JOS BAKKER TECTONIC AND CLIMATIC CONTROLS ON LATE QUATERNARY SEDIMENTARY PROCESSES IN A NEOTECTONIC INTRAMONTANE BASIN (The Pitallto Basin,South Colombia)

CENTRALE LANDBOUWCATALOGUS

00000394 7401

Cover (thesis edition) The cover symbolizes the multi-disciplinary approach of this study. The irregular, titlebearing brown box at the top illustrates the basement morphology of the Pitalito Basin going from west to east (geology). The two block diagrams in the middle part of the cover illustrate the two different types of alluvial architecture going from west to east (geomorphology). The sides of the front cover shows a hypothetical general pollen diagram (palynology). At the back cover apart of the lithological column of section 20A is shown (sedimentology). CIP-DATAKONINKLIJKEBIBLIOTHEEK,DENHAAG Bakker,Jos Tectonic andclimatic controlsonlatequaternary sedimentary processes in aneotectonic intramontane basin (ThePitalitoBasin, SouthColombia)/JosBakker.-Wageningen: Department of SoilScience& Geology,AgriculturalUniversity.- 111. ThesisWageningen.-Withref.-WithsummaryinDutchandSpanish. ISBN90-9003479-X SISOam-colo565UDC[552.5:627.81](861)(043.3) Subjectheading:basinresearch;Colombia. f

Author's address J.G.M.Bakker Department ofSoilScience and Geology Agricultural University of Wageningen P.O.Box37 6700AA Wageningen The Netherlands

Promotoren:

Dr.S.B.Kroonenberg,hoogleraar inde geologie Dr.T. vanderHammen,emeritushoogleraar inde palynologie aan de Universiteitvan Amsterdam (UvA)

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JOS BAKKER TECTONIC AND CLIMATIC CONTROLS ON LATE QUATERNARY SEDIMENTARY PROCESSES IN A NEOTECTONIC INTRAMONTANE BASIN (The Pitalito Basin,South Colombia)

PROEFSCHRIFT TERVERKRIJGINGVAN DEGRAADVAN DOCTOR IN DELANDBOUW- ENMILIEUWETENSCHAPPEN, OPGEZAG VAN DERECTORMAGNIFICUS, DR.H.C. VAN DER PLAS, IN HETOPENBAAR TE VERDEDIGENOP WOENSDAG20JUNI 1990 DESNAMIDDAGS TEVIERUUR INDEAULA VAN DELANDBOUWUNIVERSITEIT TE WAGENINGEN

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Fig. 18. Tectonic-sedimentary evolution of the Pltallto Basin explained within the framework of a transtensional regime along a segment of the dextral strike-slip Garzon-Suaza fault system. In the left-hand figures the paleoenvironment is depicted. The right-hand figures represent profiles along the SW/NE-basln axis and correspond to. the outlined phases. The arrow points to the stratlgraphical position of the sediments which were deposited during the respective phases. For the legend of the sediments see Fig. 16.

