Delpomdor F., 1 Préat A. and 2 Tack L. Dpt. Earth Sciences and Environments, University of Brussels (ULB), Brussels (Belgium)) 2

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1 Royal Museum of the Central Africa, Tervuren (Belgium) Palaeoenvironments of Neoproterozoic carbonates in part of the Schisto- Calcaire Subgroup, West Congo Belt, Democratic Republic of Congo (DRC): lithostratigraphy, sedimentology and diagenesis Delpomdor F., Préat A. and Tack L. Dpt. Earth Sciences and Environments, University of Brussels (ULB), Brussels (Belgium)) Royal Museum of the Central Africa, Tervuren (Belgium)

2 Objectives Lithostratigraphic and palaeoenvironmental study from boreholes crossing the lower part of the Schisto-Calcaire Subgroup in Bas-Congo (Democratic Republic of Congo) - Characterize the sedimentary environments and their evolution in the Neoproterozoic ( to My) in Bas-Congo - Characterize the sedimentary environment in the geologic and geodynamic contexts of the panafrican West-Congolian belts (ca. My) in the border of the Congo craton - Determine the role of microbial organisms in the carbonate deposits (early mineralizations and hydrocarbons) Geologic map of the West-Congolian belt and cross-section (SW-NE) of the belt (Frimmel et al. ) Delpomdor et al. 8 th congress of African Geology, Hammamet 8

3 Lithostratigraphy: After Alvarez et al. 99 Limestones with oolites and cyanobacterial mats Limestones with stromatolites Limestones with oolites and pisolites Limestones with cyanobacterial mats Delpomdor et al. 8 th Congress of African Geology, Hammamet 8 Synthetic lithostratigraphic subdivision of the West-Congolian belt (West-Congo Supergroup) (Modified after Tack et al. in progress) Rb/Sr isotopes datation (Poidevin 7) SHRIMP datation on zircons (Frimmel et al. ) «CAP CARBONATE» Marinoan diamictites

4 Localization of boreholes in Bas-Congo, Kimpense area Ca Cb «CILU» quarry in Lukala. The red lign represent the limit between the Cb/Ca Photography of the border of the Kwilu river ) Laminar texture in limestones. Right bank of the Kwilu river ; ) finely laminar green limestones. Right bank of the Kwilu river Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

5 Schisto-Calcaire Subgroup (C and C) Petrographic description and interpretation From analyze of thin sections, 9 microfacies have been identified (MF à MF9) and constitute the standard sequence This sequence represents the evolution of the subtidal zone with detrital materials, cyanobacterial mats and intraformational conglomerates (MF, MF, MF, MF) to an evaporative supratidal zone - sebkha - (MF, MF, MF7) strongly altered by diagenesis (MF8, MF9). BC, core 9 Schisto-Calcaire Subgroup (C): Faisceau du Kwilu CI Kwilu borehole Core Schisto-Calcaire Subgroup (C): Faisceau du Kwilu CI Lukala borehole Standard sequence of microfacies for the Kwilu (S, et ) and Lukala boreholes. SUB = subtidal, INTER = intertidal, SUPRA = supratidal. Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

6 MICROFACIES STANDARD SEQUENCE µm 9 µm 9 µm 9 µm MF MF MF MF 9 µm 9 µm 9 µm 9 µm MF MFa MFb MFc 9 µm 9 µm µm 9 µm MF7a MF7b MF8 MF9 Subtidal to intertidal zones Supratidal zone with supratidal channels Sebhka Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

7 TIDAL FLAT PALAEOENVIRONMENT Sedimentary model of the tidal flat in Kwilu-Lukala area in Bas-Congo (Democratic republic of Congo). The numbers ( à 9) correspond to microfacies of the standard sequence (modified after Ginsburg and Hardie 979, Kinsman 9). Indicative vertical scales and no horizontal scale. Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

8 SEM analysis of Microfacies : CYANOBACTERIAL BOUNDSTONES µm µm µm Cyanobacterial mats grouped in bushes Cyanobacterial mats oriented perpendiculary to the stratification Cyanobacteria and associated EPS Normalized Wt. C-K % fr()_pt 7. O-K. Ca-K. Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

