INVESTIGATING THE STRATIGRAPHIC ARCHITECTURE OF A POTENTIAL TSUNAMI DEPOSIT

The “Blue Bed” is an unusual sedimentary deposit that occurs in Mesoproterozoic (~1.1 Ga) strata of the Atar Group, Mauritania. This 2 to 6 meter-thick deposit set atop a thin (<30 cm thick) interval variably displaying ball-and-pillow structures and completely fluidized sediment; the main body of the deposit consists of amalgamated intervals of plastically deformed strata, breccia, and thin intervals of cross-bedded grainstone. The “Blue Bed” can be traced for >20 km southeast of the town of Atar, and has been recognized in the interdune region, more than 150 km north of Atar. Similar deposits also occur in coeval strata near the Mauritania-Algeria border, more than 1000 km northeast of Atar. These deposits have previously been interpreted as a tsunami deposit, potentially deriving from an extraterrestrial impact (Kah et al., 2007).

Here we explore the stratigraphic architecture of drill core that preserved deposition of the “Blue Bed” in the far interior reaches of the craton, using hand sample inspection, petrographic, and cathodoluminescence imaging. In these environments, we expect that individual tsunami waves would have interacted with the coastal paleoenvironment. Our drill core preserves a 5.29 meter thick deposit that consists primarily of 6 facies, including: (1) fine-grained detrital carbonate sediment with cm-scale bedding [often hosting enigmatic “molar-tooth” structures], (2) thin, often deformed deposits of black shale and silt; (3) coarse breccias consisting of ripped up and redeposited molar-tooth fragments; and (4) fine-grained breccias with more rounded detrital clasts. By mapping out the stratigraphic occurrence of these facies, we find that the core records 20 transitions between different inferred water energies. Whereas low-energy deposits occur in 9 distinct intervals, high-energy breccias occur 11 distinct intervals, and represent 62% of the core thickness, suggesting that multiple high energy events dominated “Blue Bed” deposition.