Stratigraphic, sedimentologic, and palaeontologic investigation of Karoo and Cretaceous-aged

sedimentary cover sequences in the south-central

Congo Basin, Democratic Republic of Congo

Prepared by:

Dr. Eric M. Roberts

School of Geosciences

University of the Witwatersrand

Final Report November, 2008

SCOPE AND PURPOSE OF INVESTIGATION

This report outlines the sedimentological and paleontological findings from an

investigation of Karoo and Cretaceous-aged sedimentary cover sequences in the

south-central Congo Basin. Data was collected during a 15 day field visit to the

Kananga and Tshikapa areas. The investigation focuses on cores recovered from drill-

site localities to the south of Kabinda, Mbuji-Mayi, Kananga and Tshikapa. Detailed

examination and logging was performed on 17 cores and several outcrops.

Numerous artisinal alluvial diggings also were investigated in the Kananga and

Tshikapa areas. Samples for petrographic, geochemical, and paleontological analysis

were collected from core and outcrop at Kananga and Tshikapa, however only the

Kananga samples were studied since samples from Tshikapa never arrived.

Preliminary assessments of microfossils and bivalves collected from several of the

cores were performed; however these interpretations are pending more accurate

taxonomic determinations by invertebrate specialists Dr. Leif Tapanila, Dr. Oscar

Gallego and Dr. JP. Colin. Additionally, two calcrete samples were collected for U/Pb

radiometric dating and sent to Dr. Sam Bowring at MIT. The radiometric results are

also are still pending.

Previous Work

This study represents an expansion upon the facies analysis and sandstone

provenance investigation by Emily Hansen in 2007 and was designed to provide

additional insight into the stratigraphy and depositional environments of cover

sequences in the Congo Basin. In some cases, cores examined by Hansen (2007) were

reexamined and resampled for paleontological data and to obtain more detailed

information concerning depositional environments and stratigraphy. The results of

this study have been used to develop a 2-D basin model to improve stratigraphic

correlations for the south-central Congo Basin, from Kabinda, in the east, to Tshikapa,

in the west. This data can then be used to more reliably develop a 3-D model for the

entire basin.

GEOLOGICAL BACKGROUND

The Congo Basin is one of the more poorly known large continental sedimentary

basins in the world (Daly et al., 1992). As outlined by Jelsma (2006) and Hansen (2007),

the stratigraphy of cover sequences across the basin is poorly understood due to

limited previous investigations, inconsistent nomenclature between different regions

(Angola; DRC; Congo), poor age control, and a variety of other factors that obfuscate

the depositional history of the basin. A somewhat layer-cake style stratigraphy has

generally been applied to the Congo Basin, with gross subdivision of Palaeozoic and

Mesozoic cover sequences in the southern DRC into the lower Karoo (Lukuga Group),

middle Karoo (Haute Lueki Fm), upper Karoo (Lualaba

Group[including the Stanleyville Fm), Lower Cretaceous (Loia Group, Camina Series,

Calonda Fm), Upper Cretaceous (Bokungu or Kwango Group) and PostCretaceous

(Gres Polymorphs/Kalahari Group). Although this is useful for broad, regional-scale

correlations, examination of cores from a relatively small portion of the basin

(between Tshikapa and Mbuji-Mayi) reveals considerable facies, provenance and

thickness variations suggesting a much more complex depositional and stratigraphic

history. Moreover, this study strongly suggests that the Congo Basin represents a

more structurally complicated basin than perhaps previously appreciated, with

multiple fault bounded basement highs and depocenters. The scope and duration of

this study does not yet permit full resolution of the complex history of the

sedimentary cover units, but a clearer picture is beginning to develop.

Figure 1. Cratonic map of Southern Africa showing prominent basement structures.

Field area located shown in box (From Pereira, 2003).

CORE LOGGING AND FACIES ANALYSIS

Methods

Cores were studied in detail with the goal of identifying sedimentary features that

may permit better correlation locally and regionally. Detailed core logs were

constructed for 20 different bore holes from Kabinda to Tshikapa (see Appendix A).

