Sedimentary Geology 216 (2009) 125–137

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Sedimentary Geology

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The influence of sea-level changes on tropical coastal lowlands; the PleistoceneCoropina Formation, Suriname

Th.E. Wong a, R. de Kramer a, P.L. de Boer a,⁎, C. Langereis a, J. Sew-A-Tjon b

a Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlandsb N.V. BHP Billiton Maatschappij Suriname, P.O. Box 10053, Onverdacht/Para District, Suriname

⁎ Corresponding author.E-mail address: pdeboer@geo.uu.nl (P.L. de Boer).

0037-0738/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.sedgeo.2009.02.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 January 2006Received in revised form 18 February 2009Accepted 27 February 2009

Keywords:SurinamePleistoceneQuaternaryPalaeomagnetismTropical coastal plainCheniers

The Pleistocene Coropina Formation largely constitutes the Old Coastal Plain of Suriname. It is exposed fullyonly in open-pit bauxite mines in the central coastal plain as part of the unconsolidated overburden ofPaleocene–Eocene bauxites. This study deals with the stratigraphy, sedimentology and chronology of thisformation, and is based on a study in the recently closed Lelydorp-III bauxite mine operated by N.V. BHPBilliton Maatschappij Suriname.The Coropina Formation consists of the Para and Lelydorp Members. We present a detailed lithologicalsubdivision of these members. In the Para Member, four units are discerned which are grouped in twotransgressive cycles, both rangingupward fromterrestrial towards chenier and coastalmudflatdeposits reflectingglacio-eustatic sea-level changes. The sandy sediments representfluviatile and beach-bar (chenier) deposits, andwere supplied by rivers from the Precambrian basement and to a lesser extent by westward longshore coastaldrift. Clays, largely derived from the Amazon River and transported alongshore over the shelf, were deposited inextensive coastal mudflats. The Lelydorp Member, also comprising four units, represents a depositional systemthat is closely comparable to the recent Suriname coastal setting, i.e., a lateral and vertical alternation of mudflatand chenier deposits formed over a period characterised by more or less constant sea level.Palaeomagnetic data indicate a dominantly reversedmagnetic polarity in the ParaMember,whereas the LelydorpMember shows a normal magnetic polarity with a minor reversed polarity overprint. The reversed polarities ofthe Para Member exclude a Brunhes Chron (0.78–0.0 Ma) age, and thus assign it to the Matuyama Chron (2.58–0.78 Ma). This implies that the Coropina Formation is much older than hitherto assumed, and that one or more(long-term) hiatuses are not recognizable in the lithological succession.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

A general problem in terrestrial and coastal sedimentology andstratigraphy is the common lack of precise dating of the non-marinesedimentary record. Stacked sequences of terrestrial and shallow-marine sedimentary facies, interrupted by soil formation and long-term erosional surfaces usually do not allow dating due to alterationsand decay or original lack of adequate material. Here we analyse thesedimentology of a Pleistocene–Holocene succession in the coastalplain of Suriname and use palaeomagnetic dating in order to putconstraints on the age of the deposits.

All along the north coast of South America, from the mouth of theAmazon River to Venezuela, suspended mud and fine sand aretransported alongshore by the Guyana Current, and are broughtshoreward by bottomwaters flowing up the slope and across the shelf(cf. Gibbs, 1976). Along the shore, extensive shoreface-attachedmudflats alternating with sandy cheniers dominate the coast.

ll rights reserved.

Attachment of mud and cheniers to the coast is a continuous processcausing a general progradation of the coastline (Eisma and Van derMarel, 1971; Augustinus, 1980), and occurred ever since the uplift ofthe Andes started to yield abundant fine-grained sediment to theAmazon River. During glacial periodswith a low sea level the shelf wasexposed, and the Amazon spilled its sediment load directly into deepwater (Lopez, 2001). Tidal mud flats are protected against erosion bybiofilms, andmangrove vegetation shelters the coastline from erosion.Minor advances and retrogradations of the coastline due to variationsin tidal current strength on a bi-decadal time scale (Gratiot et al.,2008) wipe out the effects of each other over longer time spans.

The major part of Suriname consists of Precambrian crystallinerocks of the Guiana Shield, forming a hilly landscape covered bytropical rain forest. Basement rocks dip to the north, and are coveredby unconsolidated coastal-plain deposits (Fig. 1). The coastal plain hastraditionally been divided into two geomorphological units: theYoung Coastal Plain (northern part) with Holocene sediments and theOld Coastal Plain (southern part). The latter is a dissected chenierplainwith predominantly Pleistocene sediments (Veen,1970). Locally,incised gullies have been filled with organic clays of the HoloceneMara Formation. In the south the Old Coastal Plain is bounded by the

Fig. 1. Geological map of the coastal plain of Suriname. Note the east–west orientation of the sandy units (cheniers) within the Holocene and Pleistocene sediments of the coastalplain (after Wong, 1989).

126 T.E. Wong et al. / Sedimentary Geology 216 (2009) 125–137

Savannah Belt which comprises mainly Pliocene terrestrial sediments(Zanderij Fm) and minor residual weathering products of thePrecambrian basement.