57

Non-exposed basin features

rates withhigherratesin theeasternpartcompared tothewesternpart.Loading effects mayhaveplayedaroleinthisdifferentation; thepresenceofthethicksedimentpileinthe eastmighthaveleadtohigher subsidenceratesinthispartof thebasincomapred tothe westernpartwhere such athickpilewaslacking.Phase2lastedfrom =1.2Mato=200 Ka if the described sediments for this phase are assumed to be situated between approximately300and50mdepth. -Phase3 (Fig. 18C) The branching splay from the northern oblique-slip fault in the outermostNWpartofthestudy area(no.4;Fig. 12)suggeststheexistenceofayounger phase of westward basin extension. This normal fault may be considered as the 'third generation'of splays.Phase3describes theperiod from =200,000years B.P.to 17,000 yearsB.P.Duringthisphasematerialwasdeposited which constitutestheactual surface andnear-surface sedimentsof thebasin.Theyrepresent thefinal stageof basin fill and areoffluvial origin.Their sedimentological characteristicswillbediscussedindetailin Chapter4.Fromthegeoelectricaldatathefollowing conclusioncanbedrawn: West from the actual position of the Guarapas river, coarse-grained sediments were depositedontopoftheunderlyingfluvio-lacustrine sedimentsfromphase2(coarseningupward megacycle). This is explained by an eastward prograding fluvial system. The progradation canbeascribed todecreasing subsidence ratesintheentirebasinortothe increaseof sediment supply,leadingtoastatewherebasininfilling outpaced subsidence. According to McClean & Jerzykiewicz (1978) such a state would produce maximum alluviationdistally,andpossiblyerosion andreworkingproximally. Similarly,theRidge Basinof California isfilled byfluvial sedimentsinitslaststage(Crowell&Link 1982). Totheeastthegrain sizeofthematerialdeposited bythisriversystemdecreased.Except fortheoccasionaldeposition of sandinthisarea,detritalinflux waslow andenabled the accumulation ofvery fine material likeclay andpeat.Thetransition between thecoarse sedimentsinthewestandthefine-grainedsedimentsintheeastislocated alongtheline oftheNW/SE-oriented normalfault (no.5;Fig. 12).Thismeansthattectoniccontrolstill playedanimportantroleinthedistributionofthesediments. -Phase4 (Appendix I) Thisphaseisreflected bythemodern surface morphology and thepresentprocesses that takeplaceinthePitalitoBasin.Themodern drainage system shows aNWflow direction and the river bedsare incised deeply into the accumulated sediments.The90°-turnofflow direction (section 2.2.1)andthereversion ofdeposition toerosion,markthetransitionfrom phase3tophase4.Sucharegionalchangemightbe explained by someallocyclic control,e.g.tectonicsoraclimaticchange.According the 14CdatingsofPIT 11andPIT2,phase4started somewhere between 17,000and7,000 years B.P. 3.4 Discussion Thedevelopment andmorphology ofthePitalitoBasinisanalogous tothatof theLittle Sulphur Creek Basins (McLauglin &Nilsen 1980, 1982;Nilsen etal. 1980;Nilsen & McLaughlin 1985).These similarities areoutlined inFig. 19.TheLittleSulphur Creek Basins developed as a result of oblique pull-apart extension along the right-lateral Maacama fault zone. Although the lateral offset along the northern basin margin is 58

Non-exposed basin features

unknown, thePitalitoBasin showsmany characteristics which areapplicable to basins whichdevelopadjacent tostrike-slipfaults (Nilsen&McLauglin 1985): - ThePitalitoBasinisformed alongtheGarzón-Suazafault zone:azoneshowingdextral strike-slipmovement. - Of allthefault-controlled basin margins,thestrike-slipfault alongthenorthernoneis themostprominent - Thebasin is asymmetrical, withits structurally deepest partclosetotheactivestrikeslipmargin. - The basin fill is dominated by axial infilling (from west to east), subparallel to the northern strike-slipbasinmargin. - Thebasinfillischaracterizedbyabruptlateralfadeschanges. - Theabruptlateralfadeschangesarestablein time. - Thesmall-sizedbasincontainsaverythicksedimentarysequence. - Thebasin-margin deposits aredistinctive: alongtheactive strike-slip margin,debrisflow-dominated fansoccuroflimitedextentwhereasalongtheinactivesouthernmargin largerstreamflow-dominated fansarepresent. - Basin extension (towards the west) is opposite to that of the general flow of paleocurrent(towardstheeast).Thisfeatureiscompatiblewithfindingsfrom theDead SeaRift andRidgeBasin(Manspeizer 1985;Nilsen&McLaughlin 1985,respectively). Animportant assumption inthetectonicmodelis thehigher subsidenceratesintheeast compared tothewesternpartofthebasin.Thisisnotonly supported bythedistribution ofthebasin infill butalsobythec. 10mverticalstepintheactualtopography alongthe line of the Guarapas river (Fig. 4A). This step partly explains the position of the Guarapas river: If this river was not influenced by the actual surface topography one wouldexpectafarmoredirectcoursetowardstheNWoutlet Thequestionifandtowhat extent theeasternpartreally showshigher subsidenceratescan besolvedbydating the sedimentsat severaldepthsintheeastern aswellasinthewesternpart.Astheavailable daringsarerestrictedtotheupperc.20mof sedimentstheyyieldnoinformation for this problem. Besides the effects on subsidence caused by movements along the fault patterns, the proposedtectonicmodelfor thePitalitoBasinincludesloadingaseffect onsubsidenceas well. McClean &Jerzykiewicz (1978) pointed out that loading may have a substantial influence on local subsidence aswellason subsidence of the surrounding areas('distal loading effect'). If distal loading played a role in the Pitalito Basin, the enormous sedimentaccumulationandsubsequentlocalloadingeffects intheeastmayhaveinduced theinitialdownwarpofthewest.Thisleadstosedimentaccumulationbeyondtheorignal basin marginsand subsequentprogressive subsidencetowardsthewest.Inotherwords, distalloading mayhavehaditsinfluence ontheenlargement ofthePitalitoBasininthe opposite direction of the general paleocurrent. Similar basin extension and depocentre migration is found in several other strike-slip basins (Reading 1980; Nilsen & McLaughlin 1985). Although in all these cases tectonics is undoubtfully the major controllingfactor inthemigration ofthedepocentre anditsdirection,loadingmayhave

59

Non-exposed basinfeatures

LITTLE SULPHUR CREEK

PITALITO

EXPLANATION Debris-flow-dominated alluvialfans Streamflow-dominated alluvialfans Lacustrinedeposits Holocenepeatdeposits Alluvialplainand fan-delta deposits

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Fig. 19. Tectonic and sedimentary comparison of the Little Sulphur Creek basins in California (redrawn from Nilsen & McLaughlin 1985) and the Pitalito Basin. The faults are drawn as if not concealed. For easy comparison both basins are drawn roughly at the same size and are reoriented.

playedaroleaswell. Intheproposedtectonicmodeltwophasesofwestwardextensioncanbedistinguished whicharedelineatedbythefault zones3and4,respectively.Faultzone4istheyoungest zoneandtectonicactivityalongthis'thirdgeneration'ofbranchingfaultsprobablyleadto the90°-turnofthedrainagepatternandaloweringofthelocalbaselevelwhichmarkthe transition from phase3tophase4. Reading (1980)distinguished threephasesintheformation of astrike-slipbasin: (1)a phaseoftranstension,(2)aphaseofbasininfilling and(3)aphaseoftranspression.The transition from phase 2 tophase 3 shows a change in the basin fill characterized by a basinwide coarsening-upward sequence due to a decreasing subsidence rate. The last (0

Non-exposed basin features

stageofinfilling ofthePitalitoBasin(phase3)mightreflect thistransitionalperiod Theaveragesedimentationratefor thesurface sediments,calculatedonthebasisof 14C datingsinpeats,is0.25-0.30 m/1000yearsover thelast 60,000years (section 5.2.2.3). Generally,averagesedimentationratesinstrike-slipbasinsarehigherthanthecalculated ones for thePitalito Basin (Schwab 1976;Miall 1978).When this calculated figure is accepted for theentireinfill of 1200m,thiswouldimply that subsidence of thePitalito Basin started around 4.5Ma ago.However, afaster/slower deposition ratefor thebulk of the sediments (that are beyond 14 C reach), cannot be excluded. This rate would increase for instance when phases of non-deposition are taken into account and not includedincalculationsof averagevalues.Suchphases,which aredescribedbyPitman i n & Andrews (1985), take place especially when stretching begins. Furthermore, compactionoftheorganic-rich sedimentsonwhichthecalculationsforthePitalitoBasin arebasedcanalsoresultinloweraccumulationrates. A final remark concerns the negligible role of sediment input of the Guarapas river in former times;throughouttheentireregisteredperiodofbasininfill thereisnoevidenceof animportant sedimentary influx coming from the narrow Guarapas Valley. One would expectarelativelycoarse-grained,fan-shaped bodydirectlySEofPitalitotownwherethe Guarapas river leaves its narrow valley. Such a body has not been detected by the geoelectrical survey. It seems that the water flow velocity of the Guarapas river is not able to transport coarse material beyond its narrow valley into the Pitalito Basin. The presentsituationisquitesimilar:coarseelasticsintheactualbedoftheGuarapasriverare restrictedtotheareaoftheshallowvalleyandalongthenorthernbasinmargin (Appendix I).Whencrossingthebasinplainonlyfineelasticsarefound.

