Spatial Expression of Architectural Elements and Packages in a Large-Scale Outcropping Siliciclastic Mudstone Succession: The Mancos Shale, Book Cliffs, Utah

Joe H. S. Macquaker¹, Kevin G. Taylor², and Robert L. Gawthorpe^1
1 Manchester University, Manchester, United Kingdom
² Manchester Metropolitan University, Manchester, United Kingdom

The architectural elements present, and controls on sedimentological variability in proximal sand-dominated, shallow marine successions are well known. In comparison, these characteristics are rather poorly known in distal “shale”-dominated successions. This paper aims to address this problem by examining facies variability at bed to systems tract scale in siliciclastic mudstones to constrain lithofacies variability and different architectural elements present.

The Upper Cretaceous Mancos Shale, time-equivalent units to Grassy and overlying Desert Members (Blackhawk Formation) is spectacularly exposed in the Book Cliffs, Utah, and is ideal for this study as continuous exposures from updip sandstones to down-dip mudstones are available. Data on mudstone variability were obtained using field, optical, electron optical and geochemical techniques.

Detailed thin section analyses reveal that 2.0 to 5.0 m thick parasequences are present in this siliciclastic mudstone-dominated succession. Flooding surfaces are marked by significant fining of grain-size, and commonly by thin, laterally extensive, carbonate cements. Superimposed sequence boundaries / transgressive surfaces are characterised by a marked decrease in grain-size, together with development of concretionary dolomites (δ18O range -4 to –7 ‰ V-PDB) up to 2 m below the surface.

These data suggest that the most appropriate architectural elements in this succession are the parasequence sets. The mudstones within these units are genetically related and fine down-dip. Outwith these parasequence sets, mudstones exhibit subtly different mineralogies related to differences in productivity and sediment supply and are not part of the same stratal unit. They therefore should not be directly compared with one and another.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005