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

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; Conglomerate

Geochemistry of Mafic-Intermediate Intrusive Rocks from the Twangiza-Namoya Gold Belt, Eastern Democratic Republic of Congo: Trace Element Constraints on their Origin, Petrogenesis and Tectonic Setting

Bulk geochemical data for the Twangiza-Namoya Gold Belt, Democratic Republic of Congo, are presented in order to classify the rock assemblages, elucidate their petrogenesis and tectonic settings. Nb/Yb and Zr/Ti constraints reveal two suites for the rocks: sub-alkaline to andesitic basalts (Suite-1); and alkaline basalts (Suite-2).  Ratios of Ti/Zr = 22–70; Zr/Hf = 35–42; Nb/Ta = 12–13; Nb/Th = 1–2 and La/Nb = 2.4–3.8 suggest arc-generated mantle melts with crustal inputs and arc-fluids metasomatism for Suite-1. The samples have positive LILE and negative Nb, Ta, Ti anomalies, enrichments in the LREE over middle and HREE (La/Yb)CN = 3.6–8.9 with negative Eu anomalies (0.7–0.9) in Chondrite-normalized diagrams. Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest active continental margin settings, whereas Ta/Hf-Th/Ta and Nb/Yb-Th/Yb diagrams suggest a depleted MORB source. Ti/Zr, Zr/Hf, Nb/Ta, Nb/Th and La/Nb constraints for Suite-2 suggest MORB and oceanic plateau basaltic sources. REE patterns show enrichments in the LREE with negative to positive Eu anomalies (0.7– 1.3). Yb-Th/Ta and Ta/Yb-Th/Yb ratios suggest a within plate tectonic setting. It is suggested that Suite-2 rocks were sourced from mantle-derived melts with a composition between an enriched MORB to OIB. Keywords: Twangiza-Namoya; Petrogenesis; Tectonic setting; Trace element ratio

Constraining the Origin and Age of the Thermal and Cold Water in the Lake Natron Basin, Northern Tanzania

Springs on the eastern and western shores of Lake Natron Basin (LNB), located within the eastern branch of the East Africa Rift System (EARS) in Northern Tanzania had a discharge temperature that ranged between 34.0 °C and 51.2 °C, while the pH varied from 8.0 to 10.7. The electrical conductivity (EC) ranged between 5,007 µS/cm and 49,200 µS/cm. Cold waters had a temperature of 31.9 °C to 32.5 °C, while the pH ranged between 8.0 and 8.3, and the EC ranged between 1,401 µS/cm and 3,806 µS/cm. The stable isotope composition varied between -2.4 ‰ and -5.3 ‰ for δ18O, and -15.5 ‰ to -29.3 ‰ for δ2H. The isotopic composition of thermal and cold water of LNB indicates a significant contribution of meteoric water in the recharge of the hydrothermal system. However, thermal water is affected by evaporation, water-rock interaction, carbon dioxide (CO2) exchange and condensation processes. Tritium analysis indicated that the spring water in the LNB hydrothermal system has a residence time of more than 50 years. Keywords:    thermal water; Lake Natron Basin; stable isotopes; springs. &nbsp