Epeirogenic Movements Research Articles

A possible mechanism for epeirogenic uplift

Continental geology is dominated by vertical movements, some of these are the result of crustal shortening and extension associated with large horizontal displacements, involving processes which are now understood, at least in outline. However, both uplift and subsidence can occur without associated shortening or extension of the continental crust. Most investigations of such epeirogenic movements have been concerned with subsidence, partly because of the economic importance of the resulting sedimentary basins, and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2–4. However, uplift has been relatively neglected and cannot be produced by the same mechanism. The purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust. This suggestion was considered as a possible mechanism for the regional uplift of the Colorado Plateau by Gilbert5 more than 100 years ago, and also by Holmes6 (though both rejected the idea). However, a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process, even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3,8,9. Although the suggestion is not new, the argument used here to support it is, and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation.

PERMIAN AND TRIASSIC GEOLOGICAL HISTORY AND TECTONICS OF THE MIDDLE EAST

The paleogeography postulated from the distribution of Permian and Triassic sedimentary rocks in the Middle East is shown and related to the paleostructure of the region. The Middle East region as defined here includes the Arabian Peninsula, Iran, Iraq, SE Turkey, Syria, Lebanon, Jordan, West Jordan, and the Sinai Peninsula. Within the limits of the area included in this study, a relatively stable pre‐Late Triassic tectonic regime can be recognized and distinguished from a succession of diastrophic events of the Late Triassic epoch, which caused marked changes in the types and distribution of facies.Excluding NE Iran, the Middle East was stable from Late Cambrian to Middle Triassic times, as it formed a part of the Arabian Massif and much of the Middle East Platform, which is a broad shelf bordering the positive Afro‐Arabian Massif to the NE and east. The sediments of this plarform have undergone no strong deformation and have been subjected only to epeirogenic movements which interrupted sedimentation locally or regionally. However, from Upper Permian through Middle Triassic times, the predominant facies on this shelf was carbonates which unconformably overlie the non‐marine or extremely shallow marine elastics of the Lower Permian or older strata in most of the region.Upper Triassic strata in the area are markedly different in facies from older beds because of the movement of blocks in Turkey and northern and eastern Iraq and Iran related to the enlargement and broadening of the Tethys. Asa result, much of northern and eastern Iran were separated from the area to the south and west, which continued to be a shelf flanking the Afro‐Arabian Massif. Consequently, three major structural elements existed in the region: the Arabian Shelf, the Tethyan rift zone and the Paleo‐Tethyan trough.

Relationship of Epeirogeny and Sedimentation in Kansas: ABSTRACT

Regional subsurface and lithofacies mapping has revealed that epeirogenic movement, particularly involving subtle, periodic reactivation of preexisting structural weaknesses has affected sedimentation through time in the Mid-Continent. Similarly, regional outcrop studies have provided many examples where paleostructures have influenced sedimentation. Detailed measurements of the earth's gravity and magnetic field intensity, remote sensing, and geomorphologic analysis also have identified surface and subsurface features which can be used to infer discontinuities in the composition and structure of basement rocks. Faults, folds, and possible fracture systems have propagated up through the sedimentary section, are expressed in the topography of the present land surface, and ontrol stream and river drainage patterns. Structural elements, large and small, appear to have influenced regional and local sedimentation, erosion, and diagenetic patterns through long periods of time, as suggested by the geologic record in Kansas. In some situations, bathymetric and topographic highs related to preexisting structural elements have affected markedly the distribution of petroleum reservoir-quality carbonates and sandstones from the Cambrian through the Cretaceous. A review of four, vertically stacked carbonate-dominated cyclothems from the Upper Pennsylvanian Kansas City Group in central and western Kansas reveals an evolving display of time and location dependent features caused by the combined effects of epeirogenic and recurrent structural movement, sedimentologic controls such as clastic influx, and eustatic changes in sea level. Favorable reservoir facies trends of the carbonates and early freshwater diagenetic patterns can be explained, in part, by variations in the configuration of the Pennsylvania epeiric shelf. In addition to trap formation, subtle structural development also may affect indirectly the reservoir distribution by influencing markedly the processes of sedimentation and diagenesis. Inasmuch as the subtle expression of deep structure usually can be detected in shallow and surface records, it follows that remote sensing and geomorphologic analysis can assist conventional geophysics and subsurface geology in the development of new energy and mineral prospects. End_of_Article - Last_Page 1328------------

The Western Canada sedimentary basin

The Western Canada Sedimentary Basin, a simple northeasterly tapering wedge of sedimentary rocks more than 6 km thick, extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt. Its internal structure and the lateral variations in its shape reflect a long and complex history of development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. This history, which is inextricably linked to the evolution of the Canadian Cordillera, can be outlined succinctly with reference to the unconformity-bounded transgressive-regressive stratigraphic sequences established by Sloss ( Bull. geol. Soc. Am . 74, 93 (1963)), each of which has a distinctive character in Western Canada. The continental terrace wedge was established with the deposition of the Proterozoic Purcell (1500-1350 Ma) and Windermere (850-600 Ma) sequences, but the first record of the platformal phase is the early Palaeozoic transgressive onlap of the early Proterozoic ( > 1750 Ma) crystalline basement by the Sauk sequence. Early Palaeozoic subsidence of the margin of the craton may have been due to cooling of the lithosphere after renewed stretching at the ancient rifted western margin of the Precambrian craton, and to isostatic flexure of the lithosphere under the weight of the sediment that had accumulated at the margin in the oceanward prograding continental terrace wedge. During a subsequent Middle Ordovician to Middle Jurassic phase, the cratonic platform became differentiated into an intersecting network of epeirogenic arches with intervening basins. Development of the basins was as much a result of erosion and uplift of the arches between transgressive-regressive cycles as it was a result of differential subsidence of the basins during the cycles. The cause of the long (> 300 Ma) episode of intermittent epeirogenic movements that produced the basins and arches is a major unsolved problem. The foreland basin developed in two stages, in Middle Jurassic to early Cretaceous and late Cretaceous to Palaeocene time, as a result of collisions between North America and two pieces of a tectonic collage of oceanic terranes that were accreted to its western margin. During these two collisions, the continental terrace wedge, which had accumulated outboard from the rifted margin of the continental craton, was compressed and displaced over the western margin of the craton. Part of the supracrustal cover was scraped off the craton and accreted to the overriding mass to form a wedge of imbricate thrust fault slices that was tectonically prograded over the margin of the continental craton. Isostatic flexure of the continental lithosphere in response to the tectonic loading imposed on it by the displaced continental terrace wedge and the accretionary wedge of thrust slices produced the migrating moat in which the outwash of clastic detritus from the evolving thrust belt was trapped to form the foreland basin.

Hydrogeological conditions in the Middle East

Summary The geology of Middle East is summarized under the subheadings: Precambrian basement, epicontinental sediments, geosynclinal and shelf deposits, Tertiary volcanics and Quaternary cover. The main tectonic episodes including epeirogenic movements, rifting and the Tertiary orogeny, are reviewed. The imposition of hydrometeorolocal and climatic conditions upon the regional geology provides the setting for the hydrogeological discussion. Five factors which influence infiltration to aquifers under conditions of low precipitation and high potential evaportranspiration are discussed. The predominance of fossil groundwater is the most striking hydrogeological phenomenon occurring on a regional scale in the Middle East. Its mode of formation during the pluvials is outlined and the isotopic evidence is reviewed. The main physical and chemical characteristics of fossil ground-waters are described. It is conservatively estimated that some 65 000 km 3 of good- to medium-quality groundwater are stored in the great artesian basins of the Near East. These fossil ground-waters are a non-renewable natural resource. Current annual abstraction is, as yet, a small percentage of the total reserves but economic factors rather than the volume of reserves will determine the ultimate extent of their exploitation. The renewable groundwater resources of the Middle East tend, by comparison, to be of local rather than regional significance. Some originate outside the Middle East, coming in as surface flows in the Nile and Tigris-Euphrates and infiltrating into the sediments in and adjacent to the flood plains. Other renewable resources accumulate within the region where high precipitation and mountainous relief are associated. Such areas include the Djebel Akhdar of Cyrenacia, the Tertiary fold mountains from the Taurus through the Zagros to the Oman ranges, and the volcanic and basement highlands of Yemen, Asir and Ethiopa. Locally, in areas of lower precipitation, lenses of recent fresh groundwater float on regional more saline groundwater. In some areas subsurface flows towards and through wadi systems are also of importance.

Epeirogenic Plate Movements

Consideration is given to the kinematics of epeirogenic movements over the recent geologic past (late Pleistocene) in a region for which there is an abundance of geological and geophysical information-the continental shelf off the East Coast of North America. It is found that there has been a relative change in elevation of 140 m downward toward the northeast over this longitudinal distance of 3000 km during the past 18,000 years. Specifically, Miami has been elevated at a rate of 0.8 cm/yr with respect to the Scotian shelf continental margin. The confirming data include the shelf break corrected for progradation, reef building, and glacio-isostatic adjustment; late Pleistocene shores on the continental shelf; earlier Pleistocene unconformity surfaces; dated geologic samples of known shallow water origin; buried Pleistocene channels; and recent leveling analysis.

Plio-pleistocene volcano-tectonic evolution of la Reforma Caldera, Baja California, Mexico

La Reforma volcanic complex, in east-central Baja California, shows a characteristic caldera structure, 10 km in diameter. The first eruptive stage, during the Pliocene, was manifested by ash and pumice falls and by subaqueous pumitic flows. In a second stage basic flows were deposited in a near-shore environment (subaerial and pillow lavas). During the early Pleistocene a large ignimbritic eruption, producing mainly pantelleritic tuffs, immediately predated the formation of the caldera itself. Afterwards, along marginal fractures of the caldera, some rhyolitic domes and flows partially covered the thick ignimbritic sheet. A block of Miocene substratum, in the center of the caldera, has been uplifted, nearly 1 km, by “resurgent doming”. Small outcrops of diorite might constitute the top of coarse-grained crystallized magmatic bodies, and thus support the “resurgent doming” interpretation. A few basaltic cones were finally built on the flanks of the caldera complex; the latter are not related to the caldera history but to the extension tectonics of the Gulf of California which are also responsible for the Tortuga Island and the Holocene Tres Virgenes tholeiitic cones. South of la Reforma are found the highest (+300 m) Pleistocene marine deposits of the Gulf coast of Baja California. The uplift of this area is due in part to the positive epeirogenic movements of the whole peninsular crustal block, and also to the late doming of the caldera. On the coastal (eastern) flank of La Reforma complex up to seven stepped wave-cut terraces have been preserved, the highest reaching more than +150 m and the lowest ones +25 m. Lateral correlations of the marine terraces along the whole Gulf of California suggest that this volcano-tectonic uplift, that is still active, is of the order of 240 mm/10 3 y. The set of terraces is interpreted to be Middle (700−125 × 10 3 y) to Upper (125−80 × 10 3 y) Pleistocene, and is tentatively correlated with the paleoclimatic chronology of deep-sea cores.

Recent crustal movements and intra-plate earthquakes in India

The peninsular shield of India in the interior of the Indian Plate is characterized by intra-plate earthquakes. The major earthquakes in the peninsula are associated with prominent fault zones in the margins of the peninsular shield, especially along the Bombay—Ratnagiri coast, the western parts of the Narmada Rift zone and the southeastern parts of the Precambrian shield. Mild seismicity is also observed in several parts in the interior portions of the shield. Prominent recent crustal movements have been observed in the peninsular shield as evidenced by surface geologic and geomorphic features as well as prominent features in the lineament maps, including aeromagnetic lineaments. While the major intra-plate earthquakes are associated with tectonic zones characterized by deep crustal faults, the mild seismicity in the interior of the shield is associated with epeirogenic movements involving the movement of crustal blocks. The regional gravity and magnetic maps of the Deccan-trap region of the shield indicate prominent zones of uplift and subsidence involving the crust. The movements and dislocation with time in the deeper parts of the crust in the major zones of subsidence, as typified by the Koyna earthquake region which is bounded by a major fault zone on its west, obviously also involve displacements in the superincumbent mass of the Deccan trap. This latter being of much higher density and magnetic susceptibility, causes appreciable changes in the observed gravity and magnetic fields on the surface in addition to surface level changes observed by high-precision geodetic levelling. These studies are now in progress as an aid for earthquake prediction. The role of thermal processes in the upper mantle in these crustal movements is also discussed.

