Stratal Patterns within Fluvial Strata of the Upper Cretaceous Hunter Canyon/Williams Fork Formation and their Implication for Foreland Basin Evolution

Kamola, Diane; Ost, Rebekah C.

Stratal patterns within fluvial strata of the Upper Cretaceous Hunter Canyon (Williams Fork) Formation of Colorado record the repeated, punctuated development of accommodation and subsequent deposition within a foreland basin. The Hunter Canyon Formation was deposited during the final stage of basin fill in the Cretaceous Sevier foreland basin. The stratal pattern consists of complicated, repetitive fluvial successions. Each succession shows an upward increase in accommodation and an overall decrease in fluvial energy. The stratal succession is 3-fold and initiates with multi-story, laterally-continuous fluvial sandstones which are the highest energy fluvial deposits in the succession. These are overlain by laterally-amalgamated single-story channel-fill sandstones separated by overbank fines. These are then overlain by an interval of aggradationally-stacked, isolated channel-fill sandstones that are encased in overbank fines. Stratal successions are interpreted as depositional sequences and are bracketed by sequence boundaries. Stratal successions are laterally continuous across the extent of outcrop exposure (up to 10 km), and form a distinctive and repetitive stratal pattern; sixteen stratal packages are identified in stratigraphic succession.

The contact between each stratal package is sharp and is overlain by an abrupt grain size increase and an increase in cross bed thickness. This is interpreted to represent a sudden increase in the gradient of the fluvial system. The vertical trend in the succession of upward decrease in fluvial energy, and progression from multi-story to laterally-continuous single-story, to isolated single story deposits encased in overbank fines, represents an upward increase in accommodation. The successive stratal packages record a repetitive change between base level fall (at the start of the stratal succession, which decreases accommodation) and subsequent base level rise (which increases accommodation). These are interpreted as the continental equivalent to the numerous depositional sequences in marine strata in the foreland basin. Each stratal succession documents the punctuated nature of basin fill over time, where sediment supply is great enough to record the interplay between base level fall and accommodation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013