Adjacent Grabens Research Articles

Evaluation of alkalinity sources to Cryogenian cap carbonates, and implications for cap carbonate formation models

Current cap carbonate formation models invoke a number of different alkalinity sources for the Neoproterozoic deglacial ocean (e.g., weathering alkalinity, hydrothermal alkalinity, microbial sulfate reduction (MSR) alkalinity, and methane oxidation alkalinity). Assessment of the competing models depends in part on an evaluation of their proposed alkalinity source(s). In the Nanhua rift basin of South China, recently discovered Sturtian cap carbonates on horsts have the potential to address this issue owing to sedimentological and geochemical differences between them and correlative deepwater deposits from adjacent grabens. Cap carbonates at three sites in northeastern Guizhou Province, China were selected for petrographic, XRD, XRF, ICP-MS, in-situ LA-ICP-MS, and Sr and Nd isotope analyses to evaluate the conditions and processes under which they formed: (1) JJS, a high-energy shallow-water (platform) setting that accumulated white laminated dolostone and pisolitic grainstone, (2) ZK01, an intermediate-depth (upper slope) setting that accumulated dark-gray laminated dolostone, and (3) ZK4207, a deep-water (basinal) setting that accumulated organic-rich Mn‑carbonates of the basal Datangpo Formation. The salinity proxy B/Ga records brackish conditions at JJS (4.7 ± 1.2) versus fully marine conditions at ZK01 (7.5 ± 0.8) and ZK4207 (9.1 ± 2.0). Hydrothermal influences, as proxied by high Eu/Eu*, trace-metal (Co, Ni, Cu, V) enrichments, and radiogenic SrNd isotopic compositions, were strong in the latter sections but absent at JJS. These observations document the existence in the post-Sturtian Nanhua Basin of a reduced-salinity surface layer presumably containing a large glacial meltwater component and lacking hydrothermal inputs, and a fully marine deep watermass subject to strong hydrothermal inputs. Carbonate δ13C variation between JJS (−2 to +2‰), ZK01 (−4 to 0‰), and ZK4207 (−7 to −6‰) records a pronounced vertical carbon-isotope gradient in the Nanhua Basin, consistent with elevated marine productivity in the context of strong salinity/density stratification. Massive inputs of alkalinity from meltwater sources are supported by a ∼ 3.5× greater flux to surface-layer dolomitic cap carbonates than to basinal Mn‑carbonates during the Sturtian deglaciation, although strong correlations between intermediate/deep-water carbonates and hydrothermal proxies imply that basin-floor vents were an additional source of alkalinity. Our study provides insight into the mechanisms of formation of cap carbonates generally, with implications for both Sturtian and Marinoan cap carbonate deposits on other cratons.

Depositional history of the Mesoproterozoic Chhattisgarh Basin, central India: insights from geochemical provenance of siliciclastic sediments

ABSTRACT The Chhattisgarh Basin is one of the most important Proterozoic basins of peninsular India. Owing to its deposition in the Mesoproterozoic, the largely undeformed and unmetamorphosed sedimentary sequence of this basin is believed to hold vital clues to our understanding of the evolution of the Indian craton subsequent to the disintegration of the supercontinent Columbia. Despite its importance, only limited studies have been carried out to decipher the depositional history of the basin and to correlate various litho-units in it. Here, we present results of quantitative provenance analysis of the sediments in the basin using trace element and Nd isotopic ratios and discuss their implications for the evolution of the basin in the context of regional tectonics. The sediment provenance analysis reveals that the spatially extensive ~2.5 Ga old granitoids of the Bastar craton were the major contributors (45% to 65%) of the total sediment budget into the basin during its initial stage of evolution. As sedimentation progressed the contribution from the Bastar granitoids was restricted to less than 30% of the total budget with the remaining supplied by younger 1.6–1.7 Ga magmatic rocks. The overall provenance of sediments appears to have remained constant throughout the evolutionary history of the basin; however, the relative contributions of various sources did change with time. The latter is highly conspicuous across the stratigraphic boundary between the Singhora and Chandarpur groups, in the lower part of the Chhattisgarh Supergroup. The geochemical data suggest that the basin developed as a result of either rift or sag into which the sea incursion occurred through the adjacent grabens during the Mesoproterozoic marine transgression.

