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<p>The Ruvu Basin is one of the Tanzanian coastal basins that developed in response to the breakup of Gondwana and the subsequent opening of the Indian Ocean, a process that occurred from the Permo-Carboniferous to the Middle Jurassic. Previous studies conducted in the region have demonstrated that sea-level fluctuations and terrigenous influxes strongly influenced the deposition of organic-rich sediments. However, this relationship has not yet been systematically investigated within the Ruvu Basin. The present study documents Jurassic organic-rich intervals in the Ruvu Basin and aims to evaluate their organic richness and identify the key factors that controlled their deposition. This was accomplished through analysis of total organic carbon (TOC), sedimentological and mineralogical composition, and spectral gamma-ray data (Th/U ratios and U). Comparison of these parameters with TOC enabled the reconstruction of terrigenous influxes and redox conditions across the Jurassic stratigraphic succession. Geochemical results indicate that the Lower Jurassic strata are organically enriched (TOC &gt; 1 wt%), with an average TOC of 2 wt% and a maximum value of 3.3 wt%. In contrast, the Middle and Upper Jurassic intervals are organically lean (TOC &lt; 1 wt%). Correlation of TOC data with sedimentological, mineralogical, and Th/U ratio data suggests that stratigraphic variations in organic carbon content were primarily governed by fluctuations in terrigenous input and paleo-redox conditions. The elevated TOC values in the Lower Jurassic interval reflect enhanced preservation of organic matter under more reducing conditions, likely linked to the Early Jurassic transgressive phase. Conversely, the Middle and Upper Jurassic intervals are characterized by high terrigenous influxes (clastic content &gt; 70%), indicative of shallow, well-mixed depositional environments that were unfavorable for organic matter preservation, as further supported by Th/U ratios, U concentration and sediment composition and mineralogical data. These findings suggest that although much of the Tanzanian coastal basin was submerged by a marine transgression during the Middle Jurassic, in the Ruvu Basin, this transgression was interrupted by strong terrigenous influxes. These influxes not only diluted organic matter but also introduced oxygenated waters that oxidized it, creating conditions unfavorable for its preservation. This insight is important for understanding the factors governing spatial variations of source rocks within the region, which is critical for predicting areas where hydrocarbons may accumulate.</p>

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Organic carbon-rich Jurassic rocks of the Ruvu Basin, Tanzania: richness, paleoredox conditions, and terrigenous influx insights

  • 期刊论文

  • 发表日期：

    2026年5月14日

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  • John William Gama

摘要

The Ruvu Basin is one of the Tanzanian coastal basins that developed in response to the breakup of Gondwana and the subsequent opening of the Indian Ocean, a process that occurred from the Permo-Carboniferous to the Middle Jurassic. Previous studies conducted in the region have demonstrated that sea-level fluctuations and terrigenous influxes strongly influenced the deposition of organic-rich sediments. However, this relationship has not yet been systematically investigated within the Ruvu Basin. The present study documents Jurassic organic-rich intervals in the Ruvu Basin and aims to evaluate their organic richness and identify the key factors that controlled their deposition. This was accomplished through analysis of total organic carbon (TOC), sedimentological and mineralogical composition, and spectral gamma-ray data (Th/U ratios and U). Comparison of these parameters with TOC enabled the reconstruction of terrigenous influxes and redox conditions across the Jurassic stratigraphic succession. Geochemical results indicate that the Lower Jurassic strata are organically enriched (TOC > 1 wt%), with an average TOC of 2 wt% and a maximum value of 3.3 wt%. In contrast, the Middle and Upper Jurassic intervals are organically lean (TOC < 1 wt%). Correlation of TOC data with sedimentological, mineralogical, and Th/U ratio data suggests that stratigraphic variations in organic carbon content were primarily governed by fluctuations in terrigenous input and paleo-redox conditions. The elevated TOC values in the Lower Jurassic interval reflect enhanced preservation of organic matter under more reducing conditions, likely linked to the Early Jurassic transgressive phase. Conversely, the Middle and Upper Jurassic intervals are characterized by high terrigenous influxes (clastic content > 70%), indicative of shallow, well-mixed depositional environments that were unfavorable for organic matter preservation, as further supported by Th/U ratios, U concentration and sediment composition and mineralogical data. These findings suggest that although much of the Tanzanian coastal basin was submerged by a marine transgression during the Middle Jurassic, in the Ruvu Basin, this transgression was interrupted by strong terrigenous influxes. These influxes not only diluted organic matter but also introduced oxygenated waters that oxidized it, creating conditions unfavorable for its preservation. This insight is important for understanding the factors governing spatial variations of source rocks within the region, which is critical for predicting areas where hydrocarbons may accumulate.