Estratigrafía y bioestratigrafía (charophyta) del Cretácico inferior en el sector occidental de la Cuenca de Cameros (Cordillera Ibérica)

Résumé

The Western Cameros Basin yields a complete charophyte fossil record which enables us to characterise chronostratigraphically the formations and depositional sequences described within the basin. The stratigraphic framework obtained in Western Cameros basin is proposed as a model for the Eastern Cameros Basin, where biostratigraphic data are comparatively scarce. From Tithonian to Barremian five depositional sequences developed in the Western Cameros Basin. The Depositional Sequence I is represented basinwide and yields one to three stratigraphic minor order cycles which are composed of a basal alluvial formation (Sra. de Brezales Fm, Jaramillo Fm and Rio Salceda! Fm) passing laterally and upwards to a carbonatic lacustrine formation (Boleras Fm, Campolara Fm and Río de San Marcos Fm). The lowest formations are Tithonian to Early Berriasian in age (charophyte biozone Maillardii) whereas the upper part of the sequen ce reaches the Middle Berriasian (Incrassatus biozone). At least, Tera, Magaña and Sierra Matute Fms represent this sequence in the Eastern Cameros Basin. In Western Cameros there is no record of units belonging to the Nurrensis biozone (Upper Berriasian to Lower Valanginian) whereas in Eastern Cameros at least the "Lower Heterolithic" Fm belongs to this age. The Depositional Sequence II (Valanginian-Hauterivian) was recorded in the South of the Western Cameros Basin (Peñacoba Fm). This Depositional Sequence is also poorly represented in Eastern Cameros, where it has been recorded in the Cabretón Limestones. The Depositional Sequence III is largely represented in Western Cameros by Pinilla de los Moros and Hortigüela Fms, which yield charophytes from the Neimongolensis charophyte biozone (Early Barremian). However in the area of Soria this depositional sequence includes the Golmayo Fm, which is relatively older (Late Hauterivian - Earlymost Barremian). The biostratigraphy of the Western Cameros Basin during the same time interval is poorly known. Depositional Sequences IV and V have not yet yielded charophyte biostratigraphic data.