Three genetic types of dolomite grains occur in Cretaceous sandstones of the Western Interior and Alaska. They are:
(1) Detrital—clastic fragments eroded from dolomite rocks in the sediment-source terrain. These are well-rounded polycrystalline aggregates with relict internal textures inherited from the source rocks.
(2) Secondary—formed after deposition of the enclosing sediment. These are small euhedral rhombs that replaced calcite cement in lenticular marine sandstones.
(3) Primary—formed within the depositional basin prior to final settling-down and burial of the sediment. These are single rhombic crystals abraded to various degrees of roundness and sorted to the size of associated clastic sand grains. The fabric relationships are of the depositional type, with no evidence of relict internal structures. This ––––eliminates a possible secondary origin. A detrital origin is eliminated by the absence of inherited textures and by the distribution patterns, for primary-dolomite grains are restricted to marine sandstones. Within these sandstones, the primary-dolomite grains are most abundant in the basinward portions. The grains are widely distributed in marine Cretaceous sandstones throughout the Western Interior, regardless of variations in lithology of sediment-source terrains. The recognition and interpretation of primary-dolomite grains is the major contribution of this paper.