Recognition and Origin of Exotic Salt Structures in Eugene Island Area, South

HART, W., C. GARDNER, and D. KIKKERT

Centrally located in the emerging offshore subsalt trend, the Eugene Island Area conspicuously lags industry subsalt activity, testimony to its complex structural fabric. Vintage 2-D seismic data hinted at discordant features underlying the pervasive shallow sills and decollement surfaces, but were inadequate to resolve the structures. Recent large volume 3-D data sets provide enhanced coverage and resolution, revealing a multitiered honeycomb system of horizontal and inclined salt deflation surfaces (welds) and a concomitant array of distinctive flank geometries. Notable features include upward bifurcating welds, perched sediment blocks, suspended collapse grabens, pseudo-thrust faults and salt remnants of various configurations. Descriptive terminology includes "snake heads", "rat tails", "triple junctions", "salt noogies", and "zipper structures".

Schematic 2-D restorations were constructed to deduce the origin of the observed features. In preparation, areas of salt/sediment attachment were distinguished from areas of salt/sediment detachment, thereby defining separate regimes of vertical and rotational sediment collapse along deflating salt surfaces. Models were referenced to the sea floor, which served as the regional datum for all horizons. The inclination angles of singular and bifurcating welds were reconstructed by varying, respectively, the unilateral and bilateral migration rates of salt induced bathymetric highs along the sea floor. Vertical welds having asymmetrical flanks were reconstructed by progressive downward lengthening and thinning of discrete salt volumes, culminating in the juxtaposition of attached and detached sediment blocks.

As a result, restorations illustrate how specific salt inflation/deflation sequences, within a passive downbuilding framework, resulted in the varied geometries observed in Eugene Island Area, without invoking compressional tectonics, upbuilding kinematics, or out of plane salt withdrawal.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.