Sains Ma1aysiana 25(4) 1996: 47-68                                                       Sains Bumi/

                                                                                                               Earth Sciences

Diagenesis dan Asal Mula Dolomit dan

Kuarza di dalam Batu Kapur Kodiang, Kedah

(Diagenetic and origin of dolomite and quartz in Kodiang Limestone, Kedah)

Che Aziz Ali & Kamal Roslan Mohamed

Jabatan Geologi

Fakulti Sains Fizis dan Gunaan

Universiti Kebangsaan Malaysia

43600 UKM Bangi Selangor D.E.

ABSTRAK

Batu Kapur Kodiang yang tersingkap di Kodiang dan Alor Setar telah banyak dikaji dan didokumentasikan. Berbagai aspek geologi formasi batu kapur ini telah dilaporkan termasuk stratigrafi, startigrafi jujukan, sedimentologi dan paleontologi. Walau bagaimanapun aspek diagenesis formasi batuan ini tidak pernah dibincangkan secara khusus. Batu kapur Kodiang ini telah melalui sejarah diagenesis yang rumit yang bermula sebaik saja sedimen dimendapkan di atas permukaan dasar laut. Berbagai hasil diagenesis telah dikenal pasti termasuk penyimenan samudera awal, penyimenan meterorik dan penyimenan di bawah permukaan yang lebih dalam. Secara asas fitur-fitur yang dikenalpasti adalah seperti berikut; simen kalsit berserat, kalsit spar dan poikilotopik, tumbuhlampau kalsit, dolomit, simen silika dan evaporit. Pemeriksaan rapi pada singkapan menunjukkan dolomit hadir pada beberapa aras di dalam jujukan. Kehadirannya secara bersistematik boleh dikorelasikan pada setiap singkapan. Kajian petrografi menunjukkan dolomit wujud dalam berbagai mod yang boleh diklaskan sebagai dolomit berstrata dan dolomit tidak berstrata. Dua klas ini boleh dikaitkan dengan dua fasa pendolomitan. Dolomit berstrata ditafsirkan telah terbentuk awal di dalam sejarah diagenesis batu kapur. Dolomit tidak berstrata pula dikaitkan dengan pendolomitan semasa timbusan dalam di bawah keadaan fiziko-kimia bawah permukaan.

ABSTRACT

The Kodiang Formation which outcrops in Kodiang and Alor Setar areas has been well studied and documented. Various geological aspects of the limestone formation have been reported in detail including its stratigraphy, sequence startigraphy, sedimentology and paleontology. Diagenetic aspect of this rock formation, however, had not been specifically discussed before. The Kodiang limestone has undergone a complicated diagenetic history which started immediately after the sediments were laid down on the sea bed. Various diagenetic products have been identified including, early marine cement, meteoric cement, and deeper subsurface cement. The diagenetic features identified are as follows; Fibrous calcite cement, sparry and poikilotopic calcites, calcite overgrowth, dolomite, silica cements and evaporites. Close examination at outcrops shows that dolomite occurs at several levels in the sequence. Their systematic occurrences can be correlated across the area. Petrographic studies show that dolomite occurs in several modes as stratified dolomite and as non-stratified dolomite. The two dolomite groups are attributed to two phases of dolomitizations. The stratified dolomites are interpreted to have been formed early in the limestone diagenetic history. The non-stratified dolomite type is, however, attributed to dolomitization during deep burial under the control of deeper subsurface physico-chemical conditions.

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