Evolution of extensional fault related structures in the Southern Lokichar Basin, Kenya.

Abstract

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The South Lokichar Basin is an east facing half graben that is 60 km long, 30 km wide and has an area of 1200 km². It is located in the Northern Kenyan Rift, which is situated broadly in the middle of the 2200 km long eastern limb of the East African Rift System. The hangingwall is floored with crystalline Precambrian basement and has a 6-7 km syn-rift fill of Paleogene to Quaternary interbedded fluvio-deltaic to fluvio-lacustrine sediments, capped by basaltic flows. The basin is bounded by the N-S striking extensional Lokichar Fault. The fault and basin originally formed in the Oligo-Miocene by regional rifting events which allowed pre-existing basement weaknesses to reactivate and form several En-Echelon configured half grabens. The segmented paleo-Lokichar Fault grew through periods of lower strain rate to form structures in its transfer zones known as relay ramps. These acted as conduits for E-W rivers to supply sediment into the basin. With increased strain, these relay ramps hard linked to form breached relay structures, which cut off the access of W-E rivers into the basin. Thus, the dominate drainage pathway switched from W-E to S-N in the mid-Miocene, allowing for mature and well sorted sediment to be transported into the basin by axial rivers. These linkage event correspond directly to periods of regional tectonic rifting across East Africa, which were the mid-Miocene (σ3 = E-W) and the Plio-Pleistocene (σ3= NW-SE). A footwall entirely composed of basement and fault propagation folding of the hangingwall has created the perfect scenario for a hangingwall Vs. basement trap to form. Coupled with good quality fluvio-deltaic reservoir and basin wide lacustrine shales, formed by large paleo-lakes, a world class on-shore petroleum system has developed.

Interpretation of 2D seismic, semblance data, depth-structure-maps and isopach mapping was used to reconstruct the tectono-stratigraphic evolution of the South Lokichar Basin and analyse fault geometries. Gravity & magnetics data and fault displacement models were utilised to pinpoint the location of the Lokcihar Fault linkage zones. Hangingwall, footwall and dual breached relay ramps were characterised with 2D seismic and their relative time of formation was assigned using onlapping syn-kinematic growth strata.

 List of contents

• Abstract
• Acknowledgements
• List of contents
• List of figures
• Chapter 1: Introduction
  • 1.1 – Introduction and location of the study area
  • 1.2 – Regional overview
  • 1.3 – Aims and objectives
• Chapter 2: Review – regional geology of the research area
  • 2.1 – Introduction to the East African Rift System (EARS)
  • 2.2 – Tectonic setting of the East African Rift System
  • 2.3 – Regional tectono-stratigraphy
  • 2.4 – Regional stratigraphy and structural geology
  • 2.5 – Rifting processes
  • 2.6 – Relay ramps and extensional fault related structures
  • 2.7 – Exploration history and Hydrocarbon systems
• Chapter 3: Research methodologies
  • 3.1 – Introduction
  • 3.2 – Project workflow
  • 3.3 – Datasets
  • 3.4 – Software used
  • 3.5 –  Limitations
• Chapter 4: Seismic interpretation – Results
  • 4.1 – Introduction
  • 4.2 – 2D seismic lines – regional cross-sections across the survey
  • 4.3 – Seismic stratigraphy and tectono-stratigraphy
  • 4.4 – Summary
• Chapter 5: Analysis of Fault and Fold Systems
  • 5.1 – Introduction
  • 5.2 – Fault geometries
  • 5.3 – Fault Linkages
  • 5.4 – Displacement Analysis
  • 5.5 – Summary
• Chapter 6: Discussion
  • 6.1 – Introduction
  • 6.2 – Tectono-stratigraphy and structural evolution of the Lokichar Basin
  • 6.3 – Comparative examples and analogues
  • 6.4 – Implications for hydrocarbon exploration and production
  • 6.5 – Future research

