Late Early Cretaceous Time Research Articles

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic melange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values ( Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Mixed fluvial systems of the Messak Sandstone, a deposit of the Nubian lithofacies, southwestern Libya

The Messak Sandstone is a coarse-grained to pebbly, tabular-crossbedded deposit of the widespread nubian lithofacies. It was deposited during Late Jurassic and/or Early Cretaceous time at the northern edge of the Murzuq basin, in southwestern Libya. Although the sedimentary record is predominantly one of braided fluvial systems, a common subfacies within the formation is interpreted to record the passage of straight-crested sand waves across laterally migrating point bars in sinuous rivers, similar to parts of the modern Ganga and Yamuna rivers. Because the sand waves were larger on the lower parts of the point bar, lateral migration created diagnostic thinning-upward cosets of tabular crossbeds, as well as fining-upward grain-size trends. Common thick, interbedded claystones, deposited in associated paludal and lacustrine environments, and high variance in crossbed dispersion patterns, also suggest the local presence of sinuous fluvial systems within the overall braided regime. The Messak Sandstone contains some of the features that led to the proposal of an unconventional low-sinuosity fluvial environment for the Nubian lithofacies in Egypt, and the continuously high water levels of this model may explain channel-scale clay drapes and overturned crossbeds in the Messak. However, most of the Messak characteristics are incompatible with a low-sinuosity model, suggesting instead that the fluvial channels in the Murzuq basin alternated between braided and high-sinuosity channel patterns.

Northward translation of “BAJA British Columbia” along the Late Cretaceous to Paleocene margin of western North America

Paleomagnetic data suggest that a large part of British Columbia and southern Alaska was at the latitude of Baja California, Mexico, in early Late Cretaceous time. This allochthonous block (Baja BC) includes the Insular (Talkeetna in Alaska) and Intermontane composite terranes, the miniterranes between them, and possibly part of the Omineca metamorphic belt. Assuming that Baja BC was adjacent to North America by middle Cretaceous, an analysis using plate kinematics of the Pacific basin produces a model of northward movement of Baja BC which is compatible with the known geology of the reconstructed margin of western North America. Baja BC moved north 2400 km as part of the KuIa plate along a transform margin with North America from 85 to 66 Ma. This time interval is marked by the cessation of igneous activity in the Sierra Nevada, development of wrench faults and basins along coastal California, and initiation of Laramide‐style tectonics to the east. Within Baja BC, diminished igneous activity to the west was coeval with possible wrench‐tectonic basins along its eastern margin. By the end of the Cretaceous, Baja BC was positioned from Oregon to northern British Columbia. Between 66 and 56 Ma, Baja BC became attached to North America in a dextral‐transpressive stage, during which the Kula‐‐North America plate boundary expanded to encompass the whole block with a series of northwest trending strike‐slip faults, a fold and thrust belt in the east, and large‐scale uplifts and the formation of an incipient volcanic arc on the west. Southern Alaska was the site of convergence during the whole of the period of northward translation of Baja BC. A Late Cretaceous subduction zone in Alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of Baja BC collided with interior Alaska. The collision culminated in the Paleocene with formation of the McKinley granites and coeval dextral slip on the Denali fault system.

Review of radiometric data from the Yukon Crystalline Terrane, Alaska and Yukon Territory

The results of more than 20 years of geochronological studies in the Yukon Crystalline Terrane in east-central Alaska and the western Yukon Territory suggest at least six igneous and thermal (metamorphic?) events. Plutonism during Mississippian, Early Jurassic, mid-Cretaceous, Late Cretaceous, and early Tertiary times is indicated. Evidence also indicates that Mississippian, Early Jurassic, late Early Cretaceous, and late Cretaceous thermal (metamorphic?) events have affected parts of the terrane. The western part of the terrane was affected by a significant regional metamorphic event in late Early Cretaceous time, followed by a terrane-wide mid-Cretaceous plutonic event. The pattern of K–Ar ages allows division of the terrane into domains, bounded by northeast-trending lineaments.