9 XRD : MAIN MINERALOGICAL CONSTITUENTS OF THE CARBONATES (MF-MF9) 8 BC_ -Theta - Scale calcite, quartz, chlorite, polyhalite, celestite (+ albite) BC_ 7 -Theta - Scale ) s t n u o C ( in L ) s t n u o C ( in L ) s t n u o C ( in L mixed - File: bc.raw - Type: Th alone - Start: 7. - End: 9. - Step:. - Step time:. s - Temp.: C (Room) - Time Started: s - -Theta: 7. - Theta:. - Chi: -. - P Operations: Background.7,. Import -8 (*) - Calcite, syn - CaCO - Y: 9.8 % - d x by:. - WL:. - (*) - Quartz, syn - SiO - Y:. % - d x by:. - WL:. - (D) - Chlorite - MgAl(SiAl)O(O)8 - Y: 9.7 % - d x by:. - WL:. - (D) - Polyhalite - KCaMg(SO) HO - Y:. % - d x by:. - WL:. -9 (*) - Celestine, syn - SrSO - Y:. % - d x by:. - WL:. -Theta - Scale X-Ray diffraction on the sample BC (core BC-9, Lukala borehole) mixed - File: bc.raw - Type: Th alone - Start: 7. - End: 9. - Step:. - Step time:. s - Temp.: C (Room) - Time Started: s - -Theta: 7. - Theta:. - Chi: -. - Phi:.7 - X: -.9 Operations: Background.7,. Import -8 (*) - Calcite, syn - CaCO - Y:.8 % - d x by:. - WL:. - (*) - Quartz, syn - SiO - Y:. % - d x by:. - WL:. - (D) - Chlorite - MgAl(SiAl)O(O)8 - Y:.8 % - d x by:. - WL:. -Theta - Scale - (D) - Polyhalite - KCaMg(SO) HO - Y:.88 % - d x by:. - WL:. 9- (D) - Illite, trioctahedral - K.(Al,Fe,Mg)(Si,Al)O(OH) - Y:.8 % - d x by:. - WL:. X-Ray diffraction on the sample BC (core S-, Kwilu S borehole) Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

10 Diagenesis Early diagenesis - Sedimentation (original muds - micrite-, gypsum) - Dolomitisation (replacement of micrite) - Anhydritisation (replacement of gypsum by anhydrite) - Calcitisation-microsparitisation (replacement of dolomite) Intermediate to late diagenesis - Calcitisation-microsparitisation (replacement of dolomite) - Silcification (replacement of dolomite and calcite) - Pression-dissolution (stylolites) diagenetic sequence mineral cement occluded porosity shape size time. micrite. gypsum. dolomicrite. anhydrite. microspar. Silica 7. pression - dissolution Schisto-Calcaire SCHISTO -CALCAIRE SUBGROUP Subgroup (DRC) original calcite or aragonite original gypsum replacing calcite or aragonite replacing gypsum replacing dolomite replacing dolomite and calcite - Diagenetic sequence of Kwilu n S,, and Lukala boreholes. The figured depths are indicative. anhedral euhedral anhedral, rhomb, Idiotop ic, mosaic subhedral Mosaic, planar microcrystalline - < µm µm - µm > µm - µm >-> µm > µm - Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

11 Isotopic data - carbon and oxygen Oxygen 8 O Negative values (-8 to - ) indicate a meteoric influence or a increase of temperature resulting from burial No evaporitic influence (except BC8). Carbon C Negative values (- to - ) suggest an atmospheric influence of CO in a vadose environment. δ C V-PDB / 8 O V-PDB diagram of the Kwilu (S, et ) (stratigraphic interval C) and Lukala boreholes (stratigraphic interval C) Possible influence of bacterial sulfatoreduction reaction. No evaporitic influence in the C and C in Bas-Congo. δ C V-PDB / 8 O V-PDB isotopic diagram containing the data of Frimmel et al. () (stratigraphic interval C, C et C), the Kwilu (stratigraphic interval C) and Lukala boreholes (stratigraphic interval C). The composition of the Neoproterozoic seawater is from Veizer and Hoefs (97), Lindsay et al. (), Jacobsen and Kaufman (999). Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

12 Delpomdor et al. 8 th Congress of African Geology, Hammamet 8 Sequence stratigraphy The recognition of a rd order sequence is impossible and the stratigraphic resolution is too complex (truncated and masked sequences by the diagenesis) th et th order parasequences The petrography allows to etablish basic cycles (cycle A or cycle B) orelementary parasequences of th order). The cycles record local bathymetric variations of seawater. In conclusion, impossibility to define a stratonomic evolution for the parasequences of th order. The correlations of th and th orders are impossible to establish between the different boreholes in the sedimentary basin Representation of basal cycles of th order. Stratigraphic log of Lukala borehole. Column a = position of analysed samples (petrography), Column b = amalgam of sequences of th and th orders, Column c = parasequences of th order, Column d = standard sequence of microfacies to 9. The arrow corresponds to the bathymetric evolution

13 Conclusion Sedimentary model consists to semirestricted to restricted lagoonal subtidal environments evolving to evaporitic supratidal environment with sebkha. The diagenesis is complex and subjected to polyphased and overlapped stages. The diagenesis contains an early diagenesis with sedimentation, dolomitisation, anhydritisation and microsparitisation; intermediate to late diagenesis with microsparitization, silicification and pressure-dissolution. The isotopes of carbon and oxygen indicate the increase of temperature resulting from the regional burial (δ 8 O de -8 à - ) and a meteoritic influence resulting from vadose conditions during sedimentation (δ C de - à - ). Sedimentary model of the tidal flat of the Kwilu-Lukala deposits in Bas-Congo (Democratic Republic of Congo). The numbers ( à 9) correspond to microfacies of the standard sequence (modified after Ginsburg and Hardie 979, Kinsman 9). Indicativee vertical scale and no horizontal scale. Delpomdor et al. 8 th Congress of African Geology, Hammamet 8

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