UPDATING THE TAPHONOMIC WINDOW OF WRINKLE STRUCTURE FORMATION; MICROBIALLY-INDUCED SEDIMENTARY STRUCTURES IN JURASSIC TURBIDITES FROM MOROCCO

Wrinkle structures – irregular, millimeter-to-centimeter scale ridges or pits – are microbially induced sedimentary structures attributed to microbial mats. Although microbial mats or biofilms can be found in many environments, macrofauna and bioturbation typically destroy them before lithification. Therefore, wrinkle structures (or runzelmarken) are rare in Phanerozoic and modern marine subtidal environments but are commonly preserved in the Precambrian. Extreme events, such as mass extinctions, can result in depauperate faunas and anachronistic facies that open the taphonomic window for the preservation of wrinkle structures. Previous work identified two environments in which wrinkle structures typically form and are preserved in the absence of bioturbation: intertidal environments or storm-dominated environments of the offshore transition.

Here, we document Early Jurassic wrinkle structures following the Pliensbachian/Toarcian extinction within turbidites of the Tagoudite Formation (Dadès Valley, Central High Atlas Mountains). Shortly after the stage boundary, an influx of terrestrial material poisoned the carbonate platform, resulting in a carbonate crisis and marine extinction. The Tagoudite Fm. was deposited in the aftermath of this collapse, and wrinkle structures are found on bedding planes of rippled sandstones within prodelta deposits that prograded into the basin (earliest Toarcian). Although wrinkle structures are typically indicative of severe biotic depletion, the Tagoudite Fm. wrinkles do not coincide with a significant extinction of grazing taxa. The macrofauna is, however, facies controlled, so we suggest that the sedimentological and hydrographic conditions of the turbidite deposits briefly excluded grazers allowing wrinkle structures to develop. Given the depth of these deposits (~200m), mats were not likely composed of photoautotrophs; instead, biofilms were likely composed of chemoautotrophic and/or heterotrophic microbial communities. Notably, other examples of wrinkle structures have been observed in turbidite deposits from the Cambrian, Silurian, and Middle Jurassic. Therefore, we suggest that wrinkle structures may be more common than previously assumed and turbidites may represent a third taphonomic window for wrinkle structure preservation.