Beach ridges and prograded beach deposits as palaeoenvironment records

Abstract

Beach ridges are landforms commonly developed on prograded coasts with beach shorelines. A sequence of beach ridges, coupled with their subsurface deposits, can be regarded as a time series of coastal evolution. Methodological advances in field surveying and chronology applicable to beach ridges have led to detailed palaeoenvironmental reconstructions to be derived from such sequences. This paper reconsiders the basic aspects of beach ridges and deposits, which need to be properly understood for their comprehensive interpretation in a palaeo-environmental context. It also reviews case studies in which beach-ridge sequences have been used to unveil past sea-level history, catastrophic events, and climate changes.Proposed formative processes of beach ridges include: 1) progradation of sandy beach and berm formations in relation to fairweather waves, coupled with aeolian foredune accumulation; 2) building of gravel ridges by storm waves; 3) welding of longshore bars. Beach-ridge formation through sea-level oscillation is thought to be questionable and caution is suggested for this process when undertaking palaeoenvironmental reconstruction. Beach deposit stratification is known to dip either landwards or seawards, but landward dips are uncommon. Seaward dipping stratification is formed in relation to beachface progradation, and is usually dissected in places by erosion surfaces resulting from episodic beach retreat. The boundary between the foreshore and the underlying shoreface is well defined only in the case that longshore bars lead to complex bedding structure relative to that of the foreshore. Reliable chronology of beach ridges can be determined by radiocarbon and optically-stimulated luminescence (OSL) dating. Radiocarbon dating of articulated shells, which are considered not to be extensively reworked, provides robust results, but OSL dating is more useful as it enables direct dating of sediment grains. It is noted that there are restrictions in chronological resolution and continuity inherent to beach ridge and beach deposits. The plan-view geomorphic expression of beach ridges typically consists of ridge sets with multi-decadal intervals, whereas their internal sedimentary structures define shorter time scales. Records of beach sedimentation and erosion are likely to be reworked by episodic high-magnitude beach retreat, and the resultant record of the net progradation is likely to be sporadic and discontinuous.The height of sandy beach ridges is often variable due to differing degrees of aeolian sand accumulation, and they are thus not used as sea-level indicators unless purely wave-built. Gravel ridge height is a relatively reliable indicator of sea level, but can vary in response to storminess fluctuations. Subsurface sediment facies boundaries are preferred as sea-level indicators, and those proposed include: boundaries of aeolian/beach, foreshore/shoreface, and upper/lower shorefaces.Catastrophic events are expressed in both erosional and depositional records. Erosion surfaces, or scarp imprints, revealed in a cross section of beach deposits, indicate storm or tsunami events. However, erosional events are likely to rework previous records of sedimentation and even other erosional events, and thus the apparent history decoded from the resultant deposits tends to be biased. Several attempts for estimating the frequency and intensity of prehistoric cyclones rely on assumed relationships between the level of coarse sand beach ridges and cyclone inundation. The formative process of coarse sand ridges remains uncertain and needs to be clarified, as it constitutes the fundamental basis of these attempts.The growth rates of beach-ridge systems are expected to reflect fluctuations in river sediment discharge to the coast and in aeolian sand flux due to onshore winds, both of which are affected by climate change. Assessment of the growth rate is potentially improved by ground-penetrating radar survey of subsurface structure and by detailed chronology. Orientation of beach ridges reflects long-term trends in wave direction. Inferred relationships between beach ridges and cyclic fluctuations of sea level and climate rely on weak assumptions and are not substantiated by rigid chronological evidence, and thus remain highly questionable. Correlation between the inherently decadal signal of beach-ridge intervals and possible climate cycles, such as sunspot activity, is probably coincidental as it lacks causal explanations.