163-5 New Insights into the Age and Extent of the 200-Kilometer Roosevelt Impact Structure from Suevites, Shocked Granophyres, Pseudotachylites, Spherules, and Seismic Data

Session: Impact Cratering Processes Across the Solar System: In Memory of Dr. Bevan M. French

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Prior ideas of the size and shape of the Roosevelt Impact Structure (RIS) in west-central Georgia and eastern Alabama were based on the mapped extent of the oblong Pine Mountain massif and associated charnockites, which we interpreted as the eroded remains of the central uplift and melt sheet of a highly oblique crater formed c. 800 Ma. The quartzites capping the massif exhibit evidence of shatter cleavage, multiply striated surfaces, and shatter cones. The charnockites contain blocks of the uplifted quartzite, which exhibit abundant PDFs, along with shocked clasts of gneiss, schist, and granite. The subsequent discovery of PGE-enriched pseudotachylitic breccia dikes striking parallel to the uplift axis, containing quartzite clasts that exhibit both PDFs and feather features, further supported our conclusions.

Determining the structure’s dimensions additionally was influenced by the belief that the Towaliga Fault (TF) had displaced and/or destroyed the northern half of the RIS (similar to the southern edge of the Sudbury impact structure). But recent access and review of COCORP data suggest that assumption was incorrect. Instead, the seismic reflection profiles show that the crater north of the TF is intact and partially buried beneath remnants of a late Proterozoic thrust sheet. The portions of the RIS south of the TF appears to have been uplifted at least several kilometers in the Phanerozoic, which has aided in their more rapid exhumation. Combining the seismic data with SRTM imagery reveals that the RIS is nearly circular with a diameter exceeding 200 km. The RIS extends N-S from Atlanta to Andersonville and E-W from east of Macon to Auburn. However, it does have an elongate central uplift consistent with an impact angle between 20 and 30 degrees.

Using the revised geophysical/geodetic map of the RIS, we made predictions about where different structural features and stratigraphic facies of the structure might occur. This has led to the discovery of layered melt deposits inside the modeled rim, including an intensely shocked granophyre and reddish suevite that contains weakly to moderately shocked quartz and feldspar. Just beyond the rim we have found a Bunte-like breccia in sequence with red boulders of welded accretionary lapilli composed of aggregated condensate spherules, mineral fragments, and altered glass shards. These units may correlate to some enigmatic global strata. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4767

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