Skip to content

Genesis and morphological evolution of coastal talus-platforms (fajãs) with lagoons: The case study of the newly-formed Fajã dos Milagres (Corvo Island, Azores)

Research output: Contribution to journalArticle

Original languageEnglish
Pages (from-to)138-152
Number of pages15
JournalGeomorphology
Volume310
Early online date10 Mar 2018
DOIs
DateAccepted/In press - 7 Mar 2018
DateE-pub ahead of print - 10 Mar 2018
DatePublished (current) - 1 Jun 2018

Abstract

Supratidal talus-platforms are low-relief subaerial accumulations of debris produced by mass wasting along high coastal cliffs, being particularly abundant at reefless volcanic islands subjected to high wave energy. Known as “fajãs” across the Portuguese-speaking Atlantic archipelagos, these coastal features, on rare occasions, may exhibit lagoons, constituting sites of high geological, biological, landscape, and social value. Whilst the origin of fajãs is firmly established as being the product of coastal landslides, little is known about the processes that shape fajãs with lagoons. In particular, doubts still remain concerning whether fajãs featuring lagoons are a fortuitous product of mass wasting, or result from marine reworking (by waves and currents) after emplacement. On October 30, 2012, a coastal landslide (~0.001 km3) occurred on Corvo Island, Azores Archipelago, forming a nearshore gravel islet that later migrated to the island's coast, resulting in a fajã with an ephemeral lagoon (Fajã dos Milagres). This event provided a unique opportunity to study the generation and development of fajãs with lagoons, and therefore a 3-year survey was carried out to record its evolution. This GIS-based study used aerial oblique photography and satellite optical imagery, complemented with a land survey for a more precise topographic reconstruction. Analysis of data concerning bathymetry, precipitation, and wave regime was also employed to investigate the driving forces behind the morphodynamic evolution of the deposit. “Fajã dos Milagres” evolved very rapidly, through an evolutionary pattern with five main stages: 1) “islet stage”; 2) “gravel spit stage”; 3) “early lagoon stage”; 4) “mature lagoon stage”; and 5) “fajã (without lagoon) stage”. Our reconstructions show that, for fajãs with lagoons to be formed, several factors should converge: a) presence of high coastal cliffs, made up of composite volcanic sequences, capable of producing large landslides that supply sufficient mobile sediment to the shelf; b) presence of a shallow, wide insular shelf where the sediments can be transported without significant loss to the submarine slopes; and c) a wave-dominated, high-energy regime, capable of significant cross-shore and longshore sediment drift. These observations allowed us to propose a preliminary conceptual evolutionary model for the generation of fajãs with lagoons, where marine reworking plays a fundamental role. Finally, this study documents the generation and very rapid subsequent evolution of a clastic coastal morphology solely driven by the action of waves and currents, and without interference from relative sea level and/or external sediment replenishment, with possible implications to other settings.

    Research areas

  • Azores, Coastal morphology, Coastal talus-platforms (fajãs), Lagoons, Spit bars & coastal barriers, Topographic reconstruction, Volcanic islands

Download statistics

No data available

Documents

Documents

  • Full-text PDF (accepted author manuscript)

    Rights statement: This is the author accepted manuscript (AAM). The final published version (version of record) is available online via Elsevier at https://www.sciencedirect.com/science/article/pii/S0169555X18301077?via%3Dihub. Please refer to any applicable terms of use of the publisher.

    Accepted author manuscript, 1.39 MB, PDF document

    Licence: CC BY-NC-ND

DOI

View research connections

Related faculties, schools or groups