61

CHAPTER 4

SEDIMENTARY

CHARACTERISTICS

OF

THE LAST-STAGE BASIN INFILL The actual near-surface and surface sediments constitute thefinal stageof sedimentary infill ofthebasin.Geoelectricaldataindicatethatthesesedimentsarecharacterized bya transgressiveoverlapofcoarsematerialfrom westtoeast (Chapter 3).Thiscoarseningupward megasequence (tens of meters thick) is interpreted as an eastward prograding ancient fluvial system which also has its expression in the morphology of the plain (Appendix I).The sedimentary sequenceoftheuppermost 40metersofthese sediments have been studied in numerous clay pits,borings and outcrops along theriverbanksof the incised Guarapas and Guachicos river and will be discussed in this chapter. The exposed sediments arereferred toas sectionsfollowed bya number (e.g. Section 20A) whereas the borings are abbreviated as PIT followed by a number (e.g. PIT 11). The positionofthedescribedlithological sequencesareshowninAppendixIV. 4.1Arealfaciesdistribution As was stated inChapter 3thePitalitoBasin can be subdivided intoa shallow western part and adeepeastern part.The transition between bothpartsliesalongthelineofthe Guarapasriver.Itappeared that thistwofold division could alsobeusedfor the surface morphology anddrainagepattern(AppendixI):westoftheGuarapasriverapockmarked relief with small seasonally ponded depressions and east of the Guarapas river interconnectedridgesthatenclosebadlydraineddepressions (pages22and23). Additionally,basedontheirlithologicalcontentandcharacteristics,thesamedivisionin awesternandeasternpartcouldbeusedfor thesurface andnear-surface sediments. 4.1.1 WESTERN PART

The surface sediments inthe western part, viz. west of the Guarapas river, are roughly made up by two types of sediments (Figs. 20 to 23): 1/ coarse-grained sediments (pebbles,gravel and sand)and 2/fine elasticswhich encasethefirst-mentionedtypeof sediments andform thebulk ofthetotallithofacies inthewesternpartof thebasin with about60%. Coarse-grainedsediments Description—The coarse sediments range from fine sand to clast supported cobbles (max. diameter of c. 10cm).Although small isolated lens-shaped bodies are present, theycommonlyhaveasheet-likeappearancewithamaximumthicknessofabout3mand 63

