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Aurora Phosphate Mine

Geology, Aurora Phosphate Mine, NC Coastal Plain, NC Minerals

The geology of the area near the town of Aurora, North Carolina takes us back millions of years ago when the ocean covered the area of what is now part of the coastal plain. The complex sequence of sediments that make up the record of the past is exposed at the Nutrien phosphate mine located near the town of Aurora. The various layers of earth, called formations, can be seen in succession going from the surface down to the base of the valuable phosphate ore that is recovered. The layers are exposed allowing the geology to be studied in ways that could not be done without the mining operation. The information collected is valuable to science and allows us the opportunity to take an amazing journey back into the distant past of geologic time. Our trip will take us from existing land surface to the bottom of the Aurora phosphate mine, at approximately 160 feet below sea level.

The Geology of Aurora, North Carolina - From the Top...Down

Background: Let's start the geologic journey by first understanding how the mining sequence works. The sediments above the phosphate ore being mined are collectively called overburden. The overburden is removed in two steps. The upper overburden is first removed to a depth of 35 to 50 feet. This step in the mining process is referred to as pre-stripping. The mining operations currently use backhoe excavators and 40-ton haul trucks to accomplish this. The upper overburden is removed by the excavators and placed into the haulage trucks and transferred to a feeder that breaks up the material before sending it on a conveyor system. The material is conveyed to a machine called a spreader, which places the upper overburden behind the area of mining and starts the reclamation process. The removal of the upper overburden allows for the development of a stable, well-drained mine bench, some 30 to 40 feet above sea level.

 

Three large mining machines, called draglines, excavate the remaining overburden from the mine bench down to the top of the phosphate ore and place the overburden in previously mined out areas. This process, called stripping, exposes the phosphate ore, which is then excavated by the draglines and placed on the mine bench where it is stockpiled. The stockpiles of phosphate ore are mixed with recycled water to form a slurry that is pumped to the Mill. The Mill removes material from the phosphate ore that is either too coarse or too fine and then separates the sand from the phosphate, resulting in a phosphate concentrate. The resulting concentrate is the raw material and is used in many phosphate products that are created at the Aurora chemical complex.



Upper Overburden: The upper overburden, exposed by pre-stripping at the mine, is made up of a complex sequence of sediments that contain a variety of fossils. The upper overburden takes us from relatively recent geologic time through the Pleistocene. This was a time of rapid change in earth history with great fluctuations in sea level.

The Holocene: The Holocene geologic time ranges from recent to 11,700 years ago. From the surface down (some 10 to 15 feet), we see the most recent layers commonly known as the Farmers Clay and Surface Sands. These sediments can contain large buried trees, which are relatively well preserved. The Farmers Clay is best known for its mottled appearance with reddish brown to gray sandy clays. Occasionally, horse teeth, Mastodon teeth, and other large terrestrial mammal fossils can be found in the near surface deposits of Surface Sands excavated by the pre-stripping operations.



The Pleistocene: The Pleistocene geologic time ranges from 0. to 2.0 million years ago. Layers of sediments collectively referred to as the Post-Croatan Formation are below the Holocene. The Post-Croatan Formation is typically found to depths ranging from approximately 15 to 35 feet below land surface in the vicinity of the mine. The Post-Croatan contains layers ranging from organic rich clays, to clean coarse sands. Exposures of these sediments in the mine reveal changes in sea level. Fossils found in the soft, gray, silty clays can contain small shell fragments while the soft, plastic clays occasionally contain beds of oyster shells. Layers of white sand and gray shell laden mud are sometimes seen transitioning with each other across the expanse of the pre-stripping excavation face. The depositional environments of these sediments can be studied and compared to more recent ones.



The next layer we find going downward is the Shell Bed, or Croatan Formation. The Croatan Formation is typically found to depths ranging approximately from 35 to 55 feet. This layer contains an abundant variety of shells and coral that offer us a spectacular window into the past when a shallow sea covered the area. Whole shells of large bivalves and snails are numerous in the Shell Bed. Occasionally a cobblestone layer can be found, and is locally referred to as the Boulder Bed. This layer can contain casts and molds of shells. Thick layers of channel sands are also occasionally found in the Pleistocene that can scour the underlying Pliocene beds to greater depths.



