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Mica and shell as indicators of energy level and depositional regime on the Nigerian shelf
Marine Geology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
Letter Section Mica a n d shell as i n d i c a t o r s o f e n e r g y level a n d d e p o s i t i o n a l r e g i m e o n t h e Nigerian Shelf
OLUWAFEYISOLA S. ADEGOKE and DANIEL J. STANLEY Department of Geology, University of lfe, Ile-lfe (Nigeria) Division of Sedimentology, Smithsonian Institution, Washington, D. C (U.S.A.) (Accepted for publication October 25, 1972) ABSTRACT Adegoke, O.S. and Stanley, D.J., 1972. Mica and shell as indicators of energy level and depositional regime on the Nigerian Shelf. Mar. Geol., 13: M61 -M66. The recent sediments on the Nigerian Shelf are derived largely from the Niger and Benue rivers and distributed by the deep Guinea Current and a complex series of wave- and tide-generated shallow, long-shore currents. The distribution of sand-size mica flakes and total shell counts are used to recognize energy levels and to delineate zones of by-passing and modern terrigenous deposition in a regressive deltaic setting.
INTRODUCTION Sedimentation on the modern Nigerian Shelf is dominated by the Niger Delta, a thick depositional wedge actively prograding onto the shelf. This delta has been investigated by a number of workers (Nedeco, 1959, 1961 ; Short and St/iuble, 1967; Weber, 1971). The most comprehensive sedimentological study to date is by Allen (1965) who recognized two Quaternary sedimentary sequences: a late-glacial to early Holocene "Older Sands" facies partially buried by a Holocene "Younger Suites" facies (Fig. 1A). Allen noted that the shelf is characterized by a concentric arrangement of sedimentary facies w i t h respect to heads of passes, and that there is progressive textural fining of sediments with depth. These factors were correlated with the complex interplay of hydrological and physical oceanographic factors (Fig. 1B) and with the diminishing intensity of energy conditions away from the coast. He, however, observed a notable disparity between lithofacies and his sedimentary environments based on zones of equal processes and energy levels. Close correlation between sand-size mica flakes, sedimentary texture and sea floor processes has been recorded in a number of continental margin settings (Neiheisel, 1965; Pomerancblum, 1966; Doyle et al., 1968). Because of its hydraulic equivalence to silt, mica tends to be winnowed out of high-energy environments such as the littoral zone and carried
LETTER SECTION
M62
KM
'/
D ONITSHA
\\ ,5*
4°
SUPER - ENVIRONMENTS 3o
• SAMPLE STATION DEPTH IN FATHOMS
A
NON-DEPOSITIONAL "Older S a n d s " facies CONTINENTAL SHELF ~ yq CONTINENTAL SLOPE ] " o u n g e r 5 u i t e s " f a c i e s S=, 6°1 7°
~
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po
~CONTINENTAL ~
~
~ RIVER
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,
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GUINEA CURRENT ~ J I I I l l l l ~ ~ , ,
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i
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~" ~
LETTER SECTION
M63
Fig.2. Charts showing selected compositional parameters on Nigerian Shelf. A. Mica isopleths. B. Shell isopleths. Fig. 1. Charts showing Niger Delta and Nigerian Shelf, modified after Allen (1965). A, Distribution of the two major sediment facies and sample locations. B. Fluvial and marine parameters affecting sedimentation.
M64
LETTER SECTION
in suspension into depositional environments where lower turbulence prevails. Its absence in modern sediments was taken by Doyle et al. (1968) as probable indication of nondeposition. In this study, we demonstrate the value of counting mica and total shell content as a rapid, yet sensitive, petrologic method for determining energy levels and depositional regimen in a classic modern deltaic environment. METHODS About 340 grab samples collected by the Mees Cremer Expedition in 1958 and the University of Miami in 1964 and 1965, supplemented by over 100 samples collected by one of us (O.S.A.) from the lagoons and estuaries of southern Nigeria were used for this study. All samples were wet-sieved and the terrigenous and carbonate components of the coarser than silt fraction (0.062-2.00 mm) were examined petrographically. In each case, 300 grains were counted for the major components (light, heavy and opaque minerals, mica, glauconitic and phosphatic pellets, planktonic and benthonic Foraminifera, shell fragments and plant debris).
