Sedimentological facies analysis of the mid-Jurassic Garn Formation in Halten Terrace area, Mid-Norway Continental Shelf
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The mid-Jurassic Garn Formation in the Halten Terrace area, Mid-Norway Continental Shelf, has been studied on the basis of ~500 m of well-core samples from 9 selected wells, geophysical wireline logs from nearly 30 wide-spread wells and two selected seismic cross-sections. The aim of this sedimentological study was to improve the existing general understanding of the Garn Fm. sedimentary environment and also to assess the role of contemporaneous fault tectonics in its deposition. The Garn Formation consists of sandstones, but varies laterally in thickness and only locally reaches >100 m. On the basis of detailed sedimentological analysis, the following facies associations are recognized as the main components of the Garn Fm.: deposits of tidal sand ridges (longitudinal tidal bars), deposits of sheltered (heavily bioturbated) and non-sheltered (little- or non-bioturbated) inter-ridge swales, deposits of shoal-water subtidal sandflats and subordinate deposits of wave-worked littoral shoals. Tidal sand-ridge deposits volumetrically predominate in the studied well-core profiles. The study thus supports the palaeogeographic notion of the mid-Jurassic Halten Terrace being a tidally-dominated seaway linking the Boreal Ocean to the north with the Tethys Ocean to the south. The present study also supports the notion that the mid-Jurassic sedimentation in the Halten Terrace area was controlled by active extensional fault tectonics, which bathymetrically compartmentalized the area into an array of NE-trending incipient grabens and half-grabens. The resulting topographic configuration is thought to have greatly enhanced the action of tidal currents and controlled spatial sand dispersal. The study confirms further the general notion that the Garn Formation is a transgressive succession composed of a transgressive parasequence set. The thickness and number of the vertically stacked transgressive to normal-regressive parasequences appears to vary laterally across the Halten Terrace, depending on the local fault activity. Only 3 to 5 transgressive–regressive parasequences, ~7–58 m thick, have been recognized in the main axial zone of the Halten Terrace seaway, but up to 16 thinner coeval parasequences were recognized at the outer (NW) margin of the Halten Terrace. The marginal flank of the synclinal Halten seaway was apparently much more sensitive to the interplay of bathymetric changes and pulses of lateral sand supply. The study as a whole considerably improves our general understanding of the origin of the Garn Fm. and also bears important implications for this formation’s geo-model as a petroleum reservoir.