Discoveries of Chicxulub impact ejecta of the Albion Formation in road cuts and quarries in southern Quintana Roo, México and Belize, broaden our understanding of ejecta depositional processes in large impacts. There are numerous new exposures of ejecta near the Río Hondo in Quintana Roo México, located at distances of 330–350 km from the center of the Chicxulub crater. A single ejecta exposure was discovered near Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed of two lithostratigraphic units: the spheroid bed and diamictite bed, originally identified at Albion Island, Belize. The new spheroid bed exposures range from 2 to 5 m thick and are composed of altered glass fragments, accretionary lapilli, and pebble-sized carbonate clasts in a fine-grained calcite matrix. The base of the spheroid bed is exposed at Ramonal South in México and at Albion Island and Armenia in Belize, and at all three locations, the spheroid bed was deposited on a weathered karst land surface that had emerged in the Late Cretaceous. The new diamictite bed exposures are composed of altered glass fragments and carbonate clasts up to 9.0 × 3.2 m in size. In all but one of the new exposures, the diamictite bed extends to the surface with observed thicknesses up to 8 m. At Agua Dulce in México, the weathered top of the diamictite bed is overlain by thin-bedded Tertiary carbonates. No diamictite bed is found in Armenia, where the spheroid bed is overlain with a limestone conglomerate containing altered glass shards and shocked quartz. These discoveries indicate that ejecta are emplaced in large terrestrial impacts by at least two distinct flows: (1) an initial flow involving a volatile-rich cloud of fine debris similar to a volcanic pyroclastic flow, which extends >4.7 crater radii (the spheroid bed), and (2) a later flow of coarse debris that may not extend much beyond 3.6 crater radii (the diamictite bed). The former deposit we attribute to material entrained in the impact vapor plume, and the latter to the turbulent collapse of the ejecta curtain.

Abstract Geological interest in Earth’s orbital variations harks back to the discovery of the Pleistocene ice ages in the 1840 by Louis Agassiz, who convinced numerous prominent geologists that the “drift” that covered much of northern Europe was not a relict of the biblical deluge but of a great ice sheet ( Imbrie and Imbrie, 1979 ). What had caused this to happen?

Abstract This work presents a detailed 87 Sr/ 86 Sr isotope curve for the 7.5–9.8 Ma time interval obtained by analyzing isotope compositions of the Orbulina universa planktonic foraminifer species from the Mediterranean Gibliscemi section (southern Sicily). The available astronomical tuning of the section provided the opportunity to assess a direct control of the Milankovitch climate cyclicity on the seawater Sr isotope changes. Results of spectral analysis suggest a linear forcing of the 400 ky eccentricity component on the 87 Sr/ 86 Sr ratios of the Mediterranean seawater. The recorded amplitude of Sr isotope 400 ky cycles ranges between ± 5.5 × 10 -5 and ± 6 × 10 -5 around the long-term trend for the Tortonian at global scale. Such a ∆ 87 Sr is of the same order of magnitude of that measured by Capo and DePaolo (1990) and Dia et al. (1991) for the Pleistocene 100 ky glacial-interglacial cycles and about two times larger than that reported for site 758 by Clemens et al. (1993) for the same periodic oscillations. Mass-balance calculations applied to our dataset suggest that periodic changes of about 100–150%in the riverine inputs can account for the amplitude oscillations of 87 Sr/ 86 Sr ratios recorded in the Mediterranean during the Tortonian,thus emphasizing the high potential of this basin as good recorder of climate-induced seawater Sr isotope changes.

Abstract High-resolution cyclostratigraphy and calcareous plankton astrobiochronology have been obtained from the latest Langhian to the earliest Tortonian of the Mediterranean. The investigated areas (Malta, Tremiti, and Sicily) are located in different geological settings, and the three studied sections show different cyclicity. Direct correlation between the Laskar 90 (1.1) solution of the insolation curve and the sedimentary cycle pattern occurring in the investigated sections showed that all the sedimentary cycles are forced dominantly by Milankovitch periodicity. This forcing is also reflected in the climate-sensitive data (CaCO 3 content, and the relative abundance of the planktonic foraminifer Globigerinoides ) as shown by the application of spectral and filtering analyses. The calibration provided astronomical ages for all the sedimentary cycles and bioevents recorded in the sections. In particular, an age of 13.59 Ma was obtained for the extinction level of Sphenolithus heteromorphus , which is the best event approximating the Langhian-Serravallian boundary and an age of 10.55 Ma for the first regular occurrence of Neogloboquadrina acostaensis , the event that better approximates the Serravallian-Tortonian boundary in the Tortonian type section.