Implications of pre-mesozoic orogeny in the geological evolution of the Himalaya and Indo-Gangetic Plains

An integrated geological analysis of the Himalaya and Indo-Gangetic Plains demonstrates that the Great Vindhyan Basin incorporating large parts of these morphotectonic units were uplifted into an uneven landmass due to the Pre-Mesozoic orogenic cycle. This uneven landmass was eroded off largely during a considerable part of the Devonian and Carboniferous thereby causing partial absence of sedimentary sequences of these periods except in parts of the Tethys Himalaya. The Late Paleozoic epeirogenic movements brought about renewed sedimentation in the Lesser and Tethys Himalayas in the Krol and Tethys Basins, respectively, which was terminated by the Himalayan Orogeny during Late Cretaceous—Early Eocene.

Mississippian Shelf Margin and Carbonate Platform from Montana to Nevada: ABSTRACT

The Kinderhookian to middle Meramecian history of a carbonate platform and shelf margin, extending from Nevada to Montana, is documented through four time-rock correlation charts and five successive maps that are synchronized by foraminiferan, conodont, and coral zonations. The platform was bordered on the west by a starved basin, a flysch trough, and orogenic highlands. The history of platform development is an integral part of the sedimentary cycle of the deep-water Deseret starved basin. Antler orogenic activity produced epeirogenic movements on the craton, which affected sea level and caused episodic progradation and retreat of the carbonate shelf margin. The sequential history is: (1) in earliest Mississippian time a narrow, northeast-trending seaway bordered by low oastal plains received mostly fine clastic sediments; (2) during late Kinderhookian time, a carbonate platform and shelf margin formed as a result of eastward expansion of the seaway; (3) during early Osagean time, the shelf margin retreated and a broad, gentle (less than 0°5^prime) clinoform ramp developed; (4) during middle Osagean time, lowering of the basin and craton and rise of sea level changed the pattern and sedimentary regime of the carbonate platform. Progradation of the shelf margin over the former ramp resulted in maximum expansion of the platform concurrent with maximum deepening of the starved basin. The foreslope attained a maximum steepness of 5°; (5) in middle Meramecian time, uplift of the craton and lowering of sea level caused shoaling of the carbonate plat orm and development of a sabkha landward. With increased uplift a karst plain developed over most of the former carbonate platform, and some cratonic sands were transported westward by streams into the basin. Meanwhile, filling of the flysch trough allowed eastward spillover of distal-flysch sediments to almost completely fill the basin. End_of_Article - Last_Page 716------------

Episodic volcanism, epeirogenesis and the formation of the North Atlantic Ocean

This paper reviews the timing of volcanic and epeirogenic events in the North Atlantic area during the Palaeogene with special emphasis on East Greenland. An appraisal of existing radiometric ages leads to the conclusion that onshore volcanism was of an episodic nature, being either limited to, or concentrated around, the following ages: 65 m.y. (anomaly 30), 60 m.y. (anomaly 26), 55 m.y. (anomaly 24), 50 m.y. (anomaly 21), 35 m.y. (anomaly 13) and 28 m.y. (anomaly 9?). In some cases (55 and 35 m.y.) these episodes appear to correspond to widespread plate tectonic events, in other cases (50 and 35 m.y.) to more localized epeirogenic movements. A consideration of the physiography of East Greenland shows that at least two epeirogenic events have been significant. One of these raised a dome 300 km across and 4 km above its surroundings and the other elevated a plateau of regional extent some 2.5 km above sea level. A variety of evidence tentatively dates these events to 50 m.y. and 35 m.y. respectively. The recognition and dating of these events is a contribution towards an understanding of the fine-scale sequence of events during continental break-up and subsequent ocean basin development. The conclusions are tentative due to inadequate data at the present time, especially from the offshore area.

