Los arrecifes coralinos del Malm en la Sierra de los Cameros (La Rioja, España)

Abstract

El ultimo episodio marino del Jurásico de la Sierra de los Cameros (La Rioja sur) se caracteriza por el desarrollo local de complejos arrecifales coralinos. El estudiado aquí está constituido por una progradación de arrecifes franjeantes que presenta acreción frontal casi exclusiva, aunque con cierta acreción vertical durante la maduración de cada edificio. Su funcionamiento estaría controlado por pulsos de levantamiento tectónico con emersión (mecanismo dominante) y eustatismo positivo (mecanismo subordinado).An important development of coral reefs caracterizes the last marine jurassic sediments in the Sierra de los Cameros. The bests exposures are situated along the Rio Iregua valley, where it is possible to recognize a reefal complex prograding towards the northeast, and which is constituted by a serie of laterally attached reef buildups (fringing reefs). The complex overlies a hard ground developped on top of open marine sediments of age Middle - Upper Oxfordian (downlap), whilst overlying sediments are continental and mostly clastic deposits (tot-lap relation). An important discontinuity separates marine and continental deposits, represented by Fe - crusts and palaeokarstifications. They are clearly distinguished the core reef and fore reef facies in every genetic increment of deposit (individual buildup) and litoral calcarenitic longshore and sandy bars associated to the reef front. Both core and fore reef facies present evidences of submarine cements, infilling borings and cavities. Outbuilding is clearly predominant in the development of this reefal complex, although a small amount of upbuilding can be detected. Controlling mechanism is interpreted as a combination of successives pulses of tectonic uplifting with exposure, and continuous sea level rise.