Abstract Methodology of cyclostratigraphic analysis applied to benthic foraminifera is verified utilizing a faunal and geochemical dataset,from the Middle Miocene Ras il-Pellegrin composite section (Malta Island, central Mediterranean). Benthic data were elaborated by Q-mode varimax principal factor analysis. In this paper, spectral analysis is carried out only on two factors, which have a clear paleoecological significance: Factor 1 (loaded by Cibicidoides ungerianus and Siphonina reticulata ), indicative of oxic bottom waters, and Factor 2 (loaded by Bulimina elongata group), indicative of oxygen-stressed conditions. Results of these analyses show that Factor 1 and Factor 2 curves are,respectively, in and out of phase with maxima of the eccentricity (100 d 400 ky). We utilize the 3D paleocenographic model of Bellanca et al. (2002) as reference for the Middle Miocene Mediterranean circulation and focus our attention on the Mediterranean Intermediate water, characterized by hydrographic and hydrodynamic features similar to those presently recorded in the Levantine Intermediate Water (LIW). Consequently, we suppose that Middle Miocene Mediterranean Intermediate Water, here defined as proto-MIW, played a role similar to that of present Mediterranean Intermediate Water (MIW). Following this hypothesis, Factor 1, which is indicative of oxic bottom waters, is interpreted as a tracer of high production of proto-MIW, during periods of high eccentricity and, probably, precession minima, characterized by coldest winter seasons. These results point out a direct link between selected benthic species, long-term astronomical forcing, and deep-water response and provide a useful tool for astronomical calibration of geologic time and for paleoceanographic reconstructions.

Abstract The mid-Cretaceous (Albian) deep-water sediments (coccolith-globigerinacean marls) of the Umbria-Marche Apennines show complex rhythmic bedding. We integrated earlier work with a time-series study of a digitized and image-processed photographic log of the Piobbico core. A drab facies is viewed as recording normal stratified conditions, and a red facies as the product of downwelling warm saline (halothermal) waters. Both are pervaded by orbital (Milankovitch) rhythms. These reflect fluctuations in the composition and abundance of the calcareous plankton in the upper waters. The drab facies is overprinted by redox oscillations on the bottom, including episodic precessional anaerobic pulses (PAPs). Contrasts between the individual beds representing the alternate phases of the precessional rhythm rose and fell with orbital eccentricity, in the classical pattern of Berger’s climatic precession or precession index curve, varyingly complicated by the obliquity rhythm. We conclude that greenhouse oceans in general, and perhaps this area in particular, were very sensitive to orbital forcing. Our count of 29 406-ky eccentricity cycles yields an Albian duration of 11.8 ± 0.4 My.

Abstract Zircon dates and orbital interpretation of bedding rhythms have yielded very different estimates on the duration of Middle Triassi stages. Recently, a core was drilled in Middle Triassic basin sediments at Seceda (western Dolomites) to directly compare cyclostratigraphy with geochronologic data. Detailed study of facies, sediment sources, and transport mechanisms formed the basis of the statistical analysis of bedding rhythms that are based on a grayscale scan and a gamma-ray well log. Amplitude spectrograms reveal strong frequency components at f = 0.025 cycles/cm in the main nodular limestone interval (92–64 m core depth), corresponding to the dominant 40 cm bedding thickness. Significant spectral differences were found between the grayscale and gamma-ray bedding proxies, placing doubt on the appropriateness of the use of the latter as an effective tool in cyclostratigraphy. In the uppermost part of the succession (59–45 m core depth) calciturbidites constitute more than 50% of the rock volume. If turbidites and tuffs are removed from the rock column, the spectrogram in this interval becomes much smoother and significant peaks appear at higher frequencies. The signals of this pelagic background sedimentation were extracted by bandpass filtering and show strong similarities to Milankovitch cycles in the Quaternary. According to this cyclostrati-graphic interpretation, the dominant 40 cm bedding rhythm was produced by eccentricity, and the average sedimentation rate results in ~3.6 mm/ky. This estimate is in contrast to zircon data from volcaniclastic layers that bracket this core interval and suggest a sedimentation rate of 13.5 mm/ky. As it currently stands, neither of the two interpretations is yet fully satisfactory. Although the presence of orbital variations in the Triassic analogous to those predicted for the last 20 My remains questionable owing the presumed chaotic behavior of the planets, the zircon age data have uncertainties related to their origin that remain unaccounted for and require further investigation.

