| Popis: |
This composite standard establishes a scaled biostratigraphic sequence that incorporates all the fossil occurrences usually reported in paleontological studies of the North Sea Paleogene. All relevant taxonomic groups are represented, including benthic and planktic foraminifera, siliceous microfossils (diatoms and radiolarians), calcareous nannoplankton and palynomorphs (dinoflagellate cysts and sporomorphs). The North Sea Composite Standard was initiated in graphic correlation to a general Cenozoic composite standard with essentially complete representation of the geological record on a worldwide basis. Following this initial chronostratigraphic calibration, additional graphic correlations of area wells were done to develop a separate North Sea Composite Standard which precisely catalogues the relatively limited ranges and overall endemism of the intrabasinal biostratigraphy. The resulting database has extensive well control in the Central and South Viking grabens and provides definitive stratigraphic coverage, in that wells formally designated as lithostratigraphic types and references are included. In application, the composite standard provides a basis for consistent stratigraphic evaluations unencumbered by the incongruities of the many consultant-specific zonal schemes. The graphic correlation results clearly indicate that the Paleogene sediments accumulated in a punctuated series of regional pulses. Major time-stratigraphic breaks are defined by data terraces in relation to the composite standard scale. The length of each horizontal terrace spans an interval of composite standard units (time) that is not represented by a significant rock record in the comparison well. Geologically, horizontal terraces are related to missing section (i.e., faults and/or unconformities) or periods of sediment starvation. In the Paleogene of the North Sea, most of the terraces appear to represent periods of widespread sediment starvation because: they usually occur within a shale unit that is time-correlative throughout the North Sea; deep-marine (bathyal) conditions are indicated by associated benthic foraminifera assemblages in the sediments above and below; most are marked by a distinctive fossil assemblage (e.g., unusually high abundances of radiolarians, planktic foraminifera and calcareous nannofossils); they are confirmed by appropriate seismic stratal geometries. The terraces occur in a time-succession, each correlating in terms of composite standard units from well to well throughout the study area. Considered in the broader context of related geological and seismic data, the graphic correlation terraces appear to be marking major hiatuses resulting from widespread sediment starvation. Therefore, the stratigraphie subdivision afforded by the graphic correlation terraces may be used to define major depositional sequences within the Paleocene-Eocene. In general, the hiatuses are situated between rapidly deposited sections (i.e., steep lines of correlation) that contain major subsea fan sandstones (lowstand systems tract). The nomenclature introduced in this study for these depositional sequences emphasizes the relationship to established lithostratigraphic units based on analysis of the type and reference wells. However, the past regional application of lithostratigraphic terms does not always respect the boundaries between the biostratigraphically defined depositional sequences. Based on electric log criteria alone, genetic packages are difficult to resolve regionally due to the repetitive lithological character of the Paleogene deposits and the pronounced lenticularity of sandstone units. Consequently, the Paleogene Composite Standard has an essential role to play in guiding sequence stratigraphie interpretations. |
