ABSTRACT The Cretaceous and Paleogene pelagic limestone and marl formations of the Umbria-Marche Apennines of north-central Italy have proven to be exceptional recorders of the history of Earth and of life on Earth, and they have been the subject of numerous geological and paleontological studies over the last several decades. Founded a quarter century ago, in 1992, the Geological Observatory of Coldigioco is a research and teaching center focused on these exceptional rocks. This chapter is a historical introduction that briefly reviews the highlights of the lithologic, biostratigraphic, sedimentologic, magnetostratigraphic, impact-stratigraphic, geochemical, geochronological, time-scale, and cyclostratigraphical research done on the Umbria-Marche stratigraphic sequence, much of it facilitated by the Geological Observatory of Coldigioco. This review covers work up to the Coldigioco 25th anniversary Penrose conference in September 2017; it does not treat work presented at that conference or done since then. A remarkable irony is that a century ago, the Umbria-Marche Cretaceous–Paleogene sequence was so difficult to date that early work contained an error of ~35 m.y., but now there is a reasonable hope that this entire section may eventually be dated to an accuracy and precision of ~10,000 yr. This review begins with an homage to the little medieval city of Gubbio, its wild Festa dei Ceri, and its Bottaccione Gorge, where much of the research described here has been done. The review ends with three points of perspective. The first is the notion that sometimes geology can be done by looking up at the sky, and astronomy can be done by looking down at Earth, with much of the Coldigioco-based research being of this latter kind. The second is the observation that geology and paleontology are contributing far more new information to Big History—to our integrated knowledge of the past—than any other historical field in the humanities or sciences. The third is that three of the major scientific revolutions of geology in the twentieth century have direct connections to the Umbria-Marche stratigraphic sequence—the turbidite revolution, the development of plate tectonics, and the downfall of strict uniformitarianism.

ABSTRACT The Franciscan subduction complex formed over a protracted, ~150 m.y. period, during Late Jurassic to late Cenozoic subduction of oceanic lithosphere beneath the western margin of the North American continent. Growth of the complex occurred chiefly by progressive accretion, in which voluminous sediment was eroded from the magmatic arc and continent, deposited in the trench region, and then progressively subducted and accreted soon after deposition. The Yolla Bolly terrane, a major Franciscan subunit, has stood out as a possible exception to a progressive-accretion model. Yolla Bolly clastic rocks are almost barren of fossils, but there are ~13 localities with Late Jurassic and Early Cretaceous Buchia specimens, ~3 with mid-Cretaceous ammonites or Inoceramus , plus several with mid-Cretaceous youngest detrital-zircon populations. These ages had suggested that sediments may not have been deposited into an active trench, but instead were deposited into a relatively stable Yolla Bolly basin, which was both long-lived (ca. 150 Ma through ca. 95 Ma) and far-traveled (exotic). This basin was then accreted and metamorphosed at perhaps 92 Ma. It is surprising, however, that such a basin could have survived for ~50 m.y. along a subduction margin before being accreted. We determined detrital-zircon U-Pb ages from 31 new sandstone samples, including from key Buchia sites, and they indicate that Yolla Bolly clastic deposition actually occurred almost entirely between ca. 115 and 98 Ma. All of the Buchia specimens in the main parts of the Yolla Bolly terrane have been redeposited and the arc- and continent-sourced clastic rocks that comprise almost all of the terrane are much younger than once thought. This makes evolution of the Yolla Bolly terrane compatible with a progressive-accretion model, in which its constituent packets of clastic rocks were deposited in a native trench setting and then rapidly subducted, accreted, and metamorphosed.