Abstract: Geometry of Coal Beds in San Juan Basin, New Mexico
E. C. Beaumont, J. W. Shomaker
Late Cretaceous coal deposits in the San Juan basin of New Mexico are believed to be shore-marginal in origin. During the Late Cretaceous the shoreline advanced and retreated across the present San Juan basin area in several major episodes. Environments of deposition were transitory and the characteristics of the transitional deposits appear to reflect the rate of movement of the shoreline. Coal-bearing zones tend to be persistent but to vary in the relative concentrations of coal present within the zone. Individual coal units tend to be lenticular and discontinuous and, if the rate of shoreline shift were sufficiently regular, the coal unit may be a series of imbricate beds that appears to be a single unit.
The coal-producing swamps seem to have been interrupted laterally along the strandline as streams and rivers breached the swamp environment in seeking the sea. The lateral unloading of the streams into the swamp inhibited peat accumulation and produced a mix of sediments that, following compaction, is difficult to decipher.
Differential compaction of the sediments results in apparent relations that can be misleading. Closely spaced drill holes are required to provide correct correlations. Simple constructions with variable compactive factors illustrate the complexities imposed by compaction on the sedimentary sequence.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC