Beate Louise Stølen Leren, University of Bergen, Norway (Norway)
John Howell, University of Bergen (Norway)
Allard Martinius, StatoilHydro ASA Rotvoll (Norway)
The majority of studied deltaic systems occur on passive continental margins and are deposited during relative sea level highstands. However, deltas are not exclusive to these settings and are also deposited in a wide variety of tectonic settings under different accommodation conditions. This paper presents three case studies that consider low-accommodation coarse-grained deltaic systems deposited in tectonically active basins.
1) Case study 1 presents a study of the Eocene Roda Sandstone in the "piggyback" Tremp-Graus Basin, Spain. This study suggests that the Roda Sandstone consists of two different but coeval deltaic systems, a Gilbert-type and a shoal-water delta. Their differing internal architecture is structurally controlled and reflects different sediment transport length, hinterland characteristics and intrabasinal slope angles.
2) An Eocene aged incised valley system in the Boyabat Basin of the Central Pontides foreland basin of northern Turkey is presented in case study 2. The infill of the compound incised valley system shows several cycles of incision and progradation of Gilbert-type and shoal-water deltas. Tectonic activity and high sediment fluxes in the "piggyback" foreland basin are considered to have been the main controlling factors of the incised valley infill studied.
3) Case study 3 presents a subsurface study of the Oseberg Formation, Northern Norwegian North Sea. The study concludes that the depositional architecture of the Oseberg Formation is controlled by the onset of an initial phase of middle Jurassic rifting and rift-margin uplift. This is recorded by change in sedimentation pattern caused by relative sea level fall from a transverse, SW-WSW prograding shoal-water fan-delta deposited during forced regression to a longitudinal N-NW prograding Gilbert-type delta deposited during lowstand.
Fan-deltas are the dominant depositional environment within the studied tectonically active basins. Fan-deltas are well known to be highly sensitive recorders of variations in relative sea level and tectonic activity. However, the intermingled effects of these controlling factors can be difficult to distinguish and relies on detailed analysis of the facies association. The controlling factors highlighted by the presented studies are subsidence and uplift which are a response to regional and local tectonic activity, and sediment supply, which is also, in part controlled by local tectonics, climate and base level. Results show that tectonic influence is seen within all orders of cyclicity and controls external and intra-delta architecture. This is significant because such deltaic systems can show very rapid facies changes and complex geometries which reflect high frequency tectonic movements. These changes impact both hydrocarbon exploration and production strategies.