Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)
Abstract Microboring was investigated through thin section analysis of Oxfordian rocks from the Prebetic Zone (Betic Cordillera, southern Spain). Microboring description and distribution were analysed, as was their correlation with traits such as the type and size of bioclasts and microtaphonomic features (encrustation and fragmentation). Single and branching microboring are the two major morphological groups differentiated. Microborers were not identified but by their patterns of distribution in the bioclasts and in the environmental setting are assumed to be phototrophic. Microboring only affected bioclasts, with higher values of microboring index (MBi) registered in thicker ammonoid shells and serpulids, and lower ones in ostracods, foraminifera and small bivalves, gastropods and echinoderms. A very close, direct relationship exists between bioclast size and microboring abundance, as well as between the microboring (MBi) and encrustation indexes (Ei) used. In contrast, a clear relationship between MBi and the fragmentation index (Fi) has not been proved. Microboring activity has been interpreted to be related to both bioclast-size and time exposure of bioclasts on the seafloor, the latter resulting directly proportional to the rate of sedimentation in low energy environments. The studied features (MBi, Fi, Ei and bioclast mean size) were analysed taking into account the stratigraphic intervals investigated (I=Transversarium Zone to lower Bifurcatus Zone, II=upper Bifurcatus Zone, III=Bimammatum Zone, and IV=Planula Zone p.p.), and the trends registered were interpreted as a final consequence of relative sea-level fluctuations. Stratigraphic intervals I and IV, characterised by higher values in MBi and Ei, were related to lower sedimentary rates during increasing distality in a context of relative sea-level rise, which is consistent with the development of Transgressive System Tract (TST) conditions. Stratigraphic intervals II and III, showing lower values in MBi and Ei, are related to higher sedimentation rates and decreasing distance from shore during relative sea-level falls, which are in accordance with developing Highstand System Tract (HST) and Shelf Margin Wedge (SMW) conditions. Data obtained and interpretations agree with the general ecostratigraphic context in the studied area previously proposed by the authors.