Gravel Fossils

Within the northern half of the Florida Parishes and in an east-west belt across central Louisiana, the fossils of marine invertebrates can be found within exposures of gravel-bearing sands called the "Citronelle Formation" or the "Upland Complex" by geologists. The invertebrate fossils are found within the chert gravel that characterizes the Citronelle Formation and is mined all across the state for aggregate. The fossils consist of a wide variety of Devonian to Mississippian age marine fossils ranging in age from 320 to 480 million years. They include marine invertebrates such as brachiopods, bryozoans, cephalopods, corals, crinoids, pelecypods, and trilobites. All of these fossils occur as part of the chert gravel (Dockery 1995, Self, 1983, Smith and Meylan 1983)

Types of Fossils

Some of the Paleozoic marine fossils found as part of the chert gravel are:

I. Brachiopods - Brachiopods are marine invertebrates that lived at the bottom of warm shallow seas that covered large parts of the United States during the Paleozoic Era, 570 to 245 million years ago. They are phyla, Brachiopoda, characterized by a food-catching organ called a lophophore and a bilaterally symmetrical shell. The lophophore is a circular or horse-shoe shaped fold around the mouth that bears numerous food-gathering tentacles. An extinct group of brachiopods, order Spiriferida, had a spiral lophophore with an internal support called a "spiralium". Individual chert pebbles often exhibit the cross-section of the spiralium where a brachiopod shell has been worn down by erosion.

II. Bryozoans - Bryozoans are colonial marine invertebrates, often called "moss animals", that were abundant in shallow Paleozoic seas along with the brachiopods. They were filter-feeders that built complex lacy and frond-like structure bearing hundreds of individual bryozoans. The most common bryozoan exhibits a "fenestrate pattern" composed of spreading branches linked by cross-bars. Some have the form of a garden trellis.

III. Cephalopods - Cephalopods are squid-like marine mollusks that lived within chambered shells. The partitions in the shells strengthen the shells. Also, the partitions formed chambers that contained gas that cephalopods use to control their buoyancy as they swam through the seas. Cephalopods are generally very rare fossils within the chert gravels.

IV. Corals - Corals are marine invertebrates that belong to the Phylum Anthozoa which also include sea anemone and jellyfish. Corals, as they are in modern seas, were also reef builders in the warm shallow seas that covered large parts of North America. Two types of corals, tabulate and rugose corals occur within the chert gravels.

A. Tabulate corals - Tabulate corals are colonial corals composed of numerous small individual organisms, called "corallites." Tabulate coral consists of numerous tightly packed corallites, which exhibit a fine polygonal pattern when viewed from the top. When viewed from the side, they look like a bunch of straight tubes with numerous partitions perpendicular to the sides of the tubes. These partitions, called "tabulae," give the tabulate corals their name.

B. Rugose corals - Rugose corals, tetracorals, are corals with a well-defined four-fold symmetry. Some of the rugose corals lived as solitary individuals, commonly called "horn corals" for their horn shape. Barrel-shaped fossils with prominent ribbing are typically the worn and rounded remains of solitary rugose corals. Other rugose corals lived in colonies exhibiting the same prominent polygonal pattern of the tabulate corals. However, the colonial rugose corals

V. Crinoids - The most common fossils found in the chert gravels are of crinoids. Although they are called "sea lilies," the crinoids belong to the phyla Echinodermata, a primary group of animals including sea urchins and starfishes. A typical crinoid consists cup-like body, called a "calyx," from which tentacle-like arms radiated from the rim of the cup. The body, or calyx, of a typical crinoid is attached to the sea floor by a long, segmented stalk that ends in a holdfast that anchors it in the seafloor or to some object on it. Both the body and stalk are composed of interlocking plates. The circular to star-shaped plates that compose the segmented stalk are the most common fossil found in the chert gravels.

VI. Gastropods - Gastropods are a mollusk characterized by a coiled single-chambered shell. They are very common in both saltwater and freshwater bodies and found in soils and on vegetation. Modern examples of gastropods are snails and conchs. In the chert gravels, gastropod fossils consist usually of the inside casts of their shells.

VII. Pelecypods - Pelecypods are a mollusk characterized by a shell that has two halves that are typically mirror images of each other. Although less common in the Paleozoic shallow seas, they are currently abundant in modern saltwater and freshwater environments. Oysters and clam are modern examples of pelecypods.

VII. trilobites - Trilobites are an important group of extinct marine arthropods characterized by a segmented body. A central ridge divides the body into three lobes for its entire length. A typical trilobite processes a distinct head, mid-section, and tail. Although and abundant fossil in most sedimentary of the Paleozoic Era, they are rarely found within the chert gravels of Louisiana.

