Spatial and temporal controls on the development of heterolithic, Lower Jurassic tidal deposits (Upper Are and Tilje Formations), Haltenbanken area, Offshore Norway.
Ichaso Demianiuk, Aitor Alexander
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The stratigraphic organization of clastic successions deposited during the early synrift phase is controlled by the rates of tectonic subsidence and the growth of the master faults, which, coupled with eustatic sea-level changes, control the generation of accommodation. The highly heterolithic, Lower Jurassic Upper Åre and Tilje succession (100 to 300 m thick), which occurs in the Halten Terrace of offshore Norway, represents an excellent example of ancient synrift deposits that accumulated within a NNE-SSW-oriented structurally controlled embayment where sedimentation was dominated by tidal currents, with secondary influence by river and wave processes. Overall, the Tilje was deposited in a deltaic setting near the lowstand shoreline, forming a shallowing-upward succession, which is organized in two, thick, tabular second-order sequences. These sequences are separated by two main sequence boundaries (SB2 and SB3) associated with two main rift-related tectonic pulses. The first pulse formed SB2 and is believed to have exerted a major regional control on the geomorphology of the basin, causing a change from an open, wave-dominated setting (upper Åre Fm.) to a funnel-shaped, tide-dominated setting in the Tilje Fm. SB2 shows shallow incision into the underlying Åre Fm., and the overlying sediment accumulated predominantly in a distributary-mouth-bar environment. Sequence 3 rests erosively on Sequence 2, and is characterized by proximal tidal-fluvial distributary-channel fills and mouth-bar deposits showing at least 2 main oblique to axial fluvial input points, one from the N-NW and a second one from the NE, with overall increase in wave influence and deepening toward the S. Local rapid subsidence of elongated narrow hangingwalls associated with the active master faults exerted a subtle control on the succession thickness, as well as a local control on the location of the tidal-fluvial distributary channels by “tectonic axial funnelling” during the onset of the second-order base-level rises. The internal architecture and facies distribution are less complex than other thick tide-dominated successions worldwide, because the rate of creation of accommodation was sufficient to avoid channel amalgamation throughout most of the succession.