Several different iron compounds have been known to replace organic
matter. Many Texas limestones contain fossil snails and clams which have
had their original shell material replaced by iron compounds such as
limonite, hematite, marcasite, or pyrite. Certain of the fossiliferous
Tertiary sandstones of the Texas Gulf Coast area contain large amounts
of glauconite which commonly replaced organic material.
In some areas entire faunas have been replaced by iron compounds. Such
is the case in the famous “Pyrite Fossil Zone” of the Pawpaw formation
(Lower Cretaceous) in Tarrant County. The fossils in this part of the
formation are very small or “dwarfed” and have been replaced by
limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are
particularly abundant at this locality.