• Chapter 7: Conclusions
• Chapter 8: References and Appendices

List of figures

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Chapter 1: Introduction

1.1 Introduction & location of the study area

Kenya covers an area of 581, 309km² and is located on the East African coast at 1⁰N, 38⁰E. The Lokichar Basin is an East facing, N-S trending half graben that is 60km long, 30km wide and has an area of 1200km². Located in County Turkana southwest of Lake Turkana within a Cenozoic rift in northwest Kenya, it sits broadly in the middle of the 2200 km long Eastern Branch of the (EARS) and is bound by longitudes 35⁰30’E by 36⁰20’E and latitudes 2⁰N by 3⁰N (Talbot et al., 2004). The Lokichar region and Kenya in general have an arid climate at on average 30⁰C per year and around 120-1500mm of rainfall per year (Rhemtulla, 1970). The Turkwell and Kerio River are the main streams which propagate through the region with both their tributaries transporting sand rich sediment during heavy periods of rain only. Relief in the study area falls steadily from 1830m at Nakuru (Kenya’s fourth largest city) to 375m at Lake Turkana, which aids transportation of clastic sediment into half grabens (Kisali., 2012). The half graben has a Precambrian crystalline basement with around 7km of interbedded Paleogene to Quaternary aged fluvio-lacustrine sediments that are thickest westward towards the east-dipping Lokichar Fault and thinnest toward the Lokhone Horst at the east end of the half graben (Vetel et al., 2004; Talbot et al., 2004). The sediment fill is capped by a 300m thick basaltic flow known as the Auwerer Basalts that were dated to give an age of 12.5-10.7 Ma (Morley, 1999). The Lokichar Fault is a major bounding normal fault that has a mixture of high (45⁰-60⁰), low (20⁰-45⁰) and very low (12⁰-20⁰) angle segments and is believed to exhibit fault linkage between smaller normal faults due to continued extension or intrusions re-orientating the principal stress regime (Morley, 1998). Along the N-S striking low angle planar to listric Lokichar Fault, other smaller synthetic and antithetic faults (flower structures) aid the central antiformal high in separating the two depocenters situated along the fault, known as the Souh Lokichar Basin and the North Lokichar Basin to which fluvio-lacustrine and volcaniclastic sediments have populated the accommodation space created during rifting of the East African Ridge Systems (EARS) (Morley, 1999).

1.2 Regional overview

The East African Rift System (EARS) is a narrow and active continental rift separating two unequal plates: the Nubia and Somalia plates, which are moving slowly apart from each other as a divergent plate boundary. It previously went under the name “The Great Rift Valley,” and is approximately 6,000 km in length transecting nine African countries, including Kenya (Ebinger, 2005). EARS rifting began in the Late Oligocene-Early Miocene (25-22 Ma) and continues to diverge today at a rate of roughly 6-7 mm/yr (Fernandes et al., 2004). At some point in the future, rifting of these two plates will create a new oceanic basin. The EARS is split into two prominent rift valleys. The 2200 km long Eastern Rift Valley which includes the main Ethiopian Rift connecting the Kenyan Rift Valley to the Afar Triple Junction due NE, and the 2100 km long Western Rift Valley which includes the Albertine Rift to the valley of Lake Malawi in the south (Chorowicz, 2005).

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The geologic evolution of this continental rift zone has seen many competing theories been mooted and is still a hot topic of debate in the geoscience community. Example theories include an African superplume existing beneath the continent leading to mantle deformation was put forward by (Corti, 2009). Other academics proposed that the rifting of the EARS wasn’t initiated by tectonic activity but rather by varying crustal densities between the Nubia and Somalia tectonic plates (Logatchev et al., 1972). Probably the most widely accepted view of rift development today is the inter-play of magmatism and plate tectonics controlled by oblique rifting. Through mechanical stretching, the lithosphere was thinned during the Oligo-Miocene (34-28 Ma). This allowed development of a super plume under East Africa, resulting in volcanic activity, smaller plumes and intrusions developing along the EARS. Coupled with tectonic thinning, the magmatic processes helped to further thin the lithosphere and causing it to act like a mid-ocean ridge along East Africa (Corti, 2009). Hot asthenospheric elongate diapirs allowed rift shoulder uplift to take place forming an intracontinental ridge system that we see today (Chorowicz, 2005).

1.3 Aims and objectives

There are three key aims to this project:

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1. How does the structural geology of the Lokichar Faults transfer zones evolve through time? Where are the fault segments linked, what type of linkage occurs and when did this happen in geologic time? What is the effect of linkage on basin sedimentation?
2. Define both the tectono-stratigraphic evolution of the basin and potential structure related traps.
3. How do these extensional structures relate to the basin margin structures described in the Gulf of Suez, the NW Red Sea, Egypt (Khalil & McClay, 2016, 2006, 2002) and other examples across the world?

To achieve these aims the following objectives will be carried out:

Table 1.2: Objective list that represents a step by step guide to how the aims of the project will be achieved.