Young Formation Research Articles

The Fjallgardar volcanic ridge in NE Iceland: an aborted early stage plate boundary or a volcanically dormant zone?

Summary The extinct Fjallgardar volcanic ridge (FVR) in NE Iceland is approximately 15–20 km wide and extends for about 190 km from the Vatnajökull glacier toward the Slétta peninsula in the far N. The FVR was mainly formed subglacially and strikingly parallels the curvature of the present median line (Askja to Theistareykir centres) of the neovolcanic zone (NVZ) which lies about 40 km to the W. The basalts from the Fjallgardar and Slétta regions are dominantly evolved tholeiites as shown by major- and trace-element compositions of 42 samples. The average magnesium number is 47 (range 37–58). 90 per cent of the samples can be grouped into either ‘low-’ potassium (0.1%–0.4% K 2 O) or ‘high-’ potassium (0.5%–0.7% K 2 O) units. The high-potassium lavas in the FVR are chemically similar to Tertiary tholeiites of eastern Iceland. A test for temporal geochemical variations for three stratigraphic formations was carried out for the Mödrudalur area of the FVR. The oldest of the three formations (M3) consists of low-potassium basalts only, whereas the two younger formations (M4 and M5) consist predominantly of more evolved high-potassium basalts. Formation M5 is most probably fed into Mödrudalur from the Kverkfjöll volcanic system some 75 km further S. Volcanism in the FVR S of Slétta was short-lived and lasted for perhaps 200 000–300 000 years. Subsurface lateral flow of magma from the NVZ is probably responsible for the Recent subaerial volcanism in Slétta. In the Slétta area the older subglacially formed ridges have only a low-potassium basalt composition. The FVR may be viewed as ‘abortive’ and as the precursor to the NVZ in northeastern Iceland. During the Brunhes magnetic polarity chron (less than 0.7 Ma) a rift jump is postulated to have occurred from the FVR to the present NVZ, approximately 40 km to the W. E.-W lateral rift jumps (20–40 km) may have been common in the eastern half of Iceland in the vicinity of the Iceland hot-spot located 150–200 km E of the Reykjanes and Kolbeinsey mid-ocean ridges.

Stratigraphy and Structure of Late Archaean, Dharwar Volcanic and Sedimentary Rocks and their Basement in a Part of the Shimoga Basin, East of Bhadravathi, Karnataka

Abstract The Dharwar Supergroup in the Shimoga basin east of Bhadravathi has been subdivided into seven formations on the basis of sedimentary structures and facies. The lowest is dominated by metabasites including metalavas and tuffs, metagabbros with seams of titaniferous magnetite, serpentinites and talc schists. A range of sedimentary rocks including phyllitic greywackes, quartzites, dolomitic limestones, conglomerates and local banded ferruginous cherts characterise the formations overlying the metabasites. These formations include local metabasites, and the youngest formation contains a thick suite of metarhyolites. The Dharwar lithostratigraphy is characterised by rapid lateral variations in thickness and facies which are the consequence of its original unstable alluvial and relatively shallow marine depositional environments. The instability is marked by slump structures in the phyllitic greywackes and by debris flows at many levels in the stratigraphy. The Dharwar rocks were deposited on a basement of homogeneous granodiorite s.l. and foliated multiphase quartzo-feldspathic orthogneisses. Erosion of elevated parts of the basement and the Dharwar cover provided detritus for alluvial fans, shallow marine quartzites and more distal phyllitic greywackes. Correlation of the lowest formations in tbe Bababudan basin immediately to the south shows that the Dharwar Supergroup becomes progressively younger from south to north in this part of Karnataka. The structure is characterised by large upright synclines in the Dharwar cover with intervening domal areas of basement. Many of the cover-basement contacts are faults, but some are interpreted as rotated and steepened unconformities. Flushing of water and CO2 through much of the basement and cover took place during deformation. Buoyancy forces related to heating at deep crustal levels and concomitant compressional forces, generated perhaps in a transpressive oblique-slip regime, may account for the structure of the cover and its basement. Comparable tectonothermal instability, but with trans-tensile components, may have characterised the Dharwar volcanic and depositional phases.

