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.

Lithology, carbonate and organic carbon geochemistry, and molluscan macrofauna were investigated in fine-grained clay-dominated strata spanning the Cenomanian-Turonian Stage boundary in the southern Colorado Plateau region. Analyses of these data show basinal and onshore to offshore changes in sedimentologic and molluscan assemblage trends through the upper Cenomanian Sciponoceras gracile and Neocardioceras juddii and lower Turonian Watinoceras coloradoense Biozones. Diverse infaunal/epifaunal bivalve assemblages are best developed in the Sciponoceras gracile Biozone and at the more clay-rich nearshore sections. Gastropod and infaunal bivalve abundance and diversity decrease in an offshore and up-section direction as carbonate and organic carbon percentages generally increase. The Neocardioceras juddii Biozone precedes the stage boundary and is characterized in Utah and Arizona by pulsed increases in organic carbon and detrital-feeding gastropods, and by a shift in infaunal bivalve dominance toward species adapted for soft-substrate conditions. These data suggest that pulses of increased organic detrital resources and decreased substrate firmness were associated with the stage boundary extinctions. The stage boundary is marked by a change to depauperate epifaunal assemblages at western sections. At all sections, the base of the Watinoceras coloradoense Biozone is reflected by an abrupt increase in carbonate and decrease in organic carbon percentages as well as by the immigration of Mytiloides bivalves. Periods of progressive recovery through this biozone were associated with carbonate-rich depositional periods.

Abstract The Bridge Creek Limestone Member of the Greenhorn Formation records maximum eustatic rise of the Greenhorn Marine Cycle, and the maximum aerial extent reached by the Western Interior Seaway. Limestone-shale couplets reflecting Milankovitch-style climatic cycles, and abundant laterally persistent bentonites, within a highly refined biostratigraphic framework based primarily on rapidly evolving cosmopolitan molluscan taxa, allows the development of a very detailed system of event-isochron stratigraphy divided into intervals of 100,000 yr or less. The Bridge Creek Limestone Member can be divided into three informal subdivisions (lower, middle, and upper) based on overall lithologic characteristics and slope forming profile. These precisely correlate to the upper Hartland, Jetmore, and Pfeifer members in central Kansas and reflect changes in marine environments and sedimentation style corresponding to the periods just prior to, during, and after maximum transgression respectively. The development and preservation of cyclic sedimentation during deposition of the Bridge Creek Member is largely the result of slow rock accumulation rates of 0.5 to 1.0 cm/1000 years across the central Western Interior. The combination of widespread deposition, numerous chronostratigraphic marker-beds, and a highly refined regional and global biostratigraphy make the Bridge Creek Limestone Member a magnificent natural laboratory for detailed ecologic, evolutionary, biogeographic, and oceanographic studies.

Abstract The Cenomanian-Turonian boundary extinctions, as observed near Pueblo, Colorado, are represented by a series of events reflecting accelerated macrofaunal evolutionary and extinction rates. Four main events are recognized near Pueblo which are cumulatively responsible for 80% species-level and about 30% genera-level macrofaunal extinction. The two main extinction events near Pueblo occured about 250 and 200 Ka before the terminal extinctions at the stage boundary. In addition to the evolutionary and extinction related faunal turnover, fluctuations in the relative abundances of inoceramid bivalves, oysters and ammonites appear to be related to substrate and inferred habitat changes reflected by the climatically controlled lithologie cyclicity. The increased rates of evolution and extinction observed through the Cenomanian-Turonian boundary interval may have been driven by increased biotic stress resulting from eustatic highstand and increased climatic variability influencing temperature, salinity, and benthic oxygenation within the Western Interior seaway and globally.

Abstract The Pueblo-Canon City area provides an outstanding opportunity to study well exposed sections of the marine and marginal marine stratigraphie units which make up the Kiowa-Skull Creek (Late Albian), Greenhorn (latest Albian-Middle Turonian), and Niobrara (Late Turonian -Early Campanian) Cyclothems. Each of these cyclothems represents a third-order eustatic fluctuation that produced regionally synchronous transgression and regression in the Western Interior Seaway. Extensively studied exposures along the Canon City hogback and in the dissected Rock Canyon Anticline west of Pueblo are easily accessible and contain the most typical examples of the stratigraphie units in these cyclothems found in Colorado. Numerous, detailed and interdisciplinary studies of this Cretaceous section have been completed (see papers in this volume and references therein) which collectively allow the dynamics of large-scale Cretaceous eustatic cycles to be interpreted and modeled for epicontinental seas. In view of these factors, the Pueblo-Canon City area has become the standard mid-basin reference section for the Kiowa-Skull Creek, Greenhorn, and Niobrara Marine Cycles. The field trip will examine outcrops of each stratigraphie unit in the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems in the Pueblo-Canon City area. Field examination will focus on high-resolution stratigraphie concepts, facies characteristics, geochemical profiles, and inferred depositional environments. Discussions will emphasize a broad range of possible interpretations and current controversies regarding the depositional history of these cyclothems. The prevailing theme of this excursion will be cyclic sedimentation as expressed in Cretaceous marine and marginal marine facies of the Western Interior Seaway. Detailed discussion of each outcrop section can