Southern Continental Margin Research Articles

Tectonic synthesis and seismic risk along the Rio Grande de Santiago fault, in Jalisco, Mexico

Various kinds of geological evidence has recently been used to show the presence of a master fault along the course of the Rio Grande de Santiago (RGS). Triangulation data of small motions of survey monuments located in the neighbourhood of the Santa Rosa Dam for the period 1964–1981, show a pattern that closely matches the shape and orientation of the strain ellipsoid deduced previously from the regional structural framework. These and other observations on the mode of fracturing of both margins, and the instability of the wall rocks, indicate that the fault is active and therefore potentially dangerous to civil constructions. Petrologic parameters, sustained by major element chemical analysis and Strontium isotope data support the hypothesis of emplacement of nepheline bearing mildly alkaline basalts through fractures controlled by a complicated pattern of synthetic and antithetic faults, and a pull-apart basin development along the right lateral fault of the RGS. The area does not have a seismic history like the southern continental margin of Mexico (i.e. southern Jalisco, Guerrero, Michoacan, Oaxaca and Chiapas), yet in February 11, 1875, an earthquake of estimated magnitude 7.5, had its epicentre located in Zapopan, Jalisco, a village immediately adjacent to Guadalajara City, and only a few kilometres to the southeast of the study area. After that the region has remained relatively free of seismic activity. An important conclusion is that if the motion of two large blocks sliding past each other along the surface of the fault has been aseismic, it implies the possible accumulation of stress at depth. Seismic risk estimation should therefore be considered at a yellow alert, based on the following features: It is recommended to establish a close survey of microseismic activity, a high precision triangulation and studies of paleoseismicity. A civil protection plan should therefore be carefully carried out to prevent a major disaster.

Alpine tectonics — an overview

Summary This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

The quaternary volcanic belt of the southern continental margin of South America: Transverse structural and petrochemical variations across the segment between 38°S and 39°S

In the segment between 38°S and 39°S of the southern South American continental margin volcanic belt, the volcanoes of the Quaternary “orogenic arc” may be divided into two groups: the first forming the volcanic front, which is located within the Cordillera Principal along a trend oriented 15° east of north; the second east of the volcanic front, occurring upon an uplifted precordilleran block that is east of the Bío-Bío/Aluminé fault system, trends west of north, and merges with the Cordillera Principal near 38°S. Quaternary “back-arc” plateau lavas, consisting of basalts erupted from small cones, occur in the valley east of this precordilleran block. Quaternary intra-arc and back-arc extension appear to be occurring in this section (38–39°S) of the southern South American volcanic belt and perhaps as far north as 34°S. South of 39°S, intra-arc extension is absent and the orogenic arc is restricted to the Cordillera Principal. Differences in the structural characteristics of the arc north and south of 39°S may be due to the change in age from older to younger oceanic lithosphere being subducted beneath, respectively, the two regions. Basalts erupted from both Quaternary arc and back-arc centers located east of the volcanic front between 38°S and 39°S have higher K 20, Rb, Ba, and LREE contents, and higher La/Yb but lower Ba/La ratios, than basalts erupted along the volcanic front. 87Sr/ 86Sr ratios for arc lavas from volcanic centers both within and east of the volcanic front range from 0.7038 to 0.7041 independent of their Si0 2 content, suggesting that more silicic compositions developed from basalts by crystal-liquid fractionation without significant crustal assimilation. Back-arc plateau basalts from the region 38–39°S have Sr isotopic compositions similar to those of the arc volcanoes. Basalts from both the arc and the back-arc centers between 38°S and 39°S may form by partial melting of a large-ion-lithophile-element-enriched mantle source similar to the source of mantle-xenolith-bearing Quaternary alkali plateau basalts from southern Patagonia (south of 39°S), further modified by the addition of alkali elements derived from subducted oceanic lithosphere.

The upper paleozoic pebbly mudstone facies of peninsular Thailand and western Malaysia — Continental margin deposits of Paleoeurasia — Reply

The Phuket Group in Peninsular Thailand and the Singha Formation in NW Malaysia are of Devonian to Lower Permian age. These approximately 3000 m thick strata, which are part of the »SE-Asian pebbly mudstone belt« stretching from Southern Tibet to Sumatra, have been interpreted as continental margin deposits (Mitchell et al. 1970). In contrast, recent papers (Bunopas et al. 1978,Stauffer 1983) propose a glaciomarine origin. These authors follow the scenario that parts of mainland SE Asia (Shan-Thai Craton) rifted away from Gondwana during the Lower Carboniferous and collided with Eurasia during the Late Triassic after crossing the Tethys Ocean under clockwise rotation of more than 180 degrees. In this paper the arguments of these authors will be discussed and a sedimentological description will be given which shows that the pebbly mudstones are not glaciomarine sediments, but rather of continental margin origin. The investigations on Phuket and Langkawi Islands, when compared with newest results from Central and North Thailand, clearly show that the »Shan-Thai Craton« was a part of, or closely connected with Palaeoeurasia (Indosinia) during Carboniferous and Permian times (comp.Helmcke 1985). The pebbly mudstones were deposited on the southern continental margin of this continent (northern margin of Tethys).

