The absence of land-plant-type spores from a number of Silurian, and some Ordovician, samples is attributed to original absence; prelithification and postlithification oxidation and postlithification weathering; and alteration and destruction involving chiefly thermochemical (heat), piezochemical (shear), and possibly radiochemical (radiation) reactions.
Energy for thermal alteration and destruction (conversion to amorphous carbon and to graphite) may be generated geothermally (related to sedimentary or tectonic overburden) or by igneous activity (volcanic or intrusive), or it may be, though not invariably is, generated by tectonic activity involving folding, faulting, and shearing. Superheated steam, originating geothermally or as a result of igneous or tectonic activity, may be a potential source of heat for the thermal darkening or destruction of spores. Shear that results in physical as well as piezochemical alteration of spores and pollen may be provided, respectively, by overburden pressure or tectonic pressure. High temperatures need not be a factor. The decay of radiogenic isotopes may alter spores and pollen if radioactive minerals are present in adequate concentrations in microfossil-bearing rocks.
Shrinkage cracks developed in strongly devolatilized organic microfossils and fracturing of organic microfossils during tectonic activity both contribute to the difficulty of retrieving altered microfossils from rock samples. Brittle, highly altered microfossils are also easily broken during extraction of rock samples. Fossil specimens that are sufficiently fragmented by these means are no longer recognizable, and hence for practical purposes, they are “absent” in the rock unit.
Because the color of organic matter is a function of its decomposition, regional color plots of organic microfossils appear to have potential for predicting the properties of petroleum in the same way that they have been used to predict the properties of coal.