Gauss Normal Epoch Research Articles

Integrated stratigraphy of the Middle to Late Pliocene upper Miyazaki Group, southern Kyushu, Southwest Japan

Neogene sedimentary deposits are present in the Miyazaki region, southern Kyushu, on the Pacific side of southwestern Japan. The upper part of the Miyazaki Group, exposed in the Tsuma–Takanabe region of northern Miyazaki, is divided into the Tsuma, Sadowara, and Takanabe formations, in ascending order, and contains a planktic foraminiferal and calcareous nannofossil record of Pliocene events of the mid-latitude northwestern Pacific. The geological age of these units is based on magneto- and biostratigraphy. Five calcareous nannofossil datum horizons are recognized: the last occurrence (LO) of Reticulofenestra pseudoumbilicus, the termination of acme zone II of Reticulofenestra minutula var. C, the LO of R. minutula var. B, calcareous nannofossil datum plane A, and the termination of acme zone I of R. minutula var. C. Six planktic foraminiferal bioevents are also recognized: a coiling direction change, from sinistral to dextral, in the genus Pulleniatina; the first occurrence (FO) of Globorotalia crassaformis; the LOs of Globorotalia margaritae, Sphaeroidinellopsis seminulina, and Dentoglobigerina altispira; and the FO of Neogloboquadrina asanoi. The presence of two calcareous nannofossil bioevents, Datum A and the termination of acme zone I of R. minitula var. C, suggests that the normal and reversed polarity intervals of the upper part of the Takanabe Formation correspond to Chron C2An.1n (latest Gauss Normal Epoch) and Chron C2r.2r (earliest Matuyama Reversed Epoch), respectively. Based on combined floral and faunal data and magnetostratigraphic chron boundaries from the ATNTS2004, the upper part of the Miyazaki Group is Middle to Late Pliocene in age.

Magnetic anomoly patterns around the central rift area in the North Fiji Basin: Inversion approach for detailed structure

A detailed magnetic anomaly map around the central rift area in the North Fiji Basin was compiled from 6 starmer cruises. The data range is almost 800 km in length (between 22°S and 14°30′S), 100 km in width (between 173°E and 175°E). Detailed magnetic structure analysis was applied for the data around the rift axis, which revealed 5 or 6 segments along the 800 km long rift axis. Inversion trials were done on the grid data and good comparison with topographic features was obtained. The results of inversion indicate that the lineations which correspond to the Olduvai event were recognized in this area broadly, but not so common as those for the Gauss epoch. The central rift system seems to have started its spreading from around the end of the Gauss normal epoch or the beginning of the Matuyama reversed epoch (2.5 Ma).

黒富士火山と甲府盆地北方に分布する火山岩類の火山活動とＫ‐Ａｒ年代

Kurofuji Volcano occupies the northwestern margin of the Kofu Basin and the older Cenozoicvolcanic rocks are distributed in the eastern part. The oldest Taragatoge Volcanic Rock consisting of subaqueous andesite effused at 7.9 Ma in the central part of the area. During 6.1-4.3 Ma, the Higashiyamanashi Volcano-Plutonic Complex was formed by eruption of rhyolite-dacite welded tuffs and minor andesite lava followed by intrusion of granodiorite in the eastern part. The Mizugamori Volcanic Rock of andesite composition occupied the central part derived from many vents, while the magnetic polarity changed from Gauss Normal Epoch to Matuyama Reversed Epoch in 2.4-1.7 Ma. Kurofuji Volcano began its dacitic activity at 1.0 Ma forming the main shield of five pyroclas-tic-flow sheets and lava domes built up at the top at 0.5 Ma. The youngest andesite cone, the Kayagatake stratified cone, formed at the west flank of the main shield at 0.2 Ma. These volcanic activities suggest that the volcanic front around this area moved about 20km from the east in 8-2 Ma to the west in 1.0-0.2 Ma, farther away from the Japan Trench.

日本海東縁久六島の鮮新世アルカリ玄武岩

We obtained a K-Ar age of 3.32 Ma (Pliocene epoch) for an alkali basalt sample from Kyuroku-shima Island in the northeast margin of Japan Sea. The sample is normally magnetized and probably correlates to the Gauss normal epoch. The N-type MORB-normalized incompatible element pattern and the K2O/TiO2 ratio. indicate that the alkali basalt has a character of typical island arc. The petrography and chemistry of the alkali basalt is similar to those of basalts from present Chokai Volcanic Zone, especially those from Oshima-Oshima volcano. A regional study of Pliocene volcanoes by Shuto (1989) and ours revealed an existence of the lateral variation back then at least within our study area.