Paleozoic Disruptive Deformation in North American Continent and Its Relation to Formation and Development of Interior Basins and Deformation Within Orogenic Core: ABSTRACT

The two major disruptive tectonic events during the Paleozoic which affected the North American craton seem to be associated with Appalachian-Ouachita orogenic events. The first Paleozoic cratonic disruption was tensional rifting, which occurred during the Avalonian intrusive-metamorphic event (± 560 m.y.). Evidence continues to mount that these Cambrian rifts of the Appalachian-Ouachita foreland, such as the Rome trough of Kentucky and West Virginia, are not Cambrian aulacogens. By time and position, the rifts seem to be incipient basins along a developing back-arc trough. However, this disruptive deformation was not restricted to the developing arc trough, but extended far into the craton where it commonly involved reactivation of older rift zones. These zones form d the axial portion of the subsequently developed Paleozoic basins. The Paleozoic basins developed by epeirogenic movement after a period of relative quiescence during Late Cambrian through Early Ordovician (pre-Taconic) time. The second Paleozoic continental disruption created large upthrust blocks in the craton during the Pennsylvanian and early Permian, probably by compressional deformation. This event ties, both by time and position, to deformation within the Ouachita part of the orogenic core. Upthrust crustal blocks in the craton may be bounded by reactivated faults of precursor rifts. When they formed, the upthrusts often developed near the axial part on the middle Paleozoic basins to form the late Paleozoic yoked basins. The occurrence of axial rifts within interior and foreland basins, and of axial upthrusts in the craton-margin basins, suggests an interrelation among rifts, basin formation, and the late-forming yoked basins. The developing foreland trough (the Appalachian-Ouachita geosyncline) has a tectonic history similar to that of the cratonic basins but, along its trend, tensional bending of the basement predominated. End_of_Article - Last_Page 1588------------

Mississippian Carbonate Shelf Margin Along Overthrust Belt from Montana to Nevada: ABSTRACT

A constructional carbonate platform and a generally north-trending shelf margin in Utah and southwestern Idaho were bordered on the west by a starved basin, flysch trough, and orogenic highland during Kinderhookian to middle Meramecian time. The Antler orogeny produced epeirogenic movements, which resulted in sea-level changes that caused the carbonate platform episodically to prograde and retreat. At different times the shelf was bordered either by a narrow foreslope or by a broad ramp. The sequential history is as follows: (1) Late Devonian thrusting raised the continental margin to produce the Antler orogenic highlands, which in earliest Mississippian time had a low eastern coastal plain that bordered a narrow, shallow marine basin lacking a distinct eastern shelf. (2) Widespread marine inundation of the craton on the east was followed by a stillstand, during which a low shelf margin that turned abruptly eastward in Montana was developed and deposition of clinoform micritic limestone beds occurred in moderately deep water across a very broad ramp. (3) Increased downwarping produced an incipient starved basin, separated by a shallow carbonate bank from the flysch trough on the west and by a broad ramp from the northeast-trending shelf margin on the east; coarse encrinites were deposited alternately with micrites on the ramp. (4) Maximum deepening and expansion of the starved basin were accompanied on the west by deepening of the carbonate bank and on the east by westward progradation of a carbonate platform with a narrow, steep foreslope. (5) Lowering o sea level produced a karst plateau on the former carbonate platform and caused cratonic sands to be carried westward into the basin. Meanwhile, filling of the flysch trough allowed an eastward spillover of distal flysch sediments into the basin. The starved-basin sediments, which have organic-carbon values as high as 7% in outcrop, are considered to be source rocks. Coarse sediments of the carbonate platform, particularly where dolomitized, may serve as petroleum reservoirs. End_of_Article - Last_Page 828------------

Mesozoic-Cainozoic volcanism of Australia

Mesozoic—Cainozoic volcanism was concentrated on the youngest eastern Australian craton. Basaltic activity (with some felsic fractionation) has predominated over Mesozoic interludes of calcalkaline volcanism (rhyolites, dacites, trachytes andesites) and more isolated shoshonitic activity (now represented by appinitic, syenitic, granitic and lamprophyric complexes). Epeirogenic movements and associated sea-floor spreading and orogenic episodes at the continental margins, initiated and controlled much of the volcanism. Basin edges, faults, lineaments and their intersections were important in locating sites of volcanism; some fundamental structural lines have focussed volcanism over 300–600 km. The eastern Mesozoic basaltic volcanism shows a late Jurassic N-S trend from undersaturated to saturated compositions, with increasing intensity of melting towards a major Tasmanian-Antarctic thermo-tectonic event. A late Jurassic-late Cretaceous E-W trend may extend from possible ‘kimberlites’ through shoshonitic to calcalkaline activity with increasing proximity to orogenic movements along the New Zealand ‘Geosyncline’. Cainozoic basaltic volcanism reflects the NNE drift of Australia under Atlantic-Indian-Southern Ocean sea-floor spreading, with a debatable role for subduction along the Tasman Sea margin. The ultimate mechanisms of volcanism are not clearly understood. Drift of cratonic structural weaknesses over thermal anomalies in the mantle, with generation of magmas from a geochemically zoned Lower Velocity Zone under influence of uplifts, lithospheric thickness and periodic release of thermal energy, seems to partly explain observed patterns of E. Australian volcanism.

The Messinian Var canyon (Provence, southern France) — Paleogeographic implications

From north to south, the lower course of the Var River cuts through three tectonic units: (1) the Nice arc (“arc de Nice”), (2) the oriental part of the Castellane arc (“arc de Castellane”), and (3) the autochthonous Provençal substratum. The Nice and Castellane arcs are sub-Alpine overthrusts, emplaced near the end of the Miocene and reactivated in Plio-Quaternary times. During the Pliocene, the downstream section of the river was invaded by the sea and transformed into a ria. The Pliocene sedimentary succession (Tabianian in its oldest levels) fossilizes a deeply incised erosional relief. In order to put together an orderly geographic reconstitution of this pre-Pliocene topography, one has to exclude from the study area the two sub-Alpine units, which were affected by neotectonics, and to limit the observations to the Provençal substratum. Some modest epeirogenic uplift of the substratum and its incision by erosion provide good outcrops of its sedimentary series. The Provençal substratum consists of Mesozoic and Cenozoic sediments, the youngest (Vence succession or “série de Vence”) ranging in age from Burdigalian to Tortonian. Chronostratigraphic constraints given by the underlying and overlying sediments respectively, indicate that the erosion surface is younger than Tortonian and older than Tabianian, and can, therefore, be dated as Messinian. Geological observations at the surface and geophysical investigations in the subsurface indicate that at some 40 km from the present abyssal plain of the western Mediterranean, the thalweg of the Messinian paleovalley cuts more than 500 m deep into the Provençal substratum. At 20 km from the abyssal plain, the same thalweg lies some 700 m beneath sea level, or some 1000 m below the former Miocene sea level. Cores and dredges obtained from the continental slope south of the Var Valley suggest a progressive connection of the Pliocene ria with the abyssal plain. This gigantic incision now filled with Pliocene sediments and located along the continental shelf and the emerged land, represents a remarkable example of a Messinian canyon partially exhumed by the Plio-Quaternary epeirogenic movements. This canyon has been eroded subaerially by the paleo-Var River in a setting fairly similar to the modern margin. The subaerial erosion is a product of the endoreic regression that followed the isolation of the Mediterranean during the latest Miocene. The base level coincided with the level of the abyssal plain where evaporites were being deposited. The presence of Messinian canyons, at present more or less filled by younger sediments, along the continental margins of the Mediterranean basin (the case of the Var River is just one example among others) leads to an independent confirmation of the deep-basin desiccation hypothesis for the genesis of Messinian evaporites.

