Of all the words in geological jargon, the term “fault” probably is the best known to the general public. Geologically, a fault is a crack or break along which masses of rock have slipped past another under the earth’s surface. As structurally active points, faults are associated with earthquakes and earth tremors, and it is in this context the layman thinks about them.
So it may be disquieting to learn that a relatively “young” fault recently was discovered in the Augusta-Fort Gordon area. Named for Belair, a crossroads a few miles west of Augusta, the fault was exposed by a surface mining operation. Its earthquake potential probably is not high, but caution dictates that construction in the area take the possibility of earth tremors into account. Aside from the earthquake aspect, this fault and its configuration are very interesting and will greatly illuminate the geological history of the Augusta area.
The Belair fault is exposed at the clay pits of the Georgia Vitrified Brick Company in western Richmond County of the south side of U.S. Highway 78 and 278, just west of the Fort Gordon entrance. It was discovered in 1973 by the Earth and Water Division during the field studies portion of the Division’s state geologic map project.
The fault and its geological context at Belair are important for several reasons. In particular, the outcrops at Belair provide new scientific information, since we did not know that faults of this relatively young age occurred in metamorphic rocks this far south in the United States. Now we have a more complete understanding of the sequence of geological events in the southeast millions of years ago. More practically, the fault warns of the possibility of earth tremors in this vicinity and tell us we need to design buildings accordingly.
A series of sequential sketches suggests the geological history of the Belair-Augusta area. (Sketches deleted because of time and space) Each sketch shows what we believe the earth looked like from the surface down to several thousand feet. Each is oriented northwest-southeast, passes through Belair, and continues beyond in either direction for several miles.
The oldest rocks in Belair are exposed in the lowest portions of the clay pits below the conglomerate and sandstone. They now are phyllites, but originally were volcanic ash deposits laid down by the repeated eruption of many volcanoes. In other areas, lavas poured out onto the surface. The exact age of these volcanic deposits is not known, but they are believed to be 500 to 600 million years old. This volcanic outpouring lasted a considerable length of time (we don’t know exactly how long) and covered an extremely large area; these volcanic rocks from an almost continuous belt from Newfoundland to at least Macon, where they disappear under the younger sediments of the Coastal Plain.
Volcanic action stopped and the land sank beneath the sea, allowing volcanic deposits to be buried under thousands of feet of silt and mud. As the volcanic rock strata were buried deeper and deeper, they became hotter and the pressure grew greater. Finally they were recrystallized into phyllites. As this “metamorphism” was taking place, the rocks were being folded by tremendous forces squeezing from the sides. The folding and metamorphism stopped some 350 million years ago.
The exact origin of the squeezing force is not known, but many scientists believe it was produced by the slow irresistible westward creep of the rocks of the Atlantic Ocean floor. This theory of sea floor spreading is based on the fact that new volcanoes currently are being formed in the center of the ocean along ridges parallel to the continents.
As this happens, older lavas are pushed aside, and so the sea floor moves away from its center much like a conveyor belt. The lavas of the sea floor crowd against the margin of the continents, but since the lavas are denser, they plunge beneath, fold the continental rocks and produce new volcanic chains and earthquakes. This sequence stops when the adjacent portion of the sea floor stops spreading.
Once the folding and metamorphism stopped, the rocks no longer were being buckled down. They flexed upwards, rising up into high mountains. But the mountains immediately were exposed to erosion, and over hundreds of millions of years eventually wore down to a broad undulating plain. This plain now lies buried beneath the sediments of the Luscaloosa Formation.
By the end of the Cretaceous Period (about 70 million years ago) this plain was flooded by the sea (we are not sure whether the land sank or the ocean rose.) The coarse sands and gravels of the Tuscaloosa Formation were laid down by broad meandering rivers which carried their load of sediment down to the ocean from mountains near the present Blue Ridge.
A few million years later the seas retreated and the rivers began to carve into the “freshly” deposited Tuscaloosa sands and gravels, washing them into the ocean. This sequence of flooding and drying was repeated several times to produce the sedimentary deposits of the Coastal Plain. At Belair, however, we see only the oldest of these cycles – the Tuscaloosa Foundation.
Sometime between the end of the Cretaceous and a few thousand years ago the Belair fault formed. We know it must have been quite some time ago, because although there has been 40 feet of vertical movement along the fault, there is little evidence of it at the surface.
Immediately after the fault moved, there would have been a high ridge, a “fault scrap”. We do not see such a ridge now, which indicates the fault occurred long enough ago for erosion to have completely leveled it. However, we do not know whether the movement took place millions or only several thousands of years ago.
Indeed, we know that tremors do occur north of this area. Last August, an earthquake of approximately 4.5 magnitude (Richter scale) occurred on the west bank of the Savannah River northeast of Lincolnton, Lincoln County. And there has been quite a long history of earthquakes in the area – most of them too small to be felt, but some of them rather large. In fact, the largest earthquake in Georgia occurred in 1875, somewhere west of Lincolnton.
As far as we know, the great majority of these earthquakes occurred well north of Belair and Augusta, mostly north of Clark Hill dam. However, until we know more about the Belair Fault, until we know more about the earthquake activity of the entire region, we had best play it safe and design our buildings with that possibility in mind.