Composite micrite envelopes in the lagoon of Abu Dhabi and their application for the recognition of ancient firm- to hardgrounds

Micrite envelopes are common early diagenetic features in marine carbonates throughout the Phanerozoic. This paper documents a continuum of (sub)Recent, constructive and destructive (composite) micrite envelopes and discusses their application for the recognition of ancient marine firm- to hardgrounds. In the modern subtropical carbonate depositional environment of the Abu Dhabi coastal lagoon, micrite envelopes are actively forming and are directly assigned to a range of processes and depositional environments: (i) in the outer, high-energy lagoon, bioclasts and ooids typically contain empty microendolithic borings; (ii) in the unconsolidated sediments of the inner-to-middle lagoon, destructive micrite envelopes yield fragments of their host grains that result from boring/etching activity, and are bound and covered by biofilms; (iii) constructive envelopes are formed by high-Mg calcite micrite cements and coat destructive envelopes in the inner-to-middle lagoon, or may nucleate on aragonite needle cements in the outer lagoon. Micrite cements that form constructive envelopes are typical early marine diagenetic features and are among the main fabrics of firm- to hardgrounds. Calcitic skeletal grains, such as benthic foraminifera and coralline red algae, are less affected by microbioeroders compared to aragonitic ooids, gastropods and bivalves. High-Mg calcite constructive envelopes have a higher preservation potential relative to aragonite host particles and aragonite cements. Hence, the presence and recognition of constructive micrite envelopes in the rock record may be instrumental to identifying ancient aragonitic as well as unconventional firm- to hardgrounds that may otherwise be difficult to recognise.