Reijmer, John J.G.1, P. Schäfer2, A. Schäfer3,
(1) GEOMAR Research Center for Marine Geosciences, D-24148 Kiel, Germany
(2) University of Kiel, Kiel, Germany
(3) University of Bonn, Bonn, Germany
(4) RCOM, Bremen, Germany
ABSTRACT: Mud Mounds vs. Deltas: Sea-level Controlled Sedimentation in a Orogenic Foreland Basin (Cantabrian Zone, Upper Carboniferous, NW Spain)
The Upper Carboniferous sediments of the San Emiliano Formation (Cantabrian Mountains, NW Spain) were deposited in a foreland basin bordered to the south by the approaching Variscan orogenic front and rimmed to the north by a retreating shallow-marine carbonate platform. The sequence is characterized by an alternation of fine-grained, deltaic siliciclastics and open-marine carbonates, mostly of mud mound origin. The fine-grained siliciclastics show minor cyclic variations, which are well documented in the gamma-ray logs. Bioturbation and terrigenous organic input are present. Often, more sandy layers, forming channel-fills with restricted lateral extention, are found at the top of the siliciclastic intervals. They also occur at the base or on top of the carbonate units. These carbonate deposits show a specific stacking pattern starting with oolithic wacke- to grainstones, reaching their full stage with algal bindstones, and usually are topped by oolithic limestones. Sedimentological and microfacies data indicate that the siliciclastic-carbonate transitions all show a similar sequence of events suggesting a regular mechanism steering the deposition of these sedimentation cycles. Differences in sediment patterns occur from West (San Emiliano) to East (Valverdín) and suggest shallowest-marine, near coastal to partly fluvial depositional conditions in the West and slightly deeper-marine shelf conditions in the East. However, the alternation of siliciclastics and carbonates shows only minor environmental changes, which therefore is believed to reflect relatively small sea-level changes on a shallow, slightly dipping shelf. Not only 2nd order tectonic events, but also 3rd, 4th and 5th order sea-level - and climate-induced sedimentation variations were found.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.