Abstract We present an integrated geomagnetic polarity and stratigraphic time scale for the Triassic, Jurassic, and Cretaceous Periods of the Mesozoic Era, with age estimates and uncertainty limits for stage boundaries. The time scale uses a suite of 324 radiometric dates, including high-resolution 40 Ar/ 39 Ar age estimates. This framework involves the observed ties between (1) radiometric dates, biozones, and stage boundaries and (2) between biozones and magnetic reversals on the seafloor and in sediments. Detailed attention is given to chronostratigraphic calibration of stage boundaries using tethyan and boreal biozonations. Interpolation techniques to arrive at a geochronology include maximum likelihood estimation, smoothing cubic spline fitting, and magnetochronology. The age estimates for the 31 stage boundaries (Ma with uncertainty in my to 2 standard deviations), and the duration of the preceding stages (in my) are:

Micropaleontological study of the sedimentary record from the Montagnais impact structure, located on the shelf off Nova Scotia, has shown that the impact had neither regional nor global effects on biological diversity. This result provides new evidence for the lower threshold of extinctions due to the impacts, indicating that impacting bolides must be larger than 3 km in diameter to cause extinctions. We use this evidence to construct simplified curves to test relations between mass extinctions, bolide diameter, and impact periodicity. These data point to reoccurrence of mass extinctions of amplitude comparable to the Cretaceous/Paleogene event an average of once in every 100 to 500 m.y., whereas the probability of extinction of life on Earth triggered by an impact of a bolide larger than 60 km in diameter has an average frequency of once in 1 b.y. Data presented in this chapter do not support a direct parallel between the 26-m.y. extinction periodicity and the impact cratering record in Earth.

Abstract We present an integrated geomagnetic polarity and geologic time scale for the Jurassic to Recent interval, encompassing the age range of the modern ocean floor. The time scale is based on the most recent bio-, magneto-, and radiochronologic data available. The biostratigraphic bases for Jurassic, Cretaceous, and Cenozoic time-scales are discussed extensively elsewhere (e.g., Gradstein, this volume; Van Hinte, 1976b; Hardenbol and Berggren, 1978; Berggren and Van Couvering, 1974; Van Couvering and Berggren, 1977). Emphasis is placed here on magnetochronology and its integration with biochronology in the derivation of an internally consistent geologic time scale. The binary signal of normal and reversed geomagnetic polarity has little intrinsic absolute time value (ordinal scale), but it can be used to measure time according to its radiochronologic calibration (cardinal scale). The standard magnetic reversal sequence has a correlatable, characteristic pattern and is demonstrated to be continuous from numerous marine magnetic anomaly profiles from the world ocean. The reversal sequence is recorded by lateral accretion in sea-floor spreading and vertical accumulation in sedimentary or lava sections, allowing independent checks on the completeness and relative spacing of the reversal sequence as well as the opportunity to apply an assortment of geochronologic data for calibration. Although different phenomena and assumptions are invoked in their derivation, both magneto- and bio- chronologic time estimates involve indirect assessment according to calculated rates of sea-floor spreading, sedimentation, and biotic evolution. These extend the application of the relatively few reliable radiometric dates available, so that a continuous