Carbonate turbidite sequences deposited in rift‐basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)

ABSTRACTSyn‐rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault‐bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates.The most abundant redeposits are bio‐ and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high‐ and low‐density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow‐water carbonates and skeletal debris from nearby submarine highs.After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge‐shaped with a horizontal upper boundary and consists of a thinning‐ and fining‐upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin‐bedded turbidites and hemipelagic limestones. The thick‐bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick‐bedded turbidites, thinning‐ and fining‐upward cycles are common. The overlying thin‐bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine‐grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin‐plain association developed comprising fine‐grained and thin‐bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.

La marge meridionale de la Mer d'Alboran: Caracteres structuro-sedimentaires et evolution recente

High-resolution seismic reflection profiles (2000 km) were collected across the southern continental margin and major structural trends of the Alboran Sea. Seismic units are dated on the basis of well-known marine Neogene onshore sections and offshore dredged samples. A better assessment of margin structuration and Neogene chronology is made. A conformable succession of Pliocene-Quaternary and older series provides evidence for the non-evaporitic nature of the Upper Miocene, similar to the facies from the onshore Neogene Boudinar and Melilla basins. It also suggests that Alboran Ridge is in part sedimentary rather than only volcanic. This Ridge appears as an “en echelon” anticlinal complex. Conformable Upper Miocene and Lower Pliocene-Quaternary strata are folded and faulted. Primarily important manifestation of the movements is dated as Pliocene, but the main vertical relative movements are of Quaternary to recent age as emphasized by well-developed divergent strata lying above. Submarine volcanism, probably Miocene in age, often controls the morphology and structure of the eastern Moroccan continental margin and adjacent south Alboran Basin. The latter one is characterised by a thick sedimentary accumulation; it is tectonically bounded, except on the southwestern part seaward of the Bokkoyas cliffs. Vertical movements occur during much of Plio-Quaternary time, prevailing during the Quaternary. The structure and related morphology of the study area are mainly the result of neotectonic events.

The upper Palaeozoic pebbly mudstone facies of peninsular Thailand and western Malaysia — Continental margin deposits of Palaeoeurasia

The Phuket Group in Peninsular Thailand and the Singha Formation in NW Malaysia are of Devonian to Lower Permian age. These approximately 3000 m thick strata, which are part of the »SE-Asian pebbly mudstone belt« stretching from Southern Tibet to Sumatra, have been interpreted as continental margin deposits (Mitchell et al. 1970). In contrast, recent papers (Bunopas et al. 1978,Stauffer 1983) propose a glaciomarine origin. These authors follow the scenario that parts of mainland SE Asia (Shan-Thai Craton) rifted away from Gondwana during the Lower Carboniferous and collided with Eurasia during the Late Triassic after crossing the Tethys Ocean under clockwise rotation of more than 180 degrees. In this paper the arguments of these authors will be discussed and a sedimentological description will be given which shows that the pebbly mudstones are not glaciomarine sediments, but rather of continental margin origin. The investigations on Phuket and Langkawi Islands, when compared with newest results from Central and North Thailand, clearly show that the »Shan-Thai Craton« was a part of, or closely connected with Palaeoeurasia (Indosinia) during Carboniferous and Permian times (comp.Helmcke 1985). The pebbly mudstones were deposited on the southern continental margin of this continent (northern margin of Tethys).

Botryococcane in a new class of Australian non-marine crude oils

There are few documented examples of source-specific hydrocarbons in non-marine petroleum, and most of these are related to land-plant precursors1–3. One such biological marker of algal origin is botryococcane (C34H70), a geologically rare acyclic alkane derived from the freshwater green alga Botryococcus braunii4,5, first discovered in several Sumatran crude oils6,7 and here identified in a suite of coastal bitumens from 10 different stranding sites between Kingston, South Australia and Portland, Victoria (Table 1, Figs 1, 2). Biological marker and carbon isotope analyses of these bitumens provide evidence for a new class of Australian non-marine crude oils. The bitumens are inspissated waxy residues of paraffinic and paraffinic–naphthenic crude oils that emanate from submarine seeps in the western Otway Basin8–12, and are characterized by high concentrations of botryococcane. Unlike other high-wax Australian oils13,14, the coastal bitumens contain up to 2.6% sulphur and seem to be primarily of algal origin. Their geochemistry can be used to predict the occurrence of extensive organic-rich source beds that were deposited in deep meromictic lakes along Australia's southern continental margin.

