| Abstract |
The East Kirkton, Brigantian "limestone" was thought by Hibbcrt, (1836) to be of hot-spring origin due to the occurrence of numerous, small, spheruliticgrowths. This view was supportcd by Cadell, following his observations of active hot-springs in Yellowstone National Park and in New Zealand. The aim of the continuing research is to constrain this geological setting by examination of textural evidence and detailed geochemical analyses. The occur in most horizons of this finely laminated sequence, which includes: bituminous shales; carbonate rich and silica rich sediments; and occasional limestone bands and lenses. The regular laminae are typical of lake deposits, reflecting a low energy, environment of restricted circulation, with freshwater organisms preserved in many horizons. This study has focussed on tne silicic sediments, unusual in a fresh water lake. At East many chert textures suggest an origin as gel-like precipitates. Synsedimentary slumping without brittle fracturing indicates a silica precipitate capable of ductile movement. Cracks have formed perpendicular to bedding, confined within the silica laminae. These may be the shrinkagecracks of silica gel. Both silica and carbonate primary precipitates, though both also form several replaave phases. Cathodoluminescence fluorescence and SEM identify the presence of both carbonate and silica also differentiatea variety of carbonate phases, principally calcite and dolomite. Isotopic analysis of and 6D are presented constraining origins of the siliceous fluids. The analyses = 21.3 to 26.9960, 6D = -52.2 to fall close to the "agate line" of Fallick et and are consistent with equilibration of cherts with L. Carboniferousmeteoric water of and -20% at 40 to . Fluids in equilibrium with this silica "gel" have been trapped only as the "gel"cooled and hardened, upon mixing the lake waters, to results in fine, laminae of chert. |