Opaline chert nodules in maar lake sediments from Camp dels Ninots (La Selva Basin, NE Spain)

espanolChert nodule samples from three different well cores (CC, CP1 & CA) from the lacustrine infill of the Camp dels Ninot maar-diatreme (La Selva Basin) have been studied by means of X-ray diffraction, and optical and electron microscope technique. The chert nodules replace diatomites and carbonates layers, and varies in mineralogy between opal-A to opal-A/CT. The microtexture of the opal-A rich nodules is characterized by smooth microspheres of ~2µm in diameter that forms aggregates of amalgamated microspheres. Commonly, the nodules contain diatoms and their moulds when replacing diatomites, and dolomite or ankerite crystals and their moulds when replacing carbonates. The opal-A/CT rich nodules exhibit a microtexture consisting of microspheres of ~8µm in diameter that form aggregates with botryoidal and finger-like morphologies. Results indicate that the early diagenetic transformation of opal-A to opal-CT is not complete in the studied sediments. EnglishEn el presente trabajo se estudian muestras de 3 testigos de sondeo (CC, CP1 y CA) mediante DRX, microscopio optico y electronico a fin de caracterizar los nodulos de chert en el relleno sedimentario lacustre del maar de Camp dels Ninots (La Selva Basin,). Los nodulos de chert reemplazan capas de diatomitas y carbonato, y varian mineralogicamente entre opalo-A y opalo-A/CT. La microtextura de los nodulos ricos en opalo-A se caracteriza por microesferas lisas de ~2µm de diametro que forman agregados de microesferas amalgamadas. Frecuentemente, los nodulos contienen diatomeas y sus moldes cuando reemplazan diatomitas, y cristales de dolomita y ankerita y sus moldes cuando reemplazan carbonatos. Los nodulos rico en opalo-A/CT presentan una microtextura con microesferas de ~8µm de diametro que forman agregados con morfologia botroidal. Los resultados indican que la transformacion diagenetica de opalo-A a opalo-CT no es completa en los sedimentos estudiados.