31(4)[CD],i.56-i62 Tap chl CAC KHOA HOC VE T R A I DAT 12-2009
XAC DINH CAU TRUC HINH THAI CUA CHAU THO Ciru LONG TREN THEM LUC DIA VEN BO CA MAU
TREN CO S6 TAI LIEU DIA CHAN NONG PHAN GlAl CAO
NGUYEN TIEN HAI, PHUNG VAN PHACH, K STATTEGGER
I. MO DAU
Ddng bdng chau thd (DBCT) Cuu Long (thudc DBCT Mekong, hinh 1) Id mdt trong nhung vung ddng lire eua ddt nude, noi day diin ra nhilu boat ddng kinh tl(ndng nghiep, nudi trdng thuy san, du Ijeh...), an ninh, qudc phdng... ; cung Id nai cd nhilu dae trung vl trdm tich va su phat triln eila chau thd, dae biet Id qua Uinh phat triln, md rdng dien tieh ra phia biln d khu vuc mui Cd Mau (DBCT dupe hinh
thdnh bat ddu khi biln lui each ngay nay quang 4.500 nam vdi tde dp xam lan ra biln khoang 15 mm/nam, Nguyin Van Lap va nnk, 2000). Nghien edu cac dae trung, qua trinh va quy luat phat triln eua DBCT Mekong ndi ehung, DBCT Cuu Long va dai ven biln khu vuc ndi rieng ed y nghTa khoa hpe va thue tiln quan trpng giup tim hiiu quy luat tir nhien eua khu vue vd djnh hudng cho cdng tac phat trie'n kinh t l va khai thae tu nhien d day.
<^Hinh 1.
Luu vue va ha luu sdng Mekong (Hori, 2002)
® Ranh gidi kha nang xam nhap man vao thdi gian trilu eudng trong
mua khd
Vdi vj tri va tinh da dang, phdc tap cila Ur nhien, DBCT Mekong ndi ehung, khu vuc mui Cd Mau ndi rieng da va dang thu hut dupe su quan tam eua nhilu nganh, nhilu nhd nghien edu trong vd ngoai nude. Mdt loat ehuong trinh nghien cdu vl biln vd lan can da dupe Nhd nude tril'n khai thue hien, chang han : "Diiu tra tdng hpp vung biln ven bd Thuan Hai - Minh Hai". Du an thanh lap Ban dd dja chdt vd khoang san ddi biln ven bd (0-30 m nude) cila Cue Dia ehd't vd PChoang san Viet Nam...
Nhiing nghien edu tren da tap hpp, thu thap dupe nhieu du lieu biln ven bd vd cung vdi nhung nghien cdu tren luc dja da gdp phdn xae lap ede dae diem cua chau thd, dae dilm vung biln kican eiing nhu
lam sang td qua trinh hinh thanh cua phdn luc dja DBCT Mekong ndi ehung (DBCT Cuu Long ndi rieng). Tuy nhien, ddi vdi chau thd nay cdn tdn tai nhilu vdn dl edn dupe giai quylt hoac lam sang td hon, trong dd ed vdn dl vl hinh thai edu true vd co chl hinh thanh phdn thdp (low plain) vd chau thd ngdm (subaqueous plain) cua chau thd.
Di gdp phdn lam sang td qua trinh hinh thanh vd phat trie'n eua chau thd Mekong ndi chung, ddng bang Ciru Long ndi rieng, bdi bao dl cap din eae ylu td tiln chau thd va chan chau thd Cuu Long d vung biln nam Ca Mau (gpi tdt Id KVNC, hinh 2) chu ylu tren cp sd tai lieu dja chdn ndng phan giai cao.
t87->9t
TS7->93 0\^.-y.'i
<y'\^y^ 0=0' d- ti
Khu vu'c n g h i e n CLKU
Hinh 2. So dd vj tri khu vue nghien edu
n Cac die'm khao sat eda tdu Sonne, Dde (ehuyln SO - 187-3/2006), o Cac dilm khao sat eua tdu Nghien Cdu Biln, Viet Nam (ehuyln VG - 5/2004 vd VG - 9/2005
II. PHUONG PHAP LUAN VA CO SO TAI LIEU Ddng bdng chau thd la td hpp rdt phdc tap dupe hinh thdnh trong mdi tuong tac eua eae qud trinh, hoat ddng sdng va biln. VI hinh thai edu true, DBCT ndi ehung (delta) la mdt thue thi dja chdt ed edu true CO ban gdm 3 ylu td (theo diiu kien mdi trudng thdnh tao) : ddng bang tren can (delta plain), tiln chau thd (delta front) va chan chau thd (prodelta).
