Sedimentology and depositional setting of deep-water deposits of the Pennsylvanian Jackfork Group, Arkansas, and the paleogene wilcox formation, Gulf of Mexico, Mexico [electronic resource]
Keywords: Geological Sciences.

ABSTRACT Of the sediment transport agents on Earth, gravity-driven flows on the seafloor are among the largest; however, they are the least well understood. Driven in part by the petroleum industry's need to explore deepwater hydrocarbon reservoirs, the study of sediment gravity flows, their triggering effects, and the resulting deposits was initiated about two to three decades ago and there have been many researches working in this area (Bouma, 1962; Middleton and Hampton, 1973; Lowe, 1982; Mutti and Lucchi, 1972; Mutti and Normark, 1987, 1991; Kneller, 1995). The Gulf of Mexico (GOM) has a long hydrocarbon exploration and production history and is perhaps one of the best studied basins in the world. Recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types, many hundreds of kilometers from paleocoastlines. These sedimentary deposits often defy conventional turbidite or debrite interpretations, having a character suggestive of deposition from flows with transient turbulent-laminar rheologies. The goal of this research is to use different approaches to study various aspects and processes of deepwater sediments. The research will be based on data at multiple scales: seismic-reflection data will aid in documenting the architecture of the depositional systems. The intermediate scale will be explored by using well core, outcrop data and well logging interpretation. The finer scale will be used by studying thin sections. This dissertation consists of four chapters: two of them are studies of the Wilcox Formation in the Perdido Fold Belt, Gulf of Mexico, Mexico, and the last two are studies of the Jackfork Group in the Ouachita Mountains, Arkansas, USA. Chapter 1 documents the evolution of the deepwater deposits of the Paleogene siliciclastic column in the Perdido Fold Belt province. The Wilcox Formation in the deep-water Gulf of Mexico comprises a thick, sandstone-rich upper Paleocene-lower Eocene succession that is known to contain considerable hydrocarbon accumulations. In 2012, Petroleos Mexicanos (PEMEX) began to drill offshore exploratory wells to investigate the hydrocarbon potential of the offshore Paleogene sequence of the Perdido Fold Belt. These wells have encountered sand-rich turbidite systems throughout the Wilcox section that provided a remarkable amount of data from the clastic column about the lithologies present and the processes of sediment transport and deposition. One of the unusual aspects of the Wilcox strata is that they include not only classic turbidites but also abundant so-called slurry flow deposits, mud-rich units that show that both flow turbulence and flow cohesion played roles in sediment transport and deposition. Well cores, logs, and 3D seismic-reflection data were analyzed for defining and classifying architectural elements of the deep-water deposits in order to identify the depositional settings and architecture of the Wilcox Formation in the study area and to use the resulting characterization of depositional environments active in the Wilcox system during deposition to better constrain the mechanics, evolution, and environmental distribution of slurry flows and their deposits. The studied stratigraphic column of the Wilcox Formation includes three discrete main divisions: The Upper Wilcox, the Big Shale and the Lower Wilcox. The results show that both confined channel deposits and unconfined basin-floor sheets elements comprise larger bodies interpreted to be a distributary lobes. This research suggests that the Wilcox in the study area in the northwestern Gulf of Mexico represents deposition by turbidity currents and slurry flows in a downslope, submarine-fan lobe complex. Chapter 2 characterizes the Wilcox Formation in the Mexican portion of the Perdido Fold Belt in the Gulf of Mexico. The Wilcox Formation consists of interbedded sandstone and shale deposited during the late Paleocene and early Eocene. Interest in Wilcox depositional history and hydrocarbon potential has led to study of textures, compositions, and vertical arrangement of these sandstone and mudstone units. In the study area, the Wilcox includes mud-poor sandstone and mud-rich sandstone units. These units appear to be the deposits of two main types of sediment-gravity flows: turbidity currents and slurry flows. The latter exhibit rheologies comprising both cohesive and non-cohesive behavior. The data and conclusions presented in this study are based on descriptions and measurements of rock samples derived from cores collected from a well drilled in the Mexican portion of the Perdido Fold Belt. In order to characterize the Paleogene Wilcox Formation slurry flow deposits, we have subdivided sedimentary units observed in core based on lithology and sedimentary structures into several divisions. Based on vertical stacking analysis of these