Typical sedimentary features, including grain size, sorting, bed thickness, colour,

sedimentary structures, mineralogy and fossil content, were observed and recorded

while particular attention was paid to the distribution, thickness, thickness variation

and clast lithology of conglomerate units within the sequences. A variety of samples

were thin sectioned and investigated using a transmitted-light polarizing microscope

for sandstone provenance, mineral and fossil identification and diagenetic history.

Utilizing these features and analyses, 19 common lithofacies were identified. A

combination of stratigraphic data, coupled with lithofacies, fossil assemblage and

sandstone petrography, permit the subdivision of the sedimentary cover in the Congo

Basin into six depositional megasequences. Each depositional sequence was analyzed

in detail and interpreted in terms of depositional environment(s). A 2-D basin model

and stratigraphic correlation chart was developed utilizing core data, outcrop data

and satellite imagery to identify fault trends.

Facies Analysis

Nineteen common lithofacies were identified and described from cores in the

Tshikapa, Kananga, Mbuji-Mayi, and Kabinda areas. General descriptions for each

facies are provided below, however unique or special characteristics of each

unit/facies are provided in each core log under the description section. See Table 1 in

Appendix A for lithofacies codes and symbols used in all core logs. Lithofacies were

then used to interpret depositional environments.

Lithofacies 1A: Clast supported large pebble to cobble conglomerate. Clasts typically

range in size from 2-5 cm with larger sizes possible. Clasts tend to be angular and the

most common clast lithologies are Neoproterozoic stromatolitic limestone and basalt

clasts. Calcrete coatings present on some clasts. Red sandy mudstone to muddy

sandstone matrix.

Lithofacies 1B: Same as lithofacies 1A, except for matrix support.

Lithofacies 2A: Clast supported, small-medium pebble conglomerate. Clasts are

generally better rounded and sorted than in Lithofacies 1 and usually in the 0.5-2.5 cm

range. Sandy matrix typical. Clast lithology is variable.

Lithofacies 2B: Same as lithofacies 2A, except for matrix support.

Lithofaices 3A: Granulestone to coarse grained sandstone (previously referred to as

the “Bully Beef” unit). Commonly red in color with little or no mud. Cross-bedding

prevalent; clasts are subangular to subrounded, moderate to well sorted.

Lithofacies 3B: Same as lithofacies 3A, except muddy with a clay matrix.

Lithofacies 4A: Granulestone similar to lithofaices 3A, except massive with no visible

bedding. No mud.

Lithofacies 4B: Same as lithofaices 4A, except muddy.

Lithofacies 5A: Fine-medium grained sandstone with well-defined trough or tabular

cross-bedding; No Mud. Grains are typically well-rounded to sub-rounded; moderate

to well sorted. Colour ranges from orange to red to purple. Heavy mineral laminations

common.

Lithofacies 5B: Same as lithofacies 5A, except very muddy.

Lithofaices 6A: Fine-medium grained massive sandstone. No bedding and No Mud.

Grains typically well-rounded to sub-rounded; moderate to well sorted. Colour

ranges from orange to red to purple.

Lithofacies 6B: Same as lithofacies 6A, except very muddy.

Lithofacies 7: Red-purple siltstone to claystone. Minor remnant bedding and

bioturbation common. Desiccation cracks sometimes apparent.

Lithofacies 8A: Laminated siltstone and claystone. 1-3 mm laminae; minor

bioturbation and current ripple lamination. Desiccation cracks locally abundant.

Ostracodes, choncostrachans and small bivalves common. Typically brick red, but

varies considerably between gray, yellow and red.

Lithofacies 8B: Same as lithofacies 8, but more deeply oxidized.

Lithofacies 9: Similar to lithofacies 8, but contains rare to abundant dropstones. May

be interlaminated with thin, fine-grained sandstone lenses; Chocolate brown color to

dark reddish brown.

