Geology of the Mwadui kimberlite, Shinyanga district, Tanzania

Stiefenhofer, Johann; Farrow, D.

doi:10.1016/j.lithos.2004.04.017

Cited by 46 publications

(18 citation statements)

References 2 publications

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“…Another sequence that we also interpret in part as filling an upper diatreme is the Mwadui kimberlite, described by Stiefenhofer and Farrow (2004). The kimberlite comprises layered deposits enclosed within a wall dipping 30 to 70°, though below a depth of about 230 m, the dip of the kimberlite-wall rock contact becomes a "constant" 75°.…”

Section: Upper Diatremementioning

confidence: 78%

“…The kimberlite comprises layered deposits enclosed within a wall dipping 30 to 70°, though below a depth of about 230 m, the dip of the kimberlite-wall rock contact becomes a "constant" 75°. This depth corresponds to a major lithological change, of a "relatively sharp and sudden" nature, between the post-eruptive crater infill above, and layered pyroclastic deposits below (Stiefenhofer and Farrow, 2004). The pyroclastic deposits, which dominantly are lapilli tuffs, "closely resemble tuffisitic kimberlite breccia (TKB) in hand specimen" (see discussion on TKB below) and extend to at least 684 m vertical depth based on current drilling.…”

Section: Upper Diatremementioning

confidence: 95%

“…For example, studies of mine benches combined with that of drill cores has shown that the southern crater of the Orapa A/K1 kimberlite in Botswana (Cretaceous age) was filled by rock-fall avalanches, an externally derived pyroclastic flow, aqueous sheet floods, debris flows, and talus fans (Field et al, 1997;Gernon et al, 2009a). The post-eruptive crater of the Mwadui kimberlite in Tanzania (Tertiary age) was filled by (i) granite breccias formed by multiple crater wall collapse episodes; (ii) "resedimented volcaniclastic kimberlite deposits" formed mostly by slumping of portions of the tephra ring into the crater; (iii) kimberlitic turbidites; and (iv) shales (Stiefenhofer and Farrow, 2004). Stratigraphically below this sequence, Stiefenhofer and Farrow (2004) describe pyroclastic kimberlite which they attribute to a "lower crater" position, but we interpret these pyroclastic deposits as a bedded diatreme fill, as discussed below in the diatreme section.…”

Section: Post-eruptive Cratersmentioning

confidence: 99%

“…The post-eruptive crater of the Mwadui kimberlite in Tanzania (Tertiary age) was filled by (i) granite breccias formed by multiple crater wall collapse episodes; (ii) "resedimented volcaniclastic kimberlite deposits" formed mostly by slumping of portions of the tephra ring into the crater; (iii) kimberlitic turbidites; and (iv) shales (Stiefenhofer and Farrow, 2004). Stratigraphically below this sequence, Stiefenhofer and Farrow (2004) describe pyroclastic kimberlite which they attribute to a "lower crater" position, but we interpret these pyroclastic deposits as a bedded diatreme fill, as discussed below in the diatreme section. In Canada, sedimentation into open post-eruptive craters has produced deep-water shale and olivine-sand deposits in Diavik and Ekati kimberlites of the Lac de Gras field (Nowicki et al, 2004;Moss et al, 2008).…”

Section: Post-eruptive Cratersmentioning

confidence: 99%

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Maar-diatreme volcanoes: A review

White

Ross

2011

Journal of Volcanology and Geothermal Research

343

251

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Maar-diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock. A maar is the crater cut into the ground and surrounded by an ejecta ring, while the diatreme structure continues downward and encloses diatreme and root zone deposits. Here we attempt an evenhanded review of maardiatreme volcanology that extends from mafic to kimberlitic varieties, and from historical maar eruptions to deeply eroded or mined diatreme structures. We conclude that maar-diatreme eruptions are episodic. Ejecta rings provide invaluable insight into eruption processes and sequence, but are incomplete records of diatreme formation. Deposits within the diatreme structure include, in varying proportions, lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially, and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere. A basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock; root zones are irregular in form, and the clastic deposits are typically intruded by contorted dikes. Irregular root zone-like chaotic breccias cut by contorted dikes are also present within diatreme deposits, where they represent intra-diatreme fragmentation zones and record changes in the location of the explosion locus during eruption.

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Section: Upper Diatremementioning

confidence: 78%

Section: Upper Diatremementioning

confidence: 95%

Section: Post-eruptive Cratersmentioning

confidence: 99%

Section: Post-eruptive Cratersmentioning

confidence: 99%

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Maar-diatreme volcanoes: A review

White

Ross

2011

Journal of Volcanology and Geothermal Research

343

251

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“…1a, 1c). If repeated several times such injections -whether erupting or wholly subterranean -, accompanied by collapsecapture of a central zone of initially upward-driven material, provide a compelling explanation for the origin and characteristics of multiple cross-cutting bodies that have been documented for diatreme deposits (e.g., Hearn 1968;White, 1991;Naidoo et al, 2004;Nowicki et al, 2004;Stiefenhofer and Farrow, 2004;Webb et al, 2004;McClintock and White, 2006;Ross and White, 2006). A key conclusion from our experiments is that the geometry of cross-cutting granular deposits is not directly informative of the geometry of the "intrusions" that formed them.…”

Section: Applicability To Diatremesmentioning

confidence: 99%

Multiphase flow above explosion sites in debris-filled volcanic vents: Insights from analogue experiments

Ross

White

Zimanowski

et al. 2008

Journal of Volcanology and Geothermal Research

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Discrete explosive bursts are known from many volcanic eruptions. In maar-diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by "debris jets" driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5-2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5-1 second. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the "intrusions" that formed them. An additional conclusion is that complex structures can form quickly from discrete events.

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Maar‐diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth

Graettinger

Valentine

Sonder

et al. 2014

Geochem Geophys Geosyst

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Basaltic maar-diatreme volcanoes, which have craters cut into preeruption landscapes (maars) underlain by downward-tapering bodies of fragmental material commonly cut by hypabyssal intrusions (diatremes), are produced by multiple subsurface phreatomagmatic explosions. Although many maardiatremes have been studied, the link between explosion dynamics and the resulting deposit architecture is still poorly understood. Scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth, and the morphology and compaction of the host on the distribution of host materials in resulting ejecta, the development of subcrater structures and deposits, and the relationships between them. Experimental craters were 1-2 m wide. Analysis of high-speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth (physical depth scaled against explosion energy) and by the presence or absence of a crater. Jet properties in turn controlled the distribution of ejecta deposits outside the craters, and we infer that this is also reflected in the diverse range of deposit types at natural maars. Ejecta were dominated by material that originated above the explosion site, and the shallowest material was dispersed the farthest. Subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions. We conclude that the progressive appearance of deeper-seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts, rather than progressive deepening of explosion sites in response to draw down of aquifers.

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Geology of the Mwadui kimberlite, Shinyanga district, Tanzania

Cited by 46 publications

References 2 publications

Maar-diatreme volcanoes: A review

Maar-diatreme volcanoes: A review

Multiphase flow above explosion sites in debris-filled volcanic vents: Insights from analogue experiments

Maar‐diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth

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