The Pliocene: The Pliocene geologic time ranges from 2.58 to 5.3 million years ago. This was a time when deeper ocean waters covered Aurora as the sea level rose and the shoreline moved inland. The overburden excavated by the draglines in the mine exposes the Pliocene age sediments known as the Yorktown Formation. This formation spans a greater length of time and is correspondingly thicker than the overlying Pleistocene sediments. The Yorktown Formation is typically found to depths ranging approximately 90 to 120 feet below land surface in the vicinity of the mine.

The Yorktown Formation is typically subdivided into two units; the Upper and Lower Yorktown. The Upper and Lower Yorktown units are typically separated by a bed commonly referred to as the "Pecten Bed". The Upper Yorktown is predominately silty stiff gray clay, while the Lower Yorktown is predominately coarser loose dark clay. The Upper Yorktown contains occasional streaks of snail fossils, bone fragments, soft mushy shell debris, and an abundance of sea urchin spines. The Pecten Bed contains an abundance of large scallop shells. When this bed is encountered, we begin to see an increasing variety of material in the fossil record moving downward. The Lower Yorktown unit contains a great variety of abundant marine fossils, phosphate nodules and pebbles. Large whale vertebrae are commonly found along with an abundant assortment of shark teeth. The famous C. megalodon, forerunner of the Great White Shark, left behind some of the best examples of large fossil shark teeth to be found anywhere in the world.



The Miocene: The Miocene epoch time ranges from 5.3 to 23.03 million years ago. During Miocene time, Aurora would have been covered by a vast embayment (a recessed area in a coastline forming a bay). The overburden excavated by the draglines in the mine exposes the Miocene age sediments known as the Pungo River Formation. The Pungo River Formation is typically found to depths ranging from approximately 120 to 170 feet below land surface in the vicinity of the mine.

The Pungo River Formation is typically sub-divided into four primary units at the mine. The upper portion consists of a fossil rich hash and silt, known as Chartreuse Bed, and is considered overburden. The Chartreuse Bed is followed by the last remaining layer of overburden locally known as the Coquina Limestone. This limestone bed contains casts and molds of fossil shells, shark teeth, bone fragments, and abundant phosphate nodules and pebbles. The white Coquina Limestone overlies the rich phosphate ore and is in stark contrast to the underlying colored phosphate ore.



The phosphate ore contains a variety of abundant fossils that range from microscopic one-celled animals known as foraminifera up to some of the largest whale fossils and sharks teeth ever found. Some of the animals in the fossil record of the Pungo River Formation are now extinct. An example of one of these extinct creatures is a snail known as Ecphora. Streaks of hard dolomite found within the phosphate ore section contain trace fossils such as clam borings and borrows. The base of the phosphate ore section contains many phosphate nodules and pebbles and is rich in fossil debris. The phosphate ore is excavated down to a layer of hard gray sandstone known as Caprock. The Caprock is underlain by another Pungo River Formation bed called the Lean Ore. These last two units are not mined and act as an important buffer between the base of the mine excavation and the Castle Hayne Limestone aquifer found beneath the floor of the mine excavation.

Conclusion: It's time we begin our ascent back to the surface. We have just uncovered approximately 15 million years of our Earth's history in our geologic journey going from the top of the ground, to the bottom of the phosphate mine. Although this expanse of time seems great to us, it is only a brief portion of geologic time when considering the Earth is approximately 4.5 billion years old.

Some of the best examples of the fossil record near Aurora, North Carolina are from the Pleistocene, Pliocene, and Miocene geologic times are on exhibit at the Aurora Fossil Museum. One of the most diverse assemblages of marine fossils one can find in these sediments are found in the coarse material separated from the ore at the Mill Operations and donated to the Aurora Fossil Museum by



Content for this page provided by Tex Gilmore, Senior Mining Geologist, PotashCorp-Aurora, Retired.

