The Occurrence and Structural Characteristics of “Brick-Structure” Coal Fractures (Cleats) in Mukah-Balingian, Sarawak, Malaysia: a Probable Fault Reactivation Feature

Abstract

The Tertiary sequence of Northwest Borneo was gently folded with NNW- to SSE-trending axes during the Middle Miocene and the subsequent folding and uplift during the Pliocone-Pleistocene affected all the older stratigraphic successions (Haile, 1969). In this current study, a NNW- trending “coal-brick structure” (Figure 1) has been identified and is postulated to be related to the Pliocene-Pleistocene reactivation of preexisting structural fabrics. This interpretation is based on a number of supporting structural features which include the similar NW-trending fault bounded graben and horsts in the onshore Mukah area and the adjacent offshore area (Mazlan Madon, 1999). The “coal-brick structure” occurs at two locations; in the northern part it occurs in the Upper Miocene Balingian Formation close to the contact with the Lower Pliocene Bergih Formation, while in the south it occurs in the Upper Pliocene Liang Formation close to the contact with the Eocene Belaga Formation (Figure 2). A few faults have been mapped close to the brick structure. Based on the similar orientation of the NWtrending of the long fractures (face cleats) of the “coal-brick structure” to the fault at the northern area, as well as being similar to the NW-trending adjacent offshore regional structural trend, it is postulated that the occurrence of the “coal-brick structure” is an indication of a fault-bounded zone associated with folding, uplift or unloading of the overburden, and could indicate proximity to an unconformity. It is observed that there is a variable cleat strike postulated to be within the main faulting region close to a hinge of the fold, while further away from the hinge the “brick structures” are larger in size and display a prominent long fracture pattern parallel to the NW-trending regional structural trend including the offshore West Balingian bounding fault line. It is postulated that the folding and faulting and the associated “coal-brick structure” in the Balingian and Liang formations point to possible structural reactivation of the Early-Middle Miocene regional faults and structural fabric within the Mukah- Balingan and adjacent offshore areas. Similar fracture patterns that are parallel to pre-existing regional faults has also been observed in basalt of Quaternary age at Pantai Batu Hitam, Pahang which has been postulated to point to the possibility of reactivation of regional Pre-Tertiary faults in the Malay Basin region (Tjia, 2008). The Pliocene formations (Begrih and Liang) rest directly on the Eocene Belaga Formation. Such major unconformities are common in onshore and offshore Sarawak. In the offshore area adjacent to the Mukah-Balingian region, an angular unconformity that rests directly on the Oligocene-Early Miocene (Cycle I-Cycle II) occurs at the base of Cycle VI/VII in Balingian Province (Mazlan Madon and Abolins, 1999). In the adjacent Tatau Province, Cycle III/IV of Early–Middle Miocene strata were reported by Mazlan Madon and Redzuan Abu Hassan (1999) to rest directly on the Paleocene-Eocene basement (equivalent to onshore Belaga Formation). The shallow depth or immediate overlying proximity of the Pliocene Liang to the Eocene Belaga Formation may facilitate the Liang inheriting the Belaga fault directions and associated structural fabric upon reactivation. An important implication of the development of these cleats is their potential to act as conduits for hydrocarbons such as for coalbed methane gas.