Mapping of the Geological Structures Using Digital Elevation Model (DEM)-Derived Flow Direction: A Case Study of Rungwe Volcanic Province, Southwest Tanzania

This study aimed to assess the potentiality of Digital Elevation Model (DEM)-derived flow direction in mapping geologic structures, specifically in areas with thick overburden of soil or volcanic materials such as in the Rungwe Volcanic Province, SW Tanzania. Clustering of flow directions based on trends of structure sets found in the study area was applied and it successfully revealed a large number of geological structures through demarcated tectonic blocks from patterns created by flow directions. Northwest–southwest (NW-SE) and NE-SW- trending lineaments are the dominant structural sets in the area whereas N-S and E-W-trending lineaments constitute the minor sets. Moreover, the approach has also unveiled the NW-SE-trending faults that control most of the volcanic centers. The extracted lineaments by the method presented in this research are consistent with the known geological structures and structure surface manifestations such as volcanic centers, hot springs and earthquake epicenters. Findings of this study has revealed that the method can be applied as a robust technique to map crustal structures mostly in early phases of exploration, and has shown good results in delineating faults in areas with thick overburden such as soils and volcanic materials. Keywords: Applied Remote Sensing; Flow Direction; Geological Structures; Rungwe Volcanic Province

Monitoring Impervious Surface Area Dynamics to Assess Urbanisation of a Catchment: Msimbazi River Valley Dar es Salaam, 1989 - 2015

This work examined the effects of land use and cover change on the impervious surface as a measure of urbanisation together with its driving factors such as population growth and economic development. The spatial and temporal variations of the impervious surface area were extracted from the Landsat images of the years 1989, 1995, 2005, and 2015. Data analyses involved the selection of the endmember through Minimum Noise Fraction (MNF) and Linear Spectral Mixed Analysis (LSMA) on the images. Four dominant land cover types were mapped as results, which are forest, non-forest vegetation, bare-land, and built-up area. The non-forest vegetation and bare land were dominant cover classes in the catchment in 1989, occupying over 80% of the land use and cover. The built-up environment increased from 11% in 1989 to 53% in 2015, encroaching other covers. This correlates with the growth of population and gross domestic product as measures of economic development and driving forces for the growth. Keywords: Land use; Impervious Surface Area; Urbanisation; Msimbazi River; Minimum Noise Fractio

Geological Modeling of Hydrocarbon Reservoir Rocks in the Mafia Basin, Offshore Tanzania

This study presents a 3D geological model of reservoir rocks in the Mafia Basin based on interpretation of 2D seismic and well data towards understanding the hydrocarbon prospectivity. 2D seismic data were used to generate surface maps and therefore the subsurface configuration of the reservoir complemented with petrophysical analysis to determine lithology and reservoir properties. Structural and petrophysical properties modeling were distributed stochastically within the constructed 3D grid using Sequential Indicator Simulation (SIS) and Gaussian Random Function Simulation (GRFS) algorithms. Results from well log analysis and petrophysical models classify the reservoir under a moderate reservoir quality with 19% to 20% porosity, 6 - 7 mD permeability and 60% to 65% water saturation. The observed high values of water saturation imply that the hydrocarbon accumulation in the mapped area is insignificant. The reservoir structural model and subsurface configuration shows stratigraphical trap as the only trapping mechanism in the area.. However, 3D seismic and multiple wells are needed for effective correlation of geological information to enhance the structural configuration and lateral continuity of the reservoir.Keywords: Geological modeling; Mafia basin; 2D seismic data; reservoir rock

