EXTRA-CRATER STRUCTURAL DEFORMATION AND IMPACT TRAJECTORY AT WEAUBLEAU STRUCTURE, WEST-CENTRAL MISSOURI

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°.