Browsing by Author "Downie, Bob"

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    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.
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    Combining detrital geochronology and sedimentology to assess basin development in the Rukwa Rift of the East African Rift System
    (EGU General Assembly 2015, 2015) Hilbert-Wolf, Hannah; Roberts, Eric; Mtelela, Cassy; Downie, Bob
    We have employed a multifaceted approach to sedimentary provenance analysis in order to assess the timing and magnitude of tectonic events, sedimentation, and landscape development in theWestern Branch of the East African Rift System. Our approach, termed ‘Sedimentary Triple Dating’, integrates: (1) U-Pb dating via LA-ICPMS; (2) fission track; and (3) (U-Th)/He thermochronology of detrital zircon and apatite. We integrate geochronology, thermochronology, and provenance analysis to relate the initiation of rifting events to regional dynamic uplift, sedimentation patterns, and interpret the far-reaching climatic and evolutionary effects of fluctuating rift flank topography in the Rukwa Rift, a segment of theWestern Branch. This work provides additional data to support the recent concept of synchronous development of the Western and Eastern branches of the East African Rift System 25 Ma, and better constrains the age, location and provenance of subsequent rifting and sedimentation events in the Rukwa Rift Basin. Investigation of well cuttings and outcrop samples from the Neogene-Recent Lake Beds Succession in the Rukwa Rift Basin revealed a suite of previously unrecognized tuffaceous deposits at the base of the succession. A population of euhedral, magmatic zircons from a basal Lake Beds tuff and Miocene-Pliocene detrital zircons from well cuttings suggest that Neogene rift reactivation and volcanism began 9-10 Ma. This timing is consistent with demonstrated rifting in Uganda and Malawi, as well as with the initiation of volcanism in the Rungwe Volcanic Province at the southern end of the Rukwa Rift, and the estimated development of Lake Tanganyika to the north. Moreover, there appear to be a suite of unconformity bounded stratigraphic units that make up the Lower Lake Beds succession, and detrital zircon maximum depositional ages from these units suggests episodic sedimentation in the rift, punctuated by long hiatuses or uplift, rather than steady subsidence and sedimentation. A distinct, upward-younging trend in detrital zircon populations associated with each stratigraphic interval suggests that volcanism was also episodic through the Late Miocene-Pliocene, and linked to periods of rifting and basin filling. Detrital zircon populations are dominated by Paleoproterozoic grains of the same age as the metamorphic Ubendian Belt that underlies the rift basin and forms the flanks. This provenance, volcaniclastic-dominated sedimentation, and clasts from the rift flanks suggest an internally draining basin and high rift flanks associated with the most recent rifting episode. There are also dominant populations of Neoproterozoic and Mesoproterozoic zircons, likely reworked from the underlying Cretaceous sandstones and derived from younger metamorphic terranes of the Ubendian Belt. Volcanic pulses associated with rifting are responsible for the young magmatic zircons, and suggest the initiation of a late Cenozoic rifting event, further constraining the timing of rifting and basin development in the Western Branch, as well as the timing of landscape change associated with erosion and uplift. Our dates additionally provide important temporal context for the rich vertebrate record described from the East African Rift, illuminating the tectonic backdrop of important large-scale faunal shifts in East Africa.
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    INTERPLAY OF STRUCTURAL, CLIMATIC, AND VOLCANIC CONTROLS ON LATE QUATERNARY LACUSTRINE–DELTAIC SEDIMENTATION PATTERNS IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM, RUKWA RIFT BASIN, TANZANIA
    (Society for Sedimentary Geology (SEPM), 2016-08) MTELELA, CASSY; ROBERTS, ERIC M.; Downie, Bob; HENDRIX, MARC S.