The geology of the area near the town of Aurora, North Carolina takes us back millions of years ago when the ocean covered the area of what is now part of the coastal plain. The complex sequence of sediments that make up the record of the past is exposed at the Nutrien phosphate mine located near the town of Aurora. The various layers of earth, called formations, can be seen in succession going from the surface down to the base of the valuable phosphate ore that is recovered. The layers are exposed allowing the geology to be studied in ways that could not be done without the mining operation. The information collected is valuable to science and allows us the opportunity to take an amazing journey back into the distant past of geologic time. Our trip will take us from existing land surface to the bottom of the Aurora phosphate mine, at approximately 160 feet below sea level.Let's start the geologic journey by first understanding how the mining sequence works. The sediments above the phosphate ore being mined are collectively called overburden. The overburden is removed in two steps. The upper overburden is first removed to a depth of 35 to 50 feet. This step in the mining process is referred to as pre-stripping. The mining operations currently use backhoe excavators and 40-ton haul trucks to accomplish this. The upper overburden is removed by the excavators and placed into the haulage trucks and transferred to a feeder that breaks up the material before sending it on a conveyor system. The material is conveyed to a machine called a spreader, which places the upper overburden behind the area of mining and starts the reclamation process. The removal of the upper overburden allows for the development of a stable, well-drained mine bench, some 30 to 40 feet above sea level.Three large mining machines, called draglines, excavate the remaining overburden from the mine bench down to the top of the phosphate ore and place the overburden in previously mined out areas. This process, called stripping, exposes the phosphate ore, which is then excavated by the draglines and placed on the mine bench where it is stockpiled. The stockpiles of phosphate ore are mixed with recycled water to form a slurry that is pumped to the Mill. The Mill removes material from the phosphate ore that is either too coarse or too fine and then separates the sand from the phosphate, resulting in a phosphate concentrate. The resulting concentrate is the raw material and is used in many phosphate products that are created at the Aurora chemical complex.The upper overburden, exposed by pre-stripping at the mine, is made up of a complex sequence of sediments that contain a variety of fossils. The upper overburden takes us from relatively recent geologic time through the Pleistocene. This was a time of rapid change in earth history with great fluctuations in sea level.The Holocene geologic time ranges from recent to 11,700 years ago. From the surface down (some 10 to 15 feet), we see the most recent layers commonly known as the Farmers Clay and Surface Sands. These sediments can contain large buried trees, which are relatively well preserved. The Farmers Clay is best known for its mottled appearance with reddish brown to gray sandy clays. Occasionally, horse teeth, Mastodon teeth, and other large terrestrial mammal fossils can be found in the near surface deposits of Surface Sands excavated by the pre-stripping operations.The Pleistocene geologic time ranges from 0. to 2.0 million years ago. Layers of sediments collectively referred to as the Post-Croatan Formation are below the Holocene. The Post-Croatan Formation is typically found to depths ranging from approximately 15 to 35 feet below land surface in the vicinity of the mine. The Post-Croatan contains layers ranging from organic rich clays, to clean coarse sands. Exposures of these sediments in the mine reveal changes in sea level. Fossils found in the soft, gray, silty clays can contain small shell fragments while the soft, plastic clays occasionally contain beds of oyster shells. Layers of white sand and gray shell laden mud are sometimes seen transitioning with each other across the expanse of the pre-stripping excavation face. The depositional environments of these sediments can be studied and compared to more recent ones.The next layer we find going downward is the Shell Bed, or Croatan Formation. The Croatan Formation is typically found to depths ranging approximately from 35 to 55 feet. This layer contains an abundant variety of shells and coral that offer us a spectacular window into the past when a shallow sea covered the area. Whole shells of large bivalves and snails are numerous in the Shell Bed. Occasionally a cobblestone layer can be found, and is locally referred to as the