Lowermost Kimmeridgian Research Articles

Calcareous nannofossils from the uppermost Oxfordian and lowermost Kimmeridgian of Staffin Bay, Isle of Skye, northern Scotland

Twenty six samples from the sections at Flodigarry and Digg at Staffin Bay, Isle of Skye, northern Scotland, spanning the stratigraphical interval from the uppermost Oxfordian to the lowermost Kimmeridgian were examined for their calcareous nannofossil content. The study revealed the presence of an assemblage typical of the nannofossil zone NJ15 of the northern European zonation.

Notes on the evolution of the ammonite families Aulacostephanidae and Cardioceratidae and the stratigraphy of the uppermost Oxfordian and lowermost Kimmeridgian in the Staffin Bay sections (Isle of Skye, Northern Scotland)

Detailed study of all the ammonite collections gathered by the authors in the Staffin Bay sections has resulted in minor changes in the distribution of ammonite taxa, and slight modification of the position of the Oxfordian/Kimmeridgian boundary. Most significant is the discovery of Pictonia (Triozites) cf. seminudata which results in the placing of the stage boundary 0.16 m below the level formerly proposed. This study discusses the evolution of the Subboreal family Aulacostephanidae, and the Boreal family Cardioceratidae, indicating changes in the patterns of individual development in the evolution of both families in terms of heterochrony. The Oxfordian/ Kimmeridgian boundary interval shows major morphological changes in both ammonite families which were released from phylogenetic constraints by heterochrony, closely related to changes in environmental conditions.

Carbon-isotope stratigraphy and pelagic biofacies of the Middle–Upper Jurassic transition in the Tethys–Central Atlantic connection

A bulk carbon-isotope stratigraphy, based on high-resolution sampling of five stratigraphic ammonite-dated sections from pelagic swells of the Subbetic basin (External Zones of the Betic Cordillera, southern Spain) is presented. The studied sections are characterized by Callovian–Oxfordian stratigraphic successions located in the South Iberian palaeomargin, a key area connecting the Central Atlantic to the Tethys oceans. The rocks are mainly nodular limestones with common extreme condensation (rosso ammonitico facies). Discontinuities with hiatuses of variable duration, submarine “hardgrounds”, Fe-Mn ooids, limonite crusts, and neptunian dykes are observed around the Callovian/Oxfordian boundary. The stratigraphic record, although very time-averaged, allows for a consistent and accurate ammonite chronostratigraphy. The carbon isotopes of marine carbonates show a marked trend towards high δ13C values from Lower Oxfordian to the Middle Oxfordian (~4.3‰, near the Plicatilis/Transversarium boundary). The isotopic values stand between 2.7 and 3.0‰ at the Upper Oxfordian (Bifurcatus and Hypselum Zones) and lowermost Kimmeridgian (Bimammatum Zone); afterward δ13C values decrease and reach a relative minimum (~2.3–2.5‰) in the Lower Kimmeridgian (Bimammatum/Planula boundary). Finally, isotopic values increase again (~2.6–2.7‰) in the lower Kimmeridgian (mid part of the Planula Zone). Comparison of carbon-isotope stratigraphy between Subbetic and other Tethyan areas shows similar trends. The replacement of thin-shelled bivalves by planktonic foraminifers and radiolarians that took place in the Callovian/Oxfordian boundary can be associated to the widening of the trans-Pangaean seaway. This, in turn, triggered the global warming which increased nutrients concentration in upper water column due to intensified nutrient input by river plumes. These phenomena are concomitant with the major Oxfordian δ13C positive excursion and with a transgressive cycle.

