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.