Application of three-dimensional fault stress models for assessment of fault stability for CO2 storage sites

Carbon Capture and Storage (CCS) is a key technology for a low-carbon energy future and will have an important role on the economic future of the UK Continental Shelf (UKCS). The East Irish Sea Basin (EISB) is a prospective area for CCS in the western UKCS. 3D seismic from the EISB were used in this study to characterise the fault network and potential fault reactivation risks associated with CO2 injection. Two main structural domains are present: a Northern domain with NW-SE faults, and a Southern domain with faults following a N-S orientation. The main storage sites consist of structural closures in Triassic strata of the Sherwood Sandstone Formation (SSF), overlain by alternations of mudstones and evaporites of the Triassic Mercia Mudstone Group (MMG). The closures occur predominantly at fault-bounded horsts, with adjacent grabens filled by thick sequences of the Triassic Mercia Mudstone Group (MMG). The fault framework was used to test, in 3D, the stress model published for the EISB and assess the fault reactivation risk associated with CO2 storage. Slip tendency values were predominantly below 0.6, suggesting the presence of stable structures in the EISB. Under the tested conditions, faults are capable of withstanding pressure increases between 3 MPa and 10 MPa before the onset of slip. The limited fault reactivation risk suggests CCS operations are suitable prospects for the EISB. This work demonstrates the additional value gained from integration of accurately constrained fault geometries in 3D stress models.

New insights on the tectonic evolution of the Miocene gap grabens of Sers–Siliana (Tunisian Atlas) during Neogene to Quaternary: Contribution of chronology of the regional tectonic events

The NW–SE trending grabens of Sers–Siliana, located in the Central Tunisian Atlas, show a peculiarity, compared to adjacent grabens, which comes from the existence of an early Miocene sedimentary gap. This paper discusses the structural evolution of these grabens, during the Neogene–Quaternary episode, which have been poorly studied in previous studies. To better understand the chronology of the two grabens, a set of direct and indirect pieces of evidence are given in order. The results show that the collapse is of Aquitanian–Tortonian age which is synchronous with the regional collapse of the Alpine chain foreland grabens. In addition, our field observations show that the E-trending faults affect the incompetent materials (marls) and assured the mechanical junction between these two grabens. The chronology of tectonic events, during the Neogene–Quaternary time, in the Sers–Siliana area coincides with that described at the regional scale for the Tunisian Atlas domain, allowing a better understanding of the role played by the convergence between African and Eurasian plates.

Newly discovered Sturtian cap carbonate in the Nanhua Basin, South China

Sturtian cap carbonate deposits, previously unknown in South China, are documented in this study from the base of the Datangpo Formation in the Nanhua Basin (Guizhou Province). The age of these deposits is constrained by an LA-ICP-MS U-Pb zircon age of 662.7±6.2Ma from the overlying Mn-shale layer, which is approximately coeval with the termination of the Sturtian glaciation globally. The newly discovered cap carbonates are composed of massive, laminated, pisolitic, and sandy dolomites, and formed subtidally on the summits of horsts within the Nanhua Basin. Two δ13Ccarb profiles (from the JJS-1 and ZK01 sections) show positive shifts of 4–5‰ from the base to the top of the cap carbonates with a ∼2‰ offset between the profiles. These features suggest both a large global perturbation as well as local influences on carbonate carbon isotopes during the Sturtian deglaciation, which is consistent with aspects of both the Snowball Earth and gas hydrate destabilization hypotheses for the origin of cap carbonates.The newly discovered cap carbonates are inferred to be correlative with rhodochrosite (Mn-carbonate)-bearing deposits of the basal Datangpo Formation in adjacent grabens of the Nanhua Basin. This correlation is based on (1) the equivalent stratigraphic positions of the cap carbonates and Mn-deposits, both of which are found at the base of the Datangpo Formation, and (2) consistent U-Pb dates of 667–663Ma for tuff layers interbedded with these deposits at two locales. The Mn-deposits show significantly higher TOC content (1–3% vs <0.7%) and more negative δ13Ccarb values (−10.5 to −5.5‰ vs −4.0 to +2.5‰) than the cap carbonates. The upsection trend toward higher δ13Ccarb in the cap carbonates may reflect increases in marine productivity and the burial rate of organic carbon following the Sturtian glaciation. Despite coeval formation, the dolomitic cap carbonates deposited on horsts and the Mn-carbonate sediments deposited in grabens are heterogeneous facies with different formation mechanisms.

