CHESAPEAKE BAY AND WETUMPKA: STRATIGRAPHY AND PROCESSES IN MARINE IMPACT STRUCTURES

ICDP-USGS cores show that the 85 km Chesapeake Bay impact structure (CBIS) is filled by >1.3 km of sands, breccias, and megabreccias. The lower part of the core (1770-1551 rmcd) consists of blocks and megablocks of cataclastic and non-cataclastic schist, gneiss, and granite which contain suevitic veins and which may be imbued with dark carbonaceous material. The next interval (1551-1393 rmcd) consists of layers of suevite and polymict breccia intercalated with blocks of gneiss underlying a sequence of very coarse sands (1393-1371 rmcd). Granite megablocks (1371-1095 rmcd) overlie those sands. Above the granites (1095-818 rmcd) are sands composed of slumped and partly disaggregated megablocks of the Potomac Formation, separated and intruded by dikes of impactoclastic sands. Between 818-527 rmcd is mostly Potomac-sourced impactoclastic sands containing clay blocks and gravel layers and 527-451 rmcd consists of impactoclastic breccias. Finally, the uppermost impact interval (451-444 rmcd) consists of glauconitic sands terminating in a 0.5 m zone of laminated silty clays.

We suggest that the lower part of this succession is the product of early mixing of wall-hugging suevites with comminuted and fractured basement breccias. Coarse sands below the granite megablock contains weakly to unshocked sedimentary clasts and may represent initial resurge and collapse of the higher transient crater wall. The granite megablock, we interpret, subsequently foundered off the central peak. Continued collapse and resurge produced a sequence of sedimentary blocks and sands capped by settling deposits and washed-back fallout ejecta.

Breccias in the center of Wetumpka (7.6 km) consist of >100 m of sands and megabreccias underlain by blocks and megablocks of sedimentary and crystalline target, intercalated with polymict breccias. Although the CBIS is larger and formed in deeper water than Wetumpka, the similar targets (unconsolidated formations overlying crystalline basement) resulted in similar structure-filling stratigraphy. They share the same dichotomy between the upper slump and resurge megabreccias and lower mixtures of target blocks and polymict breccias (with relatively little melt). This dichotomy indicates a profound change in modification-stage processes driven in large part by the return of sea water.