Tiiu ban: Khoa hoc Trdi ddt lSBN:978-604-913-016-8
TIEN HOA CHAU THO SONG ClTU LONG GIAI DOAN HOLOCEN- HIEN TAI TirONG iTNG VOI THAY DOI Ml/C Nl/OC BIEN
• • •
VA TAC DONG CUA CON NGU^OI
EVOLUTION OF THE MEKONG RIVER DELTA IN HOLOCENE- RECENT RESPONDING TO
SEA-LEVEL CHANGE AND HUMAN IMPACTS
Nguyen Van Lap, Ta Thi Kim Oanh
Vien Dia Ly Tii nguyen TP.HCM, Vien Khoa bge v i Cdng nghe VN 01 Mae DTnh Chi, Q.OI, TP. HCM, e-mail: nvlap(g)vast-hcm.ac.vn Tom tat:
Chdu thd sdng Ciru Long bdi ldn nhanh ra bien trong sudt 6.000 ndm cudi khi muc nuac bien dgt mirc cao vd lid sau do. Tuy duac thdnh tgo trong thai gian ngdn nhung tien hda chdu thd cd sir thay ddi. Khodng 3.000 ndm cdch nay, chdu thd sdng Cim Long thay ddi tir dgng trieu ini the sang trieu- sdng ini the a khu vuc giua sdng Tien vd sdng Hgu, trong khi ria ddng bdng a Cd Mau dgc trung bai trieu ini the. Nhirng thay ddi ndy dugc xdc djnh bai qud trinh trdm tich vd cdc ddc tnmg dja mgo. Tdc do bdi ldn chdu thd dgng trieu- sdng uu the trong khodng 2.000- 3.000 ndm cudi, khodng 8- 20 m/nam, nhd hem so vai dgng trieu ini the khodng 40- 60 m/nam.
Anh huang ciia thay ddi khi hgu todn cdu vd muc nuac bien ddng den ba bien chdu thd sdng Ciru Long dugc xdc djnh. Khodng 50- 60 ndm gdn ddy, xdi la ba bien xdy ra khd nghiem trgng a mot sd nai trong tinh Ben Tre, Trd Vinh vd Cd Mau. Cdc hgat ddng khai thdc tdi nguyen vd mdi truang ven bien ciia con nguai dd lam trdm trgng han qud trinh xdi la ba bien. Nghien ciru tien hda chdu thd trong dieu kien tu nhien vd thay ddi bd bien khodng vdi chuc den trdm ndm gan day gdp phdn quan trgng ddnh gid tdc ddng ciia bien ddi khi hgu vd muc mrac bien ddng den dai ba bien nhdm du bdo xu huang bien ddi trong tuang lai.
Abstract:
The Mekong River Delta (MRD) has been prograding during the sea-level highstand and the subsequent period of slightly falling in the last 6 ky. Since the last 3 ky, the progradational delta has evolved from a tide-dominated delta to the present mixed tide- and wave- dominated delta among Mekong and Bassac river
area, meanwhile, tide-dominated delta has been characterized in the Ca Mau delta margin. These changes are well recognized by the depositional process and geo-morphological characteristics. Progradational rate is approximately of 8- 20 m/y in the mixed tide- and wave- dominated delta to be smaller than those of the tide-dominated delta reaching 40- 60 m/y in the last 2.000- 3.000years.
Impacts of global climate change and sea-level rising to the coastline in the MRD are recognized. In the last 50- 60 years, coastal erosion has been occurred severely at some places in Ben Tre, Tra Vinh and Ca Mau provinces. Coastal erosion has become more sereous due to human activities such as deforestation.
Hgi nghi Khoa hgc ky niem 35 ndm Vien Khoa hgc vd Cong nghe Viet Nam - Hd Ngi 10 2010
sand mining at the coastal areas. Compiling delta evolution under natural conditions and coastline variation around several ten to hundred year scale plays an important role to assess the impacts of global climate change and sea- level rising to coastal zone to predict its tendency in future.
