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ABSTRACT

New 240-fold, 24 km aperture, 2-ship seismic data tied to OBS refraction, gravity/magnetic and deep well data has been integrated with more conventional onshore and offshore seismic data to elucidate the deep structure and early history of the Texas rifted margin. We present a 330 km offshore composite seismic interpretation and cross-section from the Perdido foldbelt in Alaminos Canyon through a 200 km wide salt allochthon extending entirely across the Texas continental slope to the extensional growth fault complex in the Brazos shelf. Our data and interpretation are extended an additional 150 km onshore through Matagorda, Wharton and Colorado counties past the lower Cretaceous Sligo and Stuart City shelf edges. The Texas continental margin developed as an unstable progradational depositional complex built out over Jurassic transitional and oceanic mafic crust lying seaward of a detachment-faulted, rifted continental edge. Callovian Louann salt, up to 5 km thick in the cores of Perdido foldbelt anticlines, gradually thins northwestward. Autochthonous salt is thin or absent under the Texas shelf and coastal plain. A lower Cretaceous post-Callovian section 1.5 - 3.0 km thick underlies the prominent “MCU” event which can be traced in our data from the Cretaceous shelf edge into the Perdido foldbelt. A prominent sequence boundary at the top of the lower Eocene (49.5 m.y.) acts as a major decollement surface in the Texas coastal plain, largely decoupling a younger, greatly extended section overlying up to 6 km of upper Cretaceous, Paleocene and lower Eocene rocks. The 49.5 m.y. sequence boundary extends offshore underneath the allochthonous salt which intruded up-section across time lines and has accommodated massive amounts of updip extension from the early Tertiary to the present. Regional free-air gravity and OBS refraction data suggest the crust under the salt gradually thickens from 5 km (oceanic) in the Perdido foldbelt through 10 km (transitional?) under the Texas slope to 22 km (continental) under the coastal plain. No prominent magnetic anomaly distinguishes an oceanic/continental boundary. The oceanic crust is a magnetic “quiet zone”. The 300 nanotesla Houston magnetic anomaly overlies continental crust and may be speculatively attributed to a late Paleozoic (?) feature. Our data suggest that the presumably deepwater clastic packages under the salt allochthon are relatively simply-structured with huge volumes of unexplored and highly prospective section. The sub-allochthon play in the Texas slope must be considered as one of the last exploration frontiers left in the continental United States.

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