WEAUBLEAU IMPACT STRUCTURE
by: Charles O’Dale
- Type: Complex
- Age Ma: 320 – 340a – MISSISSIPPIAN
- Diameter: 19 km
- Location: N 38° 00’ W 093° 36’
- Shock Metamorphism: pdf
a Geologic estimated age: Plymate 2004.
EXTRA-CRATER STRUCTURAL DEFORMATION AND IMPACT TRAJECTORY AT WEAUBLEAU STRUCTURE, WEST-CENTRAL MISSOURI – 2015
Impact cratering commonly is seen as a surficial process, where the distribution of strain in target rocks occurs mostly within the immediate vicinity of the crater rim. However, studies of low-angle impacts, such as Weaubleau and a few other structures, show the momentum of the impactor can be transferred in the down-range trajectory and obliquely well beyond the inferred crater rim. At Weaubleau, the extra-crater deformation field is fan-shaped and extends to a distance roughly equal to the diameter of the crater. Previous studies have shown that the Weaubleau structure resulted from the impact of a large meteorite (estimated ~450 m diameter) during the medial Mississippian subperiod. It contains shocked quartz grains that show multiple directions of planar deformational features (PDFs). Sedimentological features suggest it likely was a marine impact. The surface of the structure consists of two quasi-circular topographic, ring-like drainages, 8 km and 19 km in diameter, which are eccentrically located to one another. The inner ring is an area of extensive brecciation and is interpreted as the main impact area, likely near the inferred limits of the crater rim.
The outer ring encompasses a heterogeneous field of structural deformation, where rocks have been partly brecciated, folded, faulted, but locally may show little effect of the impact event. This outer ring marks the lateral extent of structural deformation, the tectonic rim. Brecciation resulted from dilatational stress, and shear and constrictional stress components locally resulted in thrust faults, recumbent folds, broken recumbent folds, and enigmatic lineations in coarse carbonates that resemble L-tectonites, slickenlines, and stylolites. Variation in material strength of the target rock succession contributed to the shallow distribution of strain beyond the main impact area.
The eccentricity of the inner and outer rings and heterogeneous distribution of deformation indicates an impact trajectory from the paleo-west (present-day southwest) with a low angle of incidence, between 15°-30°. The main impact area of Weaubleau is only slightly elliptical, whereas markedly elliptical craters typically develop when the angle of incidence is between 10°-15°.
[See: Crater Shape]
Approximately 10% of the coarse quartz sand within the fall-back impact breccia within the Weaubleau structure exhibit multiple sets of parallel fractures and pdf’s that remain unindexed. Shocked quartz grains commonly are “toasted” with a semi-opaque yellow brown appearance.
Surrounding the impact area are spherical rock formations of varying size, called the Missouri rock balls, which probably formed at the time of the impact. Nearly perfectly round, they are also sometimes referred to locally simply as “geodes,” “round rocks,” or “Weaubleau eggs.” The round rocks may have formed when the impact threw shale away from the center of the crater and silica-saturated waters subsequently formed silica around the shale fragments.
The phenomenon of the “round rocks” is not restricted to the Weaubleau-Osceola impact structure but the formation of spheroidal clasts and larger nodular bodies is observed also in the Spanish large impact structures of Azuara and Rubielos de la Cérida and in the Ries impact crater.
The Weaubleau-Osceola impact structure overlies the Burlington-Keokuk Limestone (undivided), is cut by paleokarst, and overlain by Pennsylvanian shales, chert breccia, and channel sands. Clast constituents of the “Weaubleau Breccia” include lower Ordovician and lower to middle Mississippian material as well as exotic clasts of granite. Cambrian rocks currently have not been recovered from the breccia, but presence of granite clasts indicates a minimum of 400 m of uplift in the central part of the structure. The age stratigraphy of this impact site is discussed in greater detail in the road log below. We consider this impact to be approximately 320-340 million years old. (Plymate 2004)
Evans, K.R.: Extra-Crater Structural Deformation and Impact Trajectory at Weaubleau Structure, West-Central Missouri Department of Geography, Geology and Planning, Missouri State University 2015
Evans, K.R., Mickus, K.L., Rovey II, C.W., and Davis, G.H. (2004): Field Trip I: The Weaubleau-Osceola Structure: Evidence of a Mississippian Meteorite impact site in Southwestern Missouri. – Association of Missouri Geologists Field Trip Guidebook, 50th Annual Meeting, Springfield, Missouri, September 26-27, 2003, Missouri Department of Natural resources, Geological Survey and Resource Assessment Division, p. 1-30.
Thomas G. Plymate; The Weaubleau-Osceola Structure: Evidence of a Mississippian Meteorite Impact in Southwestern Missouri Department of Geography, Geology, and Planning Southwest Missouri State University 2004