37-1 It’s the Faults’ Fault: Kilometer-Scale Folds and Internal Deformation in the Chuckanut Formation, NW Washington State, USA

Session: Geoscience Research Poster Showcase by 2YC and 4YCU Undergraduate Students

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Abstract:

When tectonic stresses exceed elastic limits at low temperatures and pressures, rocks fail by brittle deformation, leading to faulting and jointing. So how do folds form in the upper crust? The Chuckanut Formation is a brittlely deformed Eocene fluvial sedimentary formation in northwest Washington composed of sandstone, siltstone, shale, and coal. The formation underwent significant SW-NE contraction due to the accretion of the Siletzia terrane about 50 Ma, which created gently plunging, upright, tight-to-close, concentric, kilometer-scale folds, with no apparent grain-scale deformation. Details of the internal deformation are obscured in the field due to heavy forest cover, but are beautifully exposed in LiDAR imagery. For this investigation, we used Geologic Map Data Extractor (GMDE) to trace bedding formlines, extract bedding attitudes, and identify previously unmapped faults within the formation on a 250 square kilometer texture shade LiDAR image of the southwestern Chuckanut Formation. We found evidence of abundant brittle faulting, including splay faults branching from the basal unconformity with the underlying Easton Metamorphic Suite, which we interpret to be a detachment fault. The intensity of faulting increases in the cores of folds, where there are also secondary folds, some of which are themselves internally faulted. Stereonet analysis of bedding attitudes suggests that the formation likely underwent two phases of cylindrical folding: folding first around an axis with an average trend of 325° and plunge of approximately 15° to create the dominant fold pattern, which was later warped around a sub-vertical fold axis trending approximately 160°. The minimum shortening of the formation, estimated from the line-length change of the primary kilometer-scale syncline-anticline pair, is about 30% from SW to NE, but including internal deformation, the total contraction is much larger. The many alternating layers of stronger sandstones and siltstones with weaker shale and coal likely permit flexural slip that facilitated fault-related folding.

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