Proximal–distal ichnofabric changes in a siliciclastic shelf, Early Pliocene, Guadalquivir Basin, southwest Spain

The lower Pliocene deposits, cropping out along the Atlantic coast of Cadiz, in the Conil–Cabo Roche area, formed on a shelf gently deepening towards the W–NW. Up to five types of facies can be recognized, distributed in belts from proximal to distal parts of the shelf: 1) Coarse-grained sandstone with trough cross-stratification facies on the most proximal parts of the shelf (inner-shelf deposits). 2) Medium-grained sandstone with low-angle planar cross-stratification facies deposited in a more distal position due to storm currents transporting sediment offshore. This facies intercalates with the previous trough cross-bedded sandstone. 3) Massive fine-grained sandstone and siltstone facies laterally and vertically related with the previous facies type and representing sediments formed in the middle shelf close to the storm wave-base. 4) Tabular sandstone bed facies corresponding to storm-bed deposits intercalated within the two previous facies. 5) Greenish–bluish shale facies accumulated on the outer shelf below storm wave-base.

Three ichnofabrics associated with these facies are recognized: 1) The two cross-bedded facies are dominated by a Macaronichnus ichnofabric, characterized by Macaronichnus and Bichordites traces that represent colonization of a shifting sandy substrate by polychaetes (Macaronichnus tracemakers) and by sea urchins (Bichordites producers). 2) A Rosselia ichnofabric, consisting of vertical traces of Rosselia cross-cutting Macaronichnus, characterizes the interval of massive fine-grained sandstone and silt facies and interbedded tabular sandstones. This ichnofabric formed under high sedimentation rates with Rosselia traces exhibiting stacked patterns that indicate vertical readjustment following deposition. 3) Massive fine-grained sandstone and silt facies are distinguished by a Cylindrichnus ichnofabric. This fabric was produced by the activity of a moderately diverse community dominated by sessile, burrow-dwelling, probably superficial-detritus-feeding polychaetes (Rosselia and Cylindrichnus tracemakers) and vagile, deposit-feeding polychaetes (Planolites producers). In addition, infaunal deposit/detritus-feeders producing spreiten structures (Teichichnus) are found in distal areas. Pervasive indistinct mottling of the sediment of this facies was probably due to the activity of very shallow burrowers in the uppermost sediment tiers.

This proximal–distal replacement of ichnofabrics reflects the change of benthic communities across the shelf. This distribution does not fit the traditional Seilacherian ichnofacies model for soft-ground shallow-marine settings, in which proximal, vertical-dwelling burrows of the Skolithos ichnofacies dominate high-energy environments. In the Pliocene of Cadiz, shallow-tier burrows of deposit/detritus-feeders characterize the high-energy facies instead, which fit better with the Cruziana ichnofacies. This deviation can be related with the particular palaeoenvironmental conditions. Thus, vagile, opportunistic deposit-feeders colonized the dune foreset when the dunes were stable, and the progradation of the structure favored the preservation of the traces.