Giovanni Napoleone

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.

Paleogene magnetic stratigraphy in Umbrian pelagic carbonate rocks: The Contessa sections, Gubbio

The Umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between Paleogene biostratigraphy and geomagnetic polarity history. The red-to-pink, Paleocene to middle Eocene Scaglia Rossa limestone, the varicolored upper Eocene Scaglia Variegata limestone, and the gray-green Oligocene Scaglia Cinerea marlstone form a 250-m-thick, continuous exposure in the Contessa Valley near Gubbio, Italy. Magnetostratigraphic, lithostratigraphic, and biostratigraphic investigations in three sections covering the entire Paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period. The ferromagnetic mineral in the Scaglia Cinerea is magnetite and AF demagnetization to peak fields of 20 mT is sufficient to define the characteristic remanent magnetization. The Scaglia Variegata and pink Scaglia Rossa samples contain an additional hematite component which is very pronounced in dark red Paleocene Scaglia Rossa samples. Changes of magnetic mineralogy take place in these limestones during heating, especially above 500 °C. However, thermal demagnetization is effective in isolating the characteristic remanence vectors, which form almost antipodal clusters of directions representing normal and reversed polarities. The directions are rotated counterclockwise, partly due to post-Oligocene tectonism. Paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation, but the major epoch and stage 29, as in the Gubbio Bottaccione section; Paleocene-Eocene, just above anomaly 25; Eocene-Oligocene, between anomalies 13 and 15; Oligocene-Miocene, just below anomaly 6C. These correlations require slight modifications to previous conclusions on Paleogene sea-floor spreading rates.

A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea

One of the most contentious topics in the study of human evolution is that of the time, place and mode of origin of Homo sapiens. The discovery in the Northern Danakil (Afar) Depression, Eritrea, of a well-preserved Homo cranium with a mixture of characters typical of H. erectus and H. sapiens contributes significantly to this debate. The cranium was found in a succession of fluvio-deltaic and lacustrine deposits and is associated with a rich mammalian fauna of early to early-middle Pleistocene age. A magnetostratigraphic survey indicates two reversed and two normal magnetozones. The layer in which the cranium was found is near the top of the lower normal magnetozone, which is identified as the Jaramillo subchron. Consequently, the human remains can be dated at ∼1 million years before present.

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-----------------------

Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy IV. Upper Maastrichtian-Paleocene Magnetic Stratigraphy

Pelagic limestones ranging in age from mid-Maastrichtian through Paleocene yield several well-defined magnetic polarity zones. These zones are tied to the planktic foraminiferal zonation at this site and are correlated with magnetic anomalies 26 through 31.

The Eocene-Oligocene Boundary in the Umbrian Pelagic Sequences, Italy

Publisher Summary Detailed biostratigraphic and paleomagnetic studies on Eocene–Oligocene pelagic sequences from Umbria (Central Italy) revealed that deposition across the Eocene–Oligocene boundary was continuous and undisturbed in that area. This chapter presents the data from eight stratigraphic sequences spanning the interval from the Early to Middle Eocene boundary through the Early Oligocene with specific emphasis on the events at the Eocene–Oligocene boundary. To have a complete biostratigraphic and paleomagnetic record, the Contessa highway section was sampled upwards into the northwestern quarry—that mirrors the Contessa quarry of Lowrie—where the Eocene–Oligocene boundary is undisturbed. Lithologically, the scaglia formation is very monotonous except for the major color changes. Visual correlations among sections are limited to minor changes that are not easily detected. Some of these changes, however, appear to be of larger (not local) significance.

Cesi, an early Middle Pleistocene site in the Colfiorito Basin (Umbro-Marchean Apennine), central Italy

Near the village of Cesi, at the head of the Chienti River, in the Colfiorito Basin (Umbro-Marchean Apennines, central Italy), fluvio-lacustrine deposits have yielded mammal fossil remains. The results of a multidisciplinary investigation indicate that the vertebrate-bearing sediments date about 700 ka and accordingly provide a fossil assemblage for the Middle–Late Galerian. Palynological investigations carried out from sediments underlying the fossiliferous level suggest predominantly cold and dry conditions, whereas the fauna suggests a slight climatic amelioration towards cool and moist conditions in the uppermost part of the sequence.

MAGNETOSTRATIGRAPHY OF THE HOMO -BEARING PLEISTOCENE DANDIERO BASIN (DANAKIL DEPRESSION, ERITREA)

Four magnetozones have been found in the 530 m thick profile of the Dandiero Group. The lower unit, the Bukra Sand and Gravel, extends in the R1 reversed magnetozone from 150 m below the tephra level which was used as the reference marker between the sampled sections. The normal magnetozone N1 is almost completely covered by the lacustrine and deltaic sediments of the Alat Formation, while the following reversed magnetozone contains both the Wara Sand and Gravel and the lacustrine Goreya Fm. The N2 polarity zone is completely occupied by the Aro Sand. This polarity sequence has been calibrated to the geomagnetic time scale using the Early to Middle Pleistocene age of the associated vertebrate fauna and fission-track dating. The four magnetozones were thus regarded as representing the chrons by which the Pleistocene is correlated with magnetochronology. Their three reversal boundaries provided the dates of 1.07, 0.99 and 0.78 Ma, allowing to determine average sedimentation rates close to 1 m/ky. Cyclostratigraphy of the magnetic signal, analysed by the spectral analysis of the time series across the Jaramillo and late Matuyama chrons, confirmed that value. The evidenced cyclicities were directly related to the alternating lithofacies, and both to the astronomical parameters driving the climate changes during the deposition of the Dandiero group (some five hundred thousand years). The section with the Homo site covers the Jaramillo/Matuyama boundary, and the Homo bed located 2 m below this limit is dated 0.992 Ma.

PALEOMAGNETIC AND PALYNOLOGIC INVESTIGATIONS IN THE UPPER VALDARNO BASIN (CENTRAL ITALY): CALIBRATION OF AN EARLY VILLAFRANCHIAN FAUNA

The silty clays embedding an early Villafranchian mammal fauna of the Triversa faunal unit (f.u.) have been paleomagnetically and palynologically studied in a continuous sequence exposed in the Santa Barbara quarry.The study has allowed to date the earliest occurrence in Italy of a faunal assemblage of this unit and to define the corresponding climatic conditions. The sampled section has provided a magnetic polarity sequence of the late Gauss,where the fossiliferous layer fits the Kaena reversed interval.Its age of ca. 3.1 Ma,during subtropical climate conditions correlatable to the Reuverian of Netherlands, suggests an older age for the beginning of the Villafranchian, possibly associated to a more dramatic scenario able to trigger the faunal turnover.