A detailed petrographic examination of the limestones of the Deschembault Formation (Trenton Group, Middle Ordovician) of the St. Lawrence Lowland of Quebec was undertaken to shed light on the geologic history of this formation and to contribute to limestone petrography in general.

Acetate peels were successfully employed during microscopic studies of samples taken from fourteen described stratigraphic sections. The Deschambault limestones are richly bioclastic, often pelleted, and occasionally conglomeratic. Common skeletal carbonate grains include fragments of echinoderms, brachiopods, bryozoans, ostracods, and trilobites. Interstitial calcite consists of both microspar (microcrystalline calcite mosaics) and sparry cement. The described textures of the former are considered as evidence that microspar represents recrystallized calcareous mud. The textures of the calcite cements indicate that individual grain types controlled some of the characteristics of adjacent porefilling cement crystals. Textures also supplied evidence of both external and internal replacement of carbonate grains by sparry calcite, some of which may not be cement calcite. A discussion of other diagenetic phenomena, including sediment compaction, stylolites, and chert formations is given.

The limestones were classified into eight rock types which were compared to modern calcareous sediments from the Persian Gulf. The close comparisons, along with an isopach map, paleocurrent data, sedimentary structures, etc., suggest deposition occurred upon a shallow, marine shelf which deepened toward the southeast.

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