Sedimentology of Neoproterozoic storm-influenced braid deltas, Varanger Peninsula, North Norway

The oldest part of the upper Riphean (Cryogenian) sedimentary succession of the Varanger Peninsula, North Norway, includes a series of sandstone-dominated units of fluvio-deltaic to shallow-marine origin and thinner, intercalated mudstone-rich units. The sedimentology and stratigraphic understanding of the mudstone-rich units are less complete compared to their sandstone-dominated counterparts. In this outcrop-based study, we report on the sedimentology of the mudstone-rich Klubbnasen and Andersby formations in the lower part of the Vadsø Group. Our facies analysis reveals that both units comprise offshore, storm-influenced prodelta, and fluvial-dominated to storm-affected delta-front deposits organised into two large-scale coarsening-upward cycles, each capped by >100 m-thick, cross-bedded sandstone units of braidplain affinity (belonging to the Fugleberget and Paddeby formations, respectively). An ever-dominant eastward palaeocurrent direction parallelling the Archaean crystalline basement terrane and the coinciding basin margin south of the study area, and a major regional lineament to the north, suggests structural control on sediment routing, possibly governed by differential subsidence steering the position of the palaeo-drainage system. Together, the investigated units form two vertically stacked fluvio-deltaic couplets which record the successive basin filling by eastward-prograding braid deltas. A multitude of event beds occurring in the prodelta deposits, including turbidites, wave-modified turbidites and hummocky cross-stratified tempestites, indicates that the deltas built into a shallow basin occasionally swept by storms. The shallow nature of the basin in combination with vertically extended river effluents enabled dunes to prograde far onto the slope. Here, they eventually became liquefied and collapsed, initiating sediment-gravity flows which contributed to sand deposition on the basin floor. Moreover, we describe a wide range of soft-sediment deformation structures attributed to recurrent tectonically induced seismic activity as well as gravity-driven deformation of the prodelta slope. As such, we explore sedimentary response to potential late Riphean post-rift tectonism, differential subsidence, and the interaction of fluvial and deltaic processes on a shallow, pericratonic basin on the previously rifted, Timanian margin of Baltica.