Marked accumulation patterns characteristic of Lower Devonian stromatoporoid bindstone: Palaeoecological interactions between skeletal organisms and microbes

Abstract

Stromatoporoid bindstones consisting of coarse alternations of both laminar stromatoporoids (“ Aculatostroma” and Syringodictyon) and microbialites (thrombolites and leiolites) are developed in a biohermal limestone of the Lower Devonian Garra Formation at the Catombal section, south of Wellington, New South Wales, Australia. The stromatoporoid “ Aculatostroma” typically shows a thinly laminar and ragged appearance, producing main frameworks having cryptic spaces within them, and Syringodictyon is also very thinly laminar. Thrombolites are composed of an association of peloids and Girvanella, whereas leiolites are made up of dense micrites with intensive microborings. Similar accumulation patterns are repeatedly shown at each observation scale: 1) the main framework-building “ Aculatostroma” and a combination of secondary framework ( Syringodictyon) and microbialites (leiolites and thrombolites) mesoscopically; 2) Syringodictyon and overlying leiolites and thrombolites at optical microscopic level; and 3) skeletons and intercalating dense micrite layers (leiolites) within Syringodictyon itself at SEM level. Microbialite accumulations on “ Aculatostroma” suggest disruptions of skeletal growth, corresponding to periods of harsh habitat environments, whereas the recurrent appearance of Syringodictyon over microbialites suggests somewhat ameliorated conditions. The resurgence of “ Aculatostroma” as a main frame-builder over a combination of secondary framework and microbialites suggests habitat recovery as well as the competitive superiority of this stromatoporoid. Successions such as “ Aculatostroma” – Syringodictyon – leiolites–thrombolites reflect differences in the tolerances of each responsible organism, and correspond to fluctuating environments. The growth history of stromatoporoid bindstones is as follows: 1) the laminar stromatoporoid “ Aculatostroma” first grew laterally, thereby stabilizing bioclastic sediments; 2) the stromatoporoid growths were partly or totally interrupted in relation to deteriorating habitat conditions, where micritization (leiolites with microborings), and encrustation by thrombolites occurred in succession. However, somewhat ameliorated conditions enabled Syringodictyon to temporarily dominate; 3) surviving parts of “ Aculatostroma” again spread vertically and laterally upon the recovery of optimum conditions and; 4) the ceilings of cryptic spaces produced within “ Aculatostroma” are affected by micritization (leiolites) and then encrustation of thrombolites. Repeated accumulations at various scales indicate antagonistic interrelationships within skeletal organisms and between skeletal organisms and microbes within their habitable ranges. These accumulation patterns thus provide invaluable clues for biotic interactions between skeletal frameworks and microbes, biological successions in accordance with microenvironmental changes, and resultant depositional sequences of bindstones.