Sedimentology, seismic facies and stratigraphy of a Holocene spit-platform complex interpreted from high-resolution shallow seismics, Lysegrund, southern Kattegat, Denmark

Eighty kilometres of high-resolution seismic profile data (8 km/km²) were collected in the Lysegrund area, southern Kattegat, Denmark to reconstruct the seismic stratigraphy of a Holocene spit-platform complex. Comparison of two sets of seismic data shows a significant difference in reflection pattern. By using lap style analyses, four Holocene units were recognised in boomer profiles (0.8-2.0 kHz). They are closely examined in this paper and their geometry is illustrated in a series of isopach maps, drawn to understand the development of the deposit in time and space. Three vibra cores provide the stratigraphic control and demonstrate the presence of three lithofacies with gradational boundaries in the interval that correlates with the three upper boomer units. The three lithofacies are part of a single lithofacies succession. In contrast to the boomer profiles, the subbottom profiles (35 kHz) demonstrate continuous accretion surfaces within the lithofacies succession. Comparisons of the boomer and subbottom profile data therefore indicate that the three boomer units are genetically coupled and that there are no significant time gaps between units. The three boomer units are interpreted to represent a spit-platform complex that developed in three phases during a general rapid rise of relative sea-level (> 10 mm/a) under conditions of excess sediment supply. The source materials were mobilised from glacial-fluvial sediments in a headland west of and close to the spit-platform. During phase 1, spit development occurred in shallow water. This acted as a core to the development of a spit-platform during phase 2 that consisted of three seismic facies identifiable on 3.5 kHz subbottom profiles. During phase 3, true foresets and bottomsets developed. Growth of the platform surface kept pace with sea-level rise and the entire spit-platform complex formed in less than a thousand years. Focusing on differences in the seismic data helps avoid pitfalls in choosing methods to interpret data. The closely spaced seismic line-net forms the basis of a 3D dynamic model of the spit-platform that resembles the model of Meistrell [Meistrell, F.J., 1972. The split-platform concept: laboratory observations of spit development. In: Schwartz, M.L. (Ed.), Spits and Bars, Dowden, Hutchinson and Ross, Stroudsberg, 225-283. (C) 2000 Elsevier Science B.V.