GEOLOGICAL LEGACY OF TRANSGRESSIVE LITHOSOMES IN MICROTIDAL (FLORIDA AND NEW JERSEY, USA) AND NON-TIDAL (BLACK SEA, UKRAINE) BARRIERS

Home to one of the first models of transgressive barrier stratigraphy, the non-tidal coast of the northwestern Black Sea, Ukraine, is composed of highstand lithosomes, with high proportion of mixture of bioclastic components spanning Late Pleistocene and Holocene. High-frequency georadar (800 MHz GPR) images capture a range of contexts, from thin (<4 m) transgressive and channel-fill sequences to a suite of erosional indicators in prograded strandplains and baymouth barriers. With a similar mixed siliciclastic-carbonate composition and an enclosed nature of the Florida Gulf Coast, the evolution of the microtidal Cayo Costa barrier exhibits alternation of Late Holocene progradation phases and incomplete welding of transgressive lithosomes. The width and height of the fronting barrier dictate the relative role of overwash and wetland (lagoon, mangrove) deposition in reducing the accommodation space of the intervening swale. Within blocked wetlands, burial of the detached shoreface surface by aeolian or reworked berm sands produces regressive stratigraphy, despite landward-directed sand transport. This results in complex depositional modes in a regime of net progradation, with classic beachface accretion sequences (seaward offlap; >3 m thick) alternating with transgressive tracts (landward offlap/onlap; 0.5-2.5 m). Along a more wave-dominated coast of central New Jersey, landward sediment transfer by aeolian action is punctuated by localized incision and widespread aggradation during storms. Except for sections compromised by Hurricane Irene (2011), most of the Hurricane Sandy (2012) deposition thickened the barrier lithosome by up to 20%, with clear landward-dipping GPR reflections diagnostic of overtop deposition. While undergoing compaction and increasing backbarrier accommodation space, fine-grained lagoonal or valley-fill facies (mud, peat) may be exposed above the wave during long-term barrier retrogradation. Aside from bioclastic fraction, these deposits do not contribute to beach/dune sediment budget and this fact must be considered in coastal models. Accurate assessment of backbarrier hypsometry is one of the key factors in predicting the vulnerability of barrier superstructure in a regime of reduced sediment supply, accelerated sea-level rise, and increased storminess.