The nature and field relations of the Ordovician Maumtrasna Formation, County Mayo, Ireland

AbstractThe nature and field relationships of the Maumtrasna Formation of the Partry Mountains, Co. Mayo, show that the formation underlies the regionally mappable Glenummera Formation, which is of Llanvirn age, and it is laterally equivalent to the Rosroe and Derrylea Formations, also of Llanvirn age. The formation rests unconformably on two different volcanic domains, the basic volcanics of the Bohaun Volcanic Formation to the northwest (of unknown age) and the largely acidic volcanics and sediments of the Tourmakeady Volcanic succession to the southeast (Arenig). It is not the youngest formation in the Ordovician succession of the South Mayo Trough, as was previously thought.This largely conglomeratic unit forms a coarsening upward sequence deposited on alluvial fans which were inundated by a marine transgression at the base of the Glenummera Formation. Clasts in the conglomerates are mainly of granite and porphyry with a minor metamorphic component. The source is interpreted as an arc rooted on a metamorphic basement which lay to the south and east. The suggestion that this area was present day Connemara finds little support. This area may expose some of the ‘hidden’ geology of the Midland Valley of Scotland.

Depositional Environments and Diagenetic Features of a Cretaceous Clastic Sequence, Fox Hills Sandstone of Northern Great Plains Province: ABSTRACT

The Fox Hills Sandstone, youngest formation of the Montana Group, is a geographically extensive, thin unit that is composed primarily of silty sandstone and shale. This formation records the final withdrawal of the Upper Cretaceous epicontinental sea from the Western Interior of the US. It extends from central Canada southward through Montana, the Dakotas, Wyoming, and Colorado. This typical shoreline sequence is exposed along the flanks of the Cedar Creek anticline, which trends southeastward from eastern Montana into North and South Dakota. The upper Fox Hills consists of stacked, upward-fining, trough to planar cross-bedded sandstone bodies. These sequences grade laterally and vertically into interbedded clay and silt units containing root casts and are interpreted as deposits of a braided-stream system. A matrix mineral assemblage of chlorite, montmorillonite, calcite, and minor dolomite is characteristic of distributary channels and river mouth-bar facies. The lower Fox Hills consists of either irregularly stacked upward-coarsening, trough cross-bedded sand sequences or massive sand bodies separated by erosional surfaces. Outcrops locally contain the trace fossil Ophiomorpha or limestone concretions containing marine to brackish-water fossils, and have been interpreted as deposits of a rapidly advancing shoreline and strand plain. In the matrix, illite, kaolinite, and montmorillonite with minormore » calcite characterize beach, subtidal, and crevasse-splay environments. Diagenetic features include quartz overgrowths, clay alterations of potassium feldspar and rock fragments, and authigenic clay rims. Petrographic evidence indicates multiple source areas for these sediments and the primary influence of depositional environments on clay formation.« less

Genetic interpretation of lead-isotopic data from the Columbia River Basalt Group, Oregon, Washington, and Idaho

Lead-isotopic signatures of basalts from the Columbia River Basalt Group of central Oregon, Washington, and western Idaho, and of the high-alumina olivine basalts from southeastern Oregon plot mostly in the field of Pb-isotopic data from volcanic rocks of the Cascade Range. In contrast to the mid-ocean ridge basalts (MORB), the Pb-isotopic data for several of the formations form small, tight clusters, and the Nd- and Sr-isotopic data show discrete variation between these basalt groups. Pseudoisochrons are not observed. A single model will account for the observed isotopic and trace-element data from samples from most of the Columbia River Basalt Group. This model calls for partial melting of the convecting oceanic-type mantle and mixing with, or contamination by, fluids derived from continental-derived sediments. It requires that crustal components are not only subducted along the trench, but that they are also transported in the low-velocity zone of the mantle at least 400 km behind the active volcanic arc into the back-arc environment represented by the Columbia Plateaus province. Lead isotopes in the Saddle Mountains Basalt, the youngest formation, suggest that these magmas were formed by partial melting of a 2.6-b.y.-old subcontinental lithosphere that extends as far west as south-central Washington near Ice Harbor Dam. The source material for these flows is characterized by high Th/U, Th/Pb, Rb/Sr, and Nd/Sm ratios and light REE enrichment. An additional component, presumably granitic crust, is necessary to explain the Pb-isotopic data of the Umatilla and perhaps the Pomona Members of the Saddle Mountains Basalt. This contamination model is supported by published δ18O data and petrological interpretation. Published studies of basalts from the Deccan Traps, the Tasmanian dolerites, the Snake River Group, and the volcanics of the Yellowstone Plateau province also suggest that melting of old subcontinental lithosphere beneath the continental crust may be an important process in basalt genesis on a global scale.

