Kai-Uwe Gräfe

Initial India-Asia Continental Collision and Foreland Basin Evolution in the Tethyan Himalaya of Tibet: Evidence from Stratigraphy and Paleontology

The onset of the India-Asia continent-continent collision in Tibet was accompanied by the elimination of oceanic crust and the development of a peripheral foreland basin on the subducting continental crust. However, owing to the progressively southward migration of this foreland basin, the appearance of flexural uplift (forebulge) predated flexural subsidence (foredeep) at the distal location, such as at Gamba and Tingri in the southern Tethyan Himalaya. Consequently, to trace and date this forebulge may provide a better constraint on the India-Asia collision. At Gamba and Tingri, our studies on the stratigraphy, paleontology, and paleoenvironment show that depositional cessation of the limestones occurred at Shallow Benthic Zone 7 (SBZ 7, ∼54–55 Ma) in the Gamba area and at the base of SBZ 10 (∼52.8 Ma) in the Tingri area. At Gamba, a conglomerate layer within the upper Zongpu Formation is suggested to represent the formation of the forebulge at the onset of the foreland basin, and the coincidence of the conglomerate layer with the Carbon Isotope Excursion (CIE) provides a precise age of ∼56 Ma (the Paleocene/Eocene boundary) for the possible initial India-Asia continental collision. Our results not only provide a reliable and precise age for the India-Asia collision but, for the first time, report time equivalence between the India-Asia collision and the CIE in Tibet.

Reconstruction of mid-Cenomanian orbitally forced palaeoenvironmental changes based on calcareous dinoflagellate cysts

Abstract Mid-Cenomanian, precession-controlled (21 ka) chalk–marl couplets of the Cap Blanc Nez section (Anglo–Paris Basin) have been studied with focus on the effects which Milankovitch cycles have had on the palaeoenvironment. In this paper, we present micropalaeontological and lithological proxies that enable the reconstruction of both the cycle architecture and the transformation of the orbitally forced signal into the sediment. A palaeoecological reconstruction based on changes in calcareous dinoflagellate cysts (c-dinocysts) assemblages was carried out, in which two characteristic ecological assemblages of c-dinocysts were identified. Gradual changes in absolute and relative abundance of the cyst species in these assemblages over several couplets depict a bundling pattern which is interpreted to reflect the modulation of the intensity of the precession cycle by the eccentricity cycle (100 ka). The stacking pattern in the natural gamma ray signal and the carbonate and TOC content has the same period and provides lithological support of the bundling. A shelf basin circulation model is proposed to explain the relation between orbitally forced climate change, its palaeoenvironmental consequences and the resulting sedimentary cyclicity. Variations in surface water circulation are reflected in the sediment by the chalk–marl couplets, the most distinctive couplets ocurring at the base and top of the bundles. While the chalks reflect well-mixed surface water conditions, the marls, particularly those at the bundle boundaries, can be interpreted as the sedimentary expression of stratified water masses. During deposition of these marls, reduced oceanic mixing due to low seasonality during strong precession maxima at the eccentricity maxima caused periods of water column stratification that in turn led to nutrient depletion and decreased productivity in the surface water masses.

The upper cretaceousLacazina limestone in the Basco-Cantabrian and Iberian basins of northern Spain: Cold-water grain associations in warm-water environments

SummaryThe Upper Santonian to Lower CampanianLacazina Limestone consists of massive, often amalgamated beds of packstones and grainstones which were deposited in a shallow marine environment. The most abundant skeletal components are miliolid foraminifera, echinoderm, bivalve and bryozoan fragments, peloids and sparse red algae. Small, solitary corals only occur sporadically. Hermatypic corals, sponges and green algae are missing. The series which reaches thicknesses between 60 m and 160 m, was sampled at intervals of 0.5 m at five localities. The petrographic features throughout the series are mainly a product of changing depositional energy. The limestones are well cemented. Diagenesis is characterized by a transition from marine phreatic to burial cementation. Syntaxial and blocky calcite cements predominate over early acicular to bladed and microgranular cements.The faunal association within theLacazina Limestone exhibits features typical for temperate water i.e.foramol carbonates. On the other hand, oxygen (δ18O =-1.7 to −6.3 ‰ PDB) and carbon (δ13C to 3.0‰ PDB) isotope values of diagentically unaltered bivalve shells indicate warm surface waters corresponding better to the palaeogeographical situation of theLacazina Limestone. Nutrient-surplus is proposed as a limiting factor preventing the development of reefs and finally ofchlorozoan sediments.In the sense of sequence stratigraphy, theLacazina Limestone is interpreted to contain transgressive and highstand systems tracts. This interpretation fits well into theHaq-Vail-curve. The series shows no visible high-frequency cyclicity in the field. Several cycles were found by means of principle component analysis and spectral analysis. Their relationships to the Milankovitch spectrum are discussed.The ammonite fauna of the unit and of preceding sediments (late Coniacian to early Campanian) is described and some inoceramids are figured. They permit—for the first time—the exact dating of theLacazina Limestone in the Basco-Cantabrian Basin (BCB) and throw light on a prominent faunal change at the Coniacian/Santonian boundary. The Cenomanian to Coniacian ammonite faunas which were dominated by endemic Tethyan pseudoceratitic faunas are replaced by cosmopolitan species dominated by Madagascan elements. This drastic change permits speculations about the installation of a new oceanic current system in the Santonian.