Lithofacies 10: Diamicite. Granule, pebble and cobble size clasts floating in a sandy

mudstone matrix. Range of large, angular clasts, including red sandstones,

limestones, basalt, dolerite, quartzite and granitic clasts.

Lithofacies 11: Kimberlite. Not studied in detail.

SANDSTONE PETROGRAPHY

A selection of samples collected from boreholes across the field for thin section

preparation and petrographic analysis. Some of these samples were point-counted

using a modified Gazzi-Dickinson (1988) approach similar to that applied by Emily

Hansen (2007) however most were studied qualitatively for comparison with the

provenance interpretations of Hansen (2007). Special attention was devoted to

observing sedimentological characteristics of sandstones and other rock types for

aiding in palaeoenvironmental interpretations.

GEOCHEMISTRY AND RADIOMETRIC DATING

A variety of samples were collected for geochemical analysis aimed at

palaeoenvironmental reconstruction and radiometric dating. Unfortunately, many of

the more interesting samples from the Tshikapa area (Core 169-X020) never arrived.

However, several other samples containing unusual authigenic minerals were

collected for XRD analyses to determine their mineralogical composition and origin.

Radiometric Dating

In a number of cores, particularly 173-X030 and 173-X009, exist limestone and basalt

clasts covered by a 5-10 mm coating of calcrete. In some instances the calcrete is

considerably thicker on one side, indicating the stratigraphic up of these cobbles.

Moreover, the overthickening of calcrete on one surface indicates that the calcrete

precipitated in situ on the cobble at some point after deposition of those cobbles. This

presents an unusual opportunity to try to date these rocks, since were have relative

temporal constraints on the timing of calcrete growth. In an effort to obtain absolute

age data, two samples were collected and sent to Dr. Sam Bowring for U/Pb dating

on the calcrete coating these cobbles. As yet, the results are unavailable.

Figure 2. Calcrete coated pebbles collected for radiometric dating. A. core x173-030, unit 7. B. Core sample 173-x009, near base.

PALAEONTOLOGY

Only one core, 173 X-030, contained vertebrate fossil remains in the study area. In

upper part of this core, a number of fish bones were observed. The bones appear to

be mostly spines and isolated cranial bones. The material is too fragmentary and

isolated to permit detailed taxonomic assessment, however their presence is useful for

general correlation with fish-bearing units in the north and west and in Angola.

Fig. 3. Isolated fish bone (cranial element?) from core 173-X030.

The single invertebrate macrofossil locality identified was a bivalve shell

accumulation in well 155-X040 near the base of the core, at ~115 m from surface.

Multiple partial specimens were found and several of these were collected for further

taxonomic identification. At least two of the specimens appear to be articulated.

Based on the schizodont dentition, a thick, robust shell and the oblate shape of the

bivalves, they are interpreted as members of the Unionoidea, a family a freshwater

(rarely brackishwater) clams (mussels) that commonly live in fluvial and lacustrine

environments. Moreover, the thick shell morphotype of these bivalves is consistent

with a higher-energy fluvial mode of life. Unioniods display amazing ecomorphic

plasticity in their shells, which make them ideal for use in paleoenvironmental

reconstruction, although this same trait makes them particularly difficult to diagnose

taxonomically (Roberts et al., 2008). The interpretation of this taxon as a fluvial form

is consistent with the general sedimentology of the site and the sequence in general.

This taxon appears to be a new species (L. Tapanila, pers. Comm. 2008), limiting its

biostratigraphic utility; however these findings are significant because they clearly

demonstrate that these deposits are freshwater. This is important because a marine

bivalve identified as Pteria sp. was reported from an outcrop in the Tshikaya

(Tshikapa?) and Luebo region from the Kwango Series by Cahen (1954). The

taxonomic assignment of Pteria sp. in the Kwango Series is considered questionable

based on the sedimentological investigation of Cretaceous strata in this region. It

seems plausible that the forms identified by Cahen (1954) may be referable to this

fluvial form.

