The ups and downs of “Tectonic Quiescence”—recognizing differential subsidence in the epicontinental sea of the Oxfordian in the Swiss Jura Mountains

Recent studies in northern Switzerland have shown that epicontinental areas thought to have been tectonically stable during the Mesozoic were not necessarily as rigid as presumed. By comparing Oxfordian facies boundaries and depocenters in their palinspastic position with known faults in the basement, a direct relationship between the two can be demonstrated. Previously, the lack of obvious synsedimentary tectonic features has lulled scientists into believing that the realm of the Swiss Jura was tectonically stable during the Mesozoic. However, it can be shown that facies and sedimentary structures are largely influenced by tectonics. Subsurface data provide evidence for the presence of Paleozoic troughs in the basement which, apparently, were prone to reactivation during the Pan-European stress-field reorganization taking place in the Late Jurassic. This led to differential subsidence along pre-existing lineaments within the study area, which can be recognized in the distribution of Oxfordian epicontinental basins and their coeval shallow-water counterparts. Eustatic sea-level fluctuations played an important role in the development of shallow-water facies patterns, but a subordinate role in the control of accommodation space in basins.

While tectonic activity is often recorded in the sedimentary record in the form of platform break-ups and associated sedimentary debris, more subtle indicators may be overlooked or even misinterpreted. Sedimentary structures and isopach maps, as well as subsurface data in the study area suggest that subtle synsedimentary tectonic movements led to the formation of two shallow, diachronous epicontinental basins during the Late Jurassic. It becomes possible to recognize and differentiate the combined effects of local and regional tectonism, eustasy and sedimentation.     

dC Variations in Late Jurassic Carbonates, Jaisalmer Formation, Western India

The carbon isotope measurements, carried out on subsurface carbonate samples from Oxfordian Jaisalmer Formation, western India, yield positive d¹³C values up to +3.17%. The most positive Oxfordian C-isotope value corresponds to the carbon isotope excursion measured in samples from from other late Jurassic basins of world. The latest Oxfordian C-isotope values of Jaisalmer Basin fluctuate around 2% while the C-isotope values of 1.50% mark the base of Kimmeridgian. The Oxfordian C-isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level.     

Astronomical time scale for the Middle Oxfordian to Late Kimmeridgian in the Swiss and French Jura Mountains

Detailed investigation of facies and sedimentary structures reveals that, during the Middle Oxfordian to Late Kimmeridgian, the shallow carbonate platform of the Swiss and French Jura Mountains recorded high-frequency sea-level fluctuations quite faithfully. The cyclostratigraphic analysis within the established biostratigraphic and sequence-chronostratigraphic framework implies that the resulting hierarchically stacked depositional sequences formed in tune with the orbital cycles of precession (20 kyr) and eccentricity (100 and 400 kyr). The astronomical time scale presented here is based on the correlation of 19 platform sections and 4 hemipelagic sections from south-eastern France where good biostratigraphic control is available. The cyclostratigraphic interpretation suggests that the interval between sequence boundaries Ox4 and Kim1 (early Middle Oxfordian to earliest Kimmeridgian) lasted 3.2 myr and that the Kimmeridgian sensu gallico has a duration of 3.2 to 3.3 myr. The astronomical time scale proposed here is compared to time scales established by other authors in other regions and the discrepancies are discussed. Despite these discrepancies, there is a potential to estimate the durations of ammonite zones and depositional sequences more precisely and to better evaluate the rates of sedimentary, ecological and diagenetic processes. Editorial handling: Hanspeter Funk, Helmut Weissert, Stefan Bucher     

Carbon cycle and sea-water palaeotemperature evolution at the Middle–Late Jurassic transition,eastern Paris Basin (France)

