Storm-Related Rhodolith Deposits from the Upper Pleistocene and Recycled Coastal Holocene on Sal Island (Cabo Verde Archipelago)

Abstract

This project examines the role of tropical storms in the northeast Atlantic Ocean related to the post-mortem deposition of rhodoliths in coastal settings during Neogene to Holocene time with primary emphasis on Sal Island in the Cabo Verde Archipelago located 600 km off the coast of Senegal in northwest Africa. Fossil rhodoliths from 10 to 15 cm in diameter are equal in size to contemporary rhodoliths that survive for a century or more at water depths undisturbed by all but the most energetic storms. The shape of large rhodoliths makes them susceptible to rare disturbances with sufficient energy to export them beyond their preferred habitat into extreme environments that include supratidal settings. The methodology of this study gauges the relative sphericity of rhodoliths based on measurements across three axes perpendicular to one another, plots size variations on bar graphs, and considers whether or not individual nodules are nucleated around rock cores eroded from proximal rocky shores. Sal Island is impacted on a steady basis by wave swell generated from the Northeast Trade Winds, but Pleistocene and Holocene deposits with large rhodoliths on the Island’s windward coast are interpreted as the result of major storms of hurricane intensity. Comparison of Sal Island rhodoliths with Pliocene and Miocene examples from other insular localities in the Northeast Atlantic considers evidence for displacement of the Inter-Tropical Convergence Zone (ITCZ) into more northern latitudes as an influence on past hurricane tracks that are less common today.

Trough cross-bedded rhodolith limestones in the Atlantic-linked Ronda Basin (Messinian, Southern Spain)

“…Hummocky and swaley cross-stratification in rhodolith-bearing packstones are also characteristics of Pleistocene storm deposits in Santiago (Cape Verde) (Johnson et al, 2012) and Miocene-Lower Pliocene tempestites from Santa Maria in the Azores archipelago (Johnson et al, 2017). Other storm-related rhodolith concentrations in Cape Verde Islands are supratidal deposits resulting from the storm activity that transported the rhodoliths inland from their sites of growth or even from older fossil rhodolith beds (Johnson et al, 2017(Johnson et al, , 2020.…”