ALBERT

Description: Relative sea level curves contain coupled information about absolute sea level change and vertical lithospheric movement. Such curves may be constructed based on, for example tide gauge data for the most recent times and different types of geological data for ancient times. Correct account for vertical lithospheric movement is essential for estimation of reliable values of absolute sea level change from relative sea level data and vise versa. For modern times, estimates of vertical lithospheric movement may be constrained by data (e.g. GPS-based measurements), which are independent from the relative sea level data. Similar independent data do not exist for ancient times. The purpose of this study is to test two simple inversion approaches for simultaneous estimation of lithospheric uplift rates and absolute sea level change rates for ancient times in areas where a dense coverage of relative sea level data exists and well-constrained average lithospheric movement values are known from, for example glacial isostatic adjustment (GIA) models. The inversion approaches are tested and used for simultaneous estimation of lithospheric uplift rates and absolute sea level change rates in southwest Scandinavia from modern relative sea level data series that cover the period from 1900 to 2000. In both approaches, a priori information is required to solve the inverse problem. A priori information about the average vertical lithospheric movement in the area of interest is critical for the quality of the obtained results. The two tested inversion schemes result in estimated absolute sea level rise of ~1.2/1.3 mm yr –1 and vertical uplift rates ranging from approximately –1.4/–1.2 mm yr –1 (subsidence) to about 5.0/5.2 mm yr –1 if an a priori value of 1 mm yr –1 is used for the vertical lithospheric movement throughout the study area. In case the studied time interval is broken into two time intervals (before and after 1970), absolute sea level rise values of ~0.8/1.2 mm yr –1 (before 1970) and ~2.0 mm yr –1 (after 1970) are found. The uplift patterns resulting from the different inversions suggest that the lithospheric post-GIA response changes near the border between the Danish Basin and the Fennoscandian Shield. The obtained patterns of vertical lithospheric movement rates are comparable to results from other studies based on different and similar data types. Main differences between the inversion results and the results from other studies are caused by factors such as the simplifications included in the inversion approach, such as neglecting local sea level variation caused by the dominant wind patterns, and the a priori values chosen for the vertical uplift rates. The tests of the inversion schemes reveal that realistic values of absolute sea level rise and lithospheric uplift may be simultaneously estimated provided that reliable prior knowledge regarding the overall lithospheric uplift in the study area is available beforehand. In the presented parametrizations, only one absolute sea level change rate value is estimated for each studied time interval while several vertical movement rates are found, and the inverse estimate of absolute sea level change rate is practically insensitive with respect to the choice of a priori value of absolute sea level change, as long as the uncertainty assigned to this a priori value is kept sufficiently high.

Description: The large Storegga slide, which occurred on the Norwegian Atlantic shelf ~8150 yr ago, triggered a tsunami that has been identified in sediment deposits along the coasts of Greenland, Norway, the Faroe Islands, the Shetland Islands, Scotland, and the northernmost coasts of England, but hitherto not along the southeastern shores of the North Sea. It has generally been assumed that the shallow continental shelf of the North Sea attenuated and dissipated the energy of the tsunami before it reached those coastlines. We used radiocarbon and optically stimulated luminescence dating as well as stratigraphic, lithologic, chemical, and palynological analyses of sediment cores to identify tsunami deposits on the barrier island of Rømø located on the southwestern North Sea coast of Denmark. We show that tsunami sediments were deposited in a freshwater paleolake that is located ~16 m below present-day mean sea level. The tsunami sediment run-up was between 1.5 m and 5.5 m above the contemporaneous sea level. Our results demonstrate that the Storegga slide tsunami propagated across the wide (〉500 km) and relatively shallow (depth 〈95 m) continental shelf of the North Sea and resulted in run-up along adjacent coastlines. In contradiction to earlier theoretical studies, the coastline of the southeastern North Sea cannot be regarded as being sheltered from impacts of North Atlantic tsunami incidents.

Description: The Mesozoic around the northern North Atlantic was characterized by a succession of extensional tectonic events separated by periods of regional subsidence governed by thermal contraction. Sedimentation was controlled by tectonic subsidence, sediment and water loading, isostatic compensation, sediment influx, climate, and eustatic and relative sea-level variations at several scales. A series of examples from the Upper Triassic–Lower Cretaceous from East Greenland and the Northern North Sea have been selected to illustrate the relative importance of the controlling factors on depositional systems of different temporal and spatial scale. A long-term eustatic rise, superimposed by short-term cycles thus started in the Hettangian and culminated in the Kimmeridgian, whereas the main tectonic events include uplift of a major dome in the Early Jurassic, subsidence of the dome in the Middle Jurassic, and domal collapse and rifting in the Late Jurassic. The syn- to early post-rift Wollaston Forland Group (Volgian–Valanginian) is an example of a strongly tectonically controlled coarse-grained deep-water system deposited on tilted fault blocks. Deposition of the partly contemporaneous Fynselv Member (Raukelv. Formation) took place during uniform regional subsidence mainly governed by thermal contraction; relative sea-level variations appear to be the dominant factor controlling the sedimentary architecture. The two successions differ dramatically in geometry, facies and sedimentary processes. The tilted fault block example seems, however, to include the same number of roughly contemporaneous large-scale cycles as the unit deposited in the uniformly subsiding basin. This suggests that a relative sea-level signal can be recognized also in the tectonically controlled deep-water succession. The effects of sea-level control on contemporaneous shallow marine successions are illustrated by the Middle Jurassic Vardekløft Formation and the Brent Group of the Northern North Sea. The two units show great similarity and contemporaneity in terms of facies, parasequence stacking patterns, and systems tract successions. This allows formulation of a combined age and lithological model, which can be used in prediction of spatially separated systems tracts, some of which may contain unrecognized reservoir bodies. The similar development is highly suggestive of eustatic control. However, both regions were affected by large-scale doming, and the uniform development of the two systems may thus be controlled by relative sea-level changes reflecting large-scale tectonic processes. Finally, a high resolution sequence stratigraphic analysis of the lacustrine Rhaetian–Sinemurian Kap Stewart Formation is presented to demonstrate a hitherto unrecognized play type. Correct identification of sequence stratigraphic elements in this formation allows prediction of significant deltaic reservoir sandstones interbedded with black organic-rich lacustrine mudstones with excellent source rock potential. Combined low and high resolution sequence stratigraphy can be used in large-scale lithological prediction and comparison between systems situated far apart, and in detailed within-system predictions on a reservoir scale.