A revised classification and terminology for stacked and amalgamated turbidites in environments dominated by (hemi)pelagic sedimentation

Abstract

Stacked or amalgamated turbidites provide an opportunity to infer the synchronous triggering of multiple slope failures, which is a criterion often used to attribute these slope failures to earthquake shaking; and such turbidites are thus a proxy for reconstructing long-term earthquake recurrence. However, other processes, such as erosion, reflecting turbidity currents and seiching, may produce similar amalgamated/stacked deposits. Here we study two turbidites from Lake Challa, a crater lake on the lower slopes of Kilimanjaro (Kenya/Tanzania). The occurrence in Lake Challa of both single slope failures and basin-wide landslide events, all accompanied by distal turbidites, provides an excellent opportunity to assess the characteristics and significance of amalgamated/stacked turbidites in an enclosed lake basin with diatomaceous sediments, reflecting hemipelagic sedimentation in offshore areas. We also compare the characteristics of amalgamated/stacked turbidites in basins other than Lake Challa to discuss potential causes of different amalgamation patterns (stacked or multi-pulsed character). The low density and elongated shape of diatom frustules increases grain-to-grain interaction and thereby damps turbulence, resulting in faster bed aggradation and a stacked character of the amalgamated turbidites. Finally, as currently both synchronously and non-synchronously triggered turbidites are in literature referred to as “stacked turbidite”, we propose a revised terminology that differentiates an “amalgamated turbidite” from a “turbidite stack”. In sedimentary environments that are dominated by (hemi)pelagic sedimentation, and where turbidity currents are anomalous events, an “amalgamated turbidite” can often be shown to be the result of synchronous triggering, while a “turbidite stack” must always result from a succession of discrete events.