Beachrock
Carbonate-cemented shoreline rock formed rapidly in the intertidal zone.
Beachrock is a friable to well-cemented sedimentary rock formed along a shoreline, composed of gravel-, sand-, and silt-sized sediment cemented with carbonate minerals. It typically develops within the intertidal zone of tropical or semitropical regions, though Quaternary beachrock has been found as far north and south as 60° latitude. Its rapid formation and intertidal origin make it a valuable indicator of past sea levels.
Quick Facts
- Width
- up to 300 meters
- Height
- 30 cm (1 ft) to 3 meters
- Length
- meters to kilometers
- Latitude range
- found as far north and south as 60°
- Inclination to sea
- 50–150
Facts from the source article.
Overview
Beachrock units form under a thin sediment cover atop unconsolidated sand, often comprising multiple units from repeated cementation and exposure. The cement is mainly high-magnesium calcite or aragonite. Cementation occurs through seawater evaporation, groundwater CO2 degassing, mixing of marine and meteoric waters, and microbial precipitation of micritic calcium carbonate. On retreating coasts, offshore beachrock outcrops can act as barriers against erosion, while their presence may cause sediment deficiency and alter wave regimes.
Cementation and position of beachrock
Beachrocks lie parallel to the coastline, typically a few meters offshore, and are often separated into several levels representing different generations of cementation. Older zones occupy the outer part of the formation, younger ones lie nearer the beach, possibly under sand. The formations generally incline seaward at 50–150 and are characterized by multiple cracks and gaps, resulting in interrupted blocks. Length ranges from meters to kilometers, width up to 300 meters, height from 30 cm to 3 meters. Coastal erosion, from sea level rise or sedimentary deficit, can uncover beachrock; if cementation continues, new beachrock forms in a new intertidal position. Successive sea level changes may produce sequential beachrock zones.
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