--> Upstream Control in a Coastal-Plain Fluvial Setting: An Architectural and Stratigraphic Analysis of the Campanian Masuk Formation

AAPG ACE 2018

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Upstream Control in a Coastal-Plain Fluvial Setting: An Architectural and Stratigraphic Analysis of the Campanian Masuk Formation

Abstract

Sequence stratigraphic models are commonly used to help predict reservoir presence, quality, distribution, and connectivity beyond what is immediately visible in cores, outcrops, or on well logs. While many of these models are based around the premise that relative changes in sea level (RSL) control stacking patterns in continental-margin settings, ancient examples of upstream controlled systems are largely undocumented in scientific literature. Analyses of the Masuk Formation in southern Utah provides a unique opportunity to study a coastal-plain fluvial system that appears to be controlled predominantly by upstream factors.

Climate change over geological timeframes can cause variations in relative water discharge (RQW), or the ratio of water to sediment discharge. 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 study combines UAV-based photogrammetry and three-dimensional, high-resolution digital geological data from outcrops with standard field investigation techniques to determine the lateral extent and geometry of sequence boundaries within the Masuk Formation.

Within the ~100 km2 field area, major composite sandstone bodies, and their associated bounding surfaces, display planar, sheet-like geometries. Together, these bodies record a series of high-frequency sequences that are laterally continuous throughout the study area. Frequently observed within individual sequences is a repetitive trend in vertical facies change from a basal chaotic sandstone admixed with mudrock and siltstone clasts transitioning upward to a more organized cross-bedded and stratified sandstone. This trend is interpreted to represent cyclical variations in RQW, or cycles from high to low period of precipitation. Because these packages reflect hydrological cycles more so than relative changes in sea level, a more appropriate classification scheme for the high-frequency sequences within the Masuk Formation is one based around discharge regime, i.e. high discharge and low discharge sequence tracts. This study aims to expand upon the currently limited range of sequence stratigraphic models in order to help elucidate how architectural and stratigraphic elements may vary in a range of allogenically controlled systems.