Evolution of the continental margin of southern Spain and the Alboran Sea

Seismic reflection profiles and magnetic intensity measurements were collected across the southern continental margin of Spain and the Alboran basin between Spain and Africa. Correlation of the distinct seismic stratigraphy observed in the profiles to stratigraphic information obtained from cores at Deep Sea Drilling Project site 121 allows effective dating of tectonic events. The Alboran Sea basin occupies a zone of motion between the African and Iberian lithospheric plates that probably began to form by extension in late Miocene time (Tortonian). At the end of Miocene time (end of Messinian) profiles show that an angular unconformity was cut, and then the strata were block faulted before subsequent deposition. The erosion of the unconformity probably resulted from lowering of Mediterranean sea level by evaporation when the previous channel between the Mediterranean and Atlantic was closed. Continued extension probably caused the block faulting and, eventually the opening of the present channel to the Atlantic through the Strait of Gibraltar and the reflooding of the Mediterranean. Minor tectonic movements at the end of Calabrian time (early Pleistocene) apparently resulted in minor faulting, extensive transgression in southeastern Spain, and major changes in the sedimentary environment of the Alboran basin. Active faulting observed at five locations on seismic profiles seems to form a NNE zone of transcurrent movement across the Alboran Sea. This inferred fault trend is coincident with some bathymetric, magnetic and seismicity trends and colinear with active faults that have been mapped on-shore in Morocco and Spain. The faults were probably caused by stresses related to plate movements, and their direction was modified by inherited fractures in the lithosphere that floors the Alboran Sea.

Phosphate occurrences on the western and southern coastal areas and continental shelves of southern Africa

Several different varieties of sedimentary phosphatic deposits of Neogene age occur on the southern and western continental margins of southern Africa. Their origin and distribution were influenced by major transgressions and regressions identifiable in offshore seismic reflection profiles and confirmed by onshore observations. Offshore deposits consist of nodules eroded from outcropping Neogene strata and mixed glauconite/apatite pellets. In addition concretionary masses occur on the South West African shelf associated with siliceous muds. On the western coastal plain the oldest deposit is a Miocene phosphatic sandstone which is unconformably overlain by a Pliocene unit containing reworked Miocene material. Small volumes of phosphate-cemented surface sands are found, and a number of deposits of aluminium phosphates occur in the vicinity of Saldanha Bay.The deposits are of the platform type. No tectonic control of mineralization has been identified, but local topographic features have influenced the distribution of ores. Several modes of origin including direct precipitation, replacement, diagenesis, allochemical and lithochemical, have been identfied.

Evolution of sedimentary basins from Mesozoic times in Australia's continental slope and shelf

Three basic tectonic styles are described from structural trends and sedimentary sequences within sedimentary basins in the Australian continental slope and shelf. These tectonic styles are related to sea-floor spreading events and plate-tectonic movements within the adjacent ocean floor. The same tectonic styles occur within sedimentary basins of different ages; Mesozoic and early Tertiary basins contain rift valley sequences and late Cainozoic basins contain geosynclinal sedimentary suites. Northwestern, western and southern continental margins reflect spreading events explained by an Atlantic-type model in which there are rift-valley sedimentary sequences. The oldest rift valleys in the northwest and the youngest rifts in the south formed ahead of Gondwanaland break-up. After sea-floor spreading commenced, the rate of continental margin collapse varied from place to place. The eastern and northeastern slopes and shelves border marginal seas and do not contain recognizable rift-valley sequences, except for tensional splays (triple junctions) in the Tasman Sea. Short-lived spreading within marginal seas started in the Late Cretaceous in the south and in the Paleocene in the northeast. The tectonism of the northern margin is mainly recorded on land in Timor, Irian Jaya and Papua New Guinea, where, in the Neogene to Holocene, the Australian continent collided with the Asian Plate at the Banda Arc and the sub-plates of the western Pacific at the Louisiade and Bismarck Arcs.

A geophysical study of the southern continental margin of Australia: Great Australian Bight and western sections

Research Article| July 01, 1977 A geophysical study of the southern continental margin of Australia: Great Australian Bight and western sections MICHAEL KÖNIG; MICHAEL KÖNIG 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Department of Geological Sciences, Columbia University, New York, New York 10027 Search for other works by this author on: GSW Google Scholar MANIK TALWANI MANIK TALWANI 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Department of Geological Sciences, Columbia University, New York, New York 10027 Search for other works by this author on: GSW Google Scholar Author and Article Information MICHAEL KÖNIG 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Department of Geological Sciences, Columbia University, New York, New York 10027 MANIK TALWANI 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 109642Department of Geological Sciences, Columbia University, New York, New York 10027 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (7): 1000–1014. https://doi.org/10.1130/0016-7606(1977)88<1000:AGSOTS>2.0.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 MICHAEL KÖNIG, MANIK TALWANI; A geophysical study of the southern continental margin of Australia: Great Australian Bight and western sections. GSA Bulletin 1977;; 88 (7): 1000–1014. doi: https://doi.org/10.1130/0016-7606(1977)88<1000:AGSOTS>2.0.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 Marine gravity, magnetic, bathymetric, and seismic reflection and refraction measurements, most of them made aboard U.S.N.S. Eltanin, were used to study the western and central sections of the southern continental margin of Australia. The data are presented as profiles across the margin.A magnetic quiet zone extends from west of Tasmania to the western border of the Great Australian Bight. In the Great Australian Bight area, the seaward quiet-zone boundary parallels the oldest marine magnetic lineation south of Australia, anomaly 22. An isostatic gravity high very nearly coincides with this isochron. There are no corresponding anomalous topographic features, but seismic profiler records generally show basement peaks occurring in the vicinity and seaward of anomaly 22. A conspicuous magnetic low is located near the landward boundary of the quiet zone; it separates the subdued signature of the magnetic quiet zone proper from high-amplitude, short-wavelength magnetic anomalies of shallow origin on its continentward side. It is associated with an isostatic gravity high and a significant topographic break of the continental slope. Most seismic profiler records show probable shallow basement corresponding to the rough magnetic signature landward of the prominent magnetic low. Seismic profiler and limited refraction data indicate greater depths to basement underlying the magnetic quiet zone than on either side.To the west of the Great Australian Bight, the magnetic lineations older than anomaly 19 are not well defined. The prominent magnetic low is continuous along the western and central margin, as are the isostatic gravity high and the topographic break in the slope associated with it. The area immediately seaward of the magnetic low is occupied by irregular, noncorrelatable magnetic anomalies. Available seismic data indicate a greater thickness of sediments for this zone than for the adjacent seaward area.We have examined several magnetics and gravity models to explain the geophysical anomalies of the Great Australian Bight area. One possible model assumes that non-oceanic subsided basement underlies the magnetic quiet zone, that the seaward quiet-zone boundary marks the juxtaposition of shallow oceanic and deeper non-oceanic basement, and the landward boundary represents a structural discontinuity, probably a normal fault, within continental-type basement. In this model, the basement subsidence of the magnetic quiet zone is isostatically compensated by a lateral change in density. It is not possible to eliminate all other models for the origin of the area occupied by the magnetic quiet zone. 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.

The separation of Australia from other continents

Palaeomagnetism provides the chief constraints on models of the separation of Australia from other continents. Palaeomagnetic studies suggest that the various cratons that comprise the main Australian platform have maintained the same relative position from 1800 m.y. to 750 m.y. ago, and, in greater detail, that the Australian platform, excluding the Tasman Fold Belt, has maintained its physical integrity from 750 m.y. to 450 m.y. ago. Furthermore, palaeomagnetic studies in Australia and the other Gondwanaland fragments indicate that Gondwanaland must have existed at least 750 m.y. ago until its disintegration about 150 m.y. ago. Geological studies suggest that the first clearly recorded separation of other continents from Australia occurred at the end of the Precambrian with the inception of the eastern (Pacific) and northwestern (Tethyan) margins by plate divergence between Australia and unknown continents. Thereafter, the active Pacific margin migrated eastward by the accretion of island-arc material, and the passive Tethyan margin, after its complete development in the Early Ordovician, was static until the Mesozoic. In contrast with this early phase of continental separation, the modern (Mesozoic-Palaeocene) separation of Australia from its Gondwanaland neighbours is traceable by information preserved on the existing ocean floor as well as on the formerly contiguous continents. In detail, the only obscurity remains on the Pacific margin due to a complex pattern of divergence. Greater India separated from Antarctica/Australia early in the Cretaceous (130 m.y. ago) and Antarctica separated from Australia at the end of the Palaeocene (53 m.y. ago). Each of these events was preceded by a period, 170 m.y. long between India and Australia and 110 m.y. long between Antarctica and Australia, of uplift, erosion, and thick terrigenous deposition in rift valleys. It is the subsequent deposition along the western and southern continental margins of fine marine sediments on porous coarse non-marine rift valley sediments that has led to the accumulation of Australia's main known oil and gas resources. The Lord Howe Rise and the New Zealand Plateau separated from Australia in the Late Cretaceous and Palaeocene, as denoted by the generation of the floor of the Tasman Sea, and southeast Papua separated from Australia in the early Eocene, as denoted by the generation of the floor of the Coral Sea. In separating from Antarctica, Australia moved northward and, as shown by palaeomagnetic studies, collided with Southeast Asia no earlier than the Late Cenozoic.

