Hierarchical framework and cyclicity in a fluvial-lacustrine basin-fill succession, Middle Wasatch Formation, Uinta Basin, Utah

Abstract

A three-level hierarchical framework for describing meso- to macro-scale architecture of fluvial systems was developed and tested utilizing exceptionally well exposed, three-dimensional outcrops of the Eocene middle Wasatch Formation. From smallest to largest, this hierarchical framework consists of storys, elements and archetypes. Each hierarchal level is composed of different types or combinations of components from the lower hierarchical level, which accounts for the variability in sedimentation styles recognized in fluvial systems. Three distinct associations between channel-belt elements and their adjacent splays are documented: 1) splays that are spatially isolated from channel-belt elements (unassociated splays); 2) splays that are laterally adjacent and physically connected to a channel-belt element (associated coeval splays); and 3) splays that underlie the channel-belt element (associated non-coeval splays). Deconstructing the sequential evolution of archetypes yielded two distinct upward stacking patterns that differentiate braided and meandering archetypes. Four hierarchical-orders of cyclicity are documented in the middle Wasatch Formation: 1) archetype cycle; 2) small-scale cycle; 3) intermediate-scale cycle; and 4) large-scale cycle. Each increase in hierarchical level is associated with an increase in size, time-span of existence, cross-cutting relationships, correlation length, and lateral shift in the axis of deposition of stratigraphically adjacent cycles. Archetype though intermediate-scale cycles document scale-dependent compensational stacking resulting in lateral changes in upward patterns across axis-to-margin profiles through the fluvial system. This pattern represents an end-member in fluvial stratigraphy with the other being driven by allogenic controls which display persistent upward patterns in the stratigraphy across axis-to-margin profiles through the fluvial system. A 77-mile transect up the Green River in Desolation Canyon documents a semi-Waltherian progression interpreted to reflect systematic basinward and landward shifts in a fluvial-lacustrine depositional system. This fluvial-lacustrine system encompasses the Tertiary Flagstaff, lower Wasatch, middle Wasatch, lower Wasatch, Lower Green River and Middle Green River Formations. These basinward and landward shifts contain upward stratigraphic patterns from fluvial, deltaic, lacustrine, and open lacustrine depositional environments. Regionally extensive, compound paleosols bounding the middle Wasatch Formation interrupt this Waltherian progression.