Fossilization of ammonites and sedimentary events in deep environments of carbonate platform (highest Middle to lowest Upper Oxfordian, Iberian Range, Spain).

摘要

In the Ricla area (Zaragoza, Aragonese Branch ofudthe Iberian Range), at the top of the Yátova Formation, grey-reddishudwackestone limestones grade into yellow-green siliciclastic limestonesudof the Aldealpozo Formation. These changes of facies between the twoudsuccessive formations are associated with syndepositional palaeoreliefsuddeveloped during the Late Oxfordian. The uppermost deposits of theudYátova Formation represent an Oxfordian condensed section, from theudupper Bifurcatus Zone (Middle Oxfordian) and Hypselum Zone (UpperudOxfordian). These deposits are interpreted as developed in an openudmarine, moderately deep carbonate platform, showing uniform lowenergyudconditions with extremely reduced carbonate and terrigenousudbackground sedimentation, and very low sedimentation rates. The lowuddiversity of the benthic fauna, scarce development of sponge biohermsudand ammonite populations inhabiting the platform are palaeobiologicaludcriteria which corroborate these palaeoenvironmental conditions.udAmmonite assemblages are composed of Sub-Mediterraneanudtaxa. Over 900 ammonite specimens have been collected from the upperudBifurcatus and Hypselum zones. Oppeliidae (45,2%) and Perisphinctidaeud(37,9 %) are dominant. Aspidoceratidae (14,3%) are common. Haploceratidaeud(2,2%) are scarce. Two phylloceratids and a lytoceratid haveudbeen found. Ammonoids are commonly preserved as concretionary calcareousudinternal moulds of reelaborated elements. Resedimented shellsudare scarce. The degree of packing of ammonite remains and the stratigraphicaludpersistence display high values. Taphonomic features indicativeudof sedimentary starving in deep carbonate platform environments are: 1)udhigh concentrations of reelaborated ammonites, 2) taphonic populationudof type two, 3) phragmocones completely filled with sediment, and 4)udhomogeneous concretionary internal moulds, bearing no signs of abrasion,udbioerosion or dense encrusting by organisms (such as serpulids,udbryozoans or oysters). In conclusion, the occurrence of these ammoniteudassociations confirms the development of an advanced deepeningudphase, within a 3rd order deepening/shallowing cycle, in the Aragoneseudplatform, during the late Bifurcatus to Hypselum zones.