Sedimentology of coarse-clastic beach-ridge deposits, Essex, southeast England

Abstract

The distinction of cheniers from other types of beach ridge can often be problematic. The stratigraphy, sedimentology and geomorphological development of sand- and gravel-rich beach ridges at three sites on the northern Essex coast, England were determined using ground-penetrating radar (GPR), ground-truthing trenches and auger holes, and historical–aerial photograph analysis. The 900 MHz GPR system used achieved a maximum vertical resolution between 0.02 and 0.06 m. There was good correspondence between the radar stratigraphy obtained from time-migrated radar reflection profiles and the nature and form of bounding surfaces, sets of lamination, beds and bedsets observed in trenches. Data for one of the study sites (Colne Point) illustrate a complex beach-ridge stratigraphy and sequence of development that confirms they are not true cheniers. Instead, the ridges form part of both retrogradational and progradational barrier-spit sequences. Chenier designation at the other two study sites (Foulton Hall and Stone Point) is more straightforward, as the beach ridges lie at the junction between actively eroding mudflat and saltmarsh. However, despite the distinction between the barrier-spit beach ridges and the cheniers, the same types of deposit are recognised in both, indicating similar formative processes. Washover-sheet deposits consist of low-angle (<5°), sub-parallel, landward-dipping stratification, which is concordant with bounding surfaces above and below. Washover sheets develop when high-wave energy/storm-related overwash moves landward onto unflooded marsh or lagoonal surfaces. Washover-delta deposits are characterised by high-angle cross-stratification (up to 28°) that downlaps onto the underlying marsh or lagoonal surface. Washover deltas develop when overwash enters a significant body of standing water to landward, such as a flooded marsh or lagoon. Alternations between washover-sheet and washover-delta development are seen in many instances, but their time scale is unclear due to paucity of detailed information regarding overwash sedimentation rates, or chenier and berm-ridge migration rates. Similarity of the internal structure in both the cheniers and barrier-spit beach ridges confirms that this criterion alone cannot be used to distinguish different types of beach-ridge deposit. Detailed stratigraphic data, preferably complemented by direct evidence for the geomorphological context in which the ridge developed, are also required.