Generalized bedrock geologic map of Wake County, North Carolina.
AGES OF ROCKS & GEOLOGIC EVENTS IN WAKE COUNTY AREA
The geological evolution of the Wake County area mainly involves three distinct periods of geological time, during which major tectonic events occurred (leaving out Coastal Plain).
I. Neoproterozoic (620 - 540 million yrs ago)-- This period of time includes the very last of the Precambrian and the very beginning of the Paleozoic. Rocks of this age range are the oldest in the region. Probably all of these rocks are related in some way to volcanic arc activity of the Carolina slate belt or other volcanic arcs. These rocks originally formed far away from ancient North America, near the other side of an ocean that no longer exists. Subduction of parts of the oceanic crust was responsible for the formation of magmas that resulted in the volcanism. In between volcanic eruptions, sediment was deposited on the flanks of the volcanoes, on land and under water between volcanic islands. Some of the magmas never reached the surface, but crystallized beneath the volcanoes as plutons. Later (during the Alleghanian), ALL of these rocks were metamorphosed and deformed (affected by folding or faulting). So now these rocks are metamorphic rocks, though the metamorphic intensity varies. For example, the original nature of some of the rocks is still apparent; these may be referred to as metavolcanic, metaplutonic, or metasedimentary.
II. Late Paleozoic (a.k.a. Alleghanian; 320-280 m. y. ago)--This is the time of the Alleghanian orogeny, a major mountain-building episode. This is when the ancient North American continent collided with the ancient counterparts of Europe and Africa. This is the huge collision that formed the Appalachian mountain belt (compare to India crashing into Asia today). There was a tremendous amount of compression that resulted in folding and faulting (including the Blue Ridge thrust). The huge amount of crustal thickening that occurred caused rocks to be buried deep in the earth, resulting in intense regional metamorphism. Also, heat generated during the collision caused melting, and granite magmas formed and moved upward, finally stopping and cooling in the middle levels of the crust, to form plutons such as the Rolesville batholith. About the same time as the granite bodies were forming, the big collision of continents slowed and instead evolved to a sideways motion (compression changing to shear). This sideways motion resulted in a number of right-lateral strike-slip faults, such as the Nutbush Creek fault.
III. Early Mesozoic (210-190 m. y. ago)--This segment of time records the breakup of the supercontinent Pangaea; this occurred during the Triassic period and the early Jurassic. As the continent began to stretch, normal faults developed (tensional forces) throughout the region. An example of such a fault is the Jonesboro fault. Some of these continued to move so that long rift valleys formed. These rift valleys caused changes in the pattern of rivers and streams at the time, so that streams flowed down out of the mountainous areas on the upthrown side of the fault (foot wall) and into the valleys (hanging wall). They deposited sediment in the valleys. Later this sediment was lithified, and is now represented by clastic sedimentary rocks. Of course, one of these rift valleys is continuing to open today; it is the Atlantic Ocean. When the Atlantic began to open, mantle rock (peridotite) began moving upward and underwent partial melting. This resulted in the formation of basaltic magma, and the magma intruded into many of the faults and fractures. They are now diabase dikes. Some of the magma intruded along bedding planes of the sedimentary rocks (making sills); and farther north, some of it was erupted as lava flows. This process of manufacturing basalt magma continues today, along the Mid-Atlantic Ridge. The entire Atlantic Ocean crust was formed in this way, beginning in the early Mesozoic.
