Gary D. Johnson

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.

Development of the Himalayan frontal thrust zone: Salt Range, Pakistan

The Salt Range is the active frontal thrust zone of the Himalaya in Pakistan. Seismic reflection data show that a 1 km offset of the basement acted as a buttress that caused the central Salt Range-Potwar Plateau thrust sheet to ramp to the surface, exposing Mesozoic and Paleozoic strata. The frontal part of the thrust sheet was folded passively as it overrode the subthrust surface on a ductile layer of Eocambrian salt. Lack of internal deformation of the rear part of the thrust sheet is due to decoupling of sediments from the basement along this salt layer. Early to middle Pliocene (~4.5 Ma) conglomerate deposition in the southern Potwar Plateau, previously interpreted in terms of compressional deformation, may instead document uplift related to basement normal faulting. Stratigraphic evidence, paleomagnetic dating of unconformities, and sediment-accumulation rates suggest that the thrust sheet began to override the basement offset from 2.1 to 1.6 Ma. Cross-section balancing demonstrates at least 20 to 23 km of shortening across the ramp. The rate of Himalayan convergence that can be attributed to underthrusting of Indian basement beneath sediments in the Pakistan foreland is therefore at least 9-14 mm/yr, about 20%-35% of the total plate convergence rate.

Oxygen isotope evidence of climate change from pedogenic clay minerals in the Himalayan molasse

Abstract In this study, we examine the oxygen isotope ratios of smectite and kaolinite in a stratigraphic sequence of paleosols in the Siwalik Himalayan molasse on the Potwar Plateau, northern Pakistan. There is a 3–4%o increase in the smectite δ 18 O values synchronous with a ∼ 3.5%o increase in δ 18 O values of pedogenic calcite at 8.5–6.5 Ma (previously observed by Quade et al., 1989). The covariation of the δ 18 O values of smectite with calcite gd 18 O values through time strongly suggests a pedogenic origin of smectite and that its δ 18 O values reflect the oxygen isotope ratio of the soil water present during pedogenesis. These data show that the δ 18 O values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators. Kaolinite δ 18 O values are similar to smectite δ 18 O values, as would be expected for pedogenic kaolinite, but there is greater variation in these data which we attribute to incomplete mineral separation. The measured permil oxygen isotope fractionation between calcite and smectite within individual paleosols is, on average, ∼3%o higher than the equilibrium fractionation. This isotopic disequilibrium can be explained by the calcite forming predominantly during the dry winter season and the smectite forming predominantly during the wet summer season. The synchronous 3.5%o increase in oxygen isotope ratios of smectite and calcite at 8.5–6.5 Ma indicates that this 18 O/ 16 O increase is not due to diagenesis, but rather results from a climate change. The most likely causes of the 18 O/ 16 O increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation (thus 18 O enrichment) and/or a change from more continental to more marine-sourced precipitation. The Tibetan Plateau, which is suggested to have undergone rapid uplift around 8 Ma (e.g., Harrison et al., 1992), may have acted as an effective orographic barrier to low δ 18 O precipitation originating from central Asia, causing higher δ 18 O values of the precipitation falling on the Potwar Plateau. Currently it is impossible to distinguish between these two potential causes of the 18 O/ 16 O increase at 8.5–6.5 Ma, but these clay mineral δ 18 O values support that there was a significant climate change in this region.

The late cenozoic chronologic and stratigraphic development of the Kashmir intermontane basin, Northwestern Himalaya