Spatiotemporal relationships of deep-marine, axial, and transverse depositional systems from the synrift Upper Jurassic of the central North Sea

Distinguishing axial and lateral sedimentary systems in rift basins is crucial for predicting reservoir distribution and quality, particularly where synrift strata are interrupted by mass transport complexes (MTCs). Upper Jurassic deep-marine synrift successions in the central North Sea have been studied to assess the temporal and spatial relationships of sediments and controls on reservoir quality. In the Late Jurassic, the central graben experienced erosion at rift margins, whereas adjacent grabens were starved and underfilled with marine sediments, supplied by axial and transverse systems. This study focused on sediments adjacent to a major intrabasinal high, the Josephine ridge. Data included seismic, wireline logs from 16 wells, and biostratigraphic and sedimentological analysis of 144 m (472 ft) of core. Synrift strata are dominated by mudstones but include MTCs interbedded with coarse sandstones at the rift margin and fine-grained turbidite sandstones in basinal depocenters. Petrographic and heavy mineral data indicate different provenance between MTCs and basinal turbidites. Turbidites correlate with periods of lowered relative sea level, during the initial rift phase, and record axial sediment supply. The composition of the MTCs corresponds to in situ strata on the adjacent Jade and Judy horsts. The distribution of MTCs implies formation by crestal collapse horsts during the rift climax and represents a transverse system, with no genetic relationship to axial turbidites. In starved deep-marine basins, fine-grained, well-sorted axial systems may provide the most extensive reservoirs. Transverse systems derived from isolated horsts are typically coarse-grained, poorly sorted, and spatially restricted, being unlikely to provide significant reservoir material.

Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea

Understanding rift topography is essential for determining source areas, sediment pathways, and the type of sediment delivered to a rift basin; factors essential for interpreting petroleum systems in ancient rifts. Here we investigate Upper Jurassic sediments from the Josephine Ridge region of the Central Graben, North Sea, by integrating geophysical, palynological, petrophysical and sedimentological datasets to analyse the palaeoenvironments of the Jade and Judy horsts, the tops of which are not preserved. Interpretation of geophysical and petrophysical data together with core descriptions shows study wells to step progressively away from the Josephine Ridge into adjacent grabens. One hundred and five palynological samples from six wells range from the Oxfordian to the Lower Tithonian, spanning the syn-rift period of the Central Graben. Samples from the adjacent grabens and the Jade Horst are rich in dinoflagellate cysts and possess <20% terrestrial palynomorphs. Samples from the Judy Horst contain a wide range of terrestrial palynomorphs, dominated by lycopsid, fern and moss spores, representing c.50% of the recovered palynomorphs. Correspondence analysis of the assemblages implies that Jade did not possess a terrestrial ecosystem; Judy samples define seven groupings of related miospores, all interpreted to represent very low lying, relatively early successional type environments. This implies subaerial exposure of the Judy Horst during the Late Jurassic, which is interpreted to have formed an isolated, low relief, footwall crest island. This study provides a new methodology for investigating rift topography, particularly in cases where the tops of horsts were subsequently removed by erosion. The Judy Island would have separated the Central Graben into its eastern and western arms earlier than previously predicted, in the Late Oxfordian, with consequences for distribution of shallow and deep-marine reservoir quality sediments.

Crustal and basin evolution of the southwestern Barents Sea: From Caledonian orogeny to continental breakup

AbstractA new generation of aeromagnetic data documents the post‐Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent‐ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well‐constrained onshore, swing from a NE‐SW trend onshore Norway to NW‐SE/NNW‐SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post‐Caledonian development of the late Paleozoic basins. Farther west, the pre‐breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep‐seated high‐density body, interpreted as exhumed high‐grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganization of the crustal extension toward a second necking zone defined at the level of the western volcanic sheared margin and proto‐breakup axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west.

