Everett H. Lindsay

Magnetic polarity stratigraphy and ages of Siwalik group rocks of the potwar plateau, Pakistan

Abstract Six magnetic polarity sections have been established over the Potwar Plateau region of Pakistan, including the major stratotypes of the Siwalik Group. In all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone, which is contained within the Nagri Formation. This conspicuous normal polarity zone has been radiometrically dated at 9.5 ± 0.6 m.y., which identifies it as magnetic Chron 9. Radiometric dates from the Upper Siwalik Formation have also been used to identify the Chron 2–3 boundary in two of the sections. The magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale, so that the ages of the local stratigraphy are indexed accordingly. Based exclusively on data from stratotype sections, the Chinji, Nagri and Dhok Pathan Formations have nominal age ranges of 10.1–13.1, 7.9–10.1 and 5.1–7.9 m.y. Age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype. Mean sediment accumulation rates during the Chinji, Nagri and Dhok Pathan interval range from 13 to 52 cm/103 yr. Essentially linear sediment accumulations are locally maintained over time intervals of several million years. The Chinji-Nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the Potwar Plateau, indicating a fundamental sedimentary-tectonic change at this time.

Magnetic polarity stratigraphy and vertebrate paleontology of the upper siwalik subgroup of northern Pakistan

Two hundred and fifty three sites consisting of three samples per site were taken from eight separate stratigraphic sections from sediments of the Upper Siwalik subgroup of northern Pakistan. All samples have been partially demagnetized in alternating fields of from 150 to 300 Oe. The sediments are dipping at angles of up to 75° in a variety of directions, and the locality mean directions improve significantly after correcting for bedding attitude, providing a statistically significant fold test at the 95% level. The magnetic stratigraphy was obtained based on the statistically significant well-grouped partially demagnetized data. Two prominent bentonitized tuffs occur in several sections above and below a prominent reversal of the earths magnetic field. Radiometric dates of 2.3 ± 0.4 m.y.B.P. and 2.5 ± 0.4 m.y.B.P. have been obtained by fission track on zircons from these tuffs. This allows the magnetic stratigraphy to be correlated to the standard reversal chronology of the Pliocene and Pleistocene. The longest stratigraphic section (1.87 km in thickness) can be shown to span the time between 0.6 m.y.B.P. and 5.5 m.y.B.P. The individual stratigraphic sections can be correlated on the basis of magnetic stratigraphy. All sections contain important vertebrate fossil localities which can be placed relative to each other in the time stratigraphic framework provided by the magnetic stratigraphy. The change from the Pinjor fauna to the Tatrot fauna occurs at about 2.47 m.y.B.P. (the Gauss/Matuyama boundary) based on the simultaneous occurrence of Equus-Elephas, Bos and cervids with antlers. Hipparion, the three-toed horse, persists to the lowermost Pleistocene just after the termination of the Olduvai event. The ranges of other important vertebrate faunas are also discussed. The folding of the eastern Salt Range has been very recent, beginning within the Brunhes normal magnetic epoch, since sediments of lower Brunhes age are folded on the flanks of the Pabbi and Rohtas anticlines and on the flanks of Chambal Ridge. The rate of uplift of these structures above the present base level is estimated to have a minimum rate of four meters per thousand years for the Rohtas anticline and three meters per thousand years for the Pabbi Hills. The rates of subsidence (i.e., rates of sedimentation) of the various sectios are estimated to range from 0.35 m/103 yr to 0.50 m/103 yr. The first occurrence of conglomerate-containing clasts derived from the Himalayan uplift occurs just prior to the Olduvai event at the abse of the Pleistocene.

Magnetic Polarity Stratigraphy of Pliocene-Pleistocene Terrestrial Deposits and Vertebrate Faunas, San Pedro Valley, Arizona

Horizontally stratified magnetic zones are observed throughout the Pliocene-Pleistocene valley fill of the upper San Pedro Valley, Arizona. Within one detailed 150-m stratigraphic section (Curtis Ranch), twelve superposed magnetic polarity zones are identified spanning the time from the upper Gilbert Epoch through the Brunhes Epoch. These magnetic polarity zones extend over a 64-km stretch of the valley and serve as convenient chronostratigraphic units. Based on the position of mammalian fossils within the chronostratigraphic zones, we have defined four mammal datum planes for the upper San Pedro Valley, including the first appearance of Lepus sp. at 1.9 m.y. Lepus sp. marks the first occurrence of a definitive Irvingtonian land mammal in the San Pedro Valley and thus places the local boundary of the Irvingtonian Land Mammal Age shortly below the Olduvai event. This position equates approximately with the Pliocene-Pleistocene boundary of the pelagic marine record.

