Structural geology of Datai beds and Macincang Formation, Langkawi

702001-101104-906-B
Author : H.D. Tjia
Publication : Bulletin of the Geological Society of Malaysia
Page : 85-120
Volume Number : 23
DOI : https://doi.org/10.7186/bgsm23198906

Geological Society of Malaysia, Volume 23, August 1989, pp. 85 – 120

 

Structural geology of Datai beds and Macincang Formation, Langkawi

H.D. TJIA

Department of Geology, Universiti Kebangsaan Malaysia, 46300 Bangi, Selangor

 

Abstract: In the Macincang terrain of northwest Langkawi are a few isolated outcrops of probably Precambrian, newly named Datai beds that consist of meta-arenite and meta-argillite intercalations. Parts of the beds are dismembered, all thoroughly fractured and strongly deformed into steeply and vertically plunging tight folds. The Datai beds are overlain by regularly bedded, moderately inclined Macincang strata. At Pasir Tengkorak the contact between the rock groups is an angular unconformity; at Teluk Anak Datai, the separation consists of a 4 m thick mylonite zone of the Datai thrust fault. Five photo-litho units and a number of major fault zones striking north and east were traced from aerial photographs and later confirmed in coastal outcrops. The Macincang beds were deformed into three major folds, the ENE-trending Buta anticline and the north-striking Datai anticline and syncline. Reliable indicators for stratigraphic facing show that in general the Macincang layers are still right-side up. This simple structural style contrasts sharply with the tight structures and overfolds that characterize the Palaeozoic beds outcropping in a narrow strip along eastern Langkawi. The situation suggests that the Macincang terrain may represent the eastern border of a micro-continent buried at the north entrance of the Strait of Malacca. Quartz clasts within quartz-mylonite zones that are parallel or subparallel to Macincang layers often exhibit results of three phases of activity; the clasts were coaxially deformed into rotated sigmoids and later warped about a different axis. Equal-area projections of structural elements also suggest that the Macincang rocks experienced three folding phases. The oldest maximum principal stress direction was NNW-SSE and developed the Buta anticline. The second phase had compression acting NE-SW and resulted in tectonic transport towards SW along the Datai thrust and along a number of other low-angle reverse faults. The youngest traceable compression direction was east-west and formed the Datai anticline and syncline. It also re-orientated earlier structures to trend in NNE-SSW direction.

 

Abstrak: Di beberapa tempat pada kawasan Macincang di sebelah barat laut Pulau Langkawi terdapat Lapisan Datai yang mungkin berusia Prakambria. Batuannya terdiri daripada selang lapis meta-arenit tebal dengan meta-argilit. Sebahagian lapisan terrabak, direntasi padat oleh retakan dan tercangga kuat kepada lipatan ketat yang bertunjam curam hingga menegak. Lapisan Datai ditindihi Lapisan Macincang yang berlapis teratur dan berkemiringan sederhana. Sempadan antara kedua kumpulan batuan bersifat ketakselarasan bersudut di Pasir Tengkorak, manakala di Teluk Anak Datai zon milonit sesar sungkup Datai setebal 4 m menjadi pemisah. Lima unit fotolitologi serta sejumlah sesar utama berjurus utara dan timur telah dikesan dari foto udara dan kemudian dikenalpasti pada singkapan pantai. Batuan Macincang telah dicangga kepada tiga lipatan terbuka lagi besar, iaitu antiklin Buta berjurus timur timur-Iaut dan antiklin serta sinklin Datai yang berjurus utara. Atas-atau-bawah stratigrafi ditunjukkan oleh sejumlah ciri yang boleh dipercayai dan mencadangkan bahawa umumnya lapisan Macincang belum terbalik. Gaya canggaan yang begitu mudah bagi kawasan Macincang sangat berlainan dibanding dengan struktur ketat yang mencirikan batuan Paleozoik pada jalur timur Kepulauan Langkawi. Keadaan demikian mencadangkan bahawa kawasan Macincang merupakan sempadan timur daripada sebuah mikro-benua Prakambria yang terpendam di kawasan utara Selat Melaka. Klasta kuarza dalam zon milonit yang selari atau hampir selari dengan perlapisan batuan Macincang sering menunjukkan kesan akibat tiga fasa kegiatan; klasta terlipat semula kepada bentuk sigmoid bergulung. Unjuran samaluas daripada unsur struktur mencadangkan pula bahawa Formasi Macincang telah mengalami tiga fasa perlipatan. Yang paling tua telah bertindak utara-Iaut dan menghasilkan antiklin Buta. Fasa canggaan kedua bertindak selari arah timur laut ke barat daya dan menyebabkan  pengangkutan tektonik ke barat daya menerusi sesar sungkup Datai serta sejumlah sesar naik landai yang lain. Mampatan yang paling muda bertindak timur-barat dan menghasilkan lipatan Datai serta merubah kedudukan beberapa struktur lebih tua kepada jurus utara timur-Iaut. 

https://doi.org/10.7186/bgsm23198906