Abstract Detailed sedimentological and carbon-isotope data have allowed us to propose a high-resolution regional correlation between four Lower Cretaceous carbonate-platform successions, Early Aptian to Early Albian in age, cropping out in the southern Apennines. These successions, formed in open to restricted lagoonal and peritidal-supratidal settings, reveal a high-frequency cyclic recurrence of depositional and early meteoric (karstic and/or pedogenetic) features. The latter are normally superimposed on subtidal deposits, suggesting that the above cyclicity may be linked to sea-level changes. In the studied sections, elementary cycles are grouped into bundles, which in turn are grouped into superbundles. Although bundles and superbundles appear to be related to the Earth’s orbital short-eccentricity and long-eccentricity signal, respectively, elementary cycles seem to record either the precession or a combination of the precession and obliquity periodicity. Moreover, the stacking pattern of the orbitally controlled cycles suggests that they are superimposed on lower-frequency sea-level fluctuations (transgressive-regressive facies trends). The δC curves, established throughout the sections, show the same carbon-isotope pattern as the time-equivalent pelagic strata (two positive carbon-isotope episodes separated by an interval with lower carbon-isotope values). On the basis of this correspondence, and integrating the cyclostratigraphy and the carbon-isotope stratigraphy with the sequence stratigraphy, we propose a high-precision regional correlation and a chronostratigraphic chart and suggest a duration of 7.8 My for the studied interval. Moreover, on the basis of sequence stratigraphy and isotope geochemical criteria, and using our orbital chronostratigraphy as a reference frame, a correlation with current global scales is here proposed.

Abstract A detailed carbon-isotope stratigraphy has been generated from Barremian to Lower Aptian shallow-water carbonate sections in the Campania Apennines (Monte Raggeto, southern Italy). The new isotope curve is correlated with the magnetostratigraphically and biostratigraphically dated pelagic carbon-isotope stratigraphy from the Cismon locality (Southern Alps, northern Italy). All the major positive and negative carbon-isotope excursions that characterize the Barremian and Early Aptian carbon-isotope stratigraphy can be recognized in the shallow-water curve. Cyclostratigraphy, which was established earlier at the Monte Raggeto section, is used as an age calibration tool for the Barremian and Early Aptian isotope stratigraphy. The duration of the isotopically calibrated stratigraphic interval between the top of Chron M3 and the base of Chron M0 is estimated as 4 My. These time calculations are in good agreement with cyclostratigraphic data from the Cismon locality but differ from estimates based on a magnetic anomaly block model for the interval between M3 and M0 that yield only 3 My. We have also calculated that the Selli Level Equivalent (SLE) at the Monte Raggetto locality was deposited within 1.2 My. Our results demonstrate that the combination of chemostratigraphic and cyclostratigraphic studies can contribute significantly to the calibration of the Mesozoic time scale.

Abstract The Berriasian Pierre-Châtel Formation in the Swiss and French Jura Mountains is dominated by shallow-marine carbonates tha overlie lacustrine and marginal-marine sediments with a major transgressive surface. Detailed facies analysis of five sections allows the definition of elementary and small-scale depositional sequences, which commonlexhibit deepening-shallowing trends. Benthic foraminifera and rare ammonites on the platform, as well as a sequence-stratigraphic correlation with a well-dated deeper-water section,furnish the biostratigraphic framework. Thus, the large-scale sequence boundaries below and at the top of the Pierre-Châtel Formation can be correlated with dated boundaries in other European basins. This time constraint and the hie archical stacking pattern on the platform as well as in thebasin suggest that the sea-level fluctuations influencing the formation of the depositional sequences were controlled,at least partly, by Milankovitch cycles. The elementary sequences correspond to the 20 ky precession cycle, and the small-scale sequences to the 100 ky eccentricity cycle. Uncertainties in the definition of sequences exist if facies contrasts are too low to develop clearly marked sequence boundaries or maximum-flooding intervals. Nevertheless, a best-fit solution for the correlation of the small-scale sequences between the studied sections can be proposed. Thelowermost three small-scale sequences of the Pierre-Châtel Formation are analyzed in detail. They are decompacted and correlated on the level of the elementary sequences. Within this relatively precise time frame, the flooding of the Jura platform (following the early Berriasian sea-level lowstand) can be monitored. It is seen that the transgression occurred stepwise: every 20 ky, atransgressive pulse established marine facies farther towards the platform interior. This study demonstrates that the cyclostratigraphical approach makes it possible to construct a narrow time frame, within which the rates of sedimentary, ecological, and diagenetic processes can be evaluated, phases of differential subsidence identified, and the durations of stratigraphic gaps estimated. The complex and dynamic evolution of an ancient carbonate platform can thus be studied with a time resolution of 20 to 100 ky.

