Bedding and bedding surfaces in carbonate and mixed carbonate–siliciclastic successions part 2: processes, identification and implications of diagenetic bedding

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Bedding and bedding surfaces in carbonate and mixed siliciclastic–carbonate successions can be diagenetic, not depositional, in origin and require interrogation to determine their true origins. This study reviews the different models and implications associated with diagenetic bedding and diagenetic bedding surfaces in such successions. Weathering, especially where strong lithological and mechanical differences occur, can make distinguishing primary and diagenetic bedding more difficult, and criteria are reviewed. Early, shallow diagenetic processes linked to mineral instability of aragonite, mediated by biochemical processes, are capable of producing bedding and bedding surfaces. Zones of early cementation within sediments can produce discrete beds and surfaces exemplified in the formation of limestone–marl alternations (LMAs). Variations in the degree of early cementation (‘stratified cementation’) in shallow buried argillaceous carbonate sediments affected by later compaction can produce layering, with beds showing little compaction separated by highly compacted, argillaceous fissile interbeds which develop bedding surfaces within those fissile units (‘couplets’). Where such diagenetic limestone beds form intrastratally, as in LMAs, they do not obey the principle of superposition and, by virtue of never having been deposited laterally adjacent to their associated deposits, do not comply with Walther's Law of Facies. Bedding surfaces associated with LMAs only become true surfaces when exhumed to produce some types of firmgrounds and hardgrounds.