Stratigraphy of the Upper Devonian argillite successions in north Cornwall

AbstractSome 880 m of grey, green and purple argillites are estimated to be present in complexly deformed Upper Devonian successions which occur in several overlapping coastal sections. Four lithostratigraphical formations are formally defined, in order of decreasing age: the Marble Cliff Limestone; Longcarrow Cove Slate; Harbour Cove Slate; and Polzeath Slate Formations. Improved correlation and biostratigraphical control is available for the two younger formations which are studied in detail. The formations range in age from lowest Frasnian to mid Famennian. Despite the intense deformation there are no major structural breaks in the successions.

Influence of Strike-Slip Movement on Terrestrial Sedimentation in Upper Carboniferous of Nova Scotia and New Brunswick: ABSTRACT

Lower Pennsylvanian (Namurian and lower Westphalian) sediments of maritime Canada were deposited in alluvial and lacustrine environments. Thick (> 3.5 km, 2.1 mi) sequences of these sediments accumulated within structural basins formed between strike-slip faults. Movement of the faults during the Namurian caused uplift in compressional areas. These positive areas, consisting of crystalline rocks and older Carboniferous sediments, provided a local supply of coarse sediment to the basin. In some cases, the positive areas excluded the introduction of sediment from large rivers sourced well outside the depositional basin. Large lakes were common in the Namurian and were probably a result of the relatively small amount of sediment that entered the basins. This sediment-starvation is also indicated by sequences of stacked paleosols that provide evidence of slow rates of sedimentation. A detailed study of the sediments indicates a progressive climatic change from arid conditions, with evaporites, in the lower Namurian to humid conditions, with coal deposits, in the Westphalian. This climatic change is reflected by an increase in the size of the extra-basinal river systems in younger formations. By the mid-Westphalian, the influx of sediment to the area was so great that the topographic basins were essentially infilled. Westphalian lakes were shallow and of limited lateral extent. Anomalously thick sequences of overbank sediments and stacked point-bar deposits are present, suggesting that tectonic movements were still sufficiently strong to influence the style of fluvial architecture. End_of_Article - Last_Page 505------------

The petrology and palaeoenvironment of the Sortino Group (Miocene) of SE Sicily: evidence for periodic emergence

The Sortino Group of carbonates consists of three formations which developed on a shallow marine shelf during Tortonian and lower Messinian times. The two lower formations contain three bioherms and are intercalated with thick volcaniclastic sequences. The youngest formation is dominated by shoal water, lagoonal and peri-tidal carbonates. Early submarine lithification is well-developed within the bioherms and consists of construction by both algae and invertebrates, the development of peloidal micrites and isopachous fringe growth. Superimposed upon these are a series of both meteoric phreatic and vadose textures related to episodic emergence of the bioherms. These features include bladed cements showing cathodoluminescence, pendent cements, brecciation, alveolar texture and palaeosols. The Sicilian strata may be directly correlated with the Maltese Islands sequence, and collectively exhibit the effects of fluctuating Mediterranean sea levels at an early stage in the late Miocene salinity crisis.

Quaternary geology of the middle son valley, North Central India: Implications for prehistoric archaeology

