Basin-scale diagenetic alteration of shoreface sandstones in the Upper Cretaceous Spring Canyon and Aberdeen Members, Blackhawk Formation, Book Cliffs, Utah

Abstract

Leached zones (‘whitecaps’) and dolomite cement bodies are volumetrically and areally significant features in shoreface sandstones of the Upper Cretaceous Spring Canyon and Aberdeen Members, Blackhawk Formation, Book Cliffs, Utah. Quantitative spatial and stratigraphic data of their distribution, together with petrography and geochemistry, allow development of a model for their formation within a sequence stratigraphic framework. Leached zones, up to 5 m thick and 15 km in lateral extent, are present beneath coals capping parasequences. Concretionary dolomite cement is present in shoreface sandstones downdip of parasequence capping coals and associated “whitecaps”. These concretions are up to 2 m thick and 6 m in length and are present for up to 7 km downdip of these coals. Petrographic and field data indicate an early diagenetic timing for “whitecap” and dolomite cement formation. δ 13C data suggest carbonate derivation from detrital dolomite, whereas δ 18O data suggest that cement precipitated from porewater with a significant meteoric component, although later diagenetic re-equilibration and recyrstallization cannot be rejected. We propose that organic-acid generation during early diagenesis of coals led to the leaching of detrital dolomite from underlying sandstones, and the formation of “whitecaps”. Meteoric water flowed basinward in response to base-level lowering and sequence boundary development in the upper part of the Aberdeen Member, remobilizing dolomite into shoreface sandstones. Dolomite precipitation resulted from the mixing of these meteoric fluids with downdip marine pore fluids, and the presence of detrital dolomite in the shoreface sandstones that acted as nucleation sites for dolomite precipitation. The geometry and extent of dolomite cement in the Spring Canyon and Aberdeen Members is in marked contrast to the much greater spatial extent of such cement in low accommodation space settings, such as the stratigraphically younger Desert Member, Blackhawk Formation. The relatively simple aggradational to progradational evolution of the Spring Canyon and Aberdeen Member parasequences, with only minor sequence boundary development in its upper part, resulted in spatially limited meteoric fluid input into shoreface sandstones. This resulted in dolomite cementation of shoreface sandstones up to a maximum of 7 km downdip of leached zones. In the Desert Member, extensive meteoric fluid migration as a result of major sequence boundary development resulted in dolomite cementation of shoreface sandstones for over 25 km downdip of leached zones. This highlights that the degree and extent of early fluid diagenesis can be significantly controlled by the nature of sequence development, having clear implications for developing predictive models for porosity–permeability evolution in sandstone reservoirs.