THE ORIGIN OF ROLL-UP STRUCTURES AND SYNDEPOSITIONAL FOLDS IN A STURTIAN CAP CARBONATE, THE RASTHOF FORMATION, NORTHERN NAMIBIA

The Rasthof Formation of northern Namibia is a carbonate depositional sequence characterized by dark gray limestones and dolostones that formed during post-glacial transgression and highstand following the Sturtian Chuos glaciation. The lower Rasthof Formation consists primarily of thinly (<mm) and thickly (1—4mm) laminated microbial facies that exhibit different rheological responses to the emplacement of syndepositional dikes. The thinly laminated microbialaminite facies host cm—sized syndepositional folds of formerly cohesive sediment commonly referred to as roll-up structures. In more thickly laminated and cement-rich facies, layers are deformed into broad decimeter-sized folds, but roll-up structures are absent. Large syndepositional carbonate clastic dikes (0.5—1m wide) and smaller veins (0.1—0.5m) that contain convoluted microbial mats and abundant early marine cements cut across bedding in both the thinly and thickly laminated facies, but are absent from underlying and overlying beds. The lack of evidence for wave or current influence in the form of bedforms, intraclasts, or scour marks indicates that these microbialaminites formed below storm wave base.

Carbonate clastic dikes cross-cut the deep-water microbialaminite facies. We propose that the emplacement of the dikes, most likely due to the release of fluid into partially lithified mats, was co-genetic with broad folding in the thick microbialaminite facies and the rolling of the thinly laminated microbialaminites into smaller roll-up structures. Microbialaminite facies in the Rasthof Formation reflect the depositional processes that operated in deep water settings in the aftermath of the Sturtian glaciation and may also provide insights into the formation of roll-up structures during other intervals of Earth history.