Tiln chau thd dupe hinh thdnh ehil ylu bdi cac vat
lieu luc dja do sdng tai ra dupe ldng dpng trong mdi trudng vung eua sdng (mdi trudng sdng - biln) ; edn vat chdt tao nen chan chau thd cd nhilu ylu td biln hon ; do vay, vat chdt tao nen ddng bdng tren can chu ylu Id vat lieu vun (eat bun hoac bun cat) thd hon so vdi vat chdt bl mat chan chau thd (bun hoac bun set).
VI dae dilm hinh thai, nhin chung, bl mat eila ede edu true ed dd dde va tinh tuong phan khdc
nhau, trong dd bl mat ddng bang tren can thudng thoai nhdt nhung tinh tuang phan lai cao nhdt. So vdi tiln ehdu thd, bl mat chan chdu thd thudng phdc tap va dde han.
Nhu vay, dl phan biet va xae lap cac ylu td edu true ngdm cua chau thd la tiln chau thd va chan chau thd, edn xde djnh dupe ede dae dilm chinh hinh thai, dd dde vd vat chdt bl mat cua chung. Nhin chung, vl CP ban : tren bl mat tiln chau thd chu ye'u Id vat lieu luc nguyen cat bun hoac bun edt, edn d chan chau thd chu ylu Id bun vd bun set. Cac dae dil'm nen Id nhirng ddu hieu tdt dl nhan bilt, phan biet eae
ylu td cdu true hinh thai tiln chau thd va chan chau thd. Phdn dudi sau, ranh gidi giira tiln chau thd va chan chau thd thi hien su khae biet vl thanh phdn vat cha't, mdi trudng thdnh tao khae nhau vd su thay ddi cila bl mat chdn ehdu thd trude khi hinh thanh tiln chau thd. Ddu hieu nay thi hien rdt ro net tren mat cat dja chdn ndng ndi ehung (dja ehdn ndng phan giai cao ndi rieng).
Tai lieu chinh sir dung trong bai viet la cac sd lieu cua ehuye'n khao sat hdn hop Dice - Witt SO - 187-3 thuc hien bdi tdu Sonne (Dde) trong thang 4 vd 5 nam 2006 {hinh 3).
104°30'
8°
5^)32007-3 187-3-9^4
29113007-1
o187-3-105
Them trong
® Ranh gidi tien chau thd/chan chau thd
® Ranh gi6i chSn chau th(3/them trong
187-3-109
8°50'
Hinh 5. So dd vj tri tuyln do dja chdn ndng phan giai eao (SO-187-3/2006) ^°^
• Tuye'n do dja ehdn ndng phan giai eao, o Vj tri khao sdt Idy mdu
"40'
III. KHAI QUAT DAC DIEM TU NHIEN KHU VUC BIEN VEN B O NAM CA MAU VA
VUNG KE CAN
Chdu thd sdng Mekong (trong dd cd DBCT Cdu Long) dupe hinh thanh chil ye'u do qua trinh tuong tae giiia he thdng sdng Mekong va biln.
Khu vuc nghien cdu la phdn thdp, ngdm cila chau thd Ciiu Long (phan bd d vj tri biln ndng ven bd).
Nhirng nghien edu, diiu tra hien nay eho thdy, vung bie'n ndng nay cd dae dilm tu nhien kha
dae trung khae biet so vdi cac vung biln khdc a Viet Nam.
Khi hau khu vuc nay co ban la kieu khi hau ddng bang Nam Bd vdi nIn nhiet dd cao quanh nam, ehe dd mua d'm phan hda theo mua gid (gio mua Ddng Bae : tu thdng 10 dlh thang 3 nam sau va gid Tay Nam : tu thdng 5 din thang 8), khdng ed mua ddng lanh, cdn bao chu ylu xa'y ra vao thang 9 - 1 0 (mudn ban so vdi miin Bde vd miin Trung Viet Nam).
Vl ddng luc biln, ehe' dd sdng d KVNC ndi rieng (bie'n VQng Tdu - Ca mau - miin Tay ndi ehung) trong mua he cd hudng sdng thjnh hdnh la hudng ddng nam vdi tdn .sudt cao (tren 70 %), cae hudng khdc cdn lai 10-15 % ; mda gid Tay Nam, hudng sdng thjnh hanh la hudng tay nam vdi tdn sudt cao ddn v l phia nam (60-70 %). Dd cao sdng trong khu vuc dao ddng khoang 1 - 1,5 m (cue dai tdi 4 m)
Mua ddng, hudng sdng thjnh hdnh theo hudng gid bdc va ddng bdc. Dd cao sdng trung binh 0,75
1,0 m (cue dai tdi khoang 4 m).
