ROCK STRUCTURES AND FAULTS

ROCK STRUCTURES AND FAULTS

Outcrop: The outcrop of any stratum or rock is that part of it which is exposed at the surface. In most of the cases an outcrop is not visible at the surface, being covered by soil or alluvium.

Anticline and Syncline: Rocks are not always dipping at a uniform gradient, but are sometimes undulating or folding. The folding results from pressure or force which may be local, and vertical or horizontal over a wide stretch. The shape of the land surface may be quite different from the rock structure below.

The crest of a fold where the strata are bent up to form an arch is called an anticline and the beds dip away from the axis of the fold (Fig. 1.6, Left). If the folding is such that the beds dip towards a common axis or plane the structure is known as a syncline (Fig. 1.6, B). In a syncline the beds form a trough and dip towards the axis on both sides. In a dome-shaped structure the beds dip away from a common point whereas in a basin-shaped structure they dip towards a common point. The coal seams and associated rocks at Amlabad colliery (Dhanbad district) have a dome-shaped structure.

Fig 1.5 In a mineral bed A—Washout, B—Roll, C—Swilley

Washout: A washout is an irregularity where the bed thins out, generally due to erosion by a river or stream and the roof of the bed is filled up with sand or other material .(fig. 1.5).

 

Fig. 1.6. AP - axial plane aa  — axis of fold

C shows outcrops of eroded beds in a pitching anticline

Roll: This may be considered exactly opposite of washout in that the floor on which a mineral bed is deposited shows an upward extension towards the roof of the mineral bed which thins down in -the area of roll (Fig. 1.5).                                                    ^u^

Swilley: A swilley is a trough or depression in which the seam, with its otherwise undisturbed roof and floor, sags several metres below the general seam level, the seam generally being thicker in the swilley area. (fig. 1.5).

Outlier and Inlier: An outlier is an outcrop of rocks surrounded by older rocks and, therefore, entirely separated from the main mass by denudation. It is a mere patch surrounded by older strata on all sides.

An inlier is an outcrop of rocks in the form of a patch of strata surrounded by newer beds on all sides (Fig. 1.7).

 

Fig. 1.7

Fault

A fault is a fracture in rocks usually associated with lateral or vertical displacement of the fractured beds (Fig. 1.8). Faults occur when the strength of the rocks is insufficient to withstand the stress due to earth movement.

The vertical displacement CD of the fractured bed is called the throw of the fault and is measured from floor to floor of the seam. It may be from a few metres lo hundreds of metres. The horizontal displacement DE is called the lateral shift or heave and it represents the width of the ground in which the coal seam is missing. The area in which coal seam is thus missing is called want or barren ground. The plane of fracture FF is called Fault plane. The angle DCE is called the Angle of hade or underlies of the fault and is always measured from the vertical. It is usually from 10° to"40°.

A fault is called down throw or up throw according to the side from which one travels towards the fault plane. A man travelling over bed M towards the fault would meet a down-throw fault, whereas a man travelling over bed N towards the fault plane would meet an up throw fault. Throw of a particular fault is not always the same at all places.

A Normal fault is one in which the ends of the broken strata do not overlap and the fault plane hades forward towards the displaced bed. The two parallel faults M and N form a step fault, (fig 1.9) and the intersecting faults N and S form a trough fault. Normal faults occur in areas of tension and cause an extension of the surface of the faulted region. A reverse fault is one in which the older rocks are thrust over new ones by lateral compression as at T in fig. 1.9 so that the ends of the broken strata overlap each other. In such a case a vertical borehole would pierce the same bed twice. Reverse faults occur when the strata are compressed into a space less than their original horizontal extension and are common where strata are intensively folded. In general most of the faults are normal faults and reverse faults are uncommon in coalfields.

The strike of a fault, or its true bearing is the direction of a level line in the fault plane. The direction of the fault, as shown on a mine plan, is not necessarily its true bearing because the line of the fault shown on a mine plan represents the line of intersection of the fault and the mineral bed.

A dip fault is one whose general direction or trend is parallel to the dip of the beds. It is also called, a transverse fault.

A strike fault is one whose general direction or trend is parallel to the strike of the strata. It is also called longitudinal fault. A truly dip fault or a truly strike fault is rare. A fault that runs at some angle between the dip and strike of the beds is called an oblique fault.

Fig 1.11. Lateral displacement of outcrops by faults. Right side figures show faults on a level ground after denudation, ff is fault plane

Effect of fault on outcrops

Sometimes a fault may -be traced at the surface by a line of hollows or depressions that are denuded out as the fault zone consists of broken and shattered rocks which easily weather out. More often, however, a fault has little or no effect on the surface contours.

A strike fault has the effect of causing repetition of the outcrop of some of the beds (Fig. 1.10)

A dip fault causes lateral displacement of the strata.

In F g. l.11 (i) ff is the fault plane, a the up throw side, b the down through side, and the t vertical throw. After denudation the surface of up throw side a has come in level with the surface of down throw side b. The outcrops then appear as in (Fig. ii) showing lateral displacement.