Abstract: Deformation of an Amalgamated Salt Sheet: Controls on the Structural and Stratigraphic Evolution of the Eugene Island 330 Mini-Basin

Mark G. Rowan, Paul Wiemer, Peter Fleming

The structural and stratigraphic evolution of EI-330 over the last 5-6 million years has been controlled primarily by deformation of a regional, allochthonous salt sheet. This sheet formed in latest Miocene time by amalgamation of individual salt glaciers that flowed laterally at the sea floor from the tops of deep-rooted salt diapirs. The compound sheet had an irregular base-salt topography, with lows (thick salt) over the diapiric roots and highs (thin salt) between the source diapirs. Subsequent down-slope gravitational collapse of the salt initiated basin formation.

Restorations of interpreted seismic profiles both illustrate the interaction between salt sheet deformation and sedimentation as the shelf edge pro graded past El-330 and determine the timing and pathways of hydrocarbon migration. Deposition between 5 and 2.2 Ma was dominated by bathyal turbidite systems and pro-delta muds in a restricted intra-slope basin located where the salt was originally thickest. Salt displaced from beneath the growing basin moved laterally into flanking diapirs that formed topographic highs. A salt weld formed possibly as early as 3.4 Ma, allowing initial migration of hydrocarbons into the El-330 area; thereafter, accommodation space was created primarily by growth faulting and down-dip extension. After the shelf edge shifted south between 2.2 and 1.5 Ma, depo ition was dominated by deltaic sands in a confined low between regional and counter-regional faults lust landward of the shelf edge. Eventual filling of the mini-basin and a decrease in extension after 0.9 Ma resulted in a further southward shift of the shelf edge, sediment bypass, and coastal plain deposition.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994