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1 2006

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Depositional sequences and ammonite fossilization on deep carbonate platform environments (Upper Oxfordian, Iberian Range, Spain)

Fernandez-Lopez S., Melendez G.

Volumina Jurassica

2006

Vol. 4, no. 1

87-88

The uppermost deposits of the Yatova Formation in the Ricla area represent a condensed section, 1.5 m thick, developed during the Semimammatum and Berrense subchronozones. This interval is composed of grey-reddish wackestone to packstone and boundstone beds alternating with marly intervals, bearing common sponges, ammonites and bioturbation textures. Terebratulid and rhynchonellid brachiopods, belemnites, bivalves, gastropods, serpulids, bryozoans, crinoids and echinoids are very scarce. Small sponge mud mounds, some few metres wide and less than 50 cm high, are locally developed. Limestone beds, 10 to 40 cm thick, show sharp base, but gradual size-increase or inverse grading of fossils and gradational upper boundary. Marly intervals, under 40 cm thick, display planar fabric, being normal grading of fossils more common than inverse grading. Hardground surfaces, ferruginous crusts and glauconite grains are common on the limestone beds. In contrast, hardground surfaces are not developed within marly intervals, although reworked concretions and remobilization surfaces are common, often capping the underlying argillaceous sediments. Parasequences show a less distinct development than in underlying intervals. Thickening and coarsening upwards sequences of metric thickness are common. Thinning and fining upwards sequences are scarce, generally developed between the last sponge mounds and associated with the thickest intervals of condensed deposits. This condensed interval is interpreted as formed in an open marine, moderately deep carbonate platform, below effective wave base, showing generally low-energy conditions with extremely low rates of carbonate and terrigenous sedimentation. Marly deposits represent background sedimentation, with very low rates of sediment accumulation, which may be due to sedimentary starving as well as to winnowing action on the seafloor. In contrast, limestone beds correspond to event sedimentation, with relatively high rates of sediment accumulation, probably distal tempestites. Lasting episodes of background sedimentation give rise to clay deposits showing no evidence of basal discontinuity, whereas brief events of turbulence lead to carbonate deposits with sharp base. The low diversity of the benthic fauna, scarce development of sponge bioherms and microbial crusts, as well as ammonite populations inhabiting the platform are all palaeobiological criteria confirming these deep and distal palaeoenvironments. Taphonomic features of ammonite assemblages indicative of sedimentary starving are the occurrence of: 1 - high concentrations of reelaborated ammonites showing very low values of taphonomic condensation; 2 - taphonic populations of type-2; 3 - predominant internal moulds of phragmocones completely filled with homogeneous sediment up to the innermost whorls; and 4 - reworked fossils bearing no signs of abrasion, bioerosion or dense encrusting. These condensed deposits characterize the last phase of a deepening half-cycle, attaining the maximum deepening environments during Upper Jurassic, within a 3rd order deepening/shallowing cycle developed in the Iberian platform system.