Comparison of Small Salt-Cored Diapirs (Northern Gulf of Mexico Slope) With Green Knoll Beyond The Sigsbee Escarpment: Variability in Surface Expression and Seismic Character
Authors: Roberts Harry H. 
Published: October 2008

Mounds of various sizes occur as part of the surface geology of the northern Gulf of Mexico continental slope. The origins of these mounds vary from buildups created by sediment extrusion, to shale and salt diapirs forced by sediment/salt deformation associated with gas hydrate and authigenic carbonate formation. Recognizing the characteristics that distinguish these mound types is a fundamental part of evaluating seafloor features and their geohazards potential.

This paper contrasts two mound-like features on the upper slope that have salt cores which penetrate to near the modern seafloor. The two salt-cored features occur in the Garden Banks (GB) lease area (blocks 189 and 427). The surface geology and to a lesser extent biology of these two mounds is compared to that of a much larger salt-cored diapir that is located seaward of the Sigsbee Escarpment, Green Knoll. The purpose of this comparison is to assess whether these three salt-related mounds share a common surficial geology-biology.

High resolution seismic as well as exploration scale 3D-seismic data indicate that all three mounds have a very high amplitude reflector, interpreted as salt, near the modern seafloor. Salt is interpreted to be ?25 ms from the seafloor in GB189, 40–50 ms in GB427, and &lt;5 ms in Green Knoll. Only the GB427 mound has localized 3D-seismic surface amplitude anomalies suggesting hardgrounds and gas in near-surface sediments. Both the GB189 mound and much larger Green Knoll examples display surface amplitudes equivalent to the surrounding seafloor. Direct observational and sampling data from each site confirms the lack of crude oil and gas flux to the sediment-water interface in the GB189 and Green Knoll cases, even though brines associated with dissolution of salt were observed at Green Knoll. Short-lived fluid and gas expulsion events have characterized past history of the GB427 mound. All cases lacked complex chemosynthetic communities and only GB427 displayed evidence of authigenic carbonate deposition forced by microbial utilization of hydrocarbons. Unlike its upper slope counterparts, the large Green Knoll had abundant evidence of slope instability and dissolution of near-surface salt.

Introduction

Continued interest in deep water oil and gas exploration in the Gulf of Mexico (GOM) has presented geoscientists and engineers with a new set of challenges uniquely associated with little known areas of the deep water continental slope environment. Exploration and production activities on the northern GOM slope are being conducted in one of the most geological complex deep water settings in today's oceans. Since Cretaceous times, the continued introduction of large volumes of sediment and compensating deformation of Jurassic salt has created a geologic setting characterized by numerous salt withdrawal basins, salt bodies with a wide variety of shapes and sizes, and complex fault patterns. The modern sea floor reflects this complexity, exhibiting well-defined interslope basins flanked by more complex and higher relief areas which basically reflect underlying salt (Fig. 1).