Last-stage basin infill

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Type646:Palmae-type3.PlateI. Pollen grains monocolpate or trichotomocolpate, bilaterally symmetrical or radially symmetrical,ambelliptical orsub-triangularwithconvex walls;semi-tectate,reticulate. Colpus c. 30.5 um long and 4.2 um wide, psilate margins. Exine c. 1.5 um thick. Luminaepolygonal shaped(c.2urnwide).Size:c.41.2(2.1)x28.5(1.3) um. Type647:Palmae-type4.PlateI. Pollen grains monocolpate, bilaterally symmetrical, amb elliptical; intectate, echinate. Colpus of moderate length (c. 26.5 um long, 6.6 um wide) with ragged margins. Echinae (length 2.5to3.2 um,diam.at base 1.5 to2.1 um) areevenly distributed and seatedinpockets,perpendicular onexine.Thenecksof theechinae are sharply pointed anddonotbendorhardly so.Exinec. 1.3 (imthick. Size:32.3(1.7)x26.1(0.9) um. Type648:Deleted Type649:Papillionidae.PlateI. Pollen grains triporate, subsphaeroidal to oblate, radially symmetrical; semi-tectate, coarse-reticulate.Poricircular (diameterc.8.0 um).Exinec. 1.4 umthick.Luminamore orlesssphaeroidalandisodiametric (c.6umwide).Relatively smallmuri(0.5(im).Size (n=l):c. 38x32um. Occurred inPIT 11 atdepth 12.45m. Type650:Phyllanthuscf. caroliensis(Euphorbiaceae).PlateI. Pollen grainstricolporate,prolate,radially symmetrical,ambcircular; tectate,scabrate. Colpilongwithcostae,endoporielliptical.Exinec.0.8 urnthick,thickening towardsthe pori and the poles to c. 1.5 um. Size (n=l): c. 25.2 x 11.5 um. Lit. Punt & Bentrop (1974). Type651:Begoniaceae.PlateI. Pollen grains tricolporate,prolate toperprolate, bilaterally symmetrical, amb circular, semi-tectate,striate.Exinec.0.5 umthick.Size: 18.2(1.7)x8.3(0.9)um. Type652:Clusia (Clusiaceae).PlateI. Pollen grains tricolporate, prolate, radially symmetrical, amb circular; tectate, microreticulate to reticulate. Colpi long with costae. Endopori distinct (diameter c. 3 um). Exinec.2umthick.Size 25(1.7) x 17.6(2) um. Type653:Campanulaceae.PlateI. Pollengrainsperi(5)porate,circular,radially symmetrical,ambcircular, intectate,microPLATEH Type655:Menispermaceae.Type656: cf.Menispermaceae.Type657:Norantea-typc(Marcgraviaceae). Type658:Proteaceae.Type659:C3P3 scabr.Type660:C3P3ret.Type661:C3P3ech.Type 662: C3P3microret.Type663:C3P3 ret.Type664:C3 psi.Type665:C3P3verr.Type666:C3P3 scabr. Type667: C3HP3ret.

140

Vegetcrtionalandclimatic history

PLATE II

141

Vegetationen and climatic history

echinate. Large pori (diam. c. 5.3 (im) with indistinct annuli. Exine c. 2 \im thick. Echinaearesparselydistributedandindistinct.Equatorialdiameter(n=3):c.37.5|xm. IncludedinthecurveofPollenDiv. Type654:Bunchosiacf.pseudinitida (Malpighiaceae).PlateI. Pollengrainsperi(9)porate,circular,radiallysymmetrical,ambcircular,intectate,psilate. Poriwellrounded(diameterc.2pm).Exinec. 1 \ixathick.Equatorialdiameterc. 13|im. IncludedinthecurveoftheMalpighiaceae.Occurred inPIT 11between9.75 and 10.95 m. Type655:Menispermaceae.PlateÜ. Pollen grains tricolporate,radially symmetrical,.subsphaeroidal, amb triangular; semitectate,reticulate.Colpilongwithdistinctandsmoothmargin.Endoporicircular (diam. c.2\ua) andindistinct.Muri narrow.Lumina moreorlessangular and irregular (diam. c. 1.5 \im). Exine c. 1.7 |xm thick. Short but distinct columellae. Size: 16.0(1.5) x 15.0(2.1)pm.Lit.Thanikaimoni (1984):Plate67. Type656:cf. Menispermaceae.PlateII. Pollen grains tricol(por?)ate, subsphaeroidal to circular radially symmetrical, amb circular;semi-tectate,reticulate.Colpilonganddistinct.Endoporiindistinctwithragged margins.Exinec. 1.5 \im thick.Lumina moreor less sphaeroidal,differing in size (1.5 Hm< 0