The late Quaternary alluvial history of the Son is very similar to that of the Nile, the Amazon and the Murray—Murrumbridgee. During the terminal Pleistocene cros-bedded sands were laid down as bedload by a highly seasonal, low-sinuosity stream. Early Holocene deposition of horizontally stratified silts and clays is the suspended load of regular overbank flooding by a less seasonal, more sinuous stream. The upward-fining sequence of the late Quaternary Son coincides with a global change in climate in the intertropical zone, from cold and dry in the terminal Pleistocene (⩽25,000 B.P.) to warm and wet in the Early Holocene. Two earlier alluvial formations are present in the middle Son valley. One, directly overlying eroded bedrock, comprises fanglomerates and clays of a mantled pediment sequence developed when river level was low and the climate dry. Lower Palaeolithic artifacts of upper Acheulian aspect are common near the fanglomerate—clay contact. The clay may be reworked aeolian dust. The other, younger formation is fluviatile. It represents an influx of allocthonous gravels under high flow regime during the Late Pleistocene (⩽75,000 B.P.). Middle Palaeolithic implements fashioned from the exotic gravels are found in situ in this formation produced by a return to wetter conditions. Earlier two-dimensional models which equate the north-central Indian Lower, Middle and Upper Palaeolithic, respectively with Gravels I, II and III of the Belan valley sequence now require amendment. The three-dimensional model proposed here to account for the Quaternary alluvial formations in the middle Son valley also has important implications for archaeological prospecting. Primary-context microlithic sites are generally confined to the outer margin of the early Holocene floodplain, along the piedmont zone of the Kaimur hills. Derived Middle Palaeolithic flakes and tools are scattered throughout the terminal Pleistocene cross-bedded sands of the Son; relatively undisturbed Middle Palaeolithic assemblages are stratified within the preceding alluvial formation.

Late tortonian—lower messinian (miocene) palaeogeography of SE Sicily: information from two new formations of the sorting group

A thick volcanoclastic, biohermal and lagoonal sequence of strata succeeds the Siracusa Limestone Member (Monti Climiti Formation) of SE Sicily. This overlying sequence may be divided into two new formations on the basis of lithostratigraphy, petrology and palaeontology. The older formation, here named the Carlentini Formation, is predominantly a thick volcanoclastic sequence but with two intervening carbonate horizons. Faunas are rare within the volcanic strata but may be abundant and associated with corals and patch-reefs within the two carbonate levels. The younger formation, here named the Monte Carrubba Formation, is almost exclusively composed of oolitic grainstones and lagoonal wackestones. Peritidal carbonates are developed in the northeast of the area. Faunas are frequently of low diversity. Euryhaline mollusc associations are dominant, although Pecten vigolenesis occurs in the lower beds. Collectively the new formations span the late Tortonian—lower Messinian time interval and may be correlated with the Calcare di Base of the main sicilian basin to the west. The palaeoenvironmental reconstruction suggests that both formations were deposited around the SW margins to a low island or landmass lying to the north of the Hyblean region.

Inferred Eastern Extent, Overthrust Belt, Central Utah: ABSTRACT

Structural and stratigraphic relations in the Wasatch Plateau, the Gunnison Plateau, and the northwestern Cedar Hills suggest a new interpretation of the easternmost limit of thrusting during Late Cretaceous foreland deformation in central Utah. Steep to overturned dips in the Upper Cretaceous Indianola Group within this region are interpreted to reflect involvement of foreland basin clastics in thrust-related structures that deformed the flank of the foreland basin. A subhorizontal overlap assemblage of inferred Paleocene age indicates that the post-thrust erosion surface sloped gently eastward into the foreland basin. Northeast-striking, southeast-facing homoclines of Indianola Group strata rest disconformably on Jurassic beds. However, favorable horizons within the Arapien Shale of the Jurassic sequence served as a zone of regional decollement, along which younger formations were detached and transported eastward with respect to underlying autochthonous units. The decollement zone was deformed internally by multiple thrusts and by east-vergent isoclinal folds subsequently complicated by local diapiric modifications. Most exposures of the Indianola Group are thus wholly or partly allochthonous. Folds involving Indianola strata apparently include: (1) fully detached ramp anticlines associated with subsurface thrust faults that underlie the southern Wasatch Mountains, Gunnison Plateau, and Sanpete Va ley; and (2) partly detached frontal anticlines associated with blind thrusts that approach the surface beneath the Sanpete Valley and the western margin of the Wasatch Plateau. Structures inferred locally are consistent with documented patterns of deformation involving foreland clastics elsewhere in the overthrust belt and should influence exploration strategy in central Utah. End_of_Article - Last_Page 592------------

A seismic velocity inversion on Bjørnøya — The western Barents Shelf

Seismic velocity measurements on the exposed Triassic to Cambro-Silurian sedimentary sequence on Bjørnøya show a velocity inversion in the stratigraphic section with a minimum velocity (3.8 km/s) in the Devonian fluvial sandstones of the Røedvika Formation, which also has the highest porosity (∼ 17%). The low porosity in the younger formations is due to silicification and dolomitization probably by chemical alterations of the pore water rather than an overburden effect. Sonobuoy measurements in the vicinity of Bjørnøya show seismic velocities at the sea floor comparable with velocities of the exposed formations on the island itself.

