Published evidence indicates that Late Cretaceous shorelines trended northwest through west-central New Mexico and adjacent Arizona. Our investigations delineate these shorelines through time and relate them to the prominent northwest-trending monoclinal flexures in the Zuni and southwestern San Juan basins. We related the transgressive (T)-regressive (R) marine cycles (T2-R2, T3-R3, T4-R4) of C. M. Molenaar to deep-rooted monoclinal or asymmetric anticlinal structures. The T2-R2 turn-around is coincident with the Pinon Springs anticline in the northern part of the Zuni basin and appears to be controlled by the Atarque and Gallestina monoclines in the southern part of this basin. Shoreline configurations during the T3 and T4 transgressive maximums coincide with the axis of the Nutria monocline and relate to some subtle pre-Laramide movements along this structure. The R2 regression is unique to New Mexico, suggesting local tectonic control on the configuration of the seaway. The subsequent T3 transgression, which was a major widespread event elsewhere in the Western Interior, was abbreviated in west-central New Mexico near the location of the Nutria monocline.
The T2-R2 through T4-R4 shoreline turnarounds produced numerous paralic basins favorable for the accumulation of organic detritus. A turn-around probably represents a period of slow rates of shoreline migration which allowed a thicker, more extensive accumulation of plant material and hence thicker coals. The present and most of the past coal production in the Zuni and southwestern San Juan basins is from coals formed in paralic basins just landward of the turnarounds caused by pre-Laramide tectonics.