No abstract

Thin‐bedded delta‐front and prodelta facies of the Upper Cretaceous Ferron Notom Delta Complex near Hanksville in southern Utah, USA, show significant along‐strike facies variability. Primary initiation processes that form these thin beds include surge‐type turbidity currents, hyperpycnal flows and storm surges. The relative proportion of sedimentary structures generated by each of these depositional processes/events has been calculated from a series of measured sedimentological sections within a single parasequence (PS6–1) which is exposed continuously along depositional strike. For each measured section, sedimentological data including grain size, lithology, bedding thickness, sedimentary structures and ichnological suites have been documented. Parasequence 6–1 shows a strong along‐strike variation with a wave‐dominated environment in the north, passing abruptly into a fluvial‐dominated area, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave‐dominated environment further to the south‐east. The lateral facies variations integrated with palaeocurrent data indicate that parasequence 6–1 is deposited as a storm‐dominated symmetrical delta with a large river‐dominated bayhead system linked to an updip fluvial feeder valley. This article indicates that it is practical to quantify the relative importance of depositional processes and determine the along‐strike variation within an ancient delta system using thin‐bedded facies analysis. The wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents (i.e. surge‐type turbidity currents and hyperpycnal flows) and storm surges, and shows that there are significant interactions and linkages of these often paired processes.

No abstract

Despite recent advances in understanding the complex dynamics of mud deposition, it remains a challenging task to characterize the grain size, origin of different components, and sedimentary textures of mudstones through detailed petrographic analysis. In this study, the Tununk Shale in Utah has been examined by optical and scanning electron microscopy (SEM) to determine how variations in petrographic characteristics (e.g., composition, texture) of this shelf mudstone succession reflect changing depositional environments. In the context of the general depositional setting, detailed petrographic studies indicate that most mud in the Tununk system were transported in bedload as silt-to sand-size mud-dominated composite particles (MCPs), rather than specific components (e.g., clay minerals, silt grains, fossil fragments) of smaller size (micrometers to tens of micrometers). Three types of MCPs in the Tununk Shale can be identified and distinguished from each other. These include fecal pellets, altered volcanic rock fragments, and shale lithics. Two other types of MCPs, namely floccules and soft mud rip-up clasts, likely contributed significantly to the formation of the precursor mud matrix of the Tununk Shale. Due to their water-rich nature, however, floccules and mud rip-up clasts suffer significant compaction. Except in fortunate circumstances, they are therefore no longer discernible in the rock record. MCPs and their role in the formation of fine-grained sedimentary successions has largely gone unnoticed in previous studies. The recognition criteria, as well as petrographic characteristics of each type of MCP in different depositional environments of the Tununk Shale, are summarized here, with the intent that they may benefit future studies of other mudstone successions. The complex variability in the characteristics of different types of MCPs illustrated in this case study, however, highlights the need for additional systematic petrographic studies (integrating both optical and SEM) in order to develop and refine the current recognition criteria of MCPs in fine-grained sedimentary rocks. Detailed petrographic examination of mudstones, though labor intensive, can yield critical information regarding their provenance and depositional setting, as well as provide general insights into the underlying causes for mudstone heterogeneity.

Mantle-induced dynamic topography (i.e., subsidence and uplift) has been increasingly recognized as an important process in foreland basin development. However, characterizing and distinguishing the effects (i.e., location, extent and magnitude) of dynamic topography in ancient foreland basins remains challenging because the spatio-temporal footprint of dynamic topography and flexural topography (i.e., generated by topographic loading) can overlap. This study employs 3D flexural backstripping of Upper Cretaceous strata in the central part of the North American Cordilleran foreland basin (CFB) to better quantify the effects of dynamic topography. The extensive stratigraphic database and good age control of the CFB permit the regional application of 3D flexural backstripping in this basin for the first time. Dynamic topography started to influence the development of the CFB during the late Turonian to middle Campanian (90.2-80.2 Ma) and became the dominant subsidence mechanism during the middle to late Campanian (80.2-74.6 Ma). The area influenced by >100 m dynamic subsid-ence is approximately 400 by 500 km, within which significant (>200 m) dynamic subsidence occurs in an irregular-shaped (i.e., lunate) subregion. The maximum magnitude of dynamic subsidence is 300 ± 100 m based on the 80.2-74.6 Ma tectonic subsidence maps. With the maximum magnitude of dynamic uplift being constrained to be 200-300 m, the gross amount of dynamic topography in the Late Cretaceous CFB is 500-600 m. Although the location of dynamic subsidence revealed by tectonic subsidence maps is generally consistent with isopach map trends, tectonic subsidence maps developed through 3D flexural backstripping provide more accurate constraints of the areal extent, magnitude and rate of dynamic topography (as well as flexural topography) in the CFB through the Late Cretaceous. This improved understanding of dynamic topography in the CFB is critical for refining current geodynamic models of foreland basins and understanding the surface expression of mantle processes.