The Coon Creek Formation is a geologic formation

Geologic formation

A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

 located in western Tennessee

Tennessee

Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...

 and extreme northeast Mississippi

Mississippi

Mississippi is a U.S. state located in the Southern United States. Jackson is the state capital and largest city. The name of the state derives from the Mississippi River, which flows along its western boundary, whose name comes from the Ojibwe word misi-ziibi...

. It is a sedimentary sandy marl

Marl

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...

 deposit, Late Cretaceous

Late Cretaceous

The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

 (Maastrichtian

Maastrichtian

The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

) in age, about 70 million years old. The formation is known for producing mosasaur

Mosasaur

Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s and plesiosaur

Plesiosaur

Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s, particularly at Coon Creek in McNairy County, Tennessee, which the formation is named for. Additionally, the formation produces many other marine invertebrates such as Turritella

Turritella

Turritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. They have tightly coiled shells, whose overall shape is basically that of an elongated cone....

and the state fossil

State fossil

Most American states have made a state fossil designation, in many cases during the 1980s. It is common to designate one species in which fossilization has occurred, rather than a single specimen, or a category of fossils not limited to a single species....

 of Tennessee, the bivalve

Bivalvia

Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...

 Pterotrigonia thoracica, as well as other fossils such as crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s.

Beginnings

The story of Coon Creek began near the end of the Cretaceous Period, around 71 million years ago. At that time western Tennessee, eastern Arkansas

Arkansas

Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...

, western Kentucky

Kentucky

The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...

, and southeast Missouri

Missouri

Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

 were submerged beneath the Mississippi Embayment, a bay of the Gulf of Mexico

Gulf of Mexico

The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

. Coon Creek was formed in shallow coastal water probably less than 100 feet deep (Russell and Parks 1975). The sea floor was heavily populated with shellfish

Shellfish

Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found only in freshwater...

, crabs, and lobsters. Huge plesiosaurs, marine crocodiles, sea turtles, and mosasaurs shared the waters with shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s and fierce fanged-tooth fishes. The climate was warmer than today. Coon Creek was semi-tropical, like present-day southern Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

 (Wade 1926). Heavy waves from severe tropical storms constantly churned up shallower parts of the sea floor.

A couple of miles to the east lay a marshy lowland bordering the limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 bluffs

Cliff

In geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are usually...

 of the Western Highland Rim of the Nashville Done, home to duckbill

Duckbill

Duckbill may refer to:* Platypus, Ornithorhynchus anatinus* Duckbill dinosaurs, family Hadrosauridae* Duckbill fishes, family Percophidae* Duckbill eels, family Nettastomatidae* Duckbill valve, a valve used to prevent backflow...

 and theropod dinosaurs. Sluggish rivers annually washed tons of driftwood, along with the occasional dinosaur carcass, from this heavily forested area into the bay (Russell and Parks 1975).

The Geology Of The Mississippi Embayment And Coon Creek

This thumb print-shaped bay extends roughly north-south from central Mississippi to southern Illinois. It was formed when rocks, weakened by gradual spreading of the North American continent, sagged into a wide, shallow trough. The Gulf of Mexico filled the trough on several occasions between 80 and 50 million years ago. The Mississippi Embayment stretched West from the Tennessee Valley to the area of Little Rock, Arkansas. It may have been 1,000 feet deep where Memphis is now. The embayment gradually filled with sand, clay, and gravel brought in by rivers on uplands to the north, east, and west (Wade 1926).

The margins of the bay teemed with marine life. Crabs, snails, lobsters, clams, scallops, whelks, nautilus, sharks, and other familiar animals lived in the warm, shallow sea, eating, reproducing, and being eaten (Sohl 1960 and 1964). Giant reptilian mosasaurs, highly ornamented cephalopods, and other less familiar sea creatures lived in the water. Their shells, bones, carapaces, teeth, and other hard parts were constantly being buried in the sandy mud of the sea floor. The lack of distinct layering indicates that clams, shrimps, and other burrowing organisms mixed the bottom sediments. Periodic hurricanes may have brought in heavy loads of river sediment to bury the plants and animals living there. Conditions for life were ideal; the water was warm and of normal salinity (Wade 1926). Wave action ensured sufficient oxygenation for animal life.

Ancient Life

Many rivers fed into the sea bringing leaves and driftwood from the land. These served as the base of the food chain. Bacteria and other microscopic scavengers ate the decaying wood. Plankton ate the bacteria. Clams filtered the small plankton. Snails ate the clams and were eaten in turn by crabs and fish. Mosasaurs and cephalopods ate the fish and crabs. Some organisms were swimmers or floaters, but most lived on or in the sandy mud of the sea bottom. This layer of sandy clay, bones, and shells became the Coon Creek Formation.

Fossils

Most organisms are not preserved as fossils. Unless covered quickly after death, their bodies are consumed by other animals and plants or destroyed by weather. Even the bones and shells of animals which were quickly buried after death may be later dissolved by groundwater. The clay in the sediment at Coon Creek sealed off the fragile fossils from the corrosive action of water and the hard parts of the clams, snails, crabs, and shrimps were well preserved. Occasionally the bones of vertebrates are found, as well as the cartilaginous vertebrae of sharks. Fish scales and plant leaves are sometimes found as well (Wade 1926).

Dinosaurs

Dinosaur fossils have not been found in the Coon Creek Formation. While dinosaurs did roam the land of central Tennessee during the same time period, they were land animals and did not live in the sea. Sometimes the dried carcasses of dinosaurs were washed out to sea by rivers. Dinosaur bones and teeth have been found in marine deposits in Mississippi. It is possible that dinosaur bones will turn up at Coon Creek someday. The remains of at least two mosasaurs have been found. They were not dinosaurs but large aquatic lizards that could reach lengths of up to 45 feet. They were carnivorous and would have been the top of the food chain in the Coon Creek area.

Importance

Coon Creek has been named as one of the country's top twelve fossils sites for several reasons (Noble 1996).

• The fossils are found in their original state. The hard shells have not been permeated by groundwater and not replaced by minerals.

• The number of fossils is stupendous. Many times you will literally find fossils on top of fossils. Most fossil sites require concentrated efforts to find a representative sample of the fossils.

• There is a rich diversity of the animals with over 600 different species of organisms found.

• Because the Coon Creek Formation sediment is unconsolidated, it makes it very easy to collect and prepare the fossils.

Age

Geologists employ biostratigraphy

Biostratigraphy

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...

, the use of index fossils, for dating sedimentary rock units like the Coon Creek. Index fossils are species of plants or animals that existed over a wide area for a geologically short period of time (Russell and Parks 1975). The cephalopod Jeletzkytes

Jeletzkytes

Jeletzkytes is an extinct genus of ammonite....

nodosus is a time-sensitive fossil found in rocks a little younger than 70.6 million years old in the western United States. Other index fossils from Coon Creek date a little older than 70.6 million years. The overlap indicates that Coon Creek sediments were probably deposited between 70 and 71 million years ago.