Also, Tertiary age petrified wood is present in the gravels of the Citronelle Formation. Generally, it occurs as pebbles scattered throughout the chert gravel. However, where the Citronelle Formation lies on the Tertiary formation bearing the petrified wood, cobble and larger size pieces of petrified wood can be found.

Occurrence of Fossils in the Gravels

This gravel and the above fossils, which it contains, are found mainly in the Citronelle Formation. The Citronelle Formation, which is also mapped as the "Upland Complex" on the "Geologic Map of Louisiana," consists of sandy gravels, muddy (clayey) sandy gravels, silty sands, muddy gravelly sands, and red hemantitic clay. Typically, at least 5 percent, by weight, of the Citronelle Formation consists of mostly pebble-size gravel composed of chert and quartz. It is the chert gravel that contains the above fossils (Saucier 1994, Self 1983).

The layers sands and gravel that comprise the Citronelle Formation exhibit cross-bedding and other structures like those found in the point and other bars of modern streams and rivers. The gravelly layers have indistinct bedding. The sand layers generally exhibit cross-bedding of both the planar and trough types. Cut and fill structures and scoured channels are abundant. Within some beds, buried fine-grained, fossil soils, called "paleosols," up to a few feet thick can be found. The presence of the cross-bedding, the paleosols, the cut and fill structures, the channel scours, and the distribution of graveliferous beds clearly show that ancient river and streams deposited the gravels of the Citronelle (Saucier 1994, Self 1983).

The chert gravel and the fossils also occur in younger alluvial deposits. Where younger river and streams have eroded the Citronelle Formation, the gravels and the fossils that they contain have redeposited in younger alluvial sediments. These deposits including the Prairie Formation (Terraces), Deweyville Terraces, and modern alluvium. Thus, gravel fossils can be found in gravel pits developed in the Prairie Formation (Terrace), Deweyville Terrace, and flood plains of modern rivers and streams. In the Florida Parishes, fossils can be found the gravel covering modern bars of rivers and streams.

Age and Origins

Most of the invertebrate fossils found in the chert gravels of Louisiana are of Devonian to Mississippian age, between 320 to 480 million years old. During this time, large shallow seas covered large parts of the Central United States. In these shallow seas, the animals now found as fossils, lived, died, and the shells accumulated on the sea bottom. The accumulation of shells and calcareous mud buried and preserved some of these shells in sediments composed of calcite and aragonite.

After deposition, marine, lagoonal, and, later, ground-waters moved through these calcareous sediments. The movement of these waters through these sediments cemented them into limestone or altered them to dolomite. However, these waters also replaced a small, but significant, percentage of these sediments and the fossils in them with chert.

By Late Tertiary, erosion had exposed beds of limestone and dolomite containing the chert within the Central United States. Weathering dissolved the limestone and dolomite leaving chert as a residue that later washed into local streams. Then, ancient streams and rivers carried the chert down from Kentucky and Tennessee into Mississippi and Southeastern Louisiana. Within western Louisiana, ancient rivers and streams carried other Paleozoic chert from the Ouachita Mountains. Later coastal river and streams have eroded the Citronelle Formation and redeposited the gravels into younger alluvial deposits, e.g. Prairie Formation and modern river bars (Dockery 1995, Self 1993).

Identifying These Fossils

The best guides for identifying these fossils are Dockery (1995, 1996).

References Cited

Dockery, David T., III, 1995, Rocks and Fossils Collected from Mississippi Gravel. Mississippi Geology. vol. 16, no. 2, pp. 1-42.

Dockery III, David T., III, 1996, More rocks and fossils from Mississippi gravel. Mississippi Geology. vol. 17, no. 4, pp. 65-74.

Saucier, Roger T., 1994, Geomorphology and Quaternary Geologic History of the Lower Mississippi Valley. Mississippi River Valley Commission, U. S. Army Corps of Engineers, Vicksburg, Mississippi.

Self, R. P., 1983, Petrologic variation in Pliocene to Quaternary gravels of southeastern Louisiana. Transactions of the Gulf Coast Association of Geological Societies. vol. 33, pp. 407-415.

Self, R. P., 1993, Late Tertiary to early Quaternary sedimentation in the Gulf Coastal Plain and Lower Mississippi Valley. Southeastern geology. vol. 33, no. 2, pp. 99-100.

Smith, M. L., and Meylan, M., 1983, Red Bluff, Marion County, Mississippi: a Citronelle braided stream deposit. Transactions of the Gulf Coast Association of Geological Societies. vol. 33, pp. 419-433.

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Dec 18, 2001

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