--> Shale Mechanics of Growth Faults in Deltaic Parasequences of the Cretaceous Ferron Notom Delta of Utah, USA

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Shale Mechanics of Growth Faults in Deltaic Parasequences of the Cretaceous Ferron Notom Delta of Utah, USA

Abstract

Passive continental margins host regional-scale growth faults which form hydrocarbon traps and locally thickened reservoir compartments. Self-similarity of scales suggests that analysis of growth faults in outcrops may be applied to regional-scale systems. This study used detailed sedimentological and stratigraphic data, and a photomosaic covering 100 m laterally, to analyze shale deformation mechanics within small-scale growth faults at the base of the Turonian Notom delta of the Ferron Sandstone in Central Utah. Five growth-fault blocks are exposed in strike and dip views and include sandy growth strata and pre-growth muddy facies. The highly curved faults sole out into bedding parallel decollement surfaces associated with deformed prodelta shales, which also form diapirs. Throw on individual faults averages 4 m and heave increases with depth from 2.5 to 5 m. Four sedimentological sections indicate deposition in a river-dominated, storm-influenced delta that prograded northeastward into a protected embayment. Three parasequences are represented in the pregrowth strata and comprise mixed heterolithic facies deposited in a prodelta and distal-delta front environment. Growth strata are primarily planar and quasi-planar fine sandstones deposited as upstream and downstream accreting mouth bars. Deposition of mouth bar sands formed a “critical wedge” that initiated growth faulting at the sand/mud interface. Low density mobile prodelta muds, guided by a pressure gradient, accommodate the fault strain and deform to produce diapirs that penetrate the sediment-water interface. Motion on the primary fault surface ceases when growth strata contact higher-strength, dewatered pre-growth strata. Sands at the fault contact in the growth strata lock onto higher-strength pre-growth muds. Further deformation of the growth strata are accommodated by synthetic microfaulting in the growth sands in the terminal phase of motion. Smaller-scale growth faulting may have different triggers than regional-scale faults, which are driven by critical-taper dynamics at the continental margin. This example emphasizes the importance of single point stress sources associated with sandy mouth bar deposits, which act as fault initiation points, versus the regional stress regime associated with regional-scale growth faults. This can lead to expanded, but localized, reservoir compartments within river-dominated deltaic parasequences, such as those found in Prudhoe Bay Field, Alaska.