ENTOLIUM BEDS - HIATAL SHELL CONCENTRATIONS IN STARVED PELAGIC SETTINGS (MIDDLE LIASSIC, SE SPAIN)

Lower Domerian (upper Pliensbachian) Entolium shell concentrations cha racterise the transition between shallow-water carbonates and pelagic marls and marry limestones in the middle Liassic from the southern par t of the Median Subbetic, one of the subdomains of the southern passiv e margin of the Iberian Massif. Rudstones and floatstones of Entolium valves probably formed below storm-wave base in a sediment-starved pal aeoenvironment. Valve disarticulation and the scarce shell fragmentati on were caused by bioturbation. Low sedimentation rate resulted in lon g preburial residence times that probably favoured the differential di ssolution of aragonitic skeletons that are absent or scarce in the Ent olium floatstones and rudstones but occur in the overlying crinoidal p ackstones to grainstones with Entolium fragments. These latter are mad e up of highly fragmented and abraded bioclasts incorporated in trough cross-beds and probably formed in shallower settings affected by stor ms. Some Entolium packstone laminae intercalated in the Entolium conce ntrations may represent distal storm deposits. The Entolium beds were deposited on the southern parr of a bank separated from the Iberian Ma ssif mainland by a trough located in the External Subbetic subdomain. Pelitic sediments were trapped in that trough, while to the south only condensed limestones and hard-grounds formed. This Median Subbetic ba nk was already active in the middle Carixian and deepened during the m iddle Domerian, eventually disappearing. The absence or scarcity of re mains of other benthic organisms with calcitic skeletons and the small average size of E. lunare shells point to unfavourable sea-floor cond itions that inhibited the growth of a diverse benthic community. Entol ium is considered to be characteristic of dysaerobic environments, and the reported Entolium shell concentrations probably formed in a dysae robic environment below storm-wave base. Taphonomic processes favoured the differential preservation of the calcitic E. lunare valves, thus enhancing their concentration. The Entolium beds were deposited at a t ime of global rising or high sea level. They can be considered as hiat al shell concentrations associated with transgressive or highstand reg imes.