Basin development and deposition of the Bongaya Formation in the Pitas Area, Northern Sabah.

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1 Ge. Sc. Malaysia, Bulletin 29, July 1991; pp Basin develpment and depsitin f the Bngaya Frmatin in the Pitas Area, Nrthern Sabah. F. TONGKUL Earth Science Dept., Universiti Kebangsaan Malaysia, Sabah Abstract: Develpment f a small, shallw basin fr the depsitin f the Bngaya sediments f late Micene age in the Pitas regin was cntrlled by earlier faults trending NE-SW, N-S and NW-SE n the underlying rcks f the area (Crcker and Chert-Spilite Frmatins). This nearly circular-shaped basin was filled by sediments surced frm lder uplifted rcks t the east, suth and sutheast by the verlapping f numerus fiuvi-deltaic fan lbes. INTRODUCTION The ccurrence f Upper Tertiary basins fr the depsitin f thick sediments in Sabah is widespread, yet studies n them is quite scarce. This is prbably due t the inaccessibility f mst f these areas in the past and previus effrts have been mainly cncentrated n Upper Tertiary ffshre basins where il ccurs. The presence f new rad cuttings due t lgging activities prvide a new avenue fr studying these sediments. One such basin ccurs in the Pitas regin in the Bengkka Peninsular f Nrthern Sabah (Fig. 1), where the Bngaya sediments have been depsited. Gd expsures f the frmatin in this regin and the availability f gd aerial phtgraphs prvided an pprtunity t study the prbable develpment mechanism f this basin. The Bngaya Frmatin in Nrthern Sabah ccurs in several islated basins lying uncnfrmably n lder defrmed sediments which includes the Crcker, Kudat and Chert-Spilite Frmatins (see Fig. 1). The Bngaya sediments have been dated as Upper Micene in age based n framinifera and was depsited in a shallw water envirnment (Wilsn, 1960). In the Pitas regin the Bngaya sediments rest with marked uncnfrmity n the Crcker Frmatin (Fig. 2).. SEDIMENTOLOGICAL FEATURES The Bngaya sediments cnsist mainly f grey sandstnes interbedded with carbnaceus shales with minr ccurrences f cnglmerate beds. The sandstnes are texturally immature, cnsisting mstly f quartz grains with mud matrix as its cement. The cnglmerate clasts mainly f sandstnes and cherts which is typical f the Crcker and Chert-Spilite sediments.

2 184 F. TONGKUL 'E Suth China Blmbngn Is. Sea lwli Is. Sulu Sea N I M~RAK PARAK AGE FOR MATION RECENT 1:--;::1 ALLUVIUM LINEAMENTS / DIP PLlO.-PLEIST. li:h::i TIMOHING I / / FAULTS MID.-UPP. MIOC. IK{1 MID EOC. - L. MIOC. L:::::::.J r/'1 BONGAYA / SOUTH BA NGGI (Sb) ~ KUDAT (K)jCROCK ER (C) GEOLOGICAL BOUNDARY? MAJOR RIVERS L.CR ET- L. EOC. BASEMENT (CHERT-SPllITE) I i MAJOR ROADS ;.i "" Fig 1. Gelgical map f Nrthern Sabah shwing the ccurrence f the Bngaya Frmatin as islated circular basins.

3 N t 17 OO' E MARU O u BAY 07 45' N Basement ROC k /Chert- spdite Fm. (Early Cretceus - Ea rly Ecene ) J'. Fld O ~ IS /1 Fault s / Thrusts /4~ Fra c t ures ~ Strike / dip ROCk Bun daries j( f {, ~. Rivers R ds Sc l e Q 2 i Km Pleflw Inlier td» C/l Z Cl < r "0 :s: Z -J» z -0 C/l :i z z -J :r: ""d ~ C/l >- ;;0» CROSS-SEC non Fig 2. Gelgical ma p f the Pitas area. The un def rmed Bngaya sediments sit uncnfrmably n the flded-faulted Crcker sediments. The nearly circular-shape f the Bngaya basin was cntrll ed by majr faults trending N-S, NW-SE and NE-SW. f-' 00 01

