Browsing by Author "Stevens, Nancy J."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Application of U–Pb detrital zircon geochronology to drill cuttings for age control in hydrocarbon exploration wells: A case study from the Rukwa Rift Basin, Tanzania
    (The American Association of Petroleum Geologists, 2017-02) Hilbert-Wolf, Hannah; Roberts, Eric; Downie, Bob; Mtelela, Cassy; Stevens, Nancy J.; O’Connor, Patrick
    Precise dating and correlation of drilled wells through continental successions is challenging for hydrocarbon exploration, especially where preservation and recovery of age-diagnostic fossils is poor. As a complement or alternative to biostratigraphic dating we demonstrate the effectiveness of U–Pb geochronology via laser ablation–inductively coupled plasma–mass spectrometry on detrital zircon from well cuttings. In basins with syndepositional volcanic input, the youngest zircons in a stratigraphic interval can refine and serve as a proxy for the age of deposition. We demonstrate the reliability of this technique when applied to hydrocarbon exploration wells by analyzing drill cuttings through a continental interval of the Galula-1 well in the Rukwa Rift Basin, East African rift system, Tanzania, which previously yielded conflicting biostratigraphy results. The lower third of the well section reveals a late Miocene to Pliocene up-hole younging trend in the youngest detrital zircon populations, which matches new radioisotopic ages on volcanic tuffs from a correlative outcrop section. This is followed by an interval with recycled young zircons, followed by a zircon-free interval, interpreted to correspond to changes in magma composition of the nearbyRungwe volcanic province. This study provides the first radioisotopic age constraints for the Lake Beds in the Rukwa riftand demonstrates that sedimentation in the basin began by 8.7 Ma, critical for burial and thermal history modeling and establishing the probability of a working hydrocarbon system. Correspondence in age and zircon preservation between well and outcrop samples from the same intervals provides strong support for applying U–Pb detrital zircon geochronology to well cuttings, as a rapid, inexpensive approach for hydrocarbon exploration.
  • Thumbnail Image
    Item
    Oligocene Termite Nests with In Situ Fungus Gardens from the Rukwa Rift Basin, Tanzania, Support a Paleogene African Origin for Insect Agriculture
    (2016-06-22) Roberts, Eric M.; Todd, Christopher N.; Aanen, Duur K.; Nobre, Tânia; HilbertWolf, Hannah L.; O’Connor, Patrick M.; Tapanila, Leif; Mtelela, Cassy; Stevens, Nancy J.
    Based on molecular dating, the origin of insect agriculture is hypothesized to have taken place independently in three clades of fungus-farming insects: the termites, ants or mbrosia beetles during the Paleogene (66–24 Ma). Yet, definitive fossil evidence of fungus-growing behavior has been elusive, with no unequivocal records prior to the late Miocene (7–10Ma). Here we report fossil evidence of insect agriculture in the form of fossil fungus gardens, preserved within 25 Ma termite nests from southwestern Tanzania. Using these well-dated fossil fungus gardens, we have recalibrated molecular divergence estimates for the origins of termite agriculture to around 31 Ma, lending support to hypotheses suggesting an African Paleogene origin for termite-fungus symbiosis; perhaps coinciding with rift initiation and changes in the African landscape.
  • Thumbnail Image
    Item
    Sedimentology and paleoenvironments of a new fossiliferous late Miocene-Pliocene sedimentary succession in the Rukwa Rift Basin, Tanzania
    (elsevier, 2017-01) Mtelela, Cassy; Roberts, Eric M.; Hilbert-Wolf, Hannah L.; Downie, Bob; Hendrix, Marc S.; O’Connor, Patrick M.; Stevens, Nancy J.
    This paper presents a detailed sedimentologic investigation of a newly identified, fossiliferous Late Neogene sedimentary succession in the Rukwa Rift Basin, southwestern Tanzania. This synrift deposit is a rare and significant new example of a fossiliferous succession of this age in the Western Branch of East Africa Rift System. The unit, informally termed the lower Lake Beds succession, is late Miocene to Pliocene in age based on cross-cutting relationships, preliminary biostratigraphy, and U-Pb geochronology. An angular unconformity separates the lower Lake Beds from underlying Cretaceous and Oligocene strata. Deposition was controlled by rapid generation of accommodation space and increased sediment supply associated with late Cenozoic tectonic reactivation of the Rukwa Rift and synchronous initiation of the Rungwe Volcanic Centre. The lower Lake Beds, which have thus far only been identified in three localities throughout the Rukwa Rift Basin, are characterized by two discrete lithologic members (herein A and B). The lower Member A is a volcanic-rich succession composed mostly of devitrified volcanic tuffs, and volcaniclastic mudstones and sandstones with minor conglomerates. The upper Member B is a siliciclastic-dominated succession of conglomerates, sandstones, mudstones and minor volcanic tuffs. Detailed facies analysis of the lower Lake Beds reveals various distinctive depositional environments that can be grouped into three categories: 1) alluvial fan; 2) fluvial channel; and 3) flood basin environments, characterized by volcanoclastic-filled lakes and ponds, abandoned channel-fills and pedogenically modified floodplains. Member A represents a shallow lacustrine setting filled by tuffaceous sediments, which grade up into a system of alluvial fans and high-energy, proximal gravel-bed braided rivers. An unconformity marks the contact between the two members. Member B shows an upward transition from a high-energy, gravel-bed braided river system to a sandy braided river system with increasingly abundant floodplain deposits and well-developed paleosols. Vertebrate fossils are sparse in member A, but common in member B, preserved both within pedogenic soil horizons and as isolated elements and microsites within fluvial channel facies associations. Faunal remains include fishes, turtles and crocodylians, along with well-preserved mammal cranial and post-cranial remains. In addition, freshwater gastropod shells are locally present in member A and continental trace fossils, including abundant fossilized termite nests, are present in both members