R/VPillsbury of the
DISTRIBUTIONOF MICA AND BIOGENICFRACTION lsopleths for mica and for total shell - planktonic and benthonic Foraminifera plus shell fragments - (in parts per 300 grains counted) are shown in Fig.2A,B. Mica isopleths are roughly parallel to the coast line. A narrow band, 2 - 3 miles wide in the littoral zone, containing less than 2 mica/300 grains is succeeded by a relatively broad zone of high mica concentration between the 3 and the 3 0 - 5 0 fathom contour intervals. The mica content in this band increases abruptly from about 3 to 250 mica/300 grains. Particularly high concentrations were recorded in four areas on the shelf north of the head of Mahin Canyon; off the mouth of the Forcados River; off Bonny River and off the Cross River (see Fig.2A). This mica-rich band ranges in width from about 5 miles in the western part of the study area off Lagos to over 30 miles, being widest on the axis of the delta off the Nun River. It lies within the outer delta-front platform, prodelta slope and inner open shelf of Allen (1965, fig.7) where surficial sediments consist largely of layered time silty sand and mud. It is conspicuously interrupted in two places: in the vicinity of Avons Deep and in the Bight of Biafra off the mouth of the Imo River and west of Fernando Poo (Fig.2A). The mica content decreases rapidly ( < 3/300 grains) on the outer open shelf and upper glope. Miea is consistently rare in the zone of the "Older Sands" facies and its distribution is highly irregular along rivers and in estuaries. Isopleths for total shell also parallel the coastline and bathymetric contours on the shelf with a bulge on the delta axis (Fig.2B). A narrow zone of low shell ( 0 - 1 0 shell/300 grains), ranging in width between 1 and 12 miles, borders the coastline. There is a general seaward increase in carbonate content beyond this zone. Beyond a depth of 2 0 - 5 0 fathoms the carbonate content increases to over 200 shell/300 grains and is mostly composed of foraminiferal tests (see Adegoke et al., 1971). The biogenic fraction of the relict "Older
LETTER SECTION
M65
Sands" facies, on the other hand, is composed mostly of fragmented shells much of which is probably of pre-Holocene age (Allen, 1965).
DISCUSSION Since the inception of the modem Niger Delta during the Cretaceous, sediment supply has dominated over subsidence and base level changes. The long-term result of sedimentation has produced a steady southward progradation of the deltaic wedge (Short and St/iuble, 1967; Weber, 1971) and the consequent bowing out of topographic contours in front of the subaerial delta axis. The river Niger and its tributaries presently supply about 25,000,000 tons of sediment to the shelf annually (Nedeco, 1961). These sediments are dispersed by the combined effect of major deep currents, notably the Guinea Current, opposed by the wave- and tide-generated longshore currents. The latter are less effective in eroding the sea floor beyond the littoral and delta-front zones (Fig. 1B). Mica, transported from the geologically complex hinterland into the turbid, highenergy coastal zone, is effectively winnowed seaward by current action. It by-passes the clean sand and silt littoral and inner delta-front platform and is selectively deposited with Freer interbedded clayey silt and f'me sand in deeper water. The > 2 mica/300 grains isopleth at a depth of 3-5 fathoms thus marks the limit beyond which active deposition of f'me terrigenous material takes place. The zone of mica abundance on the outer delta-front platform, prodelta slope and inner open shelf defines the area of predominant accumulation of fluviaUy-derived elastics. This belt coincides with the area of decreasing energy levels and def'mes the limit of the regressive Holocene blanket prograding seaward across much of the shelf beyond which there is slow accumulation of bioturbated mud and calcareous ooze (Adegoke et al., 1971). The two anomalous breaks in the mica-rich zone (Fig.2A) occur at the points of convergence of the opposing long-shore currents and the Guinea Current. These are relatively high-energy, non-depositional areas in which the mica is lost by winnowing, and the sands brought in by the long-shore currents are siphoned into the active submarine canyons (Burke, 1971). The isopleths for total shell also support these inferences. The coincidence of the high shell (> 200 shell/300 grains)-low mica isopleths is significant. Both define the present terrigenous depositional-null depositional boundary. The extent of this zone of sediment accumulation varies systematically: it is widest on the nose of the delta (about 30-35 miles wide) and is only 5 miles wide off the nondeltaic coast of Lagos. The Fingerlike projection of the high mica zone into the Cross River estuary is caused by the proximity of the source area, the volcanic and undifferentiated Basement Complex rocks of the Oban Massif. This method may hace potential application in the sedimentological study of ancient deltaic regimes.
