HOLOCENE TRANSGRESSION OF THE RHINE RIVER-MOUTH AREA, THE NETHERLANDS/SOUTHERN NORTH SEA: PALAEOGEOGRAPHY AND SEQUENCE STRATIGRAPHY

We present a detailed reconstruction of the palaeogeography of the Rhine valley (western Netherlands) during the Holocene transgression with systems tracts placed in a precise sea-level context. A high level of detail could be reached because of 1) favourable antecedent topography and subsidence setting; 2) long history of coring, mapping, dating, establishing facies models; 3) New 3D-techniques of dealing with the wealth of data and 4) key papers produced in recent years. The reconstruction permits comparison of actual versus conceptual boundaries of the lowstand, transgressive, and highstand systems tracts. It shows that the inland position of the highstand Rhine river-mouth on a wide, low-gradient continental shelf meant that base-level changes were the dominant control on sedimentation for a relatively short period of the last glacial cycle. Systems in such inland positions predominantly record changes in the balance between river discharge and sediment load and preserve excellent records of climatic changes or other catchment-induced forcing.

We furthermore show that the Transgressive Systems Tract-part of the coastal prism formed in three stages: (1) The millennium before 8.45 ka BP, when the area was dominated by fluvial environments with extensive wetlands; (2) The millennium after 8.45 ka, characterized by strong erosion, increasing tidal amplitudes and bay-head delta development and (3) the period between 7.5-6.3 ka BP when the Rhine avulsed multiple times and the maximum flooding surface formed. The diachroneity of the transgressive surface is strongly suppressed because of a pulse of accelerated sea-level rise at 8.45 ka BP. That event not only had a strong effect on preservation, but has circum-oceanic stratigraphical relevance as it divides the early and middle Holocene parts of coastal successions worldwide. We propose that a marine deltaic sequence recording the 8.45-8.25 event could well host the Global Stratigraphical Section and Point for the Middle Holocene.