The San Juan basin is a post-Pennsylvanian tectonic subsidence area closely related in its early sedimentational history to the Paradox geosyncline. During early Pennsylvanian time, this region was subjected to widespread erosion, but received high-shelf Pinkerton Trail carbonates and fine gray clastics related to the Cordilleran geosyncline, followed by Cherokee subsidence to form a through-going seaway in whose deeper parts evaporites of the Paradox formation were deposited, succeeded by coarse arkosic clastic invasion from the strongly activated San Luis and Penasco positives on the east and fine-grained red clastics from the Zuni-Defiance positives on the west and southwest. Normal marine waters persisted during the deposition of the Honaker Trail-late Madera sediment , succeeded by filling of the seaway with red continental clastics across the Pennsylvanian-Permian temporal boundary everywhere except in the medial parts of the seaway where marine Wolfcamp carbonates were deposited. The southwest and west shelf of this seaway, with its sediment-controlling "spurs" and "sags," appears to be a locale where commercial oil should be present in Pennsylvanian strata.