Approaching trophic structure in Late Jurassic neritic shelves: A western Tethys example from southern Iberia

The palaeoenvironmental conditions and trophic structure of a mid-outer neritic biota (microfossils, mainly forams, and macroinvertebrate assemblages) have been approached in middle Oxfordian–lowermost Kimmeridgian deposits from the Prebetic Zone (Betic Cordillera) in south-eastern Spain. According to relationships between fossil assemblages and lithofacies, a general seaward trend is identified which displays decreasing sedimentation rates and nutrient inputs, but increasing substrate consistency and presumably depth. Midshelf, terrigenous-rich deposits in the External Prebetic relate to the highest sedimentation rates and nutrient availability. These two parameters correlate with the highest content in vagile-benthic, calcareous perforate, epifaunal forams, as well as with potentially deep infaunal forams and infaunal macroinvertebrates. Outer-shelf lumpy deposits in the Internal Prebetic show the lowest sedimentation rates and nutrient availability and the highest records for macro–micro nektonics and planktics. In contrast, vagile-benthic, calcareous perforate epifaunal and potentially deep infaunal forams are scarcer in the midshelf environments. Colonial encrusting forams, benthic microbial communities and sessile benthic macro-invertebrates increase from the middle to outer shelf. Trophic-analysis structuring through the integration of benthic microbial communities, foraminiferal and macroinvertebrate fossil assemblages makes it possible to interpret: (a) a trophic-level frame composed of producers and primary and secondary consumers; (b) a main trophic-group differentiation in suspension-feeders, detritus-feeders, browsers, grazers, carnivores and scavengers; (c) a preliminary approach to food-chain structure supported by suspension-feeders, deposit-feeders and predators (active prey-selection carnivores); and (d) a food-pyramid model, which takes into account both recorded fossils and envisaged —i.e., ecologically inferred-organisms.

Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)

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 ( MB i) 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 ( MB i) and encrustation indexes ( E i) used. In contrast, a clear relationship between MB i and the fragmentation index ( F i) 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 ( MB i, F i, E i 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 MB i and E i, 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 MB i and E i, 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.

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.

Macroinvertebrate assemblages andecostratigraphic structuration within a Highstand System Tract. An example from the lower Kimmeridgian in Southern Iberia

Selected sections belonging to epicontinental areas from the southern paleomargin of the Iberian subplate havebeen studied in order to examine the possibilities for identification of ecostratigraphic evolution within System Tracts. The chosen example we present is that of the Highstand System Tract in which sedimentation was organized during the lowermost Kimmeridgian (Platynota Zone). To evaluate the significance of ecostratigraphic information obtained from benthic and vagile macrofaunas we compared sections in which depositional frames were sufficiently differentiated. The analysis of faunal spectra shows the existence of recognizable trends in the recorded composition of macroinvertebrate assemblages, which must ultimately be related to ecological fluctuations induced by changes in relative sea-level. Over-all fluctuations in the benthic-vagile rate correlate well with variations in the gradient of proximality-distality induced by eustasy and oscillations in relative sea-level. Concerning ammonites, a rather complex and still not well known ecological dynamics seems to have been the cause of the areally and stratigraphically recurrent substitutions in the recorded assemblages. “Deviations” in the general patterns recognized from the evolution in the composition of ammonite spectra are related with the record of the Phylloceratina-Lytoceratina group. These “deviations” are interpreted to be induced by ecological differentiations and sometimes also by biostratinomic processes.

The Oxfordian-Kimmeridgian stage boundary (Upper Jurassic): Dinoflagellate cyst assemblages from the Harome Borehole, North Yorkshire, England

The Harome Borehole, North Yorkshire is an important reference section for the Boreal Upper Oxfordian and overlying Lower Kimmeridgian because a fully substantiated sequence of standard ammonite zones was proved. Forty-three palynological samples examined from the Upper Oxfordian Serratum Zone to the Lower Kimmeridgian Cymodoce Zone all yielded rich and diverse dinoflagellate cyst floras; sixty-nine taxa are listed. Atopodinium haromense sp.nov. is described and figured, and Tehamadinium aculeatum (Klement, 1960) comb.nov. is discussed. Comparison of the ranges of selected taxa having range tops or bases near the Oxfordian/Kimmeridgian stage boundary at Harome with those proven in other sections in eastern England shows broad agreement but with some significant variations. The ranges of Occisucysta balia and Dingodinium tuberosum, taxa previously believed to have range bases in the lowermost Kimmeridgian, are shown to extend into the Upper Oxfordian.