This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity-the seaward extension of a subaerial unconformity. The exceptionally well-exposed transitional interval between the Tununk Shale and the Ferron Sandstone offers an excellent opportunity to pinpoint the T-F sequence boundary, as well as to closely examine its sedimentary character and lateral variability along depositional strike. 2012 Li and Schieber 2020), providing useful constraints on the stratigraphic position of the sequence boundary separating the two genetically different depositional systems (hereafter referred to as the T-F sequence boundary). Detailed sequence stratigraphic frameworks of both members have been developed (Zhu et al. The main objectives of this study are to characterize the sedimentary facies of the mudstone-dominated transitional interval between the Tununk Shale and the Ferron Sandstone in detail, and to develop a depositional and sequence stratigraphic framework within which the sedimentary character of the sequence boundary between the Tununk and Ferron (the T-F sequence boundary) can be more closely examined. In addition, an Organic Matter (OM) index, which is the ratio of bolivinid-type taxa versus trochospiral taxa shows relationships with original probable OM content of the sediment inferred from the lithofacies and the plant macrofossils. A salinity index, which is a ratio of trochospiral taxa versus testate amoebae taxa, provides a useful measure of coastal proximity that matches lithofacies trends. Nonetheless, the assemblages provide useful information that correlates with previously documented, lithofacies data. It appears that landward and coastward transport of tests led to a taphonomic bias. The Blue Hills outcrop represents the most landward and thus the lowest salinity environment (tidally-influenced backwater), Steamboat is more coast proximal with a slightly higher salinity found in the delta front and the fluvial/estuarine environment, while the Caineville North outcrop represents the most coast proximal with the highest salinity of the prodelta/delta front and fluvial/estuarine environment. Despite this possible taphonomic bias, the agglutinated foraminifera and testate amoebae morphogroups show plausible trends of salinity both among the outcrops and stratigraphically within each outcrop. Due to a possible fossilization biases with the calcareous foraminiferal fraction, the biodiversity index is only used as relative measure in defining biofacies. Morphotypes were used to define the main biofacies of the study sections using cluster analysis. Morphological analysis showed four morphogroups with eleven morphotypes of foraminifera and testate amoebae reflecting relationships between test morphology and depositional environment. Sixty-six hand samples of mudstone were collected from the three outcrops (Caineville North, Blue Hills, and Steamboat) along strike and dip profiles. This study investigates associations between ancient deltaic environments and foraminiferal and testate amoebae assemblages with an assessment of their ecological zonation based on their extant Holocene counterparts. Morphogroup and biofacies analyses were conducted on strata of the Upper Cretaceous (Turonian) Ferron-Notom delta, Central Utah, USA.
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