BASAL INDUAN (EARLY TRIASSIC) GIANT SPONGE-MICROBIAL BUILD-UPS IN ARMENIA: MICROFACIES ANALYSES AND CARBON ISOTOPE STUDIES

The Transcaucasia area (central Armenia) offers the opportunity to study a distinctive sponge-microbial community development in the aftermath of the end-Permian mass extinction (Permian-Triassic Boundary sponge-microbialites PTBSM): isolated communities started to built vertical edifices up to 12 m high during Griesbachian times (Induan, Lower Triassic), and show two growth phases within a pelagic carbonate ramp. The first sponge-microbial build-ups, which are rich in sponge spicules, co-occur with impressive calcium carbonate crystal fans (CCFs) that vary in thickness between 5 cm to 2 m. A comparison between the δ¹³C values from the PTBSM that formed during the first growth phase and the surrounding sediment revealed a remarkable differences: the δ¹³Cmicrobialite values are up to 2.3‰ more positive than the corresponding δ¹³Csediment values.

During the second sponge-microbial growth phase, numerous thrombolitic domes with specific internal structures were formed. The highest growth reaches a total height of up to 12 m with an 8 m top head diameter. Its asymmetry indicates a steady bottom current condition, which also contributes to the concomitant deposition of thin bedded bioclastic lime-wackestone containing ostracods, foraminifers, gastropods, bivalves as well as ammonoids, and to the embedding of the PTBSM. Within these buildups the δ¹³Cmicrobialite and δ¹³Csediment values are nearly the same, which is a remarkable contrast to the sponge-microbial build-ups that formed during the first microbial growth phase.

Regarding the growth phase duration, instead of a Lower Griesbachian microbialite short existence found in South China as in South Turkey, the Armenian PTBSM growth extends all over the Griesbachian from parvus to krystyni conodont zones, which is at least twice as long.