Sedimentology of a linear prograding coastline followed by three high-destructive delta-complexes (Cambro-Ordovician, Cantabrian Mountains, NW Spain)

In the Luna area of the Cantabrian Zone the Cambro-Ordovician Oville and Barrios Formations were studied. Five facies groups were distinguished, each consisting of one or more subfacies. Facies group I represents littoral deposits and consists of four subfacies: barrier-beach deposits, beach deposits surrounded by tidal deposits, sub-beach deposits and littoral-channel fills (rip-current channels). Facies group II represents tidal deposits, consisting of high-energy tidal-channel deposits, low-energy tidal-channel deposits, non-channel tidal deposits and highly wave-influenced tidal deposits. Fluvially influenced deposits form facies group III. Three subfacies were distinguished: fluvial-channel fills, fluvio-tidal deposits and fluvial-overbank deposits. Shelf, pro-delta and delta-slope deposits form facies group IV, four subfacies are found: pro-delta deposits, lower delta-slope deposits, upper delta-slope deposits and shelf deposits. The subfacies of this group form sequential units: delta-slope sequences, shelf sequences or combined shelf and delta-slope sequences, all of which are regressive. Facies group V represents lagoonal deposits. The Oville Formation can be subdivided into four members, the Barrios Formation into seven members. The Oville and Barrios Formations together consist of four large sequences. Sequence I, in which the Griotte Member of the underlying Lancara Formation is included, begins with deposits of a carbonate-clay shelf environment, followed by argillaceous mottled shelf deposits. The top of the sequence consists mainly of littoral deposits. The sequence represents a linear prograding coastline. Sequence II consists of a lower part formed by pro-delta and delta-slope deposits and an upper part formed by tidal delta-top deposits. It represents a high-destructive tide-dominated delta. Part of the area was not influenced by the delta, here a tide-controlled prograding coastline existed. Sequence III begins with a thin transgressive sandstone sheet, which is followed in part of the area by tidal-delta deposits. After the transgression a regressive sequence was formed: tidal-delta deposits, littoral deposits (barrier-bar complex), tidal-flat deposits and fluvial or fluvio-tidal deposits. In part of the area the latter two are absent. The sequence represents a high-destructive wave- and tide-controlled delta, which was bordered on both sides by a coastal plain, consisting of beach ridges. Sequence III was followed by a period of subaerial exposure. Sequence IV begins in some sections with lagoonal deposits, which are overlain by barrier-bar deposits. Fluvial channels are locally found in the top of the sequence. Sequence IV possesses only small lateral continuity and is absent or extremely thin in the larger part of the area. Sequence IV represents a high-destructive wave-dominated delta, bordered on both sides by emerged areas of non-deposition.