Regional Stratigraphy of the Deepwater Gulf of Mexico
Authors: Watkins Joel S. 
Published: October 2008

The stratigraphy of the deepwater Gulf of Mexico can be divided into four major phases as follows. (1) Deposition of synrift fluvial and lacustrine red beds during rifting and crustal extension. This phase ended with the deposition of the Louann salt in late Jurassic. (2) Invasion of marine waters, retreat of the continental shelfedge and deposition of shallow-marine carbonates on the shelf and upper slopes while marl and shale were deposited in deeper water. This phase ended with a widespread unconformity evident in seismic data as the Mid-Cretaceous Sequence Boundary. The MCSB is a prominent seismic marker in the deepwater Gulf. (3) Sediment starvation over much of the northeastern Gulf while moderate thicknesses of carbonate and clastic sediments were deposited in the western Gulf. Clastic sediments prograde across the former shelves in the northwestern and north-central Gulf. This phase lasted throughout part of the Late Cretaceous and all of the Paleogene. (4) Neogene sediment flood in the northern Gulf of Mexico. During the Miocene, clastic depocenters translated rapidly from offshore central Texas to the region of the present-day Mississippi River. Sedimentation rates increased and clastic turbidites reached well out into the center of the Gulf. In the Plio-Pleistocene, depocenters swung around toward the west and the sedimentation rate increased again. Turbidites from the Mississippi lapped onto the continental rises of the southern Gulf. Excellent source rocks were deposited along with Jurassic, Lower Cretaceous and Upper Cretaceous carbonates. Additional source rocks were incorporated within Cenozoic shales. Sediment starvation in the northeastern Gulf delayed maturation of Mesozoic source rocks until they were deeply buried by the Neogene sediment flood. Neogene turbidites comprise the reservoirs of several large recent deepwater discoveries.

Introduction

The deep Gulf of Mexico stratigraphy is similar to that of other low latitude rifted margins of the world. The warm climate resulted in deposition of thick salt following initial stages of rifting and also resulted in dominantly carbonate deposition for several tens of millions of years following initiation of seafloor spreading. The stratigraphy differs from other areas in that the Gulf is surrounded by buoyant continental crust that inhibited communication with the world ocean. These sills limited free circulation to the uppermost waters of the Gulf, and created an environment favorable to the preservation of organic matter. The Laramide Orogeny to the west and northwest of the Gulf peaked at the end of the Cretaceous. Uplift and erosion during this event resulted in the deposition of thick clastic sediments in the west and northwest of the Gulf. Sediment thickness data indicate that the Rio Grande River was the principal sediment pathway. Stream capture during the Cenozoic gradually directed erosional debris into the Mississippi River, culminating in a flood of sediment during the Neogene. The flood peaked during the Pleistocene as glaciation blocked streams formerly draining into Hudson's Bay and added additional sediment through glacial erosion. The Paleogene and especially the Neogene saw the migration of large, shallow, allothonous salt sheets into slope sediments of thenorthern Gulf and Campeche Bay. Channels between coalescing salt sheets funneled sediments into local deepwater fans. The foregoing events produced several good hydrocarbon sources as well as good traps and seals. This paper reviews these events in greater detail and shows examples of seismic signature associated with them.