regional geology of khwisero district, kenya april 2012 scott patterson

Regional Geology of Khwisero District, Kenya

April 2012

Scott Patterson

Geologic Map of Kenya

Khwisero

Geologic Map of Western Kenya

Wilson and Gaciri, 1995

Khwisero

Geologic Map of Khwisero District


Stratigraphic Column of Archaean Rocks of Western Kenya

Mudaa Formation - Shale deposited in marine environment

Shivakala Formation - Conglomerate deposited in braided stream

Igukhu Formation - Sandstone (greywacke) deposited in turbidite desposit

Nyanzian Group - Volcanic rocks; mafic tholeiitic basalts, calc-alkaline dacites and rhyolites

Mumias and Maragoli Granites - granitic rocks



Shivakala Formation

Description •Conglomerate composed of 2.5 to 20 cm size clastscomposed 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 intrudethe Kavirondian Group because of timing considerations• Deposited in a braided stream environment



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



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 grainedand 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 distalportion of submarine fans• These fine grain rocks exhibit Bouma C, D and E intervals


Timeline of Significant Events

• 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

Gniess - This is a quartz rich gniess. It could be metamorphosed sandstone of Igukhu Fm.

Slate - This is metamorphosed shale know has slate,from the side one can see fine layers. It is likely from the Mudaa Formation.

Igneous Hand Samples from Khwisero

Basalt - This is fairly weathered basalt, it wasfound outcroppping on the road between Jackson’s and the Khwisero Market.

Weathered Volcanic Ash - This rock has a chalkytexture and is fine grained (clay), it was uncoveredwhile 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 rockthat was related to volcanic activity. It is likely fromthe Nyvanzian Formation.

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,

regional geology of khwisero district, kenya april 2012 scott patterson

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