Interpretations of the magnetostratigraphy of the Hadar hominid site, Ethiopia

As part of an investigation to determine the age of the hominidbearing strata of the Hadar Formation of Ethiopia we have determined the magnetostratigraphy of two separate sections of the strata. Two major zones of reversed polarity occur within the predominantly normally magnetized sequence, one at the top and the other in the middle. There are difficulties with the absolute age determinations of volcanic units within the strata which result in uncertainties in correlating the Hadar magnetostratigraphy with the standard magnetic reversal time scale. The age of the upper part of the section is established by the BKT-2 tuff at ∼2.8 Myr (refs 1–4). The K–Ar dating of the Kadada Moumou basalt which lies in the middle of the sequence yields variable results ranging from 3.1 to 3.6 Myr (refs 1–4). Regional geochemical5 and bio-stratigraphical6 studies suggest the younger age for the Kadada Moumou basalt. We show here that this is also the age that results in the most consistent interpretation of the observed magneto-stratigraphy of the formation, which is that the Hadar Formation was deposited during the Gauss Normal epoch and that the two reversals represent the Kaena and Mammoth events within that epoch.

Palaeomagnetism and the age of the Makapansgat hominid site

The time of deposition of the Makapansgat Limeworks site in South Africa, containing several hominid fossils, has an important bearing on hominid phylogeny in Africa. Hence, in an attempt to determine this age, a palaeomagnetic analysis of the site has been performed and the results are reported here. Member 3 (from which most of the important hominid fossils have been recovered) appears to have an age greater than 2.90 Ma and possibly greater than 3.06 Ma but less than 3.32 Ma. Previously it has not been possible to delineate the age of this important member with any degree of accuracy. The most likely position for the underlying Member 2 is in that section of the Gauss normal epoch predating the Mammoth event and having age limits of 3.06–3.32 Ma.

Magnetic polarity stratigraphy of the Upper Siwalik deposits, Pabbi Hills, Pakistan

Over 1000 m of fluvial molasse, exhibiting a stable detrital remanent magnetization, is exposed in a mammal-bearing sequence in the Upper Siwalik Group of the Pabbi Hills, Pakistan. The magnetic polarity chronology reveals that the sequence records a time period of 2.6 m.y., extending from the early Gauss Normal Epoch into the Brunhes Normal Epoch. During this period, sedimentation rates increased upward in time from 0.25 m/1000 yr to 0.45 m/1000 yr. The sudden disappearance of red beds and a change in the lithoclastic composition of basal channel sands suggests that about 800,000 years ago the primary source area began shifting from the metamorphic terrane of the Himalayan Orogen to a more local sedimentary terrane on the folded margins of the Himalayan foredeep. About 500,000 years ago the anticlinal Pabbi Hills attained surface expression. Uplift continued at a minimum rate of 1 m/1000 yr. A local Pliocene/Pleistocene boundary based on the Olduvai Normal Event is clearly recognized. Local fossil finds reveal that Equus, diagnostic element of the Pinjor faunal zone, appeared locally about 1.8 m.y. ago and that Hipparion, a faunal element of the Tatrot and earlier faunal zones, persisted locally until at least 3.0 m.y. ago.

Paleomagnetism and Potassium-Argon Ages of the Sonoma Volcanics, California

Paleomagnetic data and potassium-argon ages indicate that the Sonoma Volcanics was erupted during the Pliocene Gilbert reversed and Gauss normal polarity epochs. The Gilbert reversed epoch is represented in the Howell Mountains east of Napa and east of St. Helena, in the mountains immediately east of the Valley of the Moon, and on the hill just north of Santa Rosa. The Gauss normal epoch is represented by the rocks from Mount St. Helena and possibly by the flows from the upper part of Sonoma Mountain and the rhyolite flows north of Sonoma. The age of the volcanic rocks ranges from 5.3 m.y. to about 2.9 m.y. Volcanic rocks approximately 11.8 m.y. old occur at Burdell Mountain northwest of Novato. This volcanic sequence is probably part of the Tolay Volcanics of the Petaluma area.

