Anna Lisa CESCON, J. Andrew G. COOPER, Derek W.T. JACKSON [email protected] Centre for Coastal and Marine Research, University of Ulster, United Kingdom Beach ridge sedimentology: field observation and palaeoenvironmental interpretation for Anegada Island, British Virgin Islands. West Walkover Introduction End Set Bay Windlass Bight Sandy beach ridges have been identified in tropical environments1, however there is little reference to Caribbean beach ridges in the literature. A discussion exists about their wavelenght or aeolian origin2,3,4 which impacts on their use as paleostorm archives. This study presents a first description of the stratigraphy and the sedimentology of Anegada’s sandy beach ridges (British Virgin Islands, Caribbean Sea [Fig 1]). There are more than 30 beach ridges in the Atlantic-facing plain and around 10 in the south, Caribbean-facing plain [Fig 2]. Fig 1: British Virgin Islands Map N S Fig 2: West Anegada beach ridge plains Stratigraphy Composition and grain shape During our fieldwork in November 2012 we studied The main components are broken carbonates from A B different Anegada’s beach ridges [Fig 2] and their diverse origin (coral, shells, foraminifera, sponges). C sections. Primarily, the sections present low angle Under the optic microscope, the beach ridges grains 1 m seaward-dipping bedding. When identified, the have an higher roundness and lower size compared to Degree and direction of bedding landward-dipping layers have higher angles but a the modern beach samples. Further analyses of the limited extent. The sand fraction is dominant in the sediment grain size are intended . Soil development stratigraphy with few intact macro shells. A soil level has been identified on Anegada’s beach Beach Beach Ridge ridges as the upper part of the sections. The limitation Walkover 1 m of the soil extent and the relative absence of animal Set Bay Modern beach deposits 1 mm 500 μm and roots reworking point to an observed stratigraphy Recent swash West End Fig 3: West End beach N S C 0.4 m B in the beach ridge sections corresponding to the one Profile A A N S N S ridge section: during their implementation. Upper part , the 10 m 40 cm 30 cm long dip section; Aeolian deposits may represent the upper part of the beach ridge. They seem associated with one or two Windlass Centre, stratigraphy Bight 500 μm of the dip section; limited soil levels as a stabilization of the beach ridge 500 μm Lower part, detailed with ongoing accretion. First berm 0.1 m 20 cm sections A, B C West End beach ridges Windlass Bight beach ridges The modern coastal erosion reveals the fine beach ridge stratigraphy in West End. It is visible We observe coral pebbles in the beach ridge sections. along a section of 60 meters long and up to 3 meters high. Fig 3 presents the stratigraphy of A similar deposit of reworked coral pebbles has also one beach ridge dip section exposed along 10 meters. Layers with coarser material are clearly been identified along the modern shoreline. The identifiable as more prominent. Their slopes have a low angle dipping with values comprise coarse elements identified in Windlass Bight are between 0° and 18° for the seaward [Fig. 3 A and 3 B] and up to 24° for the landward probably due to wavelength action during the Context photo directions [Fig. 3 C]. Beach ridge dip section implementation of the beach ridge. Along the shore Beach ridge strike section Bibliography [1] Tamura, T. (2012) Beach ridges and prograded beach deposits as palaeoenvironment records. Earth-Science Reviews, 114, pp. 279-297. Conclusion [2] Hesp, P. (2004) Sand Beach Ridges: Definitions and Re-Definition. Journal of Coastal Research, SI 39, 1, pp. 72-75 [3] Nott, J. (2010) A theory (involving tropical cyclones) on the formation of coarse-grained sand beach ridges in NE Australia. Geological Anegada Island is an unique opportunity to study sandy beach ridges without influence of terrestrial sediment Society, London, Special Publications, 346, pp. 7-22 [4] Tamura,T. (2014) Aeolian transport of coarse sand over beach ridges in NE Australia: A reply to a discussion of "Beach ridges and sources. The beach ridges present low dipping layers and coral pebble elements that question their wavelenght prograded beach deposits as palaeoenvironment records“ Earth-Science Reviews,132, pp. 85 - 87 origin. Further investigation needs to be done on possible secondary aeolian transport. Is this aeolian transport process ongoing simultaneous than the beach ridge construction or during successive phases? Another question The first author thanks the EGU and IAS for travel support. that remains open on whatever beach ridge s form by a single or a multiple wave event . .