Análisis de facies de la Formación Guara en las Sierras Exteriores Occidentales (Cuenca Surpirenaica, Eoceno Medio)

Abstract

A detailed facies analysis of the Guara Formation, deposited during Lutetian times (Middle Eocene) in the Jaca-Pamplona basin (central South-Pyrenean Basin), is presented herein. Sedimentological and paleontological studies of three bed-by-bed logs located next to La Peña reservoir (External Sierras), provides a paleoenvironmental reconstruction of the carbonate ramp system, mainly the inner and middle domains. The inner ramp was covered by seagrass meadows, as evidenced by the abundance of epiphytic and encrusting organisms. Mid ramp enviroments were dominated by the presence of rhodophytes, and large Nummulites. External ramp deposits, poorly represented in the study area, are characterised by the occurrence of Assilina, Discocyclina and Nummulites. The Guara limestones were deposited under high-energy conditions, probably in a storm-dominated carbonate ramp system, as evidenced by the grain-supported textures and sedimentary structures. The sequential evolution shows a general transgressive trend, composed of successive deepening upward sequences, bounded by discontinuity surfaces linked to abrupt stages of accommodation gain, which resulted in rapid platform flooding.