Xainxa ultramafic rocks, central Tibet, China: Tectonic environment and geodynamic significance

Research Article| May 01, 1985 Xainxa ultramafic rocks, central Tibet, China: Tectonic environment and geodynamic significance J. Girardeau; J. Girardeau 1Institut de Physique du Globe de Paris, 4 Place Jussieu, 75230 Paris cedex 05, France Search for other works by this author on: GSW Google Scholar J. Marcoux; J. Marcoux 2Sciences Physiques de la Terre, Paris 7, 4 Place Jussieu, 75230 Paris cedex 05, France Search for other works by this author on: GSW Google Scholar E. Fourcade; E. Fourcade 3Laboratoire de Stratigraphie, Université Paris 6, 75005 Paris, France Search for other works by this author on: GSW Google Scholar J. P. Bassoullet; J. P. Bassoullet 4Laboratoire de Géologie Stratigraphique et Structurale, Université de Poitiers, 40 Avenue du recteur Pineau, 86022 Poitiers, France Search for other works by this author on: GSW Google Scholar Tang Youking Tang Youking 5Ministry of Geology, Chinese Academy of Geological Sciences, Beijing, China Search for other works by this author on: GSW Google Scholar Author and Article Information J. Girardeau 1Institut de Physique du Globe de Paris, 4 Place Jussieu, 75230 Paris cedex 05, France J. Marcoux 2Sciences Physiques de la Terre, Paris 7, 4 Place Jussieu, 75230 Paris cedex 05, France E. Fourcade 3Laboratoire de Stratigraphie, Université Paris 6, 75005 Paris, France J. P. Bassoullet 4Laboratoire de Géologie Stratigraphique et Structurale, Université de Poitiers, 40 Avenue du recteur Pineau, 86022 Poitiers, France Tang Youking 5Ministry of Geology, Chinese Academy of Geological Sciences, Beijing, China Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1985) 13 (5): 330–333. https://doi.org/10.1130/0091-7613(1985)13<330:XURCTC>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 J. Girardeau, J. Marcoux, E. Fourcade, J. P. Bassoullet, Tang Youking; Xainxa ultramafic rocks, central Tibet, China: Tectonic environment and geodynamic significance. Geology 1985;; 13 (5): 330–333. doi: https://doi.org/10.1130/0091-7613(1985)13<330:XURCTC>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 SocietyGeology Search Advanced Search Abstract The Xainxa ultramafic rocks, 200 km south of the Donqiao ophiolite, Bangong–Nu Jiang suture zone, central Tibet, form a 1-km-thick sheet in tectonic contact with Paleozoic sediments to the south and with Mesozoic sediments to the north through roughly east-west, south-dipping reverse faults. The ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1-m-sized deposits of podiform chromite. Mafic rocks, represented by a few isotropic gabbros and dolerites, occur in only minor amounts. These rocks, which may have underlined a suture zone within the Lhasa block, have lithologies and tectonics very similar to those occurring in the Donqiao area along the Bangong-Nu Jiang suture zone and therefore can be considered as a klippe of this ophiolite.The Xainxa ultramafic rocks were emplaced in Early Cretaceous time. The main tectonic event observed in that area (northward folding and overthrusting) occurred after late Early Cretaceous time. 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.

Lemhi Arch, a Late Proterozoic and Early Paleozoic Landmass, Central Idaho: ABSTRACT

The northwest-trending Lemhi arch of central Idaho first formed in late middle Proterozoic time, and as much as 4,500 m (14,760 ft) of middle Proterozoic clastic rocks were eroded in later Proterozoic time. The west flank of the arch was partly covered in late Proterozoic(?) and Early Cambrian time by the Wilbert Formation. On the east flank, westward-thinning marine sedimentation began with deposition of the Middle Cambrian Flathead Formation, and continued through the Late Cambrian. During Ordovician and Silurian times, the east flank of the arch was dry. The west flank was submerged in the Ordovician, and the Summerhouse Formation, Kinnikinnic Quartzite, and Saturday Mountain Formation were deposited. The west flank of the arch was briefly exposed after deposition of t e Saturday Mountain Formation, but was partly submerged later in the Silurian, when the Laketown Dolomite was deposited. During the Middle and Late Devonian, deposition was renewed on the west flank of the arch, where the Jefferson Formation indicates eastward transgression. The east flank was exposed until the Late Devonian, when a thin sequence of the Jefferson and Three Forks Formations was deposited across the top of the arch, and marine sedimentation was continuous from the miogeocline far onto the craton. The Lemhi arch continued to influence marine deposition even after it was submerged, separating shelf deposits in southwest Montana and east-central Idaho from miogeoclinal deposits in central Idaho. The arch was overridden by the Medicine Lodge thrust in late Early and Late Cretaceous times. End_of_Article - Last_Page 864------------