Rock phosphate and lime for small-scale farming in Tanzania, East Africa

Poor soils are a major cause of poverty in sub-Saharan Africa, and thus restoration of soil fertility is a significant challenge for sustainable agriculture. Some of the main resources required, e.g. phosphate and lime, are present in many African countries and can be used by smallholder farmers in a relatively unprocessed form instead of expensive commercial fertilisers. Here we present a small study of the Mbeya region in Tanzania, which locally has both phosphate and lime. Most soils in sub-Saharan Africa are losing nutrients necessary for sustainable agriculture. This is mainly due to intensive farming and the fact that the nutrients are not replaced adequately. Further reasons for nutrient losses are leaching, soil erosion and fixation by iron and aluminium oxides. Vast areas experience moderate to acute phosphorus deficiency (Vanlauwe & Giller 2006). The Mbeya region in south-western Tanzania (Fig. 1) is characterised by intensive smallholder plots along with several local sources of phosphate-bearing rocks and limestone. The former were examined in the 1980s (Chesworth et al.1988, 1989), but have never been utilised (Kalvig et al. 2010)

A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Maselli, V., Oppo, D., Moore, A. L., Gusman, A. R., Mtelela, C., Iacopini, D., Taviani, M., Mjema, E., Mulaya, E., Che, M., Tomioka, A. L., Mshiu, E., & Ortiz, J. D. A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa. Geology, 48(8), (2020): 808-813, doi:10.1130/G47257.1.The December 2004 Sumatra-Andaman tsunami prompted an unprecedented research effort to find ancient precursors and quantify the recurrence time of such a deadly natural disaster. This effort, however, has focused primarily along the northern and eastern Indian Ocean coastlines, in proximal areas hardest hit by the tsunami. No studies have been made to quantify the recurrence of tsunamis along the coastlines of the western Indian Ocean, leading to an underestimation of the tsunami risk in East Africa. Here, we document a 1000-yr-old sand layer hosting archaeological remains of an ancient coastal Swahili settlement in Tanzania. The sedimentary facies, grain-size distribution, and faunal assemblages indicate a tsunami wave as the most likely cause for the deposition of this sand layer. The tsunami in Tanzania is coeval with analogous deposits discovered at eastern Indian Ocean coastal sites. Numerical simulations of tsunami wave propagation indicate a megathrust earthquake generated by a large rupture of the Sumatra-Andaman subduction zone as the likely tsunami source. Our findings provide evidence that teletsunamis represent a serious threat to coastal societies along the western Indian Ocean, with implications for future tsunami hazard and risk assessments in East Africa.This work was funded by the National Geographic Society (grant N. CP-R008–17). Maselli acknowledges support from the Canada First Research Excellence Fund through the Ocean Frontier Institute. We are extremely grateful to the editor, two anonymous reviewers, J. Bourgeois, G. Eberli, A. Prendergast, and C. Gouramanis for all the suggestions provided, which greatly improved the quality of the manuscript. We would like to thank the United Republic of Tanzania and the University of Dar es Salaam for allowing us to perform the field work activity. This is ISMAR Bologna scientific contribution number 2024

A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa: reply

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Maselli, V., Oppo, D., Moore, A. L., Gusman, A. R., Mtelela, C., Iacopini, D., Taviani, M., Mjema, E., Mulaya, E., Che, M., Tomioka, A. L., Mshiu, E., & Ortiz, J. D. A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa: reply. Geology, 49(1), (2021): E516-E516, https://doi.org/10.1130/G48585Y.1.We appreciate Somerville’s (2020) interest in our work, and the opportunity to further expand the discussion about the occurrence of a trans-oceanic tsunami in the Indian Ocean generated by a megathrust earthquake ~1000 years ago. Somerville suggests a connection between the inferred tsunami deposit presented by us (Maselli et al., 2020) and a tsunami event reported to have occurred in Nagapattinam (India) in the year 900 CE and described in Kalaki Krishnamurty’s book (Rastogi and Jaiswal, 2006)

Rock phosphate and lime for small-scale farming in Tanzania, East Africa

Poor soils are a major cause of poverty in sub-Saharan Africa, and thus restoration of soil fertility is a significant challenge for sustainable agriculture. Some of the main resources required, e.g. phosphate and lime, are present in many African countries and can be used by smallholder farmers in a relatively unprocessed form instead of expensive commercial fertilisers. Here we present a small study of the Mbeya region in Tanzania, which locally has both phosphate and lime. Most soils in sub-Saharan Africa are losing nutrients necessary for sustainable agriculture. This is mainly due to intensive farming and the fact that the nutrients are not replaced adequately. Further reasons for nutrient losses are leaching, soil erosion and fixation by iron and aluminium oxides. Vast areas experience moderate to acute phosphorus deficiency (Vanlauwe & Giller 2006). The Mbeya region in south-western Tanzania (Fig. 1) is characterised by intensive smallholder plots along with several local sources of phosphate-bearing rocks and limestone. The former were examined in the 1980s (Chesworth et al.1988, 1989), but have never been utilised (Kalvig et al. 2010)

