The Link Between Hyperpycnal Flows and Growth Faults in Steeply Dipping Delta-Front Clinoforms

Outcrop examples of flood-dominated mouth-bars and associated hyperpycnal prodelta deposits within the Cretaceous Ferron Notom delta in central Utah show delta-front clinoforms that dip at approximately 7°. Mouth-bars consist of meters-thick medium-gained sandstone. In some examples, sandstones show abundant medium-to large-scale cross stratification interpreted as the deposits of dunes and mouth-bars respectively. Climbing dune-scale cross strata suggest landward accretion at diffluence zones on the upstream ends of mouth bars. In other examples, growth strata comprise primarily upper-plane bed or “humpback” stratification, thought to represent upper flow regime conditions. The growth faulted mouth bars store 5 to 7 times more sediment then the clinoforms found in non-growth-faulted bars.

The bars overlie several-meter thick successions of heterolithic silty mudstones and sandstones. Normal and inverse grading, climbing ripple cross lamination, small-scale soft sediment deformation, and a lack of burrowing, suggest high sedimentation rates in a hyperpycnal-dominated prodelta setting. These deposits form a highly porous and easily fluidized mud substrate, as indicated by abundant diapirs, which forms the underlying platform for strain accommodation. When this is coupled with the rapid deposition of the mouth-bar during a hyperpycnal event, over-pressuring of the initially high porosity mudstones occurs. The throw on the faults in the study area are probably controlled by the thickness of this substrate. The over-pressured muds formed shale ridges, which compartmentalized the longitudinal extent of the fault and allowed depletion of the substrate, which allowed the system to prograde without continued movement on the fault.

In these ramp-type margins, strain accommodation depth relates to parasequence thickness (usually < 10m) and clinoform slope resulting in highly listric faults with a few meters of throw. In shelf-edge deltas, where strain accommodation depth relates to the continental slope length, growth faults can achieve regional scale and the initiation mechanisms may be masked due to excessive offset.