Abstract New data has recently become available on the length and duration of the Permo-Carboniferous reversed superchron (PCRS, Kiaman). This data set is reviewed, and the upper boundary is found to occur at approximately 260 Ma and the lower boundary at about 315 Ma. The frequency of reversal before and after the onset of the PCRS is similar to that observed before and after the Cretaceous quiet zone (KQZ). The earth’s magnetic field seems to have two states, one with reversal frequencies exceeding 1 per million years and the other with no or few reversals for 10’s of millions of years. The KQZ occurred at a time of continental dispersal and the PCRS at a time of continental amalgamation.

Abstract We discuss how the chronology of terrestrial sediments can be refined when sound systematics of the contained fossils is combined with paleomagnetic data. In the absence of other means of dating Neogene deposits of China, strata can be ordered in time on the basis of their contained fossils. Biochronology alone attains good but unqualified accuracy in a biogeographically restricted system, such as China; dating the biochronology requires relationship in time to other systems of known age. This can be achieved by building a long composite of successive assemblages, faunal correlation at times of low endemism, identification of tie points in the chronology by using index fossils, and age estimation through paleomagnetic correlation. Late Neogene examples from China illustrate how that sequence of ordered faunas can be placed in a time scale through faunal comparison with the Yushe Basin (Shanxi Province) magnetostratigraphic sequence, and how the increased information content can be applied to new problems in geochronology. New data from Yushe Basin identify an Ertemte-like assemblage at Jiayucun in reversely magnetized rock. This is correlated with early Gilbert Chron. The Jingle and Youhe reference faunas are correlated with Gauss Chron Yushe assemblages. Nihewan-like assemblages are correlated with the Matuyama Chron, some being pre-Olduvai Subchron, and therefore Pliocene in age.

Several diatom datum levels are proposed for latest Miocene/earliest Pliocene sediments in the equatorial and north Pacific. These datum levels are tied directly to the magnetostratigraphy and may be summarized as follows: 1) Last occurrence of Denticulopsis hustedtii —middle of Chron 10 in the equatorial Pacific. 2) First occurrence of Thalassiosira burckliana —lower part of Chron 9 in the equatorial Pacific. 3) First occurrence of Coscinodiscus nodulifer var. cyclopus —middle of Chron B in the equatorial Pacific. 4) Last occurrence of Thalassiosira burckliana —lowest part of Chron 7 in the equatorial Pacific. 5) First occurrence of Rossiella praepaleacea —lower part of Chron 6 in the central Pacific. 6) First occurrence of Nitzschia miocenica var. elongata —lower part of Chron 6 in the equatorial Pacific. 7) Last occurrence of Rossiella praepaleacea —upper part of Chron 6 in the equatorial Pacific. 8) Last occurrence of Nitzschia miocenica var. elongata —middle part of Chron 5 in the equatorial Pacific. 9) First occurrence of Denticulopsis kamtschatica —lower part of Chron 6 in the high latitude North Pacific. 10) Last occurrence of Rouxia californica —middle part of Chron 6 in the high latitude North Pacific. Within the context of a more refined biostratigraphy and improved correlation between high and low latitudes, we can trace the latitudinal changes in distribution of selected diatom species as a function of time. Actinocyclus ingens is seen as a cosmopolitan species in the middle Miocene and by the Late Miocene it has retreated to the higher latitutes of the north and south Pacific. Denticulopsis kamtschatica, on the other hand, is shown to make its first appearance in the high latitude north Pacific at about 6.3 Ma and extend its range southward during the succeeding 1.5 Ma.

From the Sierra Nevada of Santa Marta at the northern terminus of the Andes, preliminary paleomagnetic results from ten sites in two formations characteristically have stable directions with northerly declinations and moderate positive inclinations. Most site mean directions are close in attitude to the present Earth's field and are not indicative of large tectonic rotations. An antipodally reversed direction is obtained at one site by thermal demagnetization, suggesting magnetization in an ancient paleofield nearly coaxial with the present field. Few cratonic Jurassic directions have been reported from South America, but at least one such study shows Jurassic paleofield directions close to the present field directions. In defining the Mesozoic polar wander path for South America, distinguishing Jurassic from present field directions will be a continuing challenge.

A paleomagnetic study has been carried out on 114 piston cores from the Pacific Ocean north of 20° N. lat. The cores were sampled at 10 cm intervals, and the direction of magnetization was determined after partial demagnetization in fields of from 50 to 150 oersted peak field. The coercivity of the red clays from the central part of the North Pacific was found to decrease with depth in the cores so that in red clay cores magnetic stratigraphy is often undecipherable even after partial a.f. demagnetization. In other types of lithologies, a.f. demagnetization was successful in removing secondary components. The magnetic stratigraphy from the cores was interpreted in terms of the radiometrically derived time scale for the last 4.5 m.y. of earth history. Confusion has recently arisen as to whether the large event, which is often identified in the bottom part of the Matuyama series, is best correlated with the Olduvai or Gilsa event. The age of this event in 13 North Pacific cores was determined by interpolation between the Brunhes/Matuyama and Gauss/Matuyama boundary. The average age of the upper and lower boundaries of the event are 1.71 and 1.86 m.y. with a standard deviation exceeding ± 0.1 m.y. The original date on the Olduvai event is 1.91 ± 0.06 m.y. There is no significant difference between the range of the event as seen in marine sediments and the age of the Olduvai event at its type locality. Therefore, on the basis of priority, it seems proper to correlate this event with the Olduvai event. Nine stratigraphic sections were constructed on the basis of magnetic stratigraphy, and from the cores used in these sections, rates of sedimentation were calculated using a date of 0.69 m.y. for the Brunhes/Matuyama boundary. These rates were then contoured and compared with the sediment distribution in the Pacific. The rates of sedimentation in the red clay area of the central Pacific are 3 mm/1000 yrs. The observed rates increased toward the margins of the Pacific in all cases, due to an increase in volcanic, biogenic and glacial detritus present in the cores. The effect of topography on rates of sedimentation is considered. Rates of sedimentation indicate a Miocene age for the time of cessation of turbidite deposition on the Aleutian abyssal plain. A comparison of the known rates of sedimentation with the sediment distribution favors the hypothesis of the mobility of the sea floor.