Alfred G. Fischer

Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy V. Type section for the Late Cretaceous-Paleocene geomagnetic reversal time scale

A biostratigraphically complete and well-exposed sequence of Upper Cretaceous-Paleocene pelagic limestones at Gubbio, Italy, has provided a record of geomagnetic polarity reversals that closely matches the sequence inferred from marine magnetic anomalies. Abundant foraminifera permit accurate dating of the sequence. Because of these favorable circumstances, the Gubbio locality is formally proposed as the magnetostratigraphic type section for the Upper Cretaceous and Paleocene.

Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy I. Lithostratigraphy and sedimentology

An essentially complete section of middle Cretaceous to Paleocene, calcareous, pelagic sediments is exposed at Gubbio, in the Umbrian Apennines of Italy. This, the first of five papers, presents the lithostratigraphic and sedimentological basis for subsequent discussions of biostratigraphy and magnetic stratigraphy. Biogenic coccolith-globigerinid oozes were deposited along with a constant but low background of fine terrigenous detritus at a mean sedimentation rate (compacted) of 6.6 m/m.y. from Albian to Eocene time; individual stages deviate from this by a factor of two. Short-term variations in carbonate input yielded a sequence of limestone beds punctuated by thin shale interbeds. These shale-limestone couplets accumulated in time spans between 20,000 and 100,000 yr, and the thin shales may represent up to one-half of the total time. Modification by diagenetic solution, tectonic shear, and weathering render shale interbeds unsuitable for magnetic work. The limestone beds, first bioturbated and then compacted and cemented during diagenesis, appear likely to have recorded magnetic history at intervals on the order of 50,000 yr, sufficient to provide a good record of polarity changes. Moderate to high states of oxidation prevailed except in Aptian-Albian and a portion of Turonian to Santonian times.

CLIMATIC OSCILLATIONS IN THE BIOSPHERE

The history of the biosphere is a peculiar mixture of patterned and random processes, which are not easily untangled. Three sets of patterned processes are here discussed: A major climatic cycle with a period (?) of around 300 m.y.; an intermediate one in the 30 m.y. range; and a family of cycles in the 10,000 to 100,000 year domain. The long cycle is here interpreted as one in atmospheric carbon dioxide pressure, leading to an alternation of “greenhouse” and “icehouse” states. The origin is attributed to changes in the patterns and vigor of mantle convection, and two related phenomena: volcanism and fluctuations in sea level. The causes of the 30 m.y. rhythm remain enigmatic. The 10,000-100,000 year rhythms seem to be related to the Earths orbital perturbations, as suggested long ago. These environmental fluctuations stress organic communities in various ways, and have been instrumental in steering their evolution.

Carbonate Lithification on the Sea Floor

Indurated Globigerina-rich sediments have been recorded from more than thirty localities on sea floors around the world, between depths of 200 and 3,500 m. Many are stained by iron and manganese minerals or are associated with manganese nodules or crusts, suggesting that such lithification occurs along profiles of sedimentational equilibrium. Samples from the eastern Mediterranean and from off Barbados, here described and illustrated with photomicrographs and electron micrographs, contain high-magnesian calcite and dolomite, and show recrystallization of the micritic matrix, as well as calcitic cavity fillings. We conclude, contrary to widespread opinion, that calcite can be precipitated chemically in seawater, and that carbonate sediments can become lithified on the sea floor.

Enso and Sunspot Cycles in Varved Eocene Oil Shales from Image Analysis

ABSTRACT On the premise that sequential changes in varve thickness offer a proxy for climatic variations, we investigated varve thickness in three core segments from the distal lacustrine oil shales (Tipton and Laney members) of the Green River Formation, by means of an image analysis program. Of two strong bimodal periodicities one, at 4.8-5.6 years, is interpreted as an El Nino type (ENSO) phenomenon of atmospheric dynamics, while the other, at 10.4-14.7 years, is interpreted as the sunspot cycle, originally recognized in this formation by Bradley (1929, 1931). Weaker periodicities may exist at ca. 8 and 33 years--the latter also recognized by Bradley. Taken in conjunction with the work of Bradley (1929, 1931) and of Crowley et al. (1986), this suggests that some but not all of the oil shale of the Green River Formation is truly varved and can be used to infer climatic time-series. Automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time, but careful selection of facies in their geological framework is necessary.

