DETERMINING A FORMATIONAL MECHANISM FOR A MAURITANIAN MESOPROTEROZOIC (1.1 GA) BRECCIA

An anomalous breccia unit exists within the Mesoproterozoic-aged Atar Group on the West African Craton in present-day Mauritania and Algeria. This unit, informally known as the Tawaz Breccia, is unusual because its sedimentary features record a high energy event in what was an otherwise calm, shallow marine, depositional environment. In order to ascribe a depositional mechanism for this unit, fourteen common breccia formational mechanisms (glacial, tsunami, storm, turbidite, debris flow/alluvial fan, rock fall/talus slope, fault, pyroclastic flow, auto-brecciation, karst collapse, evaporite collapse, lithic ejecta, suevite ejecta, and impact tsunami) were selected. Specific characteristics were used to describe these breccias; for example, clast size and angularity, and deposits thickness and morphology, among many others. An extensive literature search and field observations revealed the likelihood for a characteristic to result from a particular breccia generating mechanism.

We performed a series of correlation analyses to evaluate each breccia forming mechanism for its potential match with respect to initial observations of the Tawaz Breccia. Negative correlations were removed, and a frequency distribution histogram was graphed, which allowed the characteristics that were more discriminating between breccia types to be retained, and the least discriminating to be incrementally eliminated with each new correlation analysis.. The outcome was a percent agreement between each known mechanism and the Tawaz Breccia according to their physical characteristics. We also performed correlation analyses between each breccia-generating mechanism, independent of the Tawaz breccia to identify the most distinguishing breccia characteristics overall. These characteristics were then used in a separate correlation analyses between each breccia type and the Tawaz Breccia. For both sets of correlation analyses, regardless of the number of distinguishing characteristics included in the analysis, breccias generated by either an impact or non-impact tsunami correlate best with the Tawaz Breccia. Upcoming geochemical analyses should allow us to further discriminate between endogenic/exogenic tsunami-generating mechanisms.