Physical Stratigraphy and Sedimentology of an Alluvial Fan Delta Complex (South Pyrenean Basin, Spain)

P. Crumeyrolle

The Santa Liestra depositional sequence as defined by Mutti occurs in the eastern part of the Eocene South Pyrenean foreland basin. This sequence was deposited on the back of an active thrust system, and synsedimentary tectonics played a key role in determining sedimentation styles and stratigraphic relationships.

During this time, three contemporaneous depositional systems accumulated in a restricted area. On the northern active limb of the foreland basin, an alluvial fan delta formed, prograding south and southeast, perpendicular to the basin axis. This alluvial fan coalesced to the south with an eastward-directed fluvial system, flowing along the basin axis. Both of these systems supplied sediments to a north-south-oriented coastal deltaic system which passes distally into nummulitic marls and channel-levee turbidites. The paleogeographic distribution of these three main sedimentary systems was directly controlled by the structural framework.

The Santa Liestra is comprised of four main stratigraphic units with a major stratigraphic unconformity separating unit 2 from unit 3. This unconformity is expressed by an abrupt facies change and large-scale shelf instability features, and it probably represents a depositional sequence boundary related to a phase of thrust movement within the Santa Liestra sequence.

Field correlations on transverse cross sections through the basin show a typical foreland asymmetrical clastic wedge; these correlations also illustrate the sheet-like geometry of the fan delta-front deposits.

This study of a large fan delta-front system furnishes insight into the complex interactions governing the geometry and sedimentological characteristics of potential petroleum reservoirs in foreland basin settings.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.