The surface and subsurface sediments of the coastal-plain areaform the narrow on-shore part of the large Guiana Basin that ispresent north of Guyana, Suriname and French Guiana. In Surinamethese sediments have been grouped into the Corantijn Group (Fig. 2).The group consists of a homoclinally dipping (ca. 1° North) successionof mainly clastic sediments, varying in thickness from 2000 m at themouth of the Corantijn River in the west to 200 m at the MarowijneRiver in the east. The age of the Corantijn Group ranges from LateCretaceous to Holocene. The Late Cretaceous sediments, unconform-ably overlying the Precambrian basement locally in the subsurface,belong to the Nickerie Formation. This formation is locally overlainwith an unconformable contact by Paleocene–Eocene sediments ofthe originally coarse–clastic alluvial Onverdacht Formation, whichmostly rests unconformably on the Precambrian basement. Towardsthe north the Paleocene–Eocene sediments are represented by thelateral equivalent of the Onverdacht Formation, the fluvio-deltaicSaramacca Formation and the marine Alliance Formation, bothextending offshore. The Onverdacht Formation forms a non-exposed10 km wide belt, some 40 km inshore, from about 40 km west of theSuriname River to the east into French Guiana.

During a long period of non-deposition, known as the Late Eoceneto Oligocene “Bauxite Hiatus”, intense weathering of the OnverdachtFormation resulted in bauxitization of its upper part. The resultinglandscape was characterized by isolated erosion surfaces and bauxitehardcaps, surrounded and commonly covered by Pleistocene deposits(Wong, 1992). During the Oligocene, Miocene and Pliocene, theBurnside, Coesewijne and Zanderij Formations have been deposited,with an unconformable contact. All formations are characterized bymedium to course, angular kaolinitic quartz sand with interbeddedclay. The Zanderij Fm is the lowermost formation above thePrecambrian basement that is exposed (Figs. 1 and 2). The PleistoceneCoropina Formation comprises the Para and Lelydorp Members. ThePara Member consists of coarse braided fluvial deposits, cheniers, andfluvial and lagoonal (silty) clays. The Lelydorp Member is made up ofan alternation of mudflat and very fine sandy chenier deposits. Duringthe Holocene the coastal-plain sediments of the Mara Formation(N6000 years) and Coronie Formation (b6000 years) have beendeposited (Wong, 1989). The tripartition of the Coronie Formation(Fig. 1) is based on age, soil and geomorphological characteristics.Present-day coastal sediments, the Comowine Deposits, form theupper part of the Coronie Formation.

The present coast of Suriname is under the influence of theAmazon River (Eisma and Van der Marel, 1971). Amazon River

Fig. 2. Stratigraphical division of the Corantijn Group (modified afterWong,1986,1992). North of the Onverdacht Fm are the time-equivalent deltaic Paleocene–Eocene Saramacca Fmand the marine Alliance Fm.

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sediments consisting of a mixture of clay and very fine sand aretransported westward, alongshore towards the Orinoco River inVenezuela through a system of extended migrating mudflats,alternating with sandy shores with cheniers (Augustinus, 1994). Thelongshore current is westward, and the NE-trade winds cause wavesto approach the shore at an angle of about 45°. The clays of themudflats are made up of a mixture of smectite, illite, kaolinite, andsome quartz (Table 1, association A). Clay is winnowed by wave actionfrom the upstream side of migrating mudflats and transported bycurrents to the frontal side of downstream mudflats, where high mudconcentrations result in the resettling of the clay particles (Augusti-nus, 1983; Eisma, 1992). This mechanism causes mudflat migrationalong the shore. With a spacing of 45 km of the fronts of adjacentmudflats and a migration rate of about 1.5 km/year, the coastexperiences a cyclicity of about 30 years. The retained sandsaccumulate and form nearshore sandbars which are pushed towardsthe high-water line by incoming waves, forming cheniers that migrateover the clay deposits of the mudflats (Augustinus, 1980) (Fig. 3). TheAmazon River has existed since mid-Miocene times more or less as itis today. During sea-level highstands the coast between the mouth ofthe Amazon River in Brazil and the Orinoco River in Venezuela musthave been comparable to the present-day situation. During sea-levellowstands, the shelf was exposed and Amazon-derived (muddy)sediments must have been shed directly to deep water (Lopez, 2001).

Past studies of the Pleistocene Coropina Formation and of thegeology of the coastal plain (Schols & Cohen, 1951, 1953; Montagne,1964; Brinkman & Pons, 1968; Aleva et al., 1969; Nota, 1969; Veen,1970, Krook, 1970, 1979, 1994; Wong, 1986, 1989, 1992) stressed thatthe sedimentary history of the Coropina Formation has been governedby global sea-level changes due to Pleistocene glaciations. In this

Table 1The dominant claymineral composition of five different areas, ranging from the AmazonRiver estuary to the Precambrian Guiana Shield in Suriname (after Brinkman, 1967).

