Rock Type:
Metamorphosed volcanic tuff
Geologic terrane or major geologic element: Carolina terrane
Age: Late Proterozoic
to Cambrian – approximately 630-520 million years old
Location: GoogleMaps Link
USGS 7.5-minute Quadrangle:
Southeast Durham
Site Access: From Glenwood Ave. (US 70) turn in to Umstead
State Park. Proceed past the Visitor’s
Center and Maintenance Drive to the first parking lot on the left. Park at the north end of the lot and follow
the Oak Rock Trail markers. The outcrop
is near the northern end of the loop trail.
Other outcrops can be seen along the Potts Branch Trail.
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Technical Information: Blake and others, 2001, A Temporal
view of terranes and structures in the eastern North Carolina Piedmont, in Geological Society of America,
Southeastern Section, Field Trip Guide for 2001. See especially description for Stop 7 on p.
173-174.
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Introduction
The Carolina terrane (formerly
known as the Carolina slate belt) is a major geological region in central North
Carolina. It consists of lightly
metamorphosed volcanic and related igneous and sedimentary rocks of Late
Proterozoic to Early Paleozoic age, or approximately 630-520 million years old. These rocks were formed as part of an ancient
volcanic arc (Figure 1) on the far side of an ocean that no longer exists. Later, when earth movements caused that ocean
to close, the volcanic arc collided with ancient North America, helping to form
the Appalachian mountain chain. The
eastern edge of the Carolina terrane runs through Cary; the western edge is
near Greensboro. Carolina terrane rocks
are well exposed in the Uwharrie Mountains to the west, but In Wake County, Umstead
State Park is a good place to see them.
Figure 1. A volcanic island arc forms above a
subduction zone. Eventually, the ocean
will close, causing plates to collide.
This is the primary way that mountain belts form. The Carolina terrane is an ancient volcanic
arc.
Big Lake – Raven Rock schist
The Big Lake – Raven Rock schist
is an extensive rock unit within the easternmost part of the Carolina
terrane. It is named for exposures near Big
Lake in Umstead State Park and others at Raven Rock to the south in Harnett
County. This rock originated mainly as
dacitic tuff, formed by explosive volcanic eruptions. These eruptions involved varying amounts of
crystals, rock fragments, and ash that were blown into the air and accumulated
on the flanks of volcanoes. Lithic tuff
features rock fragments, crystal tuff features phenocrysts, or crystals, of
quartz and feldspar; crystal-lithic tuff has both.
These volcanic features have been
somewhat obscured by later geological events, namely deformation and
metamorphism that came as the result of the Appalachian mountain-building
events later in the Paleozoic era. This
modified the volcanic rocks into metamorphic phyllite and schist with the
growth and alignment of the platy minerals mica and chlorite. The direction of the lined-up minerals is the
foliation of the metamorphic rock.
At Oak Rock (Figure 2), the
foliation is steeply dipping, nearly vertical. As you explore the park’s trails, you will
quickly see that the foliation is not consistent. The reason for the variation is that these
rocks have been bent into folds by the collision between plates that formed the
Appalachians.
If you look closely, and ignore
the lichen on the rock, you may see patches of the rock that have slightly
differing color. These are the flattened
rock fragments of this metamorphosed lithic tuff.
Figure 2. Oak Rock,
near the north end of the Oak Rock Trail.
To the southeast of Oak Rock, the Potts Branch Trail crosses
over a very nice exposure of crystal tuff (Figure 3), in which you can see
bumpy white phenocrysts of quartz and feldspar.
Figure 3. Exposure of
metamorphosed crystal tuff on the Potts Branch Trail.
