Weathering and biokarst development on marine terraces, northwest Morocco

Abstract

Weathering features are described from an arid coastal area in northern Morocco which are indicative of invasive chasmolithic and endolithic microbial communities. Active weathering of marine terraces and karst development is associated with endolithic and epilithic algae responsible for boring, disintegration of matrix and mineral fabrics, solution and biomineralization that undermines the marine carbonate platform. Evidence of a range of biological weathering agents remains preserved, specifically calcified filaments and sporangial material. An abundance of filaments and spores representative of a consortium of algae, fungi, cyanobacteria and lichens is associated with the most denuded outcrops. The array of microbes contributes to the formation of the stromatolites to depths of 0·5 m within the limestone substrate. The preservation of stromatolites is supported by calcification of spores and filaments, with trapping and binding of carbonate held in suspension by vadose waters. The pervasive weathering on this sector of coastline has important practical implications for coastal planning and development authorities in Morocco.

Pelagosite and coniatolite crusts associated with zones of sea spray on coastal terraces, Mohammedia (Northwest Morocco)

“…The coastal terraces along Morocco's northern shores (north of Casablanca, Mohammedia to Rabat) are areas of considerable diagenetic activity indicated by the widespread occurrence of biominerals and phytokarst development (Duane 2001;Duane et al 2003). north to Rabat, are composed of Quaternary calcareous sandstones exhibiting cavernous, pitted, and honeycombed appearances especially along exposed headlands.…”

“…The spray adds Ca to the surface of the terraces. The ultimate cause ofprecipitation ofpelagosite-coniatolite in and on sand surfaces was supersaturation of the trapped seawater laden with CO 2 from vadose waters entering the cliffs (Duane et al 2003). The loss of CO 2 by degassing is often the main driving force causing very high levels of supersaturation (Chafetz et al 1991) leading to the precipitation of calcium carbonate.…”

“…The term 'spherulitic bodies' usually refer to calcium carbonate polycrystals (Lacroix 1901;Pobeguin 1954) common in travertine, lacustrian, and marine settings but rare in marine vadose environments. This study documents the characteristics and explores the origin and significance of Recent carbonate deposits (see Duane et al 2003) that occur on weathered sandstone prominences in the Quaternary coastal vadose zone, northwest Morocco. Laminated carbonates in sand-sized lithologies rarely get preserved (Hagadorn and Bottjer 1997;Schieber 1999) and thus several unique considerations are required to explain the process.…”

Geomorphic zoning and eogenetic karst on limestones within the supratidal environment: San Salvador, Bahamas

“…Duane et al (2006) further suggested that a combination of biological weathering processes, abrasion, wave action, salt spray, and wetting/drying resulted in dissolution of the rocks, with the magnitude of platform deflation decreasing with distance from the platform edge. The interdependence of biologic communities to tolerable habitat conditions, in conjunction with spatial variations in the extent of physical and chemical weathering has led to the development of geomorphic gradients along low-lying coastal rocky exposures (Duane et al, 2003;Moses, 2003). In a study examining meso-and micro-karst morphologies within supratidal exposures of calcarenite on Lord Howe Island, Australia, Moses (2003) recognized three distinct geomorphic zones extending landward from the platform edge.…”

“…Coastal karst morphologies may vary as a function of geology, temporal and spatial locations, and the influences of physical, biological and chemical processes (Schneider and Torunski, 1983;Duane et al, 2003;Moses, 2003;Taborosi et al, 2004;De Waele et al, 2009). Physical and textural parameters such as composition, grain sorting, and packing play a significant role in determining the extent of dissolution or erosion.…”