Fibrous calcite beds containing cone-in-cone structures occur in outcrops and in core and cuttings from the subsurface, at a constant stratigraphic position near the base of the Hare Indian Formation, throughout parts of Anderson, Peel, Mackenzie, and Great Bear Plains in the District of Mackenzie. These remarkably persistent beds have a maximum thickness of 2.75 in. (7 cm) and occur throughout an area of about 15 000 mi2 (24 000 km2). Their most northerly observed exposure is near the junction of Anderson and Wolverine Rivers, and the most southerly is at Powell Creek about 220 mi (354 km) away. The fibrous calcite marker beds are rock stratigraphie units that probably developed in partly consolidated muds close below the water/sediment interface in response to undetermined changes in the physicochemical environment. Organic material and plant remains in the surrounding sediments probably acted as catalysts to initiate nucleation and growth of the calcite fibers. The zone of fibrous calcite occurs within and delineates the approximate areal distribution of a sequence of sediments rich in organic remains, sediments favorable to the formation of petroleum source beds. Closely associated strata are likely to be of interest to industry.