| Summary: | Upper Permian deposits from the eastern margin of the Helgeland Basin, offshore Norway, are known to contain two organic-carbon-rich (OCR) intervals within the Lower Turbidite Unit. These intervals were so far not proven in the deeper basinal areas and their depositional conditions in the margin areas were not fully understood. In order to understand the general depositional conditions and lateral extents of the two organic rich intervals, Upper Devonian-Triassic successions have been investigated. Core logging, analysis of element proxies, qualitative and quantitative seismic interpretation and intercept and gradient (IG) crossplotting were used to investigate these successions in the Trøndelag Platform, specifically the Helgeland and Froan basins and partly the Nordland Ridge. Seismic interpretation, both qualitative and quantitative, and IG crossplotting were used to investigate possible geophysical manifestations of these upper Permian OCR sediments in the deeper basinal areas. Qualitative seismic interpretation, which focused on the analysis of the Upper Devonian-Triassic sedimentary fill geometries and fault-strata relationships, and a combination of core logging and element proxies were used to assess conditions that controlled deposition and preservation of the OCR sediments. These conditions include local tectonic developments, which are not as well understood as the regional development, and variation in oxygen levels in the deep basinal areas. A portable X-Ray Fluorescence (PXRF) scanner was used to determine major, minor, and trace element concentrations. The measurements of these element concentrations on core started below the lower organiccarbon-rich interval and continued above the upper organic-carbon-rich interval to track changes over time that might indicate what caused deposition to change. Changes in element values were then correlated to grain size, deposit types and organic content to understand the sedimentological influence on element distributions. Results show that the Trøndelag Platform has a long history of rifting with several tectonic pulses from the Carboniferous to the late Triassic. These tectonic pulses include two local late Permian and two Early Triassic tectonic events. Thus, the upper Permian organic carbon rich intervals were deposited during a period of active tectonics. The seismic analysis showed that organic-carbon-rich rocks are present in the deep basin areas, and not just on the margins where previous work has shown their existence. These organic rich rocks are present in most of the Helgeland Basin, and present but spatially restricted in the Froan Basin since the Froan Basin was highly compartmentalized by the late Permian tectonic events. Depositional processes, oxygen concentration and fluvial and terrestrial influx varied significantly during deposition, as indicated by the element concentrations. The lower organic-carbon-rich interval was deposited in the deep basinal areas under anoxic conditions that may have resulted from restricted oxygen circulation caused by tectonically induced isolation of the sub-basins from the main ocean. Sediment input into the basin changed between the deposition of the lower and upper organic-carbon-rich intervals with a more constant input of fine silt and sand. If this sediment influx was from fluvial input, this could potentially have caused stratification of the water column and subsequently anoxia. Another feature which is found in this work is the possible presence of upper Permian carbonate build-ups in the Helgeland Basin. These carbonates were deposited on the structural highs showing that anoxia was not developed there, in contrast to the deeper basinal areas where organic-rich rocks were laid down. This may explain the patchy occurrence of the upper Permian organic-carbon-rich layers.
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