Form of Deposit or Seam

Form of Deposit or Seam:-

Coal seams in Australia are generally found as tabular deposits which extend 

over large areas, usually at low dip angles (i.e. fairly flat, not steeply inclined).

Seam thicknesses vary, but usually on a gradual basis except where 

local "washouts" occur, where a portion of seam has been eroded away and 

refilled with stone deposits in the distant past. Seams vary from less than 1m 

thick up to over 30m. The minimum thickness worked in Australia has been 

around 1.3m and few mines work less than around 1.8m for extended areas. 

Few of the very thick seams are of consistent quality throughout their thickness 

and most mines working in such seams only mine a portion of the full thickness. 

Few, if any mines mine more than around 5m in height.

Seams are generally extensive over large areas but may be broken-up by 

discontinuities such as "faults" (where strata has fractured and been displaced 

each side of the fracture) or by igneous intrusions such as "dykes" (where igneous material has flowed into and through a seam forming a wall of rock, 

often burning or "cindering" the coal close to it). In some coalfields it is 

common for seams to split into 2 or more sections or seams or for separate seams to "coalesce" into one.

The depth below surface can vary from zero, where the seam outcrops to 

around 450-500m. Deeper seams exist but have not yet been worked in 

Australia.