Sedimentary dynamics of modern and fossil rhodolith beds from the Gulf of California and North Atlantic islands
Markes E. Johnson
Geosciences Department, Williams College, Williamstown, MA 01267 USA
Living rhodoliths in the Macaronesian realm (North Atlantic Ocean) and the Gulf of California are distributed around islands with extensive rocky shores and narrow shelves that rapidly drop off at the shelf break normally beyond the 50-m isobath. Wind and wave erosion is intense on north-facing shores due to the prevailing northeasterly trade winds. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern and western 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 this survey include Santiago, Maio, São Nicolau, and Sal from the Cape Verde Islands, Fuerteventura in the Canary Islands, and Porto Santo in the Madeira Islands. Gulf of California islands include Isla Coronados and Isla del Carmen. In direct comparison with living counterparts, the objective is to show that rhodoliths from these islands enter the fossil record as taphofacies defined by the degree of breakage and corrosion also characterized by sedimentological criteria regarding the amount of matrix and degree of packing among bioclasts. Fossil rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, they were subject to transportation and post-mortem disintegration resulting in the accumulation of 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 offshore, but exposed strata indicate shoreward migration of taphofacies under a range of different energy regimes (shown below).
Taphofacies inferred from sedimentary dynamics in carbonate deposits dominated by rhodoliths and degraded rhodolith materials in the Macaronesian realm.
Rhodolith beds provide a habitat for many species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths. Some encrusting and boring activities occurred while the host rhodolith was still alive, but others after the host was transported onto rocky shores far from the optimum life habitat.
About the Author
Makes E. Johnson is the Charles L. MacMillan Professor of Natural Science, Emeritus, at Williams College in Williamstown, Massachusetts, where he taught courses in historical geology, paleontology, and stratigraphy in the Geosciences Department over a 35-year career. His undergraduate education in geology concluded with a BA degree (1971) from the University of Iowa and his advanced training in paleoecology culminated with a PhD degree (1977) through the Department of Geophysical Sciences at the University of Chicago. With 25 years of field experience in Baja California, Johnson has been a semi-annual visitor to the frontier states of Mexico where he habitually led field courses and supervised thesis projects for students from Williams College. He is an authority on the geology of ancient shorelines and the evolution of inter-tidal life through geologic time based on studies conducted around the world from Western Australia to China’s Inner Mongolia to the fringe of Arctic lands across Siberia, Norway, and Canada, as well as comparatively young island groups such as the Seychelles in the Indian Ocean and the Cape Verdes in the North Atlantic. Whether on explorations near or far away, this traveler has always been drawn back to the wild islands in the western Gulf of California and their associated peninsular shores. The author lives with his spouse, Gudveig Baarli, in Williamstown, Massachusetts, where they maintain an active and mutually supportive schedule of ongoing research and writing projects. Current interests and journal publications relate to the travels of Charles Darwin and his formative growth as a field geologist in the Cape Verde Islands.