Grain-size and botanical analysis of two coal beds from the South Island of New Zealand

Abstract

Two coal beds from the South Island of New Zealand were examined both from a sedimentological (grain-size) and botanical (plant part and palynological) perspective. The Cretaceous “Morley” coal bed contains thick (up to 4 cm) bright (vitrain/xylite) bands whereas the Eocene “Brunner” coal bed is virtually non banded. Macroscopic point counting and grain-size measurements show that the bright bands comprise up to 25% of the Morley coal and that those bands have a mean size of −1.73π (3.35 mm) when size measures are converted to −log 2 or phi (π) scale. Microscopic examination of the bright bands show that they are derived from secondary xylem of probable gymnosperm origin. Although the macroscopic texture of the Brunner and Morley coal beds differ substantially, the fine-grained constituents of both beds are comprised of roots with primary growth, stem/roots with secondary growth, leaves, secondary xylem tissue, cork tissue and unidentified plant tissue. The beds differ in plant organs/tissues mainly in that the Morley contains slightly larger and higher concentrations of microscopic plant parts than does the Brunner coal. In addition the type (morphology) of the cork tissue and stem/root material differs significantly. Stem/roots in the Morley are composed predominantly of gray cell walls in addition to suberinized cell walls whereas the Brunner stem/roots are composed mostly of cork cell fillings and gray cell walls. Microscopic plant part sizes in both coal beds range from 5.6 to 2.2π (0.02 to 0.22 mm). Matrix components consists of fragments of cell walls and fillings, spores/pollen, cuticle, resins, algae, unidentifiable fragments of waxy/resinous material, unidentifiable fragments of oxidized tissue, fungal remains, amorphous humic gels and bitumen. Other than the Brunner containing bitumen and a higher concentration of cell fillings, the matrices of the two coal beds are similar. Size distributions for particulate matrix components range from 9.0 to 5.9π (0.002 to 0.017 mm).