Re-evaluating the Origin of an Upper Eocene Diamictite in the Coastal Plain of East-central Georgia: an Impactoclastic Layer?

Introduction: An Upper Eocene diamictite, unique in the Coastal Plain stratigraphy of east-central Georgia, has been described as an altered volcaniclastic deposit [1-4] resulting from widespread ashfall or pyroclastic flows [1,2]. While the breccia bed does appear strikingly similar to weathered silicic tuffs (Figure 1), previous authors have never speculated where the Late Eocene volcanic source might have been. Gibson and Towe [5] suggest that MidAmerican or Caribbean volcanoes contributed some volcanic components to Coastal Plain sediments during the Eocene, but such sources seem insufficient to produce a relatively thick (≈ 0.5 to 1.5 m) deposit off the eastern shores of Georgia. Prior to the early 1990’s [6], most stratigraphers were unaware of the 35 Ma Chesapeake Bay Impact and the Late Eocene cataclysm that must have had major consequences up and down the Atlantic seaboard. The conspicuous diamictite should be re-evaluated to determine if it could have had an impact-related origin. Where roadcuts and mines expose the diamictite, along the 60 km from Gibson to Augusta, it overlies the massive commercial kaolin deposits of the Middle Eocene Huber Formation. Huddlestun and Hetrick [4,7] assign the diamictite and related sediments (the Albion Member) to the Clinchfield Sand Formation, too early to be associated with the impact [8]. Pickering [9] asserts that the Albion Member actually represents the basal portion of the Dry Branch Formation, exactly where Albin and Wampler [8] predict that a Chesapeake Bay impact horizon might be found. Recent reconnaissance in active mines seems to support Pickering’s claim and suggests that the breccia correlates 8 km up-dip with the coarse sand horizon in which Harris et al. [10] and Schroeder et al. [11] have identified possible shocked quartz. Here I report the initial results of my investigation of the diamictite. Composition: The diamictite is composed of a kaolin matrix supporting cream-colored kaolinite clasts and dark-gray smectite clasts (Figure 1). The kaolinite clasts appear in a variety of shapes from angular to rounded. They range in size from <<1 mm to more than 4 cm. Commonly, the larger clasts display internal fracturing. The sizes of the smectite clasts are more restricted, ranging from approximately 0.5 mm to about 3 mm. They typically exist as angular fragments, but some rounded and bleb-shaped particles were observed. In one case, an elongate clast appears to have been stretched around a larger kaolinite clast. The kaolin matrix contains minor amounts of quartz and mica. Sand-size quartz grains are very angular and relatively scarce. They have not been searched yet systematically for evidence of shock metamorphism. Where previous authors have described this unit [1-4,9], the breccia is opalized and occurs close to the modern surface. In J.M. Huber’s Braswell Mine, 9 km NE of Wrens, mining has exposed the unit in the subsurface where it remains unsilicified.