SEDIMENT-AVALANCHE DEPOSITS IN THE LATE EOCENE CHESAPEAKE BAY IMPACT STRUCTURE: EVIDENCE FROM THE ICDP-USGS EYREVILLE CORES, VIRGINIA, USA

Transported sediment-clast breccias attributed to catastrophic ocean-resurge sedimentation are ubiquitous in the upper part of the impactite section throughout the late Eocene Chesapeake Bay impact structure. Parautochthonous sediment-clast breccias deformed in situ within the sedimentary target layer underlie the resurge breccias in the annular trough outside the collapsed transient crater. Within the collapsed transient crater, a third type of sediment-clast breccia is present in the ICDP-USGS Eyreville cores A and B between depths of ~867 m and 1,096 m. These matrix-poor breccias contrast compositionally and texturally with the matrix-rich resurge breccias found higher in the Eyreville cores.

The material in the studied breccia consists entirely of variably deformed Cretaceous target sediments (Potomac Fm.). The larger clasts (cobbles, boulders, and blocks; max. 25 m) display primary Cretaceous stratification that dips at a consistent low, moderate or high angle within individual clasts. Some large clasts consist only of sand, silt, or clay, whereas others consist of interbedded sand, silt, and clay. The sands are weakly indurated; the clay-silts are moderately indurated and densely fractured, but most are not disaggregated. Sand dikes and poorly indurated, massive sand layers indicate local sand liquefaction and fluidization. The matrix between clasts consists entirely of sand and fine gravel produced by comminution of the Cretaceous sediments. Muddy quartz-glauconite sands typical of the matrix in the resurge breccias, clasts of Tertiary target sediments, and crystalline-rock and melt clasts are absent.

These characteristics suggest rapid mass transport and deposition of water-saturated target sediments as a sediment avalanche initiated in the upper part of the transient-crater wall during crater collapse. The good preservation of primary stratification in weakly indurated sediment clasts suggests transport in an internal zone of low and (or) spatially localized shear stress (plug flow). The location of the avalanche base is uncertain; shear-induced basal features (e.g. substrate folds, mixed substrate and breccia) were not observed. The base may be located within an underlying 275-m-thick granitic slab or more likely in a thin unit of quartz sand below the granite.