Preliminary results, Central Gneiss Complex of the Coast Range batholith, southeastern Alaska: the roots of a high-K, calc-alkaline arc?

The Central Gneiss Complex (CGC) of the Coast Range batholith is the oldest unit of the batholith east of Ketchikan, Alaska, being dated by the zircon UPb method (by T.W. Stern) at 128–140 Ma. Heterogeneous, layered, commonly migmatitic, orthogneiss of hornblende-biotite quartz diorite, tonalite, quartz monzodiorite and granodiorite compositions (IUGS terminology) form the major part of the CGC. These gneisses show a range of 50–65% SiO 2 and are high in Al 2O 3 (c. 15–19%), K 2O (1.5–4%) and Sr (800–900 ppm). Most major elements show coherent, typically magmatic trends with SiO 2. La and Rb show maxima at ∼ 58% SiO 2. Initial 87Sr/ 86Sr ratios are relatively high and range from 0.7052 to 0.7066. Wallrocks of the CGC are mostly metagraywacke, pelite and metavolcanic rocks at amphibolite facies; they are geochemically dissimilar to the CGC. Major and minor elements of the CGC are very similar to those of high-K orogenic, calc-alkaline andesitic suites. The CGC may have formed largely by fractionation of mantle-derived, high AlKSr basaltic liquid in an ascending diapir, having hornblende, plagioclase, and biotite as major precipitating phases. The CGC probably represents the plutonic equivalent of a continental-margin or Andean arc that formed when the Taku terrane of the Insular belt on the west collided with the previously emplaced (but also allochthonous) Stikine terrane on the east in Late Jurassic or Early Cretaceous time.

Metamorphism and plutonism in the central part of the Okanogan Range, Washington

This report is concerned with the bedrock geology of a 1,000-km 2 area in the central part of the Okanogan Range in north-central Washington. Most of the rocks underlying this area were formed under conditions of high-grade amphibolite-facies metamorphism and/or plutonism. The metamorphism, accompanied by metasomatic granitization and anatexis, occurred between Late Triassic and late Early Cretaceous time. Plutonic rocks in the area are grouped into three temporal suites: (1) an early premetamorphic suite represented by inclusions of mafic to granitic basement rock in gneisses of a trondhjemitic batholith which also is believed to have served as basement to supracrustal rocks deposited in a late Paleozoic-early Mesozoic geosyncline; (2) a synmetamorphic suite of gneisses, including both concordant and discordant intrusions; and (3) a post-metamorphic suite of massive rocks which generally are discordant. The igneous rocks range in composition from peridotitic to granitic; trondhjemites and granodiorites are most abundant. Many of these units are somewhat leucocratic for their respective compositions. Younger intrusive rocks are more felsic than older ones. After the plutonic rocks had been deroofed, probably in early Tertiary time, the eastern part of the area was covered by quartz-keratophyre flows. These flows, widely scattered diatremes, and other hypabyssal intrusives are the youngest bedrock elements. Structures generated during metamorphism and plutonism generally trend northwest and indicate lateral compression. Some younger (middle and late Tertiary?) structures followed the established northwest grain, but most formed athwart it. These structures represented a basin-and-range type block-faulting episode in the region—a period of presumed lateral extension. The strike of these fault-block structures seldom varies more than a few degrees from north.