Fig. 4. Fluvial Unionoid bivalves from Core 155-x040.

During the course of the core investigation and logging, microfossils were

observed from numerous cores at various levels. Samples were collected at various

locations, with the goal of using this data to provide improved biostratigraphic and

paleoenvironmental constraints on these deposits. Cores 172-X146, 173-X009,

173X030, 157-X016, and 172-X156 all preserve microfossils and it is highly suspected

that if a more intensive sampling effort were conducted, that many more cores would

also yield microfossils.

A preliminary assessment of the microfossils demonstrates the presence of both

ostrocodes and conchostracans. Preliminary taxonomic identification of the

conchostrachan and ostracode fauna has been performed and will be highlighted

below, but more detailed taxonomic assessments are ongoing with taxonomic

specialists. Towards this end, I have identified three workers, Leif Tapanila

(molluscs and general biostratigraphy), Oscar Gallego and his students

(conchostrachans), Jean-Paul Colin (ostracodes) who are interested in examining the

material, however we have not yet worked how or where to process the samples.

Through my own efforts and those of Dr. Tapanila, we have identified at least two

different taxa of conchostrachans, both of which belong in the group Afrograpta

(formerly called the Estheriellids). Various Afrograptids have been identified from

fresh and brackish water deposits in Central Africa and South America and are most

common within the Middle-Late Jurassic to the Early Cretaceous. However, several

forms extend back into the Triassic, while some extend into the early Late Cretaceous.

Fig. 5. Apparent Afrograptid Conchostrachan fauna from Core 172-X146 (35 m).

Brief examination of the ostracodes also indicates the presents of multiple species. The

most common form, which is notable for its smooth (unornamented) carapace is

reminiscent of Early Cretaceous members of the Cyprididae. Similar ostracodes have

been described from nearby, in the Aptian Dinosaur Beds of Malawi (Colin and

Jacobs, 1990), however the taxonomic confirmation and detailed species level

determination of these specimens have not yet been confirmed by Dr. Colin.

Figure 6. A. Ostracodes from Core 173 X-030 ~35 m level. B. Close-up of an

unornamented member of the Family Cyprididae.

Once the taxonomy of both microfossil groups has been completed, their

biostratigraphic and paleoenvironmental significance can be fully explored.

Numerous microfossil studies have been focused on specimens recovered from the

Dekese and Samba boreholes drilled to the north of the field area, as well as on

samples collected from outcrop in the Brazzavile and Kishasa areas. The Upper

Jurassic (Kimmeridgian) Stanleyville Group (Series) has yield abundant ostracode

and conchostrachan material, and will provide a good basis for comparison.

Certainly, many of microfossil bearing laminated siltstones and shales from the wells

studied in this facies bear a general resemblance to descriptions of the Stanleyville

Group. This strongly supports lithological data that indicate that lower Stanleyville

Group strata is present in isolated, fault bounded depocenters in the central and

eastern margins of the study area.

SEDIMENTOLOGICAL AND STRATIGRAPHIC INTERPRETATIONS OF CORES

A wide range of depositional environments, including glacial, fluvial, lacustrine and

aeolian were identified based on distinctive lithofacies, lithofacies assemblages,

petrography, sedimentary structures and fossils. One significant omission is the lack

of diagnostic marine depositional environments (possibly excluding the basal Lukugu

Group) in the study area. This is particularly interesting given that a variety of

previous investigations have suggested the presence of marine sequences in the basin

(see Giresse, 2005).

