Classification of paraglacial barrier systems: coastal New England, USA

Abstract

The New England coast harbours a wide variety of barrier forms, which we organize into six barrier‐coastline types. The barriers develop in response to the relative importance of several spatially and temporally variable parameters, particularly antecedent topography and geology, sediment abundance and size, exposure to wave and tidal energy and sea‐level history. The six coastline types can also be identified in other paraglacial regions. Existing barrier‐coastline classification schemes do not allow consistent subdivision of paraglacial barrier coasts. This paper presents a new scheme that is applicable to paraglacial and non‐paraglacial barrier coasts alike. Aside from hydrodynamic regime, which forms the basis of the barrier classification most commonly used to date, it includes a compartmentalization factor. Sediment‐starved ‘isolated’ (‘type 1’) barrier coastlines are characterized by short, widely spaced barriers. Small, localized updrift and offshore sources have provided sediment for short barriers along ‘clustered headland‐separated’ (‘type 2’) barrier coastlines. Various amounts of sediment from larger updrift and offshore glaciofluvial deposits or directly from rivers have formed the longer barriers along ‘wave‐dominated mainland‐segmented’ (‘type 3a’), ‘mixed‐energy mainland‐segmented’ (‘type 3b’), ‘wave‐dominated inlet‐segmented’ (‘type 4a’) and ‘mixed‐energy inlet‐segmented’ (‘type 4b’) barrier coastlines. Geomorphic form, grain size and stratigraphy can be used to characterize individual barriers along barrier coastlines. Most paraglacial barriers form as spits, but many are transformed subsequently. Welded barriers are common along ‘type 1’ and ‘type 2’ coasts. Baymouth barriers are characteristic of ‘type 3’ coasts, and barrier islands occur exclusively along ‘type 4’ coasts. The coarsest grained barriers are located along ‘type 1’ and ‘type 2’ coasts. Progradational barrier sections are concentrated along ‘type 3’ and ‘type 4’ coasts with abundant sediment supply, but are also present along ‘type 2’ coasts. Temporary increases in sediment supply, common in paraglacial regions, result in transitions between retrogradational and progradational barrier behaviour, which may be recognized on shore‐perpendicular stratigraphic cross‐sections.