Facies architecture and depositional processes of the Holocene-Recent accretionary forced regressive Skagen spit system, Denmark

Spit systems are seldom recognized in the pre-Quaternary sedimentary record compared to their common occurrence along present-day coasts and in Quaternary successions. This lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of spit systems and their subaqueous platforms in particular. The Skagen spit system is a large active system that began to form 7150 yr bp and from 5500 bp to Recent times it has prograded 4 m year−1 and accumulated 3·5 × 109 m3 of sand. The spit system provides a unique opportunity for establishing a well-constrained depositional model because uplift and erosion have made large windows into the preserved facies, while active spit-forming processes can be examined at the young prograding end of the same system. The depositional model presented here thus builds on excellent outcrops, surface morphology, a well-defined palaeogeography and detailed C14 age control supplemented with observations from continuous well cores and profiles obtained by ground-penetrating radar and transient electromagnetic surveys. The factors that have governed the development of the spit system, such as relative sea-level change, wave and current climate, tidal range, sediment transport and depositional rates are also well-understood. The sedimentary facies of the spit system are grouped into four principal units consisting from below of thick storm sand beds, dune and bar-trough deposits, beach deposits and peat beds. These four units form a coarsening and shallowing upward sand-dominated succession, up to 32 m thick, which overlies offshore silt with a transition zone and is topped by a diastem overlain by young aeolian dune sand. The sedimentary structures and depositional processes are described in detail and integrated into a depositional model, which is compared to other spit systems and linear shoreface models.