Sample class Dominant claymaterial associations

Amazon estuary A (75%), B (25%)Marine mud on continental shelf between AmazonRiver and Essequibo River

A (87%)

Estuarine mud from rivers in Suriname and Guyana A (91%)Marine young coastal plain A (85%), B–C (13%)Residual Guiana Shield (Precambrian basement) D (61%), D–C (23%)

A: 20–30% smectite, 20% illite, 40% kaolinite and some quartz.B–C: 10% chlorite–vermiculite, 20–30% illite, 40–50% kaolinite and some quartz.D–C: kaolinite, some quartz and a trace of illite.D: kaolinite and some quartz.

context, deposition took place at high sea levels corresponding tointerglacial periods, whereas erosional periods represent sea-levellowstands during glacial periods. It thus was assumed that the Paraand Lelydorp Members of the Coropina Formation were depositedduring transgressions and sea-level highstands during the two lastinterglacials (120–20 ka BP), while soil formation and erosionoccurred during the intervening glacial (see above references). Dueto the lack of reliable age dating, these age assignments could never betested. For the present study, we have taken palaeomagnetic samplesto determine a magnetic polarity stratigraphy, in order to betterconstrain the age of the Coropina Formation.

The study area was confined to a previous open-pit mine as noother, natural outcrops occur in the area. Due to the temporarycharacter of this mine, the studied sections are not exposed anymore.From unpublished drilling data for bauxite and water prospectingelsewhere in Northern Suriname, it is known that in the coast-parallelzone the character of the succession is similar to that in the mine. Asthe previous open-pit mine is located on a bauxite-covered hill datingfrom Eocene times, the sedimentological development in thisparticular area may have been influenced by the topographical high.

2. Methods

The open-pit-bauxite mining operations of N.V. BHP BillitonMaatschappij Suriname in the recently closed Lelydorp-III Mine in thecoastal plain of Suriname (Fig. 1) yielded the unique opportunity toperformdetailed lithological andpalaeomagnetic studies of the20–30mthick sedimentary sequence covering the bauxite. These sedimentsmainly belong to the Coropina Fm, and were described and sampled ingreat detail. Palaeomagnetic measurements were done at the Paleo-magnetic Laboratory of Utrecht University.

3. Results

3.1. Sedimentology of the Coropina Formation in the Lelydorp area

We have subdivided each of the Para and Lelydorp Members of theCoropina Formation into four lithological units (Fig. 4).

3.1.1. Para Member (PM)The contact between the Paleocene–Eocene Onverdacht Forma-

tion, consisting of partly bauxitisized coarse clastic alluvial deposits,and the ParaMember is a sharp, irregular unconformity. The basal partof the Para Member onlaps the palaeotopography of bauxite andkaolinitic clays of the Onverdacht Formation (Fig. 5).

Fig. 3. At the upper left a developing small chenier along the present-day coast of Suriname. Location: Weg naar Zee.

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3.1.1.1. Lower sand unit (PM-ls)3.1.1.1.1. Sedimentological description. The unit is subdivided into

a lower and an upper interval, mainly based on the difference insedimentary structures. The lower interval consists of middle to verycoarse-grained, angular–subangular, ochre–brownish quartz sandwith large-scale (max. height±25 cm), high-angle (20°) tangentialforesets (Fig. 6). The foresets dip to the north, with the exception ofthe top part where the set height decreases, and some more variationin current directions occurs. This interval is characterised by sharplower and upper boundaries. Moreover, long (up to 50 cm) and small-diameter (1 cm) rootlet traces occur in the basal part.

The upper interval comprises very coarse to middle-grainedstructureless, white, clayey sand. A very high content of kaoliniticclay and the absence of visible sedimentary structures characterizethis interval, and accompany an overall fining upwards from very-coarse sand at the basis to middle-grained sand at the top. Thetransition towards the overlying unit is gradual.

3.1.1.1.2. Depositional environment. The large-scale, coarse-grained, northward-dipping cross bedding and numerous internaltruncations (Fig. 4) indicate a braided-river origin, the decreasing setheight in the top of the lower interval indicating a decrease inaccommodation space. The more variable foreset directions towardsthe top may be due to a more meandering character of the fluvialsystem and/or to the accretion of a chenier-fed spit which forced theriver to deviate, comparable to the evolution of the Mana River inFrench Guiana since 1785 AD (cf. Augustinus, 1978; Aleva & Krook,1998). The high kaolinite-clay content in the upper interval must bedue to chemical weathering of feldspars and pedogenesis whichblurred sedimentary structures so that the origin of the fining upwardin the upper part (e.g., point bar) cannot be established.

Fig. 4. Section 1: lithology and palaeomagnetic data of the Coropina Fm. The palaeomagneticexamples in Fig. 10). The Lelydorp Member samples often show a reversed high-coercive cacquired during the Matuyama Chron. The Para Member shows a reversed polarity, with a noand open circles indicate a reversed polarity overprint.

3.1.1.2. Lower clay unit (PM-lc)3.1.1.2.1. Sedimentological description. This unit is a firm, white–

greyish sandy kaolinitic clay (with about 15% middle to very coarse-grained sand). A strong, red mottling is apparent in the top 25 cm. Theboundary with the overlying unit is sharp.

