SEDIMENTOLOGY OF BAYFILLS IN THE NESLEN FORMATION, BOOK CLIFFS, USA, WITH SPECIAL FOCUS ON THE IMPACT FROM EARLY COMPACTION ON THICKNESS- AND LITHOFACIES DISTRIBUTION
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- Master theses 
The Neslen Formation of the Mesaverde Group (eastern Book Cliffs, Utah) accumulated along the western margins of the Western Interior Seaway. The formation is subdivided into Lower-, Middle-, and Upper Neslen Interval. The Lower Neslen Interval is composed of fluvial channel-fills and floodplain muds, overlain the marginal-marine interdistributary bay deposits of the Middle Neslen Interval, characterized by six vertically stacked bayfill units. Fluvial channel-fills and floodplain muds dominate the Upper Neslen Interval. During burial of coastal plain deposits, sand-rich channel-fill deposits undergo less compaction than the surrounding floodplain muds, resulting in differential compaction. Early differential compaction is extensively documented in Holocene delta plain sediments, whereas fewer studies address early compaction in ancient delta plains and its effect on thickness and lithofacies characteristics of overlying deposits. This MSc thesis investigates the vertical and lateral controls on thickness and lithofacies distribution of the three lowermost bayfill units within the Middle Neslen Interval, focusing on early differential compaction of the Lower Neslen Interval. Detailed documentation of lithofacies has been carried out by sedimentary logging and the larger-scale sedimentary architecture has been demonstrated through a number of correlation panels. Eleven lithofacies are identified in the lower delta plain deposits, and the interdistributary bay deposits are assigned to three facies associations; wave-dominated bayfill, bayhead delta, and sub-bay. An interdistributary bayfill unit is a thin (1.7 - 6.3 m), upward-coarsening unit, a “parasequence”, bounded by allogenically controlled flooding surfaces. As the tectonic subsidence is considered uniform in the study area, the stacking of bayfill units is driven by low-amplitude, high-frequency relative sea-level changes which had a profound effect in this low gradient and low relief environment. This study finds that the bayfill units become thinner and more sandstone-rich upwards in the succession, suggesting an overall decreasing rate of generation of accommodation space. Analyses of the sand- and shale-body geometries show that lateral variations in bayfill unit thickness and lithofacies distribution are vulnerable to early differential compaction of Lower Neslen Interval. The bayfill units exhibit a thinning above sandstone-rich channel-fill deposits concurrently with an increased abundance of sandstones relative to mudstones and increase in the amount of wave-generated structures. The effect decreases upwards in the bayfill successions and is usually absent after 7 - 8 meters of overburden. Lateral thickness variations are to some extent caused by autocompaction within the bayfill units and compaction of coals and organic-rich mudstones. Adding knowledge of the effect of early differential compaction on bayfill unit thickness and lithofacies distribution will increase the understanding of reservoir architecture within lower delta plain reservoirs. A good understanding of reservoir architecture is crucial for improved quality of static and dynamic reservoir models. An improved understanding will also support drainage strategies and well planning.