Alkaline lake systems with stromatolitic shorelines in the Ediacaran volcanosedimentary Ouarzazate Supergroup, Anti-Atlas, Morocco

Abstract

In the Zenaga and Saghro inliers of the Anti-Atlas, Morocco, the Ediacaran Ouarzazate volcanosedimentary Supergroup preserves a record of the complex interplay between effusion of lava floods and explosive volcanic activity from numerous separate volcanic centres. Volcanic units consist of basaltic and andesitic, lava flows and subvolcanic sills and dikes, and dacitic/rhyolitic ignimbrites and tuffs that represent a post-collisional, high-K calc-alkaline magmatism. Volcanic events are separated by erosive and low-angle unconformities (related to regional tilting), which mark significant hiatii in the timing of deposition and are locally marked by carbonate production in palaeoweathered lowlands. Infill of scattered, decametre-scale, carbonate-bearing troughs, less than 1 km across, started with the dominantly local derivation of weathered and eroded lava debris, followed by the migration of scattered subaqueous bottomset and foreset structures, and final nucleation of carbonate productivity. Microbial mats developed along the shorelines of these ponds. When bathymetry increased the stromatolites developed domal to columnar morphologies that coalesced laterally to form metre-thick biostromes. The lack of evidence for significant fluvial and deltaic deposits suggests that the streams that entered the ponds were intermittent and thus not a significant source of water. It appears that groundwater, rather than surface water, was the dominant source of water to sustain the ephemeral ponds in the low-lying weathered palaeodepressions preserved in the Ouarzazate Supergroup. Fluids passing through the surrounding volcanic landscape and the volcanic ashes interbedded within the lacustrine infill supplied silica-enriched fluids, giving the ponds the characteristics of alkaline lake systems. Their lacustrine affinity is supported by the subaerial character of many of the volcanic products that surround them. REE composition of partly silicified, dolomitized and undolomitized stromatolites is entirely consistent with freshwater (lacustrine) or shallow lagoonal (with a strong freshwater input) depositional settings, but they also display a strong contamination from the rhyolitic tuffs that overly them.