Abstract On the basis of cyclostratigraphy and sequence stratigraphy criteria, an orbital chronostratigraphy is hereproposed for the Upper Valanginian-Lower Hauterivian (Lower Cretaceous) stratigraphic interval. For such a purpose two biostratigraphically constrainedcarbonate platform section were measured and sampled at centimeter scale in southern Italy: (1) San Lorenzello, Matese Mountains in theCampania Apennines, and (2) Sferracavallo, Palermo Mountains in northwestern Sicily. The former has a thickness of about 87 m,and thelatter totals about 40 m. Analysis of depositional and early diagenetic features has shown that both the successions are characterized byinternal cyclicity, consistent with orbital (Milankovitch) forcing. This high-frequency cyclicity, in which the elementary cycles areorganized in bundles and in groups of bundles (superbundles), appears to be superimposed on three longer (from ~ 800 ky to ~ 1200 km transgressive-regressive facies trends (T∕RFTs). Whereas the elementary cycles record the precession and∕or the obliquity signal (or a combination of them), the bundles and the superbundles correspond to the short- and long - eccentricity signals (~ 100 ky and 400 ky cycles), respectively. Sequencestratigraphic criteria, used to interpret the superbundles and the T∕RFTs in terms of depositional-sequence equivalents, allowed us to propose chronostratigraphic diagrams of the studied intervals and to attempt regional- to global-scale physical correlations. At regional scale, the Sferracavallo section was correlated, bundle by bundle, with the time-correspondent segment of the San Lorenzello section. The chronostrat igraphic correlation shows that only one main gap, calculated as about 200 ky, occurs in the studied intervals. This allows us to assume that the sedimentary record can be considered quasi -continuous, at least at the superbundle scale (~ 400 ky).The assembled orbital chronostratigraphy suggests that the time recorded in the San Lorenzello and in the Sferracavallo sections is 2.9 and 1.9 My, respectively. At global scale, a physical correlation is proposed with the time-equivalent third-order sequences of Haq et al. 1987 and Jacquin et al. (1998) , taking into account the stratigraphic position of the Valanginian-Hauterivian (Early Cretaceous) boundary in the standard time scales, as well as in our orbital chronostratigraphy. This correlation shows that there is a good agreement between the 2.6 My time duration of the stratigraphic interval spanning from the Valanginian 4 to the Hauterivian 1 sequence boundaries (Jacquin et al., 1998) and the 2.8 My interval estimated on the basis of our orbital chronostratigraphy.

Abstract A 160-m-long section measured in the lagoonal facies of the Middle Triassic Latemar platform (Dolomites, Italy) reveals a set of frequency components that we interpret as a strong Milankovitch signal. In this interpretation, all principal frequencies associated with the theoretical Middle Triassic precession index, P1 = 1/(21.7 ky), P2 = 1/(17.6 ky), and its modulations, E1 = 1/(400 ky), E2 = 1/(95 ky),and E3 = 1/(125 ky), were detected in a time-frequency evaluation of the cycles. A weak obliquity signal is also present in part ofthe section.Thus, the Latemar cycles appear to have recorded the clearest orbital forcing signal yet found in a carbonate platform. This astronomical calibration indicates that the section was deposited in ca. 3.1 My and therefore that the entire Latemar cyclic succession (~470 m) took at least 9 My to form. However, the calibration also leads to serious conflicts with other interpreted geological data: U/Pb radiometric ages of zircons collected from tuffites within theLatemar lagoon and in coeval basinal sediments point to a timescale that is five times shorter than this astronomically calibrated estimate; similar discrepancies arise when the average duration of Triassic ammonoid biozones or the sedimentation rates of coeval basinal series are considered. Nonetheless,all of the methods that have been used to estimate the time of formation of the Latemar platform continue to have shortcomings, and the contradictions among these different geologicalcalibrations remain unresolved.