Ve ddng chay bien, d vung biln Vung Tdu - Cd Mau : ddng ehd'y chju tae ddng manh eua hoan luu b l mat dupe tao nen bdi e h i dd gid mua ; miia he, ddng ehd'y tdng mat cd hudng ddng bde vdi tde dd nhd hon 25 erri/s ; mua ddng, ddng ehdy tdng mat ed hudng nam vdi tde dd cd t h i dat tdi 50 cm/s (Dang Ngpc Thanh vd nnk, 1980). Nhu vay, d KVNC trong nam hudng ddng ehd'y uu t h i Id ddng ehd'y do gid mua Ddng Bde. Rieng vung biln miin Tay, tde dd ddng ehdy khdng ldn, nhung khd phdc tap va cd su khae biet vl hudng giua vung ven bd vd ngoai khoi.
Mua he, ddng ehd'y ed hudng thjnh hanh tu Hd Tien v l Ca Mau (tde dd 8 -10 cm/s), nhung de'n gdn mui Ca Mau, ddng ehdy ehuyln hudng sang phia ddng d l nhap vao hoan luu tu phia nam ; edn trong mua ddng, ddng ehdy thudng ky cd xu hudng tach ra vd di vl phia tay d l nhap vao hodn luu td bae di xud'ng qua mui Cd Mau. 0 ven bd, tdn tai hai he thdng ddng ehdy ngupe nhau : mua ddng ddng ehdy ed hudng Hd Tien -^ Ca Mau (tde dd trung binh 5-8 cm/s) va mua he, hudng ddng ehdy ngupc lai (tde dd trung binh 10-15 cm/s) [IJ.
VI thily trilu, trong KVNC cP ban la e h i d d nhat trilu khdng dlu vdi dd ldn thuy h-ilu trung binh 1,5 - 2 m (eao nhdt trong ky nude cudng 3 - 4 m) vd giam ddn theo hudng td Vung Tdu din Cd Mau.
Rieng bil'n miin Tay, dd ldn trung binh thuy trilu khoang 1 m.
VI dja hinh day bil'n KVNC, nhirng nghien cdu trude day va theo sd lieu do dae eua tdu Sonne [1, 2, 5...], b l mat dia hinh d day kha thoai vd dan dieu, tinh tuong phan dja hinh thdp {hinh 4 va 5). Tao nen b l mat day bil'n chu ye'u la vat lieu bun set va set [1.2,6],
Mdt trong nhOng ylii td chinh hang ddu tac ddng de'n mdi trudng tu nhien khu vuc - dd la he thdng sdng trdn luc dja - he thdng sdng Mekong.
Sdng Mekong Id mdt trong nhung ddng sdng ldn nhdt t h i gidi, bdt ngudn td eao nguyen Tay Tang (Trung Qudc) va ehdy qua eae nude Trung Qudc, Lao, Thai Lan, Campuchia vd Viet Nam vdi tdng chilu ddi khoang 4.230 km (xlp thd 11 fren t h i gidi), dien tich luu vue ehdng 160.000 km (hinh 1), luu lupng nude quang 470 km/nam, ehuyen tai mdi nam tren 160 trieu tdn vat lieu phu sa (trong dd ed gdn 16 trieu tdn dupe dua ra biln (K.O. Ta va nnk, 2002). 0 Viet Nam, sdng Mekong tao thanh hai nhanh chinh la sdng Tiln vd sdng Hau vdi mtSt loat chi luu nhd chang ehjt, nhung dd ra biln vdi chin eua chinh d each xa vl phia ddng bde KVNC.
Vdi nhQng dae dil'm tren, phdn nao da ly giai tai sao d vung biln ven bd nam Ca Mau lai cd dja hinh ddy biln kha bang phdng, don dieu va thoai vdi vat chdt tao nen b l mat ddy biln chu ylu la bun set ma khdng phai la eat bun.
IV. PHAN TICH, LUAN GIAI CAU TRUC NGAM DELTA ClTU LONG
Nhu uen da neu, ede ylu td chinh de xae djnh va phan biet tiln chau thd vd chan chau thd la thdnh phdn vat chdt, dae dil'm hinh thai dja hinh va sir thay ddi vat chdt giira chung.
