Abstract
Most Ordovician source rocks consist of accumulation of a colonial marine microorganism,
Gloeocapsomorpha prisca (G. prisca) whose nature, ecology and affinity with extant organisms have been in dispute for years. Furthermore, recent studies have shown major differences in phenol moieties between two
G. prisca-rich samples. Examination of five
G. prisca-rich kerogens by electron microscopy and pyrolysis studies revealed (i) the occurrence of two markedly distinct “morpho/chemical” types: a “closed/phenol-rich” type (Baltic samples) and an “open/phenol-poor” one (North American samples) and (ii) the selective preservation of the resistant micromolecular material building up the thick cell walls in the original organism. Comparison with extant
Botryococcus braunii (a widespread green microalga) grown on media of increasing salinity suggests that
G. prisca is likely to be a planktonic green microalga related to
B. braunii, which can adapt to large salinity variations which, in turn, control its polymorphism. The large differences in colony morphology and in the content of phenol moieties observed in fossil
G. prisca and the resulting occurrence of two “morpho/chemical” types, should therefore reflect depositional environments with different salinities. The presence of thick, highly aliphatic, resistant walls in
G. prisca selectively preserved during fossilization, accounts for the major contribution of this organism to Ordovician organic-rich sediments and for the resulting typical signature of Ordovician oils.