3.1.1.2.2. Depositional environment. This unit forms the logicalcontinuation of the fluvial deposits in the underlying unit and reflectsdeposition after a shift of the river and abandonment of this site and/or in the floodplain. The dominance of kaolinite in the clay fraction inboth the lower sand and clay units of the Para Member indicates thatthe crystalline basement in the south remained a dominant source ofsediment (cf. Table 1). The strong mottling at the top points to anextended period of non-deposition.

3.1.1.3. Upper sand unit (PM-us)3.1.1.3.1. Sedimentological description. In the eastern part of the

Lelydorp-III Mine the unit is subdivided into a lower and an upperinterval. The lower interval consists of well cemented middle to verycoarse-grained, structureless, white–greyish, kaolinitic sand. The claypercentage is about 20%. A few dispersed pebbles (Ø max. 0.8 cm)occur. The interval is made up of gently southward-dipping sand bedsconstituting a series of fining-upward sequences. The sharp upper andlower boundaries also dip gently to the South.

The upper interval onlaps the southward-dipping upper boundaryof the lower interval (Fig. 7). In the basal part, wavy to parallel-laminated, blue–greyish silty clay beds with red mottling alternatewith poorly sorted,middle- to coarse-grained, parallel-laminated sandbeds. The wavy lamination is made up of wave ripples. The higher partof the interval consists ofmiddle to very coarse-grained, ochre–reddishclayey sands with high-angle cross-stratified, wedge-shaped beds

data are given as declination of the characteristic remanent magnetisation (ChRM; seeomponent (Fig. 10), suggesting that at least part of the magnetisation must have beenrmal polarity interval in the middle part. Closed circles represent remanent magnetism,

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Fig. 4 (continued).

130 T.E. Wong et al. / Sedimentary Geology 216 (2009) 125–137

(tangential foresets), which alternatewith the clay beds. The thicknessof the cross-stratified beds increases upward, and towards the top claybeds die out. Foresets have a uniform northward dip (Fig. 4), with theexception of the top part where also westward directions occur. Theinterval coarsens upwards to very coarse-grained sand with smallpebbles (Ø 0.5 cm),with a gradational boundary to the overlying upperclay unit (PM-uc). Ophiomorpha-type burrows occur.

Towards the western part of the mine the upper part of the PM-usunit shows a gradually changing character over a distance of about500m (Figs.1 and 8). Here (lower part of section 3 in Fig. 8), the unit iscomposed of a few, large-scale, high-angle, cross-stratified sets ofmiddle-grained sand. Set thicknesses vary from 0.4 m–2.8 m, anddecrease towards the top. The sets are made up of middle-grained,clayey sand with some dispersed very coarse grains and clay clasts(Ø 10 cm). Clay laminae are intercalated at various levels. The top 50 cmconsists of large-scale, high-angle cross-stratified beds with foresetsdipping towards the N, W, and ESE. The beds truncate each other andconsist of an interbedding of cm- to dm-scale fine- to middle-grainedclayey sands and brown–grey clays. The upper boundary is gradational.

3.1.1.3.2. Depositional environment. Southward to westward dip-ping sand beds in the lower interval are interpreted as chenier deposits.The fining upward seen in these south(land)ward dipping beds (Fig. 4;20–18m)fits to observationsof cheniers byAugustinus (1978) that after

Fig. 5. The top of the Onverdacht Fm forms a pronounced palaeorelief in the Lelydorp-III Mindicated. Note the onlap of the PM-ls unit onto the Onverdacht FM. Palaeotopography (A), b(E).

the springtidewashoveractiondiminishes, that topsets and correspond-ing foresets from this final stage in sedimentation are preserved andcontain finer material due to the lower energy conditions.

In the interval above, the coarser grain size in the easternpart and thefiner towards the west fits to descriptions of the present-day coastalsystemwhere cheniers are coarse-grained close to areas where course-grained sediment from crystalline source areas in the south are broughtinto the system (cf. Augustinus, 1978), and fining to the west relates tothe alongshore transport. The brown–grey colours and burrowing in thefine-grained intervals indicate deposition in coastal mudflats.

3.1.1.4. Upper clay unit (PM-uc)3.1.1.4.1. Sedimentological description. This unit consists of a firm,

structureless, white–grey kaolinitic clay which is strongly red-mottled. The mottling intensity increases towards the top and hasoverprinted any possible lamination. The gradational lower boundary(Fig. 4) consists of an alternation of the coarse-grained, cross-stratified sand beds of the Upper sand (PM-us) unit and thestructureless clays of the Upper clay unit (PM-uc).

3.1.1.4.2. Depositional environment. Kaolinitic clays represent theproduct of chemical weathering of feldspatic sandstones supplied byfluvial processes from source areas in the south and were depositedunder quiet conditions in alluvial floodplains. Considering the intense

ine. The Lelydorp Member and the different lithological units of the Para Member areauxite (C), and the more or less horizontal lower boundary (D) of the resilificated clays

Fig. 6. High-angle, large-scale cross-stratified, very coarse sandy sets in the lowerinterval of the Para Member PM-ls unit. Note the tangential foresets and the wedge-shaped lamina sets. Alternation of white and brownish foresets is related to grain-sizevariations: brownish foresets consist of coarse to very-coarse-grained sands, whichgradually pass into white, medium-grained sands with a high kaolinite content.

