Laramide Foreland Structures in Zuni Basin and Northwestern New Mexico: Exploration Significance: ABSTRACT

Abstract

Mapping and coal resource evaluation in the Zuni basin, west-central New Mexico, have led to the recognition of Rocky Mountain foreland type structures. These structures consist of narrow, northwest-trending monoclines, which are the result of northeast-directed compression. Late Cretaceous coal resources have been found to be localized along the downwarped side of these monoclines; moreover, the advances of three Late Cretaceous marine transgressions were halted at the sites of these present-day monoclines. Basement-involved tectonism was initiated during the late Turonian (90 Ma) along the trend of the Atarque monocline, as evidenced by a marine turnaround and associated paralic coal deposits. The timing is consistent with a tectonic event that produced intense compression in southeastern Arizona. This deformation provided the source area for the coarse-grained upper member of the Gallup Sandstone (latest Turonian). As compressive tectonism progressed northeastward, movement along the Nutria monocline localized marine turnarounds as well as sites for peat accumulation during the Coniacian (88-89 Ma) and again during the Santonian (86-87 Ma). Monoclinal deformation farther to the north, along the defiance and Hogback monoclines, exerted no syndepositional influence on coal-bearing sequences of late Campanian age (about 75 Ma). Thus, tectonism here was initiated later and may have takenmore » the form of late Laramide right-lateral slip. The segmented nature of the Zuni basin monoclines is evidence for compartmental deformation. Abrupt termination of the folds along strike demands some lateral slip between adjacent blocks, hence a compartmental fault. This lateral slip has occurred along a N56/sup 0/-58/sup 0/E trend. Total structural balance and crustal shortening must be maintained across these faults, and compensating fold structures can be demonstrated on adjacent blocks in the Zuni basin.« less