Taphonomy of cephalopod assemblages from Middle Jurassic hardgrounds of pelagic swells (South-Iberian Palaeomargin, Western Tethys)

Abstract

Taphonomic analysis of cephalopod assemblages, together with analyses of ichnologic and sedimentologic features, are useful tools in the interpretation of the history of unconformities in Jurassic pelagic swells. The External Subbetic presents a complex evolution during the Middle Bathonian–Middle Oxfordian interval that includes hardgrounds and condensed levels rich in ammonoids. Bathonian and Callovian hardgrounds, separated by a condensed calcarenitic bed, can be differentiated. In the Bathonian hardground, ammonoids are preserved with unfilled phragmocone chambers, as a consequence of initial fast burial. Neomorphic calcite in the ammonoids is related to early diagenesis, indicating a stop in sedimentation. The ichnological features testify to an early diagenesis and progressive cementation of the sea-bottom, which is corroborated by the record of neptunian dykes. Subsequently, sediment by-passing and erosion produced reorientation of the ammonoids and belemnite remains and exhumation of moulds, during which faceting and reelaboration took place. Finally, the sedimentation began resulting in the deposition of a condensed calcarenite bed with Fe–Mn macro-oncoids. The Callovian hardground developed at the top of the calcarenite bed, where the cephalopods show signs of long exposure on the sea-bottom such as colonization by chemosynthetic microbial communities and sessile foraminifera. The study proves that the Middle Bathonian–Middle Oxfordian represents a time interval of strongly reduced and highly discontinuous sedimentation, interrupted by periods of non-deposition and erosion.