S. K. Tandon

Fluvial response to Late Quaternary climate changes, western India

We review here the fluvial response to Late Quaternary climatic changes in the semi-arid regions of western India, i.e. the Thar Desert and its margins, and Maharashtra. The rivers in the Thar Desert show a spectrum of environments from gravel bedload braided to ephemeral sand-bed streams to sheet flows and sheet floods, while those in the Thar margin and Maharashtra show dominantly two modes, i.e. braided and meandering rivers. The streams became defunct and aeolian sands were deposited during periods of relatively greater aridity in the Thar Desert and its margins. Incisions occurred during the wet phases in OIS 1. It is inferred that changes in the monsoon precipitation led to relative changes in the discharge and sediment supply, which were routed through those in the vegetation cover, and played a key role in determining the fluvial style. An inter-comparison of the Late Quaternary alluvial deposits in the semi-arid regions of western India suggests a continuum of fluvial environments interrelated through a precipitation gradient. Further, a global comparison indicates that the response of semi-arid western Indian rivers was synchronous with the rivers in other climatic settings.

Climate control on erosion distribution over the Himalaya during the past ~100 ka

Sediment samples from a 50-m-long core representing ~100 ka of deposition, taken from the Ganga Plain on the campus of the Indian Institute of Technology, Kanpur, were analyzed for Sr and Nd isotope compositions. Both 87 Sr/ 86 Sr and ϵ Nd vary significantly with depth in the core, 0.72701–0.76708 and −14.4 to −16.6, respectively, within the range for silicate rocks of the Higher and the Lesser Himalaya. The variations in the isotope compositions reflect variations in the mixing proportion of sediments from the Higher and Lesser Himalaya, the two major sediment sources to the Ganga. The opposite trends in 87 Sr/ 86 Sr and ϵ Nd depth profiles further confirm this hypothesis. The isotope profiles exhibit two major excursions, ca. 20 ka and ca. 70 ka ago, coinciding with periods of precipitation minima and larger glacial cover. These excursions are the result of a decrease in the proportion of sediment from the Higher Himalaya due to a decrease in monsoon precipitation and an increase in glacial cover that are in turn caused by lower solar insolation. This study highlights the significant influence of climate on erosion in the Himalaya.

Concentration of carbon dioxide in the Late Cretaceous atmosphere

Stable carbon isotope data from Late Cretaceous (Maastrichtian) palaeosols in India are used to estimate the concentration of carbon dioxide in the Late Cretaceous atmosphere. We show that the Maastrichtian atmosphere is unlikely to have contained more than about 1300 ppm by volume of CO2.This value agrees with an independently modeled value of CO2 in the Late Cretaceous atmosphere. A low concentration of the greenhouse gas carbon dioxide in the Maastrichtian atmosphere (relative to concentrations in the earlier Cretaceous) is consistent with palaeotemperature information from terrestrial plant and marine fossils, which suggest that the global climate cooled toward the end of the Cretaceous Period.

Palaeoenvironments of the dinosaur-bearing Lameta Beds (Maastrichtian), Narmada Valley, Central India

Abstract The Maastrichtian Lameta Beds of central India are intimately associated with the Deccan lavas and are critical to determining Upper Cretaceous palaeoenvironments and palaeogeography of the area. In the type Jabalpur sub-region, four mappable units of the Lameta Beds are recognised. The basal Green Sandstone is interpreted as a braided stream deposit. The Lower Limestone, characterised by brecciation and shrinkage cracks, is interpreted as a sub-aerially exposed palustrine flat with calcrete formation occurring on topographic highs of low relief plains. The overlying Mottled Nodular Beds exhibit a variable range of calcrete fabrics and morphologies including circum-granular and linear cracks, root casts and nodules. These are interpreted as pedogenically modified sheet wash deposits of a semiarid alluvial plain. The Upper Sandstone is a sheet flood deposit, again pedogenically modified before arrival of the basal lava flows. Overall the Lameta beds are considered to represent a regionally extensive Maastrichtian regolith. Carbon and oxygen isotope analyses of calcareous components are entirely consistent with soil-zone environments. The δ13C values are low, typically −8 to −9‰ PDB, demonstrating a strong input of carbon from the decay of terrestrial land plants. Calcrete δ18O values are variable, −5 to −10‰ PDB consistent with precipitation from meteoric water, some of which was evaporatively modified in pools on the alluvial/palustrine flat. The Lameta Beds are well known for sauropod nesting sites and sedimentological analyses of these sites suggest that the animals selected topographic highs, usually in marly or sandy, soft sediment. Multiple nests with similar egg types probably indicate colonial nesting. Isotopic analyses of eggshell carbonate agree with earlier work suggesting that the sauropods ate a “C3” plant food.

