Evidence for synsedimentary differential tectonic movements in a low-subsidence setting: Early Jurassic in northwestern Switzerland

During the Early Jurassic (lasting c. 27 Myr) only thin deposits (mostly ca. 30-50 m) of the Staffelegg Formation accumulated in wide parts of NW Switzerland while sea-level rise was in the range of c. 60 m. Isopach and facies patterns provide clear evidence of differential subsidence while faults that formed in the basement during the late Palaeozoic became reactivated. Orientation of many relative thickness minima and maxima follows faults constituting either the Rhenish Lineament or the North Swiss Permo-Carboniferous Trough. Such pattern is seen on the isopach maps of the Schambelen, Beggingen, Weissenstein, Frick, Fasiswald, Mt. Terri, Breitenmatt, Rickenbach, Rietheim and Gross Wolf members of the Staffelegg Formation, independently upon if the individual lithostratigraphic units are condensed or display somewhat enhanced thickness. Onto a general trend of decreasing thickness to the S, often isopach anomalies of small areal extension are superimposed. They suggest that localized strike-slip movements affected a mosaic of basement blocks. Reactivation of faults in the basement during the Early Jurassic is also evidenced by temporally enhanced hydrothermal activity as documented by chronometric ages of veins and mineral alterations

Ichnofabrics elucidate the accumulation history of a condensed interval containing a vertically emplaced ichthyosaur skull

A three-dimensionally preserved skull and parts of the postcranial skeleton of an ichthyosaur (Leptonectes) was found vertically oriented within on-average slowly deposited (0.5 rn/My) Lower Jurassic shallow-water marls. The ichthyosaur sank headfirst into the seafloor because of its center of gravity, as anatomically similar comparably preserved specimens suggest. The skull penetrated into the soupy to soft substrate until the fins touched the seafloor. There is no evidence either for active penetration of the ichthyosaur during death agony or an acceleration by explosive release of sewer gas that would have pushed the skull into the substrate. Ichnofabrics and crosscutting relationships an-tong trace fossils preserved therein allow analysis of stratigraphic completeness. In spite of on-average slow accumulation, the ichthyosaur-hosting sediments formed rapidly during three distinct but similar deposition-bioturbation phases. First, 10 to 15 cm of mud accumulated rapidly. Biodeformational structures subsequently produced therein imply a soupy consistency. As sedimentation slowed down, muds slightly dewatered and consolidated, as reflected by trace fossils with distinct outlines (Palaeophycus and Planolites, thereafter Thalassinoides and Chondrites). The contact with the overlying depositional interval is obliterated by biodeformational structures. Hence, the previously rapidly deposited mud must still have been soft. A short time after the third deposition-bioturbation phase, the ichthyosaur parts penetrated into the still-soft mud and started to be degraded microbially. Below the bioturbated zone, but before compaction, a concretion started to form around the ichthyosaur parts and led to their excellent preservation. During further burial, the skull-hosting concretion experienced differential compaction and moved downward relative to the underlying beds. The skull-hosting concretion penetrated through condensed deposits representing three ammonite zones. Restoring differential compaction, the initial porosity of the sediment can be estimated to have been < 70 analogues, such muds are soft, as ichnofabrics imply

Palynology of Triassic–Jurassic boundary sections in northern Switzerland

A first palynostratigraphic scheme of Upper Triassic deposits in northern Switzerland was established based on spore-pollen associations and dinoflagellate cyst records from the upper part of the Upper Triassic Klettgau Formation and the lower part of the Lower Jurassic Staffelegg Formation. Drill cores from the Adlerberg region (Basel Tabular Jura) and from Weiach (northern part of Canton Zurich) as well as from an outcrop at the Chilchzimmersattel (Basel Folded Jura) were studied and five informal palynological associations are distinguished. These palynological associations correlate with palynological association of the Central European Epicontinental Basin and the Tethyan realm and provide a stratigraphic framework for the uppermost Triassic sediments in northern Switzerland. Throughout the uppermost Triassic to Jurassic palynological succession a remarkable prominence of Classopollis spp. is observed. Besides Classopollis spp. the three Rhaetian palynological associations A to C from the Upper Triassic Belchen Member include typical Rhaetian spore-pollen and dinoflagellate taxa (e.g., Rhaetipollis germanicus, Geopollis zwolinskae, Rhaetogonyaulax rhaetica, and Dapcodinium priscum). Association B differs from association A in a higher relative abundance of the sporomorph taxa Perinopollenites spp. and the consistent occurrence of Granuloperculatipollis rudis and Ricciisporites tuberculatus. Spore diversity is highest in the late Rhaetian palynological association C and includes Polypodiisporites polymicroforatus. A Rhaetian age for the Belchen Member is confirmed by palynological associations A–C, but there is no record of the latest Rhaetian and the earliest Jurassic. In contrast to the Rhaetian palynological associations the Early Jurassic associations W and D include Pinuspollenites spp., Trachysporites fuscus (in association W), and Ischyosporites variegatus. In the view of the end-Triassic mass extinction and contemporaneous environmental changes the described palynofloral succession represents the pre-extinction phase (associations A and B) including a distinct transgression, the extinction phase (association C) associated with a regression, and the post-extinction phase (association W)

Float, explode or sink: postmortem fate of lung-breathing marine vertebrates

What happens after the death of a marine tetrapod in seawater? Palaeontologists and neontologists have claimed that large lung-breathing marine tetrapods such as ichthyosaurs had a lower density than seawater, implying that their carcasses floated at the surface after death and sank subsequently after leakage of putrefaction gases (or ‘‘carcass explosions’’). Such explosions would thus account for the skeletal disarticulation observed frequently in the fossil record. We examined the taphonomy and sedimentary environment of numerous ichthyosaur skeletons and compared them to living marine tetrapods, principally cetaceans, and measured abdominal pressures in human carcasses. Our data and a review of the literature demonstrate that carcasses sink and do not explode (and spread skeletal elements). We argue that the normally slightly negatively buoyant carcasses of ichthyosaurs would have sunk to the sea floor and risen to the surface only when they remained in shallow water above a certain temperature and at a low scavenging rate. Once surfaced, prolonged floating may have occurred and a carcass have decomposed gradually. Our conclusions are of significance to the understanding of the inclusion of carcasses of lung-breathing vertebrates in marine nutrient recycling. The postmortem fate has essential implications for the interpretation of vertebrate fossil preservation (the existence of complete, disarticulated fossil skeletons is not explained by previous hypotheses), palaeobathymetry, the physiology of modern marine lung-breathing tetrapods and their conservation, and the recovery of human bodies from seawater