Three Upper Carboniferous, limestone-rich, high-destructive, delta systems with submarine fan deposits, Cantabrian Mountains, Spain

In the eastern part of the Cantabrian Mountains, northwestern Spain, Upper Carboniferous strata crop out. In the Pisuerga area an Upper Moscovian (\u2248 Westphalian D) limestone-rich sequence is exposed in a number of structural units. Correlations between the various structural units are based on lithostratigraphic characteristics and paleontological dating with fusulinids and calcareous algae. The various limestone units show rapid lateral transitions into siliciclastic deposits. The siliciclastic deposits are mainly interpreted as deltaic deposits. Three distinct delta systems are distinguished which, like the recent Rhône delta, are all of the wave-dominated, high-destructive type. The oldest delta system is associated with important turbidite deposits, which indicates that the delta prograded into relatively deep water (i.e. \u2265 125 m). The second delta system is associated with only minor turbidite sequences but is relatively rich in coals. These two delta systems prograded from approximately SW to NE. To the NW open marine deposits with limestones were formed, to the NE fine-grained shelf deposits and limestones were deposited. After deposition of the second delta system, tectonic tilting in the southwestern part of the area caused formation of the Vergaño disconformity. In the remainder of the area this level can be recognized as a deepening or transgressive sequence. In the northeastern part of the area important slumping movements were caused by the tectonic movements. After this tilting the river forming the delta was diverted to another area and the only source of siliciclastic sediments was a competent longshore drift system. The existence of such a longshore drift system is proved by the presence of quartz arenitic pebbles and cobbles in compositionally mature sandstones, as the deltaic deposits do not contain such coarse siliciclastics and are furthermore of lithic arenitic composition. During this period a zone with shallow marine deposits in the southwestern part of the area can be distinguished from submarine canyon and fan deposits (turbidites) in the southern and northeastern parts of the area. The third wave-dominated, high-destructive delta system prograded from SW to NE over these deposits. Open marine, shelf and shelf slope deposits are again present to the NW and NE. After deposition of the third delta system the Leonian phase gave rise to the Leonian disconformity, and a new basin configuration resulted. In the interval studied a basin margin to the southwest of the area studied can be inferred from the facies distributions. The presence is proved of a synsedimentary fault, which separates the present Casavegas Syncline from the remainder of the area. This fault mainly influenced thickness distributions, and had but little effect upon the facies distributions. The limestones were deposited in a wide range of environments, i.e. lagoonal to open shelf. The rapid lateral transitions into coarse siliciclastics are interpreted as indicating that the limestones were formed in slightly shallower water than the surrounding siliciclastics. Together with the generally muddy character, the absence of an organic framework and the presence of all kinds of algae, this indicates that the limestones are biogenetic bank deposits. The data collected have led to a redefinition of the Vañes Formation and to replacement of the Sierra Corisa Formation by the Vergaño and Covarres Formations.