4 186 F. TONGKUL The sandstne beds vary in thickness frm a few centimetre t several metres thick. The sandstne beds mstly pinched ut laterally (Fig. 3). Sme f the thick sandstne beds have cnglmeratic bases. Sedimentary structures mst cmmnly fund in the sandstne beds are crss-beddings and crsslaminatins (Fig. 4). Small lbe structures are als quite cmmn (Fig. 5). Current directins derived frm these structures varies frm abut N3300E t N600E (see Fig. 2). This wide distributin f paleflw prbably indicate lcal changes in the surce area and depsitinal system during the filling f the basin. The shale beds als vary in thickness frm a few cm t several metres. They are mstly carbnaceus shwing crss liminatins and parallel laminatins. Vertical trace fssils (Fig. 6) are quite cmmn in them indicating a shallw water depsitinal envirnment. Measured sectins frm different parts f the regin indicate that the thickness f the Bngaya sediments varies frm ne area t anther, usually thicker twards the side f the basin. Its ttal thickness is uncertain but is estimated t be less than 200 metres. The stratigraphic sequence cnsists f several thick sandy and shaly sequences (each abut metres thick) alternating with each ther. Sequence analysis indicates the presence f Fig 3. Pinching ut f sandstne beds interpreted t represent lbe migratins. Lcality: Near Kg. Senaja.

5 B ASIN DEVELOPMENT A D DEPOSITION IN T HE PITAS AREA 187 Fig 4. Carbnaceus plana r crss-bedding. Paleflw directin measured here trends N-S. Lcality: Near Pitas Schl. several fining upward sequences (FUS) interpreted t represent several migrating channels r fan lbes (Fig. 7). STRUCTURAL FEATURES The Bngaya sediments ccur as gently dipping beds (5-25 ) with a general strike directin trending apprximately NE-SW, NW-SE and E-W in the sutheastern, nrtheastern and nrthern part respectively frming a nearly circular shaped-basin. A majr NE-SW trending fault clearly bunds the sutheastern part f the Bngaya Frmatin frm the underlying rcks as seen n aerial phtgraphs. Parallel t this majr fault several smaller NE-SW trending faults affecting the Bngaya sediments were bserved in the field (Fig. 8). Steeper dips ccur near this majr fault. Lcal structures fund in different parts f the regin are mstly nrmal faults trending N-S, NW-SE and NE-SW with displacements ranging frm a few cm t a few metres (Fig. 9). The small faults are mstly syn-depsitinal while the bigger nes are clearly pst-depsitinal. The gently dipping nature f the Bngaya sediments and the absence f flds suggest that it has nt experienced intense cmpressinal defrmatin. The presence f numerus nrmal faults suggests that the Bngaya sediments may have undergne extensinal phase during and after its depsitin.

6 188 F. TONGKUL Fig 5. Nearly hrizntal sandstne and shale beds shwing small lbe structures. Lcality: Near Pitas Schl. Fig 6. Vertical trace fssils in grey shale beds f the Bngaya Frmatin indicating shallw water depsitinal envirnment. Lcality: Near Kg. Senaja.

7 BASIN DEVELOPMENT AND DEPOSITION IN THE PITAS AREA Trugn crss.bedding 20.~~ FUS FUS FUS r FUS M 5 Mud clasts Crban I laminatin I M 5 ~;::~I Grin Size KEY ~~cz:l1sl!lli:b2 ~3 LL.J4 ~5 ~6 1i17 Grin Size Fig 7. Lithstratigraphic sequence f the Bngaya sediments shwing several fining upward sequences (FUS) prbably representing migrating lbes r channels. 1. Cnglmerate 2. Sandstne 3. Shale 4. Trugh crss-bedding 5. Planar crss-bedding 6. Vertical trace fssils 7. Laminatins.