I. MO D A U
Chiu thd li viing dit thip bing phing dugc hinh thinh noi cira sdng vdi dudng bd ldi ra phia bien. Day li ket qua eiia sy dich chuyen dudng bd budng ra bien nhd ngudn cung cip vit lieu trim tich tir sdng mang den. Chiu thd hinh thanh vi bdi lin ra bien khi vit lieu trim tich duge eung cip ddi dio tir bdn lun vyc mang den ha luu, mye nude bien dung yen hoac liii - bien liii. Ngugc lai khi mdt trong hai yeu td niy thay ddi, vi du giim ngudn vit lieu trim tieh mang den ha lun, vi / hoac mye nude bien ding len, thi bien lin dit lien vi chiu thd se bi phi buy. Bien Iin dit lien thudng bit diu bing qui trinh bd bien chiu thd bi xdi Id, nude man xim nhip sau vio ndi dia, dien tieh dit bi thu hep dan vi den thdi diem nio dd chiu thd se bi phi buy hoac nhin chim dudi mye nude bien. Day li qui trinh ty nhien xiy ra trong lich sir tien hda chiu thd, lien quan den thay ddi khi hiu va eic bing ky thdi gian dia chit. Tuong ty cic chau thd ldn d Chiu A nhu Hoing Ha, Duong Tir, Chaophraya, Ganges ..., viec nghien cim sy phit trien chiu thd giai doan Holocen- hien tai da duge quan tim d chiu thd sdng Hdng vi sdng Cim Long. Bii bio niy ed mye tieu nghien cuu xiy dyng md binh phit trien chiu thd sdng Cim Long giai doan Holocen- hien tai trong dieu kien ty nhien vi tie ddng eiia eon ngudi de eung cip co sd khoa bgc, dinh gii va dy bio xu hudng phit trien chiu thd trong tuong lai.
Sdng Mekdng, mdt trong nbijng con sdng ldn nhit Ddng Nam A vdi chieu dii 4.500 km, lugng phii sa khoing 160 trieu tin/nim eung cip cho chiu thd sdng Cim Long (SCL) hien nay [4]. Ngudn vit lieu eung cip chii yeu Ii set, bdt vi cit trim tich trong dieu kien che do thuy trieu thay ddi cua Bien Ddng vi die trung bdi ehe do gid miia. Tren co sd eic ket qui phin tieh ve die diem thach bgc, ciu tnic trim tich, vi cd sinh vi tudi tuyet ddi C tir cic Id khoan d chiu thd SCL, thay ddi mdi trudng trim tieh tuong img thay ddi mye nude bien va tien hda chiu thd tir cudi Pleistoeen den Holocen da duge xie dinh. Giai doan bien tien cudi De Tir ghi nhin tir loat trim tieh lip diy thung liing bio mdn gdm mdi trudng cira sdng, vinh bien cd tudi tir 13.000- 6.000 nim cich nay vi giai doan tien hda chiu thd bat diu tir khoing 6.000 nim cich nay khi myc nude bien dat mirc cao nhit (high standing) vi bien liii sau dd [4, 6].
Ciing vdi bien liii, ngudn eung cip vit lieu trim tich ddi dio gdp phin quan trgng de chiu thd SCL bdi Ian hon 200 km tir bien gidi Campuchia den bd bien hien tai trong khoing 6.000 nim cudi. Tuy nhien, bd bien chiu thd SCL bi xdi Id khi nghiem trgng khoing 100 nim gin diy [3]. Mdt sd ket qui nghien cuu cho thiy viec xiy dip thiiy dien d thugng lun sdng Mekdng di lim giim ngudn phii sa tir thugng lun mang den chiu thd. Bien ddi khi hiu vi myc nude bien ding lam qui trinh bien "xim chiem" dit lien, die biet hien tugng thdi tiet eye doan lim gia tang pham vi vi cudng do xdi Id bd bien.
II. KET QUA
1) Tram tich bien tien lap day thung lung bao mon
Dia mao- trim tieh chiu thd SCL da duge xie dinh tren co sd phin tich inh vien thim, anh miy bay va mdi trudng trim tich be mat chiu thd. Kit qui phan tieh mdi trudng trim tich tren 220 hd khoan (khoan tay) cd do sau 3- 8 m/ hd khoan gdp phin quan trgng thinh lap so dd dja mao trim tieh chau thd SCL [1]. Thay ddi mdi trudng trim tieh dudi mat dit dugc xac dinh tren ca sd phin tich 25 hd khoan (khoan miy) cd do siu 30- 100 m/ hd
Tieu ban: Khoa hoc Trdi ddt lSBN:978-604-913-016-8
khoan. Kit qui nghien cim cho thiy cd mdi lien quan giira dia mao- trim tich be mat chau thi SCL vdi sy tiln hda chiu thd tuong iing vdi thay ddi muc nude bien khoing 6.000 nim cich nay den hien tai.
Vio khoang 18.000-20.000 nim cich nay, mye nude biin thip nhit vao khoing -120 m d vimg Ddng Nam A. Sy ha thip mye nude bien lim ha mye gdc cua sdng, dio khdet ldng sdng va thinh tao thung liing bio mdn, trong dd vet tieh thung IGng bio mdn SCL di dugc xie dinh tir Ding Thip din Bin Tre [4, 6] vi keo dii vl phia ddng thugc thim lyc dia Viet Nam [9].