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mottling, the top of this unit represents a long period of non-deposition and possibly some erosion.

3.1.2. Lelydorp Member (LM)The boundary between the Para Member and the Lelydorp

Member in the study area is characterized by laterally continuousand strongly mottled red kaolinitic clay which forms the top of thePara Member (Brinkman and Pons, 1968).

Fig. 7. Onlapping sequence of the PM-us upper interval characterized by the alternation of bland spacing between the clay layers varies. The unit becomes gradually thinner northward afirst marine influence recognized in the Lelydorp Member.

3.1.2.1. Lower clay unit (LM-lc)3.1.2.1.1. Sedimentological description. This is a local unit with

cm-scale interbedding of soft, silty, reddish clay beds and ochre, veryfine sandy laminae, wedging out to the west.

3.1.2.1.2. Depositional environment. Similarly to the top of thePara Member this interval also reflects very quiet depositionalconditions. Relatively fast sedimentation and/or climate-inducedabsence of vegetation should be the cause of the absence of mottlingand preservation of the fine sand laminae.

3.1.2.2. Lower sand unit (LM-ls)3.1.2.2.1. Sedimentological description. This unit consists of large-

scale, low-angle cross-stratified sands. The low-angle foresets aredirected both seaward and landward. The unit is composed ofunconsolidated very fine-grained, ochre, parallel-laminated sandbeds. Two types of beds are intercalated at various levels:

a) Blue–grey clay beds, consisting of a mm scale interbedding of clayand clayey silt laminaewith wavy to parallel lamination. Wave andcurrent ripples are abundant in the clayey silt beds. From thesebeds large (up to 90 cm length, Ø max 8 cm) Ophiomorpha-type(cf. Frey et al., 1978) knobby branching burrows penetrate theunderlying sediments.

b) Large-scale, fine-grained low- to high-angle cross-stratified sandbeds. High concentrations of organic matter and clay pebbles (Ø4 cm) are present along and primarily at the toes of the foresets.These cross-stratified beds occur as wedge-shaped, isolated stackedbeds with a width up to some metres and a thickness up to 60 cm.Additionally, clay flasers occur at discrete levels, especially in thebasal parts of the sand beds just above the underlying clay beds. Thecross section through this part of the succession (Fig. 8) with section

ueish and silty clay layers, and middle- to coarse-grained sands to the top. The thicknesss it onlaps onto the underlying PM-us lower interval. The clay composition indicates the

Fig. 8. Composite SSE–NNW cross section. See Fig. 1 for location. Section 1 (Fig. 4) is located some 500 m to the East and was projected on the section line.

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1 projected on the section line (Fig. 1) gives an idea of themorphology and dimensions of the chenier.

3.1.2.2.2. Depositional environment. The succession represents analternation of cheniers and clayey sediments deposited in coastalmudflats (cf. Augustinus, 1980). The latter were protected by cheniersat the seaward side, fairly similar to the sedimentary system that ispresently active along the coast of Suriname.

3.1.2.3. Upper clay unit (LM-uc)3.1.2.3.1. Sedimentological description. In section 1, the Lelydorp

Member Upper clay unit is subdivided into three intervals:

a) Striped yellow–blue lower interval (Fig. 9), consisting of cm-scaleinterbedding of yellow clayey silt to very fine sand beds (max.some cm thick), and blue–greyish, soft, silty clay beds up to a fewcm thick, with an equal sand/clay distribution. Interbedding hasbeen only slightly disturbed by root and burrow traces, both smalland large. Ophiomorpha burrows have a diameter of at maximum5 cm as above.

b) Grey middle interval. This interval has a comparable compositionas the lower interval, but differs with respect to the dominance ofsand (sand/clay ratio=70/30). Interbedding has been stronglydistorted by root traces and small and large Ophiomorpha burrows,and only traces of the original interbedding occur.

c) The grey upper interval shows a centimetre-scale interbedding ofyellow–grey clayey silt beds with greyish silty clay beds. The silt/clay ratio is 60/40. The interbedding has been strongly disturbed,but root and burrow traces cannot be distinguished.

In seaward direction (Fig. 8, sections 2–4) similar deposits alsoshow mottling and somewhat more reddish colours.

Fig. 9. Characteristic appearance of the Lelydorp Member LM-uc/lower interval. Thisinterval is made up of cm-scale interbedding of blue–grey, silty clay beds and yellowclayey to silty, very fine sand beds. Note the presence of clearly visible burrow traces andthe overall distortion of the sediment.

3.1.2.4. Upper sand unit (LM-us)3.1.2.4.1. Sedimentological description. This unit mainly consists

of grey–whitish, clayey silt. There is a minor content of very fine-grained sand and dispersed clay flasers. Ophiomorpha is common. Theunit has been strongly disturbed by bioturbation.

The top of the Lelydorp Member has been incised by numeroussmall gullies (width max. 10 m, depth up to 2 m), filled with peatyclays and peats of the Mara Fm.

