THE STRUCTURAL GEOMETRY AND EVOLUTION OF THE SALT-RELATED PERDIDO FOLD BELT, ALAMINOS CANYON, NORTHWESTERN DEEP GULF OF MEXICO

Abstract

ABSTRACT The Perdido fold belt, located in the northwest part of the Gulf of Mexico basin, is defined by a series of large-scale fold structures that extend southwest into Mexican waters and northeast beneath the Sigsbee salt nappe. Within the Alaminos Canyon OCS lease area, the fold belt consists of northeast-southwest trending, sub-parallel, concentric, box folds cut on one or both of their flanks by high-angle reverse faults. The folds are slightly asymmetric and verge both landward and basinward, a geometry typical of contractional fold belts formed above a weak detachment layer. The folds uplift the regional middle Cretaceous sequence boundary (MCSB) by up to 3 km, with a basinward decrease in height and amplitude of the folds. Detailed structural mapping has led to a model for the structural evolution of the Perdido fold belt that is consistent with sequence stratigraphic analysis of the seismic data. Minor salt movement occurred during the Late Jurassic and Early Cretaceous, as indicated by onlapping and thickness variations within the relatively thin overlying section at that time. Salt mobilization before the main phase of shortening led to early growth of some fold structures during the Eocene-early Oligocene. The main phase of compressional deformation occurred during the late Oligocene-early Miocene by gravity sliding on a detachment within the Jurassic Louann Salt. The basinward limit of autochthonous salt deposition defined the southeastern margin of the foldbelt. Detailed analyses of onlapping middle to upper Miocene strata indicate that separate folds had different evolutionary histories and developed variable along-strike geometries. Subsequent Pliocene to present-day reactivation of the highest-relief structures further modified the fold geometries. Topographic relief over the highest folds has in turn influenced the evolution of the allochthonous Sigsbee salt nappe. The advancing salt nappe has been deflected around the highest fold structures, resulting in a complex allochthonous salt sheet geometry. Previous studies of the Perdido fold belt have produced conflicting interpretations for the evolution of the fold geometries. These include salt or shale-cored interpretations and the development of the fold geometries by imbrication and fault-bend folding. Our interpretation favours an origin as salt-cored detachment folds, with late modification by re-mobilization of salt in the cores of the folds.