Q. Geologically speaking, how old is the Cape Fear River? Can the riverbed be tested to determine how long it has flowed?
A. The answer isn’t quite as simple as finding out what birthday to celebrate. One geologist estimates that the age of the Lower Cape Fear drainage system might be estimated at around 2.75 million years.
Roger D. Shew, of the Department of Geography and Geology at the University of North Carolina Wilmington, shared his insights with MyReporter. Here is what he wrote:
The Cape Fear is a pretty interesting river with a long history.
Unfortunately, its age is a complex topic with no absolute facts to provide a definitive answer but there are some statements that can be made. Many of these comments are based on USGS data but there are numerous contributors to parts of the story over the years.
Here are some comments:
1. First, let me address your question of “Can the riverbed be tested to determine how long it has flowed?”
Because rivers migrate and downcut, looking at the base of the river will show what sediments or rocks are being cut at this time. However, the variation in sediment composition vertically in the river deposits may reflect what was being cut in the past and thereby give an indirect line of evidence to the time of erosion but not an absolute age.
The river has eroded through rocks and sediments that range from recent deposits to ones that are hundreds of millions of years old. These oldest deposits occur in the Piedmont and in the Triassic Basins all the way to the Fall Line (boundary between the Piedmont and Coastal Plain) near Lillington.
From the Fall Line to the mouth of the Cape Fear Estuary, there is a progression seaward from Cretaceous age (70+ million years old) rocks to modern beach deposits that are cut by the Cape Fear River. Although we can look at the sediments and determine their origins based on mineralogical/compositional variations, using that to determine a specific age relationship is problematic.
2. What is the age of the Cape Fear River?
Some form of ancestral Cape Fear River has likely been present since Cretaceous time, when coastal sediments began to be deposited seaward of the Fall Line. Belts of Cretaceous, coast-parallel, deltaic and shallow marine deposits are present inland and are also present in the subsurface below younger rocks of the Coastal Plain. The story, of course, is complicated by repeated sea level rises and falls since that time.
During falls in sea level, any river that was present would flow further seaward to the ocean and this has happened numerous times. The last one of these to occur was the fall of sea level of up to 120 meters (420 feet) approximately 18,000 years ago, at the last maximum glaciation. At that time you would have had to go around 95 kilometers (60 miles) to the east to reach the beach. And of course that is what the Cape Fear River and other rivers in North Carolina did – flow southeastward across the continental shelf to the beach.
Of course, rises in sea level would have inundated the old stream courses as the beaches migrated westward. In short, the history and formation of the N.C. Coastal Plain is one of a battle between rising and falling seas and this is reflected in the variety of marine (limestone and limey sand deposits) and continental (river and dune more quartz, sand-rich deposits) sediments and rocks present.
But I think your question is one of when was the likely formation of the Lower Cape Fear River in the Lower Coastal Plain that we see today. Evidence to this origin was given by Soller (1988 – USGS) and by subsequent workers. In looking at the Cape Fear and the surrounding Cape Fear River Valley, there are five river terraces present that become progressively older away from the river (based on isotopic age data).
These are correlated with strandline deposits, which are old beach locations of the same ages as the river terraces; remember the sea level fluctuations mentioned previously and these led to recognizable ancient beach locations at different topographic levels on the Coastal Plain.
The oldest of these river terraces that is present in the Cape Fear River Valley is approximately 2.75 million years old, which is in the Pliocene Epoch. Therefore it is inferred that the Lower Cape Fear River drainage system, though evolving, has been present since that time.
I might add that all of these terraces occur on the north side of the Cape Fear River. Because of uplift of portions of the Cape Fear Arch, which trends northwest to southeast, the Cape Fear has been “forced” to migrate southwestward. This is why there are higher bluffs on the south side of the river and the floodplain is dominantly on the north side. The river is cutting into the banks on the south side and depositing floodplain sediments on the north.
Date posted: November 13, 2013
User-contributed question by: