Structural Evolution of Northern South Marsh Island, Northern Gulf of Mexico: Analysis of a Hybrid Roho-Stepped Counter-Regional Salt System, and Kinematic Evolution of Normal Fault Growth and Linkage Patterns

Sean M. O’Connor and Paul Weimer
ConocoPhillips, Houston, TX, and University of Colorado, Boulder, CO

Northern South Marsh Island, northern Gulf of Mexico has been divided into three structural provinces comprising two distinct sets of listric normal faults that sole into evacuated salt sheets. Southeast-dipping faults of the northern structural province sole into a salt base lying below the 3-dimensional seismic data used in this study, six-second two-way travel time. This province is interpreted as a roho salt system.

The southern and western provinces comprise south-dipping normal faults that sole into a single, complex salt weld. It is convex in the west and flattens easterly into the southern province. Half-turtle structures and counter-regional faults of the western province indicate a stepped counter-regional, end-member salt system. Roller faults soling into a sub-horizontal salt weld in the southern province suggest a roho end-member salt system. Because the evacuated salt exhibits characteristics of both systems, it is a “hybrid roho-stepped counter-regional salt system”.

Restorations in the western area address the evolution of the fault system. Linkage patterns of fault segments through time reveal fault linkage points. However, the timing of linkages is questionable. Anomalous isochron patterns characteristic of relay zones were identified in the 9.50-13.55 Ma stratigraphic interval, in close proximity to relay zones occurring at 5.10 Ma. The age discrepancy between the isochron pattern and the fault geometries indicates that the map view restoration method may be limited in predicting fault linkage chronology. Such chronology is important because relay zones at linkage points can act as migration conduits for hydrocarbons across fault zones; their absence can infer trapping in hanging walls and the termination of active migration across relay zones.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004