M66
LETTER SECTION
ACKNOWLEDGEMENTS We thank Dr. C. Kruit of Koninklijke/Shell, The Hague, and Dr. Gilbert L. Voss of the University of Miami for providing samples used in this study. The on-going investigation is supported by research funds of the University of Ife, Ile-Ife, Nigeria, and the Smithsonian Research Foundation (FY 1972-472350). REFERENCES Adegoke, O.S., Dessauvagie,T.F.J. and Kogbe, C.A., 1971. Planktonic Foraminifera in Gulf of Guinea sediments. Micropaleontology, 17: 197-213. Allen, J.R.L., 1965. Late Quaternary Niger Delta, and adjacent areas: sedimentary environments and lithofacies. Bull. Am. Assoc. Petrol. Geologists, 49: 547-600. Burke, K., 1971. Longshore drift and submarine canyons in the development of the Niger Delta (Abstract). In: Program with Abstracts, VllI lnternational Sedimentological Congress 1971, pp. 15-16. Doyle, L.J., Cleary, W.J. and Pilkey, O.H., 1968. Mica: its use in determining shelf-depositional regimes. Mar. Geol., 6" 381-389. Nedeco (Netherlands Engineering Consultants), 1959. River Studies and Recommendations on Improvement of Niger and Benue. Amsterdam, 1000 pp. Nedeco (Netherlands Engineering Consultants), 1961. The Waters of the Niger Delta. The Hague, 317 pp. Neiheisel, J., 1965. Source and distribution of sediments at Brunswick Harbor and vicinity of Georgia. U.S. Coastal Eng. Res. Center, Tech. Mere., 12:21 pp. Pomerancblum, M., 1966. The distribution of heavy minerals and their hydraulic equivalents in sediments of the Mediterranean continental shelf of Israel. 3". Sediment. PetroL, 36: 162-174. Short, K.C. and St/iuble, A.J., 1967. Outline of geology of Niger Delta. Bull. Am. Assoc. Petrol. Geologists, 51: 761-779. Weber, K.J., 1971. Sedimentological aspects of oil fields in the Niger Delta. Geol. Mijnbouw, 50: 559-576.
Letter Section Mica a n d shell as i n d i c a t o r s o f e n e r g y level a n d d e p o s i t i o n a l r e g i m e o n t h e Nigerian Shelf
OLUWAFEYISOLA S. ADEGOKE and DANIEL J. STANLEY Department of Geology, University of lfe, Ile-lfe (Nigeria) Division of Sedimentology, Smithsonian Institution, Washington, D. C (U.S.A.) (Accepted for publication October 25, 1972) ABSTRACT Adegoke, O.S. and Stanley, D.J., 1972. Mica and shell as indicators of energy level and depositional regime on the Nigerian Shelf. Mar. Geol., 13: M61 -M66. The recent sediments on the Nigerian Shelf are derived largely from the Niger and Benue rivers and distributed by the deep Guinea Current and a complex series of wave- and tide-generated shallow, long-shore currents. The distribution of sand-size mica flakes and total shell counts are used to recognize energy levels and to delineate zones of by-passing and modern terrigenous deposition in a regressive deltaic setting.
INTRODUCTION Sedimentation on the modern Nigerian Shelf is dominated by the Niger Delta, a thick depositional wedge actively prograding onto the shelf. This delta has been investigated by a number of workers (Nedeco, 1959, 1961 ; Short and St/iuble, 1967; Weber, 1971). The most comprehensive sedimentological study to date is by Allen (1965) who recognized two Quaternary sedimentary sequences: a late-glacial to early Holocene "Older Sands" facies partially buried by a Holocene "Younger Suites" facies (Fig. 1A). Allen noted that the shelf is characterized by a concentric arrangement of sedimentary facies w i t h respect to heads of passes, and that there is progressive textural fining of sediments with depth. These factors were correlated with the complex interplay of hydrological and physical oceanographic factors (Fig. 1B) and with the diminishing intensity of energy conditions away from the coast. He, however, observed a notable disparity between lithofacies and his sedimentary environments based on zones of equal processes and energy levels. Close correlation between sand-size mica flakes, sedimentary texture and sea floor processes has been recorded in a number of continental margin settings (Neiheisel, 1965; Pomerancblum, 1966; Doyle et al., 1968). Because of its hydraulic equivalence to silt, mica tends to be winnowed out of high-energy environments such as the littoral zone and carried
LETTER SECTION
M62
KM
'/
D ONITSHA
\\ ,5*
4°
SUPER - ENVIRONMENTS 3o
• SAMPLE STATION DEPTH IN FATHOMS
A
NON-DEPOSITIONAL "Older S a n d s " facies CONTINENTAL SHELF ~ yq CONTINENTAL SLOPE ] " o u n g e r 5 u i t e s " f a c i e s S=, 6°1 7°
~
4o !
