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MAPPING THE 200+ KILOMETER ROOSEVELT IMPACT STRUCTURE IN CENTRAL GEORGIA AND EASTERN ALABAMA

Abstract

We previously have reported evidence for one of the largest asteroid or comet impact structures in the world preserved in the Neoproterozoic rocks of west-central Georgia and east-central Alabama. Our observations include shocked target blocks in a thick charnockitic melt rock, shocked quartzite fragments in altered pseudotachylitic breccias, and shatter cones/shatter cleavage in the quartzites at the crest of the Pine Mountain massif. Surface mapping of the melt rocks and deformation outlined a structure approximately 180 x 140 kilometers, centered on southeastern Meriwether County. U/Pb dating of zircons in the melt rock constrain the age of the impact at about 809 ±65 Ma, a time at which independent studies have shown a major decline in biologic productivity and a possible comet bombardment on the Moon. Our more recent examination of seismic reflection data recorded in the 1970s now suggests that the full extent of the Roosevelt impact structure may be much larger, with a peak-ring diameter close to 70 kilometers and an outer rim almost 220 kilometers across. It possibly extends from Atlanta to Andersonville and from Milledgeville to Auburn, Alabama. Approximately half of the crater is buried beneath southern Metro Atlanta. Modeling the ring and rim from those data has led to predicative discoveries of fallback ejecta facies, including suevite, impact melt breccias, impact spherules, and polymict lithic breccias situated both near the outer edges of the structure and atop the peak ring. We also have identified deposits associated with the gravitational collapse of the original rim. Those include unique rollover folds (against inward-dipping listric faults) in finely laminated granular flow deposits that contain “drop stones” of ejected Grenvillian basement. Based on data currently available, we estimate that the asteroid or comet had a diameter around 30-35 kilometers and collided at an angle between 10⁰and 15⁰ from the horizontal.

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