Lab 6: Modes of Fossilization
The purpose of this lab is become familiar with examples of the different way in which biological organisms (or evidence of their activity) is preserved in sedimentary material over geological timescales.
Sketch and describe each set of samples at each station, and use those observations to determine which station is an example of which fossilization mode.
Internal Mold: cavities become filled with sediment during or shortly after burial. This infilling sediment can then retain the shape of the cavity which it filled.
Quartz: The starting biological material is typically calcite, in the case of shells or coral, or apatite in the case of bones and teeth. Fluids percolating through the host rock dissolve away the original mineral and simultaneously precipitate silica.
Siderite: Siderite, or FeCO3, is the ferrous analog to calcite. It is a moderately dense, brown, fine-grained mineral which commonly forms oddly shaped concretions in sandstones.
Pseudo-fossil: Manganese oxide phases (e.g. desert varnish) can precipitate onto surfaces or between bedding planes, creating shapes which seem biological in origin, but are not.
Recrystallization: Typically a phase change (commonly calcite -> aragonite) resulting from increasing pressure from burial. Any sediments contained within cavities in the sample will be recrystallized as well.
Carbonization: A slight degree of metamorphic heating drives off the volatile hydrogen in the hydrocarbons making up the organism, leaving graphite behind in the original shape of the organism.
Trace fossils: A couple examples of this were already seen in lab 2 (the bird & worm tracks). It is not a fossil of the actual organism, but a fossil of some way in which they disturbed the soil, such as a footprint or a burrow.
Impression Fossils: preservation of the shape which the organism’s remains tamped down in the sediment.
Casting: Similar to internal mold, but the infilling sediment is all which remains.
Permineralization: pore spaces become filled with a precipitate such as calcite or silica.
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