PRODUCTION OF WRINKLE STRUCTURES AND LINEAR TRAILS BY MOVING MICROBIAL AGGREGATES

Wrinkle structures and linear trails abound in late Ediacaran and early Paleozoic sandstones and siltstones. Wrinkle structures are sometimes interpreted as remnants of microbial mats, whereas some of the trails are attributed to early grazing animals. Recently, we have used laboratory experiments to demonstrate a plausible mechanism for the formation of wrinkle structures: both ridged and pitted morphologies are produced by fragments of microbial mats smaller than about 5 mm that move back and forth over a sand bed. These patterns emerge when the flow over the sediment bed cannot move the heavy sediment grains directly, but can mobilize light microbial aggregates. Here, we show that a combination of oscillatory and residual flow can move 10-20 millimeter-wide microbial aggregates and produce elongated surface trails on sand beds. These trails do not penetrate the substrate vertically and resemble various trace fossils that predate the rise of complex animals. Most trails left by microbial aggregates are simple, single, decimeter-long, millimeter-deep and 2-5 millimeter-wide grooves with raised levees. Irregularly shaped microbial aggregates rocking over sand can also produce trails with multiple grooves, or sets of small, regularly spaced circular to ellipsoidal depressions aligned along a curvilinear path. The trails can zig-zag, reverse, curve, intersect with other trails, terminate abruptly and restart few centimeters apart. The shaping of wrinkle structures and trails by interactions between the flow, microbial aggregates and sediment grains casts doubt on the interpretations of some Proterozoic trace fossils as indicators of protistan or early animal locomotion.