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Rhodolith/Maërl Beds: A Global Perspective

Volume 15 of the series Coastal Research Library pp 221-261

Date:

Taphonomic Range and Sedimentary Dynamics of Modern and Fossil Rhodolith Beds: Macaronesian Realm (North Atlantic Ocean)

  • Markes E. JohnsonAffiliated withDepartment of Geosciences, Williams College Email author 
  • , Jorge Ledesma-VázquezAffiliated withFacultad de Ciencias Marinas, Universidad Autónoma de Baja California
  • , Ricardo S. RamalhoAffiliated withSchool of Earth Sciences, University of BristolLamont-Doherty Earth Observatory at Columbia University
  • , Carlos M. da SilvaAffiliated withDepartamento de Geologia e Centro de Geologia, Faculdade de Ciências da Universidade de Lisboa
  • , Ana Cristina RebeloAffiliated withDepartamento de Biologia, Universidade dos AçoresStaatliches Museum für Naturkunde
  • , Ana SantosAffiliated withDepartamento de Geodinámica y Paleontología, Facultad de Ciencias Experimentales, Universidad de Huelva
  • , B. Gudveig BaarliAffiliated withDepartment of Geosciences, Williams College
  • , Eduardo MayoralAffiliated withDepartamento de Geodinámica y Paleontología, Facultad de Ciencias Experimentales, Universidad de Huelva
  • , Mário CachãoAffiliated withDepartamento de Geologia e Centro de Geologia, Faculdade de Ciências da Universidade de Lisboa

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Abstract

Distribution of living rhodoliths in the Macaronesian realm is limited by extensive rocky shores and narrow insular shelves that rapidly drop off beyond the 50-m isobath. Wind and wave erosion is most intense on north and northeast-facing shores due to the prevailing northeasterly trade winds over much of the region. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern shores with strong long-shore currents and southeastern shores that benefit from wave refraction. Rhodoliths are not entirely absent off northern shores, but may fail to reach maximum size before being washed ashore to make berms and beaches. Islands considered in greater detail in this survey include Santiago, Maio, and Sal from the Cape Verde Islands, Fuerteventura and the related islet of Lobos in the Canary Islands, Selvagem Grande and Pequena from the Savage Islands, Porto Santo in the Madeira Islands, and Santa Maria in the Azores. This contribution expands on the concept that living rhodoliths enter the fossil record through a range of taphofacies defined by the degree of breakage and corrosion and further characterized by sedimentological criteria regarding the amount of matrix and packing among bioclasts. Rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, rhodoliths are subject to transportation and post-mortem disintegration resulting in the accumulation of rhodolith materials captured by subtidal storm deposits, tidal pools and platform over-wash deposits, as well as beachrock, beach, berm, hurricane, tsunami, and coastal dune deposits. Some of this material is transferred farther offshore, but exposed island strata show a tendency for shoreward migration of taphofacies. Rhodolith beds provide a habitat for some species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths and these features play a role in rhodolith biostratinomy.

Keywords

Rhodolith taphonomy Depositional environments Northeast trade winds Macaronesian archipelagos North Atlantic Ocean