Magnetic and micropaleontological study of deep-sea sediments from the west-central equatorial Pacific

A paleomagnetic study of four oriented deep-sea cores collected by the R.V. Hakuho Maru in the west-central equatorial Pacific Ocean has revealed clear sequences of normal and reversed magnetic polarities. Stability in direction and intensity of the natural remanent magnetization has been examined in some detail to confirm the reliability of the paleomagnetic applications. Two of the cores taken from sites close to the equator contain siliceous microfossils. It has been shown from magnetic stratigraphy aided by paleontological control that one core extends from Recent to the Gauss Normal Epoch while the other core has a stratigraphic hiatus near the top below which the Gauss to Gilbert Epochs are recognized. Ranges of two radiolarian species ( Pterocanium prismatium, Eucyrtidium elongatum peregrinum), five diatom species ( Thalassiosira convexa, Rhizosolenia praebergonii, Actinocyclus ellipticus f. lanceolata, Pseudoeunotia doliolus and Nitzschia fossilis). and a silicoflagellate ( Mesocena elliptica) have been determined for a time span from the late Gilbert Epoch to Recent and compared with the eastern tropical Pacific record. Magnetic stratigraphy of two other cores which have no microfossils seems to indicate that the bottom of these cores is in the late Gauss Epoch. The rates of sedimentation thus calculated with the four cores are 2·0 mm/1000 years to 8·8 mm/1000 years, being roughly consistent with those of the eastern equatorial Pacific cores.

Planktonic Foraminiferal Datums and Late Neogene Epoch Boundaries in Mediterranean, Caribbean, and Gulf of Mexico: ABSTRACT

Late Neogene planktonic foraminiferal datums and epoch boundaries in Italy, as proposed by the Committee on Mediterranean Neogene Stratigraphy, are compared with those in the Caribbean and Gulf of Mexico. Datums recognizable in these regions include (1) appearance of Globorotalia margaritae in early Pliocene, (2) appearance of the G. crassaformis lineage in middle Pliocene, (3) appearance of G. tosaensis and Sphaeroidinella dehiscens in very late Pliocene, and (4) appearance of Globorotalia truncatulinoides within the early Pleistocene (middle or late Calabrian). Initiation of the Pleistocene in these regions is recognized by abrupt onset of climatic deterioration, as indicated by marked changes in the planktonic and benthonic foraminiferal populations, and marked eustatic lowering of sea level, as demonstrated by regressive facies, nondeposition, or deep-water turbidites, depending End_Page 2038------------------------------ on basinal position. Evidence given by paleontology (typologic studies), paleoclimate records, eustatism, and geomagnetic dating of earliest Pleistocene glacial rocks favors a Pleistocene Epoch beginning 2.5-3.0 m.y. ago within the Gauss normal epoch. This time corresponds to the extinction horizon of Globoquadrina altispira and withdrawal of other warm-water species from the Caribbean and Gulf of Mexico. There is no reason to assume that worldwide onset of Pleistocene climate deterioration was diachronous, as would have to be postulated if the Pleistocene had begun in warm-water regions only 2.0 m.y. ago, as postulated by some workers. A new species is described and discussed. End_of_Article - Last_Page 2039------------

Geomagnetic Polarity Epochs: Sierra Nevada Data, 3

Available paleomagnetic and radiometric results for about 60 igneous rocks 3.6 m.y. old or less are consistent with the following geomagnetic polarity epoch time scale: Brunhes normal polarity epoch, 0.0 to 0.85±0.15 million years; Matuyama reversed polarity epoch, 0.85±0.15 to 2.4±0.1 million years; Gauss normal polarity epoch, 2.4±0.1 to 3.35±0.1 million years; Gilbert reversed polarity epoch, 3.35±0.1 million years to (?). A brief interval of normal polarity (the Olduvai event) occurs within the Matuyama reversed epoch at about 1.9 m.y., and a brief interval of reversed polarity (the Mammoth event) occurs within the Gauss normal epoch at about 3.06 m.y. Data for twelve units from eastern California and Nevada, reported here for the first time, are particularly useful in defining the Gauss-Gilbert boundary and for recognizing the Mammoth event.