Salt Architecture and Its Impact from Deposition, Ultra-Deepwater Congo Basin, Angola

Mark G. Rowan¹, Elena N. Zhurina², Michael F. Liebelt², and Wade D. Hutchings^2
1Rowan Consulting, Inc., Boulder, CO
²Marathon Oil Co., Houston, TX

The ultra-deepwater Congo Basin of Angola is characterized by contractional structures and associated allochthonous salt that formed in response to proximal loading and extension and the consequent gravity spreading of the margin. Interpretation of 3-D, pre-stack depth-migrated seismic data shows four dominant structural styles: 1) symmetrical folds connected to squeezed salt diapirs; 2) thrusted folds linked to thrusted diapirs; 3) thrust sheets riding on allochthonous salt with secondary diapirs and minibasins; and 4) areas of inflated autochthonous salt and shallow contractional structures. Stratal growth geometries indicate that the main shortening phase postdated diapirism and was contemporaneous with allochthonous salt extrusion. There is no obvious pattern to the distribution of the different styles, either in dip or strike directions. Strike-parallel juxtaposition of different styles with varying amounts of shortening requires strike-slip structural elements such as tear faults and lateral ramps.

The structural architecture played a profound role in controlling the distribution of channelized turbidites and depocenters. In several areas Lower Tertiary turbidites entered the area from more proximal positions along main fairways controlled by salt withdrawal and dip-oriented normal faults. In the ultra-deepwater area, sediment-transport pathways and depocenters became increasingly convoluted due to the complex pattern of diapirs and contractional structures that created paleotopographic highs. A subregional Oligocene amplitude extraction example shows such features as ponded fill updip of barriers, channel systems snaking around and between barriers, and likely fan-lobe deposition just downdip of constrictions between diapirs and/or incipient folds.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery