Homoclinal Structure Research Articles

Multivariate morphological characteristics and classification of first-order basins in the Siwaliks, Nepal

This study identifies and analyzes a group of interrelated morphometric variables that contribute to the morphology of first-order basins in the Siwaliks and the adjacent part of the Lesser Himalaya in the Far West Nepal. First-order basins were identified and their various morphometric indices (21 variables) such as shape, area, pattern, roughness, height, gradient, angle, and distance were obtained from topographic maps (scale 1:25,000) and a digital elevation model using a geographic information system. The study also identifies the basin types based on morphometric properties and assesses their association with lithology, structure, and uplift rates. A principal component analysis identified seven major components that accounted for 78% of the total variance explained by the original 21 morphometric variables. Variables' loading reveals that the components are the expressions of slope, size, valley/depression, basin shape, relative massiveness, the convergence index, and basin asymmetry. Of these, slope and size components explain 48% of the total variance. The sets of variables represented by each major component for the entire study area were mostly common to all lithostratigraphic units. Some variables such as drainage density, junction angle, and the sediment transport index have associations with more than one group. This implies that complex geomorphic processes are involved in the development of basin morphology. Using the seven major components, five basin types were recognized by cluster analysis. The distributions of these basin types are similar in the Lower and Middle Siwaliks which have similar geological structures. The geometric relationship between the dip and the topography of a homoclinal structure significantly affects the proportion of basin types, as evidenced by the contrasting proportions between the cuesta and hogback and among dip, anti-dip and orthoclinal slopes. Correlation analysis also confirmed the effect of structure (dipping) on the morphometric variables, and indicated the influence of uplift rates on basin slope, stream slope, elevation range, mean elevation, valley angle, the shape index and the convergence index associated with the active thrust.

한반도 남동부 경주시 양북면 마이오세 입천소분지의 형성과 발달사

입천소분지는 기반암과 단층으로 경계되며 북동 방향으로 길쭉한 형태의 비대칭 지구의 기하를 가지는 분지로 인근 와읍과 어일분지와는 기반암에 의해 격리되어 있는 독립된 소규모 마이오세 분지이다. 분지충전물의 층리는 대부분 북서 내지 서북서 방향으로 경사지며, 분지 북동부에는 데사이트질 화산물질을 다량 포함하는 전기 마이오세 퇴적물이 그리고 남서부에는 사암을 협재하는 비화산성 중기 마이오세 육성 역암이 분포한다. 또한 분지충전물 내 퇴적동시기 공액상 정단층들은 분지가 서북서-동남동 방향으로 확장하였음을 지시한다. 이러한 특징들은 인근 와읍 및 어일분지와 것들과 매우 유사한 것으로, 분지의 확장이 북서부 경계단층에 의해 주도되었으며 분지의 열개가 북서에서 남동으로 전파되었음을 지시한다. 한편, 입천소분지 내에는 어일분지에서 흔히 나타나는 현무암질 화산물질이 관찰되지 않는다. 슬랩과 경하관찰 결과, 입천소 분지의 데사이트질 응회암과 응회질 이암은 와읍분지의 용동리응회암과 매우 유사한 특징을 보인다. 중기 마이오세의 비화산성 퇴적층은 와읍과 어일분지 그리고 입천소분지에 공통적으로 분지의 남서부에 분포한다. 따라서 입천소분지의 확장은 22 Ma 경 와읍분지의 확장과 함께 시작되어 다량의 데사이트질 화산물질이 유입되었으며, 이후 어일분지의 주 확장시기인 20~18 Ma 사이에는 확장을 멈추었다가, 약 17 Ma에 이르러 연일 구조선의 운동과 함께 분지의 열개가 남서쪽으로 전파되면서 중기 마이오세 초의 연일층군에 해당하는 비화산성 역암이 분지의 남서부에 퇴적된 것으로 결론지어진다. The Ipcheon Subbasin is an isolated Miocene basin in SE Korea, which has the geometry of an asymmetric graben elongated in the NE-SW direction. It is in contact with basement rocks by faults and separated from adjacent Waup and Eoil basins by the basement. The strata of the basin fills have an overall homoclinal structure, dipping toward NW or WNW. The basin fills consist of Early Miocene sediments rich in dacitic volcanic and volcaniclastic deposits and Middle Miocene non-volcanic and nonmarine conglomerates intercalated with sand layers, which are distributed in the northeastern and southwestern parts of the basin, respectively. Kinematic analysis of syndepositional conjugate faults in the basin fills indicates WNW-ESE extension of the basin. These features are very similar to those of the adjacent Waup and Eoil basins, indicating that the basin extension was governed by the NE-trending northwestern border faults and that the basin experienced a propagating rifting from NE to SW. Basaltic materials, which occur abundantly in the Eoil Basin, are totally absent in the Ipcheon Subbasin. The observations of the dacitic tuff and tuffaceous mudstone in the subbasin, on slabs and under microscope, suggest that they have lithologies very similar to those of the Yondongri Tuff in the Waup Basin. The Middle Miocene non-volcanic sediments of the Waup and Eoil basins and the Ipcheon Subbasin are distributed consistently in the southwestern part of each basin. It is thus concluded that the extension of the Ipcheon Subbasin began at about 22 Ma together with the Waup Basin and was lulled during the main extension period of the Eoil Basin between 20-18 Ma. At about 17 Ma, the subbasin was re-extended due to the activation of the Yeonil Tectonic Line associated with the propagating rifting toward SW. This event is interpreted to have provided new sedimentation space for the Middle Miocene sediments in the southwestern parts of the Waup and Eoil basins and the Ipcheon Subbasin as well.

Relationship between geologic structure and the source fault of the 2004 Mid-Niigata Prefecture Earthquake, central Japan

Abstract The geologic structure of the Uonuma Hills, the site of the 2004 Mid-Niigata Prefecture Earthquake (MJMA 6.8), Japan, is examined based on published geologic maps, drill hole data and seismic reflection profiles. The Uonuma Hills are located in an active folding area on the eastern margin of the Niigata sedimentary basin, and are divided into three discrete regions from north to south based on the geologic structure. The southern extent is marked by a west-dipping homoclinal structure produced by reverse faulting of the west-dipping Muikamachi active fault. The central section exhibits a regional anticlinorium produced by west-dipping reverse faulting with a steep angle at depth (> 5 km) and a more shallow angle (30°) near the surface. This fault can be correlated with the source fault responsible for the main shock. The Muikamachi fault consists of two fault segments and the source faults of the northern and the southern segments show discrete geometry forming an step over. Thus, judging from the geologic structure, the 2004 Mid-Niigata Prefecture Earthquake ruptured to the maximum possible extent in this fault segment in the central part. Furthermore, this study reveals that the activity on the Muikamachi fault, which has an apparently continuous surface trace, is produced by two different source faults at depth, with important implications for seismic risk assessment.

Stratigraphy of the Kanmantoo Group: A critical element of the Adelaide Fold Belt and the Palaeo‐Pacific plate margin, Eastern Gondwana

The Kanmantoo Group, occupying a key position along the Cambrian Palaeo‐Pacific margin of southern Australia Gondwana, comprises two distinct successions (from bottom to top): (i) the Keynes Subgroup comprising the Carrickalinga Head and Backstairs Passage Formations (lowstand systems tract, transgressive systems tract, highstand systems tract); and (ii) the unconformably overlying Bollaparudda Subgroup (new name) comprising the Talisker, Tapanappa, Tunkalilla, Balquhidder, Petrel Cove and Middleton Formations. Both conventional stratigraphy and recent isotopic work point to the base of the Kanmantoo Group as representing a major change in the depositional history of the Adelaide Geosyncline. The type section of the Kanmantoo Group, along the south coast of the Fleurieu Peninsula, is now recognised as being characterised by structural complexity (shear zones, folding), contrary to the essentially largely non‐faulted homoclinal structure described by most earlier authors. Many of the formation boundaries in the type section, previously described as conformable, are in fact faulted. As a result, subsidiary type sections are proposed for the Talisker and Tunkalilla Formations.

The structure and relief of the bedrock sequence in the Gotland-Hiiumaa area, northern Baltic Sea

Based on high resolution seismic reflection profiling, structural and relief maps of the sedimentary bedrock between Gotland and Hiiumaa in the Baltic Sea have been composed and analysed. The general structure and relief of the submarine Lower Palaeozoic succession reveal a westward extension of the homoclinal structure distinguished in the Estonian mainland. The main bedrock structures offshore are 1-4 km wide, and several tens of kilometres long, linear zones of disturbances. On the structural map, these disturbances appear as submeridional zones of contour changes, up to several tens of metres in offset. The seismic profiles usually reveal a faint flexure-like bending of the layers through the zone. Locally, this flexure can be intersected by small faults. These bedrock structures are ascribed to fault movements in the crystalline basement. Two different bedrock relief systems were superposed on the region during the Cenozoic uplift and the Pleistocene glaciations. The first event resulted in the formation of a subparallel cuesta-like system of alternating erosional scarps and plains. Glacial erosion created submeridional valleys and troughs. Today three large bedrock forms, namely the Baltic and the Silurian clints and the Ordovician plateau, characterize the area. The outlines of the cuesta relief, and the amount of eroded sediments, advocate a regional increase in erosional activity from the St. Petersburg district to the area of the Baltic-Bothnian mobile zone northeast of Gotland. This zone existed as a subsided meridional lower ground during the Cenozoic, accommodating a main river that collected water both from the craton margins and the inner platform areas.

Hydrogeology of the Gunungsewu karstic area, Central Java, Indonesia: a conceptual model

The Gunungsewu Area in Central Java, Indonesia, has been suffering from water shortage since time immemorial, although the precipitation in the region concerned and surroundings is known to be adequate (2,000 mm/year average). This region is controlled by a karstic geology with conduits, dolines, caves, and subterraneous rivers, and very permeable rock formation, causing most of the rainwater in this area to be directly absorbed into the ground. There is a homoclinal structure dipping southward that conducts groundwater to discharge enormously into the Indian Ocean. The Gunungsewu limestone is composed of reefs and bioclastics , which based on their different physical characteristics in the field, can be classified into chalky limestone called caliche and karstic limestone. Groundwater level in the bioclastic limestone is at 5-10 m depth, whereas in the reef limestone it is able to reach 150 m depth or more. The existence of caliche and karst in Gunungsewu enable the rock formation to be divided into non-karstic aquifer with diffuse flow and karstic aquifer with conduit flow.

Folded basement-cored tectonic wedges along the northern edge of the Amadeus Basin, Central Australia: evaluation of orogenic shortening

Intracontinental deformation at the northern margin of the Amadeus Basin, Central Australia, is characterised by the presence of basement-cored, downward-facing antiformal synclines. These structures were formed during the Devonian–Carboniferous Alice Springs Orogeny and in the past were explained as erosional relics of fold nappes indicating large horizontal crustal shortening within the Australian continent. This notion is in stark contrast with both the uncomplicated homoclinal structure that contains the `nappes' and with seismic data suggestive of a simple crustal structure formed during only modest intraplate shortening. Balanced and sequentially restored sections across the Razorback `nappe' and its vicinity show that such downward-facing synformal anticlines can be produced by emplacement of a basement cored tectonic wedge along shortcut thrusts in the footwall of a crustal-scale (Redbank) shear zone which initially formed during the Mesoproterozoic. The forelandward-propagating wedge proper is formed by basement and the lower two successions of the Amadeus Basin. Higher sedimentary successions accommodate shortening by hinterlandward backthrusting above a detachment horizon which is hosted in carbonates and evaporites of the Bitter Springs Formation. During the formation of the homocline which characterises much of the northern margin of the Amadeus Basin, the tectonic wedge was rotated through 90° and now forms a downward-facing antiformal syncline. The orogenic shortening indicated by this succession of structural events is about 19 km which is compatible with seismic data. The proposed model explains the existence of downward-facing synformal anticlines as folded basement-cored tectonic wedges; although they resemble parts of fold nappes, these folded tectonic wedges do not necessarily imply large amounts of crustal shortening.

Characteristics of, Topography, Geology and Alteration in Slide Areas

Characteristics of twenty-nine ancient slides which were surveyed in the eastern region of the Ikutahara district in northeastern Hokkaido, Japan are as follows;The scale of the slides is about 120-1, 170 m wide and 190-1, 280 m long. Most of the slides can be classified as “Concave Hill Type”, “Slides Occurring in Hydrothermal Alteration Zones” and “Detritus and Clayey Soil Slides” based on their geomorphological features, bedrock geology and displaced materials, respectively.It is believed that the “Slides Occurring in Hydrothermal Alteration Zones” situated on volcaniclastic rocks of the Upper Miocene Ikutahara Formation, moved due to a stage I vein-related interstratified sericite/smectite zone and a stage II vein-related argillized kaolin mineral zone in hydrothermal alteration haloes and a homoclinal structure, while those of the Upper Miocene Yahagi Formation moved due to a stage I regional smectite zone. The slides located on mudstones of the Ikutahara Formation are believed to have occurred due to complex folding structures and distribution of the mudstones which make up the interstratified sericite/smectite zone. “Slides Occurring in Hydrothermal Alteration Zones” are closely related in space to interstratified sericite/smectite, smectite, and argillized kaolin mineral zones.“Slides Occurring in Crushed Zones” situated on rocks of the Upper Cretaceous -Paleocene Yubetsu Group, are thought to have occurred due to a fault, a homoclinal structure, and distribution of shales.“Granitic Rocks -Type Slides” probably occurred due to weathering along fault and joint planes in the hydrothermally altered or the least altered nevaditic biotite rhyolite intrusives.

Constraining seismic parameters with a controlled-source audio-magnetotelluric method (CSAMT)

SUMMARY Precise seismic two-way traveltime (TWT) determinations are often limited by the knowledge of the overburden thickness and its seismic velocity. Values between 500 and 1800 m s−1 are conceivable, according to the water content. The CSAMT method has been successfully applied to determine the thickness of the Quaternary fluvio-glacial deposits and Tertiary Molasse (interbedded marls and siltstones) overlying Mesozoic carbonates along a 3 km profile through a syncline in the Swiss Jura fold and thrust belt. The homoclinal structure within the syncline is well constrained by a petroleum exploration Vibroseis profile which indicates a SSE-dipping Mesozoic series and a similarly oriented increasing thickness of Molasse and Quaternary. The CSAMT places tight constraints on the apparent resistivities of the three formations: Quaternary—300–1000 ωm; Molasse—5–15 ωm; Mesozoic—100–1000 ωm. Furthermore, modelling of the CSAMT sounding data provides absolute thicknesses for both the Quaternary (80–20 m) and the Molasse (10–210 m) layers, revealing a true dip of the top Mesozoic substratum of 7° to the SSE. The seismic velocities of the Quaternary (weathering layer) and the Molasse can thus be determined at 800 and 2100 m s−1, respectively.

A new angle on the tectonic evolution of the Ridge basin, a "strike-slip" basin in southern California

Research Article| October 01, 1993 A new angle on the tectonic evolution of the Ridge basin, a "strike-slip" basin in southern California STEVEN R. MAY; STEVEN R. MAY 1Exxon Production Research Co., Houston, Texas 77252-2189 Search for other works by this author on: GSW Google Scholar KENNETH D. EHMAN; KENNETH D. EHMAN 1Exxon Production Research Co., Houston, Texas 77252-2189 Search for other works by this author on: GSW Google Scholar GARY G. GRAY; GARY G. GRAY 1Exxon Production Research Co., Houston, Texas 77252-2189 Search for other works by this author on: GSW Google Scholar JOHN C. CROWELL JOHN C. CROWELL 2Institute for Crustal Studies, University of California, Santa Barbara, California 93106-1100 Search for other works by this author on: GSW Google Scholar Author and Article Information STEVEN R. MAY 1Exxon Production Research Co., Houston, Texas 77252-2189 KENNETH D. EHMAN 1Exxon Production Research Co., Houston, Texas 77252-2189 GARY G. GRAY 1Exxon Production Research Co., Houston, Texas 77252-2189 JOHN C. CROWELL 2Institute for Crustal Studies, University of California, Santa Barbara, California 93106-1100 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1357–1372. https://doi.org/10.1130/0016-7606(1993)105<1357:ANAOTT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEVEN R. MAY, KENNETH D. EHMAN, GARY G. GRAY, JOHN C. CROWELL; A new angle on the tectonic evolution of the Ridge basin, a "strike-slip" basin in southern California. GSA Bulletin 1993;; 105 (10): 1357–1372. doi: https://doi.org/10.1130/0016-7606(1993)105<1357:ANAOTT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Ridge basin, located in southern California between the San Andreas and San Gabriel faults, contains as much as 14 km of Mio-Pliocene marine and nonmarine sedimentary fill. These strata are exposed as a northwest-dipping homoclinal sequence that becomes increasingly younger to the northwest. Pliocene to Holocene deformation, uplift, and dissection of this basin provide an opportunity to study stratal geometry and sedimentary facies in what is commonly cited as a classic "strike-slip" basin. Newly acquired seismic data from the northern Ridge basin provide new evidence for the subsurface basin geometry. These data lead us to conclude that the principal northern strand of the San Gabriel fault is a listric, east-southeast-dipping, oblique-slip fault rather than a subvertical, strike-slip fault.The principal San Gabriel fault appears to become subhorizontal at the southeastern end of the seismic line at a present-day sub-surface depth of ∼4 km. This interpretation is consistent with preliminary two-dimensional gravity models across the basin and with the homoclinal structure of basin fill. Subsidence within the Ridge basin is explained as the result of northwest- southeast extension locally accommodated along the listric, right-lateral San Gabriel and associated late Miocene strike-slip faults. As a consequence of oblique normal slip on the gently dipping San Gabriel fault, sedimentary strata were continuously tilted and moved southeastward in the hanging wall away from the depocenter. This mechanism of basin formation allowed 14 km of sediment to accumulate without deep burial. The geometry and displacement history of the San Gabriel fault controlled sediment accommodation and resultant stratal geometries within the basin. Slip along the San Gabriel fault and associated subsidence of the Ridge basin correspond temporally with regional tectonic events in southern California, reflecting both changes in the relative motion of Pacific and North American plates and the evolution of strain partitioning across this plate boundary. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin of the inverted metamorphism of the lower Himalayas, Central Nepal

The southern slope of the central Nepal Himalayas is divided geologically by the Main Central Thrust zone into the underlying Midland metasediment zone (Lower Himalayas) in the south and the overlying Himalayan gneiss zone (Higher Himalayas) in the north. The former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium, and the latter zone consists of high-grade crystalline rocks with a north-dipping homoclinal structure and is further overlain by the Tethys Himalayas, composed of non- or slightly metamorphosed Tethyan sediments. The Main Central Thrust zone shows a characteristic lithology such as calcareous schist, quartzite, phyllitic schist and augen gneiss, and is intensely mylonitized. The mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the Midland metasediment to the Main Central Thrust zones, that is, the metamorphic grade of the Midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies, and the greenschist-amphibolite transitional zone is attained in the thrust zone. The Himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone. On the basis of the compositional zoning of garnets and muscovite compositions, however, it is concluded that the Himalayan gneisses suffered a polymetamorphism. The younger metamorphism resulted in a retrograde metamorphism of the Himalayan gneisses together with the above-mentioned inverted metamorphism and an anatectic melting in depth. This was caused by shear heating due to the thrust movement along the Main Central Thrust zone during the Alpine tectonism. The older metamorphism, which seems to be Precambrian in age, was much higher in metamorphic grade than the younger one and formed the pyrope-rich cores of garnets in the Himalayan gneisses. The maximum metamorphic temperature of 550°C, estimated from the mineral paragenesis, in the thrust zone, is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5–10 cm/yr, maximum stress of 300 bars and thrusting distance of 250 km (Bird, 1978).

A Surface Vitrinite Reflectance Anomaly Related to Bell Creek Oil Field, Montana U.S.A.: ABSTRACT

Vitrinite reflectance measurements from surface samples of mudrock and coal show anomalously high values over the Bell Creek oil field. The average vitrinite reflectance (Rm) increases to a peak of 1.2% over the field against background values of about 0.3%. The Rm anomaly coincides with a geochemical anomaly indicated by ^dgr13C in carbonate-cemented sandstones. These samples were taken from the Upper Cretaceous Lance and Paleocene Fort Union formations, which form an essentially conformable sequence. The depositional environment is apparently similar in both formations, and we expect little variation in the source and composition of the organic matter. Rm should be rather constant across the field if conditions of diagenesis w re uniform. The limited topographic relief (< 1,000 ft or 305 m) over the shallow-dipping homoclinal structure of the field and the poor correlation coefficient of Rm regressed against sample locality elevation (r = 0.2) indicate that the Rm anomaly is not due to burial, deformation, and subsequent erosion. Temperature studies over local oil fields with similar geologic conditions suggest the expected thermal anomaly would be less than 10°C (50°F), which is too small to account for the significantly higher rank over the field. Coal clinkers are rare in the vicinity of the Bell Creek and widespread heating by burning of coal seams is unlikely. We suggest that activity by petroleum-metabolizing bacteria is a possible explanation of the Rm anoma y. Microseepages from oil fields support large colonies of these organisms, which could also metabolize aliphatic side-chains on the kerogen molecule. The loss of these side-chains increases the aromaticity of the vitrinite and consequently increases its reflectance. End_of_Article - Last_Page 418------------

Geologic Interpretation of Space Shuttle Radar Images of Indonesia

The National Aeronautics and Space Administration (NASA) space shuttle mission in November 1981 acquired images of parts of the earth with a synthetic aperture radar system at a wavelength of 23.5 cm (9.3 in.) and spatial resolution of 38 m (125 ft). This report describes the geologic interpretation of 1:250,000-scale images of Irian Jaya and eastern Kalimantan, Indonesia, where the all-weather capability of radar penetrates the persistent cloud cover. The inclined look direction of radar enhances subtle topographic features that may be the expression of geologic structures. On the Indonesian images, the following terrain categories are recognizable for geologic mapping: carbonate, clastic, volcanic, alluvial and coastal, melange, and metamorphic, as well as undifferentia ed bedrock. Regional and local geologic structures are well expressed on the images. In the Vogelkop region of Irian Jaya, the major tectonic elements (Tamrau Mountains, Sorong fault, Kemum block, Bintuni basin, and Lengguru foldbelt) are readily mapped. On the image of the Sorong fault, geomorphic features (offset drainage, shutterridges, aligned notches) provide clear evidence for left-lateral strike-slip displacement. Several lineaments on the image correlate with previously mapped faults. Other lineaments may be the expression of previously unrecognized faults. On the image of the mainland of Irian Jaya, a belt of metamorphic and melange terrain marks the zone of collision between the Pacific plate and the Australian plate. The high Central Range of Irian Jaya is located south of the metamorphic and melange belt. The generally homoclinal structure of the Central Range is interrupted in the Paniai Lake region by a series of southward directed thrust plates indicated by belts of carbonate terrain (New Guinea Limestone Group) that alternate with belts of clastic terrain (Kembelangan Formation). In the image of eastern Kalimantan, several major foldbelts are clearly expressed. These limited examples of space shuttle radar images demonstrate their value for geologic interpretation of Indonesia. It is recommended that future shuttle missions acquire complete radar coverage of the main islands of Indonesia for use in geologic mapping and energy exploration. Shuttle radar images would also be valuable for other cloud-covered forested regions.

Structural Framework, Stratigraphy, and Evolution of Brazilian Marginal Basins

The structural framework of the Brazilian continental margin is basically composed of eight structural types: antithetic tilted step-fault blocks, synthetic untilted step-fault blocks, structural inversion axes, hinges with compensation grabens, homoclinal structures, growth faults with rollovers, diapirs, and igneous structures. The antithetic titled and synthetic untilted step-fault blocks are considered as synchronous, complementary structural systems, separated by an inversion axis. They originated during the rift phase of the South American and African plates; genesis of the hinge and the homocline is related to the divergent migration of those plates. Two evaporitic cycles (Paripueira and Ibura) were differentiated in the Sergipe-Alagoas type basin and tentatively correlated to the evaporitic section of other Brazilian marginal basins. It is assumed that the Paripueira evaporites could have been continuous with their African correlatives, but the Ibura evaporites are marginal deposits and were never continuous with their west African equivalents. Four phases are considered in the evolution of the Brazilian marginal basins: pre-rift, rift, transitional, and drift. During the pre-rift phase (Late Jurassic-Early Cretaceous), continental sediments were deposited in peripheral intracratonic basins. In the rift phase (Early Cretaceous), the breakup of the continental crust of the Gondwana continent gave rise to a central graben and rift valleys where lacustrine sediments were deposited. The transitional phase (Aptian) developed under relative tectonic stability, when evaporitic and clastic lacustrine sequences were being deposited. In the drift phase (Albian to Holocene), a regional homoclinal structure developed, consisting of two distinct sedimentary sequences, a lower clastic-carbonate and an upper clastic. From the Albian to the Holocene Epoch, structures associated to plastic displacement of salt or shale developed in many Brazilian marginal basins. The diapirism deformed mainly Upper Cretaceous strata in proximal areas, and younger strata in distal areas lying, in some places, on the ocean floor. Two phases of major igneous activity occurred: one in the Early Cretaceous associated with the rift phase of the Gondwana continent, and the other in the Tertiary (Oligocene-Miocene) during the migration phase of the South American and African plates.

Sedimentological Studies of the Quaternary System in Tokyo District with Special Reference to the Thickness Changes in Sedimentary Layers during Compaction History

The area of Tokyo Metropolis occupies low hills, plateaus, fluvial and coastal plain which are underlain by thick Quaternary sediments. Tokyo is situated in the west central part of the Kanto Tectonic Basin which has been developed since the Late Neogene, and the greatest thickness of the Japanese Pleistocene sediments was attained in this basin. On the other hand, concentrated disasters by earthquakes and landsubsidence have been caused from thick unconsolidated sediments of the lowland area of Tokyo.The present results were obtained from subsurface prospecting for the geologic structure and groundwater during the past about 10 years, in which the writer has been engaged. Twenty-two wells were drilled specially for the purpose of this investigation, among which the eight were located on the low plateau, the Musashino Upland, and fourteen on the fluvial and coastal plains, the Shitamachi Lowland. Mechanical and paleontological analyses were made on the drilled cores, and density logging was made at wells for porosities of the layered sediments.This article dealt with subsurface geology, as to the problem of the basin forming movements. The main results are as follows.(1) The Quaternary subsurface sediments are divided stratigraphically into two groups, the lower is the Kazusa Group and the upper the Tokyo Group. The Kazusa Group comprises the Kitatama and Higashikurume Formations, in ascending order. The Tokyo Group comprises the Toneri, Edogawa and Takasago Formations, in ascending order. The Kitatama Formation is mainly of massive siltstone, and the Higashikurume Formation of sandstone with siltstone intercalations. The Toneri, Edogawa and Takasago Formations consist of sand-rich alternations of silt and sand layers with gravels.(2) A gently upheaved zone is recognized from Setagaya Ward to the northern part of Katsushika Ward. On the west of this zone, the strata generally strike E-W with very low dip to the north, and an undulated structure is found in the Kitatama area. A gentle homoclinal structure is developed on the east of the upheaved zone with general strike of NNW-SSE. The center of the sedimentary basin is considered to be in the northern part of Chiba Prefecture to the studied area.(3) Evaluation of the initial thickness of these formations was based on the relation between porosity and burial depth. The relation was figured on silt and sand separately from the results of density logging and mechanical analyses of the present material referring to the published ones.The initial porosity of silt from the surface to 10 m in burial depth seems to be 64 percent. The porosity decreases gradually with depth to 41 percent at 1, 000m, and 34 percent at 2, 000m. The initial porosity of sand from surface to 10m in burial depth seems to be 50 percent, then decreases with depth to 43 percent at 500m and 41 percent at 900m in burial depth.(4) Decompaction number (D) indicates the ratio of initial thickness of a layer (Te) to thickness at a given burial depth (Tp) as follow.D=Te/TpThe decompaction number is considered to vary with burial depth, particle size of the sediments, and some other factors. The average decompaction number is about 1.53 in the upper part of the Kitatama Formation consisting mainly of siltstone, 1.13 in the Higashikurume Formation of sandstone, 1.13 in the Toneri Formation, 1.08 in the Edogawa Formation and 1.07 in the Takasago Formation.(5) During deposition of additional layers, the surface of each formations had been sinking according to compaction of the lower formation and the own. At a sedimentation stage, both values of initial thickness and final one and compaction values, are generally larger in the northern part than in the southern part of the area.

Lateral Diagenesis in Monterey Shale, Santa Barbara Coast, California: ABSTRACT

Lateral differences in Monterey strata along the Santa Barbara coast indicate increased diagenesis toward the west. Westward, silica alters from biogenic amorphous opal (in diatom frustules) to diagenetic opal-CT, and then to diagenetic quartz. Continuous exposure for 50 km, simple homoclinal structure, paleogeographic setting, and detailed analysis of 14 stratigraphic sections together show that sediments were laterally equivalent, as originally deposited. Distinctive stratigraphic differences in sediment composition, informally divided into five members, are also laterally age-constant. Because silica phases differ in rocks of the same age, same depositional environment, and with identical bulk chemical composition, the differences must reflect post-depositional conditions. Overburden thicknesses and thermal changes in organic matter indicate that diagenesis increased westward owing to greater burial temperature. Study of sample sets taken laterally shows that both silica phase changes occurred by rapid solution-precipitation accompanied by significant compaction, and by little movement of silica between beds. Distinctive field characteristics (hardness, brittleness, bulk density, and luster) changed mainly during opal-CT formation. Although these changes affected rocks of nearly all compositions, details varied--even in carbonate-rich rocks--with the proportion of detrital material to biogenic or diagenetic silica. As this proportion increased, opal-CT formation was retarded, accompanying compaction decreased, ordering of opal-CT increased, and quartz formation was promoted. Nearly identical timing relation in associated calcareous, dolomitic, and carbonate-free rocks show that silica diagenesis was unaffected by carbonate except in rocks containing at least 10 times more silica than detrital material. End_of_Article - Last_Page 473------------

Offshore Geology and Sediment Distribution of the El Capitan-Gaviota Continental Shelf, Northern Santa Barbara Channel, California

The Santa Barbara Channel region, the westernmost part of the Transverse Range Province, is the submerged offshore extension of the Ventura basin. It is a topographic depression containing more than 15,250 metres of strata ranging in age from Early Cretaceous to Holocene. Core samples reveal that only the Late Neogene sedimentary section is exposed along the El Capitan Gaviota portion of the shelf. This includes the Miocene Monterey Formation, the Late Miocene/Early Pliocene Sisquoc Formation, the Pliocene Repetto Formation, the Plio-Pleistocene Pico Formation, and Holocene marine sediments. Continuous seismic surveying investigations show the area to be a continuation of the southward-dipping homoclinal structure of the Santa Ynez Range. Superimposed on this structure are the Molino, Gaviota Offshore, and Elwood Offshore fold trends. Associated with the east--west anticlinal trends are subparallel frontal faults and north--south cross or tear faults. Petroleum development of all three trends has occurred. Folding may have started as early as late Oligocene time with episodic growth through Holocene time. The structure is controlled by a single north--south compressive force with two components of stress, one shallow and one deep in the earth9s crust. Patchy distribution of the unconsolidated overburden sediments is controlled by paleotopography of an erosional bedrock surface. Minor scouring of the unconsolidated material with subsequent redeposition is apparent. During the last twenty years a 1 metre average decrease in overburden cover has occurred.

Photogeologic fracture trace study in Northern part of the Kashmir valley

Photogeological fracture traces were analysed in the Sopur area, North Kashmir valley. Azimuthal frequency diagram of the fracture traces show two preferred fracture trends indicating joint or fault pattern caused by tension. The study revealed a homoclinal structure beneath the valley alluvium.

On the Easternmost Part of the Aritagawa Graben in the Kii Peninsula, Southwest Japan

The name “Aritagawa. graben” was newly given to the area which is occupied by the Cretaceous system along the medial zone of the Chichibu terrain in the western Kii Peninsula. The Aritagawa graben is bounded on the east by a normal fault (Itao fault) trending N-S and dipping to the west ; to the east of it, the Cretaceous system is not distributed. The easternmost part of the Aritagawa graben including the Itao fault was re-examined.On the north and the south of the graben are exposed the basement Paleozoic Chichibu. complex and the Upper Jurassic Idani formation, respectively ; the latter may belong either to the Chichibu terrain or the Shimanto terrain. The Cretaceous system in the graben, consisting exclusively of the Futakawa formation (late Urakawanearly Hetonaian epoch in age), forms several anticlines and synclines (major folds), each of which is marked with the abbreviated symbol and is described. These folds are not crumpled at their axial parts irrespective of the angle between the northern and southern limbs and have, in most cases, axes plunging more or less to the west. Each stratum of the Futakawa formation displays, on this account, the arched pattern convex to the east (in the case of a syncline) or to the west (in the case of an anticline), on the geological map.Furthermore, on the eastern extension of one of the anticlines (A-VIII), a small block consisting of the Chichibu complex is exposed within the graben (hereafter, called simply as the “small block”). Faults which demarcate the “small block” on the north and the south converge to one another into the eastern end of the A-VIII axis. The northern boundary fault, without any marked sheared zone along it, trends NE-SW and dips gently (40°±) to the north. On the contrary, the southern one, trending E-W and standing essentially vertically, is represented not by a single fault but by several ill-continuous faults. Lenticular bodies, about 20 m in width, of the Chichibu complex and of the Futakawa formation crop out repeatedly between these faults. Both the northern and the southern boundary fault are formed at or near the basal part of the Futakawa formation. Each stratum of the Futakawa formation proximal to the faults takes the attitude essentially parallel to that of the fault planes, whereas the Chichibu complex in the “small block” forms a homoclinal structure trending approximately E-W.These facts observed in and near the “small block” lead to the following conclusions (1) In all probability, the major folding structures of the Futakawa formation within the graben reflect the superficial relief of the underlying basement. Namely, they were formed as a result of the differential vertical movement of the blocks into which the basement had been separated by the closely spaced small faults. (2) After the folding structures had been accomplished, another block movement took place in the basement. In the easternmost part of the graben, each block tilted more or less to the west. The eastern half of the Futakawa formation which had participated in the anticline of the A-VIII was eroded away and the basement became to crop out to form the present “small block”. The northern boundary fault of the “small block” might be regarded as a sort of the plane of slip which occurred near the basal part of the Futakawa formation. The southern one is probably the fault which originally bounded the block of the basement under the A-VIII.

Geology of South-Central Tejon Oil Field: ABSTRACT

The South-Central Tejon oil field is at the southern edge of the San Joaquin Valley, approximately 1 mi. southeast of the Central Tejon oil field. It was discovered by the Drilling and Production Company in June, 1963. The oil-bearing section is the upper Miocene Reserve zone, which is 90 to 170 feet thick in the field and is composed of several sandstones separated by shale layers. Currently, 5 wells produce from depths of 2,200 feet or less. Initial production of these wells ranged from 25 to 135 BOPD; at present they average 30 BOPD. The field lies within the Tejon embayment, an ill-defined trough of Tertiary and Quaternary sediments. Trapping results from stratigraphic pinchout across a gentle arch within an essentially homoclinal structure. Minor faults are suspected to be present and may contribute to structural closure. End_Page 1084------------------------------ Several of the more than 35 exploratory wells and core holes previously drilled in the area encountered substantial oil and gas shows in the upper Miocene Fruitvale and Reserve zones. Several were cased and tested but commercial production could not be sustained. Geological review and modern well completion techniques were mutually beneficial in achieving production. End_of_Article - Last_Page 1085------------