Existencia de estructuras nodulosas de celestina afectadas por silicificación en el Valle de Losa (N. de Burgos )

The present work describes sorne nodules of celestite, which appear in a marly clay level of the Upper Cretaceous (Santonian) in Losa Valley (North of Burgos). A great number of nodules have been in some extent, or totally pseudomorphized by quartzine and lutecite (length-slow chalcedony). These nodules have not been mentioned up to now in the research works carried out in the Basque Cantabric Basin; some nodules present elongated shapes, while some others are subspherical, In the inside, the radial arrangement shows pyramidal shapes with rhombic sections, emerging from a compact nucleus. These nodules are diagenetic structures growing expansively from the nucleus towards the periphery. In this action echinoid spines, pelecypods fragments, bryozoans, etc., are enclosed. Celestite here is highly pure; there is no barite at all. For the celestite ions the following origin in proposed. The sulphate was supplied by fluids which laterally migrate from the Triassic gypsum (Keuper). Sr seems to have been released along the transformation process of aragonite sediments to calcite. Although it can't be forgotten the amount of Sr furnished in the parcial dolomitization process suffered by the sedimentary sequence, In the same level, irregular cherts appear associated to celestite nodules. These cherts have a considerable amount of siliceous sponge spicules (litisteids?) replaced by sparry carbonate what doubtless shows the biogenic origin of the silica. The silicification process has allowed the preservation of the nodules, in spite of the strong celestite solution which afterwards originated a great loss celestite. En el presente trabajo se describen unos nódulos de celestina que aparecen concentrados en un nivel de margas arcillosas pertenecientes al Cretácico superior (Santoniense) del Valle de Losa (N. de Burgos). Gran parte de los nódulos se encuentran parcial o totalmente sustituidos por cuarzo en las formas fibrosas de cuarcina y lutecita (length-slow chalcedony). Estos nódulos, no mencionados hasta ahora en los trabajos realizados en la Cuenca Vasco-Cantábrica, se caracterizan por su aspecto alargado, a veces subesférico, Interiormente presentan una estructura radial constituida por pirámides apretadas entre sí, de sección rómbíca, que parten de un núcleo compacto. Su formación es claramente diagenética, con crecimiento expansivo desde el núcleo hacia el exterior. En este crecimiento son englobadas espinas de equínidos, restos de lamelibranquios, briozoos, etc., que existían en el sedimento y que conservan sus características morfológicas y mineralógicas.