Plan-view Facies Architecture in the Cretaceous Notom Delta, Ferron Sandstone Member, Utah, U.S.A: An In-depth Analysis of Mouth Bars, Terminal Distributary Channels and Small Scale Growth Faults

Growth faults are syndepositional normal faults that form at the same time sediment accumulates. Continental scale growth faults act as traps in hydrocarbon plays responsible for producing oil and gas. Small-scale growth faults are studied because of their analogous development to continental-scale growth fault. The Notom delta along the coast of the Cretaceous Western Interior Seaway houses small-scale growth faults easily accessible and one of the only outcrops to depict plan-view exposure of such fault systems.

Bhattacharya and Davies (2001;2004) suggested mouth bars act as the point source stress mechanism responsible for the initiation of small-scale growth faults in the Notom delta of the Ferron Sandstone Member, Utah. When enough sand accumulates as the mouth bar grows, it collapses, slumping forward and initiating growth faulting. Turbidites and hyperpycnal flows create a water saturated prodelta mud acting as a mobile substrate over which the growth faults migrate onto the landward or seaward direction. High resolution plan-view analysis using a drone is used to create a stitched panoramic view of sandstone unit A.  Laterally extensive beds are traced to create a bedding diagram used to identify architectural elements where downlap and lapout truncation relationships express major bedding packages, L1 through L6, characteristic of varying facies architecture and complex bedding geometry.

Above L1, sandstone unit A is interpreted as topsets feedings into a growing mouth bar or a floundering frontal splay. Below L1, mounded bedding packages, U-shaped architectural features, amalgamated, low-angled, wave-influenced strata dominated by lenticular packages of sandstone units and stacked bedding is interpreted as the proximal delta front with mouth bars, terminal distributary channels and hummocky-cross strata representative of flood cycles across the Notom delta during the Turonian. Mouth bars range in width from 1.3 to 9.5 meters while terminal distributary channels range in width from 2.2 to 7.7 meters. Growth faults range in width from 8.1 to 95.4 meters yet growth faults of interest – growth faults I, II and III – range in width from 14.5 to 12.8 meters respectively. The mouth bar width scales to the growth fault widths of interest on a 1 to 1.3 – 1.5-meter ratio.