Ammonites, taphonomic cycles and stratigraphic cycles in carbonate epicontinental platforms

Abstract

Some variations of the taphonomic features and the distribution of the successive recorded associations of ammonites in the carbonate epicontinental platforms enable to distinguish elementary taphonomic sequences, taphosequences, taphorecords, taphocycles, megataphosequences and supertaphocycles resulting from relative sea-level changes. The identification of such taphonomic cycles is of utmost importance in interpreting the stratigraphic cycles of Mesozoic epicontinental platforms, when no evidence of coastal onlap is preserved but fossiliferous sediments of outer platform are widely developed. Five environmental cycles of deepening/shallowing were developed in the Castilian, Aragonese and Tortosa platforms during the Middle Jurassic: four cycles of median deepening (early Aalenian, middle Aalenian early Bajocian, Bathonian and Callovian) and one cycle of advanced deepening (late Bajocian). Ammonite taphorecords of the Opalinum Biozone represent the last terms of a shallowing upwards megataphosequence which began in the late Toarcian. The maximum deepening and the maximum relative sea-level rise were reached during the Niortense Biochron (late Bajocian). Sediments of the Polygyralis Subchronozone belongs to a second-order peak transgression. Foremost shallowing episodes during the Middle Jurassic, representing peak regression of second order, correspond to middle Aalenian (Murchisonae Biochron) and late Callovian (Lamberti Biochron). Murchisonae and Lamberti biochrons conform two second-order peak regressions. Recorded associations of ammonites comprísed between these two biochrons represent a supertaphocycle of second order in the Iberian Basin. Sediments of the Middle Jurassic comprised between these two stratigraphic discontinuities, from the middle Aalenian until the upper Callovian, also represent a stratigraphic supercycle of second order in the Iberian Basin.