In many parts of the world a thin clay or marly unit marks the boundary between Cretaceous and Tertiary rocks. In marine sequences this boundary is defined by the first appearance of typically Paleocene marine plankton in the clay. In continental rocks, the boundary sediment yields the stratigraphically highest occurrence of a Cretaceous assemblage of fossil pollen. Detailed analyses of the marine boundary sediment at Caravaca, Spain, permit a three-fold subdivision: the lowest is apparently a fallout deposit of impact ejecta, preserved as a 0.5-cm lamina of red clay. The main subdivision is a black or dark gray clay or marl, containing reworked extraterrestrial debris, laid down in an oxygen-deficient environment. The uppermost boundary clay is lighter gray in color, transitional in lithology to the overlying Paleocene sediments, which were deposited after the recovery from the terminal Cretaceous convulsive event. The boundary clay unit on land, represented by a section in Raton Basin, New Mexico, consists of a lower white clay, which is apparently a fallout deposit, and an upper carbonaceous shale. Boundary sections elsewhere are similar to those sections. The sedimentology of the boundary sediment records the convulsive environmental changes at/after a terminal Cretaceous event.

Sediments containing components produced by accretionary events should contain sufficient geochemical evidence to constrain several of the variables involved in modeling the environmental consequences of such events. We consider the geochemical record expected for 3 modes of accretion: accretion from an interstellar cloud, non-impacting accretion of weak materials subjected to tidal and atmospheric disruption, and the impact of dense asteroidal or cometary materials. To constrain an accretionary event, it is important to determine: 1) the siderophile abundance patterns; 2) the duration of the event; 3) the geographic extent of the anomaly; 4) the physical nature of the extraterrestrial materials and their siderophile concentrations; and 5) the source of the terrestrial component. In attempting to constrain the Cretaceous-Tertiary and the Antarctic Basin late-Pliocene events we find that the necessary evidence is only partially available. It seems clear that the Cretaceous-Tertiary event generated fallout on a worldwide basis, and that its duration was ⩽1 ka, but other features are not yet well defined. The siderophile pattern is generally chondritic, but variations from site to site (and even sample to sample) indicate differing geochemical fractionations during deposition and make it impossible to associate the projectile with a specific group of meteorites. Future studies of unusually well-preserved basal layers at Caravaca and DSDP 465A may improve the precision with which the siderophile patterns are determined. Magnesium concentrations and isotopic studies argue against a mantle ejecta component in the boundary layer, and thus against the oceanic impact of a dense projectile.

Scanning electron microscope studies of particulate matter in Cretaceous/Tertiary boundary sediments reveal no common carrier phase larger than about 1 μ m for the supposedly extraterrestrial elements. Particles possibly related to an extraterrestrial impact are Fe-rich aluminosilicate spheres (similar to microtektites), Al-rich smooth spheres, K-rich feldspar spheroids, and Fe-Ti–rich particles. Mercury abundances in bulk sediment samples show no consistent enrichment pattern. Some of the supposedly extraterrestrial elements (e.g., Pt, Ni) are present within diagenetic sulfide grains. Terrestrial processes have modified the extraterrestrial element signature, and characterization of an impactor simply from sediment chemistry is erroneous. Deep-sea sediments not enriched in chalcophiles will probably provide the best clues for impactor identification.