Bio-mediated cementation of supratidal beach sediments associated with groundwater springs

The formation of beachrock, sensu stricto, via carbonate precipitation in the intertidal zone is widespread throughout the tropics and subtropics. While cementation of supratidal beach sediments has also been noted in several locations, it has received much less attention. Here, we report cementation of upper beach sediments promoted by discharge of mineralized groundwater springs and the associated development of microbial mats and biologically mediated tufa. These fundamentally supratidal deposits form at the co-occurrence of (a) unconsolidated clastic beach deposits, (b) carbonate-enriched groundwater and (c) bedrock or a stable substrate, where biological activity variably mediates precipitation of calcium carbonate cement. Analysis of sites in Ireland, Australia, South Africa and Scotland reveals commonalities and some differences that yield insights into the nature of cemented beach deposits and the cementation process. At some sites, primarily vadose, meteoric cementation occurs through the infiltration of beach sediments by groundwater, enabled by a stable substrate or bedrock and the infrequent mobility of back-beach deposits. At other sites, cementation by surficial biofilms or microbial mats forms microbial boundstone laminae, with mobile sediment producing multiple beds of cemented beach deposits and microbial boundstone in a multi-bed deposit. The differentiation of beachrock sensu stricto and the aforementioned groundwater spring-cemented beach deposits is key in establishing their utility as shoreline and associated sea-level indicators.