    This paper presents the first detailed, outcrop-based sedimentologic investigation of the Pleistocene–Holocene upper Lake Beds succession in the Rukwa Rift Basin, located in theWestern Branch of the East African Rift System, southwestern Tanzania. The goal of this investigation is to examine the sedimentary facies and reconstruct the depositional environments of this important archive of Quaternary climate and environmental change. Eleven diagnostic facies associations comprising 24 facies were identified and provide the basis for recognition of three key deposystems: 1) alluvial-to-fluvial channel system; 2) lake delta system; and 3) profundal lacustrine system. Analysis of paleocurrent indicators and sandstone provenance indicate widely dispersed source regions and drainage patterns that were strongly influenced by major border-fault systems and episodic volcanism. Six stratigraphic sequences (A–F), ranging from ~ 2 to 17 m thick, were identified based on stratal stacking patterns and the development of sequence-bounding unconformities and lacustrine flooding surfaces. Sedimentation processes, facies architecture, and stratigraphic packaging record a complex interplay between Quaternary climate fluctuations and intense episodic volcanism in the nearby Rungwe Volcanic Province, set against large-scale tectonic controls associated with synchronous development of the East African Rift System. Sequence stratigraphic analysis reveals that the Rukwa Rift Basin episodically shifted between a balanced-fill lake basin and an overfilled lake basin. Deep water, basin-wide lake expansion occurred at different times during the late Quaternary. The final positional sequence preserved in the basin, a fluvial–underfilled lake basin, initiated ~ 7.9 ka and has persisted to the present day. High-frequency climate change played the key role in sequence development in the upper Lake Beds. However, voluminous, rift-related olcanism and erosion of abundant labile volcanic materials from the Rungwe Volcanic Province, as well as syntectonic evolution of the rift, led to high sedimentation rates and transformation of flash floods and debris flows in the hinterlands (rift margin) to hyperpycnal flows towards the basin depocenter.
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    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
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    U-PB ZIRCON GEOCHRONOLOGY OF VOLCANIC TUFFS AND WELL CUTTINGS REVEALS INITIATION OF LATE MIOCENE-PLIOCENE SEDIMENTATION IN THE RUKWA RIFT BASIN, TANZANIA BY ~8.7 MA
    (GSA Annual Meeting in Denver, Colorado, USA - 2016, 2016-09) HILBERT-WOLF, Hannah L.; ROBERTS, Eric M.; Downie, Bob; MTELELA, Cassy; O'CONNOR, Patrick; STEVENS, Nancy J.
    U-Pb zircon geochronology confirms a late Miocene to Pliocene depositional age for newly discovered, fossiliferous deposits of the lower Lake Beds succession in the Rukwa Rift Basin in the western branch of the East African Rift System (EARS). The Lake Beds succession is the capping sedimentary unit preserved in the Rukwa Rift Basin and correlates with the latest phase of deposition in many basins along the EARS. U-Pb geochronology on zircons from newly discovered volcanic tuffs indicate that sedimentation of the lower portion of the Lake Beds succession initiated ~8.7 Ma and that deposition continued beyond 3.5 Ma. In addition, we applied detrital zircon U-Pb geochronology to legacy well cuttings from the Galula-1 well in the Rukwa Rift Basin, indicating a corresponding late Miocene to Pliocene up-well younging trend in the youngest detrital zircon populations through this interval, matching the radioisotopic ages on volcanic tuffs from outcrop. These ages match the timing of onset of the nearby Rungwe Volcanic Province. Together, this suggests coeval Neogene rift reactivation, volcanism, and sedimentation, which has important implications for our understanding of the timing of rift formation, evolution of topography and climate, and other fundamental geologic questions that have an impact on understanding vertebrate evolution along the EARS. Preliminary investigation of the Lake Beds succession has resulted in the discovery of fossiliferous horizons, with faunal remains consisting of abundant fish, some turtle and crocodilian elements, as well as rare mammalian remains. These results also demonstrate the applicability of detrital zircon U-Pb geochronology via LA-ICP-MS to well cuttings, as a tool for dating strata lacking biostratigraphic age control. In basins with syndepositional volcanic input, the youngest zircons in a stratigraphic unit can serve as a proxy for the age of deposition. This technique is applicable to other basins along the EARS and elsewhere. The detrital and tuff-derived zircons dated here provide important temporal context for the rich vertebrate record described from the East African Rift, and help to illuminate the tectonic backdrop and timing of important large-scale faunal shifts in eastern Africa.