Abstract The intermontane basin of Kashmir developed within the thrust-faulted, southern margin of the Himalayan Range. Detailed lithostratigraphies, magnetic-polarity stratigraphies, and fission-track dates on enclosed volcanic ashes were determined at four separate localities in order to develop a chronology of the Late Cenozoic evolution of the Kashmir Basin. The results indicate that sedimentation had commenced by about 4 m.y. ago. Since then, over 1300 m of sediments have aggraded at inferred average rates varying from 16 to 64 cm per 1000 yr. Lacustrine and deltaic sediments dominate the Kashmiri sequences and appear to respond sensitively to tectonic events along the basin margins. Conglomerates shed from the faulted basin margins at about 1.7, 2.1, 2.7, and 3.0–3.5 m.y. ago punctuate the predominantly low-energy, fluviolacustrine depositional record. Paleocurrent analyses indicate a switch from northeasterly derived conglomeratic facies to southwesterly derived ones about 1.7 m.y. ago. This transition reflects enhanced activity along the Main Boundary Thrust complex to the southwest and an apparent diminution of faulting along the northeastern margin of the basin. In the Pir Panjal Range, 1400–3000 m of uplift at inferred rates of up to 10 mm yr −1 have terminated widespread sedimentation within the Kashmir Basin since the middle Pleistocene.

The occurrence and fission-track ages of late neogene and quaternary volcanic sediments, Siwalik group, Northern Pakistan

Abstract Volcanic sediments, now mostly bentonites and bentonitic mudstones, occur throughout the Late Neogene and Quaternary Siwalik Group of northern Pakistan. A number of these deposits have been dated by the fission-track method, utilizing zircon phenocrysts from these deposits, and provide the chronometric constraints upon which a paleomagnetic stratigraphy is developed for the Siwalik Group. Notable in the occurrence of these altered tuff horizons is an apparent mode in their stratigraphic development from approximately 3.0 to 1.5 m.y. B.P. which coincides with the period of activity of the Dacht-e-Nawar volcanic complex of east-central Afghanistan. Fission-track ages of certain tuffs for critical areas of northern Pakistan are reported herein.

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.

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 thatEquus, diagnostic element of the Pinjor faunal zone, appeared locally about 1.8 m.y. ago and thatHipparion, a faunal element of the Tatrot and earlier faunal zones, persisted locally until at least 3.0 m.y. ago.

The magnetic polarity stratigraphy of the Siwalik Group at Haritalyangar (India) and a new last appearance datum for Ramapithecus and Sivapithecus in Asia

Abstract The Haritalyangar (India) local fauna contains type specimens of several hominoid primates which are assigned a new last appearance datum age based on a magnetic polarity stratigraphy. This youngest assigned age for Sivapithecus indicus, S. sivalensis and Ramapithecus punjabicus in Asia is about 7.0–7.5 m.y. The age of the holotype specimen of Gigantopithecus bilaspurensis is about 6.3 m.y. The Nahan Sandstone Formation is at least in part contemporary with the Nagri Formation (lithofacies) of the Potwar Plateau, Pakistan.

Distribution of noble metals across the Cretaceous/Tertiary boundary at Gubbio, Italy: Iridium variation as a constraint on the duration and nature of Cretaceous/Tertiary boundary events

Iridium, Pd, Pt, and Au were determined in sections from the Bottaccione Gorge and Contessa Valley, Gubbio, Italy, by radiochemical neutron activation. Shales and limestones were sampled from 2.85 m above to 219 m below the Cretaceous/Tertiary (K/T) boundary. Metal enrichment was evaluated by comparing the boundary shale region with the lower part of the section (background). Iridium is concentrated by 63 times in the boundary shales in comparison with the background, whereas other metals are enriched by no more than 2.2 times. The enrichment of Ir is not confined to the boundary shales but extends approximately 2 m above and below this horizon. Within this Ir-rich region there are four distinct Ir maxima in addition to the major Ir enrichment in the K/T boundary shales. Iridium maxima are stratigraphically coincident with maxima in abundances of shocked minerals characteristic of explosive volcanism. Limestones are much lower in noble metals than shales, and their Ir contents in the K/T boundary region are largely accounted for by their minor clay mineral contents. The time represented by the 4 m of Ir-rich section is at least 3 x 10 5 yr, if published sedimentation rates are used. To sustain an increased Ir flux over this period and to account for the Ir distribution near the K/T boundary, intense volcanic activity is a preferred alternative to impact of extraterrestrial material.