Depositional architecture of a low‐gradient ramp shelf in an epicontinental sea: the lower Cretaceous of Svalbard

Abstract

ABSTRACTFacies 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.