Diversity of bahamian microbialite substrates

Abstract

Stromatolites, laminated columnar and branched structures of limestone and dolomite, are the only macroscopic evidence of life for the first few billion years of earth history. These organo-sedimentary structures are a prominent constituent of Precambrian carbonate successions and occur sporadically throughout the Phanerozoic. They are hosts for metallic ores and serve as reservoir rocks for hydrocarbons. Still living Bahamian columnar forms that are counterparts of ancient microbialites (stromatolitic and thrombolitic) provide a special opportunity to examine if their substrates played a role in determining the occurrences and patterns of these remarkable structures. The cyanobacterial builders of Bahamian stromatolites can colonize almost all substrates except mobile sands. The development of columnar structures with significant relief however, requires either a hard or firm substrate. From published reports on substrates and our own observations we recognize two families of substrates: inherited, consisting of pre-existing rock surfaces and renewable, including all substrates that can develop repeatedly during accumulation. Inherited substrates in the Bahamas include Pleistocene limestone with or without palimpsest encrustations of caliche or paleosol. Renewable substrates in the marine environment include syndepositional hardgrouds, large skeletons of corals and mollusks, encrustations of coralline algae or vermetid gastropods, and firm grounds of fine-grained carbonate sediment. Recognizing the key roles of renewable substrates in determining the occurrences and age variations of modern Bahamian specimens emphasizes the need for increased attention to the foundations of microbialites in future studies.