The term stromatolite: towards an essential definition

Kalkowsky regarded stromatolites as laminated and microbial. Recognition of unlaminated microbial deposits, and also of biogenicity problems, subsequently resulted in broader definitions of the term stromatolite: (i) microbial but not necessarily laminated, and (ii) laminated but not necessarily organic. The first of these definitions does not discriminate between microbial deposits with markedly differing macrofabrics (such as thrombolite, dendrolite, etc.). The second purposely disregards origins and would apply the term stromatolite to both inorganic and organic deposits. Subsequent recognition of cognate terms (thrombolite, dendrolite, leiolite), and the umbrella term microbialite, has not resolved the question of stromatolite definition. Consequently, at least three long-standing definitions of stromatolite are available. These respectively emphasize the following features: (i) laminated and microbial, (ii) just microbial, (iii) just laminated. It is proposed to stabilize usage by adopting Kalkowsky's key points of laminated and microbial, supplemented by the adjective benthic, as expressed in the definition: ‘a stromatolite is a laminated benthic microbial deposit’. This definition excludes non-laminated microbial deposits (e.g. thrombolites) that may have had a different accretion history, and also abiogenic laminites. Doubt concerning biogenicity can be expressed by the descriptors ‘probable’ and ‘possible’ stromatolite. The alternative to stipulating a microbial origin for stromatolites would defer, but not ultimately avoid, the key question of their origin.