Geologic Framework, Evolutionary History, and Distribution of Hydrocarbon in Jizhong Depression: ABSTRACT

North China basin is a large Mesozoic to Cenozoic sedimentary basin in eastern China. A result of strong block-faulting activities, the inner part of the basin reveals the characteristics of multiple uplifts and depressions. Each depression is generally an independent exploration unit. The practice of exploration in recent years has proved the following: taking each depression as an individual unit, the basic geologic framework and evolutionary history are quickly determined. This is very important in order to achieve the best effects in petroleum exploration. Jizhong depression is located at the western part of the North China basin. This area is about 25,000 km2 (9,650 mi2). Extensive seismic surveys and several hundred exploration wells have been completed during the past few years, resulting in the discovery of Renqiu and other oil fields. Taking the Jizhong depression as an example, the writer has considered the four following problems. 1. Pre-Tertiary fault blocks and their distributional form: There are 11 primary fault blocks in the Jizhong depression. They are all the result of Himalayan movement, but they are obviously subjected to the effects of the structure lines of Yenshan stage. 2. Some characteristics of block-faulting activities: At present, most geologists think that the eastern part of the Eurasian plate, where the Jizhong depression lies, is subject to the effect of subduction of the Pacific plate, which was first compressed and uplifted in Late Jurassic and Early Cretaceous times and then fractured and sagged in Late Cretaceous and early Tertiary. The writer, considers Tertiary block-faulting activities with characteristics of pulsation, through analysis of fault-block development of the Jizhong depression. This analysis follows the generation, development, and extinction of the faults in the depression. The result is that pre-Tertiary blocks are known to have experienced many processes of disruption and union. Because the active period of each fault is at a different time, the main periods of disruption and nion in the different parts of the depression are also varied. 3. Block-faulting in relation to the controlling of Tertiary sediments and structures: Block-faulting activities controlled the differences in source area and depositional center, giving rise to regional climatic differences and leaving clear and definitive time marks in the sedimentary bodies. In the Jizhong depression, each stage has its own special sedimentary formations. By analysis of the main faults and main structures, we may make a conclusion which is somewhat different from that previously accepted--that the block-faulting activities in this area may have formed either tensile structure or compressive and compressive-shear structures. Both of these structures contain petroleum. 4. The types of oil and/or gas pools: Based on the analysis of the geoframework and evolution of the depression, there should be three main types of oil or gas pools: (a) Tertiary oil (gas) stored in Tertiary reservoirs; (b) Tertiary oil (gas) stored in older reservoirs (i.e., buried-hill oil or gas pools); and (c) sub-Sinian and early Paleozoic oil or gas stored in reservoirs of the same age. In the future, the exploration of the buried-hill gas or oil pools will continue. This should expedite the exploration for the other two types of oil or gas pools. In addition, we should pay much attention to exploration for Tertiary stratigraphic-lithologic oil or gas pools. End_of_Article - Last_Page 574------------

Radiolarian Biostratigraphy of Hawasina Complex, Northern Oman: ABSTRACT

The allochthonous Hawasina complex is a sedimentary sequence of both continental-slope and oceanic-basin deposits that were thrust up over shallow-water marine carbonates of the Arabian Shelf during the Late Cretaceous. The Hawasina tectonically overlies the autochthonous Hajar Super Group, and is overlain tectonically by the Samail Ophiolite and associated sedimentary rocks. The lower thrust units, the Hamat Duru Group, and Wahrah Al Ayn Formations, are generally interpreted as limestone and sandstone turbidites deposited on the continental rise. The Wahrah Formation and lower Zulla and upper Sid'r Formations within the Hamrat Duru Group all contain thick sequences of radiolarian-bearing chert. Samples collected from the Zulla and Wahrah (lower chert member) Formations yielded radiolarian faunas assignable to the Late Triassic (Karnian/Norian) based on key species of the genera Capnodoce, Capnuchosphaera, Eptingium, Sarla, Triassocampe, and Yeharaia. Additional samples from the Zulla Formation indicate an Early Jurassic (Pliensbachian) age based on the presence of Broctus, Canoptum, Canutus, Droltus, and Pseudoheliodiscus sp.; previous investigators sugge ted a hiatus in pelagic sedimentation during Early Jurassic time. Radiolarian faunas extracted from two measured sections of the Wahrah Formation (upper chert member) range in age from the Late Jurassic (Tithonian) to Early Cretaceous (late Valanginian/Hauterivian). No suitable radiolarian faunas were obtained from cherts of the Sid'r Formation (Hamrat Duru Group). The higher thrust units are represented by the conglomeratic Al Ayn Formation, and the deeper water Halfa and Haliw Formations. Radiolarian faunas extracted from a measured section near the type locality of the Halfa Formation range in age from the Late Jurassic (Kimmeridgian/Tithonian) to the Early Cretaceous (Hauterivian/Barremian). All the radiolarian faunas obtained thus far from the Haliw Formation are assignable to the Late Triassic (late Karnian to middle Norian) based on fragments of Capnodoce and Veghicyclia sp. Previous biostratigraphic data suggested that the thickest sections of radiolarian chert and mudstone were deposited during Late Jurassic and Early Cretaceous time. Newly obtained paleontologic evidence based on radiolarian biostratigraphy indicates that significant pelagic sedimentation occurred also during the Late Triassic and Early Jurassic. End_of_Article - Last_Page 425------------

