Core descriptions, thin-section analyses, and X-ray powder diffraction analyses of whole-rock samples and clay-sized fractions were employed to interpret the sedimentology and mineralogy of synimpact Exmore beds and the overlying Chickahominy Formation. This study attempts to explain the origin and postdepositional alteration of materials in the Eyreville core from the central zone of the Chesapeake Bay impact crater. Samples were obtained from eight zones extending from core depths of 435 to 1471 m, with emphasis on the interval from 435 to 455 m, representing the upper Exmore beds and the lower Chickahominy Formation. Qualitative clay mineral determinations were aided by peak decomposition procedures to unravel overlapping diffraction bands, and quantification was accomplished by least squares matching of actual and computed patterns.

The major facies in approximate ascending order are suevite breccias, poorly sorted conglomerate and sandstone, and upward-fining glauconitic sandstone within the Exmore beds followed by parallel laminated sandy siltstone and claystone in the Chickahominy Formation. They all contain clay minerals (mica, smectites, and some serpentine, kaolinite, and chlorite) plus quartz and feldspar. Heulandite, pyrite, calcite, and disordered silica (partly representing nanofossils and microfossils) are present in the Chickahominy Formation. The boundary beds (upper 7 m) of the Exmore beds have higher clay contents but fewer varieties of expandable clay minerals than in the Chickahominy Formation. The Exmore beds are enriched in reworked glauconite, but there are no indications of heulandite, calcite, disordered silica, or pyrite, except in the very top of the 7-m-thick boundary bed interval.

The clay fractions of the Eyreville materials are dominated by different species of expanding clay minerals (smectite, fine and coarsely crystalline nontronite, and fine and coarsely crystalline smectite-illite mixed-layered clay minerals), but dioctahedral mica and illite are also present. Amorphous material and minor amounts of quartz, chlorite, and mixed-layered smectite (0.95)/iron-rich illite (0.05) are common. The abundance of the clays in most intervals is highly variable due to the chaotic assemblage of sediments and crystalline materials from diverse sources. The boundary beds are dominated by a single smectitic mineral, nontronite, which is assumed to be the principal product of melt glass alteration. Amorphous material (melt glass) and nontronite are calculated to represent 13 vol% and 13–19 vol% of the sediments in this interval, respectively. Grain size, or clast size, has a major influence on mineralogical variability, i.e., when grain size (clast size) is large, the mineral content of adjacent samples is highly variable.