Cretaceous "Gallup" fields in the San Juan basin, northwestern New Mexico, are stratigraphically controlled oil accumulations in marine strike-valley sandstone deposited during a transgression over the pre-Niobrara erosion surface. Individual sandstone bodies are localized on the seaward side of cuesta faces formed by the outcrop of relatively resistant beds in the folded and truncated pre-Niobrara sequence. Successively younger sandstone beds in the overstepping sequence extend farther in the direction of transgression. The sands, transported along shore from beyond the area, were deposited in significant thicknesses where the advance of the sea was slowed by the increase in slope associated with the ridges.

The paleotopography of the erosion surface, as shown by the stratigraphic cross sections and isopach maps, consisted of northwest-trending cuesta-like ridges and intervening valleys with the steeper slopes facing northeast. Local relief was more than 100 ft. A subcrop map and an isopach map show that the paleotopography was controlled largely by the paleogeology at the time of transgression, and help to interpret the position and trend of ridges and valleys in areas of poor control. Even minor lithologic variations in the outcropping beds had a significant effect on the paleotopography.

Individual sandstone bodies, shown by isopach maps, are elongate parallel with the ridges and valleys on the erosion surface and with the direction of sand transport. They thin abruptly southwestward by onlap against the erosion surface and more gradually in the opposite direction largely by facies change to shale. Pre-unconformity shale and sandstone provide part of the barrier to updip migration of oil and gas accumulations. The paleotopography and paleogeology are important in evaluating potential stratigraphic traps and in predicting the geometry of the reservoirs.

Knowledge of the transport patterns of the sands, interpreted from measurements of cross-stratification dip directions in nearby outcrops, also may be helpful in regional evaluation. Toward the southeast, in the direction of sand transport, the lenticular sandstone bodies deposited during early stages of the transgression generally are thinner; the more widespread sandstone units deposited during later stages may become more discontinuous.