Tectonic implications of a late Mesozoic fold and thrust belt in northwestern Nevada

The Luning-Fencemaker fold and thrust belt of northwestern Nevada formed east of the Mesozoic Sierran arc in a back-arc setting where noncontinental crust localized marine deposition and subsequent deformation. The fold-thrust belt was active in Middle or Late Jurassic and Early Cretaceous time and evolved during major northwest-southeast contraction. Comparable shortening was not developed in the Sierran arc, and for much of the Mesozoic the arc and the back-arc regions were at least partially decoupled and separated by the regionally extensive Pine Nut fault system, which underwent substantial left-lateral displacement. Development of the fold-thrust belt and associated strike-slip fault is attributed to oblique subduction. The shortening axis of the fold-thrust belt and inferred displacement of the strike-slip fault are incompatible with right-oblique subduction and call for an interval of left-oblique convergence during part of Jurassic-Cretaceous time.

Massive Siliciclastic Deposits Juxtaposed Against Massive Carbonate Bodies: Paradox of Eastern Gulf of Mexico: ABSTRACT

The eastern Gulf of Mexico is the location of a major and dramatic transition from siliciclastic to carbonate deposits and deposition. The massive lutites that make the Mississippi Cone are juxtaposed against the equally massive carbonates intercalated with evaporites that make the western portion of the Florida Platform. The region now occupied by the continental margin of the eastern Gulf has been cut off from clastic sedimentation since Late Jurassic or Early Cretaceous time, and over 5,000 m (16,404 ft) of predominantly carbonate sediments have built up. Surficial sediments of the carbonate portion of the transition zone are called the West Florida Lime Mud Facies. A foraminifera-coccolith ooze is still being deposited in water depths as shallow as 200 m (656 ft); this facies covers over 40,000 km2 (15,444 mi2) and in many respects resembles a deep-sea ooze. However, in some places shelf carbonate debris, including whole valves of mid-shelf oysters, have cascaded over the shelf-slope break and are incorporated into the slope ooze. Composition, rates of accumulation, and relatively shallow deposition suggest that this sedimentary body may be an analog of some of the chalk deposits of northwestern Europe. It has long been considered axiomatic that planktonic foraminifera and coccolith oozes are deep-sea deposits that accumulate at rates of a few tens of millimeters per thousand years. However, radiocarbon dates show that the eastern Gulf margin ooze has accumulated rapidly. Prior to the last rise in sea level, deposition rates were as high as 65 cm/1,000 years (25 in./1,000 years); and ever since sea level began to rise sedimentation rates have still been as high as 20 cm/1,000 years (8 in./1,000 years). Such a rapid buildup of the carbonate sediments has led to widespread mass wasting and all forms, from creep to massive multiple slide deposits containing thousands of cubic kilometers of material, are represented in our set of high resolution seismic profiles. The actual contact between carbonates and siliciclastics occurs in a variety of styles. Carbonate rubble is found in a terrigenous lutite matrix at the base of the West Florida Escarpment. Farther west on the Mississippi Cone the brown and yellow-brown siliceous lutites contain carbonate turbidites with provenance the West Florida slope. Another type of contact is shown in some piston cores with carbonate ooze in normal and alternating depositional End_Page 453------------------------------ contact with Mississippi siliceous lutite. The position and timing of alternating carbonate and fine terrigenous deposition, as well as the loci of carbonate buildups on the slope, must be affected and perhaps even controlled by the positioning of the Loop Current, a major precursor to the Gulf Stream which, depending upon factors as yet not totally understood, irregularly advances into or retreats from the eastern Gulf. When the loop is up and sweeps from north to south along the West Florida slope, it blocks or deflects Mississippi sedimentation. When the loop is restricted to the area of the Florida Straits, fines derived from the Mississippi River can be deposited at the base of the escarpment and even on the slope. End_of_Article - Last_Page 454------------