Authigenic, perigenic, and allogenic glauconites from the Castle Hill Basin, North Canterbury, New Zealand

Abstract Two major vanenes of glauconite are distinguished within the Upper Cretaceous and Tertiary formations of the Castle Hill Basin. Type A is the only variety present in the Upper Cretaceous Broken River Coal Measures, is absent in the Upper Cretaceous to Eocene Iron Creek Greensand which overlies the coal measures, and is sparsely present in the younger (Oligocene to Miocene and possibly early Pliocene) detrital formations. This type is dusky yellow-green, crystallographically disordered with 40–50% expandable layers. It shows a characteristic “cauliflower” texture under the scanning electron microscope. Type A occurs mainly as a matrix/cement in the coal measures, but outlines of rounded grains can be discerned within this interstitial filling; it occurs only as partly rounded fragments in younger formations. Type B is the only variety present in the Iron Creek Greensand and is the most abundant glauconite variety in the younger formations. It is greenish black, crystallographically disordered with 15–25% expandable layers, and has more iron, potassium, magnesium, calcium, and strontium than Type A. Type B always occurs as pellets with spheroidal, ovoidal, or lobate morphology, and resembles the common glauconite described in most previous literature. Glauconite of Type B is formed originally at the sediment-water interface in a normal marine environment, i.e., it is authigenic and perigenic (transported locally). Glauconite of Type A developed after burial, under conditions of reduced marine salinity and/or influenced by organic decomposition products. In the coal measures it is authigenic, in part being the result of chemical modification of parent glauconite grains which themselves probably had formed at the sedimentwater interface. The vast majority of the glauconite in the younger formations studied has been recycled from the Broken River Coal Measures and the Iron Creek Greensand; i.e., in these occurrences the grains are allogenic. Different varieties of glauconite can be distinguished and used to decipher the stratigraphic record. Also, glauconite can be recycled, hence care must be exercised when using this mineral to provide radiometric dates for deposition of the enclosing sediments.

Developments in Arkansas, North Louisiana, and East Texas in 1977

In 1977, the level of activity in all 3 CSD Districts covered by this report (Arkansas, North Louisiana, East Texas) continued to show the strength which developed in 1976. Levels of exploratory and development drilling in several areas even showed gains from the very high 1976 figures. Successes in South Arkansas were distributed more evenly among Smackover and younger formations than in the past. Arkoma basin activity centered on field extensions. North Louisiana drilling was concentrated on the Cotton Valley and Hosston. East Texas activity was widely distributed with heavy interest in the Cotton Valley of both the east and west sides of the East Texas basin.

Stratigraphy and structure of the Mt Patriarch-Crow River area, North-West Nelson, New Zealand

Abstract The Mt Patriarch-Crow River area contains the three youngest formations in the Central Sedimentary Belt of North-West Nelson: the Baldy Formation (new), Summit Limestone, and Patriarch Formation, all of Lower Ordovician age and included in the Mt Patriarch Group (new). The Baldy Formation is the youngest formation and consists of at least 100 m of sheared grey sandstone and phyllite. The fossiliferous Summit Limestone lies conformably between the Baldy Formation and the Patriarch Formation. The latter is redefined and consists of fossiliferous medium to dark grey siltstone and fine sandstone with carbonaceous limestone and shale horizons. The Anatoki Formation of the Haupiri Group conformably underlies the Patriarch Formation. The faults of the area are divided into old (commonly rejuvenated in the late Cenozoic) generally north-striking faults, north-east-striking faults with large components of late Cenozoic movement, and low-angle thrusts. The dominant folds of the area are about north-trending subhorizontal axes, but mesoscopic structures in both the Western and Central Sedimentary Belts are consistent with there having been an earlier period of recumbent folding about east-trending axes. The presence of a major thrust within the Central Belt in the area, corresponding to the Haupiri Thrust to the north, suggests that low-angle thrusts extend the length of the belt and that the internal structure of the Central Belt is consistent with an allochthonous origin.

