Abstract

Nonmarine sequence stratigraphic models hypothesize systematic changes in fluvial style within individual sequences and across sequence boundaries, based largely on mudstone–sandstone ratios. The main purpose of the paper is to evaluate the validity of these models by documenting facies relationships and detailed bedding architecture of bar and channel deposits as well as mudstone–sandstone ratio within a compound incised-valley fill at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation in the Henry Mountains region, Utah, U.S.A.

Field photomosaics, walking out of beds, and 42 measured sections document the stratal and facies organization in the incised-valley fill. Two erosional surfaces with erosional relief up to 17 m are traced continuously for up to 7 km. These erosional surfaces are defined by onlap and truncation and show a marked basinward shift in facies, with pebbly coarse-grained and conglomeratic fluvial sandstones overlying highly burrowed marine mudstones and sandstones. These major erosional surfaces partition the valley fill into two sequences, marked as V2 and V1 in ascending order. Each sequence comprises a multistory fill that reaches a maximum thickness of about 30 m. Within each sequence there is a vertical transition in facies from fluvial, to tidal, and finally back into fluvial facies.

In the younger V1, there is a systematic vertical evolution of fluvial style from braided, to single-thread meandering, and finally to a low-sinuosity river system. In contrast, formative rivers in the older V2 were always meandering and show more marine and tidal influence. A major change in valley sedimentology is recognized from V2 to V1, as shown by a 30° eastward shift in main flow direction, overall increase in grain size and river discharge, and a clear change in fluvial style from meandering into braided streams. This is also shown by the increase in preserved dune height in fluvial bar and channel deposits across the boundary, which suggests that deeper rivers cut the younger V1 valley.

The change in river style from braided to meandering in V1 is interpreted to reflect a gradual decrease in slope and discharge as the valley was filled. The change in fluvial style across the sequence boundary reflects an increase in grain size and discharge, probably driven by a high-frequency climate change, as well as local increase in valley slope.