Reverse structures in accommodation zone and early compartmentalization of extensional system, Laminaria High (NW shelf, Australia)

The sediments dynamics and structural development associated with the Late Jurassic rifting phase represent the key factors on the accumulation of hydrocarbon in the Timor Sea. On the Laminaria High (Bonaparte Basin) the main Oxfordian–Kimmeridgian E–W fault system forms structural traps where several discoveries have been made. Recent 3D seismic analysis has been performed based on a combination of classic structural analysis, structure-sensitive seismic attributes (grid-based, surface-based, full-volume) and meta-attribute computations. The analysis has revealed secondary reverse structures associated with the main E–W fabric and prone to act as secondary hydrocarbon traps and/or as migration barriers. The main E–W fault system consists of a complex series of sub-parallel faults that connect via relay ramps or accommodation zones. One of these zones is associated with a transverse anticline resulting from the development of a positive flower structure. The model presented in this article and supported by the interpretation of the seismic data demonstrates that the flower structure developed in an extensional setting. The formation of such a reverse structure can be related to the propagation process of the main E–W fault plane which grows by addition of secondary en-echelon tip faults. Isopach analysis and displacement pattern suggest that a zone of differential displacement occurs between two segments of the main fault plane, inducing local strike-slip movements able to form transpressional uplift associated with restraining bend. This structure related to a rotation of the stress field compartmentalises the early development of the adjacent graben and then controls the distribution of the syn-rift Frigate Fm (Oxfordian–Kimmeridgian).

Late Cenozoic development of the Strouma and Mesta fluviolacustrine systems, SW Bulgaria and northern Greece

The Strouma and Mesta are two of the largest rivers that drain across SW Bulgaria and northern Greece into the northern Aegean Sea. Their modern valleys, flanked by Quaternary river terraces, are incised into a diverse landscape, which records the region's complex tectonic history. A network of lacustrine basins existed in the region in the Late Oligocene to earliest Miocene, but was disrupted by thrusting and folding related to Early Miocene transpression. This deformation was followed by a period of erosion, covering most of the Early and Middle Miocene, which probably marked the initiation of the Strouma and Mesta fluviolacustrine system, with geometries unrelated to the older systems. The first clear evidence of these river systems dates from the Middle Miocene (late Badenian to Sarmatian). The systems evolved in the Late Miocene (Maeotian to early Pontian), when lakes existed, characterized by diatomaceous algae and by occasional burial of abundant plant fragments and coal formation. Areas in the south, south of the Kerkini fault, were intermittently submerged beneath the Aegean Sea at this time. Intense localized uplift of horst blocks in late Pontian and Pliocene, associated with crustal extension, resulted in deposition of thick alluvial fans, with tilting of sedimentary successions in adjacent grabens evident by the end of the Pliocene. The highest horsts (Osogovo, Rila, Pirin, and Belasitsa) experienced additional uplift in the Pleistocene, in part as a result of regional uplift and in part through continued normal faulting. Pleistocene climate change also resulted in influxes of glacial and fluvioglacial systems. The present form of the Strouma and Mesta fluviolacustrine systems is thus the result of interplay between crustal extension, regional uplift, and global climate change.

Structural development and trap formation in the Central North Sea HP/HT play

This paper describes a new family of structures and class of tectonism; it is not wholly detached nor basement linked but falls between the two. The 3D interplay between basement extension and salt movement is used to describe the formation of a continuum of genetically linked trap styles in the Central North Sea HP/HT province. This style of deformation is progressive and manifests itself from simple low relief tilted fault blocks to large diapiric structures common to the UK eastern Central Graben. The proposed kinematic model attempts to unite basement structure, behaviour of salt, and observed map patterns into a simple continuous process. The model is composed of a succession of 6 time-ordered elements: an initial basement (Permian/Triassic) framework of north–south and east–west normal faults; Zechstein salt infill; deposition of Triassic and Jurassic strata; Late Jurassic basement extension, initiation of salt movement; grounding and rotation of Triassic/Jurassic strata about the edges of basement fault blocks; and upward movement of salt localized at the apex of the Triassic/Jurassic fault pattern. The relative amount of salt involved varies, and produces structures of similar architecture, but of varying scale. The Triassic/Jurassic fault pattern and block orientation reflect this structural process in an apparently random nature (i.e. with no dominant trend in either fault or bedding strike). The orientation of each block and the resultant faulted boundaries are mainly an indirect reflection of the underlying 3D-basement configuration. Rotation occurs about a fulcrum between the Triassic/Jurassic section and the point of first contact with a basement fault block, with subsequent rotation into an adjacent graben. This allows for a range of possible fault and bedding orientations inherited from the initial and evolving basement structure, combined with interaction between the rotating Triassic/Jurassic blocks. Although the overall process is simple, the observed map patterns may be complex. This model inherently decouples faults in the basement from the faults in the Triassic/Jurassic section. The Upper Jurassic and basement faults are different ages, and formed in response to different regional tectonics. Thus, common appeals to wrench tectonics, based on opposing offset along a single fault, seem to be unwarranted. This model is additionally supported by experimental analogues, where the seismic based observations can be reproduced in a qualitative manner.

Relations between normal-fault geometry, tilting and vertical motions in extensional terrains: an example from the southern Gulf of Suez

Earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern Gulf of Suez are approximately planar, with dips of 30–40°, from the surface to around 10 km depth. These faults, and the blocks they bound, appear to rotate about a horizontal axis as they move, causing tilting. This tilting is seen both in young vertical movements of the coastline, such as raised beaches and marshlands, and in the distribution of Middle Miocene marine rocks, which are uplifted to elevations of 400–500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens. The absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw-tooth topography on a regional subsidence caused by crustal and lithospheric thinning. The observations required for this simple model are: the present day fault dip, the amount of tilting and the width of the rotating blocks. The virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents.

CESAR

Abstract CESAR (Canadian Expedition to Study the Alpha Ridge), a program of the Department of Energy, Mines and Resources, was conducted from an ice station on the Arctic Ocean between March 30 and May 23, 1983. The base camp was established near 85 °52' north and 112 °34' west, 420 km north and west of Ellesmere Island and drifted in an irregular manner for the duration of the project. Preliminary disclosures were made at a one-day symposium (May 31, 1984) at the CMOS/COU meeting in Halifax. A digest of the geological and geophysical interpretations follows. In cooperation with the Canadian Hydrographic Service some 1300 individual depth soundings and gravity measurements were established over the Ellesmere Island Continental Shelf and Alpha Ridge to the 116 degrees west Meridian. In addition, about 120 kilometres of echo sounding and gravity recordings were established along the drift track of the CESAR station. A 100-m contour bathymetric map and a 10-m Gal contour free-air gravity anomaly map have been compiled and include data collected from Ice Island T-3 between 1969 and 1974 just before grounding for several months off the northern coast of Ellesmere Island. Unlike the Lomonosov Ridge, investigated in 1979 during Project LOREX, the topography of the Alpha Ridge is complex and rugged, showing ridges, grabens, seamounts, etc. in profusion. The minimum depth recorded over the Alpha Ridge is 1087 m. The Alpha Ridge joins the Continental Shelf at a depth of between 1800 and 2000 m. In general, the gravity free-air anomalies mirror the ridge topography and, over this portion of the Continental Shelf, the gravity map reveals similar positive elliptical free-air anomalies that have been mapped all along the Canadian Arctic and the Alaskan Continential Shelf; these are considered to be indicative of the transition between continental and oceanic crust. Based on gravity data alone, the Alpha Ridge might be interpreted as continental crust. During CESAR, sixteen Benthos piston cores and twelve gravity cores recovered as much as 6 m of sediment from the southeastern Alpha Ridge. In total to date, approximately 600 cores have been recovered and studied from the various ice stations in the Arctic Ocean. The cores provide the first information on the lithology and age of sediments covering the Alpha Ridge crest and adjacent grabens. Initial studies indicate a correlatable sequence of litho-facies that range in age from ate Cretaceous to Recent. The graben and basins on the flanks of the northern Alpha Ridge are filled with about 250 m of unconsolidated sediment of which the top 6 m is a continuous section of Lower Pliocene to Recent mud. Most facies of this mud can be correlated with U.S.G.S. cores from the Southern Ridge, western Alpha Ridge and Chukchi Rise. Planktonic and benthic foramanifera appear at intervals in the cores with coarse detrital material interspersed. Pollen has also been observed in the cores but is not as yet identified.

Geologic Outline and Oil Fields of Sergipe Basin, Brazil

In the Sergipe basin of northeastern Brazil a Lower Cretaceous unconformity marks a change in general environment and tectonic style. Below this unconformity, Carboniferous to Lower Cretaceous beds are nonmarine, whereas the overlying Lower Cretaceous to Tertiary beds are dominantly marine. Intense normal faulting tectonic activity preceding the unconformity resulted in uplift and erosion which exposed Precambrian rocks in an area north of Aracaju while, in adjacent grabens, thick wedges of syntectonic conglomerates were deposited locally over older sediments. Irregularities on this unconformable surface were filled by the Carmopolis conglomerate and coarse sandstone member of the Muribeca Formation; the more extensive overlying Ibura Member evaporites of the same formati n also covered the areas where basement was still exposed. Neo-Cretaceous tectonism is characterized by small-scale faulting; the Riachuelo-Siririzinho and Vassouras-Carmopolis oil trends resulted from a combination of northwestward subsidence of basin-margin grabens and a regional southeastward tilting that started somewhat later. Oil production in this basin comes from the Carmopolis, Siririzinho, and Riachuelo fields, mostly from the Carmopolis Member. Some oil is produced also from Lower Cretaceous reservoirs in contact with the unconformity. Depth range of all reservoirs is 400-800 m. Favorable conditions for oil accumulation are the result of adequate structural evolution during Late Cretaceous time, presence of evaporites and probable oil source organic shales at the top of the reservoirs, and younger unconformities not reaching down to the trap. Locally, lateral permeability barriers or reservoir pinchouts complement the structural trap closure. The oil is of mixed base ranging from 24 to 30.5° API. Cumulative production to December 31, 1969, with Siririzinho and Riachuelo fields still being developed, was 23.65 × 106 bbl of oil. This production comes from 205 completed wells drilled in a 400-m grid.

Geologic Outline and Oil Fields of Sergipe Basin, Brazil: ABSTRACT

In the Sergipe basin of northeastern Brazil a Lower Cretaceous unconformity marks a change in general depositional environment and tectonic style. Below this unconformity, Carboniferous to Lower Cretaceous beds are nonmarine, whereas the overlying Lower Cretaceous to Tertiary beds are dominantly marine. A period of intense normal faulting which preceded the unconformity resulted in an uplift which exposed Precambrian rocks in an area north of Aracaju, while in adjacent grabens thick wedges of syntectonic conglomerates were deposited locally over older sediments. Irregularities on this unconformable surface were filled by the Carmopolis Member of the Muribeca Formation, a conglomerate and coarse sandstone; the more extensive overlying Ibura Member evaporites of the same fo mation also covered the areas where basement was still exposed. Late Cretaceous tectonism is characterized by small-scale faulting; the Riachuelo-Siririzinho and Vassouras-Carmopolis oil trends resulted from a combination of northwestward subsidence of basin-margin grabens and a regional southeastward tilting that started somewhat later. Oil production in this basin from the Carmopolis, Siririzinho and Riachuelo fields comes mostly from the Carmopolis Member. Some oil is also produced from Lower Cretaceous reservoirs in contact with the unconformity. Depth range of all reservoirs is 400-800 m. Favorable conditions for oil accumulation are the result of adequate structural evolution during Late Cretaceous time, presence of evaporites and probable oil source organic shales at the top of the reservoirs, and younger unconformities not reaching down to the trap. Locally, lateral permeability barriers or reservoir pinchouts complement the structural trap closure. The oil is of mixed base ranging from 24 to 30.5° API. Cumulative production to December 31, 1969, with Siririzinho and Riachuelo fields still being developed, was 23.65 × 106 bbl of oil. This production comes from 205 completed wells drilled in a grid of 400 m. End_of_Article - Last_Page 353------------

