Marine Fault-Controlled Gilbert-Type Fan Deltas

Albina Colella

Gilbert-type fan deltas comprise (a) subhorizontal topset beds (subaerial alluvial fan deposits) capping (b) steeply dipping foreset beds (subaqueous delta-front deposits dipping up to 40°) and (c) tangential bottomset beds. Nine complexes of coarse-grained, marine Gilbert-type fan deltas occur in the Pliocene-Holocene Crati basin (southern Italy). The foreset unit of the Crati examples (up to 160 m thick) consists of matrix-free conglomerate and conglomerate with a mud-poor sandy matrix. The scarcity of mud (1-4%) is the result of a flow separation at the river mouth where the freshwater plumes carry suspended sediment far out to the sea and the coarse bed load is transported by avalanching and turbidity currents along the steep delta face.

The Gilbert-type fan deltas of the Crati basin preserve evidence of tectonic control on their development. Syndepositional faults, characterized by high rates of vertical motion, produced steep coastlines and segmented the basin into subbasins protected from strong longshore and tidal currents. These morphologic conditions, similar to those produced in fjords by glacial action, triggered the development of deltaic foreset beds and their extensive progradation. The style of motion of the syndepositional faults, which in the Crati examples form the back edge of the delta foreset unit, controlled the architecture of both the individual Gilbert-type sequences and the basin infill. In the extension-dominated sector of the basin, Gilbert-type fan deltas occur as individual fans and are char cterized by a simple delta foreset unit with a complex sigmoid and oblique geometry. In the strike-slip-dominated sector of the basin, Gilbert-type fan deltas occur as stacked sequences. The proximal foreset unit of each Gilbert-type sequence is composite and consists of stacked minor foresets separated by erosional surfaces.

Marine Gilbert-type fan deltas are good potential hydrocarbon reservoirs. Deltaic conglomerates and sandstones can occur interbedded between, and contiguous with, prodelta and delta-plain mudstones. The updip trap is a syndepositional fault that places fan-delta deposits against the impermeable uplifted strata of the footwall. The Crati Gilbert-type sequences are wedge-shaped or tabular bodies that reach a thickness of up to 180 m (600 m when stacked sequences) and have a lateral continuity of several kilometers. Matrix-free conglomerates and conglomerates with a mud-poor coarse sandy matrix generate high porosities and permeabilities. Therefore, these systems have good reservoir quality in terms of overall geometry, size, lateral continuity, porosity and permeability, and structural etting.

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