ABSTRACT The late Eocene Chesapeake Bay impact structure was formed in a multilayered target of seawater underlain sequentially by a sediment layer and a rock layer in a continental-shelf environment. Impact effects in the “brim” (annular trough) surrounding and adjacent to the transient crater, between the transient crater rim and the outer margin, primarily were limited to the target-sediment layer. Analysis of published and new lithostratigraphic, biostratigraphic, sedimentologic, petrologic, and mineralogic studies of three core holes, and published studies of a fourth core hole, provided information for the interpretation of the impact processes, their interactions and relative timing, their resulting products, and sedimentation in the brim. Most studies of marine impact-crater materials have focused on those found in the central crater. There are relatively few large, complex marine craters, of which most display a wide brim around the central crater. However, most have been studied using minimal data sets. The large number of core holes and seismic profiles available for study of the Chesapeake Bay impact structure presents a special opportunity for research. The physical and chronologic records supplied by study of the sediment and rock cores of the Chesapeake Bay impact indicate that the effects of the initial, short-lived contact and compression and excavation stages of the impact event primarily were limited to the transient crater. Only secondary effects of these processes are evident in the brim. The preserved record of the brim was created primarily in the subsequent modification stage. In the brim, the records of early impact processes (e.g., outgoing tsunamis, overturned flap collapse) were modified or removed by later processes. Transported and rotated, large and small clasts of target sediments, and intervals of fluidized sands indicate that seismic shaking fractured and partially fluidized the Cretaceous and Paleogene target sediments, which led to their inward transport by collapse and lateral spreading toward the transient crater. The succeeding inward seawater-resurge flow quickly overtook and interacted with the lateral spreading, further facilitating sediment transport across the brim and into the transient crater. Variations in the cohesion and relative depth of the target sediments controlled their degree of disaggregation and redistribution during these events. Melt clasts and shocked and unshocked rock clasts in the resurge sediments indicate fallout from the ejecta curtain and plume. Basal parautochthonous remnant sections of target Cretaceous sediments in the brim thin toward the collapsed transient crater. Overlying seawater-resurge deposits consist primarily of diamictons that vary laterally in thickness, and vertically and laterally in maximum grain size. After cessation of resurge flow and re-establishment of pre-impact sea level, sandy sediment gravity flows moved from the margin to the center of the partially filled impact structure (shelf basin). The uppermost unit consists of stratified sediments deposited from suspension. Postimpact clayey silts cap the crater fill and record the return to shelf sedimentation at atypically large paleodepths within the shelf basin. An unresolved question involves a section of gravel and sand that overlies Neoproterozoic granite in the inner part of the brim in one core hole. This section may represent previously unrecognized, now parautochthonous Cretaceous sediments lying nonconformably above basement granite, or it may represent target sediments that were moved significant distances by lateral spreading above basement rocks or above a granite megaclast from the overturned flap. The Chesapeake Bay impact structure is perhaps the best documented example of the small group of multilayer, marine-target impacts formed in continental shelves or beneath epeiric seas. The restriction of most impact effects to the target-sediment layer in the area outside the transient cavity, herein called the brim, and the presence of seawater-resurge sediments are characteristic features of this group. Other examples include the Montagnais (offshore Nova Scotia, Canada) and Mjølnir (offshore Norway) impact structures.

The alteration or transformation of impact melt rock to clay minerals, particularly smectite, has been recognized in several impact structures (e.g., Ries, Chicxulub, Mjølnir). We studied the experimental alteration of two natural impact melt rocks from suevite clasts that were recovered from drill cores into the Chesapeake Bay impact structure and two synthetic glasses. These experiments were conducted at hydrothermal temperature (265 °C) in order to reproduce conditions found in melt-bearing deposits in the first thousand years after deposition. The experimental results were compared to geochemical modeling (PHREEQC) of the same alteration and to original mineral assemblages in the natural melt rock samples. In the alteration experiments, clay minerals formed on the surfaces of the melt particles and as fine-grained suspended material. Authigenic expanding clay minerals (saponite and Ca-smectite) and vermiculite/chlorite (clinochlore) were identified in addition to analcime. Ferripyrophyllite was formed in three of four experiments. Comparable minerals were predicted in the PHREEQC modeling. A comparison between the phases formed in our experiments and those in the cores suggests that the natural alteration occurred under hydrothermal conditions similar to those reproduced in the experiment.

An unusually thick section of sedimentary breccias dominated by target-sediment clasts is a distinctive feature of the late Eocene Chesapeake Bay impact structure. A cored 1766-m-deep section recovered from the central part of this marine-target structure by the International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) drilling project contains 678 m of these breccias and associated sediments and an intervening 275-m-thick granite slab. Two sedimentary breccia units consist almost entirely of Cretaceous nonmarine sediments derived from the lower part of the target sediment layer. These sediments are present as coherent clasts and as autoclastic matrix between the clasts. Primary (Cretaceous) sedimentary structures are well preserved in some clasts, and liquefaction and fluidization structures produced at the site of deposition occur in the clasts and matrix. These sedimentary breccias are interpreted as one or more rock avalanches from the upper part of the transient-cavity wall. The little-deformed, unshocked granite slab probably was transported as part of an extremely large slide or avalanche. Water-saturated Cretaceous quartz sand below the slab was transported into the seafloor crater prior to, or concurrently with, the granite slab. Two sedimentary breccia units consist of polymict diamictons that contain cobbles, boulders, and blocks of Cretaceous nonmarine target sediments and less common shocked-rock and melt ejecta in an unsorted, unstratified, muddy, fossiliferous, glauconitic quartz matrix. Much of the matrix material was derived from Upper Cretaceous and Paleogene marine target sediments. These units are interpreted as the deposits of debris flows initiated by the resurge of ocean water into the seafloor crater. Interlayering of avalanche and debris-flow units indicates a partial temporal overlap of the earlier avalanche and later resurge processes. A thin unit of stratified turbidite deposits and overlying laminated fine-grained deposits at the top of the section represents the transition to normal shelf sedimentation.

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.

Organic-rich shales of Late Jurassic age make up the main source rock for oil and gas in large parts of the Arctic. These sediments, which locally may contain more than 15% total organic carbon (TOC), covered the target area of the Mjølnir impact. We suggest that the extreme richness of organic matter and highly volatile components in the target rock resulted in colossal and intense fires in the impact area, both in the air and on the seafloor. This hypothesis is supported by numerical simulations and explains the large quantities of soot that have been found in samples associated with the Mjølnir impact.