Vdi dae dil'm hinh thai dja hinh va vat chdt tao nen dja hinh it cd su thay ddi, biln ddng nhu da neu d phdn tren ; do vay, dua vdo eae tieu ehi dae dilm vd thdnh phdn vat chdt b l mat dja hinh, rdt khd xac djnh dupe eae ylu td edu true ngdm (tiln chau thd vd chan chau thd) va ranh gidi cua ehung thude delta Cuu Long, Trong trudng hpp nay, viec sd dung mat cat dja chdn ndng phan giai eao d l xde dinh ede ylu td edu true tiln chau thd va chan chau thd Id hpp ly nhdt.
Giiia cac ylii td edu true cila delta ngdm ed su khdc biet v l dae dilm va thanh phdn vat chdt. Bl mat chan chau thd thudng khdng bdng phang va cd dd dde cao so vdi tiln chau thd, vat chdt tao nen ehdng cdng nhd, mjn ban vdi cau tao chu ye'u la phan ldp ngang xien dde hon so vdi d trdm tieh tiln chau thd. Vdi dae dilm dd, trdn mat cat dia chdn ndng phan giai eao, giiia ede ylu td cdu true neu tren cd ddu hieu phan dinh ranh gidi khd rd ret ; cdn cac Idp trdm tich dupe t h i hien bdi cde ddu hidu phan Idp ngang xien dde v l phia biln. Trong cde mat cdt dja ehdn ndng phan giai eao, eae trdm tich t h i hien cdu tao phan Idp ngang vd phan Idp
29032007 03^
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29032007 03b
Hinh 4. Sa dd mat eat dja ehdn ndng phan giai cao 29032007_03b (tren) vd xde djnh ranh gidi cdu Uilc (dudi) a. Tiln chau thd, b. Chan chau thd
ngang hoi xien, nghieng vl phia bien la cae trdm tieh biln - sdng vd biln - sdng phan bd chu ye'u d khu vue tiln chau thd (a) vachan chau thd (b) ; cdn cde thdnh tao phan ldp ngang la trdm tich bil'n phan bd ehu ylu tren bl mat thim trong (e).
Cae mat edt dja ehdn ndng phan giai eao do tdu Sonne do dae thi hien rdt rd nit ranh gidi cde edu true tren. Trong bdi viet, xin neu hai mat cdt diin hinh d KVNC : mat edt 29032007_03b vd mat cdt 29032007_01 (hinh 4 va 5).
Tren mat cat 29032007_03b, theo hudng td phai sang trai (tu bd ra biln), bl mat dja hinh rdt thoai va don dieu, gdn nhu khdng phan biet dupe ranh gidi cua chan chau thd vd tiln chau thd. Nhung trong mat edt ddng, ranh gidi nay thi hien rd su thay ddi eua vat chdt va dae dilm be mat ranh gidi (trong dd bl mat chdn chdu thd phdn giap bd - tao thanh dja hinh hai triing vd dupe eae thanh tao tiln chau thd phil trum len)
0 mat -cat 29032007_01, bl mat dja hinh day biln dde hon (so vdi mat cdt tren, nhung bl mat cQng don dieu kha bdng phdng). Trong mat cdt ddng, bl mat chan chau thd the hien rdt rd vdi dae die'm kha phdc tap, Idi Idm. Dae biet, tren mat eat ndy cdn xae dinh dupe phdn ranh gidi td ria tiln chau thd ra phia biln dupe gpi la phdn thIm trong eua thIm luc dja - noi vat lieu trdm tich tao nen bl mat the hien ylu td ngudn gde bil'n la chu dao (biln md).
Ke't qua phan tieh chung cac mat cat dja chdn ndng phan giai cao thu thap d KVNC, ed thi xac lap khai quat ranh gidi giiia tiln chau thd, chan chau thd va thIm trong d khu vuc nghien cdu dupe thi hien tren hinh 3 (cae dudng ®va ®).
VI thdi gian thdnh tao chdn chau thd va tiln chau thd, theo Nguyin Tiln Hai va nnk | 3 | , d vung bil'n them Mekong cd (biln Binh Thuan - VQng Tdu), vao thdi gian khoang 8.000 nam trude (vilt tdt la BP)
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Hinh 5. So dd mat edt dja chdn ndng phan giai cao tuyln 29032007_01 (tren) vd xac djnh ranh gidi edu true (dudi) a, Tiln chau thd, b, Chan chau thd, c. Them trong
muc nude biln thdp hon muc nude hien nay khoang 20 m, vdo khoang d.OOOnam BP thdp hon nay tren dudi 10 m, khoang 5.000 - 4.500 nam BP cao hon 4 - 5 m. Sau dd muc nude ha thdp xudng vdi muc nude thdp hon ngay nay td 2,5 din 2 m vdo thdi gian khoang 2.500 - 2.000 nam BP, rdi dang eao trd lai din muc dd eao 2,5 m (eao hon ngay nay). Sau dd, muc nude biln ha din muc 1 - 2 m thdp ban ngdy nay, rdi dang eao trd lai vao khoang 1.000 nam BP eho din hien nay.