3.1.2.4.2. Depositional environment. The Upper Clay Unit (LM-uc)and the fine-grained Upper Sand Unit (LM-us) formed underprotected conditions in a lagoon to coastal mudflats. Towards theNorth (seaward) the deposits were slightly better aerated as signifiedby the red colours. Bioturbation and vegetation (mangroves) variouslydisturbed the primary sedimentary features. The incision of the top ofthe Lelydorp Member by small gullies occurred during a period oflower sea level. The Mara Formation above represents swamps andmarshes in the coastal plain formed during sea-level rise. In areaswhere presently no artificial drainage is applied, the deposition ofsuch sediments continues.

3.2. Palaeomagnetic data; methods and results

Samples for palaeomagnetic age dating were taken in the ParaMember and in the Lelydorp Member at two different locations. Onlythe fine-grained layers (clays) were sampled, since coarse-grainedlithologies usually do not preserve the original magnetisation as theyare susceptible to later overprint. Samples were taken by pushingperspex cylinders (8 cm3) into the sediment.

All samples have been analysed by stepwise alternating field (AF)demagnetisation, using maximum fields up to 300 mT; thermaldemagnetisation could not be carried out because of the perspexsampling cups. The demagnetisation diagrams (Fig. 10) show a quitevariable behaviour, but most samples indicate a straightforwardcharacteristic remanent magnetisation (ChRM) of either normal orreversed polarity. In the Para member, for example, some samples showno decay up to 100 mT (P24 and P13), and only start to decrease inintensity at the highest fields (100–300 mT). This suggests that eithervery fine-grained hematite (Dekkers and Linssen, 1989) or a magneticFe-sulphide (e.g., Dekkers, 1988) is the main carrier of the remanentmagnetisation. Other examples show two components (e.g., P37) or noreliable demagnetisation behaviour at all (P22). In the Lelydorpmember,most samples showa consistent normal polarity component, removed at60–80 mT (e.g., L15), pointing to magnetite as the main carrier. Severalsamples show a small but clearly reversed component at the highestfields (e.g., L8). Possibly, this represents a (younger) high-coerciveoverprint, for example through ongoing diagenesis in particular layersand the formation of newmagneticminerals which subsequently recordthe geomagnetic field at the time of their formation (e.g., Langereis et al.,1997; Langereis & Dekkers, 1999). Although the demagnetisationbehaviour of the individual samples is quite variable, due to the variablelithology, the polarity results show a consistent pattern (Fig. 4).

The Lelydorp member shows only normal (N) polarities, but itmust be noted that at several levels we find clearly reversed (R)–possibly younger–high-coercive components. The Para section showsthree polarity intervals: R–N–R. The same interval sampled at anotherlocation in the mine shows two reversed polarity intervals, separatedby an interval that could not be sampled. The polarity patterns of thetwo sections agree well, if we assume that the normal interval in thePara Member occurs in the interval not sampled in the other section.

4. Discussion

4.1. Palaeomagnetic age

The clearly reversed polarities–with a normal polarity interval inbetween–of the Para member exclude a Brunhes Chron age. The Para

Fig. 10. Examples of orthogonal projection diagrams of alternating field demagnetisation of samples from the Lelydorp (L) and Para (P) Members. Open (closed) symbols denoteprojection on the vertical (horizontal) plane, number refer to demagnetisation steps in milliTesla (mT). Most samples yield a straightforward polarity, but occasionally show onlyscatter at field N6 mT (P22), or a high-coercive reversed component (L8).

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member must therefore be of Matuyama Chron age. The normalinterval could then represent one of several subchrons during theMatuyama: the Jaramillo subchron (1.07–0.99 Ma), the Cobb Mt.subchron (around 1.21 Ma), the Olduvai subchron (1.95–1.77 Ma) orthe Réunion subchron (2.15–2.13 Ma) (see Lourens et al., 2005). AJaramillo age must be excluded if we assume that the LelydorpMember is older than the Brunhes as discussed above, and could havea Jaramillo age (last normal interval in the Matuyama Chron). The

normal interval in the Para Member then should have an age of CobbMt, Olduvai or even Réunion. In case of Olduvai or Réunion, the ParaMember would be partly of Pliocene age.

The distinct magnetic polarity pattern allows links to be made toregional and global events. The predominantly normal polarities ofthe Lelydorp Member indicate deposition either during the normalpolarity Brunhes Chron (0.78–0.0 Ma) or during a normal subchronduring the predominantly reversed Matuyama Chron (2.58–0.78 Ma)

135T.E. Wong et al. / Sedimentary Geology 216 (2009) 125–137

(Fig. 11). The presence of a high-coercive reversed magnetisationcomponent (Fig. 4), however, suggests that the reversed componentshave been acquired during the Matuyama Chron.

A firm correlation to the geomagnetic polarity time scale is notpossible on the basis of the present results. We conclude, however,that a) the Lelydorp Member is either of Brunhes age or, moreprobably, represents a normal subchron during the Matuyama Chron,b) the Para Member is older than the Brunhes, and must be ofMatuyama age; it includes a normal interval, probably representingone of the Matuyama normal subchrons. That subchron must then beolder than the assumed subchron of the Lelydorp Member (Fig. 11).