Abstract An integrated cyclostratigraphic approach was applied to the 460-m-thick succession in the Latemar platform interior. The approach uses new high-resolution cyclostratigraphic data from vertical sections, lateral tracing of physical surfaces over the platform top, new and existing biostratigraphic data, existing isotopic ages from volcanic ash layers, and new spectral analyses in order to develop a genetic cyclostratigraphic model. Hierarchical cycles include meter-scale shallowing-upward microcycles and 2–6 bundled thinning-upward macrocycles. Lateral tracing and correlation of microcycles and macrocycles provides a high-resolution 2D architectural model of the platform interior. The large majority of microcycles and macrocycles is physically persistent over the platform top with only moderate changes in thickness and internal facies. The platform top showed simultaneous vertical aggradation controlled by low-amplitude, high-frequency sea-level changes. Tied-in cyclostratigraphic and biochronostratigraphic data indicate that the 619–701 microcycles in the platform interior include little more than a single ammonoid biozone ( Secedensis Zone), that the total time interval is shorter than 4.10 Ma(average total time interval =1.88 My), and that the interpolated microcycle period is shorter than 5.85 ky (average interpolated microcycle period = 2.68 ky). Microcycles cannot be reconciled with precession forcing but reflect sub-Milankovitch forcing. Spectral analysis is based exclusively on accommodation cycles, which represent the only direct indication of external control on cyclic deposition. Blackman-Tukey spectral, multi-taper spectral, and harmonic analyses indicate highly similar and significant frequencies and amplitudes which are largely stationary over all subsets applied to the cyclic series. Ratios and periods indicative of orbital forcing in the Milankovitch band potentially exist at (very) high significance points with Δ t = 4.2 ky. In the Latemar cyclic succession, basic microcycles represent sub-Milankovitch forcing (4.2 ky), thinning-upward, 2–6 bundled macrocycles short- and long-precession forcing (18, 21 ky), and higher-order cycle bundles short and long obliquity (35, 45 ky) as well as short-eccentricity forcing (95–105 ky). Because of significant latitudinal temperature gradient and seasonal climate differences, the Triassic period held a significant potential for sub-Milankovitch fluctuations in coupled ocean-atmosphere circulation. They probably triggered low-amplitude, high-frequency changes in sea level and controlled the deposition of sub-Milankovitch microcycles. Previous studies of the Latemar carbonate platform favored a model-dependent approach based on smaller cyclostratigraphic datasets from single sections and spectral analyses. The resulting orbital-forcing models could not be reconciled with the existing biochronostratigraphic framework for the Triassic and the Anisian to Ladinian stages. They left a widely noted deep disagreement between biochronostratigraphic and cyclostratigraphic time scales. In contrast, the new forcing model of this study is based on a complete 2D cyclostratigraphic dataset, considers all biochronostratigraphic constraints, and includes time-calibrated spectral analyses. The model reconciles biochronostratigraphic and cyclostratigraphic time scales. The Latemar cyclic series includes the oldest explicit sub-Milankovitch signal and the oldest set of both sub-Milankovitch and Milankovitch signals yet observed in the geologic record.

Abstract The Salento continental shelf of Apulia (southern Adriatic) shows a complex stratigraphic architecture of Pleistocene prograding wedges that records the effects of the interplay of sediment accumulation, glacio-eustatic sea level changes, and regional tectonics. The interpretation of high-resolution seismic reflection profiles reveals three ravinement surfaces, extending landwards from the shelf break and showing low gradients. They have been considered to be stratigraphic markers and tentatively correlated to the oxygen curves of the isotopic stratigraphy. Even if direct dating of seismic sequences and corresponding unconformities is lacking, this correlation is well supported by the lateral extent and continuity of the ravinement surfaces, recognized in the whole investigated area. Similar qualitative correlation of unconformities with the curves of oxygen-isotope stratigraphy has been already used by other authors in different case histories of continental shelves of both active and passive margins. Moreover, in the study area the regional geological framework suggests that during the Middle Pleistocene the rate of tectonic uplift interacted with the rateof glacio-eustatic fluctuations, producing the deposition of forced-regression systems tracts. The forced-regression prograding wedge that enlarged the shelf by about fifteen kilometers is here dated as Middle Pleistocene. Starting fromthe upper part of the Middle Pleistocene, the stratigraphic architecture of the prograding wedges was controlled mainly by glacio-eustatic sea-level changes forced by short-eccentricity cycles. This is suggested by the stratigraphic architecture of the last two prograding wedges, interpreted as incomplete fourth-order depositional sequences and consisting of forced-regression, lowstand, and transgressive systems tracts. The eustatic signal as an expression of the Earth’s orbital cyclicity (short eccentricity) appears over whelming with respect to the tectonic one, and its prominence suggests a decrease in the rate of uplift of the Apulian foreland during the last 250 ky.

