Christopher R. Fielding
Date of this Version
Most sequence stratigraphic models are based on the premise that relative changes in sea level (RSL) control stacking patterns in continental-margin settings. An alternative hypothesis, however, is that upstream factors, notably variations in relative water discharge (RQW), or the ratio of water to sediment discharge, can influence or control stratal stacking patterns in fluvial systems. There are two key features observed in sequence boundaries of RQW-driven systems that differentiate them from RSL-driven systems: (1) the depth of incision increases updip, and (2) rates of erosion are spatially uniform, leading to the formation of widespread, planar sequence boundaries. This paper presents an architectural and stratigraphic analysis of the well-exposed Masuk Formation; an Upper Cretaceous coastal-plain fluvial succession that is believed to be controlled significantly by RQW. The large (~ 75 km2) study area located in the center of the Henry Mountains Syncline shows three dimensional exposures throughout complex canyon and badland terrain. Traditional field methods, combined with three-dimensional, digital geological data from outcrops are used to investigate the architectural style and possible mechanism(s) responsible for it. Seven major composite sandstone bodies, separated by intervals of finer grained elements, are recognized within the stratigraphic interval. Composite bodies display planar, sheet-like geometry and are laterally continuous to a significantly greater extent (> 10 km) than would be expected from purely autogenic channel belt construction. Together, these intervals record a series of high-frequency sequences, formed along the western margin of the Western Interior Seaway. Within each individual sequence is a repetitive facies succession from a basal chaotic sandstone with admixed mudrock and sandstone transitioning upward to a more organized cross-bedded and stratified sandstone. This study provides new insights into the architecture and stacking patterns of coastal-plain fluvial successions. In particular, this study tests the architecture of the Masuk Formation against the key features expected in an RQWdominated environment. Overall, the lower and middle members display repetitive facies changes interpreted to reflect cyclical variations in discharge regime. Additionally, paleocurrent indicators show a mean flow direction to the east. In the upper member, there is less evidence for upstream control, and the mean paleocurrent direction is to the north. The lower and middle members represent a transverse-east flowing drainage system and its sequence architecture is interpreted to be controlled primarily by RQW forces, while the upper member, however, represents a transition to an axial-north flowing drainage system with lesser control from RQW forces.
Adviser: Christopher R. Fielding