Fig. 11. Comparison of palaeomagnetic results with the geomagnetic polarity time (GPTS).interval (Fig. 4), must therefore be older than the Brunhes Chron, and most probably belongJaramillo, Cobb Mt., Olduvai or Réunion subchrons. The presence of a high-coercive reverssuggests that the normal polarity signal was acquired during the Matuyama Chron, for exampmust have been acquired during an older subchron, i.e., Cobb Mt., Olduvai or Réunion.

4.2. Sedimentary history: the interplay of sea-level and climate change

4.2.1. Para MemberAt the start of deposition of the Para Member, the landscape was

characterised by isolated hills of the Onverdacht Formation, of whichthe upper part comprised bauxite formed during the Bauxite Hiatus(Late Eocene–Oligocene). The top of this bauxite developed as hardcaps which protected underlying sediments against erosion (Wong,1989). During the Miocene and Pliocene, the valleys between thesehills were largely filled, and locally also the hills were covered, bycoarse-grained kaolinitic quartz sands and stiff kaolinitic clays of the

The Para Member shows a dominantly reversed polarity with an intermittent normals to the Matuyama Chron. The intermittent normal interval then belongs to one of theed magnetisation in the normal polarity Lelydorp Member (e.g., sample L8 in Fig. 10)le during the Jaramillo subchron. In that case, the Para Member normal polarity interval

136 T.E. Wong et al. / Sedimentary Geology 216 (2009) 125–137

Coeswijne and Zanderij Formations. Most summits may not have beencovered by these Miocene and Pliocene sediments but it should not beexcluded that such sediments were removed during the Pliocene–Pleistocene erosion period. In this latter context the protruding hillsformed topographical expressions that influenced sedimentaryprocesses. These hills ultimately were covered in the Pleistocene bysediments of the Coropina Formation.

The Lelydorp-III Mine is situated on top of such a buried bauxitehill, of which the eroded surface is irregularly shaped (Fig. 5). Theinitial sedimentation of the Para Member represents a braided-riversystem that carried sand and gravel to the north through SW–NEtrending depressions in the bauxite surface palaeorelief. As thedepressions were filled, the system could increasingly spread laterallyand hydrodynamic energy levels decreased as indicated by a variationof foreset directions in de top of the PM-ls/lower interval, and by thedecrease in set thickness. This interpretation is largely based onobservations in the open-pit mines where the summits of bauxite hillsare relatively shallow. Laterally, the top of the Precambrian basementis located at a deeper level. From these deeper levels no comparable,detailed information is available but considering unpublished drillingdata for bauxite and water prospecting, it is believed that thePleistocene succession did not experience a significantly differentdevelopment there apart from its greater thickness.

The braid-plain depositional environment is indicative of periodswith severe erosion in the source area and strongly fluctuating run-off,representing periods of low sea level caused by glacials at highlatitudes and a consequently more arid climate than today, due tolower global temperatures and also to a greater distance to the sea. Thethick cross-stratified sets in the lower part of the ParaMember indicatea (rapid) vertical aggradation of the braid-plain deposits. In addition tothe decrease in gradient, the transition froma semi-arid climate duringthe sea-level lowstand to a more humid climate during sea-level riseresulted in the transition tomore quiet depositional conditions (Pm-ls,upper interval). The near-absence of visible structures is ascribed tothe later transformation of feldspars to kaolinite, the consequentreorganisation of sand grains and compaction.

As the rise of relative sea level continued, clays and onlappingmarinesands of the basal part of the PM-us upper interval were deposited. Claycomposition indeed indicates the nearness of the open sea. Contrary topure and white kaolinite clays elsewhere in the Para Member, the basal(75 cm) part of PM-us upper interval contains blue–greyish clayindicative of a marine origin. Brinkman (1967) investigated the originof the marine clays along the coast of Suriname (Table 1). The source ofthe pure kaolinite sediments must be the Precambrian basement in thehinterland and related older fluvial deposits to the South. The clays in thePM-us upper interval basal 75 cm, on the contrary, consist of illite,kaolinite and probably smectite clay minerals, and some quartz andmuscovite. This composition corresponds to that of Amazon River clays(Brinkman, 1967). Therefore, this shallow-marine basal bed of PM-us/upper interval is believed to represent the first marine transgression intothe Lelydorp area during the Pleistocene. Deposition of Amazon claysagrees with the idea of an area dominated by an ongoing rise in relativesea level. On the contrary, during sea-level lowstands, the Amazon Riverbrought its fine-grained sediments as far as the shelf edge, where thefineswere carried immediately to deepwater, as the shelf sea needed forlongshore transport towards theGuyana areawasexposed (Lopez, 2001).

