Abstract : Today, it is virtually unthinkable to solve almost any geologic problem without microfossils, whether in terms of age control or environmental proxies. Perhaps the turning point came in 1980 with the impact hypothesis as cause for the end-Cretaceous mass extinction. Although this hypothesis initiated one of the longest running and most acrimonious scientific debates, its most lasting and positive effects include the introduction of really high-resolution biostratigraphy at the cm-scale and the beginning of truly interdisciplinary and multi-disciplinary studies. Rarely has there been a topic so popular, albeit contentious, that it attracted interest and attention by disciplines across the spectrum of earth sciences and beyond. This report highlights advances in high-resolution biostratigraphy and environmental studies based on major events in Earth’s history, such as the Oceanic Anoxic Event 2 (OAE2) and the Cretaceous-Tertiary boundary (KTB) mass extinction. It illustrates current age control, biostratigraphic resolution, stable isotope records with characteristic δ 13 C excursions and integration with other disciplines to derive environmental interpretations.

Abstract Variation in the carbon isotopic composition of organic matter in the Greenhorn Limestone are described for Black Mesa, Arizona; Pueblo, Colorado; Bunker Hill, Kansas; Ponca State Park, Nebraska; and Cone Hill, Montana. There is a 2.5 to 3.5 o/oo positive (heavy) excursion in carbon isotopic values spanning the Cenomanian-Turonian boundary at each of these localities. Carbon and oxygen isotopic compositions of inoceramid bivalve shells and whole-rock carbonate are reported for the Black Mesa and Pueblo sections. Oxygen isotopic values of whole-rock carbonate generally become more positive upward in the Greenhorn from the lower Lincoln Limestone Member through the Hartland Shale Member and into the lower Bridge Creek Limestone Member. This trend is inferred to reflect generally increasing salinity of Western Interior seawater associated with increasing water depths and less restricted oceanic connections as the Greenhorn transgression progressed. Superimposed on this overall trend are intervals with marked isotopic and geochemical fluctuations suggesting rapidly changing proportions of oceanic and riverine water or rapidly changing relative rates of evaporation and input. A particularly severe and rapid paleoenvironmental change is inferred to have occurred just prior to the Cenomanian-Turonian boundary defined by macrofossil extinctions.

Gilbert interpreted rhythmic spacing in limestone units in the Upper Cretaceous of Colorado as a geological response to a planetary cause—the 21,000 yr precession of the equinoxes. In this light, he judged the Upper Cretaceous marine sequence of Colorado to have been deposited over a span of about 21 m.y.—a figure that seems remarkably close to that yielded by modern radiometric geochronology, 24 to 35 m.y. A reexamination of three of Gilbert’s four short rhythmic sequences, using the available radiometric data of Obradovich and Cobban, in conjunction with a model of subsidence and sedimentation, yields bedding rhythms in the 18,000 to 22,000-yr range, and seems to confirm Gilbert’s hypothesis. Most rhythmic Cretaceous sequences in other parts of the world also yield bedding rhythms close to the precessional period, according to the Obradovich-Cobban time scale. The alternative Van Hinte time scale, however, yields a wider scatter of values, and suggests that only some of the rhythms are related to the precession, others seeming to be closer to the 41,000-yr period of obliquity. The equinoctial precession can affect geology only when acting in conjunction with the eccentricity of the Earth’s orbit, which waxes and wanes irregularly with a mean period of 93,000 yr. One should therefore expect precessionally caused rhythms to occur in sets averaging 4.5. That limestone-shale bedding rhythms occur in sets was shown long ago by Schwarzacher for upper Paleozoic and Mesozoic sequences. His observation that the mean number of rhythms per set lies between 5 and 6 suggests that the orbital parameters may have changed. At least two of Gilbert’s four Cretaceous sequences of Colorado are bundled in this fashion. Thus, Gilbert’s suggestion that bedding rhythms provide a basis for geochronology takes on new interest—not to compete with radiometry in the rough calibration of Earth history, but as a refinement. It may also provide a means of tracing the evolution of the Earth’s orbital behavior.