Allochthonous Blocks as Hydrocarbon Traps in the Gulf of Cadiz
Authors: Somoza L. , Maestro Adolfo , Lowrie A. 
Published: October 2008

Westward extentsion by continental of the Alboran Sea platelet in the early Neogene initiated large slump-block movements along the Iberic and Moraccan continental margins. Extensive allochthonous salt may underlye and serve as a lubricant for gravity-driven translation whose overall movement may exceed 400 km. Mobile, Late Cretaceous-Paleogene salt, Deposited in a paleo-basin with a western limit of 10°W to 12°W, appears to have become diapiric. The migrating blocks and associated down-dip debris-flows may be the trapping mechanisim for hydrocarbons. Migrating debris covers and area as great as 90,000 km2 with potential reserves riveling the Gulf of Mexico sub-salt play.

This complex geologic evolution apparently began as the Gibraltar Arc moved westward in the Neogene, and caused an overthrust that formed an accretionary wedge onto the Gulf of Cadiz. The westward Gibraltar Arc migration margins, which initiated farther westward migration of gravity-driven continental and salt-floored blocks. Ultimate emplacement was over oceanic crust. As the continental blocks migrated, normal listric faulting along the present continental shelf of the Gulf of Cadiz developed, thus accomodating upper Miocene (past 19 Ma) tectonics and deposition. Multiple stacking of thrusting wedges occurred at the foot of the rotated continental blocks. Lateral migration can generate compression zones that, in turn, can provide extensive fracturing, faulting, and jointing, suitable for further petroleum storage. This is the first proposed ocean basin hydrocarbon province based on long-distance allochthon/mass-wasting migration.

Introduction

The Gibraltar Arc is the westernmost tectonic belt of the Alpine-Mediterranean compression zone in which south-tonorth thrusting has overriden attenuated and thinned passive margins of Morocco and Iberia. The western frontal portion of the Gibraltar Arc reaches Atlantic oceanic crust (Fig. 1). The included tectono-sedimentary complexes consist of a chaotic mixture of Triassic, Cretaceous, Paleogene, and Neogene sedimentary units, overlying a Paleozoic basement. These deposits were generically defined as huyge slump blocks, highly reflective and seismically chaotic bodies, huge in area dn volume, subdivided into three areal units from E to W (Bonnin, et al., 1975; Auzende, et al., 1981). Unit I is related to the Betic-Rifian orogen, and units II and III occupy the Seine and Eastern Horseshoe abyssal plain. Slump Block I incorporates the Prerifaine nappes of the Rif and the Guadalquivir slump block in the Gulf of Cadiz, respectively representing the northern and southern parts of the gibraltar Arc (Finch, et al., 1996). The Guadalquivir block was traditionally interpreted as large mass-wasting processes proceeding westward from the Betic Cordillera (Lajat, et al., 1975). Finch, et al. (1992) interpreted the generally east-west trending Guadalquivir block as an accretionary wedge formed by the northward and westward thrust by the Alboran platelet.

Along the Atlantic continental margin and parts of the Horseshoe and Seine abyssal plains, west of the Gibraltar Arc, deep seismic lines reveal the existence of giant seismically-chaotic bodies with thicknesses exceeding 4 km. These bodies are comprised of mass-wasting deposits, and tectonic melanges whose overall length may be up to 100 km (Torelli, et al., 1997).