9-4 Field-scale Riedel Shears in Southern Ava District of Mark Twain National Forest: Kinematic indicators of left-lateral strike-slip movement in the southern Missouri Ozarks

Session: Advances in Geologic Mapping, Databases, and Dissemination: Student Posters

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Abstract:

The tectonic mechanism and timing of the Ozarks uplift remain topics of interest in midcontinent geology. Previous investigators have proposed regional models for meso-scale structural deformation with dominant strike-slip fault systems controlling block movements during uplift, but direct field-based kinematic evidence historically has been limited. This study provides a detailed structural and stratigraphic investigation of the southern Ava District of Mark Twain National Forest to address this evidentiary gap. The southern part of Ava District in Mark Twain National Forest is located in eastern Taney County and western Ozark County in southern Missouri, approximately six miles north of the Arkansas-Missouri state line. In that area, lower Ordovican Cotter Dolomite (Ibexian Series) is overlain disconformably by and enigmatic sandstone and lower to middle Mississippian limestone strata. The Mississippian forms a narrow sinuous outcrop band on a prominent ridge top but likely was contiguous with the Springfield plateau to the north. Glade Top Trail follows this narrow outcrop band and subsidiary forest roads cross the rugged hills and hollows to the north and south in this sparsely populated area.

Using high-resolution lidar (3DEP hillshade) and targeted field mapping, we analyzed a series of parallel to subparallel lineaments hypothesized to be Riedel shear fractures associated with the WSW-ENE trending Foil Fault. Preliminary geospatial analysis reveals a consistent orientation of these features of 40° to 60°, which is geometrically consistent with Riedel shears, indicative of left-lateral displacement. The Skyline Fault is like a conjugate Riedel shear that intersects the Foil Fault at a high angle. Field observations in lower Ordovician dolomitic strata confirm that these fractures often control local vegetation patterns, specifically the distribution of cedar and post oak in glades where soils are thin. Stratigraphic analysis of Mississippian strata, which vertically are offset by approximately 12 meters along the Foil Fault but lack the distinct photo-lineaments seen in older units, which seems to provide a critical constraint on the timing of deformation. By establishing a definitive kinematic link between map-scale shears and major fault traces, this research validates the regional strike-slip model and advances our understanding of the broader structural evolution of the Ozark Plateau.

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