AGS / Geology / Fossils / Pseudofossils
There are many features in rocks that appear to be fossils when they are really not. Such features or rocks are called pseudofossils - a natural object, structure or mineral of inorganic origin that may resemble or be mistaken for a fossil (Glossary of Geology, 1989). Some of the more common pseudofossils are listed below.
|Septarian concretions are sometimes mistaken for fossilized turtle shells|
Septarian concretions, which may vary in size, are characterized by an irregular polygonal pattern of cracks that are filled or partly filled by crystalline minerals, usually calcite. Its origin involves the formation of an aluminous gel, case hardening of the exterior, shrinkage cracking due to dehydration of the colloidal mass in the interior, and vein filling (Glossary of Geology, 1987). Concretions of this type are present in the Fayetteville Formation in northern Arkansas.
|This chert nodule may be mistaken to be a fossilized egg|
Chert nodules are essentially pure microcrystalline quartz that are lenticular or spheroidal in shape and generally found in limestone or dolostone. They may be widely scattered or form continuous or discontinuous irregular beds in the rock. The nodules can be concentrically banded or mottled and may have an outer rind. The chert varies in color from white, brown, black or various shades of gray. These chert nodules were collected from the Boone Formation in northwestern Arkansas. Chert nodules are also present in the Cotter Formation.
Solutioning is a process of chemical weathering by which mineral and rock material passes into solution. An example is the removal of the calcium carbonate in limestone by rainwater. Rainwater is acidic because it dissolves carbon dioxide from the atmosphere.
|The features in this weathered outcrop could be mistaken for human footprints|
Solution features such as these are usually found in limestones in north Arkansas. Limestones are easily dissolved by the action of rain and groundwater which leaves pits, depressions, and small pinnacles in the rock. The limestone formations in north Arkansas were deposited from the Ordovician through the Pennsylvanian Periods of geologic time. This was between approximately 300 to 460 million years ago – long before the age of man. These features are present in the Ordovician Plattin Limestone.
|The feature in this limestone could be misidentified as a dinosaur footprint|
This is another example of solutioning (see previous picture write-up) on a limestone in northern Arkansas. This feature was seen in the Fernvale Limestone which formed approximately 440 million years ago. This rock formation formed long before the age of dinosaurs, approximately 240 to 65 million years ago. Therefore, we would not expect any dinosaur tracks or other remains to be found in this part of the state.
|This piece of limestone could be mistaken for a dinosaur bone|
Solutioning of this piece of limestone has carved this interesting shape. Note the scalloped edges which are typical of solution features in limestones. This piece of limestone is from the Boone Formation which formed around 340 million years ago, long before the age of dinosaurs.
|These rocks are called "turtle rocks" because they resemble the shells of turtles|
The exact processes that create "turtle rocks" are poorly understood. One explanation involves spheroidal weathering. This process occurs when water, percolating through cracks and between individual grains in the rock, loosens and separates layers of the rock. The weathering acts more rapidly on the corners and edges of the rock producing a rounded shape. The weathering of the rocks is also strongly influenced by the polygonal joint pattern seen in all "turtle rocks".
Box-shaped, triangular, and round patterns are abundant in the sandstones of Arkansas. These patterns form when iron present in the rock is oxidized. Iron exists in the minerals pyrite, siderite, magnetite, and hematite. Any of these minerals could be present in sandstone. Early in the formation of the rock, water filled the pore spaces between the sand grains and came into contact with iron-rich minerals. This caused the iron to go into solution. As the rock became exposed to air or fresh water, oxygen was added to the solution. This caused the iron to oxidize and precipitate along exposed joints or bedding planes in the rock formation. Sometimes color bands result from the different oxidation states of iron. These bands are also referred to as liesegang banding or box-work by the scientific community. Often the iron bands form interesting polygonal shapes sometimes referred to as "carpet rock". Other liesegang shapes consist of concentric layers of iron bands which may be mistaken for petrified trees (see picture below).
|Liesegang banding in sandstone from northern Arkansas|
A dendrite is a surficial mineral deposit of an oxide of manganese that has crystallized in a branching pattern (Glossary of Geology, 1987). Dendrites may also consist of iron oxides and other minerals. They could be misidentified as fossils since the branching pattern resembles leaves.
|Manganese dendrites in novaculite|
|Manganese dendrites in novaculite|
|Pyrite dendrites on Cretaceous limestone|