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

Paleoenvironmental settings and assemblage changes of foraminifera and palynomorphs across the eocene-oligocene boundary of Southern Tanzania

A quantitative micropaleontological analysis was performed on outcrop and core samples across a shallow borehole drilled in the southern coastal basin of Tanzania with the aim of characterizing foraminifera and palynomorphs assemblage changes aiming at reconstructing paleoenvironmental settings across the Eocene-Oligocene transition (EOT). The data reveal high diversity and abundance of calcareous benthic foraminifera assemblages in the Late Eocene succession and a decline of their abundance and diversity across the EOT to Early Oligocene. Planktonic foraminifera assemblages were low in abundance and diversity in the Late Eocene succession and decreased through the EOT when most planktonic foraminifera species from Hantkeniide family and Turborotaloiide groups went extinct. Additionally, marine palynomorphs/dinoflagellate dominated the oldest sedimentary succession (Late Eocene). Their abundance and diversity declined towards the EOT to the Early Oligocene while terrestrial palynomorphs (spores and pollens) dominated the youngest succession. The palynomorphs assemblage changes responded rapidly to environmental variations across the Eocene-Oligocene boundary which was associated with a global cooling event. Both foraminifera (i.e. calcareous benthic foraminifera) and palynomorphs assemblages as well as planktonic/benthic ratios indicate that the EOT paleoenvironment settings were compatible with shallow marine of inner to outer shelf environments.Keywords: Eocene-Oligocene, foraminifera, palynomorphs,  paleoenvironment, Tanzani

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

From source to sink in central Gondwana: Exhumation of the Precambrian basement rocks of Tanzania and sediment accumulation in the adjacent Congo basin

Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000 m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200 m below surface. Surface samples were also analyzed. AFT dates range between 317 ± 33 Ma and 188 ± 44 Ma. Alpha (Ft)-corrected AHe dates are between 433 ± 24 Ma and 154 ± 20 Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340–220 Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70 Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9 ± 2 km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes

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

Evolution of the central West Greenland margin and the Nuussuaq Basin : Localised basin uplift along a stable continental margin proposed from thermochronological data

ACKNOWLEDGEMENTS Tim Redfield and a second anonymous reviewer are thanked for their constructive comments. The work con-tained in this publication contains work conducted during a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training(CDT) in Oil & Gas [grant number NEM00578X/1] and is sponsored by the University of Aberdeen.Peer reviewedPublisher PD

The origin of high helium concentrations in the gas fields of southwestern Tanzania

Volatile elements are concentrated at Earth's surface, forming a rich atmosphere and oceans which enabled the eventual emergence of life. However, volatiles are also abundant in solid Earth reservoirs, such as the crust and mantle, and these reservoirs play a key role in moderating volatile movement throughout the planet. Continental cratons represent a potentially large, yet under-constrained volatile reservoir. When cratonic regions are catastrophically disrupted by large volcanic and/or rifting events, they release massive amounts of volatiles into Earth's atmosphere on geologically-abrupt timescales (e.g., Lowenstern et al., 2014; Muirhead et al., 2020). Here, we report gas data (He-Ne-N2-Ar-CO2) from seeps along the flanks of the Tanzanian craton, within the western branch of the East African Rift System (EARS) - a region where the stable continental craton is actively being broken apart by rifting and simultaneously heated by plume-induced volcanism. Bulk gas and noble gas isotopic data are reported in seeps from three regions: 1) the Rukwa Rift Basin (RRB), 2) the Lupa Hydrothermal System (LHS) and 3) the Rungwe Volcanic Province (RVP). Seep gases from the RRB are dominantly comprised of N2 and He, with >90% N2 concentrations, high 4He concentrations (2.4–6.9%) and radiogenic He isotopes (0.16–0.20 RA). Seeps in the LHS - located between RRB and RVP - are characterized by little-to-no N2, high CO2 contents (72–84%), relatively low He contents (0.008–0.15%), and higher 3He/4He (0.95–0.99 RA). RVP gases have high CO2 (78%) and low 4He (0.0003%) and more mantle-like He isotopes (3.27–4.00 RA) consistent with previous findings (Pik et al., 2006; Barry et al., 2013). All neon isotopes can be explained by mixing between air, high O/F crust and depleted Mid Oceanic Ridge Basalt (MORB) mantle-like signatures. RVP neon isotope seep data potentially suggest a solar-like deep mantle contribution, consistent with findings in rocks from the area (Halldórsson et al., 2014), however we note that this signal is difficult to discern from mass dependent fractionation (MDF). The largest 40Ar/36Ar anomalies occur in RRB, with resolvable excess 40Ar derived from radiogenic production in the crust. Using a noble gas solubility model, we calculate volumetric gas to water ratios (Vg/Vw) and show that Vg/Vw values are low for RRB (0.1), consistent with longer migration distances, whereas Vg/Vw are higher for LHS (Vg/Vw = 0.1–10) and RVP (Vg/Vw = 3–12), suggesting a more direct conduit for volatiles from source to surface. In summary, these data demonstrate interaction between two distinct helium sources, one of which is crustal in origin (most prominent in RRB) and the other being mantle-derived (enriched in RVP). The extent of mixing between the two is shown to be influenced by proximity to rift-related fault structures, groundwater interaction and magmatic heat