Fluvial Environments

The most common depositional environments identified across the basin are

lowaccommodation (probably braided) fluvial environments. These environments are

particularly common with the Cretaceous portion (C1, C2, & C3) of the stratigraphy,

but also important within portions of the Karoo sequences. Fluvial environments are

typically dominated by lithofacies 5 and 6 and to a smaller degree, by lithofacies 2 and

3. Oxidized channel facies are most abundant, while there is a relative paucity of

floodbasin fine-grained deposits (lithofacies 7) implying slow generation of

accommodation space in the basin through time. Thin overbank mudstones are

moderately rare and commonly preserve features indicative of moderate to intense

pedogenesis (Fig.8b). Rare, but well-displayed evidence of calcium carbonate

accumulations, deep oxidation, and desiccation cracks suggests that semi-arid

conditions prevailed throughout much of the Jurassic through Early Cretaceous, and

that ephemeral flow conditions existed in many of these channel systems.

A distinctive characteristic associated with many of the Lower Cretaceous C2

channel facies is heavy mineral laminated foresets in cross-bedded sandstones (Fig 7).

The base of many Lower Cretaceous fluvial sequences across the field area is

characterized by thin conglomeratic units (refereable to C1) that have been proposed

to correlate to the Calonda Gravels in Angola. These congomerates are dominated by

intraformation rip-up clasts, indicating a period of modest regional incision across

much of the basin. These intraformational conglomerates are typically quite thin and

appear to signal a shift in tectonic regime responsible for deposition of Early

Cretaceous fluvial sandstones across the Congo Basin. The thin and discontinuous

nature of these beds and the abundance intraformational clasts suggest that they are

locally derived due to incision, rather than representing longdistance transport of

gravels across the basin. As such, it is difficult to envision these units as representing

a regionally extensive pediment or erosional surface developed by sheet flow from

Angola to DRC. Moreover, clast lithology across the study area varies according to

local basement lithology, which seems to support the notion that these units are

mostly locally derived, albeit the presence of such clasts as agate may indicate long

distance transport of some clasts.

Fig. 7. Trough cross-bedded and current ripple-laminated fluvial channel facies from

Lower Cretaceous C2 sequence. Also note dark heavy mineral laminations.

Fig. 8. A. C1 (Calonda Fm) pebble (dominantly intraformational) conglomerate from

Core 157-X002, commonly associated with fluvial facies at or near the base of the

Lower Cretaceous sequence. B. Pedogenically altered overbank facies with thick

calcium carbonate accumulations.

Alluvial Fan Environments

Facies interpreted to represent alluvial fan environment are relatively common in the

study area and are best appreciated when considered in a tectonic framework. The

majority of facies interpreted as such are located along inferred fault zones,

particularly within the base of the Middle Karoo Haute Lueki Fm equivalent beds.

The basal portions of both the Upper Karoo Stanleyville Fm equivalent beds and the

Lower Cretaceous C1 (Loia Gp) beds also commonly preserve deposits interpreted as

alluvial fans. The basal portions of Cores 172-X286, -X111, and –X146 (all C1,C2

Lower Cretaceous units), which appear to be part of a narrow fault-bounded horst

and graben complex, all preserve distinctive alluvial fan facies. Alluvial fan facies are

typically dominated by lithofacies 1, where clasts are poorly sorted, poorly rounded

and randomly oriented (Fig. 9). Matrix support is common in many units, suggesting

deposition by debris flows and proximal ephemeral channels. In some units, calcrete

covers many clasts indicating prolonged surface exposure under arid conditions, as

would be expected in alluvial fan settings. Most clasts associated with alluvial fan

facies are locally derived from nearby basement blocks.

Fig. 9. Alluvial fan facies associated with the base of Core 157-X016

Glacial Environments

Glacial facies including diamictites and floatstone shales (lithofacies 9 and 10) are

representative of deep water lacustrine or marine systems, while poorly sorted

conglomerates (lithofacies 1a,b) are indicative of glacial outwash deposits. Glacial

facies are rare in the study area and appear to be limited to localized depocenters.

Only the basal Lukugu Group appears to preserve glacial facies, and this is limited to

the base of Core 172-X050.

Fig. 10. Glacial features observed in the basal sequence (265 m level of Core 172X050.

