Depths of burial of eastern Southland lignites, estimated from their moisture contents

Abstract

Bed moisture Contents range from c. 64% to 41% in eastern Southland lignite fields, being lowest in the northwest. The rates of decrease of moisture with depth in drillholes vary, being greatest in the higher moisture areas. The rates are generalised from 19 drillhole sequences and extrapolated to 90% moisture in the original surface peat. This leads to estimated former maximum depths of burial of an Early Miocene pollen zone boundary ranging from 160–200 m in the south to 700–800 m in the northwest. Where subsidence was greatest, possibly towards the faulted margin ofafault-angle depression, subsequent uplift was also greatest. For lignites of average type, the ASTM Lignite A/Lignite B boundary in eastern Southland is estimated to have been reached at an original depth of burial of c. 650 m.

Keywords:

• eastern Southland
• lignite
• moisture
• depth of burial
• ASTM rank