Stratigraphic Framework and Origin of Shallow Geohazards on the Upper Slope, Northeastern Gulf of Mexico
Authors: Fillon R.H. , Roberts H.H. , Kohl B. 
Published: October 2008

Multidisciplinary analysis of a research borehole on the upper continental slope in Viosca Knoll block 774 suggests links between lowstand shelf-edge deltaic deposystems of the ancestral Mobile River and deep Gulf of Mexico fan deposition. Paleontology, 14C-dating, stable-isotopes, sedimentology, and seismic stratigraphy detail a significant break in depositional architecture 10-15 m below the seafloor. The apparent erosion or "by-pass" surface is traceable downslope as a distinctive higher-order seismic sequence boundary capping sand-rich lowstand clinoforms. Reflector signatures, seismic amplitudes, and reflector terminations immediately above the boundary define a gas-charged channel-levee system. Benthic foraminiferal faunas in the 19 ka BP clinoforms below the boundary record warmer than modern winter ocean temperatures and carbonate bank paleoenvironments. Biostratigraphic and isotopic correlations indicate that a widely reported Gulf of Mexico ?18O glacial meltwater event (11-16 ka BP) is truncated at the boundary.

Two modes of slope sedimentation are postulated for the northeastern Gulf:accretionary progradation, with distal clinoform deposition, anderosional by-pass, with development of leveed channels above truncated section.

Channel-levee deposits and paleoecological reconstructions of lowstand sea temperatures and salinities offer evidence of a direct link between lowstand shelf-edge deltas and unconfined slope-fan deposits which, when gas or fluid-charged pose a significant geohazard. Our study confirms the utility of a sequence stratigraphic and regional paleoceanographic approach to the recognition of geohazards.

Introduction

The upper continental slope of the Gulf of Mexico is a critical transition zone linking deltaic deposition on the shelf with basinal fan deposition. In the northwestern and north-central Gulf, active salt movement in the Tertiary molded the slope, leading to the deposition of small ponded fans within stairstepping, salt-rimmed mini-basins. The shallowest mini-basins would fill first, then the next deeper one, and so on1. The resulting tectonically "confined" fan deposits typify minibasin fill sequences. The majority of reservoirs in the so-called "flex-trend" play along the modern shelf-edge are assigned to these facies. Most investigations of shallow drilling hazards including, shallow flows, hardgrounds, hydrocarbon seeps, mud volcanoes, and shallow gas2, 3 have also targeted confined fan systems.

In areas where Tertiary salt movement was minimal, however, sediments moving from the shelf to the upper slope encountered, not a series of salt-bounded mini-basins, but rather a smoothly graded, gently sloping seafloor that stretched unbroken to the abyssal plain. This is the case in the northeastern Gulf of Mexico. The mini-basin provinces of the northwestern, and north-central-Gulf are separated physiographically from the eastern Gulf "unconfined" slope system by the northwest to southeast cutting Mississippi Canyon. In the Pleistocene, sediments transported via this deep trough created the Mississippi Fan complex astride the eastern Gulf abyssal plain (Fig. 1). The Mississippi Canyon system is so large that its submarine valley-fill units constitute a unique geological system with unique hydrocarbon plays and geohazards4.

The focus of this paper is the large area of relatively featureless slope situated east of the Mississippi Canyon, and west of Florida escarpment (Fig. 1).