Boulder Bed. This layer can contain casts and molds of shells. Thick layers of channel sands are also occasionally found in the Pleistocene that can scour the underlying Pliocene beds to greater depths.The Pliocene geologic time ranges from 2.58 to 5.3 million years ago. This was a time when deeper ocean waters covered Aurora as the sea level rose and the shoreline moved inland. The overburden excavated by the draglines in the mine exposes the Pliocene age sediments known as the Yorktown Formation. This formation spans a greater length of time and is correspondingly thicker than the overlying Pleistocene sediments. The Yorktown Formation is typically found to depths ranging approximately 90 to 120 feet below land surface in the vicinity of the mine.Theis typically subdivided into two units; the Upper and Lower Yorktown. The Upper and Lower Yorktown units are typically separated by a bed commonly referred to as the "Pecten Bed". The Upper Yorktown is predominately silty stiff gray clay, while the Lower Yorktown is predominately coarser loose dark clay. The Upper Yorktown contains occasional streaks of snail fossils, bone fragments, soft mushy shell debris, and an abundance of sea urchin spines. The Pecten Bed contains an abundance of large scallop shells. When this bed is encountered, we begin to see an increasing variety of material in the fossil record moving downward. The Lower Yorktown unit contains a great variety of abundant marine fossils, phosphate nodules and pebbles. Large whale vertebrae are commonly found along with an abundant assortment of shark teeth. The famous, forerunner of the Great White Shark, left behind some of the best examples of large fossil shark teeth to be found anywhere in the world.The Miocene epoch time ranges from 5.3 to 23.03 million years ago. During Miocene time, Aurora would have been covered by a vast embayment (a recessed area in a coastline forming a bay). The overburden excavated by the draglines in the mine exposes the Miocene age sediments known as the Pungo River Formation. The Pungo River Formation is typically found to depths ranging from approximately 120 to 170 feet below land surface in the vicinity of the mine.Theis typically sub-divided into four primary units at the mine. The upper portion consists of a fossil rich hash and silt, known as Chartreuse Bed, and is considered overburden. The Chartreuse Bed is followed by the last remaining layer of overburden locally known as the Coquina Limestone. This limestone bed contains casts and molds of fossil shells, shark teeth, bone fragments, and abundant phosphate nodules and pebbles. The white Coquina Limestone overlies the rich phosphate ore and is in stark contrast to the underlying colored phosphate ore.The phosphate ore contains a variety of abundant fossils that range from microscopic one-celled animals known as foraminifera up to some of the largest whale fossils and sharks teeth ever found. Some of the animals in the fossil record of the Pungo River Formation are now extinct. An example of one of these extinct creatures is a snail known as Ecphora. Streaks of hard dolomite found within the phosphate ore section contain trace fossils such as clam borings and borrows. The base of the phosphate ore section contains many phosphate nodules and pebbles and is rich in fossil debris. The phosphate ore is excavated down to a layer of hard gray sandstone known as Caprock. The Caprock is underlain by another Pungo River Formation bed called the Lean Ore. These last two units are not mined and act as an important buffer between the base of the mine excavation and the Castle Hayne Limestone aquifer found beneath the floor of the mine excavation.It's time we begin our ascent back to the surface. We have just uncovered approximately 15 million years of our Earth's history in our geologic journey going from the top of the ground, to the bottom of the phosphate mine. Although this expanse of time seems great to us, it is only a brief portion of geologic time when considering the Earth is approximately 4.5 billion years old.Some of the best examples of the fossil record near Aurora, North Carolina are from the Pleistocene, Pliocene, and Miocene geologic times are on exhibit at the Aurora Fossil Museum. One of the most diverse assemblages of marine fossils one can find in these sediments are found in the coarse material separated from the ore at the Mill Operations and donated to the Aurora Fossil Museum by Nutrien Phosphate Aurora . You can collect your own fossils when you visit the Aurora Fossil Museum and take your own journey back in geologic time. You will truly be amazed!Content for this page provided by Tex Gilmore, Senior Mining Geologist, PotashCorp-Aurora,

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