Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

Fernández López, Sixto Rafael

Meléndez Hevia, Guillermo

Docta Complutense

113Abstracts Book23rd IAS Meeting of SedimentologyA fossiliferous condensed interval at the top of the YátovaFormation, from upper Bifurcatus Zone (Middle Oxfordian)and Hypselum Zone (Upper Oxfordian), composed ofwackestone to packstone and boundstone beds alternating withmarly intervals, has been identified in the Ricla area (Zara-goza, Aragonese Branch of the Iberian Range). This marlycondensed interval, generally 1 to 1,5 m thick, bears commonsponges, ammonites and bioturbation textures. Othermacrofossils, including terebratulid and rhynchonellidbrachiopods, bivalves, gastropods, serpulids, bryozoans,crinoids and echinoids are very scarce. In comparison withMiddle Oxfordian intervals, these deposits show lowerdiversity of the benthic fauna, and scarcer development ofsponge bioherms and microbial crusts. This condensed intervalis interpreted as developed in an open marine, moderately deepcarbonate platform, showing extremely reduced carbonate andterrigenous sedimentation. Marly deposits representbackground sedimentation time intervals, which may be dueto winnowing action on the sea bottom as well as tosedimentary starving. Marls and marly limestones, showinginverse grading, and limestone beds showing gradationalupper boundary and gradual-size increase representenvironments of lowest rates of accumulation of sediments.In comparison with Middle Oxfordian intervals, the lowerdiversity of the benthic fauna, the scarcer development ofsponge bioherms and microbial crusts and the ammonitepopulations inhabiting the platform are palaeobiologicalcriteria which suggest relatively deep environments. Moreo-ver, ammonoid taphonomic analysis supply independentcriteria to corroborate this condensed deposits. Ammonoidremains are dominated by shells. Under 1% are aptychi.Ammonoid shells are commonly recorded throughout thesections, but they rarely exceed 45 mm diameter (less than10%). Main taphonomic features indicative of sedimentarystarving in deep marine environments are the occurrence of:1) high concentrations of reelaborated ammonites showingvery low values of taphonomic condensation, 2) taphonicpopulations of type-2, 3) phragmocones completely filled withhomogeneous sediment, and 4) fossils bearing no signs ofabrasion, bioerosion or dense encrusting.1) Ammonoids are generally preserved as concretionarycalcareous internal moulds of reworked elements (i.e.,reelaborated or resedimented elements). Reelaborated internalmoulds (i.e., exhumed and displaced before their final burial)are dominant (up to 98%). Resedimented shells (i.e., displacedon the sea-bottom before their burial) are locally present.Accumulated elements (i.e., showing no evidence of removalafter laying on the sea-bottom) are absent. The degree ofremoval (i.e., the ratio of reelaborated and resedimentedelements to the whole of recorded elements) and the degreeof taphonomic heritage (i.e., the ratio of reelaborated elementsto the whole of recorded elements) can reach 100%. However,the degree of taphonomic condensation (i.e., mixture of fossilsof different age or different chronostratigraphic units) reachesvery low to zero values in all cases. Ammonite mixedassemblages composed of specimens representing severalbiozones or biohorizons in a single bed or marly intercalationhave not been identified.2) Taphonic populations of type 2 are composed of mono-or polyspecific shells of the same genus showing unimodaland normal distribution of size-frequencies, with high degreeof kurtosis. Populations of this type contain a low proportionof microconchs and shells of juvenile individuals are scarce,whilst shells of adult individuals are common.3) Internal moulds of shells, completely filled with homo-geneous sediment up to the innermost whorls, are predomi-nant. Sedimentary infill and matrix are similar in petrologiccomposition and texture, but they are separated by a sharp anderosional discontinuity in the reelaborated internal moulds.4) Ammonite assemblages show high proportion ofincomplete phragmocones (up to 90%) and scarce completeshells. Fragmented specimens are abundant, but generallybearing no signs of rounding, bioerosion or encrustation.Reelaborated internal moulds commonly show angular edgesand disarticulation surfaces along septa with sharp margins.Pisolitic, oncolitic or half-lump ammonites have not beenobserved. However, some reworked concretions andconcretionary internal moulds (less than 25% of the whole)are partially encrusted by a few remains of epilithic organismsas well as local micritic crusts or microbial stromatoliticlaminae. Among encrusting organisms, serpulids are the mostcommon, followed by bryozoans, foraminifera, bivalves,brachiopods, sponges and crinoids.The occurrence of reelaborated ammonites implies thatsome form of current flow or winnowing affected theconcretionary internal moulds before final burial. Currentswere slight, but angular concretionary moulds of ammoniteswere disarticulated and azimuthally reoriented on softgroundsthrough reelaboration. In this hemipelagic environment, timeintervals of lower rates of sedimentation and accumulationfavoured a higher degree of sedimentary infilling of shells andreworking of internal moulds, but the degree of taphonomiccondensation and the diachronism between reworked fossilsand condensed deposits are negligible to the zonal scale.Taphonomic features of ammonite associations developed in deepcarbonate platform environments (Oxfordian, Iberian range, Spain)S. R. FERNÁNDEZ-LÓPEZ1 and G. MELÉNDEZ21 Dept. of Palaeontology, Complutense University of Madrid2 Dept. of Geology (Palaeontology), Zaragoza University

English

Taphonomic features of ammonite associations developed in deep carbonate platform environments (Oxfordian, Iberian Range, Spain).

http://eprints.ucm.es/23200/1/117A_04_IAS_Oxfordian_Ricla.pdf

Taphonomic features of ammonite associations developed in deep carbonate platform environments (Oxfordian, Iberian Range, Spain).

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