Les paleocourants marins du bathonien moyen au bathonien superieur dans le nord de la campagne de caen (Normandie)

In the northern half of the Campagne de Caen (Normandy), the Bathonian deposits show a succession of four marine carbonate cross-bedded formations: Calcaire de Reviers, Calcaire de Blainville (Middle Bathonian), Calcaire de Ranville and Calcaire de Langrune (Upper Bathonian). The directional sedimentary structures are compiled, described and compared. The evolution of size, shape and direction of the main current-marks are followed through the lithological succession. The close association of large-scale planar cross-bedding and trough-bedding characterized the Calcaire de Reviers, at the base of the column: these structures are interpretated as vestigial patterns of large submarine sand dunes, more than 100 m in wavelength, cut off by meandering stream channels. The formation was laid down under shallowing water conditions on the northeastern margin of the Armorican Massif and resulted from the action of both wave- and tidal-currents. Earlier in Middle Bathonian times, the dimensions of the sand-bodies decreased as well as the depth of the depositional environment, while stream velocities strengthened. During the Late Bathonian, a minor deepening of the basin followed a non-sequence and led to a recurrence of the sedimentary evolution. From the Middle to the Late Bathonian, the resultants of the direction of the main currents reversed following a dextral rotation, so by successive steps it drifts from N 40 to N 180. In the palaeogeographical evolution of this area, this drastic change of the direction of main marine currents appears to be linked to epeirogenic movements and regional climatic modifications, which gave rise to a new pattern of marine currents at the beginning of the Late Bathonian times. By submergence of a swell (or a barrier), epeirogeny would be able to favour the southern spread of colder boreal water on the northern part of the Anglo-Paris Basin, contemporaneous with a distension phase which took place on the North Sea floor at this time.

Geophysical studies of the major sedimentary basins of the indian craton, their deep structural features and evolution

The peninsular shield of India is characterized by a number of intra-cratonic sedimentary basins of which the Cuddapah and Vindhyan Basins are conspicuous. The crescent-shaped Cuddapah Basin (~1400 m.y.) covering roughly 35,000 square kilometers in the southern peninsula and enclosing the Cuddapah formations (Precambrian) includes shallow marine shales, limestones, sandstones and quartzites. These sediments are overlain by the younger Kurnool formations of Vindhyan (Upper Precambrian) age in the western and northern marginal portions of the basin and are intruded by basaltic sils and dykes. The eastern margin of the basin is characterized by an overthrust with steeply folded beds, while in the remaining parts, the formations show a gentle eastward dip. Evidence for Recent epeirogenic movements is provided by geomorphic features and current seismicity. The Great Vindhyan Basin of north-central India covering more than 100,000 square kilometers encloses Vindhyan sediments including some marine shales and limestones in the lower parts and shallow-water deposits of red sandstones and shales in the upper parts. The beds are generally horizontal, but are strongly disturbed along the southern margin. There are intrusions of basaltic dykes and kimberlite pipes. The Gondwana basins (Upper Carboniferous to Jurassic) are relatively smaller cratonic units in Archaean faulted troughs. Gravity and magnetic investigations, both regional and detailed, supplemented by deep seismic sounding profiles in the Cuddapah Basin have brought out the deep structural features of the basin, including the Moho, indicating a total thickness of generally 5–8 km with a maximum thickness of sediments of nearly 12 km in the eastern part. The beds show both a layered structure in the horizontal and block structure in the vertical, disturbed by a low-angle thrust fault on the eastern margin. In the Vindhyan Basin, the gravity and magnetic data indicate about 5000 m of sediments in the central portions, with major, roughly faults over the western and southern margins. The deep structural features of these intra-cratonic basins, as indicated by the geophysical results, are discussed in relation to the geological theories proposed for their genesis and development.

