Resumen de Pelagosite and coniatolite crusts associated with zones of sea spray on coastal terraces, Mohammedia (Northwest Morocco)
Clusters of finely laminated, pelagosite and micro-pelagosite occur on exposed terraces on the Atlantic coastline of Morocco. These structures occupy a substrate of potholed, calcareous Quaternary sandstone, which is receding rapidly due to biogenic and mechanical erosion processes. Sandstone overhangs that are weathered into craggy prominences in the tidal zone are preferential sites of carbonate precipitation. These restricted environments at times become supersaturated with carbonate. Crusts are strongly indurated and exhibit a variety of structures, the most common of which are laterally linked spheroidal aragonite crusts reminiscent of biogenic hemispheroids (LLH), and stacked hemispheroids (SH). They encrust a variety of vertical substrate irregularities occasionally forming dripstone coniatolite. Vertical relief of the spheroids varies from 10-200 µm and all forms are characterized by branching bundles of crystals with concentrical layers of radial fibrous aragonite. The vertical cliff faces provide a modern example of palagonite formed by cement precipitation facilitated by sea spray on variable microtopography. Although many of these aragonitic crusts resemble lithified algal stromatolites, their dripstone morphologies, nanostructures and lack of desiccation cracks, strongly suggest that they are formed by physico-chemical precipitation from seawater. The vertical distribution of aragonite is controlled by CO2 entrapment and release, sea-spray Ca, and the pH of the surrounding areas.
