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Facies belts have been recorded using conventional field methods in the Lower Cretaceous Helvetiafjellet Formation in Spitsbergen, Svalbard. The formation was deposited upon a regional subaerial unconformity (SU) that developed downdip from an incised valley complex to the W, NW and possibly north of Spitsbergen during fall in relative sea level and incision into the marine Rurikfjellet Formation. The unconformity likely resulted from crustal tectonics related to the formation of the proto‐Amerasian basin. The unconformity was initially covered by braided stream facies sand and locally by small bay head delta deposits. Further rise in relative sea level resulted in transgression, and the initial braidplain turned into a low‐sloping shelf ramp of coastal plain and paralic to marine environments. Balance between increase in accommodation space and rate of sediment supply gave rise to aggradational stacking architecture of the middle and upper part of the Helvetiafjellet Formation, until further rise in sea level turned the shelf again into an open marine setting with deposition of the overlying Carolinefjellet Formation. Architectural pattern of fluvial distributary channel sandstone bodies in the aggradational succession is explained within the framework of the Boreal basin, not only the Spitsbergen outcrop domain as in previous models. The revised depositional model for the Helvetiafjellet Formation implies that fluvial systems in similar emerged epicontinental basins may have large potential to carry sand several hundreds of kilometres out into the basin with formation of fluvial sandstone intervals of large regional extent, bounded by semi‐parallel stratigraphic surfaces transecting time‐lines at very low angles. Sandstone bodies with clinoform geometry are in this setting restricted to local small‐scale paralic deltas and shoreface deposits.

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The Neoproterozoic depositional histories of the Timanian and Baltoscandian, orthogonal margins of Baltica show several important differences but also some similarities. The Timanian margin comprises mainly low-grade, terrigenous sedimentary successions with a distinctive, margin-parallel fault zone separating pericratonic and basinal domains. Magmatic rocks are comparatively rare on land, but are common in deep drillcores recovered from beneath the Pechora Basin. Conversion to an active margin occurred in latest Riphean time, ultimately leading to the accretionary and transpressional regime of the Vendian-age, Timanian Orogeny. Along the Baltoscandian margin, successions of low to high metamorphic grade are preserved in diverse Caledonian nappe complexes. Three main types of palaeobasin are distinguished, based largely on sedimentary facies and basin geometry. Magmatic rocks are more common than in the Timanides, ranging from mafic dyke swarms to the voluminous Seiland Igneous Province. This margin remained passive throughout the Neoproterozoic era. The Vendian-dated dyke swarms signify the onset of Iapetus/,zEgir ocean opening at precisely the time when the orthogonal Timan margin was being deformed and telescoped during the Timanian Orogeny.

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