ABSTRACT The Upper Ordovician Juniata Formation, Central Appalachian Basin, USA, is a thick succession of cyclically bedded arenites, wackes, and mudrocks. Sedimentary facies of the formation in West Virginia, Virginia, and Maryland indicate cyclic peritidal deposition along the northern shoreline of the basin. The subsurface Juniata Formation has been drilled throughout the basin, and long, continuous well logs are available for analysis of the cyclic deposition. A 2400-ft-long (731.52-m-long) gamma-ray (GR) log from the Preston 119 well, northern West Virginia, provides a proxy of terrigenous siliciclastic fluxes originating from the Taconic highlands, from the early Ashgillian to the Ordovician–Silurian transition. Strong cycling in the GR log shows evidence for Milankovitch-forced sea-level oscillations, hypothesized to have been produced by dynamic Late Ordovician glaciation in polar (southern) Gondwana. Juniata cycle frequencies are different from those of Quaternary Milankovitch cycles, with significantly higher obliquity and precession index frequencies, consistent with a 21.5 h Ordovician day and an Earth-Moon distance that was 95% of present day. These results support John Dennison’s long-held view that Milankovitch forcing of sedimentation took place in the early Paleozoic Appalachian Basin by action of remotely generated glacio-eustatic oscillations powered by glaciation on southern Gondwana, and that this sedimentary record has tracked “Earth’s movement through space.”

Abstract The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (10 4 years up to 10 6 years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record.

Abstract Rock magnetic cyclostratigraphy was measured in the Barremian–Aptian Cupido (‘Cupidito’) Formation, northeastern Mexico. The goal was to develop an objective evaluation of palaeo-environmental variability recorded in the formation that is independent of facies analysis and interpretation. Anhysteretic remanent magnetization (ARM) was used to estimate magnetic mineral concentration variations for the upper 143 m of the formation, which is characterized by metre-scale carbonate cycles representative of inner- and middle-shelf marine environments. Isothermal remanent magnetization acquisition experiments and scanning electron microscope (SEM) examination indicate that micron-sized detrital magnetite from eolian dust carries the ARM signal. At the sampled sections from Garcia and Chico canyons, 25 km apart, ARM records a synchronous 30–35 m oscillation with maxima coinciding with fourth-order sequence boundaries, superimposed with prominent high-frequency variability. Calibrating the 30–35 m oscillation to a 405 kyr period (long eccentricity cycle) focuses the high frequencies into short eccentricity, obliquity and precession index bands; the precession-band signal modulates with an eccentricity signature. The ARM signal is correlated between sections, but decoupled from the interpreted fifth-order depositional cycles. ARM amplitudes diminish up-section with facies suggesting deepening conditions that diluted magnetite concentration. This probably signals a warming, increasingly humid climate, changing global circulation and/or greater dispersal of magnetite grains.

Abstract In this study we examine glaciogenic rhythmites from the Late Palaeozoic Itararé Group, Paraná Basin, Brazil. We conduct spectral analysis on lithological cycle (‘couplet’) thickness series, and declination of maximum axis of anisotropy of magnetic susceptibility ellipsoidal tensor (K1) data. We tested the efficiency of K1 as a palaeoclimatic proxy. To constrain the timescale of harmonic features in the data, we analysed the couplet thickness spectra, converting the spectra to the time domain using an astronomical calibration based on Milankovitch frequency ratios. Comparison of the two rhythmites provides insights into their sedimentation rate evolution and cyclicity. Millennial-scale mechanisms of climatic origin influenced the deposition of both rhythmites, generating the lithological couplets, and are consistent with millennial-scale variations recognized as triggers for large-scale climatic changes during the Late Pleistocene. The common harmonic features in the couplet thickness and K1 spectra support the view that the azimuth of the K1 axis in sedimentary fabric is a useful palaeoclimatic proxy, reflecting sedimentation processes that were directly influenced by flow-induced, sediment transport, which is linked to external climate factors.

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 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 Meter-scale shallowing-upwards cycles (SUCs) are common in the shallow marine platform carbonate deposits around the world and throughout the geologic record. Despite that, their origin remains largely unresolved. Possible mechanisms capable of producing this kind of cyclicity include periodic (orbitally driven) and random sea-level oscillations, Ginsburgian autocyclicity, and random or quasi-periodic tectonic processes. Shallow-marine carbonate deposits of the Cambro-Ordovician Aisha-Bibi seamount (Malyi Karatau, Kazakhstan) contain tens to hundreds of meter-scale SUCs. Those cycles are typically composed of (from bottom to top) (1) transgressive lag flat-pebble conglomerates, (2) subtidal cross-stratified and bioturbated peloidal grainstones with mudstone drapes, intraformational flat-pebble conglomerates, and occasional thrombolites, (3) intertidal ribbon rocks (centimeter-scale heterolithic interlayering of mudstone and grainstone), and (4) upper intertidal-supratidal mudcracked cyanobacterial laminites and stromatolites. Boundaries between the subfacies within each cycle are typically transitional, reflecting gradual change from one laterally adjacent depositional subenvironment to the other; the boundaries between the cycles, however, are sharp and commonly erosional, separating non-adjacent subfacies and indicating abrupt change in sedimentation. Although each depositional cycle represents deposition under conditions of gradual lowering of relative sea level, the uninterrupted stacks of multiple SUCs indicate occurrence of repetitive high-frequency relative sea-level oscillations during the sediment accumulation. Three continuous, well-exposed cyclic sections were measured in detail; the longest one (Ooshbas) contains 124 cycles. That allowed us to test the Aisha-Bibi SUC sedimentary record for presence of orbital periodicities. Frequency modulation analysis was applied in order to overcome problems related to variable sediment accumulation rates. Time-series analysis of the Ooshbas SUC stacking patterns revealed the presence of Cambrian precessional periodicities, registering three out of four characteristic precessional “bundling” frequencies with > 90% confidence. That suggests that the depositional cyclicity recorded in the shallow marine deposits of Aisha-Bibi was driven by Milankovitch-forced eustatic sea-level changes.