Sau giai dgan bang hi culi ciing, myc nude bien ding len vi trim tieh bien tien lip din cac thung liing bio mdn. Tap trim tieh bien tiln lip day thung liing bao mdn dien hinh bao gdm tudng cit bdt cira sdng trilu uu thi, bim dim liy bai trilu, cat cua sdng bien uu the vi cudi ciing li tudng bim vinh biin md theo thii Ur tir dudi len tren [7]. Trini tieh eit-bdt ciia sdng trilu uu thi diy hon 8m d phin dudi ciing cua lo khoan d Bin Tre li bing chirng cua vit li?u tram tich lip diy thung liing bio mdn tuong irng vdi myc nude bien dang ding. Khi myc bien tilp tyc ding cao, cira sdng dich chuyen din vio dit lien nen inh hudng cua bien eing gia tang. Mye nude bien cd le d do siu -60m vio khoing 13.200 nim cich nay tren co sd tram tich dim liy bii trieu. Vio khoing 10.000 -8.000 nam cich nay, trim tich dam liy vi cira sdng duge tieh tu trong eic thung liing bio mdn d Ben Tre vi Ddng Thip. Trong thdi gian 7.000-6.000 nam cich nay, nude bien xim chiem toin bd ddng bang sdng Cim Long nen mot vinh bien ndng duge thinh tao [1, 6]. Ldp biin nhao ed ciu tnic ddng nhit miu xim xanh day 10 -13m chira phong phii tio silie phii du nude man va triing Id tieu bieu eiia vinh bien md la bing chiing bien tien eye dai vio khoing 6.000-5.500 nim cich nay [5-8].
2) Tien hoa chau tho song Cu'u Long khoang 6.000 nam each nay den hien tai
Chiu thd SCL rdng ldn trii dii tir bien gidi Viet Nam- Campuchia den bd bien hien tai duge thinh tao trong khoing 6.000 nim khi mye nude bien dirng yen va liii sau dd. Vao khoang 5.000- 6.000 nam cich nay, Ddng Thip Mudi vi Tii giic Long Xuyen li khu vyc ven bien va rirng ngap man phit trien manh me vdi bd bien d Cai Lay tinh Tien Giang duge xac dinh 4.500 nim cich nay. Vao khoang 4.000 nim cich nay, sdng nhinh cd cua SCL cd le da dich chuyen den hudng ddng d Ben Tre, tram tieh pro-delta vi delta front da bdi lan vi phu len tren trim tich vinh bien md. Vio khoing 3.000 nimcich nay, eiing vdi sdng nhinh cd d Ben Tre, mdt sdng nhinh khie hien dien d khu vyc Can Tho theo hudng tiy bic- ddng nam, kieu phan bd eua trim tieh pro-delta vi delta front md rdng tir Gd Cdng den Ben Tre va Tra Vinh tuong ty nhu bien tai. Die diem dia mao- trim tich tren be mat va ket qui nghien cim tudng trim tich dudi mat dat cho thiy ddng bing cao duge thinh tao trong mdi tmdng chiu anh hudng trieu chii yeu trong thdi gian khoing 6.000 den 3.000 nim each nay, ddng bang thap thanh tao trong mdi tmdng inh hudng trieu vi sdng tir sau 3.000 nim den nay [4, 6].
Tien hda cua ddng bing chiu thd di thay ddi tir he thdng trieu uu the sang sdng- trieu vdi inh hudng sdng nhieu hon. Sy gia ting inh hudng eua sdng di tie ddng den Urong trim tieh ciia chau thd, dja hinh tren can va dudi nude, thinh tao eic he thdng gidng eat, tudng trim tich tho hat, tdc do bdi tieh giim, sy phin tin trim tieh dgc theo bd bien, vi delta front cd do doe ldn trong dd viing gin cira sdng nhu Gd Cdng, Ben Tre, Tra Vinh, Sdc Tring phin tin trim tich hat thd cdn d xa cira sdng; do inh hudng cua gid miia ddng bic vi ddng chiy ven bd mang vit lieu min hat ve phia nam tao thinh ddng bing ria Ci Mau- die tnmg kieu trieu uu the. Kit qua nghien ciiu cho thiy trong khoing 2.000- 3.000 nim cudi, chiu thd bdi lin ra bien vdi tie do tmng binh 8- 20m/ nam d Bin Tre- Tri Vinh vi 40- 60 m/nim d Ci Mau [6, 8].