Abstract Petrophysical datasets and related spectral analysis from Plio-Pleistocene sediments cored on the Western Antarctic continental rise during the Ocean Drilling Program, Leg 178, are discussed. It is shown that in different cores, nonharmonic wavelength peaks, when normalized, exhibit a very high correlation factor with predicted Earth’s orbital variations. It is also found that both short (~ 95–125 ky)and long (~ 400 ky) eccentricity periodicities emerge clearly from the signal during the whole Pleistocene, without an evident switch to obliquity at mid-Pleistocene (~ 0.9 Ma), as reported in the literature. This suggests that the lithological parameters, a proxy for glacial cycles, are controlled, directly or indirectly, by astronomically forced processes (Milankovitch cycles). Moreover, the good correlatability among distant coring sites, based on systematic sedimentological variations at intervals of about 140 and 370 ky, allows extension of the results to regional scale.

Abstract The Purbeck Group (Berriasian) in Dorset, England, is divisible into meter-scale rock cycles that are in turn bundled into a multi-tiered hierarchy of cycle sets (sequences). These cycles and sequences are the product of “Croll-Milankovitch” orbital forcing, by which the smallest-scale cycles (sixth order) are the product of precession and three orders of cycle bundles (sequences) are produced by modulation of precession by variation in the degree of eccentricity of the Earth’s orbit. The Purbeckian at the thickest section in Durlston Bay (over 100 m of marginal marine, brackish, and freshwater facies and paleosols) is divisible into four third-order (2 My) sequences that in turn comprise fourth-order (400 ky) and fifth-order (100 ky) sequences. All levels of this hierarchy are recognized by asymmetry in their patterns of facies distribution. Larger facies changes occur at the bases of cycles lower in sequences whereas smaller facies changes occur at cycle boundaries higher in sequences. In the Purbeck Group coarse-grained skeletal limestone is the dominant facies at the base of cycles and low in cyclic sequences whereas fine carbonates, shale, and paleosols characterize the upper parts of cycles and sequences. Clements ( 1969 , 1993 ) described the biofacies and lithofacies of 245 beds that constitute the Purbeck Group at Durlston Bay. Morter (1984) described nine molluscan salinity (~ depth) zone assemblages and used them to interpret patterns of sea-level rise and fall through the Middle Purbeck stratigraphic interval. Anderson (1973) divided the Purbeck of southern England into four ostracod assemblage zones and later (1985) into 40 ostracod faunicycles. Each faunicycle consists of a lower more-marine S-phase and an upper more-fresh-water C-phase assemblage of ostracods. These faunicycles are traceable laterally, and Anderson (1985) suggested that they might be the products of salinity fluctuations. Using Clements’ (1993) beds as markers in association with the ostracod assemblage zones it is possible to integrate Anderson’s ostracod faunicycles and Morter’s salinity-based molluscan assemblages with the lithologically determined four-tiered hierarchy of allocycles developed in this paper. Anderson’s faunicycles (1985) appear to coincide closely with fifth-order (100 ky) sequences, and the transgressive events interpreted by Morter (1984) occur at either the first or second fifth-order boundaries in fourth-order sequences. This independent paleobiological evidence corroborates the interpretation of a cyclic hierarchy determined by lithofacies analysis on the basis of assumptions of orbital forcing. In turn, recognition of the orbitally forced hierarchy of rock cycles provides an explanation for the published patterns of repeated (cyclic) faunal change.

Abstract Limnic deposits in Plio-Pleistocene graben basins in the Apennine Mountains provide detailed records of magnetostratigraphy leading into the Pleistocene. Their lacustrine clays retain both pollen that record the vegetation and, in magnetic signals, the history of magnetic reversals and a record of the orbital variations. The magnetically defined precessional units, with mean thicknesses of 3–8 m depending on accumulation rate, are modulated in amplitude by the eccentricity cycles, and are commonly associated with (and at times overshadowed by) obliquity cycles. The magnetic stratigraphy of the Upper Valdarno and Valtiberina basins serves to trace these cyclicities through the late Gauss and early Matuyama chrons, and the Olduvai. These cyclic rec rds set a chronologic framework for the history of both basins. Varied accumulation rates, beginning of oscillations between moister and drier plant assemblages, and a rise in the strengt of the obliquity signal coincide with the 400 ky eccentricity low at 2.84 Ma.