Td tren, ed the cho, ea ban phdn tao nen chan chau thd hien nay chinh Id tiln chau thd eua chau thd Mekong hinh thanh tt-ong thdi gian trude 8.000 nam BP ; edn tiln chau thd hien nay hinh thdnh trong thdi gian tix khoang 8.000 nam BPcho din ngay nay.
THAY CHO K f T LUAN
- Trong vung bil'n ven bd nam Cd Mau vd k l can, ranh gidi chau thd/ehan chau thd cua chau thd Cuu
Long phan bd d dd sau tren dudi 8-10 m, edn ranh gidi chan chau thd/thim trong - d dp sau 13-16 m.
- Cd thi sd dung hieu qua tai lieu dja chdn ndng phan giai eao d l xde dinh dae diem bl mat cac ylu td edu true ngdm eua chau thd va ranh gidi giua chung, nhdt la ddi vdi nhung chau thd ngdm eo b l mat don dieu vd thdnh phdn vat chat ddng nhdt ve dae dilm vd thanh phdn.
TAI LIEU DAN
[11 NGUYiN TI&J HAI (chu bien) va nnk, 2006 : Tiln hda ddi ven bil'n, dao ddng muc nude biln va qua trinh tieh tu vat lieu luc nguyen (phu sa) trong Holoeen d thIm liic dja vung biln giua chau thd Mekong va Nha Trang, Ddng nam Viet Nam, Bao cao d l tai khoa hpe hpp tae Viet-Dde, Vien Thdng tin tu lieu KH, Bd KH&CN, 223 tr.
[21 TRINH THE HIEU, 1996 : Trdm tieh day vung bie'n Tay Nam Viet Nam. Cae Cdng Uinh nghien edu
Dja chdt vd Dja vat ly biln, tap II, Nxb KH&KT, HaNdi, 7i9-745.
13] TA THI KIM CANH, NGUV^N VAN LAP, 2008 : Qua trinh phat triln ddng bang sdng Cuu Long trong thdi Holoeen, Tuyln tap Bao cao khoa hpe HNKH Dja chdt biln toan qude ldn thd nhdt. Ha Long 9-10/10/2008, 297 -301.
14] PHUNG VAN PHACH vd nnk, 2007 : Bdo edo ehuyln khao sat SO-187 - 3/2006, luu trd Vien DC&DVL Bie'n.
15] NGUYIN TRUNG THANH va nnk, 2007 : Mdt sd dae dil'm trdm tich va qua trinh trdm tieh hien dai vung ven bd phia ddng nam mdi Cd Mau. Bao edo dl tai KH co sd, luu trii Vien DC&DVL Bil'n.
[6] LE XUAN THUYEN, 2007 : Mdt sd bieu hien gian doan trong tap trdm tieh tiln va ria chau thd sdng Mekong, Tuyln tap eae cdng trinh nghien cdu Dja chdt vd Dja vat ly Bil'n, tap IX, Nxb KH&KT, Hd Ndi, 49 - 59.
17] NGUVfiNTHETUdNG (chil bien) va nnk, 2000:
Sd tay tra cdu Cac dae trung khi tupng thily van
vung thIm luc dja Viet Nam. Tong cue Khi tUpng - Thuy van. Nxb Ndng nghiep, 277 tr.
SUMMARY
Determination of Cuu Long delta's morphology in the Ca IVIau continental shelf area by applying high
resolution shallow seismic profile data Deltas are complicated combination of fluvial and marine processes. These structures can be grouped into three broad divisions : the deltas plain, the steeper delta front and the broad sloping prodelta.
The continental shelf of Ca Mau area adjacent part of the Cuu Long plain, where delta front and prodelta are situated. Generally, the sea floor is flatten and broaden, filled with muds and sandy muds.
On the basis of high resolution shallow seismic profiles data, the area could be divided to delta front, prodelta and open shelf in the continental shelf of Ca Mau area.
Ngdy nhdn bdi : 20-11-2009
Vien Dia chdt vd Dia vat ly Bien, Dai hoc Kiel
(CHLB Due)