A class of distributions useful in life testing and reliability : Z. Govindarajula. IEEE Trans. Reliab.R-26, (1) 67 (April 1977)

A thick volcanoclastic, biohermal and lagoonal sequence of strata succeeds the Siracusa Limestone Member (Monti Climiti Formation) of SE Sicily. This overlying sequence may be divided into two new formations on the basis of lithostratigraphy, petrology and palaeontology. The older formation, here named the Carlentini Formation, is predominantly a thick volcanoclastic sequence but with two intervening carbonate horizons. Faunas are rare within the volcanic strata but may be abundant and associated with corals and patch-reefs within the two carbonate levels.The younger formation, here named the Monte Carrubba Formation, is almost exclusively composed of oolitic grainstones and lagoonal wackestones. Peritidal carbonates are developed in the northeast of the area. Faunas are frequently of low diversity. Euryhaline mollusc associations are dominant, although Pecten vigolenesis occurs in the lower beds.Collectively the new formations span the late Tortonian—lower Messinian time interval and may be correlated with the Calcare di Base of the main sicilian basin to the west.The palaeoenvironmental reconstruction suggests that both formations were deposited around the SW margins to a low island or landmass lying to the north of the Hyblean region.

Age and correlation of late quaternary terraces in the Lower Inangahua Valley, West Coast, South Island, New Zealand

Abstract Three late Quaternary aggradational surfaces are recognised in the Inangahua valley, and are underlain by gravels correlated with the Speargrass, Tophouse, and Manuka Formations of the Upper Buller catchment. The Manuka Formation underlies a high terrace which can be followed down the Buller valley from Murchison to Inangahua, and is thought to have been deposited during the Waimaunga Glaciation (Suggate, 1965). The two younger formations cannot be confidently traced away from the Inangahua valley, and correlation with the Upper Buller formations is indirect. Pollen analyses indicate that both formations were deposited during glacial periods. A radiocarbon date of 20 200 ± 300 years B.P. indicates that the Speargrass Formation is correlated with the Kumara-22 advance of the Otira Glaciation. A sample from the Tophouse Formation was beyond the range of dating (> 47500 years B.P.), and this formation is tentatively correlated with the Waimea Glaciation.

Porphyrin Pigments and Polycyclic Aromatics in the Neogene Tertiary of the Chuetsu District

This is the fourth of a series of paper reporting the results of a study of the porphyrin pigments and polycyclic aromatics in Neogene Tertiary rocks, Northeast Japan. The present paper gives the results of analyses on 92 muddy rocks from the Chuetsu oil field, Niigata Prefecture (Fig. 1). These data, along with earlier analytical results from the previous reports (Taguchi, Sasaki & Ushijima, 1969; Taguchi, Sasaki, Endo & Sato, 1970; Taguchi, Sasaki & Ushijima, 1970) are discussed from the standpoint of sedimentological study. The tendency of the stratigraphic distribution of these compounds in the study area is generally the same as in other areas reported before. However, the average contents of the nickel porphyrin of the Teradomari Formation in Higashiyama and Nishiyama areas show rather different value from correlatives of the Teradomari in the other areas and A-type of vanadyl porphyrin is detected preferentially not only in the early or middle Miocene strata as reported previously but also in the younger formation as the Shiiya of late Miocene. Polycyclic aromatics were found with 100 per cent positive indications in the study area, and showed higher concentration in the samples of the Nanatani Formation in the Higashiyama area. Particularly, it is very interesting that histogram which shows spectral variations in wavelengths of absorption peaks for polycyclic aromatics in the Higashiyarna area has similar pattern to those in some coal fields. The results mentioned above are showed more clearly and are compared with those of other areas by employing the triangular diagrams as seen in Fig. 6. It is clear that both areas of Higashiyama, Niigata District and Kanita, Aomori District have the different patterns from those of Akita and Yamagata Districts. The two unique patterns in Higashiyama and Kanita areas are called “Higashiyama-type” and “Kanita-type” respectively. Other patterns belong to “Yashimatype” which were reported previously. The characteristics of Higashiyama-type may be explained well by the fact that Higashiyama area is located at the marginal part of the Niigata sedimentary basin. Imokawa area also lies in the marginal part, but it does not show similar pattern to the Higashiyama. This may be attributed to the fact that Imokawa area corresponds to the invasion area which have developed roughly in the NW-SE direction in the time of deposition.

