Mud-rich delta-scale compound clinoforms in the Triassic shelf of northern Pangea (Havert Formation, south-western Barents Sea)

Abstract

In recent years it has become clear that many shallow-marine heterolithic and mudstone-dominated successions are deposited as mud belts forming part of subaqueous deltas that are related to major fluvial sources either upstream or along shore. Here the Havert Formation is presented as an ancient example of this kind of system. The Havert Formation in the south-western Barents Sea represents shelf margin clinoforms consisting predominantly of heterolithic deposits. Sediments were mainly derived from the east (Ural Mountains), but a smaller system prograded northward from Fennoscandia. The Havert Formation holds a lot of interest due to: (i) its stratigraphic position, directly above the Permo–Triassic boundary and contemporaneous to the emplacement of the Siberian Traps; (ii) the fact that it represents the first siliciclastic input in the south-western Barents Sea and it shows interaction between Uralian-derived and Fennoscandian-derived sediments; and (iii) its hydrocarbon potential. This study is focused on a detailed sedimentological analysis of cored intervals of the (Ural-derived) Havert Formation, in combination with seismic interpretation, well-log correlations and palynological analysis of the Havert and overlying Klappmyss formations. The cored intervals belong to the shelf environment of the Havert shelf-margin clinoforms (300 to 500 m thick). This sedimentological analysis distinguishes six facies associations, spanning from tidally-influenced channels at the shoreline to mud-rich subaqueous platform and foresets of the subaqueous delta. Seismic lines and well-log correlations show the larger-scale evolution of the Ural-derived Havert Formation, characterized by episodes of low-accommodation and high-accommodation. The palynological analyses provide the first detailed study of the Havert Formation in the Nordkapp Basin, revising its depositional age in the region as Induan to early Olenekian (Smithian). Furthermore, they strengthen the environmental interpretation; palynofacies present on the shelf record flora of tidally-influenced coastal plains, whereas the palynofacies in the deep-water slope contain only amorphous organic matter.