Punctuated growth of microbial cones within Early Cambrian oncoids, Bayan Gol Formation, western Mongolia

Oxygen bubbles produced during photosynthesis internally deform filamentous cyanobacterial mats, producing distinctive fenestral patterns. Similar textures preserved in ancient microbialites are useful biosignatures when filaments are no longer preserved, but have typically been observed within stromatolites. This study describes bubble-associated fenestrae within oncoids from the early Cambrian Bayan Gol Formation of Mongolia. Fenestrae appear in mm-scale micritic laminae which contain dense accumulations of large (10X300 mu m) filamentous Girvanella microfossils. Many laminae are not spherical, often occurring with one flat side opposite a conical peak. Up to six generations of conical geometry are present, with each cone rotated with respect to the previous peak. We hypothesize that the oncoids experienced intermittent disturbances followed by periods of stasis and vertical growth. During resting periods, we hypothesize that flat areas formed the oncoid resting base and peaked areas the top. The presence of bubble laminae within peaks implies formation in part via entrapment of microbially produced gases. Examples of resting oncoids growing into stromatolites are well known, as well as irregularly laminated oncoids with no cones; the Bayan Gol Formation samples are intermediate between typical spherical oncoids and stromatolites. The preservation of cones also provides evidence for relatively rapid mineralization in the Cambrian ocean, as antecedent microbial tufts would likely have collapsed if disturbed before calcification.