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Abstract: Cyclicity in a Lower Cretaceous Carbonate Ramp (Gorbea Massif, Basque-Cantabrian Basin, N Iberia)

Irene Gomez-Perez, Pedro A. Fernandez-Mendiola, Joaquin Garcia-Mondejar

A proximal to distal transect of a Lower Albian carbonate ramp occurs along a distance of 15 km as part of the Aptian-Albian Gorbea carbonate platform. Its age was established by dating with ammonites and orbitolinids as lower Albian tardefurcata zone, scharameni and acuticostata subzones. This carbonate ramp system averages 100 m thick and lies on an older mixed ramp system. The top of the carbonate ramp is characterized by subaerial dissolution on proximal settings, where it is overlain by inner platform sediments of a younger rimmed carbonate system. Middle ramp sediments are capped by resedimentation of carbonate breccias and by hardground development. A distal siliciclastic shelf system overlies the ramp in middle and distal settings and pi ches out towards the proximal ramp.

Each of the ramp subenvironments is characterized by specific facies associations: limestones dominate in the proximal ramp, the middle ramp is made of marls and allochtonous limestones, and the distal ramp consist of marly sediments. Four major inner cycles with metric to decametric scale have been recognized in proximal and middle ramp settings. Shallowing upwards cycles in the proximal ramp consist of a lower marly orbitolinid interval grading upward to orbitolinid packstones and to rudist and Chondrodonta wackestones. Proximal to medium ramp transition is characterized by marly limestones grading upward to coral mounds and skeletal packstones. Middle ramp cycles deepen upwards, and consist of locally brecciated skeletal packstones, which infill gullies and grade upward to localize rudist-coral mounds and/or to marls and marly limestones. No cyclicity was recognized in distal ramp settings.

Relative sea level changes, very likely caused by synsedimentary tectonic activity, are interpreted to be responsible of reported cyclicity.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France