Upper Cretaceous inter-hemispheric correlation between the Southern Tethys and the Boreal: chemo- and biostratigraphy and paleoclimatic reconstructions from a new section in the Tethys Himalaya, S-Tibet

A new, 430 m long and mostly continuous Upper Cretaceous section from southern hemisphere low paleolatitudes in the Tethys Himalaya (Guru, Tibet) is presented. The lithology, microfacies and fossil contents of the Guru section indicate a continuous shallowing trend from open oceanic conditions at the slope during the Turonian to shallow marine inner shelf environments in the Maastrichtian, interrupted by short periods of stagnation or slight deepening. Strong variations in sedimentation rates and contents of carbonate, quartz, organic carbon and sulfur appear to reflect a combination of regional and global processes. On regional scale, the patterns of varying elastic sediment supply seem to be related to the fast northward drift of the Indian plate from the temperate climate belt in the Turonian and Coniacian, crossing the arid zone during the Santonian and early Campanian, and passing into the tropical humid belt in the late Campanian. On global scale, similar sedimentary changes as in Guru with a transition from clay-rich Turonian sediments to Coniacian carbonates are found in other regions of the world, suggesting an additional influence of global oceanic and climatic factors. Intervals of omission, erosion and reworking in upper Coniacian and lower Campanian sediments of the Guru section appear to coincide with global sea-level lowstands. Well preserved planktic foraminifera occur over most of the Guru section and provide good biostratigraphic control for correlation of stable carbon and oxygen isotopes with data from the reference section for the boreal and Maastrichtian carbon isotope fluctuations can be related to cyclic variations in carbonate showing detailed correlation of bulk sediment carbon isotopes and comparable trends in oxygen isotopes. The real white chalk in northern Germany at Lagerdorf-Kronsmoor and with the English Chalk reference section, content at Lagerdorf-Kronsmoor which are thought to be driven by long eccentricity. This indicates orbital forcing of both carbonate accumulation and carbon isotope signature, most likely mediated by global sea-level changes. The possibility of detailed delta C-13 correlation from boreal sections of the northern hemisphere to a low-latitude section of the southern hemisphere allows for global correlation with an accuracy not achieved by biostratigraphic methods so far. It further enables linking of microfossil and macrofossil biozones, facilitating precise comparison of shallow and deep water sections, which is essential for the improvement of our understanding of timing, causes and effects of climatic and oceanographic processes.

Foraminiferal evidence for Cenomanian sequence stratigraphy and palaeoceanography of the Boulonnais (Paris Basin, northern France)

Abstract A middle to upper Cenomanian chalk succession in the northwestern European Paris Basin (Escalles section) was investigated with respect to its sediment composition, biofacies, and foraminiferal content. Most planktic foraminifers exhibit a high-frequency cyclicity in their distribution pattern and show little relationship to the sequence stratigraphic subdivision. A few characteristic species of benthic foraminifers ( Tritaxia pyramidata , Ataxophragmium compactum , Dorothia levis , and Gavelinella cenomana ) are related in their frequency distribution to certain systems tracts. Other benthic foraminiferal species are more evenly distributed through the systems tracts. Cluster analysis and factor analysis allow the separation of three benthic foraminiferal biofacies domains, which exhibit a characteristic frequency distribution pattern with respect to sequence stratigraphy. Biofacies 1 is dominant in the highstand systems tract (HST), and biofacies 3 is characteristic for the transgressive systems tract (TST). Biofacies 2 is most abundant around the maximum flooding–downlap surface (mfs) and less abundant around the sequence boundary. Factor analysis shows that most of the variance in the data set is explained by variables related to plankton productivity. Variables related to changes in water depth and changes in sea-level have minor influence on the variance of the whole data set, but have some influence on the variance of the benthic foraminiferal data set. The different reaction of benthic and planktic foraminifers to relative sea-level changes is also recorded in the total number of foraminifers per gram sediment (foraminiferal number) and in the ratio between planktic and benthic foraminifers (p/b-ratio). The foraminiferal number is highest at the mfs and lowest at the sequence boundaries of the investigated depositional sequences. The p/b-ratio has no correlation with the sequence stratigraphic subdivision, but shows a high-frequency cyclicity. The abundance pattern of planktic foraminifers is controlled by changes in the productivity of calcareous plankton. The regular variation in calcareous plankton productivity leads to marl (low productivity)–chalk (high productivity) cycles that form conspicuous couplets in the section. The productivity cycles are also recorded in gamma-ray logs in the environmental setting of the Escalles section. In contrast, the abundance patterns of benthic foraminifers are more influenced by relative sea-level changes, changes in water depth, and variations in substrate composition. The stacking of probably precession-controlled development of couplets lead to a distinct 100,000-year cyclicity that does not match very well with published sequence stratigraphic subdivisions.