SEDIMENTOLOGICAL COMPARISON OF THE EXMORE BRECCIA IN THE EYREVILLE AND LANGLEY CORES, CHESAPEAKE BAY IMPACT STRUCTURE: CLUES TO THE RESURGE PROCESS

The Chesapeake Bay impact structure formed at 35.8 Ma in a shelf sea with a few tens of meters water depth on the western side to <340 m on the oceanic eastern side. The target succession included sediments resting on an eastward-dipping crystalline basement. Sediment thickness increased eastwards from ~400 m to ~1,500 m. The oldest sediments are Cretaceous oxidized and non-oxidized clays, sands, and gravels overlain by Upper Cretaceous to Tertiary shallow-marine, gray and green clays, sands, and minor limestones. The sediments were poorly consolidated, which led to a significant expansion of the crater due to slumping. Today, a 24 km wide outer annular trough (AT) surrounds a 30-38 km wide basement crater (BC). Resurging seawater carried huge amounts of debris back into the crater, forming the Exmore breccia (EB) that now covers the CBIS. The EB was known from previous drilling [e.g., Langley core (L) near the southwestern rim of the AT], and its thickness within the BC recently was confirmed by the Eyreville cores (E). We used line-logging to compare the EB between the peripheral L and the central E. Focus has been on clast size variation (>5mm) and clast frequencies. Logged interval covers 200 clasts between 236.15 m and 272.49 m in L, and 677 clasts between 444.75 m and 596.39 m in E. Underlying sediment-clast breccias were not included in the study.

Clast frequency and size curves show a distinctive change at ~527 m (E) and 267 m (L), with blocks (max. 12 m) of Cretaceous oxidized clay-silt dominating in the lower section, and a fining-up deposit in the upper section. All lithologies are represented in all parts of the logged intervals, even in the matrix between the larger blocks, indicating blending during flow. At L, clasts from the upper part of the target are common (almost absent at E), but lower target sediments increase in frequency downwards, possibly indicating rip-up from slumped units. At E, there is a higher content of crystalline ejecta than at L, likely as a result of rip-up from the BC rim and ejecta layer. Altogether, the analysis points to a scenario where extensive collapse of the sedimentary target layer surrounding the BC is initiated after the failure of the BC rim. This headward collapse spreads rapidly out from the BC at the same time seawater begins to surge back, eventually reaching the BC rim and filling its interior.