3) Thay doi bd bien chau thd song Ciru Long khoang 50- 60 nam gan day
Ve mat ty nhien, chiu thd SCL lien tyc bdi lin ra bien, song nhieu doan bd biin bi xdi Id trong dd cd noi khi nghiem trgng trong khoing 50- 60 nim gin diy. Ngoii ra theo kit qua
Hoi nghi Khoa hgc ky' niem 35 ndm Vien Khoa hgc vd Cong nghe Viet Nam - Hd Ngi 10 2010
nghien cim eua IPCC, do biin ddi khi hau, du kiln myc nude bien se ding len khoing 0,7- 0,8 m/ 100 nam eiing vdi hien tugng thdi tilt eye doan nen nhieu chau thd tren the gidi se bi anh hudng ning nl, trong dd chiu thd SCL - Viet Nam va Ganges- Bangladesh la noi bi inh hudng nghiem trgng nhit. Trong khoing 100 nim nay, bd bien chiu thd SCL thay ddi rit nhanh. Ket qui nghien ciiu budc diu cho thiy xdi Id bd bien xiy ra khi nghiem trgng d mdt sd noi quanh khu vuc sdng Tiln, sdng Hiu va tinh Ca Mau tir khoang 50- 60 nim den ngay nay. Tdc do xdi Id bd biin khoing 8- 10 m/ nim dugc ghi nhin d cac doan bd bien thudc tinh Ben Tre va Tra Vinh, die biet mdt sd noi khi eao khoing 25- 30 m/nim. Cac doan bd bien bi xdi Id thudng cau tao bdi tram tieh bd rdi, mem, deo- mem vdi thanh phin set- bdt, bdt- cit vi cat thudc mdi tmdng tram tich dam liy, dim lay rirng ngap man, gidng cit ven bien [3]. Xdi Id va bdi tu dien ra theo miia trong nam, ehu ky vii nam la qui trinh ty nhien xiy ra noi bd bien chau thd, tuy nhien khi bd bien chiu thd bi xdi Id lien tuc trong nhieu nam- vai ehue nim la hien tugng bit thudng trong qui trinh phit trien. Lien quan den nguyen nhin xdi Id bd bien chau thd SCL cin dugc quan tim nghien cim chi tiet hon de cd bd tai lieu (1) dinh lugng giim ngudn vat lieu trim tich cung cap den ha luu ddng bang chau thd do dip dip thuy dien d thugng luu sdng Mekong, (2) mire do tic ddng cua thay ddi khi hau, mye nude bien ding va hien tugng thdi tiet cue doan den khu vyc ven bien va (3) hoat ddng cua eon ngudi khai thie tai nguyen va mdi tmdng khu vuc ven bien- die biet la van de chat pha rimg ngap man vi khai thie bira bii cit ven bien. Pham vi vi miic do thay ddi cac van de vira neu ein duge tdng hgp, phin tich va dinh gii d cac giai dgan khie nhau, tir dd xie dinh mdi lien quan eua chimg den thay ddi bd bien. Tren thyc te viec xiy dip thiiy dien d thugng luu sdng Mekdng da lam giam lugng phii sa tir thugng luu mang den chiu thd, die biet tir nam 1993, sau khi cic dap thuy dien Manwan va Dachaoshan dua vio hgat ddng. Hgat ddng chit phi rirng ngap man ven bien tinh Tra Vinh va Ca Mau tir nhiing nam 1990 lien quan den xdi Id bd bien da duge xae dinh. Ngoai ra xdi Id bd bien khi nghiem trgng xiy ra do xay dung cdng trinh ke vi khai thie cat ven bien. Tir nhirng nam 2005- 2006 do viec xiy dung bd ke khu du lich Ba Ddng tinh Tri Vinh da Iam xdi Id nghiem trgng doan bd bien phia nam lien ke, vi viec khai thie cit bai bien Khai Long huyen Nggc Hien tinh Ca Mau lam xdi Id nghiem trgng bd bien noi diy [3]. Bien ddi khi hau va muc nude bien dang lam qua trinh bien "xam chiem" dat lien. Ket qua li xdi Id bd bien va xim nhap man da va dang xay ra lam thay ddi mdi tmdng khu vyc ven bien vi hgat ddng khai thac tai nguyen- mdi tmdng cua con ngudi Iam trim trgng bon van de niy.
