Geomorphology and clast assemblages of intertidal beachrock: Implications for submerged shoreline preservation

Andrew Cooper, Andrew N. Green, Helenice Vital, F.P. Lima-Filho

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Abstract

Beachrock may be considered a reliable indicator of sea level, in addition to a reliable and recognizable source foronshore movement of beachrock clasts by overwash processes associated with extreme events. Key to this is itsgeomorphologyand preservation potential, both of whichare poorly understood. Here we examine the processesand deposits associated with beachrock breakdown from Camurupim, NE Brazil and compare its preservation tobeachrocks found on continental shelves in South Africa. We study a prominent and continuous 50–60 m-widebeachrock complex that crops out along a 10 km stretch of coastline, and which mostly comprises 1.5 to 2 m-thick trough-crossbedded sandstones. These have generated large joint-bounded blocks that have beentransported landwards under extreme energy conditions (based on numerical modelling and comparisons tothe contemporary wave climate). Tabular clasts have accumulated as isolated, very coarse boulders on the sea-ward margin, small clasts in various topographic traps, boulder ridges with large landward- and seaward-dipping clasts on the uppermost part of the outcrop, and subaqueous and subaerial washover fans. Isolatedfine meso-boulders emplaced during extreme waves (Hs= ca. 12 m) exhibitfitted textures and pressure-solution contacts, pointing to subsequent rocking and shaking. Smaller clasts in boulder ridges and overwash set-tings are initially liberated and transported by waves within the annual-decadal frequency spectrum, depositedon the high points of the outcrop. Subsequent movement during less extreme wave action progressively trans-ports the clasts across the surface, where they are often trapped in thefitted fabric on top of the platform. Ourbreakdown model and overall beachrock preservation scheme is consistent with reported seismic signatures ofsubmerged Holocene beachrock shorelines. Beachrock preservation during transgression is strongly influencedby the original sedimentary facies; thick, trough-crossbedded joint-bound clasts preserve the former shoreline.Clasts from the planar units accumulate as washover fans to landwards of the ridge.
Original languageEnglish
Pages (from-to)106-118
Number of pages13
JournalGeomorphology
Volume343
Early online date9 Jul 2019
DOIs
Publication statusPublished - 15 Oct 2019

Keywords

  • beachrock
  • preservation potential
  • boulders
  • palaeoshorelines
  • Preservation potential
  • Palaeoshorelines
  • Beachrock
  • Boulders

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