Mapping of surface hydrothermal mineral alterations and geological structures related to geothermal systems in the Songwe region, SW Tanzania

Geothermal energy is one of the most valuable and preferable renewable energy sources, given that it produces sustainable, dependable, and clean energy. Understanding the geothermal systems, parameters such as heat, fluids, and structures help to identify potential geothermal systems. These parameters are manifested as hot springs, fumaroles, and alteration minerals. Hydrothermal alteration minerals are often used in geothermal system exploration to evaluate factors such as temperature, fluid, and structures. This study employed GIS and remote sensing in the investigation of hydrothermal mineral alterations and delineating geological structures which provided invaluable information for identifying prospective geothermal resource zones. ASTER was utilised to map phyllic, argillic, and propylitic minerals using SPCA, whereas ISO cluster classification was used to detect kaolinite, smectite, calcite, chlorite, and muscovite/illite-rich areas. DEM was used to extract and map geological lineaments. Results have revealed that the regions that were mapped with indicator hydrothermal alteration minerals were also associated with geological structures. The field work, XRD, and ASD laboratory analysis results indicated a good match with the findings from the remote sensing data. The approach applied in this study may be used along with other techniques during the early phases of geothermal exploration to define geothermal potential targets.</p

Fluid Flow Modeling using Geochemistry to Characterize the Songwe Medium Temperature Geothermal System-Tanzania

A geothermal area with only bicarbonate thermal water discharges at medium temperature requires a more integrated analysis than used in classical geochemical exploration. This signature is typical for steam-heated water, which commonly occurs at the margins of a geothermal system. However, these waters can also rise from carbonate rich layers in the central part of the field. Our study shows that fluid flow modeling can identify the exact source, flow pathways and temperatures of reservoir fluids based on water-rock interaction. For the first time, we present a conceptual geothermal fluid flow model based on geochemical data for the Songwe geothermal system in Tanzania. Thermal springs discharge along NW-SE fracture zones in two separate areas: the central Songwe graben (Iyola, Main springs, Rambo and Kaguri) and eastern Songwe graben (Ikumbi). The discharge temperatures of springs range between 37 and 85 oC with Na-HCO3 type, and carbonate deposits surrounding most of the springs. We estimated fluid temperatures for a depth of 2.5km by applying K-Mg and Na-K-Ca (Mg correction) geothermometers, suggesting that reservoir fluids reach temperatures between 125 and 148 oC. We reconstructed reservoir fluid characteristics for that temperatures and propose oversaturated minerals (volcanics, clays, carbonates, apatites, weathered metamophics and hydrothermal minerals) as a model result of interaction between the deep fluids and certain lithologies. Comparison between the modeled oversaturated minerals with minerals in the springs (calcite, aragonite, analcime, muscovite, and smectite) suggests that Kaguri spring water is a result of interaction between deep reservoir fluids with all lithologies, passed on the way to the surface (Metamorphics, Karoo group and Red Sandstone). The fluid signature of Kaguri springs suggest an upflow zone of the geothermal system. Further, our model with oversaturated minerals shows that the thermal water from the reservoir flows laterally along the Red Sandstone layer to the eastern part of study area. It appears as Rambo springs, south of Kaguri springs, and as Main springs and Iyola to the west. The outflow zone might be continuing towards Ikumbi springs, where the fluids also interact with volcanic units. The proposed model shows that carbonate dissolution from the Red sandstone layer is the most common water-rock interaction. The carbonate is embedded in pores and fractures and occurs as matrix in the sandstone. The water-rock interaction is dominated by HCO3- and Na and seen in carbonate depositions at all springs