The paleomagnetism and magnetic polarity stratigraphy of the mammal-bearing section of Anza Borrego State Park, California

Abstract We sampled 150 sites in fine-grained Plio-Pleistocene sediments of the Palm Springs and Imperial Formations. Sampling was confined to 3000 m of stratigraphically continuous section containing abundant vertebrate fossil remains of the Vallecito Creek, Arroyo Seco, and Layer Cake local faunas of the Irvingtonian and Blancan Land Mammal Ages. The magnetic stability of these sediments was sufficient to delineate the magnetic stratigraphy, which ranges from below the Cochiti event at the base, to the Matuyama reversed magnetic epoch at the top of the section. Eight faunal events are placed relative to the magnetic polarity sequence. They are cf. Pliohippus extinction, Geomys appearance, cf. Equus appearance, Tremarctos appearance, Hypolagus extinction, cf. Odocoileus appearance, Smilodon and ? Euceratherium appearance. The latter two faunal events characterize Irvingtonian Land Mammal Age. The transition from the Blancan to Irvingtonian Land Mammal Age occurs in the lower Matuyama magnetic epoch close to the Pliocene-Pleistocene boundary. The appearance of European migrants during the lower Matuyama magnetic epoch indicates that the Bering land bridge was exposed for animal migration between Europe and North America at this time.

Continental deposits, magnetostratigraphy and vertebrate paleontology, late Neogene of Eastern Spain

Abstract Eleven sections have been sampled for magnetostratigraphy in the fossil mammal-productive late Neogene basins of eastern Spain. The characteristic direction of magnetization was determined through progressive thermal and alternating field demagnetization. It was possible to correlate the magnetozones from the various sections to the GPTS as well as to the fossil MN zones within the sediment. The MN zone boundaries based on this correlation are as follows: MN 11/12 ⋍ 8.1 my; MN 12/13 ⋍ 6 my; MN 13/14 ⋍ 4.9 my; MN 14/15 = 4.18 my; MN 15/16 = 3.58 my. New fossil lists are presented for the Fuente del Viso and Loma del Castillo faunas in the Cabriel Basin.

Neogene Siwalik mammalian lineages: Species longevities, rates of change, and modes of speciation

Abstract A long depositional sequence from northern Pakistan provides a good fossil record of terrestrial vertebrates for the interval of 18-7 Ma. Making allowances for possible range extensions, we use this record as a direct measure of species longevities. There is a correlation between body size and longevity, smaller mammals frequently being short-lived and larger taxa showing durations up to ca. 10 m.y. The distribution of small mammal longevities indicates an exponential decrease in frequency from a high modal value in the smallest increment measured (0–200,000 yrs). The most frequent value for large mammals is also the smallest increment measured (1 m.y.), but that distribution may not be unimodal. Small mammal taxa of short duration are concentrated late in the sequence, after 9 Ma especially. The middle Miocene fauna is more stable, with species showing longer durations. The contrast in longevities corresponds with hypothesized greater environmental stability in the middle Miocene. For comparison, the Paleogene sequence of Wyoming indicates short median species durations, with few surveyed taxa lasting over 2 m.y. Siwalik mammals show diverse modes of evolution, but stasis in at least some features is usual, with species boundaries corresponding to morphological breaks. Up to half of the Siwalik rodent and artiodactyl species surveyed likely immigrated from outside the biogeographic province, and for a few, historical data are sufficient to stipulate when and by what route they came to the Indian subcontinent.