A. Varved shales with dropstones. B. Diamictite

Lacustrine Environments

Lacustrine conditions are particularly abundant in Pre-C1 or Stanleyville Fm

equivalent strata. They are characterized by finely laminated siltstones and claystones

(lithofacies 8; Fig. 11) that commonly preserve microfossils, including ostracodes and

conchostracans, in addition to rare fish bones. Dessication cracks are common, and

taken together with the presence of the preserved conchastrachans and deeply

oxidized character of the sediments, indicate ephemeral lacustrine conditions in an

arid to semi-arid climate.

Fig. 11. Lacustrine laminated shales and siltstones in Core 159-X027 (60m) and

157X016

Some lacustrine units such as in cores 173-X009 and 173-X030 also preserve

thin (1-5 cm thick), irregular zones of white, laminated sediment (Fig 12). The white,

irregular layers were found within laminated silt and claystone units, and they

commonly appeared to represent desiccated chert layers, similar to those found in

highly unusual, evaporative, alkaline lake systems, like Lake Magadi, in the East

African Rift (Eugster, 1986).

Fig 12. Samples of unusual laminated and deformed white layers interbedded with

laminated claystone and siltstone in core 173-X030 (unit 13c).

E-173-X009-2

E-173-X030-3

Fig. 13. XRD diffractograms of bedded white mineral assocaiated with finely

laminated lacustrine units in Cores 173-X009 and 173-X030 that show both samples to

be chert, with calcite also present in Core 173-X030. This supports a hypothesis that

formation of these cherts may have taken place due to ephemeral, alkaline conditions,

such as those known to produce Lake Magadi style chert.

Aeolian Environments

An interesting finding is the abundance and regional extent of fine-grained, super

mature quartz arenites across many parts of the basin, particularly the Pre-C1

Stanleyville Fm equivalent deposits. This facies can be correlated across the entire

basin in the Stanleyville equivalent megasequence and is commonly interbedded with

lacustrine units. Significantly, the presence of such diagnostic features as excellent

sorting and rounding, quartzose composition, cross-bedding, grain frosting and

inverse grading (pinstripe laminations) can be used to correlate this facies, as well as

providing strong evidence for deposition by aeolian processes. High-angle trough

cross-bedded units indicate the presence of potentially large dune foresets, while

pinstripe bedding is representative of wind ripple cross-lamination (Fig. 14 and 15).

Fig 14. Pin-stripe bedding associated with aeolian ripple cross lamination in Core 157-

X017.

Fig. 15 Aeolian sandstone samples from Core 157-X017(4x magnification). Key

characteristics include well sorted, well rounded, fine-medium grained quartz arenite.

Also note the diagnostic inversely graded stratification (pin-stripe lamination in hand

sample) characteristic of aeolian wind-ripple stratification.

STRATIGRAPHY AND DEPOSITIONAL SEQUENCES

Six mega-sequences were observed in the study area, ranging in age from

PermianCarboniferous to Paleogene. Correlation of these six sequences reveals the

complicated tectonic history and structure of the basin. The basal mega-sequence is

characterized by glacial diamicites, varved shales with dropstones and outwash

conglomerates that presumably correlate with the basal Dwyka Group in the Main

Karoo Basin of South Arica and the Lukuga Group in other parts of DRC and Angola.

This sequence is observed in Core 172-X050 in the Kabinda area. It is located at the

base of core 172-X050 and appears to be preserved within a complexly faulted area,

near the intersection of a series NW-SE and NE-SW oriented faults. Edwards (1967)

identified a similar localized sequence of Lukuga Group glacial and periglacial strata

near Tshikapa, along the Tshiumbe River.

Overlying the basal Lukuga Group equivalent mega-sequence is a series of

coarse-grained conglomerates interpreted as high-energy alluvial fan to proximal

fluvial systems. This sequence is observed in core from the Kananga and Kabinda

areas, and is refereed to the middle Karoo sequence, likely the Triassic-Jurassic Haute

Lueki Fm, although this association is uncertain.

