The Virginia Coastal Plain is underlain by a wedge of sediments that increases in thickness from a featheredge near the Fall Zone to more than 4, meters under the continental shelf. These sediments rest on an eroded surface of older rocks similar to those exposed in the Piedmont. Two-thirds of this sedimentary wedge is comprised of late Jurassic and Cretaceous clay, sand, and gravel. These materials were eroded off the Appalachian highlands, carried eastward by rivers and deposited in deltas at the margin of newly formed Atlantic Ocean basin.
Fossiliferous marine sands of Tertiary age overlie the older strata, and were deposited in warm, shallow seas during repeated marine transgressions across the Coastal Plain.
The Chesapeake Bay Impact Structure was created in a geologic instant when, 35 million years ago, a large meteorite plummeted into a shallow Atlantic Ocean, and excavated a crater more than 90 km in diameter.
For millions of years, the Appalachian Mountains eroded and many rivers and streams deposited sediments along much of the eastern coast of North America. In some places, these sediments are over 10 km thick! This formed a flat landscape that lies between the Piedmont just below the mountains and the ocean, from New Jersey to Florida.
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This is unfortunate because it means that no deposits recording the meteorite impact hypothesized to have caused the extinction of the dinosaurs are exposed in the state. Elsewhere on the Gulf Coastal Plain near Braggs, Alabama, for example , layers of the right age do occur and have been studied by scientists from all over the world.
Although they cover much more area than Cretaceous strata, Paleogene rocks show many of the same patterns: nonmarine and coastal sediments in central Georgia and deeper-water marine sediments in both eastern and western parts of the state.
Also like the Cretaceous, Paleogene strata show the effects of repeated sea-level fluctuations throughout their history. Several of these Paleogene units act as major aquifers for south Georgia and north Florida.
Additionally, Paleogene units in Georgia contain important commercial deposits of kaolinite clay and bauxite. Paleocene strata in Georgia are best exposed in the western part of the state, where they form several important groups of formations. The Clayton Formation lies directly on top of the erosional surface unconformity that cuts off the top of Cretaceous layers in Georgia. In western parts of the state, the Clayton is a fossil-bearing limestone that formed in the shallow sea covering most of south Georgia at that time.
These limestones are highly permeable and form a major aquifer system in west Georgia and east Alabama. To the east, in the Americus area, the Clayton is made up mostly of dark clays and fossil-bearing marls or loose deposits , which formed closer to the shore.
Above the Clayton in the westernmost areas of Georgia is the Porters Creek Formation, a relatively thin deposit composed mainly of dark gray clay shales. The Porters Creek thickens westward into Alabama but is not found any farther east than Americus. An erosional surface at the top of the Porters Creek Formation suggests that it may have originally been larger but was eroded away before formation of the upper Paleocene layers above it.
In middle Georgia, Paleocene-age layers are included in the lower Huber Formation. The Huber is a complex unit whose oldest layers lie on top of Cretaceous Buffalo Creek deposits and whose youngest beds are of middle Eocene age.
Huber sediments are mainly nonmarine deposits consisting of sands and kaolinite clay, thought to have formed in broad-stream and floodplain systems. The Baker Hill Formation is a coarse-grained, cross-bedded sandstone with evidence of formation along a tidally dominated coastline. The Nanafalia Formation is of the same age as the Baker Hill but formed in deeper waters farther to the south. Made up of coarse, fossil-bearing sandstone with local thin clay beds, the Nanafalia consists in the Americus area of sands and clay that formed in river and coastal settings.
In some areas, the Nanafalia also contains deposits of bauxite, some of which are large enough to be commercially important. Overlying the Baker Hill is the Tuscahoma Formation. It is composed mainly of clay, silt, and fine sand layers that probably formed in the quiet waters of a broad tidal flat or shallow lagoon. The shallow sea in which Paleocene sediments were formed continued into Eocene time, and most layers of that age in Georgia were formed either in the shallow sea in the southern part of the state or in coastal areas farther north.
In this shallow Eocene sea lived the primitive whale Georgiacetus , which represents a very early stage in whale evolution and whose hind legs and hips were still functional. Sediments of the clay-rich Huber Formation continued to form in central Georgia. The oldest Eocene layers are also part of the Wilcox Group. They are represented mainly by the Hatchitigbee Formation, a fine-grained sandstone with clay layers and abundant marine shells of animals that lived near the shore.
The top of the Hatchitigbee is cut off by another erosion surface. Middle Eocene layers, most of which are marine or coastal in origin, are found in west central Georgia and compose the Claiborne Group. The Tallahatta Formation is mainly composed of sandstone and clays that probably formed on land in river systems.
Above the Tallahatta is the Lisbon Formation, composed mainly of fossil-bearing clays, fine sands, and limestones that formed as the gradually rising sea level caused the sea to flood over the nonmarine Tallahatta. The McBean Formation in east Georgia consists of fossiliferous fine sand and marl.
In the westernmost parts of the Georgia Coastal Plain are fossil-bearing layers of the Avon Park Limestone, which is the same age as the Claiborne and represents more open-marine, continental-shelf conditions. The Late Eocene was a time of high sea levels, with marine layers over most of the Georgia Coastal Plain. The Ocala Limestone in the Dougherty Plain region of southwest Georgia is a fine-grained, white or cream-colored, fossil-bearing limestone. It is believed to have formed in the quiet, open waters of the continental shelf far from land.
The Ocala is very permeable and, along with the Avon Park beneath it and the Suwannee Limestone above it, forms the Upper Floridan aquifer, one of the most productive aquifers in the United States.
Because limestone rock is soluble in rainwater and groundwater , the area where the Ocala is found often shows features of karst topography, a type of landscape typified by numerous sinkholes, small lakes, and caverns. In central and east Georgia, layers of the same age as the Ocala form the Barnwell Group and represent nearshore and coastal conditions. The oldest Barnwell formation is the Clinchfield Sand. In east central Georgia, where it formed in shallow, coastal waters, the Clinchfield is composed of clean quartz; farther to the east, it has some limestones and probably represents offshore conditions.
On top of the Clinchfield are the highly fossiliferous layers of the Tivola Limestone, which is considered to be an extension of the Ocala during maximum high sea level of the Late Eocene. They formed in shallower water as sea level fell and the shoreline moved southward. The layers at the top of the Barnwell are named the Tobacco Road Sand and are believed to represent the soil that formed at the end of the Eocene, after sea level fell and much of the Coastal Plain was exposed to erosion.
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