Mesozoic epeirogeny at the South Atlantic margin and the Tristan hot spot

Epeirogenic structures of the eastern margin of Brazil9s Parana basin seem related to the position of the proposed Tristan hot spot in Early Cretaceous time. Several lines of evidence indicate that the hot spot was located offshore from a major structural depression, the Torres syncline, which is flanked by two structural arches about 750 km apart. Structural relief between the syncline and arches is estimated to be 4 km. Synchronism of the formative epeirogenic movements with proto-Atlantic continental rifting is indicated by the occurrence of a prominent Early Cretaceous basaltic dike swarm on the crest of one of the arches. The Torres downflexure might have originated through volcanic loading of the continental margin prior to formation of the Rio Grande Rise in the adjacent ocean basin.

Epeirogenic studies in India with reference to recent vertical movements

Epeirogenic movements of regional extent have been manifest and recorded in several parts of India from Late Proterozoic to present times through the Tertiary and Recent. The main features of these movements, especially Recent vertical movements, in typical areas in the platform and coastal areas of peninsular India are described and the salient results of the study, qualitative and quantitative, of these zones are outlined. Quantitative measurements of Recent vertical movements have been made largely by geodetic methods and precision levelling in the plateau areas as also in the mobile, orogen zones of the Himalayan and sub-Himalayan tracts and by sea-level measurements through an extensive network of coastal automatic tide-gauge stations. The annual variations of sea level have been studied for several Indian ports. The land uplift or subsidence in relation to mean sea level over the Indian coast has also been studied. The application of geophysical techniques in the identification and delineation of regions of epeirogenic movements has been discussed and illustrated by the results in two type areas, viz., the coastal sedimentary belt of Madras and the vast Deccan Trap territory in western India. In the latter area, blanketed by extensive basaltic lavas over a vast region, the gravity data have brought out a number of major zones of subsidence and uplift which have been corroborated by the results of refraction seismic soundings. One of these major zones of subsidence is in the Koyna region which witnessed a major disastrous earthquake a few years ago. The results of deep seismic sounding along a traverse across the southern part of the Indian peninsula now in progress have also been briefly mentioned. Five important areas have been selected for quantitative measurements of current epeirogenic movements, namely the Shillong Plateau in northeast India, the Chhotanagpur Plateau and Singhbhum region in Bihar, the Aravalli region of Rajasthan, the Narmada Sone Rift in central India and the Deccan Trap territory including the Koyna earthquake zone.

Stratigraphic Analysis of Kadi and Kalol Formations, Cambay Basin, India

The Kadi and Kalol Formations of early to middle Eocene age are thick clastic wedges in the northern part of the Cambay basin, India. Southward these formations intertongue with and grade into the marine Cambay Shale. The Kadi Formation has been divided into the Mandhali and Mehsana Members. The formation is restricted areally to the northern part of the Ahmedabad-Mehsana block and was deposited in fluvial, lagoonal, and deltaic environments. Unlike the Kadi, the Kalol Formation is present in a large area from Mehsana in the north to the Dadhar River in the south. The Kalol Formation has been divided into four members. The upper three (Sertha, Kansari, and Wavel) were deposited in an extensive area and the lowermost Chhatral Member is restricted areally. Along the basin a is, shallow-marine, intertidal, fluvial, and paludal to eolian environments were established during the deposition of the Sertha and Wavel Members. These environments shifted in response to oscillations of the basin floor. Delta building during deposition of the Kadi Formation was influenced by a steep paleoslope, which became gentle during Kalol deposition. The gentle slope created conditions favorable for the formation of extensive coastal plains. Vertical lithologic variations were, however, controlled by epeirogenic movements in the source area and mild oscillations of the depositional floor. The gentle paleoslope which existed during deposition of the Kalol created large areas of favorable source-reservoir facies, although the reservoir sands generally are dirty because of the presence of trap basalts in the provenance area. Sand facies with better petrophysical properties are present only in a narrow zone paralleling the strandline. Winnowed sands were localized on some of the positive features such as the Kalol, Sanand, Nawagam, and Dholka highs. Variations in the physicochemical properties of the oil in Kalol Formation are controlled both by evolutionary and secondary alteration processes. The presence of waxy oils trapped beneath coal seams suggests that the coals exercised some control in localizing such pools.