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Renewed interest in the development of substantial coal reserves in the San Juan basin has given new impetus to the determination of the potential impact of expanded mining on paleontologic resources and to a reconsideration of depositional environments of Fruitland coals.
Reanalysis of molluscan localities from the Fruitland and Kirtland Formations on lands of the Navajo Nation, in preparation for more extensive field studies, indicates that, of the 24 localities collected by C. M. Bauer in 1915, 12 are type localities for 17 species, representing a large percentage of the total fauna known from these formations. Differentiation of brackish and nonmarine environments on the basis of biostratigraphic distribution of species divides the Fruitland Formation into upper and lower units. Near the western margin of the San Juan basin, predominantly brackish environments are present near the base of the Fruitland, underlain by the littoral-marine Pictured Cliffs Sandstone, and extend up to 35.6 m in total thickness. Exclusively nonmarine sediments are present bove this horizon to the top of the Fruitland and throughout the Kirtland Formation. Nonmarine molluscan diversity seems to increase markedly at approximately 35.6 m. Below this horizon, only 7 taxa indicate freshwater environments. At 35.6 m, 8 taxa are introduced, and of these 7 occur only at this level. Above 35.6 m, only 2 taxa are introduced: Physa reesidei at about 61 m and "Unio" baueri at 123.5 m (Kirtland Formation). The only terrestrial mollusk reported is Planorbis chacoensis from near the base of the Fruitland Formation.
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