No abstract

A model for the Late Triassic tectonic development in northwest Jutland is discussed in light of gravity, aeromagnetic and reflection seismic data. A wrench pull apart concept yields a simple explanation for both the present Zechstein basement configuration and the spatial distribution of salt structures in this part of the Danish basin.

The structural development and its influence on the present land forms is discussed on the basis of the reflection seismic and gravity data. The study area covers the western part of Jutland which was not glaciated during the Weichselian and is delimited by two Weichsel main stationary lines: the line extending from west to east and the line extending from north to south (these two lines join at Dollerup). The southern border of the study area corresponds approximately to the northern limit of the buried Ringkøbing-Fyn High. In this part of west Jutland the base Zechstein faults can be traced to the surface where they control the drainage system. The faults are mainly NW-SE trending strike slip faults which are active also at the present time. Zechstein evaporates are present in the entire area discussed here.

The N~vling salt structure is located on the southern margin of the Danish Zechstein Basin. The present study of this structure is based on a detailed gravity survey, reflection and refraction seismic data, and well information. Apart from the exploratory well NØvling-l, and a shallower well Herning-l, a number of wells, mainly 20-30 m deep, were drilled in this general area (by the Geological Survey of Denmark). Through this combined study using geological and geophysical methods it is possible to distinguish several stages in the development of the N~vling structure. It is shown that these stages are closely related to faulting at the base Zechstein level. The faulting, which apparently started as normal faulting in the Triassic, triggered the first salt movements from the deeper parts of the basin to this peripheral area. The faulting was reactivated during the late Jurassic, resulting in the formation of a salt pillow. The tectonic regime changed in the post-Paleocene tertiary when transpressional movements along a prominent basement wrench fault became dominant. This ultimately resulted in the formation of a positive flower structure and the uplifting of the Saale Quatemary surface. At shallow depths the salt became dissolved creating a topographic depression above the flower structure. The present depression was formed at the beginning of the last late glacial time. Quatemary landforms in the N~vling area are therefore greatly influenced by recent tectonic events.

On the basis of seismic and well data, supported by Bouguer gravity data, the sequence of events leading to the formation of a Zechstein salt pillow – called Voldum – in east central Jutland is discussed (the study area is delimited in Fig. 1). The initial salt movements, activated by faulting in the Triassic, resulted in the formation of a minor salt pillow on the edge of a graben within the Zechstein (Upper Permian) basin. During a renewed faulting (Voldum fault) of the base Zechstein in Late Jurassic and consequent deepening of the graben, a syncline developed above the salt where thick sediments of Late Jurassic age accumulated. In the process large quantities of salt, due to differential loading, withdrew from the graben and moved laterally up-dip across the older fault scarps. Thus a new and larger salt accumulation (Voldum pillow) formed above the southern flank of the graben. A relict Triassic thin, formed during the growth of the first pillow, remains, but no salt accumulation which could account for this thin is present. The Voldum pillow continued to grow during the Cretaceous and the Tertiary, but the speed of growth decreased considerably during post-Late Cretaceous times, although there are still large quantities of, virtually undisturbed, salt present south of the pillow in the part of the basin corresponding to the Silkeborg Gravity High. The graben area apparently underwent a mild inversion at the close of the Cretaceous. The reasons why the Voldum pillow did not develop into a diapir are considered to be a strong and thick overburden which existed at the beginning of the Voldum pillow formation, the deeply buried salt which probably acted as a deterrent to the rupture of the overburden, and the nature of the Voldum fault which, apparently, had an appreciable strike-slip component in dextral sense.