DEPOSITION OF A STRUCTURALLY CONFINED SUBMARINE FAN IN AN EVOLVING BASIN : THE GUASO I TURBIDITE SYSTEM, AINSA BASIN, SPAIN

The mid-Eocene (Late Lutetian) Guaso I turbidite system was deposited in upper bathyal water depths in the Ainsa basin, a piggyback basin in the South Pyrenean foreland basin system. This study uses outcrops of the Guaso I (G1), a sub-unit within the greater Guaso system, to document spatial and temporal changes in the stratigraphic architecture of a turbidite system deposited in a small (~ 100 km²), structurally confined basin. These types of basins occur along active and passive continental margins, as well as near mid-ocean ridges, and their stratigraphic infill can record important climatic, sea-level, and tectonic changes.

The G1 system contains nearly continuous slope to basin-floor outcrops. Paleocurrent data indicate that sediment gravity-flows entered the basin from the east, and then turned sharply to the northwest, toward the basin floor, where they were deposited. The syn-depositionally active Boltaña anticline was the basin’s distal sill, preventing G1 sediment gravity-flows from escaping the Ainsa basin. The slope feeder system, which is exposed in strike-orientation, contains mudstone sheets, mass-transport deposits (MTDs), and channels. Bypass indicators, such as muddy-debrite-filled channels, are abundant. MTDs are thickest in the axis of the feeder system, and their abundance decreases downslope toward the basin floor. Outcrops nearest to the basin floor, which is also the depocenter of the G1, contain a relatively thick stratigraphic succession, comprised of: (1) basal lobe elements; (2) distributary channels and lobes, with rare MTDs; (3) interbedded siltstone and very-fine sandstone sheets. Shallow-water trace fossils in upper G1 beds suggest a shallowing-upward trend. The upper boundary of the G1 cycle is a mappable, laterally extensive mudstone unit.

Mapping and stratigraphic correlations document that G1 architectural elements increased in compensational stacking upward, revealing a distributive map pattern. This upward pattern is interpreted to be due to a reduction in accommodation relative to sediment supply. As a result, depositional area increased through time.