Patterns of faunal turnover and diversity in the Neogene Siwaliks of Northern Pakistan

Abstract The fluvial Neogene Siwalik formations of northern Pakistan contain a long and richly fossiliferous sequence of terrestrial vertebrate faunas in which patterns of faunal turnover and changes in diversity can be documented and analyzed for intervals having durations of 0.5 m.y. The complete sequence extends from circa 18.5 to 1 Ma, but the part between 18.5 and 5.5 Ma is best sampled, and most intervals within it are well represented. Thirteen orders of Siwalik mammals have been identified, with well-sampled intervals having 50 or more species. Most Siwalik mammals, however, are either rodents or artiodactyls. Bovids are the most common and most speciose of the larger mammals, while murid and “cricetid” rodents dominate the small mammal assemblages. Between 18.5 and 5.5 Ma species diversity varied considerably. Among artiodactyls and rodents the number of species first increased between 15 and 13 Ma and then fell. Data on stratigraphic ranges of rodents and artiodactyls show that faunal change in the Siwaliks was episodic, occurring during short intervals with high turnover followed by longer periods with considerably less change. Maxima of first appearances occurred at approximately 13.5 and 8.5 Ma, while maxima of last occurrences were at 12.5 and 8.0 Ma. Some of the observed faunal events can be correlated to climatic and environmental changes. The Middle Miocene diversification occurred during a period of global cooling, while the latest Miocene decline in diversity and increased turnover accompanied oxygen and carbon isotopic changes that correlate to globally increasing seasonality and aridity. Other correlations are ambiguous. The marked decrease in diversity and the major turnover events between 13 and 8 Ma do not correspond to known local or global events. The Neogene Siwaliks and Paleogene Bighorn-Crazy Mountains sequence in Wyoming and Montana share many similarities. They have equivalent levels of temporal resolution and similar levels of completeness of their fossil records. Siwalik ordinal abundance and diversity patterns differ markedly from those of the Paleogene, but generic, and probably species, diversity was approximately the same, although the Siwalik faunas may have been slightly less diverse. Over time, changes in diversity were of comparable magnitude, with monotonic trends persisting for more than 5 million years. The magnitude of faunal turnover was also similar, ranging from less than half to 3.5 times that expected. In both sequences faunal change appears to have been episodic, with strong pulses between intervals of low turnover. The Siwaliks, in contrast to the Paleogene sequence, may have had more distinct pulses and longer intervals between pulses. Neither sequence has peaks of first occurrences coinciding with peaks of last occurrences.

Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California

The kinetics of continental rifting are recorded in the late Neogene sediments of the western Imperial Valley, California. The Gulf of California opened in its present form about 4 m.y. ago, capturing the Colorado River. Some 5 km of deltaic and fluvial sediments accumulated in the Vallecito–Fish Creek area during the interval 4.3−0.9 m.y. ago. Initial sedimentation rates were 5.5 mm yr −1 , which diminished in an exponential fashion to 0.5 mm yr −1 . When isostatic adjustments are accounted for, this implies tectonic subsidence rates of 1.5 mm yr −1 , decreasing systematically to 0.1 mm yr −1 . After 0.9 m.y. ago, right-lateral shearing along the Elsinore fault zone cut across the basin, causing it to be tilted and uplifted at a mean rate of 5.9 mm yr −1 . The basin was concurrently rotated as a unit 35° clockwise. The dissected pediments and badlands that now characterize the area were formed and eroded in the past 0.9 m.y.

Paleomagnetism of the Middle Siwalik formations of northern Pakistan and rotation of the Salt Range Decollement

Abstract Sampling for paleomagnetic study has been carried out at seven localities in the Potwar Plateau region of Pakistan in the vicinity of the Salt Range. In all, 253 sites, consisting of three samples per site, were studied. The sites were taken from the Chinji, Nagri, Dhok Pathan and Upper Siwalik formations which span the last 14 m.y. of Earth history. The formations have been deformed in the ongoing Himalayan orogeny and are exposed today in sections which dip at angles of from 10° to 90° in different directions. A characteristic direction of magnetization was isolated at all sections by thermal demagnetization at temperatures of from 550° to 650°C. The sediments have been folded within the last 2 m.y. and because of a positive fold test it can be shown that the characteristic magnetization of these sediments was acquired before folding. Four of the sections have directions of magnetization which have been rotated in a counter-clockwise fashion up to 40°; however, sections taken along the Indus River, the north flank of the Soan syncline and the south flank of the Salt Range show small counter-clockwise rotations of under 10°. These observations are in agreement with previous suggestions that strata in the Salt Range have been moved to the south and rotated in a counter-clockwise fashion. The observations reported here indicate that the paleomagnetic poles derived from Cambrian and Permian formations within the Salt Range have probably also suffered rotation and should be removed from consideration in Gondwanaland reconstruction of the Paleozoic.

Upper Cretaceous–Paleocene biostratigraphy and magnetostratigraphy, Hell Creek and Tullock Formations, northeastern Montana

Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.