Evidence for a Record of Possible Paleo-Tsunami or Storm Deposits in the Fluviatile Neoproterozoic Malagarasi Supergroup

Abstract

Two different localities within the fluviatile Neoproterozoic Malagarasi Supergroup of north-western Tanzania show striking evidence for tsunami related clastic sedimentary deposits. In each locality, the outcrops are characterised by the association of fine grained, thinly laminated shales at the bottom that are overlain by thick deposits of sandstones and conglomerates whose clasts and pebbles vary in size and angularity/roundness. In each case, the two rock units are separated by an erosional surface. The basal shale layers are consistent with deposition in deep shelf environments which are in stark contrast to the immediately overlying conglomerates/sandstones that suggest reworking under high energy conditions. The consistent association of erosional surfaces coupled with the deposition of adjacent low and high energy facies are interpreted as a result of an ancient earthquake triggered tsunami or storm that abruptly changed the depositional energy at the two localities. We propose that a backwash wave transported pebbles and sediments from the shore setting towards the basin interior depositing them on the shale units. Given the limited preservation of such unusual sedimentological deposits in ancient terranes, these two localities in the Neoproterozoic Malagarasi basin provide information on the effects of tsunami or storm impacts in Precambrian basins of Tanzania.

Keywords: Malagarasi Supergroup; Tsunami deposits; Shales; Sandstones; Conglomerates.