A very high-resolution carbon and oxygen stable isotope analysis (bulk-carbonate) of a biostratigraphically well-constrained Callovian–Oxfordian series is provided here for the first time. The homogeneity of the clayey series and the weak diagenetic alteration allow the isotopic signal variations to be considered as primary in origin. A prominent and brief negative excursion in the δ¹³C curve (−2‰), occurring at the start of the Middle Callovian (Jason Zone – Obductum Subzone) and correlated regionally, suggests a possible methane release. The increasing δ¹³C values thereafter up to the Early Oxfordian, concomitant with a warming episode, highlight the burial of carbon in organic-rich layers which, in return, may have triggered a decrease in atmospheric pCO₂. At higher frequencies, observed fluctuations of the δ¹³C and δ¹⁸O values are orbitally driven (405-kyr and 100-kyr eccentricity cycles) and may correspond to the salinity and temperature variability recorded in sea water. The δ¹⁸O isotopic measurements from well-preserved diagenetically screened belemnites and bivalves along the series, compared to available data from Tethyan domains, agree with the scenario of a global cooling at the Middle-Late Jurassic transition. The well-dated δ¹⁸O isotopic curve suggests that the onset of this cooling event occurred at the end of the Coronatum Zone (Middle Callovian).     

Taxonomic revision of Cylindroporella ? lusitanica Ramalho, 1970: In search for the origins of the Family Dasycladaceae

A study of new material of Cylindroporella ? lusitanica Ramalho, 1970, from the Upper Jurassic (Oxfordian) strata of Portugal identified features not observed previously. This includes the pattern of its laterals, including the gametophores. The algal body plan is unique and justifies the taxonomic reassessment of this species and the introduction of Barattoloporellopsis n. gen. Furthermore this species is most likely a primitive Dasycladaceae, a “missing link” between the modern ones (starting from the Cretaceous times) and their Jurassic ancestors.     

The upper Bathonian ammonite zonation of East Siberia

Modifications to the upper Bathonian zonal scale for northern East Siberia provided by the newly available paleontological record on Middle Jurassic reference sections in the Arctic regions of Yakutia and by the revised earlier collections, are justified. The oldest East Siberian members of Cadoceras are found to be characteristic not of the initial Callovian age as believed by Russian paleontologists, but of the terminal Bathonian age as was previously shown in the biostratigraphic scheme of East Greenland. The succession of zones and index species analogous to that of the latter is revealed in the studied region and the zonal boundaries in Siberia and East Greenland are inferred to be synchronous. Finds of Cadoceras calyx in the upper Bathonian scale permitted, for the first time, the recognition of a corresponding zone. The Bathonian-Callovian boundary is placed between the calyx and anabarense zones. The upper Bathonian zonal scale of northern East Siberia is now in total agreement with the East Greenland zonal scale.     

Sequence Stratigraphy of the Jurassic Coal Measures in Northwestern China

The sequence stratigraphy of the Jurassic coal measures of northwestern China has been studied based on data from outcrop and borehole sections. Because of the geological background and the diversity of basin types, such as isochronisms of coal seams and recognition of key sequence boundaries, it is rare to summarize and correlate sequence structure and framework. The types and recognition characteristics of the sequence boundaries of the Jurassic coal measures are revealed by comparing climatic characteristics, structural styles and the base level cycle. A total of five third-order sequences and 15 systems tracts have been subdivided and the sequence stratigraphic framework has been reconstructed confirming that the thick coals accumulated in the late stage of transgression system tracts of sequence I (SQ I) and sequence III (SQ III). This idea is of important academic significance for instructing coal resources exploration, and enhancing geological effects of prospecting engineering. It is also of practical significance for guaranteeing construction of the large-scale coal production base in northwestern China, accelerating the westward development of the coal industry stratagem.     

The ammonite faunas of the Osmington Oolite Formation (Jurassic, Middle Oxfordian) of the Dorset coast

A detailed study of the lithologies of each of the beds present in the Osmington Oolite Formation of south Dorset is used to allocate numerous loose-collected ammonites to their correct stratigraphic horizons. Much new material has been collected by the author in addition to the limited amount of material available in museum collections. The age of the faunas of the three constituent members of the Osmington Oolite Formation is each assessed and placed into the context of Middle Oxfordian ammonite sequences elsewhere in England and in Europe.     

GEOLOGICAL SIGNIFICANT OF SPOROPOLLEN ASSEMBLAGE IN THE XUESHAN FORMATION

GEOLOGICAL SIGNIFICANT OF SPOROPOLLEN ASSEMBLAGE IN THE XUESHAN FORMATION