At the location of section 1, thedark illite-containingmarine clays atthe base of the upper interval of the PM-us unit were covered by acoarse clastic, braided system fed by increased delivery of sedimentsfrom the source area. This interval grades into finer grained sands tothe west, inferred to be due to longshore transport. The laterallycontinuous and strongly mottled clay bed at the top of the ParaMember (PM-uc) represents a floodplain or mudflat depositionalenvironment. As these clays fully cover the coarse sediments of PM-ucunit, the braid-plain system had ceased, had laterally migrated, ordeposition of coarse sediments had been displaced southwards due to

an ongoing rise of sea level. The gradual decrease of the influence of thebraided system is illustrated also by the alternation of PM-us unit claysand coarse clastic sediments at the PM-us/PM-uc boundary. Duringthe cessation of the braided system the supply of kaolinitic clayscontinued. They were deposited in brackish mudflats landward ofmarinemudflats. Although this could not be checked in the field due toa lack of exposures, clay composition is expected to change graduallyfrom pure kaolinitic (association D) to the mixed clay mineralassociation (A) towards the north. The clays underwent intensemottling during a subsequent sea-level lowstand. Despite consolida-tion some erosion occurred, leading to the pronounced unconformitybetween the top of the Para Member and the overlying LelydorpMember. Again, the above interpretation is based on observations inthe open-pit LelydorpMine. Considering the general setting there is noreason to assume a different development to the east and the west.

4.2.2. Lelydorp MemberSince the top of the underlying Para Member is a strongly

weathered surface, the sediments of the Lelydorp Member weredeposited during a next transgressive phase as already postulated byVeen (1970). Depositional processes were comparable to the present-day ones, and periods of chenier development alternated with periodswith mudflat accretion. On the basis of the available data it cannot beestablished if a 30 year cyclicity related to the westward migratingmudflats (cf. Augustinus, 1978, 1994) or an 18.6 year cyclicity due tothe lunar nodal cycle (cf. Gratiot et al., 2008) also applies for theLelydorp sedimentation phase.

The Lelydorp Member is made up of an alternation of mudflatdeposits (LM-lc and LM-uc units) and very fine sandy chenier deposits(LM-ls and LM-us units). Mudflat clays are rich in organic matter asthey are today. On themudflats, crabs were abundant as is reflected bythe numerous large, Ophiomorpha burrows penetrating downwardfrom the clay beds. Considering the high rates of deposition in this sortof environment, this vertical stack ofmudflat and chenier depositsmayhave covered a relatively short period of at some few hundred to a fewthousand years, provided that there are no unrecognised hiatuses.

The strongly weathered and eroded surface of the LelydorpMember must be due to a fall of the sea level. The gradient increased,and humid, tropical conditions changed into a dry, semi-arid climate(Krook, 1970). Rivers were active and cut deep into the Pleistoceneand older deposits, resulting in a strongly dissected landscape.Sediments deposited during sea-level highstands after formation ofthe Lelydorp Member, if any, must have been eroded in this period. Inthe Early Holocene a major marine transgression flooded the incisedchannels, and peat accumulation resulted in the formation of theMaraFormation (Brinkman & Pons, 1968; Wong, 1992).

Although the Para–Lelydorp sequence bears the characteristics of acontinuous, uninterrupted succession formed in a period with a risingsea level, it should be realized that sea level rose and fell by repetitionand that hiatuses must dominate the Pleistocene succession. Exten-sive mottling at, for example, the transition from the Para Mamber tothe Lelydorp Member indicates a long period of non-deposition andpossibly erosion. Similar boundaries at other places may have goneunnoticed. As source areas and the controls of the sedimentary systemdid not change, long-term hiatuses may be indiscernible from atransition between different subfacies or lithologies in a continuoussequence. Thus, on the basis of the lithological succession it cannot beestablished if the Para Member and the Lelydorp Member form asuccession which is also continuous in time. The palaeomagneticresults, however, indicate that hiatuses must be present.

5. Conclusions

Detailed sedimentological logging of the Para and the Lelydorpmembers of the Coropina Formation allowed us to recognize severalsmaller units, with distinct lithological characteristics. Coarse-grained,

137T.E. Wong et al. / Sedimentary Geology 216 (2009) 125–137

braided fluvial intervals of the Para Member were formed when the sealevel was lower and climate was more arid than today. Initialsedimentation of the Coropina Formation onlaps over the truncatedbauxite surface. Intermittent rise of the sea level and an accompanyingmore humid climate led to a transition from coarse-grained braided tomeandering fluvial systems. Deposition of the Lelydorp Membercharacterized by chenier and lagoonal deposits reflects a later sea-level rise and highstand. These deposits strongly resemble the present-day coastal system. Hidden hiatuses within the succession cannot bedemonstrated in the field, but are apparent from the palaeomagneticresults.

The palaeomagnetic data indicate a dominantly reversed magneticpolarity in the Para Member. The Lelydorp Member shows a normalmagnetic polarity with a minor reversed polarity overprint whichsuggests that this member is of Early Pleistocene age. The exactlinkages to the Geomagnetic Polarity Timescale are problematic, butassigning both members to the Matuyama Chron is most logical. Thisimplies that the Coropina Formation is much older than the two lastglacial periods as was hitherto assumed.

Acknowledgements

The management of N.V. BHP Billiton Maatschappij Suriname isthanked for enabling this research and for permission to publish theresults. We greatly appreciate the useful suggestions made by ananonymous reviewer.

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