Overlying this thin and localized middle Karoo megasequence is a generally

thicker sequence of arid to semi-arid laminated shales and silstones and aeolian

sandstone interpreted as a sequence of ephemeral lakes and sand dunes sequences.

Also within this mega sequence are interspersed loess deposits and rare fluvial

channel sequences. This sequence is most parsimoniously considered to be part of the

extensive Upper Karoo Lualaba Group. The lacustrine facies likely correlate with the

Stanleyville Pools Formation in the northwest of the basin. Hansen (2007) generally

referred to this sequence as the “mudstone” facies.

Above this sequence in many cores lies Mid-Loia Group (Calonda) equivelant

strata exposed across much of the basin. This megasequence is dominated by red

sandstones and generally interpreted as braided fluvial in origin. It is informally

referred to as the C1/C2 sequence in this study.

Overlying the Loia Group in many areas are Upper Cretaceous Kwango or

Bokungu deposits, termed C3 here, also of dominantly fluvial origin. Locally exposed

in the highest elevation areas and capping the succession is the final megasequence,

termed here the C4 sequence, which is likely Paleogene in age and correlative with

the Gres Polymorphs and portions of the Kalahari Group. These units appear to be of

fluvial, aeolian and lacustrine origins.

Fig. 16. Permian-Carboniferous Lukugu Group mega-sequence, shown here in Core

172-X050. A. Basal 280-260 m interval with diamictites and varved siltstones. B.

Transition from glaciolacustrine diamictites to glacial outwash conglomerates at ~223

m.

Fig. 17. Triassic-Jurassic, Middle Karoo (Haute Lueki Fm) Mega-sequence, shown here

in Core 173-X009. A. Proximal debris flow alluvial fan facies from base of core (160

m). B. Close-up of poorly sorted, angular clasts.

Fig. 18. Typical Upper Karoo equivalent Lualaba Group (Stanleyville Fm)

megasequence, shown here from Core 159-X027. Key characteristics are the presence

of oxidized laminated lacustrine mudstones (A) and pin-striped aeolian sandstones

(B).

Fig. 19. Typical Lower Cretaceous C1 and C2 (Loia Group/Calonda Fm) from Core

154-X001.

CORRELATIONS AND REGIONAL TECTONICS

Attempting too correlate cores across the Congo Basin using layer cake style

stratigraphy is ineffective. A complex network of faulting is interpreted to control the

topography and distribution of stratigraphic sequences across the basin. Given these

complexities, a 2-dimensional basin model (Appendix B, C) was developed to explain

the complex tectonic framework of the basin and to permit correlation of stratigraphic

units across the basin. A series of faults were inferred to help explain the complex

stratigraphy, and these faults were plotted on a DEM image of the fieldarea.

Remarkably good correspondence was found between the inferred location of fault

zones between boreholes and the distribution of obvious lineaments and structural

features observed in the DEM data. Two prominent sets of faults are inferred and

were mapped onto the DEM image (see Appendix B). A NW-SE oriented set of faults

appears to control stratigraphy on the eastern side of the basin, while a set of NE-SW

joints appears to be the dominant control on stratigraphy in the central and western

portion of the basin. Most faults are interpreted as extensional normal faults, creating

a series of basement highs (horst blocks) and lows (grabens). These structures grossly

explain the distribution of basement rocks exposed in the region. Prolonged and

sporadic movement appears to have taken place on multiple faults, leading to a

heavily bisected and uneven thickness and depth distributions between each mega-

sequence (See Appendix C for 2-D Basin Model).

REFERENCES

Eugster, H.P., 1986. Lake Magadi, Kenya: a model for rift valley hydrochemistry and

sedimentation? Geological Society, London Special Publications 25: Siliclastic,

chemical, Pedogenic and Organic Sediments in Contemporary Rift Enviornments,

pgs. 177-189.

Hansen, E., 2007. Characterization of Cretaceous Age Sedimentary Units from the

Kasai Craton, Democratic Republic of Congo. DeBeers Internal Report.