po
~CONTINENTAL ~
~
~ RIVER
'
,...',
I'
/ WAVE-GENERATED,"
/
~ ~ ,
~
CONTINENTAL SLOPE
//
t~"I ,
,
CURRENTS
~'
~~
~*
~,,
GUINEA CURRENT ~ J I I I l l l l ~ ~ , ,
I
// '~ '~ ~
. . , . ,,0,~' t
~
*z ~.,...
~
~
~'OEEP C°RR~,;F~ . S , " ° ' o , ~
B
SHELF
,.
,.
6.
i
t
I
' ......
7. i
I'°°
t
:
8. ,
~
~" ~
LETTER SECTION
M63
Fig.2. Charts showing selected compositional parameters on Nigerian Shelf. A. Mica isopleths. B. Shell isopleths. Fig. 1. Charts showing Niger Delta and Nigerian Shelf, modified after Allen (1965). A, Distribution of the two major sediment facies and sample locations. B. Fluvial and marine parameters affecting sedimentation.
M64
LETTER SECTION
in suspension into depositional environments where lower turbulence prevails. Its absence in modern sediments was taken by Doyle et al. (1968) as probable indication of nondeposition. In this study, we demonstrate the value of counting mica and total shell content as a rapid, yet sensitive, petrologic method for determining energy levels and depositional regimen in a classic modern deltaic environment. METHODS About 340 grab samples collected by the Mees Cremer Expedition in 1958 and the University of Miami in 1964 and 1965, supplemented by over 100 samples collected by one of us (O.S.A.) from the lagoons and estuaries of southern Nigeria were used for this study. All samples were wet-sieved and the terrigenous and carbonate components of the coarser than silt fraction (0.062-2.00 mm) were examined petrographically. In each case, 300 grains were counted for the major components (light, heavy and opaque minerals, mica, glauconitic and phosphatic pellets, planktonic and benthonic Foraminifera, shell fragments and plant debris).
R/VPillsbury of the
DISTRIBUTIONOF MICA AND BIOGENICFRACTION lsopleths for mica and for total shell - planktonic and benthonic Foraminifera plus shell fragments - (in parts per 300 grains counted) are shown in Fig.2A,B. Mica isopleths are roughly parallel to the coast line. A narrow band, 2 - 3 miles wide in the littoral zone, containing less than 2 mica/300 grains is succeeded by a relatively broad zone of high mica concentration between the 3 and the 3 0 - 5 0 fathom contour intervals. The mica content in this band increases abruptly from about 3 to 250 mica/300 grains. Particularly high concentrations were recorded in four areas on the shelf north of the head of Mahin Canyon; off the mouth of the Forcados River; off Bonny River and off the Cross River (see Fig.2A). This mica-rich band ranges in width from about 5 miles in the western part of the study area off Lagos to over 30 miles, being widest on the axis of the delta off the Nun River. It lies within the outer delta-front platform, prodelta slope and inner open shelf of Allen (1965, fig.7) where surficial sediments consist largely of layered time silty sand and mud. It is conspicuously interrupted in two places: in the vicinity of Avons Deep and in the Bight of Biafra off the mouth of the Imo River and west of Fernando Poo (Fig.2A). The mica content decreases rapidly ( < 3/300 grains) on the outer open shelf and upper glope. Miea is consistently rare in the zone of the "Older Sands" facies and its distribution is highly irregular along rivers and in estuaries. Isopleths for total shell also parallel the coastline and bathymetric contours on the shelf with a bulge on the delta axis (Fig.2B). A narrow zone of low shell ( 0 - 1 0 shell/300 grains), ranging in width between 1 and 12 miles, borders the coastline. There is a general seaward increase in carbonate content beyond this zone. Beyond a depth of 2 0 - 5 0 fathoms the carbonate content increases to over 200 shell/300 grains and is mostly composed of foraminiferal tests (see Adegoke et al., 1971). The biogenic fraction of the relict "Older
LETTER SECTION
M65
Sands" facies, on the other hand, is composed mostly of fragmented shells much of which is probably of pre-Holocene age (Allen, 1965).
