is an embankment next to an electrical substation.  Exercise caution, avoid the fenced-in enclosure and do not block access.

Here, a ridge was partially excavated to make room for the substation.  The excavation exposed part of a large diabase dike.

Diabase dikes
A dike is a sheet-like body of igneous rock intruded as molten magma that cuts across the older rocks.  In Wake County and surrounding areas, most diabase dikes are sheets that dip very steeply and trend close to north-south (Figure 1).  Diabase is a dark-colored igneous rock that is a variety of basalt, the rock type that makes up all the ocean floors of the earth.  Whereas basalt is volcanic, diabase is intrusive, meaning that the molten magma cooled below the earth's surface.  The age of the diabase in our region has been determined quite precisely; it is almost exactly 200 Ma (million years old).  In fact, our diabase was formed in response to the stretching and eventual breakup of the supercontinent Pangea, and the opening of the Atlantic Ocean.  Its age helps to date that event.  The dike exposed here runs through downtown Garner, roughly along Creech Road, passing along the west edge of Southeast Raleigh High School, and through Southgate Park.  It continues north at least as far as Millbrook Road.

Figure 1.  Example of geological map of a portion of northeastern Wake County, showing several diabase dikes.  They are the red lines labelled Jd that run N-S or NNW-SSE.  The solid red lines are dikes that have been confirmed by fieldwork and the dotted red lines are dikes that are inferred from their magnetic signatures.

Spheroidal weathering
Weathering is the gradual breakdown of hard rock at or very near the earth’s surface.  Chemical weathering transforms hard minerals like feldspar and hornblende into soft clay and iron oxide; it is key to the formation of soil.  At this site, the chemical process known as spheroidal weathering is displayed in spectacular fashion.  Imagine a body of rock, beneath the ground, that has a network of parallel horizontal and vertical cracks, intersecting at right angles (like a Rubik's Cube).  The cracks break the rock into many cubes or rectangular blocks.  Groundwater seeps into the cracks and the rock begins slowly to chemically decompose.  The process attacks the corners of the "blocks" most intensely because there are three rock surfaces in contact with the water.  Over time, this chemical weathering modifies these cubic blocks into rounded spheroids.  The “Rubik’s Cube” becomes a bunch of hard rounded rocks separated by soft decomposed material.  Erosion then can remove the soft material and only the rounded rocks remain (Figures 2 and 3).  Spheroidal weathering typically occurs in igneous rocks, including diabase, granite, and gabbro.

Figure 2.  Approach to the site, showing the electrical substation (left) and the embankment with diabase spheroids (right).

Figure 3.  Close-up of several diabase spheroids.  Such weathering commonly involves peeling off of successive layers, sometimes called “onion-skin” weathering.

Thursday, July 26, 2018

Triassic Conglomerates - Deep River Triassic Basin - Morrisville, NC

Rock Type: Conglomerate

Geologic terrane or major geologic element:  Deep River Triassic basin - Durham sub-basin

Age: Triassic - approximately 220 million years old

Location: Google Maps Link

Site Access: This is an active railroad line.  Great care must be taken while visiting.


Technical Information:  Carolina Geological Society Field Trip guide 1994 - Stop 2


Simplified Information:

The rocks exposed in the railroad cut help tell part of the geologic story of North Carolina. 

When the Supercontinent Pangea (Figure 1) began to split apart approximately 245 million years ago, a system of rift basins (similar to the modern day East African Rift system) were formed all along the east coast of North America (Figure 2).  Called the Newark Rift System, the splitting apart of Pangea formed the Atlantic Ocean and several inland fault bounded rift valleys. 

Figure 1: Sketch of the supercontinent Pangea.

Figure 2: Sketch of rift basins along the Atlantic Ocean.

Land to either side of the rift basin began to erode rapidly filling the fault bounded lowlands with boulders, cobbles, sand, silt and clay.  The deposits later turned into the red to maroon colored conglomerates, sandstones, siltstones and mudstones common in the basin (Figures 3 and 4). Excellent examples of these deposits are exposed in the railroad cut.

Figure 3: Photograph of outcrop at railroad cut showing Triassic-aged sedimentary rocks.

Figure 4: Photograph of details of outcrop at railroad cut showing alternating beds of Triassic-aged conglomerates.

The Morrisville area during the late Triassic was full of life.  North Carolina was located near the equator and had a semi-arid tropical climate. Situated between rugged mountains, the ancient area was dominated by lakes, swamps and meandering rivers and streams that would periodically dry-up.  Crocodile-like animals called phytosaurs, early dinosaur ancestors, and primitive mammals roamed the land of the Triassic basin; fish, clams and various crustaceans lived in the lakes and rivers; insects crawled on the ground and flew through the air; and abundant conifer trees and cycads grew.  Evidence of this abundant life is seen by the common occurrence of petrified wood in Triassic sediments, the occasional discovery of fossilized bones of reptiles and the footprints of early dinosaurs (Figure 5) in Triassic basin sediments throughout North Carolina.

Figure 5: Artist rendition of flora and fauna of the Triassic basins – From News and Observer.