Marine Influence on Coal-An Analytical Challenge

C. F. K. Diessel, D. J. Curry, L. C. Gammidge, R. Rigby

Coal seams which have been affected by sea water during their formation, differ from other coals in petrographic, chemical, and physical properties. These differences have consequences for the coal's utilisation, hydrocarbon generation parameters, and usefulness as marker beds, for example in onshore sequence stratigraphy. These characteristics are controlled by the duration and timing of marine influence in relation to peat accumulation. There are two basic scenarios:

1. Post-depositional (epigenetic) marine influence-peat accumulation is part of the terrestrialisation process (upward shoaling of a parasequence) and the vertical succession is from eutrophic to oligotrophic conditions. Accumulation ceases when the mire is drowned by a marine transgression and the peat inundated by sea water and covered by marine sediments.

2. Syndepositional (syngenetic) marine influences-plants grow and peat accumulates in contact with sea water which gradually inundates the mire and covers it with marine sediments. The vertical succession is from oligotrophic to eutrophic conditions.

While the two cases differ in their biochemical environments, they experience the same post-depositional conditions, and develop similar, but not necessarily identical marine signatures. In peats and most lignites, botanical evidence commonly indicates mire conditions and the onset of marine influence. However, these indicators are obliterated with increasing coal rank and age, and the evidence for syn- versus postdepositional marine influence becomes equivocal. A wide range of chemical and petrographic analyses is often required to solve this problem. Using the Permian Greta Seam of the Sydney Basin as an example, this paper discusses some of the analytical techniques which may be used to establish the relative timing of marine influence on coal.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995