Meteoric phreatic diagenesis in cyclic late Dinantian carbonates, northwest England

Abstract

Luminescent/non-luminescent zoned syntaxial overgrowths in carbonate grainstones of late Asbian (late Dinantian) age of the southern Lake District represent precipitation in barely saturated, relatively shallow (15–120 m), meteoric phreatic pore waters. The zonation reflects repeated establishment of meteoric lenses, in response to eustatic effects, within cyclic carbonate platform sediments isolated from sources of laterally-fed groundwaters. Geochemistry and petrology suggest that there was a consistent positive covariance between Eh of pore waters and precipitation of calcite during establishment of these lenses, such that cements only precipitated from suboxic and oxic meteroic waters. Mixing-zone and stagnant marine phreatic diagenetic environments are thought to have been sites of non-precipitation to slight dissolution. Cement cycles can be correlated up to 30 km across the platform and decrease in number, in a predictable fashion, up through the sequence. The stratigraphic resolution they offer within this platform is more sensitive than that of microfossils in the same sequence. Cementation patterns are more complex in the outer 5 km of the platform and increase in complexity basinwards, reflecting: the influences of lowstand sea levels; locally developed erosional and depositional slopes; lateral components within meteoric flow patterns; rapid lateral facies variations and possible trace element partitioning. The base of the meteoric lenses from which these cements precipitated was controlled by the presence of a shale aquitard. This style of cementation ceased after an early Brigantian platform drowning event.