Pastoral High-Country South Island, New Zealand: The Second Century

Pastoral High-Country South Island, New Zealand: The Second Century Howard J. Critchfield* The high-country grasslands of New Zealand's South Island have been settled for barely a century. In the early decades of their occupation the main product, Merino wool, dominated the export trade of New Zealand. The region soon began to fall behind lowland areas in its relative contribution to the national economy. The benefits of intensive grassland farming methods, refrigerated shipping, and other technological innovations were not readily extended into the high country. Deterioration of the natural grasslands caused a decline in net production and created problems not only for pastoralism but also for flood control and hydroelectric developments in the South Island. At the beginning of its second century under European land use the region is striving to regain economic importance as a producer of wool, store sheep, and cattle and to achieve a more stable balance among its natural and cultural elements. *£> ' The Setting The pastoral high country extends nearly the full length of the South Island for more than 400 miles from the rim of the Wairau Valley in Marlborough to Lake Manapouri in Southland. It lies be- * Article is based on a paper read at the Symposium on Settlement of Highland Areas, 20th International Geographical Congress, Tarradale, Scodand, 1964. Dr. Critchfield is Professor of Geography and Chairman, Department of Geography, Western Washington State College, Bellingham 98225. Research by Dr. Critchfield on the South Island High Country in 1959 and in 1966 was supported by a travel grant from the University of Canterbury, New Zealand, and the Bureau of Faculty Research, Western Washington State College. 51 52association of pacific coast geographers tween the divide of the Southern Alps on the northwest and approximately the 1000-foot contour on the southeast, varying in width from 40 miles in Canterbury to 100 miles in Otago (Figure 1). Areal limits of the high country have been variously delineated in terms of altitude, relief, vegetation, land use, land tenure, and size of pastoral runs. As a result, estimates of the area of the region range from 8 million to 20 million acres. The limits chosen here are essentially those of the region wherein extensive pastoralism depends upon natural grassland, scrub, and alpine associations. This pastoral high country comprises about 12 million acres (4,860,000 hectares) or about one-third of the South Island. The dominant characteristic of high-country land forms is rugged relief, ranging from cirques, hanging valleys, and other alpine features at the highest elevations, throughrange and basinphysiography , to steep foothills and terraced valleys that merge into the lowlands. A series of parallel streams drain the region southeastward. Although their valleys are the natural access routes, the numerous gorges, braided channels, and unstable terrace gravels accentuate the difficulties of road and bridge construction. Several valleys were dammed by glacial moraines in the upper reaches to form large lakes which afford natural storage for hydroelectric developments at gorge sites.1 In Otago the country rock is mainly schist, and faulting has been widespread, producing block mountains and adjacent grabens. To the north greywackes are predominant. When exposed to frost action and running water, these rocks readily erode. In Canterbury greywacke shingle carried from the high country has accumulated to depths of several thousand feet to form the Canterbury Plains. Except for gold mining, which reached its peak in the nineteenth century in Central Otago, the high country has not been significant for mineral production. Rainfall ranges from less than 20 inches annually at the lower elevations, especially in protected basins and on certain lee slopes, to more than 80 inches in the headwaters of main rivers. Annual precipitation exceeds potential évapotranspiration over much of the 1 Bryan H. Farrell, Power in New Zealand, (Wellington: A. H. and A. W. Reed, 1962), pp. 93-94. VOLUME 31 1 YEARBOOK i 1969 53 Figure 1. Location of South Island High Country. NEW ZEALAND South Island High Country 200 Ml region, but on sunny exposures having low rainfall, summer conditions approach true aridity.2 A desiccating foehn, the "Nor'wester," considerably increases the water need at the lower elevations, where it is a critical climatic factor. Throughout the...

Basement Control in the Structural Evolution of Southern Oklahoma: ABSTRACT

Southern Oklahoma comprises a complex of structural elements that can be related to recurrent movement on a system of basement faults that had its origin at the time of consolidation of the crystalline basement. Repeated trans-current movements, accompanied by uplift of horsts and deep subsidence of adjacent grabens delineated by this major fault system, have exerted a profound control over the stratigraphic and structural evolution of southern Oklahoma. The larger faults, the Meers fault of the Wichita Mountains, the Washita Valley and Sulphur faults of the Arbuckle Mountains, and the Choctaw fault of the Ouachita Mountains dominate the minor faults and delineate the major basement blocks, namely, the 4 Wichita block, the Tishomingo-Belton uplift, and the frontal edge of the Ouachita Mountains and the intervening basins. The interplay of these basement controlled blocks and their adjacent grabens has not only influenced sedimentation by establishing the pattern of basin development but they controlled the magnitude and type of tectonism that has occurred throughout the Paleozoic Era. Transcurrent movement, according to the wrench-fault mechanics of Moody and Hill, on the major faults which border the basement blocks has produced the stress field responsible for the structural complexity produced during the several stages of the orogenic climax in late Paleozoic time. End_of_Article - Last_Page 551------------