DISCUSSION Since the inception of the modem Niger Delta during the Cretaceous, sediment supply has dominated over subsidence and base level changes. The long-term result of sedimentation has produced a steady southward progradation of the deltaic wedge (Short and St/iuble, 1967; Weber, 1971) and the consequent bowing out of topographic contours in front of the subaerial delta axis. The river Niger and its tributaries presently supply about 25,000,000 tons of sediment to the shelf annually (Nedeco, 1961). These sediments are dispersed by the combined effect of major deep currents, notably the Guinea Current, opposed by the wave- and tide-generated longshore currents. The latter are less effective in eroding the sea floor beyond the littoral and delta-front zones (Fig. 1B). Mica, transported from the geologically complex hinterland into the turbid, highenergy coastal zone, is effectively winnowed seaward by current action. It by-passes the clean sand and silt littoral and inner delta-front platform and is selectively deposited with Freer interbedded clayey silt and f'me sand in deeper water. The > 2 mica/300 grains isopleth at a depth of 3-5 fathoms thus marks the limit beyond which active deposition of f'me terrigenous material takes place. The zone of mica abundance on the outer delta-front platform, prodelta slope and inner open shelf defines the area of predominant accumulation of fluviaUy-derived elastics. This belt coincides with the area of decreasing energy levels and def'mes the limit of the regressive Holocene blanket prograding seaward across much of the shelf beyond which there is slow accumulation of bioturbated mud and calcareous ooze (Adegoke et al., 1971). The two anomalous breaks in the mica-rich zone (Fig.2A) occur at the points of convergence of the opposing long-shore currents and the Guinea Current. These are relatively high-energy, non-depositional areas in which the mica is lost by winnowing, and the sands brought in by the long-shore currents are siphoned into the active submarine canyons (Burke, 1971). The isopleths for total shell also support these inferences. The coincidence of the high shell (> 200 shell/300 grains)-low mica isopleths is significant. Both define the present terrigenous depositional-null depositional boundary. The extent of this zone of sediment accumulation varies systematically: it is widest on the nose of the delta (about 30-35 miles wide) and is only 5 miles wide off the nondeltaic coast of Lagos. The Fingerlike projection of the high mica zone into the Cross River estuary is caused by the proximity of the source area, the volcanic and undifferentiated Basement Complex rocks of the Oban Massif. This method may hace potential application in the sedimentological study of ancient deltaic regimes.
M66
LETTER SECTION
ACKNOWLEDGEMENTS We thank Dr. C. Kruit of Koninklijke/Shell, The Hague, and Dr. Gilbert L. Voss of the University of Miami for providing samples used in this study. The on-going investigation is supported by research funds of the University of Ife, Ile-Ife, Nigeria, and the Smithsonian Research Foundation (FY 1972-472350). REFERENCES Adegoke, O.S., Dessauvagie,T.F.J. and Kogbe, C.A., 1971. Planktonic Foraminifera in Gulf of Guinea sediments. Micropaleontology, 17: 197-213. Allen, J.R.L., 1965. Late Quaternary Niger Delta, and adjacent areas: sedimentary environments and lithofacies. Bull. Am. Assoc. Petrol. Geologists, 49: 547-600. Burke, K., 1971. Longshore drift and submarine canyons in the development of the Niger Delta (Abstract). In: Program with Abstracts, VllI lnternational Sedimentological Congress 1971, pp. 15-16. Doyle, L.J., Cleary, W.J. and Pilkey, O.H., 1968. Mica: its use in determining shelf-depositional regimes. Mar. Geol., 6" 381-389. Nedeco (Netherlands Engineering Consultants), 1959. River Studies and Recommendations on Improvement of Niger and Benue. Amsterdam, 1000 pp. Nedeco (Netherlands Engineering Consultants), 1961. The Waters of the Niger Delta. The Hague, 317 pp. Neiheisel, J., 1965. Source and distribution of sediments at Brunswick Harbor and vicinity of Georgia. U.S. Coastal Eng. Res. Center, Tech. Mere., 12:21 pp. Pomerancblum, M., 1966. The distribution of heavy minerals and their hydraulic equivalents in sediments of the Mediterranean continental shelf of Israel. 3". Sediment. PetroL, 36: 162-174. Short, K.C. and St/iuble, A.J., 1967. Outline of geology of Niger Delta. Bull. Am. Assoc. Petrol. Geologists, 51: 761-779. Weber, K.J., 1971. Sedimentological aspects of oil fields in the Niger Delta. Geol. Mijnbouw, 50: 559-576.