Calcretes at sequence boundaries in upper carboniferous cyclothems of the Sydney Basin, Atlantic Canada

Abstract Nodular and associated groundwater calcretes in coal-bearing cyclothems of the Sydney Basin mark the boundary between underlying marginal-marine and overlying alluvial deposits. They are inferred to represent lowstand surfaces (sequence boundaries). The host sediments include both sand/siltstones and palustrine limestones. Microfabrics of calcretes with detrital hosts show evidence of replacement, displacement and shrinkage (alpha fabrics), suggesting that the calcretes formed under relatively arid conditions. In contrast, microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics, with root-induced brecciation and subordinate shrinkage fabrics. The calcretes and host limestones are inferred to have formed in topographic lows. Thick alluvial sediments with red calcic vertisols overlie the calcretes, and formed within the transgressive systems tract under conditions of abundant sediment supply. Coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract. The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity.

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.

Aggradation history and luminescence chronology of Late Quaternary semi-arid sequences of the Sabarmati basin, Gujarat, Western India

The Gujarat alluvial plain in the semi-arid desert margin of the Thar desert contains a record of continental Quaternary sequences. These sequences were built by drainages originating in the Aravalli Hills and flowing westward into the Indian Ocean. Stratigraphic studies of a part of the Late Quaternary succession in the Sabarmati basin have resulted in a fourfold lithostratigraphic scheme—the Waghpur Formation (Conglomeratic Member; Sand Member); the Mahesana Formation (Lower Heterolithic Member; Upper Sand Member); the Akhaj Formation, and the Sabarmati Formation. Facies analysis, distribution of conglomerates and cross-bed azimuths of the clastic facies of the Waghpur Formation were used to infer NE-SW trending semi-arid alluvial fans. The deposition of the overlying Heterolithic Member took place in ephemeral streams influenced by episodic flows and aeolian reworking. The upper member of the Mahesana Formation represents aeolian and older levee deposits. Feasibility studies of Thermoluminescence (TL) and Infra-red Stimulated Luminescence (IRSL) dating on the Sabarmati succession yielded stratigraphically consistent ages ranging from ∼ 5 ka to ≥ 300 ka. For two samples, TL ages on quartz and IRSL ages on feldspar separates, were consistent. These results, together with the stratigraphic and petrographic data reveal the following events in the accumulation history of the Sabarmati sequence: 1. (1) Significant aggradation at ≥ 300 ka (gravel-sand) and at 58 ka (sand), and 2. (2) Pedogenesis bracketed to 58-39 ka The Sabarmati river in its middle stretches follows a regional slope-deviatory course interpreted previously to be a neotectonic adjustment within the Cambay graben. Our chronological data suggest that this adjustment occurred, possibly, within the past 39 ka.

Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India

Within the Himalayan Foreland Basin, the axial Yamuna River with Himalayan headwaters lies along the northern margin of the Indian Craton, giving the impression that cratonic rivers have contributed little to the basin compared with Himalayan drainages. However, the Betwa, Chambal, and other rivers, which drain northward into the Yamuna, are vigorous monsoonal rivers with large catchments. Stratigraphic and petrographic evidence shows that sediment derived largely from these rivers extends north of the axial Yamuna River. Red feldspathic sand and gravel underlie much of the southern foreland basin at shallow depth (>25 m), where its topmost strata are dated at ca. 119 ka ago, and extend at deeper levels (>500 m) to about one-third of the distance across the foreland basin. Petrographic analysis confirms a match with modern Betwa River sands, which derive their feldspar from granitic gneisses of the Bundelkhand Complex. Along the Yamuna Valley the red alluvium is overlain by gray alluvium dated at 82–35 ka ago, which also yields a cratonic signature, with large amounts of smectite derived from the Deccan Traps. Cratonic contributions are evident in alluvium as young as 9 ka ago in a section 25 km north of the Yamuna. This gray cratonic sediment was probably deposited in part by the Chambal River, which transports high-grade metamorphic minerals from the Banded Gneiss Complex of the Aravalli belt. Cratonic sediment appears to interfinger with Himalayan detritus farther north below the Ganga-Yamuna Interfluve. With its headwaters in the tectonically unstable Indus-Ganga watershed area, the Yamuna River may have occupied its present course late in the Quaternary, and if so, cratonic rivers may have provided the basin9s axial drainage for prolonged periods. The penetration of Himalayan sediment to the distal foreland basin may reflect avulsion of orogenic rivers along the craton margin, in addition to dynamic transverse drainage systems from the Himalaya that pushed the axial drainage to the basin9s feather edge. The wide spread of cratonic sediment would have been enhanced by slow subsidence in the distal foreland basin and focusing of rivers into a basin reentrant.

