Late Ediacaran Boxonia-bearing stromatolites from the Gobi-Altay, western Mongolia

A couplet comprising a Boxonia-bearing stromatolite unit and phosphatic layers occurs widely at the base of the Zuun-Arts Formation in the Zavkhan Terrane of GobiAltay Province, western Mongolia. The stromatolite unit is late Ediacaran in age and forms bioherms of several kilometres in lateral extent. The stromatolites consist of two parts: the lower columnar Boxonia stromatolites (ca. 7 m thick) change abruptly into the upper domed stromatolites (ca. 4.5 m thick) in the Bayan Gol Gorge. The columnar stromatolites are made up of columnar structures (2-6 cm in diameter), formed by the accumulation of upward-convex laminae, and partly protrude laterally to form bridges between neighbouring columns. In contrast, the domed stromatolites (30-60 cm in width) are composed of accumulations of low convex laminae. Both types of stromatolite are characterized by alternating darker and lighter laminae, which consist of peloids, two types of micritic clot, homogeneous lime mud, and spar-filled fenestral fabrics. These stromatolites are inferred to have been deposited in a subtidal setting below the wave base, with no evidence of sediment displacement by strong currents. The stratigraphic transition from columnar to domed growth forms reflects deepening of the basin. The change in column diameter observed within the columnar stromatolites might reflect fluctuations in microbial activity. The appearance of inter-columnar bridges in the columnar stromatolites might indicate more active colonization of microbial mats, which expanded into the inter-columnar areas. Neither type of stromatolite includes microbial remains, such as filamentous or coccoidal cells. However, the various micritic components of the stromatolites (clots, peloids, and homogeneous lime mud) originated from in situ precipitation through microbial activity, and reflected differences in the timing and intensity of microbial calcification and degradation. These stromatolites, including the variety of micritic components, may provide clues to the variations in microbial metabolic activity and degradation processes that were associated with stromatolite formation.