987 resultados para Geology, Stratigraphic|yQuaternary.


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Among the five sites drilled during Ocean Drilling Program Leg 130, two deep holes (8O3D and 807C) penetrated Cretaceous sediments overlying the basaltic pillows, flows, and possibly basement rocks. Abundant, poorly preserved radiolarians with limited diversity were recovered from a few horizons within the sediments proximal to the basalt. At Site 803, three thin layers of radiolarites interbedded with claystone and clayey siltstone yielded radiolarian assemblages of late Albian age. At Site 807, several layers of radiolarian siltstones were recovered proximal to the basalt. Among them the most significant radiolarian assemblage is an Aptian fauna, located approximately 7 m above the basaltic flows. The Aptian radiolarian age for Site 807 is at least in accord with those suggested by planktonic foraminifer and paleomagnetic evidence. These Cretaceous radiolarians are the oldest assemblages recorded from the Ontong Java Plateau region.

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The rich and diverse dinocyst assemblages in Cores 162-985A-32X through 62X confirm the importance of these microfossils in unraveling the evolution of the Norwegian Sea. Cosmopolitan taxa, with well-documented stratigraphic ranges in northwest Europe, indicate the following ages: Sections 162-985A-62X-1 through 51X-2, Rupelian (early Oligocene); 50X-5, Oligocene, possibly Chattian; 48X-6, Aquitanian? (early Miocene); 48X-4 through 37X-5, Aquitanian (early Miocene); and 36X-5 through 32X-1, Burdigalian (early Miocene). This stratigraphic interpretation suggests that a major hiatus, which can be correlated with an apparently coeval hiatus at Site 643, occurs within the Chattian at Site 985. Several endemic dinocyst taxa with unusual morphology and restricted stratigraphic occurrences are present in Hole 985A and other Norwegian Sea sites, especially Site 643. By using Hole 985A data for control, the Oligocene-Miocene sediments can be correlated with some degree of confidence in the Norwegian Basin.

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A set of 43 sediment cores from around the Canary Islands is used to characterise this region, which intersects meridional climatic regimes and zonal productivity gradients in a high spatial resolution. Using rapid and nondestructive core logging techniques we carried out Fe intensity and magnetic susceptibility (MS) measurements and created a stack on the basis of five stratigraphic reference cores, for which a stratigraphic age model was available from d18O and 14C analyses on planktonic foraminifera. By correlation of the stack with the Fe and MS records of the other cores, we were able to develop age depth models at all investigated sites of the region. We present the bulk sediment accumulation rates (AR) of the Canary Islands region as an indicator of shifts in the upwelling-influenced areas for the Holocene (0-12 ky), the deglaciation (12-18 ky) and the last glacial (18-40 ky). General observations are an enhanced productivity during glacial times with highest values during the deglaciation. The main differences between the analysed time intervals we interpret as result of the sea-level effects, changes in the extent of high productivity areas, and current intensity.

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The first and last appearances of Quaternary planktonic foraminifers in the Great Australian Bight were evaluated using datum levels from magnetostatigraphy, oxygen isotope stratigraphy, and calcareous nannofossil biostratigraphy to determine whether they were synchronous or diachronous with open-ocean biostratigraphic events. The first appearance of Globorotalia truncatulinoides is diachronous at 1.6-1.7 Ma at Site 1127 and 1.1-1.2 at Sites 1129 and 1132, similar to other local appearances in high latitudes. All other datum levels, however, are synchronous with open-ocean events, including the first appearance of Globorotalia hirsuta and the last appearances of Globorotalia tosaensis and pink Globigerinoides ruber in the Indo-Pacific region. A local reappearance of Gt. hirsuta at ~0.12 Ma and the disappearance of Globorotalia crassaformis at ~0.10 Ma were found to be useful for local biostratigraphy. Age control at the bottom of all of the sections is poor at this time, but results suggest that sedimentation recommenced starting at ~1.9 Ma above the regional unconformity that marks the base of seismostratigraphic Sequence 2. Sediment accumulation is distinctly reduced in the lower Pleistocene compared to the upper Pleistocene, perhaps in part because of processes associated with several omission surfaces.