Triassic seaways and the Jurassic Tethys Ocean in the central Mediterranean area

IN the central Mediterranean area phases of middle Triassic and middle Liassic rifting are recognisable. The former produced only relatively deep pelagic basins founded on thinned continental crust; the latter, however, marked the opening of the central Tethys. The Porphyrit-Hornstein and the Diabas-Hornstein formations1 are the results of such phenomena. During Dogger and Malm times the spreading continued and a large amount of oceanic crust was formed. The Tethys Ocean opened parallel to the long axes of some Triassic deep sea basins and across the long axes of others. Consequently, the positions of Triassic seaways along the southern continental margin of the Tethys ‘geosyncline’ are defined by the effects of at least two main palaeotectonic events.

Tectonic Framework of Petroliferous Rocks in Alaska: ABSTRACT

Alaska, comprising 3.6 × 106 sq km (about 28%) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey to contain about 20% of total petroleum liquids and natural gas resources of the nation. Some 15 billion bbl of petroleum liquids and about 31 trillion cu ft of natural gas have been discovered. In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits and some ophiolitic rocks of the Brooks Range orogen were thrust northward over the depressed south margin of the Paleozoic and Mesozoic Barrow platform, on which a foredeep (the Colville geosyncline) developed in Early Cretaceous time. Cretaceous and Tertiary sediments from the Brooks Range filled this foredeep and prograded northwest and northeast to form the Chukchi and Colville delta systems and to fill the Camden coastal basin. A series of arc-trench systems developed on oceanic rocks in southern Alaska during the Jurassic and Cretaceous. These arcs were subparallel with the Mesozoic continental margin of southern Alaska. Between the arcs and the metamorphic (continental) terranes of east-central Alaska and the southern Brooks Range, a large marginal ocean basin received thick Jurassic and Cretaceous volcanic and detrital deposits. These deposits were extensively deformed and disrupted by widespread mid-Jurassic to Tertiary plutonism, Late Cretaceous and early Tertiary (Laramide) oroclinal bending, wrench faulting, and arc-related compression. The Laramide events continentalized the late Mesozoic marginal basin deposits and welded them to the older continental terranes. Subsequent sedimentation was more localized and nonmarine, except in basins along the Pacific, Arctic, and Bering coasts where thick mixed marine and nonmarine sections are present. The active Aleutian arc and associated Queen Charlotte transform-fault system were superimposed obliquely across the southern continental margin of Alaska in early Cenozoic time and have since dominated structural and depositional patterns in southern Alaska. The largest petroleum reserves in Alaska and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by upper Paleozoic to Tertiary shelf and slope clastic and carbonate deposits. The pre-Tertiary arc and marginal-sea deposits in southern and interior Alaska are either too intensely deformed or too low in porosity to offer more than modest local prospects. The Tertiary coastal onshore and offshore basins with End_Page 1446------------------------------ thick marine and nonmarine clastic rocks, and locally many large folds, are attractive for exploration. These little explored basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet. End_of_Article - Last_Page 1447------------

Large-scale Horizontal Displacement within Australo-Antarctica in the Ordovician

Crawford and Campbell have postulated1 (page 11) that the southern continental margin of Australia between longitude 131° ? and 143° ? “is the relic of a major shear which in Early Ordovician time extended still farther across what is now north-eastern Tasmania and that dextral movement of at least 300 km occurred”. We believe that this hypothesis is without foundation and that the curvature of the southern portion of the Adelaide Orogen and the curvilinear pattern of the Mount Read Volcanics in Tasmania, which were cited to support their arguments, can be explained without invoking1 (page 12) “fault-drag on an immense scale” on opposite sides of the postulated shear (see line AB in Fig. 1).