The geological development of the passive margins of the Exmouth Plateau off northwest Australia

The Exmouth Plateau is a very large sunken continental block off Australia's Northwest Shelf, whose culmination lies 800 m below sea level. It is elongated in a northeasterly direction, and bounded by abyssal plains on three sides. Its broad tectonic framework is controlled by Mesozoic rifting and transform faulting, which accompanied the breakup of Australia and “Greater India”, after which the plateau sank to bathyal depths. The pre-breakup sequence consists of more than 5000 m of Palaeozoic and early Mesozoic shallow marine to terrestrial detrital sediments. The post-breakup sequence consists of about 2000 m of late Mesozoic and Cainozoic marine sediments, mainly carbonates. The northern margin came into existence in the Callovian (155 m.y. ago), when a continental fragment separated from it and moved off to the northwest. An early phase of rifting gave rise to Triassic—Jurassic intermediate and acid volcanics (213-192 m.y. ago), which overlie a thick Triassic paralic sequence. Steady subsidence north of an east—west hinge line allowed several thousand metres of early and middle Jurassic carbonates and coal measures to accumulate before breakup. Breakup occurred along a series of rifted and sheared segments, and the margin is further complicated by northeast-trending Callovian horsts and grabens. The horsts were planed off in late Jurassic and early Cretaceous times, and the whole margin was covered by a few hundred metres of late Cretaceous and Cainozoic pelagic carbonate as it sank steadily to its present average depth of 2000–2500 m. The northeast-trending western margin formed by rifting in the Neocomian (120–125 m.y. ago), as “Greater India” moved off to the northwest. Callovian normal faults parallel the margin. A thick Triassic paralic sequence is unconformably overlain by thin late Jurassic and younger marine sequences, indicating that the area was high in the early and middle Jurassic. Thin late Cretaceous and Cainozoic pelagic carbonates cover the margin, which now lies more than 2000 m below sea level. The northwest-trending southern margin formed by shearing in the Neocomian, at the same time as the western margin. It is cut by northeast-trending normal faults, which formed in the late Triassic and Callovian, and is paralleled by Neocomian and later normal faults. A thick Triassic paralic sequence is unconformably overlain by a thick late Jurassic and Neocomian delta, suggesting that the area was high in the early and middle Jurassic, but a depocentre before and afterwards. There was thermal uplift of more than 1000 m during the Neocomian period of shearing, and igneous intrusions buttressed the margin. Later normal faulting lowered the outermost margin, and turned the uplift into a marginal anticline trending northwest. The anticline had sunk beneath the sea by late in the Cretaceous, and thereafter this margin was covered by a thin sequence of pelagic carbonate, which now lies beneath more than 1500 m of water.

Preliminary model of regional Mesozoic groundwater flow and uranium deposition in the Colorado Plateau

Qualitative and numerical simulation of regional groundwater flow in the Colorado Plateau during Late Jurassic and Early Cretaceous time has led to a model of uranium deposition in areas of upwelling solutions. Paleogeographic reconstruction shows that surface drainage and therefore groundwater flow was generally toward the northeast and east. Croundwater flowing in these directions, principally through Triassic-Jurassic and Permian sandstones, encountered sediments of variable thickness due to buried uplifted or downdropped Precambrian blocks. The buried uplifted blocks caused upward movement of groundwater around them. These inferred zones of upwelling are closely associated with concentrations of Jurassic- and Cretaceous-age uranium deposits. The results are consistent with hypotheses of an upwelling brine mixing and reacting with descending meteoric water and causing uranium precipitation at the fluid interface. Whether the uranium came from above or below the interface is an unsolved problem, but this model helps to identify better the sources of the waters and also suggests target areas for exploration.

