ABSTRACT
Younger Bermuda (Sayles, 1931) is the collection of large dune-shaped hills along Bermuda’s external shoreline. These lithified calcarenitic dunes retain their original depositional morphology and are easily distinguished from altered, older dunes which lie inland.
The stratigraphy of these young calcarenites is presented in terms of multiple systems of stratigraphic classification. The Paget Formation is the rock-stratigraphic equivalent of the morphostratigraphic unit, Younger Bermuda. The Paget is divisible in its eolian facies into upper and lower members that reflect two distinct Late Pleistocene submergences of the shallow platform surrounding Bermuda. The marine (beach and shallow-water) facies includes long beach wedges transitional with eolianite. A soil-stratigraphic unit separates the two members and represents an interval of platform emergence.
The Bermudian dunes were large accretionary mounds that extended inland no more than about 0.5 km from their source beaches. As the mounds grew, they merged laterally to produce a transverse ridge parallel to the shoreline. The growth direction of the individual dunes is reflected by their long axes or, equivalently, by the symmetry axes of sets of horizontally convex-leeward foresets that represent the individual mounds. Apparently this growth direction was determined largely by strong winds and gales. The consistent pattern of horizontally convex-leeward foresets curving around the high points of the ridge is one of several ways the accretionary transverse dune bodies of Bermuda differ from migratory transverse dunes.
Two source-sink systems leading to the formation of calcareous dunes in Bermuda are inferred from stratigraphic relations and present-day sedimentary patterns. The first is a reef-to-dune, source-sink system for shorelines close to the platform margin where reefs are established during platform submergences. The resultant dunes (South Shore) consist of clasts of reef-dwelling organisms, and, apparently, the size of the dunes reflects reef productivity. The second system is a lagoon-to-dune, source-sink system for shorelines separated from 356platform-margin reefs by a wide lagoon. Under “normal” conditions of platform submergence (like today), the reefs act as a submerged breakwater that filters out the waves capable of moving lagoonal calcarenite onto the shoreline. The lagoon becomes a source for dunes (North Shore) upon a rapid rise of sea level that nullifies the breakwater.
The rapid, 15 m rise in sea level about 125,000 yr ago that led to the formation of early Paget dunes along Bermuda’s lagoon-facing shoreline may represent a large surge of the Antarctic Ice Sheet.
