SEDIMENTOLOGICAL AND STRATIGRAPHIC STUDY OF A FALLING-STAGE DELTA COMPLEX IN THE UPPER CRETACEOUS (TURONIAN) FERRON SANDSTONE MEMBER OF THE MANCOS SHALE, SOUTH-CENTRAL UTAH, USA

The character and distribution of lithofacies in falling-stage deltas is incompletely documented. This paper 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 in the southernmost Henry Mountains of south-central Utah, USA. The interval of interest is exposed in three dimensions over a 15 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 5.7 m deep and 300 m wide. The basal parts of these gully fills preserve large-scale 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. Paleocurrent data indicate southeastward sediment dispersal, and in this direction, both gully fills and gently-dipping clinoforms within delta front facies define descending forward trajectories. These characteristics, 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.