The Molo Formation and its ‘impossible’ correlation to the time-equivalent Kai Formation explained by strong coastal currents and occasional margin collapse

Abstract

The Molo Formation appears as a characteristic feature on the Norwegian Sea continental shelf and has drawn considerable attention in the literature. Especially the relative age and stratigraphic relationship to the Kai Formation has been discussed in various articles. Its position and areal distribution along the Norwegian continental shelf are believed to have important bearings on understanding the late Cenozoic uplift and erosion of Fennoscandia and adjoining shelf areas. The sequence stratigraphy of the Molo Formation has been explained by climatic variations in the Miocene. The timing of these events have significant implications on how to understand the maturation, migration and trapping of hydrocarbons on the inner continental shelf. A correct age, and age-stratigraphic position of Neogene units in the Norwegian Sea thus have both academic and commercial interest. In this work we document an extreme along-strike continuity of sequences within the Molo Formation where sandy deposits are trapped in a landward position due to strong longshore currents. We propose a model where the close to timeequivalent Kai Formation mainly consists of fine-grained contourite deposits and coarse-grained sediments which only occasionally reach the distal basin by infrequent collapse at the front of the Molo Formation through its progradation. The coarser-grained sediments are funnelled to the distal basin position and incorporated in the Kai Formation over a bypass area and through well-defined canyon incisions. This may explain the challenging stratigraphic correlations and problematic paleogeographic models implying a widely exposed shelf in the Miocene and a rapid sea-level rise of five hundred metres before onset of late Cenozoic shelf progradation.