Lobe Element Architecture and Reservoir Geometry in a Transient Slope Fan (Villafranca Fan; Southeastern Tyrrhenian Sea)

Gamberi, Fabiano ¹; Rovere, Marzia ¹
(1) ISMAR-CNR, Bologna, Italy.

In the Gioia basin a transient slope fan is developed. It is composed by a leveed channel that feeds a depositional lobe in the lower slope. It has a maximum width of 15 km and a length that varies from 12 to 20 km from west to east, being comparable to many other terminal lobes. The Villafranca depositional lobe has an internal architecture that largely differs from that of classic fan models. At the scale of lobe elements, the Villafranca depositional lobe is not composed of distributary channels and lobes but rather consists of channels and longitudinal interchannel bars. Straight, about 500-m-wide and 10 m deep channels and longitudinal bars are the main elements of the Villafranca channel-mouth depositional lobe. The channels narrow and deepen downslope, pointing to a progressive establishment of lateral flow non-uniformity in approaching the step at the edge of the depositional lobe. In the bars, the deposition of both thick transparent layers and packages of relatively thin bedded deposits takes place. The first may be due to a high rate of suspension fallout, whereas the latters, being sometimes also arranged in bedforms, show that deposition can be accompanied by traction. In addition, small scale, tens of meters wide and less than 5 m deep channels, scour in places the bars. Lateral accretion packages are the main infill of the channels. The lateral accretion packages consists of well layered packages that, from an adjacent bar, pass to amalgamated packages in the channel floor. We thus interpret the evolution of the system as resulting from lateral bar growth and consequent channel migration and infill. The spatial arrangement of channels and bars within the depositional lobe results in a downslope increase in stratigraphic complexity. The architecture of the Villafranca fan lobe has the potential to be applied as analogue for seismics or outcrop-based studies of fans developed in topographically complex slopes and connected to a deeper base level.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.