Unconformities--Key to Major Oil Accumulations, North Slope, Alaska: ABSTRACT

The entrapment of oil and gas at an unconformity at Prudhoe Bay, North America's largest oil field, is well known. It now appears that other large oil and gas accumulations such as those in the Kuparuk River sandstones west of Prudhoe Bay are trapped at an unconformity. Prudhoe Bay field is on a large anticline on which the reservoir rocks have been truncated by an unconformity. The unconformity places organically rich Cretaceous shales in juxtaposition with Sadlerochit reservoir rocks. Cretaceous shales above the unconformity also provide the updip seal for the oil accumulation. Oil in the Jurassic Kuparuk River sandstones is trapped on a large structural nose by truncation of the reservoir rocks. The Cretaceous shales above the unconformity provide the seal and possibly the source for the oil. Uplift of the Barrow arch in Late Jurassic and Early Cretaceous time resulted in a regional unconformity. The truncation of older rocks by this unconformity, and the seal provided by the overlying shales are key factors to exploration of this arch which extends for more than 300 mi (483 km) along the Beaufort Sea coast. The delicate relations of source, reservoir, and trap, which are commonly controlled by one or more unconformities, are important to exploring this rich petroleum province. End_of_Article - Last_Page 684------------

Hydrocarbon-Trapping Structures in Southern Canadian Rockies Segment of Cordilleran Foreland Thrust Belt: ABSTRACT

The hydrocarbon reservoirs consist of upper Paleozoic platformal carbonate rocks in northeasterly verging, imbricate, listric thrust-fault wedges, and in related flexural-slip folds. They formed in Maestrichtian to early Paleogene time, after these rocks had been buried under more than 5 km of Late Jurassic to Paleogene molasse. Both the generation and entrapment of the hydrocarbons result from subduction of the floor of the Cordilleran miogeocline. Palinspastic reconstructions of the foreland thrust and foldbelt, based on balanced structure sections that take into consideration the deep crustal structure as outlined by seismic refraction, magnetic, gravity, and geomagnetic depth sounding data, show that: (1) there has been about 200 km of net horizontal convergence between the Mesozoic magmatic arc of the eastern Cordillera and the autochthonous cover on the North American craton; (2) the convergence is expressed at a shallow level, in the eastern, more external zone, by horizontal compression and vertical thickening within supracrustal rocks that overlie an unbroken basement of cratonic continental crust; but at deeper levels, in the western, more internal zone, it involved the subduction of the former basement of the miogeocli e; (3) the Cordillean miogeocline is a northeasterly tapering wedge of craton-derived sedimentary rocks that accumulated outboard from the edge of the continental craton, on oceanic or tectonically attenuated continental crust; (4) the foreland thrust and foldbelt is a shallow subduction complex that was tectonically prograded northeastward as the miogeoclinal, platformal, and exogeoclinal supra-crustal rocks were scraped off the underriding slab and accreted to the overriding slab; (5) subsidence in the migrating foredeep was due to flexure of the lithosphere under the weight of the encroaching subduction complex, and of the molasse itself. The first of two main pulses of subduction occurred outboard from the craton, during Late Jurassic and Early Cretaceous time. It involved outward verging thrusting and folding on either side of the uplifted core of the miogeocline, and it produced a thick wedge of molasse that covered the western craton. The hydrocarbon reservoirs formed during the second pulse, in Late Cretaceous and early Paleogene time, as the cratonic cover rocks were deformed and accreted to the growing subduction complex, while the continental craton moved under the detached miogeocline. End_of_Article - Last_Page 768------------

Drainage‐basin evolution and aquifer development in a karstic limestone terrain South‐Central Texas, U.S.A.

