A faulted Barakar seam in the Wardha valley: geometry, age of movement, and the incline
A mine-scale mapping project in the Wardha valley, Nagpur district, uses three boreholes 300 m apart on a north–south line. BH-1 (north) cuts the Barakar coal seam at 90 m; BH-2 (middle) at 210 m; BH-3 (south) at 95 m. In an underground roadway the seam is offset along a clay-filled, slickensided surface striking N60°E and dipping 55° NW. Drag rolls in the footwall seam indicate relative upward movement on the south-east side. Bedding in the hanging wall strikes N45°E and dips 30° NW, and 2 km east the same seam strikes N20°E and dips 12° NW. Overlying Deccan Trap lava flows in the same block show no displacement at surface. The management proposes to drive a new incline on the assumption that the structure is a gentle monocline.
1. Interpret the three boreholes: what structure do they define, and what vertical offset is involved? Comment on the adequacy of the data.
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The seam is about 120 m deeper at BH-2 than at BH-1 and about 115 m deeper than at BH-3. A single plane cannot produce that pattern - one fault would keep the seam displaced on both sides of the middle hole. The simplest reading is a narrow fault-bounded trough, a small graben or half-graben roughly 300 m wide, with throws of the order of 115–120 m, or a single fault whose effect is reversed by a second structure between BH-2 and BH-3.
The data do not allow the two possibilities to be separated, and that must be stated plainly. Three holes on 300 m spacing sample only three points on a surface; a fault, a fold hinge or a step could all pass between them. The vertical offset is a minimum, and the true stratigraphic separation is smaller than the vertical separation because the seam dips 30°.
Recommendation: infill drilling between BH-1 and BH-2 and between BH-2 and BH-3, at about 100 m spacing, plus a seismic reflection line across the inferred faults, before any structural model is committed to the mine plan.
2. Using the fault plane and bedding attitudes, together with the drag evidence, determine the sense of slip. Is the hanging wall consistent?
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The fault strikes N60°E and dips 55° NW; bedding strikes N45°E and dips 30° NW. Both dip north-west, but the fault is steeper than bedding, so the fault truncates the strata rather than lying along them - this is a cross-cutting fault, not a bedding-parallel slip surface.
The hanging wall is the block on the north-west side, because that is the direction of dip. The drag rolls show the south-east side, that is, the footwall, moved relatively upward. Hanging wall down and footwall up is a normal fault. The small 15° difference in strike means the movement is not purely dip-slip; expect a modest oblique component, which should be checked by recording the slickenside lineation trend and plunge rather than only the fault plane.
So the answer is a normal, mainly dip-slip fault downthrowing to the north-west, with a throw near 115–120 m, forming the margin of a small graben. The clay gouge and slickensides indicate shallow, water-rich movement at low temperature, so the fault is a permeable pathway and a weak plane - both matter for the incline.
3. The block is described as a Gondwana rift basin, yet the Indian plate is today under north–south compression, and the Deccan lavas above show no displacement. What does this tell you about the timing of movement?
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The absence of offset in the overlying Deccan lavas is the key observation: the fault terminates beneath them and therefore predates their eruption at about 66 Ma. The dominant movement is Permian to Early Cretaceous, when the Wardha valley subsided as a rift or graben and accumulated Barakar coal measures; the normal fault described is the basin-margin structure of that extensional phase.
The present compressional stress field does not contradict this, because it is a much later overprint. Compression reactivates steep pre-existing normal faults, usually in oblique or strike-slip mode with limited vertical offset, which is the sort of minor movement that produces a clay gouge and polished slickensides but no visible displacement of the lava pile.
The practical implications follow: do not extrapolate the fault upward into the overlying basalt as a major break, and do not assume one generation of movement. Look for two slickenside lineation sets, and use them to separate the rift-phase dip-slip from any later reactivation. The management's monocline model is wrong in kind, not merely in detail.