2020-
Permanent URI for this collectionhttps://hdl.handle.net/10217/182111
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Browsing 2020- by Author "Abukhtwa, Ahmed, author"
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Item Open Access Sedimentary character and processes in mudstone inter-lobe deposits of the Skull Creek Formation, Fort Collins, Colorado(Colorado State University. Libraries, 2025) Abukhtwa, Ahmed, author; Gallen, Sean, advisor; Schutt, Derek, committee member; Egenhoff, Sven, committee member; Aoki, Eric, committee memberLobe-fringe deposits and interlobe strata are rarely described in sedimentary systems and often neglected in descriptions of delta successions, which mostly focus on delta top, delta front, and prodelta sediments. This may partly be the case because these deposits consist mostly of mudstones, which are generally neglected in all sedimentary systems except for black shales. Here, we describe nine siliciclastic facies of predominantly mudstones from the Cretaceous Skull Creek Formation of northern Colorado, USA. These nine facies are arranged in a 4.5 m thick predominantly fine-grained unit within the overall 25 m thick Skull Creek Formation. The nine facies are very fine to fine-grained dark massive mudstone lamina (F1), very fine-grained massive mudstone (F2), massive fine- to coarse-grained mudstone (F3), massive mudstone with fecal strings (F4), normally graded coarse to medium-grained mudstone (F5), medium- to coarse- grained lenticular siltstone (F6), siltstone lamina (F7), and massive calcitic coarse silt- to coarse-grained sandstone (F8A and F8B). From all these facies, massive fine- to coarse-grained mudstone (F3), normally graded coarse to medium-grained mudstone (F5) and siltstone lamina (F7) are the most common and comprise about 80-90 vol% of the succession. The succussion consists of 16 coarsening- and fining-upward cycles, with the majority being coarsening- upwards (11), and only (5) fining-upwards. Stratigraphically, these cycles are between 5.5 and 120 mm thick, and are here subdivided into nine distinct stratigraphic zones. These zones alternate between five fine-grained and four silt-rich zones. Each zone consists of a minimum of a portion of a cycle, and/or one or more coarsening- and fining-upwards units. These facies were deposited in three depositional environments: lobe-fringe area, medial inter-lobe area, and distal inter-lobe area. The presence of both high-energy indicators, such as clay clasts, sharp erosional bases, scours, and fragmented fishbones, as well as the occurrence of sediments reflecting suspension deposition, indicates that the lobe-fringe environment was undergoing successive shifts from high to low energy conditions. Moving farther away from the lobe, sediments show overall moderate energy conditions reflected in normal grading and some erosional contact; nevertheless, moderate and low-energy conditions alternated. Furthest away from the lobe are the distal inter-lobe sediments that show tranquil sediment deposition with only minor moderate energy deposition reflected in sharp facies contacts, and some siltstone grains in facies 3. The presence of only one type of fecal string, in these sediments, and the little bioturbated nature of inter-lobe strata suggests that the environment had been likely dysoxic and stressed but was not entirely anoxic. This study indicates that the Skull Creek Formation is primarily dominated by bed-load deposition, although the presence of suspension deposition—recorded only at times—in various facies suggests that quieter conditions occurred across all depositional areas. Even the interlobe deposits are significantly influenced by bed-load transport. Furthermore, the boundaries between the three depositional areas—lobe-fringe, medial inter-lobe, and distal inter-lobe areas—are not clearly defined and are regarded as transitional rather than firm. While unique facies mark each zone, the presence of overlapping facies across these zones makes it challenging to distinguish them clearly.