KET LUAN
Tien hda chau thd SCL lien quan den thay ddi mye nude bien Holocen duge xie djnh. Khi myc nude bien dat vi tri cao vi liii sau dd, khoing 6.000 nim each nay, chiu thd bdi lan nhanh ra bien tao thanh ddng bing rdng ldn trong sudt 6.000 nam. Tuy duge thinh tao trong thdi gian ngan nhung tien hda chiu thd ed sy thay ddi. Khoing 3.000 nim cich ngiy nay, chau thd SCL thay ddi tir dang trieu uu the sang trieu - sdng uu the d khu vyc giira sdng Tien va sdng Hau, trong khi ria ddng bang chau thd d Ca Mau die trung bdi trieu uu the. Tdc do bdi lan chau thd dang uu the trieu- sdng, khoang 8- 20 m/ nam, nhd hon so vdi dang trieu uu thi khoang 40- 60 m/nam trong khoing 2.000- 3.000 nam cudi.
Anh hudng cua sy thay ddi khi hau toan cau va myc nude bien dang den bd bien chiu thd SCL dugc xac dinh. Khoing 50- 60 nim cudi, xdi Id bd bien chau thd kha nghiem trgng d mdt sd noi nhu Ben Tre, Tra Vinh vi Ca Mau. Mdt sd hgat ddng khai thac tii nguyen va mdi tmdng ven bien eua eon ngudi da larn qui trinh xdi Id bd biin xiy ra nghiem trgng hon.
Nghien cim tien hda chau thd trong dieu kien ty nhien va thay ddi bd bien khoing vii chuc nam gin day gdp phan quan trgng dinh gia tie ddng eua biin ddi khi hiu vi mye nude bien dang den ddi bd bien nham dy bao xu hudng bien ddi trong tuong lai.
Tiiu ban: Khoa hgc Trdi ddt ISBN:978-604-'J 13-016-8
TAI LIEU THAM KHAO
1. Nguyen, V.L., T.K.O. Ta, Tateishi, M., Late Holocene depositional environments and coastal evolution of the Mekong River Delta, Southern Vietnam. Joumal of Asian Earth Science 18, p. 427-439 (2000).
2. Nguyen Vin Lip, Ta Thi Kim Oanh, Mdi tnrang trdm tich Pleistocen mugn - Holocen viing Cd Mau. Tc. Cac Khoa hgc vl Trii Dit, 26 (2), 2004, trang 170-180.
3. Nguyen Vin Lip vi nnk, Nhirng thay ddi hien tgi vd tdc dgng ciia con ngirai den viing ven ba biin dong bdng sdng Ciru Long, Viet Nam. Bio cio khoa hgc de tii hgp tie Quoc tl VAST- JSPS Nhit Bin, 2009, 30 trang.
4. Ta, T.K.O., Nguyen, V.L., Tateishi, M., Iwao, K., and Saito, Y., Holocene delta evolution and sediment discharge of the Mekong River, southern Vietnam, Quaternary Science Reviews, v. 21, p. 1807-1819. (2002a).
5. Ta, T.K.O., Nguyen, V.L., Tateishi, M., Iwao, K., and Saito, Y., Sediment fades and late Holocene progradation of the Mekong River Delta in Bentre Province, southern
Vietnam: an example of a tide- and wave-dominated delta. Sedimentary Geology, 152, p.313-325 (2002b)
6. Ta, T.K.O., Nguyen, V.L., Tateishi, M., Konayashi, I, Saito, Y., Holocene delta evolution and depositional models of the Mekong River Delta, southern Vietnam. In Giosan, L., and. Bhattacharya, J. P., eds.. River Deltas- Concepts, Models and Examples, SEPM special publication no. 83, p. 453-466. (2005).
7. Ta Thi Kim Oanh, Nguyen Vin Lap, Trdm tich Pleistocen mugn- Holocen vd sir phdt trien ciia tam gidc chdu sdng Ciru Long a Ben Tre. Tc. Cic Khoa hgc ve Trii Dit, 24 (2). 2002, trang 103-110.
8. Ta Thi Kim Oanh, Nguyen Vin Lap, Tirang trdm tich Holocen tirang img vdi dao ddng muc nuac bien viing VTnh Long - Trd Vinh, ddng bdng sdng Cim Long. Te. Cic khoa hgc vl Trii Dit, 28 (3), 2006, trang 329-335.
9. Stattegger, K., Holocene e^'olution and actual geologic processes in the coasted zone of south Vietnam. Tuyen tap bio cio khoa bge Hdi nghi Khoa hgc Dia chit biin toan quoc lan thir nhat: Dia chit biin Viet Nam va phit triln ben virng, 2008, trang 42-53.