Transcript Geology and Hydrology of Khwisero District, Kenya

Regional Geology of Khwisero District, Kenya
April 2012
Scott Patterson
Geologic Map of Kenya
Khwisero
Geologic Map of Western Kenya
Khwisero
Wilson and Gaciri, 1995
Geologic Map of Khwisero District
Wilson and Gaciri, 1995
Stratigraphic Column of Archaean Rocks of Western Kenya
Mumias and Maragoli Granites - granitic rocks
Mudaa Formation - Shale deposited in marine environment
Igukhu Formation - Sandstone (greywacke) deposited in turbidite desposit
Shivakala Formation - Conglomerate deposited in braided stream
Nyanzian Group - Volcanic rocks; mafic tholeiitic basalts, calc-alkaline
dacites and rhyolites
Wilson and Gaciri, 1995
Stratigraphic Column of Archaean Rocks of Western Kenya
Shivakala Formation
Description
•Conglomerate composed of 2.5 to 20 cm size clasts
composed of basalt, dacite, andesite, rhyolite and granite
• Minimum thickness is 260 m
• Well exposed around Shivakala, Malinya, Regea and Khwisero
• Unconformably overlie Nyanzian Group
• Basal unit of Kavirondian Group
• Intruded in north by Mumias Granite
• Overlain by Igukhu Fm with a sharp conformable contact
• Bed thickness is 12 to 27 cm
• Display imbricated clasts
• Pebbles are sub-rounded to rounded
• Cemented with fine volcanclastic matrix
• Occasitionally conglomerate is interbedded with sandstone
• Lower bed surfaces are irregular and display scour marks
Interpretation
• Source of volcanic clasts in Shivakala Fm are Nyanzian Group
volcanics because they are chemically and compositionally similar
• Erosion and deposition of Nyanzian Group volcanics was between
2710+/-340 and 2611+/-311 Ma
• Granites found in Shivakala Fm are not the same granites that intrude
the Kavirondian Group because of timing considerations
• Deposited in a braided stream environment
Wilson and Gaciri, 1995
Stratigraphic Column of Archaean Rocks of Western Kenya
Igukhu Formation
Description
• A textually immature greywacke (sandstone)
• Lower boundary is sharp conformable surface overlaying Shivakala Fm
• Abundance of rock fragments (25% by volume)
• Clasts are volcanic and mudrock in composition
• High percentage of matrix
• Angularity of coarser clasts
• High percentage of feldspar grains
• 35% of greywacke is quartz and occurs as angular to subangular grains
• 20% of rock is matrix
• Thick bed units display well developed normal grading, scour marks
• Thin bed units display sharp basal contacts normal grading
• Sandstone beds are commonly overlain by laminated siltstone and
shales
• Beds are laterally continuous throughout outcrops
Interpretation
• Source for volcanic clasts are the Nyanzian Group
• Deposited as turbidite deposition in marine environment
• Exhibit most traits of the classic Bouma sequence
Wilson and Gaciri, 1995
Stratigraphic Column of Archaean Rocks of Western Kenya
Mudaa Formation
Description
• Composed of laminated shales and mudstones
• Commonly interbedded with thin sandstone beds (<10 cm thick)
• Minimum thickness is 280 m thick
• Lower boundary is typically gradational with Igukhu Fm
• Mudstones are often soft dark brown rocks in beds up to 40 cm that
grade laterally to siltstone and greywacke
• Another facies of mudstone are interbedded light brown coarser grained
and are normally graded with dark brown finer grained with a higher
percentage of clay
• Slump structures, syn-sedimentary faults, convolute bedding and flame
structures are observed in the interbedded mudstone facies
Interpretation
• The shales and mudstones were deposited by turbidity currents in distal
portion of submarine fans
• These fine grain rocks exhibit Bouma C, D and E intervals
Wilson and Gaciri, 1995
Timeline of Significant Events
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Tanzania Plate formed
Volcanism forming the Nyanzian Group
Creation of Kavirondian Basin
Erosion of basin flanking volcanic and granitic rocks
Filling of basin with Kavirondian Group
The Kavirondian Basin is above sea level as demonstrated by the
Shivakala Fm and its deposition by braided streams that are carrying
coarse material from basin flanks to the basin
The Kavirondian Basin is then below sea level during deposition of
Igukhu and Mudaa Fm
The Igukhu and Mudaa Fm were both deposited by turbidity currents
and the Igukhu is more proximal in the system than Mudaa Fm
Intrusion of granites after Kavirondian basin is filled
Contact and regional metamorphism of Nyanzian and Kavirondian
Groups
East African Rift
Metamorphic Hand Samples from Visit to Khwisero
Slate - This is metamorphosed shale know has slate,
from the side one can see fine layers. It is likely from
the Mudaa Formation.
Gniess - This is a quartz rich gniess. It could be
metamorphosed sandstone of Igukhu Fm.
Igneous Hand Samples from Khwisero
Weathered Volcanic Ash - This rock has a chalky
texture and is fine grained (clay), it was uncovered
while constructing Jackson’s hand dug well (<30 m
in depth). This could be from the Igukhu Formation.
It is likely reworked ash from the Nyvanzian
Formation.
Dacite Tuff - This is an intrusive igneous rock
that was related to volcanic activity. It is likely from
the Nyvanzian Formation.
Basalt - This is fairly weathered basalt, it was
found outcroppping on the road between
Jackson’s and the Khwisero Market.
References
Ngecu, Wilson M. and Gaciri, Steve J., 1995, Lithostratigraphy, provenance and facies
distribution of Archaean cratonic successions in western Kenya, in Journal of African Earch
Science, v 21, no 3, p 359-372.
Geologic Atlas of Africa,