Name/Title
Fern FossilEntry/Object ID
2024.58.24Description
Assemblage Zone: shale
Chemical Composition: Si O2
Crystal System: Subhedral
Description: Genus: Neuropteris
Common name: Fern
Age: Pennsylvanian
Location data: Mazon Creek, Illinois USA
Formation: Frances Creek Shale Fm.
Description: Fern in ironstone concretion (2) halves, shows mold and cast of fern, from a coal deposit in Mason Co. IL.
Size: 2.750"x 1.125"x .500"
** Pennsylvanian Period (299 to 318 million years ago) In the Pennsylvanian, swamps of lush tropical vegetation covered most of the continents. Because there was no seasonal variations in climate or growing conditions, the foliage was dense, highly productive, and grew rapidly and abundantly. The coal mined today in the eastern and central United States was formed form the layers of decaying vegetation that accumulated in these swamps. Coal seams, as a general rule, contain only unrecognizable plant remains because the plants were crushed, compacted and altered by deep burial. The well-preserved plant fossils are usually found in the shale beds that occur either above or below the coal seams. Fossil ferns from the Pennsylvanian are often in concretions; eggshaped nodules of hardened shale bound together by somall amounts if iron; locally these are called "ironstones." When hit with a hammer, the concretions split in half exposing the two sides of the fossil fern impression.
Type of Fossils Present: Plants
Fracture: conchoidal
Grain Size: Fine
Hardness: 7 Quartz
Lithofacies: delta system
Luster: Vitreous
Occurrence: The Mazon Creek fossils are found in the Francis Creek Shale: the type locality is Mazon Creek, a tributary of the Illinois River near Morris, Grundy County, Illinois. The 25 to 30 meters of shale was formed approximately 300 mya, during the Pennsylvanian period. The fossiliferous concretion are usually found within the thickest deposits of Francis Creek. The concretion occur in locallized deposits within the silty to sandy mudstones on the lower four metres of the formation. The paleoecosystem is believed to be a large river delta system deposited by at least one major river system flowing from the northeast. The sediments are believed to derive from the Appalachian orogene events. The delta had a tropical climate, a result of the area being within 10 degree north latitude of the equator during the Pennsylvanian.
The remains of plants and animals were rapidly buried by the sediment deposited in the deltaic system. Bacterial decomposition of the remains produced carbon dioxide that combined with dissolved iron from groundwater. This process formed siderite in the sediments surrounding the remains, forming detailed casts of their structure. Lithification of the sediments formed protective nodules of ironstone around the now fossilezed remains. The ironstone concretions are recovered from exposures along streams, roadcuts, and in active or abandoned coal mine areas.
Pressure: Moderate
Rock Color: Dark
Rock Origin: Marine/Freshwater
Rock Type: Sedimentary
Specific Gravity: 2.6
Surface Process: Not apparent
Temperature: Low
Texture: fine grain
Variety: Leaf fossils in this case are carbon films that remain on the bedding surfaces of fine-grained sediments. Fossilization happened after the leaves sank to the bottom of a pond or lake and were later buried by additional fine-grained sediments. With burial, the leaves were compressed and the sediment surrounding them solidified. Eventually these fine-grained sediments became shale or mudstone. When all the volatile organic material in the living leaf was pressed out, only a thin carbon residue was left in the sediment, marking the outline and veins of the leaf.