Subdivision and petrology of the Mesozoic rocks Of Coromandel (Manaia Hill Group)

Abstract The original unconformable threefold subdivision of the sedimentary Mesozoic basement rocks of Coromandel is redefined as two conformable formations (Moehau and Tokatea Hill) within Manaia Hill Group on the basis of gradational detrital content and structural continuity; the new sequence is inverted with respect to previous subdivision. Standard sections are described for both formations (Moehau 520–610 m minimum; Tokatea Hill 530 m maximum). The older formation (Moehau) is characterised by a lithic volcanic greywacke and sub-greywacke suite derived from calc-alkaline volcanic and plutonic rocks, and minor sedimentary rocks, whereas the younger formation (Tokatea Hill) is a feldspathic greywacke suite, almost devoid of volcanic detritus except near the base, derived from a more mature calc-alkaline plutonic landmass with minor sedimentary rocks. Moehau Formation and the lower part of the Tokatea Hill Formation are slope-deposited turbidites, whereas the upper Tokatea Hill Formation resulted from ...

Palynology of the tertiary subcrops of upper Assam.

A brief account of the palynological studies carried out on the subcrops of Upper Assam during the last decade and more, mostly by one of us (D.B.), is presented. One representative well, each from Borholla, Naginijan, Teok, Amguri, Disangmukh, Rudrasagar, Geleki, Naz-ira and Lakwa structures, in Sibsagar district of Upper Assam, have been incorporated in the present
 report. On the basis of the palynofossils recorded, the subsurface sequence has been divided into three or four broad palynological zones, depending on the formations penetrated. The lowest zone, Paly. Zone IV, has been dated as Eocene (Mid. to Upp.) on the basis of the common occurrence of Retialetes spp., Monolites mawkmaensis, Palmaeplloenites eocenicus, Palaeorubiaceaepites sp., Palaeocaesalpiniaceaepites eoceniea, Polyeolpites spp., Nothofagidites spp., Sehizosporis spp., Hystrichospheridium spp. etc. In Geleki, Nazira and Lakwa structures, wells have not been drilled deep enough to encounter this zone. Sylhet and Kopili Formations are included in this zone. The next higher zone, Paly. Zone III, has been dated as Oligocene and includes the Barail Formation. The palynomorphs of common occurrence are Meyeripollis sp., Foldexina inaperturata, Cieatrieosisporites sp., Disuleites sp., Palaeoeaesalpiniaeeaepites sp., Polyeolpites spp. (not more than 5-6 colpi), Simsangia trispinosa etc. Paly. Zone II has been dated as Miocene (Mid. to Upp.) and is characterised by the occurrence of Dieksoniaeeaesporites sp., Cieatricosisporites spp., Pinuspollenites spp., Podoearpidites sp., Castaneapollenites sp., Alnipollenites sp., Erieipites sp., Stephanoeolpites sp., etc. Tipam Formation is included in this zone. The Surma Group, if present, has not been confirmed so far; Bhuban Formation is not represented in the sequence studied so far. The topmost zone, Paly. Zone I, includes Girujan, Namsang and younger formations. This zone has been dated as Plio-Pleistocene to Recent and the taxa occurring commonly are Pteridacidites, Scabratriletes, Sporites circulus, Cyathidites, Graminidites, Juglanspollenites, Compositoipollenites, Umbelliferaepites, Impatiensidites etc.
 The distribution of the taxa in different zones is more or less the same in all the wells under report, which suggests the prevalence of largely similar ecological conditions during deposition of successive zones. Some of the important phenomena noted are:
 
 the frequent occurrence of typical Barail microflora in the Namsang,
 the consistent occurrence of typical Lr. Gondwana (Permo-Carb.) micro flora in the Miocene; and
 the decrease in the frequency of microplanktonic elements from west to east of the area.
 
 From the distribution of the different taxa, it has been concluded that the basin was deeper towards the northeast from Borholla, the westernmost well. Largely shallow-marine to brackish water conditions existed during Paleogene times and there was luxuriant vegetation growing in swamps, fresh-water lakes, along the coast and inland near-shore areas in a warm, humid, tropical to sub-tropical climate. The Neogene flora was essentially inland terrestrial, growing in moist, shady lowlands and cool uplands in a temperate to sub-tropical climate. This change in the microfloral contents in the Paleogene and Neogene is interpreted as indicative of orogenic activities in the northern parts of the area and regression of the sea southwards.