Davies, Roy C.1, John A Howell2, Stephen Flint1, Claus
Diessel3, Ron Boyd3
(1) University of Liverpool, Liverpool, United Kingdom
(2) University of Bergen, Bergen, Norway
(3) University of Newcastle, Callaghan, Australia
Coal seams are highly sensitive indicators of changes in accommodation and organic
productivity. Changes in inorganic mineral and inertinite content enable sub-parasequence
scale base-level changes to be identified and correlated over tens of kilometres of
depositional dip. Similarly, changes in vitrinite to inertinite ratios record high
frequency climatic cycles, which can be detected through spectral analysis of coal seam
Detailed petrographic analysis of over 300 samples from the Sunnyside coal seam of the Cretaceous Blackhawk Formation in Eastern Utah has enabled us to identify an extremely high-resolution record of base-level change throughout the formation of the seam. Laterally extensive coal seam splits that open out basinward into packages of shallow-marine sediments enable us to integrate this record with changes in marine base-level observed in the laterally equivalent shallow-marine strata. This integration between marine and non-marine records is central to understanding sequence stratigraphic expressions in non-marine systems.
Using various spectral analysis techniques it is possible to detect cyclic variations in the vitrinite to inertinite ratios through the seam, the most significant of these occur at approximately 0.6 m-1 and 5.0 m-1 in a 2.5 metre thick section. The exact duration of these cycles is difficult to determine as peat accumulation rates are not constant and removal of peat due to wild fires and erosion may occur during excessively dry periods. However, based on estimates of 60 Ka – 120 Ka for the Sunnyside coal to form, they may correspond to obliquity (41 Ka) and hemi-precession (~ 10 Ka) respectively.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.