Geological Structure Analysis to Determine the Direction of the Main Stress at Western Part of Kolok Mudik, Barangin District, Sawahlunto, West Sumatera

On the study area there are three types of structure, those are fault, fold and joint. Types of fault were found  in the study area, reverse fault with the strike/dip is N215oE/75o, normal fault has a fault directions N22oE and N200oE with pitch 35o, and dextral fault with pitch 10o and strike N219oE. Fold and joint structures used to determine the direction of the main stress on the study area. Further, an analysis used stereonet for data folds and joints. So that from the data got three directions of main stress, those are Northeast – Southwest (T1), North – South (T2) and Southeast – Northwest (T3). On the Northeast – Southwest (T1) stress there are four geological structures, anticline fold at ST.3 , syncline folds at ST. 13a, ST. 13b, ST. 13c and ST. 33, chevron fold at ST. 44 and joint at ST. 2. On the North – South (T2) stress there are three geological structures, those are syncline fold at ST. 35, anticline fold at ST. 54 and joints at ST. 41, ST. 46 and ST. 47. On the Southeast – Northwest (T3) stress were also three geological structures, those are chevron fold at ST 42a, overturned fold at ST. 42b, syncline fold at ST. 42c and joints at ST. 5 and ST. 34.


Int r oduct i on
The study area is located at Western part of Kolok M udik, Kecam atan Barangin, Kotamadya Saw ahlunto, Sumatera Barat. The topography of Saw ahlunto is hilly areas w it h elevation about 250 650 met ers above sea level (Pebri aldi, 2015). The topography grow n could be interpreted that on t he study area be affected by tectonic activity such as fold or fault (Koesoemadinata and M atasak, 1981). Its can be seen from shape of river that nudge, indicated that river formed due to crack or fracture is relatively w eak zone and then eroded along fracture (Fig 2). Hilly area w ould be described that this area has occured uplift and then formed a fold (Koesomadinata and M atasak, 1981).
The study area is w ithin ombilin basin. Overall the strucuture of the basin ombilin show ed transtensional duplex or pull apart duplex systems. Woodcock and Fischer (1986) said in Situmorang, et.,al (1991). subduction of geometry from duplex faults w ould be meet in sub basin becam e single shear zone. This condition is so different w ith the geological structure of the northern part of Central Sumatra Basin that w as developing at the time of Neogen and asymmetrical shape that led northw est-southeast (NW -SE) w hich is a pattern of young structure (Kausarian, 2017).

Gener al Geology and St r at i gr aphy
Based on Peta Geologi Lembar Solok (1995), the study area has tw o formation and one deposition, that are Silungkang Formation, Brani Formation and Porphyry Deposition. The Silungkang Formation there are andesit hornblende, andesit argit, meta-andesit w ith thin interbedded tuff, limestone, shale and clay shale (PH. Silitonga and Kastow o, 1995). There is also a Fusulinida fossil (Koesomadinata and M atasak, 1981). The Brani Formation there is conglomerate interbedded w ith sand. The Porphyry Deposition consist of porphyry quartz w ith quartz phenocrysts and feldspar (PH. Silitonga and Kastow o, 1995). Furthermore, w olud be found fractures on the body of rocks and alteration that happened on w all rocks (Berger, et.al,2008).
After observation on the study area, w e found three formation, that are Silungkang Formation, Brani Formation and Sangkarew ang Formation. Silungkang Formation consists of lim estone w ith mudstone type, sandstone and conglomerate. On the Brani Formation there are conglomerate, sandstone and shale. M eanw hile Sangkarew ang Formation consists shale, sandstone, slump and crossbedding.

M et hodology
The methodology used in t his research w ere literature review , field survey and st ereonet analysis. The first literature review to know n structure types on the study area. The study area in the field show ed by plotting points to generate the geological mapping (Kausarian, 2016). After that field survey w as done to get the data that needed to determine the direction of main stress from geological structure. The last analysis used streonet from the data. All of data has plotted on st ereonet to get the direction of main stress at the study area (Adi Suryadi, 2016 Geological structures on the study area are faults, folds and joints (Fig 3). Indication of fault is difficult to found in the field, because the rocks on the study area have been a heavily w eathering (Rizky prata, 2011; (Putra and Choanji, 2016). On the study area, there are t hree types of fault, these are reverse fault, normal fault and dextral fault. Reverse fault located in southw est the study area w ith value fault plane is N215oE/75oon station 2. Normal fault located at central of study area, these are in station 10 w ith direction N200oE and pitch 35o, station 56 and station 57 has sam e direction is N22oE. Dextral fault located in north of study area w ith pitch 10o. Fault w ould be know n if w e found indications in the field, like w aterfall, displacement of bedding and slickenside et c.

Folds
On the study area w ere found four types of folds, there are anticline fold, syncline fold, chevron fold and overturned fold. The all of data w ould be plotted in stereonet to know n the direction of main stress (Fig 5). Based on stereonet analysis result, there are three direction of main stress, those are Northeast Southw est, North South and Southeast Northw est (Table 1).

Joi nt s
On the study area, joints data took at southw est north of the study area. Based on joints data that has been obtained and analysis used st ereonet (Figure 6), there are t hree direction of main stress, Northeast Southw est, North South and Southeast Northw est (Table2).    Based on analysis of geological structure w ere found three main stress that deform the geological condition of study area. The first stress is Northeast Southw est (T1) this stress generated some structures, these are syncline folds, ant icline folds, chevron fold and joint. Another stress is from North South (T2) that resulted syncline folds, anticline folds and joints. The last main stress w ith direction Southeast Northw est (T3) w as form chevron fold, overturned fold, syncline fold and joints.

Acknow ledgem ent s
We w ould like to say thanks for our parent s that alw ays prayed us. For our advisor is M r. Adi Suryadi, B.Sc(Hons)., M .Sc that has been faithful guided us until this paper are Adlan Rahmat, Arsyad and Yogi Aditya that alw ays have companied us w hen w e are collected data in the field.