8 190 F. TONGKUL Fig 8. Hrizntal fault trending N40E cutting thrugh the Bngaya sediments with an blique sinistral mvement. Lcality: Near Kg. Kabatasan. DISCUSSION: DEPOSITIONAL MODEL The sedimentlgical and structural features f the Bngaya sediments suggest that the depsitin f these sediments was clsely cntrlled by earlier majr structures. Based n earlier studies (Tngkul, 1989), the underlying sediments were fund t have been flded and thrust-faulted with their fld axes trending apprximately NW-SE. These NW-SE trending lineaments are crss-cut by several sets f N-S and NE-SW hrizntal faults as a cnsequence f cntinued N-S cmpressin in this regin up t late middle Micene. The extensinal features bserved in the Bngaya sediments having a similar trend t the structures n the underlying rcks, indicate that the frmer was a manifestatin f the latter (inverse structure). This is seen t be the case in ther parts f Sabah (Tngkul, 1990). The existence f the earlier fault systems significantly cntrlled the shape f the basin and subsequently the depsitinal system (Fig. 10). The Bngaya basin prbably started t develp by extensin during the uplift f the underlying rcks as a late stage accmmdatin structure. Extensin utilized the existing N-S and NE-SW fault system here t frm a shallw V-shaped graben-type basin. The narrw sutheastern part f the basin was prbably relatively shallwer cmpared t the nrtheastern part f

9 BASIN DEV ELOPMENT AND DEPOSITION TN THE PITAS AREA 191 " ~ - Fig 9. Several nrmal faults and fractures cutting thrugh gently dipping Bngaya sediment. Vertical displacement is abut a metre. Lcality: near Kg. Tambilidn.

10 192 F. TONGKUL _ km km 1 (A) Mid. Micene (8) Late Micene Open shallw sea (C) Plicene [[ill...." I.... KEY Fig 10. Schematic diagram t illustrate the prbable develpment f the Bngaya basin. (a) Structural features present n the underlying rck prir t the develpment f the basin; NW-SE trending faults and fld axis; N-S and NE-SW hrizntal faults. (b) Extensin in this regin utilizing the existing majr faults prducing inverse structures (graben-type). The NE-SW and N-S hrizntal faults marked the edge f the basin. (c) The narrw and elngate shape f the basin cntrlled the depsitinal system. (1) Bngaya Frmatin (2) Crcker Frmatin (3) Basement Rck (Chert-Spilite Frmatin) (4) Hrizntal fault (5) Thrust and fld axis (6) Current directins (7) Lbe migratins.

11 BASIN DEVELOPMENT AND DEPOSITION IN THE PITAS AREA 193 the basin which cnnected with the pen sea during this perid. This basin cnfiguratin cntrlled the majr flw f sediments frm the sutheast fanning-ut twards the nrtheast. Fluvial sediments represented by the cnglmeratic beds filled the suthwestern and sutheastern side f the basin which gradually gave way t deltaic fan lbe sediments twards the deeper part f the basin. The cntinuus depsitin f sediments frm the uplifted surrunding rcks (Crcker and Chert-Spilite Frmatins) by the verlapping r calescing f several deltaic lbes gradually filled the entire shallw basin. As a result f this filling the basin prbably sagged a bit causing steepening f beds twards the side f the basin and the nrmal faults. Overall uplift f the Bengkka regin gradually expsed this sediment. CONCLUSION This case study has demnstrated that the develpment f a particular basin and subsequently the depsitin f its sediments are clsely related t earlier majr structures. In trying t understand and recnstruct the depsitinal histry f the Upper Tertiary sediments, especially the circular-shaped basin in Central Sabah, it is necessary t knw the majr structures present n the underlying rcks. ACKNOWLEDGEMENTS This research was funded by grants frm UKM 6/85. The authr is grateful t his clleagues and final year students fr their cntributin during the field wrk. REFERENCES WILSON, R.A.M., The gelgy and mineral resurces f the Banggi Island and Sugut River area, Nrth Brne. Gel. Surv. Dept. Brit. Terr. Mem.15. TONGKUL, F., The structural styles and tectnics f Nrthern and Western Sabah. Petrleum Gelgy Seminar 1989 (abstract): 19. Gelgical Sciety f Malaysia. TONGKUL, F., The tectnic evlutin f Sabah, Malaysia. Orgenesis in Actin: Tectnics and Prcesses in the West Equatrial Pacific Margin (abstract): 57. Gelgical Sciety f Lndn. Manuscript received 12 Nvember 1990