This paper concerns the heavy mineral composition of Jurassic, Cretaceous and Paleogene sedimentary successions in the Mandawa Basin. The nature of the heavy mineral assemblages in 38 samples was investigated to assess changes in sediment provenance through time. Further, the geochemistry of detrital amphiboles and garnets and the detrital zircon populations were used to determine possible sediment source terranes. Based on the heavy mineral compositions the sandstones were grouped into four heavy mineral assemblages: garnetdominated, amphibole-dominated, epidote-dominated and zircon-dominated. Garnet-dominated sandstones are abundant in most Middle Jurassic to Middle Eocene samples and represent the main sediment input into the Mandawa Basin. Amphibole-dominated sandstones occur in a few Lower Cretaceous samples deposited in close proximity to their sediment source in the Masasi Spur area. A change in provenance is observed in the epidotedominated sandstones of Middle Eocene and Early Oligocene age. This change coincides with a climatic shift towards a wetter and cooler climate associated with an uplift phase in East Africa. The detrital zircon population in the investigated samples share the same age fractions and are indistinguishable within analytical error. Mineral chemistries and zircon ages imply that the sediments deposited in the Mandawa Basin were mainly derived from several high-grade sources within the Neoproterozoic Mozambique Belt to the west of the basin.

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The shallow-marine Upper Jurassic–Lower Cretaceous sedimentary successions of the Mandawa Basin, coastal Tanzania, are located about 80 km away from the offshore gas discoveries of Block 2, Tanzania. In this paper we present petroleum geochemical data, including bitumen extracted from outcrop samples which are relevant to the understanding of the onshore ‘Petroleum System’ and possibly also to the offshore basin. Despite some biodegradation and weathering, common to all outcrop samples, most bitumen samples analysed contain mature migrated oil. The maturity span of geomarkers (C13–C15 range) covers the entire oil and condensate/wet gas window (Rc = 0.7–2% Rc, where Rc is the calculated vitrinite reflectance), with the biomarkers generally indicating the oil window (Rc = 0.7–1.3% Rc). This suggests that the bitumen extracts represent several phases of migrated oil and condensate, which shows that the samples are part of an active or recently active migration regime or ‘Petroleum System’. The source-rock facies inferred for the bitumen is Type II/III kerogen of siliciclastic to carbonate facies. This is oil-prone kerogen, typical for a marine depositional system with an influx of proximal-derived terrigenous material blended in with in situ marine algal organic matter (OM). Application of age-specific biomarkers such as the C28/C29-steranes, extended tricyclic terpane ratio (ETR), nordiacholestanes and the aromatic steroids suggest that more than one source rock have contributed to the bitumen. Possible ages are limited to the Mesozoic (i.e. excluding the Late Paleozoic), with the most likely source rock belonging to the Early Jurassic. More geochemical and geological studies should be undertaken to further develop the general understanding of the petroleum system of the Mandawa Basin and its implications to the ‘Petroleum Systems’ both offshore and onshore. This paper also presents a reinterpretation of published gas composition and isotope data on the Pande, Temane and Inhassoro gas fields (Mozambique) with implications for possible oil discoveries in the gas-dominated region.

The Mandawa Basin of Coastal Tanzania is located onshore, adjacent (about 80 km offshore) to giant offshore gas discoveries in Lower Cretaceous to Miocene formations. The Mandawa Basin Project is a research and educational project organised between the Universities of Oslo and Dar Es Salaam, the Tanzania Petroleum Development Corporation (TPDC) and Statoil (Tanzania).