Sedimentology of plio-pleistocene late orogenic deposits associated with intraplate subduction—the Upper Siwalik Subgroup of a part of Panjab Sub-Himalaya, India

Abstract This study provides a detailed sedimentologic documentation of the lithofacies, cyclicity, sandbody geometry, palaeoflow variability and depositional models of the youngest late orogenic deposits of the Siwalik molasse. These deposits represent one of the important examples of sedimentation in the zone of intraplate subduction of the Himalayan collision belt. Lithofacies analysis has resulted in the recognition of four conglomerate facies (massive, matrix-supported conglomerate; crudely stratified conglomerate; trough cross-stratified conglomerate; planar cross-stratified conglomerate), five sandstone facies (erosional scour and fill; pebbly sandstone; medium- to coarse-grained sandstone; fine- to medium-grained ripple drift laminated sandstone; bioturbated calcareous fine-grained sandstone), and six mudstone facies (parallel-laminated bioturbated mudstone; reddish brown massive mudstone; variegated mudstone; bluish-grey to violet mudstone; buff-coloured mudstone; pebbly mudstone). In addition, hard calcareous layers have been recognised as a separate facies state. The deduced cyclic sequences of the Tatrot and Pinjor Formations of the Upper Siwalik Subgroup show fining-upward characters from CS (coarse sandstone) to MS (mudstone) through FS (fine sandstone) and SS (alternations of sandstone and siltstone). The sandbodies of the Tatrot and Pinjor Formations reveal a sheet-like geometry and an internal dominance of inter-storey scour surfaces. The presence of intraformational debris, mud drapes, reactivation surfaces and a general fining upward of grain size indicates fluctuating hydrodynamic conditions with frequent channel abandonment and re-occupation, resulting in net vertical aggradation. The dominant style of sedimentation both in the channel and overbank facies is through vertical aggradation. Data on the palaeoflow variability indicates that the Upper Siwalik Subgroup can be subdivided into four and three sediment-vector sequences in the Khetpurali and Ghaggar sections, respectively. The palaeodrainage pattern in the Khetpurali section reveals a major influence of northerly drainage twice in the stratigraphic column. The high percentage of overbank deposits in individual cycles, the lack of vegetal debris, organic-poor palaeosol units, and abundant syngenetic CaCO 3 activity warrant comparison with the modern anastomosed deposits described from arid Central Australia. Some intervals of the sequences are comparable to the sublinear, subparallel drainages with local anastomosis of the modern piedmont zone of the Himalaya. The higher parts of the stratigraphic column indicate proximal braided stream deposits and finally proximal alluvial fan deposits. Syntectonic activity has controlled the deposition of the megasequences which are a response to episodicity in the activity of basin margin-basin margin or basin margin-intrabasin structures. The vertical stacking of the Siwalik megasequences appears to be episodic in character.

Calcretes in the Thar desert: Genesis, chronology and palaeoenvironment

The calcretes in the Thar desert occur in a variety of settings, including the piedmonts, sheetwash aggraded plains; and this study adds calcretes in regolith and colluvio-alluvial plains to the group of settings in which calcretes occur in the region. Field logs, morphological details and analytical data such as petrographic, cathodoluminescence and geochemical characteristics are described along with a discussion on their implications. Sand dunes and sandy plains dating to < 20 ka have weakly developed calcretes. The better-developed calcrete horizons occur in piedmonts, interdunes or in areas that have sufficient groundwater. Deep sections in the region show phases of calcrete development in aeolian sand aggradation at ∼ 150, ∼ 100, ∼ 60 and 27–14 ka. The extensive sheetwash plains have mature calcretes and date to mid-Pleistocene. Our studies indicate that these calcretes represent a hybrid process, where carbonate enrichment of the originally calcareous host occurred due to periodically raised groundwaters, and its differentiation into nodules occurred under subaerial environment i.e., after recession of groundwater. Deep sections also show a stack of discrete calcretes that developed in individual aggradation episodes with hiatuses as indicated by ESR dating results. Nodules display a multiplicity of carbonate precipi tation events and internal reorganization of calcitic groundmass. The process is accompanied by degradation and transformation of unstable minerals, particularly clays and with a neosynthesis of palygorskite.The ancient calcretes are dated from the beginning of the Quaternary to ∼ 600 ka and show more evolved morphologies marked by brecciation, dissolution, laminar growth on brecciated surfaces, pisolites and several generations of re-cementation. Mica/chlorite schists and such other rocks are particularly vulnerable to replacement by carbonate. In an extreme case, replacement of quartzose sandstone was observed also. The presence of stretches of alluvio-colluvial plains in an area presently devoid of drainage bespeaks of occasional high-energy fluvial regime, under a semi-arid climate. The mid-Pleistocene period saw a shift towards more arid climate and this facilitated sheetwash aggradation. Finally, during the late Pleistocene, aggradation of aeolian sands indicated a progressively drier climate. However, this does not find its reflection in stable isotope data. The amount of carbonate in the form of calcretes is substantial. The present studies indicate that aeolian dust or rainwater are minor contributors to the carbonate budget. A more important source was provided by the pre-existing calcretes in the sheetwash aggraded plains and detrital carbonate in the aeolian sediments. The original source of carbonate in the region, however, remains unresolved and will need further investigations. Electron spin resonance protocols for the dating of calcretes were developed as a part of this study and the results accorded well with geological reasoning