Alabound reported in a 2014 Master's Thesis for the University of Nebraska that the character and distribution of lithofacies in falling-stage deltas are
incompletely documented. This thesis presents a sedimentological and stratigraphic evaluation of a superbly-exposed interval of Cretaceous deltaic strata that are believed to be of falling stage origin. The studied interval forms part of the Upper Cretaceous (Turonian) Ferron Sandstone Member of the Mancos Shale in the southernmost Henry Mountains Basin of south-central Utah, USA.
The interval of interest is exposed in three dimensions over a 20 km2 area in a series of canyon walls. Observed facies include fine-grained mudrocks (offshore basin), mudrocks with thinly interlaminated sandstone (prodelta), thinly interbedded siltstones and fine-grained sandstones (distal delta front), thickly interbedded siltstones, coarse-grained siltstones and fine-grained sandstones (medial delta front), and amalgamated fine- to medium-grained sandstones (proximal delta front). No facies of interpreted delta top origin were observed within the studied succession. The proximal delta front facies includes spectacular, chaotic sandstone-filled gullies up to 6 m deep and 300 m wide. The basal parts of these gully fills preserve largescale convolute bedding, rotational failures and, in numerous places, growth faults. At least three such gully fills were noted, incised into different levels within the succession.
The existence of three gully fills indicates episodes of delta progradation and sea level fall. Paleocurrent data indicate southeastward sediment dispersal. Facies stacking patterns and incised gully fills, together with the lack of a delta plain topset, argue strongly for a falling stage origin for the described succession. This study provides new insights into the facies architecture and stacking patterns of shallow-water, falling-stage deltas. In particular, the study illustrates a style of falling-stage deltaic system in which a descending regressive trajectory is difficult to discern.
He concluded that The Turonian Ferron Sandstone Member of the Mancos Shale accumulated in a foreland basin by deltas prograding eastward into the Cretaceous Western Interior Seaway. A detailed stratigraphic study of one component deltaic unit within the Ferron Sandstone outcrop in south-central Utah allowed recognition of facies stacking patterns, sequence stratigraphy, and depositional environment interpretations. Observed facies include fine-grained mudstones (offshore), mudstone with thinly interlaminated sandstone (prodelta), thinly interbedded siltstones and fine-grained sandstones (distal
delta front), thickly interbedded siltstones, coarse-grained siltstones and fine-grained sandstones (medial delta front), and amalgamated fine-to medium-grained sandstones (proximal delta front). Vertical measured sections show change from offshore marine mudstones, through interbedded siltstone and sandstone, to amalgamated and trough cross-bedded sandstones. Facies in the study area indicate progradation of deltas into offshore environments under low accommodation. The observed sedimentary structures within the studied interval indicate fluvial and wave influences on broadly deltaic deposits.
Three gully fills in different stratigraphic positions were observed, mapped, and described. The gully fills are up to 6 m deep and 300 m wide. The basal parts of these gully fills preserve convolute bedding, rotational failures and growth faults. The presence of these gully fills suggests episodes of delta progradation and sea level drawdown.
Detailed description of the gully fills led to the interpretation that they were cut and filled during sea level fall, and not during transgression. The paleoshoreline was oriented north-south and the deltas prograded into the east depositing the Ferron sandstone (Fielding, 2010). Paleocurrent data within delta front facies in the study area indicate southeast sediment dispersal. Sediments were deflected in a south-southeastward direction as a result of the Coriolis Effect upon prevailing winds and water currents upon entering the KWIS. The facies stacking pattern and the lack of delta plain topset argues for deposition within falling-stage systems tracts. Incision and downcutting into underlying distal deltaic facies also support the falling-stage interpretation. The depositional dip cross-section, however, does not show a descending regressive trajectory. Forced regression is implied by facies associations and incision,
despite the lack of descending regressive trajectory in the depositional dip direction.
See original working URL:
https://digitalcommons.unl.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1054&context=geoscidiss#:~:text=The%20top%20of%20the%20Ferron,et%20al.%2C%202012).&text=Figure%204%3A%20Location%20of%20the,Utah%20State%20highlighted%
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