The Influence of a Pre-Existing Late Jurassic Transtensional Basin on the Structure and Stratigraphy of a Cretaceous Foreland Basin: The East Potrillo Mountains, New Mexico

Published maps of the East Potrillo Mountains (EPM) show that the range is tilted to the southwest revealing a section of Mesozoic rocks disconformably resting on Late Paleozoic strata. The section is cut by thrust faults and shows folds that probably formed during Laramide inversion, typical of southern New Mexico, Arizona, and northern Mexican states. This tectonic setting differs from those recognized in the adjacent US Rockies and Sierra Madre Oriental where deformation is generally characterized by gently dipping thrust faults. Recent mapping within the EPM suggests that these structures were superposed on the infill of a Late Jurassic transtensional basin.

The northeastern front on the EPM exposes the floor of the basin. Above the massive carbonate strata the basin fill comprises four tectonostratigraphic units: 1) carbonate beds disrupted by shear zones sub-parallel to bedding, development of local breccia, and open, asymmetric folds; 2) a section composed of brecciated carbonate units and other calcareous debris that accumulated by mass-gravity processes and fine into muddy carbonate and silicified breccia, which is transitional into clast-supported conglomerate comparable to the Glance found at base of the Bisbee Group; 3) the Hell-to-Finish Formation that has sandy carbonate at its base, contains soft-sediment folds and some intermixed, thin, volcaniclastic layers overlain by calcareous units and that is locally strongly disrupted by penecontemporaneous slides and folds from about the middle of the unit downward into the underlying conglomerate; 4) massive, detached calcareous rocks including fossil-rich units that moved southward above ductile surfaces along the axis of the pre-existing basin. The latter process caused flattening and shearing of weak units hundreds of meters below the base. Some south-directed slides were folded during northeast-directed Laramide thrusting. The infilling of the basin by southward-directed mass-gravity processes and Mesozoic rejuvenation of basin margins during Cordilleran orogenesis were principal events.