EVIDENCE FOR PUNCTUATED GROWTH OF MICROBIAL CONES WITHIN EARLY CAMBRIAN ONCOIDS, BAYAN GOL FORMATION, GOBI-ALTAI, MONGOLIA

In modern cyanobacterial mats, fenestrae separated by thin, hourglass-shaped structures can result when filament bundles encompass oxygen bubbles created during photosynthesis. Similar “hourglass structures” are preserved in some ancient stromatolites, providing a useful tool for evaluating biogenicity. Here we report similar structures within unique growth patterns in oncoids from the Bayan Gol Formation (early Cambrian) of Mongolia, and discuss how these patterns indicate the processes of oncoid formation.

The hourglass structures appear in mm-scale micritic laminations which commonly contain dense accumulations of large (10x300 μm) filamentous microfossils (e.g., Girvanella). The laminae are not spherical; instead, they form multi-mm scale bundles with a flat region on one side and a conical region on the other. Starting from the center and working outward, up to six generations of flat-bottom/peaked top layers are present, with each peak rotated with respect to the underlying peak. We hypothesize that the flat areas formed the base of the oncoid when it came to rest, and the peaked areas the top. Our interpretation is that the oncoid did not roll around continuously, but rather flopped over at times, followed by some period of stasis when the structure grew vertically before another disturbance occurred. The presence of hourglass structures within these peaks implies that the structures formed in part via entrapment of microbially-produced gases.

The formation of microbial filament cones on multiple layers of the Bayan Gol oncoids indicates that motion was intermittent, allowing for upward growth during longer calm periods. Examples of oncoids coming to rest and growing into stromatolites are well known, as well as irregularly laminated oncoids with no cones; the Bayan Gol Formation example is intermediate between typical spherical oncoids and stromatolites. The preservation of these cones also provides evidence for relatively rapid mineralization, as the antecedent microbial tufts would likely have collapsed if disturbed before calcification.