Cyclicity in the Green River Formation (lacustrine Eocene) of Wyoming

ABSTRACT Basinal facies of the Green River Formation have two main modes, lacustrine and playa. The lacustrine mode (Tipton and Laney members) accumulated mainly varved oil shale. Here annual cycles are recorded as varves. Variations in verve thickness demonstrate El Nino (ENSO)-type and sunspot cycles (Ripepe et al., this volume). Milankovitch-scale cycles are not obvious in lithic variations, but gamma ray logs record 1) precessional variations with a mean period (varve-timed) of 19.5 ka, and 2) a bundling of these in the ca. 100 ka eccentricity cycle. In the playa mode (Wilkins Peak Member), the lithic succession oil shale-trona-dolomitic marlstone records the precessional drying up of a lake and is again bundled in sets of 5, by the 100 ka eccentricity rhythm. The Tipton Member persisted f r 450 ka, the Wilkins Peak Member for ca. 1 Ma.

Geological history of the western North Pacific

A considerable portion of the abyssal floor of the western North Pacific was already receiving pelagic sediment in late Jurassic time. Carbonate sediments were later replaced by abyssal clays as the basin deepened and bottom waters became more aggressive. The resulting facies boundary, which can be recognized on seismic profiles, is broadly transgressive; it ranges in age from mid-Cretaceous in the western Pacific to Oligocene in the central Pacific. Cherts are encountered at and below the major facies boundary and appear to have been formed by postdepositional processes.

Astronomical tuning of the Aptian Stage from Italian reference sections

A high-resolution grayscale series of the pelagic Fucoid Marls (Piobbico core, central Italy) shows strong, pervasive lithological rhythms throughout the Aptian interval. A hierarchy of centimeter- to meter-scale cycles characterizes the rhythms; when calibrating ~1 m cycles to Earth’s 405 k.y. orbital eccentricity cycle, these rhythms correspond to the periods of the eccentricity, obliquity, and precession index. Tuning to orbital eccentricity cycles provides a high-resolution time scale for the Aptian. Correlation to the Cismon core (northern Italy) extends the tuning to the Aptian-Barremian boundary. The tuning indicates a minimum duration of 13.42 m.y. for the Aptian Stage, where previous estimates range from 6.4 to 13.8 m.y. The combined Aptian–Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-m.y.-long astronomically calibrated time scale for Earth history.

Eocene magnetic stratigraphy at Gubbio, Italy, and its implications for Paleogene geochronology

The sequence of polarity zones established by Alvarez and others for the Cretaceous and Paleocene in the Scaglia Formation in the Bottaccione Gorge at Gubbio has now been extended through the uppermost Paleocene into the upper Eocene Globigerinatheka semiinvoluta zone (= anomaly 18). The relationships between the standard Paleogene foraminiferal zones and the sea-floor anomalies are defined, with the exception of the mid-Eocene zones of Morozovella lehneri , Orbulinoides beckmanni , and the base of the Truncorota-loides rohri zone, which are obscured by inadequate preservation and a fault gap. The base of the Eocene lies above anomaly 25; the lower to middle Eocene contact (base Lutetian) lies in the upper part of anomaly 22; the base of the Bartonian lies below the base of anomaly 18; and the base of the Priabonian (upper Eocene) lies within anomaly 18. Radiometric ages appear to diverge systematically from a geochronology based on steady sea-floor-spreading rates, suggesting that spreading may have been as much as a factor of 3 slower in the Paleocene and the late Eocene than in the early and middle Eocene.

Albian Pelagic Rhythms (Piobbico Core)

ABSTRACT The Piobbico core of Aptian-Albian pelagic rhythmites in Italy has been used to explore ways of extracting quantitative time-series of geological, chemical, physical and biological parameters from stratigraphic sequences. Recognition of the precession, obliquity, and eccentricity cycles (the Option frequencies) has permitted time-resolution to the 104-year level. Individually, these time-series curves furnish a potential basis for precise correlations. Collectively, they track the evolution of the depositional system in detail, including its response to orbital forcing. Orbital cycles primarily influenced oceanic fertility and carbonate productivity by foraminifera and coccoliths, as well as the aeration of bottom waters (redox cycle). Three short magnetic reversals occur w thin the Ticinella praeticinensis zone. Frequency spectra of variations in magnetic intensity, inclination and declination yield Milankovitch frequencies which are only partly explained as responses to mineralogy. A computer simulation used Bergers astronomically calculate precession index to drive 1) carbonate flux and 2) depth of bioturbation. This achieved a stratigraphy remarkably similar to that observed. Each step in the computer simulation transfers spectral power from precession to eccentricity. Work on the Piobbico core continues, while Project ALBICORE aims to explore the patterns of rhythmicity in the praeticinensis zone of other areas. End_Page 1164-----------------------