Publication: Taphonomic analysis and sequence stratigraphy of the Albarracinites beds
(lower Bajocian, Iberian range, Spain).
An example of shallow condensed section
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2011
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Société géologique de France
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Abstract
Several ammonite fossil assemblages from Sierra de Albarracín (Teruel, Castilian Branch) contain abundant
specimens of the Ovale and Laeviuscula zones (lower Bajocian, Middle Jurassic), including microconchs and
macroconchs of the characteristic genus Albarracinites. Over 1500 ammonites from the type horizon of the species A.
albarraciniensis, in the outcrop of Masada Toyuela, have been studied.
Ammonites are commonly preserved as phosphatized, calcareous, concretionary internal moulds (mean size =
50.5 mm) of unflattened shells, partially or completely filled with relatively heterogeneous mudstone to wackestone
sediment. Incomplete, fragmented phragmocones with calcitic septa are the dominant remains, generally bearing traces
of rounding, incrustation and bioerosion. Taphonomic data, such as the predominant taphonomic populations of type 3,
composed of reelaborated, relatively heterogeneous concretionary internal moulds, isolated concretionary body chambers
and hollow ammonites, bearing traces of abrasion, bioerosion and encrusting organisms, are indicative of low rate
of sedimentation and occasional high rate of sediment accumulation, due to sedimentary winnowing and bypassing interrupted
by storm depositional events, in shallow-water marine environments. The exceptional occurrence of taphonic
populations of type 2 belonging to Albarracinites and Hebetoxyites, with dominant shells of pre-adults and absence of
juveniles, suggests autochthonous biogenic production of shells by indigenous populations of Sub-Mediterranean taxa
at the Iberian platform system.
In the outcrop of Masada Toyuela, the Albarracinites beds correspond to a condensed section, developed in shallow-
water, open-marine, carbonate environments of the External Castilian platform. The sharp, irregular base of beds
and the normal grading of reworked elements suggest these carbonate deposits were affected by tractive currents, scouring
and redeposition. Limestone beds represent storm-related sedimentary events, whereas the local marly intervals represent
background-sedimentation time-intervals of winnowing and bypassing on the seafloor. This condensed section
is composed of at least four, decimetric or centimetric, expanded-deposit intervals, stacked and showing an overall thinning
upward, which constitute a deepening sequence developed during two biochrons. Taphonomic results also corroborate
the development of an incipient-deepening phase, which represents the first episode of a deepening half-cycle of
third order, in the Albarracín area within the Castilian platform, during the Ovale and Laeviuscula biochrons (early
Bajocian).