divisions, a deposit model is proposed to characterize the slurry beds in the Wilcox Formation. This model includes four basic divisions that include, from base to top, a mud-poor sandstone, a heterogeneously-structured mud-rich sandstone division, a thin, fine-grained clean sandstone, capped by a mudstone layer. Slurry beds were deposited by flows that were transitional between cohesionless turbidity currents and cohesive debris flows due to, among several controls, mud-content changes through time. Mud contents as well as clay type characterization within the slurry bed divisions were documented using petrography and X-ray diffraction techniques. The wide variability in the character of these slurry flow deposits provides further clues to their mode of origin and can grouped in two main types based on their architecture. Chapter 3 documents the sedimentary rocks of the Pennsylvanian Jackfork Group exposed in the DeGray Lake Spillway, Arkansas, which were deposited by three types of sediment gravity flows: turbidity currents, debris flows, and slurry flows. Previous studies based on modern and ancient fan systems have suggested that slurry flows exhibiting both turbulent and cohesive behavior are commonly deposited as lobes or sheets in unconfined distal lobe or lobe fringe environments, and that they are usually absent in more proximal parts of submarine depositional systems. Based on lithology, sedimentary structures, and stacking patterns, the spillway outcrop can be divided into 6 separate lithofacies and corresponding depositional environments. Overall, this sequence is interpreted to represent the vertical stacking of sedimentary components that made up one or more large channel-levee-overbank complexes. The evolution of depositional environments can be described in terms of six main facies associations (FA) and depositional stages: (FA1) an initial period of mud sedimentation in a sediment-starved basin; (FA2) development of a crevasse-splay complex; (FA3) the spread of a levee succession capped by a major debris-flow unit that probably buried much of the existing turbidity-current-generated topography; (FA4) deposition of lobe sequence across the debris-flow-deposit surface; (FA5) development of overbank and levee deposits, and (FA6) formation and filling of a submarine channel complex. An assemblage of interbedded turbidity current and slurry flow deposits in the lower part of the section is interpreted to represent an avulsion splay formed as the initial deposits of channel avulsion. Mud-rich sand and mud-clast-rich debris-flow components of the slurry flows in FA2 appear to have developed from initial relatively clean, sandy turbidity currents through erosion of muds from the levee tops and from the bottoms and sides of crevasses. This study shows a specific example of how slurry flows can develop and slurry-flow deposits accumulate within more upslope parts of submarine depositional systems. Chapter 4 characterizes the deep-water deposits of the lower Pennsylvanian Jackfork Group exposed in the DeGray Lake spillway in Arkansas, which comprise a relatively thick sequence of contrasting mud-poor and mud-rich sandstones. We interpret these beds to have been deposited by turbidity currents and slurry flows, respectively, based on sedimentary structures and inferred mud content. The objective of this study is to provide a quantitative characterization and interpretation of the lithological changes in sedimentary rocks of the Jackfork Group with special emphasis on the slurry flow deposits. Petrography and X-ray fluorescence spectrometry (XRF) systems are used to directly determine and compare the rock composition, especially the mud contents, of the contrasting deposit types in order to constrain and compare effects of entrained mud on the properties of the flows and depositional architecture. The mud content estimation obtained by petrography and geochemical techniques is described and discussed. By comparing results obtained from benchtop and handheld XRF analyses, this study also assesses the reliability of portable XRF technology as a method to characterize exposures of sedimentary rock at a fine resolution. Slurry flow deposits at DeGray spillway are characterized by an upward sequence of four well-defined, sedimentary structure divisions. The ideal slurry bed includes from base upward a mud-poor sandstone division, a mud-rich sandstone division, a mud-clast breccia division, and a capping relatively thin mudstone division. Mud-rich sandstone divisions vary widely in mud content, the muddiest sandstone may contain up to 40% mud, whereas the cleanest mud-rich sandstone may comprise only 15% mud. Debrite divisions also display a wide range of mud contents from 30 to 60%, while mudstone divisions are characterized by more than 60% mud. The result of the present study suggest that at a critical value of 15% concentration of mud or 5 wt% of Al2O3 in the Jackfork Group sandstone was sufficient to provide the strength to the parent flow, damping tu ... .