Pleistocene aragonite crust diagenesis mimics microbialite fabrics (Danakil Depression, Ethiopia)

Abstract

Fibrous aragonite crusts occur in two consecutive Pleistocene successions in the Danakil Depression (Afar, Ethiopia). Lateral transitions between pristine and altered fibrous aragonite crusts document changes in texture associated with diagenesis. Crusts formed as essentially abiotic seafloor precipitates at the transition from marine to evaporitic conditions. Diagenesis started with the dissolution of aragonite fans at the interface between single fans in non-laminated crusts and along lamination planes in isopachous, irregular, or crudely laminated crusts. Incomplete dissolution resulted in the development of secondary porosity within a matrix of undissolved aragonite fibers. Subsequently, the porosity was filled with calcite that systematically encased remaining aragonite crystals. This was followed by the dissolution of remnant aragonite fibers, producing a network of elongated inter- and intracrystalline pores that were eventually filled with low-Mg calcite. The stepwise substitution of fibrous aragonite by low-Mg calcite resulted in sparry, sparry-cloudy, sparry-micritic (including clotted micrite), and peloidal textures, which obscure the fibrous nature of the original deposits. Stable C- and O-isotope compositions suggest that early diagenesis was driven by meteoric and evaporative fluids. These observations unequivocally demonstrate destructive diagenesis, resulting in secondary textures, which mimic micritic and grumous (peloidal and clotted) textures associated with sparry microfabrics.This suggests that these textures, classically interpreted as primary microbial precipitates and used as evidence of biogenicity in ancient microbialites, might be diagenetic products in some cases, even though at some stage, microbial processes and/or degradation of organic matter could have been involved in the diagenetic process.