Sedimentology, diagenesis and
geochemistry of the Great Limestone,
Carboniferous, northern England.

Abstract

Yoredale type cyclothems of the Mid-Carboniferous of north east England were deposited as a result of glacio-eustatic fluctuations arising from waxing and waning of the Gondwana ice sheets present in the southern hemisphere. Rhythmic alternations of areas of maximum cyclothem thickness have been recognised in the Scar to Little Cyclothems which are attributed to localised differential subsidence, flexuring and uplift of the Alston Block of the northern Pennines.

A detailed study of one cyclothem, the Great Limestone Cyclothem of the Alston Block, reveals that within the transgressive carbonates, the beds form two and a half thinning-upward to thickening-upward bed-sets with the individual beds and the bed-sets being correlatable across the region. Inevitable diagenetic alteration of the Great limestone has occurred and resulted in resetting of some initial geochemical values. However, it is proposed that in the case of δ18O and several trace elements their trends through the limestone do in fact track an original pattern, namely that of the bed-thickness pattern.

It is suggested here that the cyclothems are attributable to the short eccentricity Milankovitch rhythm, the bed-sets, within the Great Limestone, to the range of either the obliquity and precession rhythms, with the beds in the Great Limestone being deposited in periods of the sub-Milankovitch millennial time-scales.

The biostromes within the Great Limestone, the Chaetetes band, Brunton band and the Frosterly band are typical of shallow-marine environments as are all grains seen in thin-section analysis. All limestone beds are a similar bioclastic wackestone to packstone with no observable changes in the proportions of the various elements throughout the thickness of the Great Limestone.