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Tuesday, 21 November 2000

Pliocene Fossils

Pliocene Fossils


Introduction

Outcropping strata of Pliocene age, 1.8 to 5.2 million years old, consist of reddish and orange fluvial sands and gravels mapped as the Willis Formation, Citronelle Formation, or the Upland Complex. Typically, these sediments consist of slightly unconsolidated and weakly cemented quartz sands and chert gravels. These coarse-grained sediments commonly exhibit channels, cut and fill structures, scour surfaces, and large-scale cross-bedding. Minor beds of silt and clay are interbedded with the coarse-grained sediments. The general consensus is that the Citronelle Formation / Upland Complex was deposited as part of a gently sloping coastal plain was built up by the coalesced flood plains of numerous braided streams and rivers (Manning and McFarlan 1989, Parsons 1967).

Gravel Fossils

Gravel fossils are invertebrate fossils found in chert gravel within the Willis and Citronelle formations. They are much older than it and originated outside of Louisiana. These Paleozoic marine invertebrate fossils were initially buried about 330 to 425 million years ago within limestones that were partially silicified to form chert. Later, the cherts and its fossils were eroded, transported by rivers, and eventually deposited within Louisiana and Mississippi as chert gravel (Dockery 1995).

Vertebrate Fossils

The teeth and metacarpals of the early Pleistocene, three-toed horse Coromohipparion emsliei have been reported as having come from Pliocene deposits the Tunica Hills of West Feliciana Parish, Louisiana (Manning and McFarlan 1989). However, since they were found loose in bed of a modern stream, their association with the Citronelle formation is uncertain. Additional finds will be needed to establish whether these fossils came from Pliocene Age strata.

References Cited

Dockery, III., David T., 1995, Rocks and Fossils Collected from Mississippi Gravel. Mississippi Geology, vol. 16, no. 2, pp. 25-42.
Manning, E. M., and MacFadden, B. J., 1989, Pliocene three-toed horses from Louisiana, with comments on the Citronelle Formation. Tulane Studies in Geology, vol. 22, no. 2, pp. 35-46.

Final Note

Any vertebrate or plant fossils found within the either the Willis or Citronelle formations would be of great scientific interest to geologists and paleontologists. If you know of any fossils found within the Citronelle Formation of Louisiana or Mississippi, please, drop a letter or e-mail to;
Paul V. Heinrich,
Research Geologist
Louisiana Geological Survey
Energy, Coast and Environment
Building, Room 3079
Baton Rouge, Louisiana 70803
225-578-4398 - workdays

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