AbstractThe Edwards artesian aquifer occurs in cavernous limestones of Cretaceous (Albian) age within the Balcones fault zone in south‐central Texas. The major recharge and discharge zones of the aquifer are contained within the upper reaches of three river systems: the Nueces, the San Antonio, and the Guadalupe. Within these watersheds, recharge dominates in the semiarid Nueces basin to the west while most discharge occurs farther east from wells in the subhumid San Antonio basin and from springs in the subhumid Guadalupe basin. This long‐distance transfer of ground water (up to 240 km) is a result of several factors: depositional and early diagenetic history of the limestone host rock, geometry and magnitudes of fault displacement, and physiographic responses to faulting.The loci of greatest discharge from the aquifer occur in an area that was exposed subaerially with concomitant porosity enhancement due to dissolution of limestone during late Early Cretaceous time. This area also was subjected to the greatest fault displacement during Miocene time. Thus, faults and associated joints superimposed additional avenues for porosity and permeability development onto an area that already had considerable secondary porosity. Further determinants on aquifer properties resulted from late Tertiary and Quaternary drainage evolution in response to faulting along the Balcones trend. The strike of the fault zone lay at acute angles to the courses of the main trunk streams in the ancestral Guadalupe and San Antonio River systems, whereas in the Nueces basin the trend of the fault zone was normal to the courses of the main streams. Thus, as a fault‐line scarp began to form in the eatern basins, scarp‐normal streams were incised rapidly into northwest‐trending canyons. These steep‐gradient streams captured the eastward‐flowing major streams in the easten watersheds. These pirate streams incised into the aquifer at the lowest topographic levels within the region because of: 1. The sudden acquisition of extensive catchment areas in a subhumid area; and 2. Steep stream gradients that reflected the larger fault displacement in the east. The low topographic points of discharge became the loci of major springs.Recharge is dominant in the Nueces basin mainly because streams cross permeable limestone units at higher topographic levels than in the San Antonio and Guadalupe basins. The topographic characteristics of the Nueces watershed resulted from a combination of diverse factors: lesser fault displacement, no major stream piracy, and less vigorous erosion because of a semiarid climate.

Subduction, Buoyant Braking, Flipping, and Strike-Slip Faulting in the Northern Caribbean

The distribution of metamorphic rocks and their division into continental and oceanic types, the distribution of ophiolites and alpine ultramafic rocks, and the distribution and age of volcanic rocks in the Greater Antilles indicates that a northward-dipping subduction zone may have been the first orogenic event in the northern Caribbean. Buoyant continental crust entering the zone from the south stopped that subduction in Late Jurassic or Early Cretaceous time (120-130 m.y.b.p.), as evidenced by lithologic types and metamorphic rock ages in Cuba, Dominican Republic, and Puerto Rico. Continued compression resulted in a new, "flipped" subduction zone dipping southward on the north side of the old island arc. The second episode of subduction began about 110 m.y. ago and formed most of the exposed island arc. Two alternative models are proposed: 1) continuous subduction on a variably-dipping plane such that the western segment dipped too gently to permit volcanism after 85 m.y.b.p.; 2) subduction converted about 85 m.y.b.p. into strike-slip motion with anatectic volcanism. In both models, intense deformation ended at 45 m.y.b.p., volcanism ended in the west at 85 m.y.b.p. and in the east at 45 m.y.b.p. Still-continued compression of the Caribbean microplate since 45 m.y.b.p. has resolved itself into an eastward transform motion which continues to the present.

PALEOMAGNETISM OF SOUTH AMERICAN AND AFRICAN ROCKS AND THE AGE OF THE SOUTH ATLANTIC

The late Palaeozoic polar path, common to the South American-African block, defines a continuous shift of about 60° from the Late Carboniferous to the Middle Permian. During this episode that block moved away from the South pole. In either-early Middle or late Early Permian time, such movement stopped and a quasi-static period started for the South American-African block which lasted up to the Jurassic. A new and reversible polar shift (about 35°) relative to the South American African block started in either late Jurassic or early Early Cretaceous time; previous palaeomagnetic studies did not recognize this Early Cretaceous polar shift common to the South American-African blook. In late Early Cretaceous time such movement stopped for South American and a quasi-static period started for this continent, which lasted up to the late Late Cretaceous. Palaeomagnetic data for Africa suggest that the Cretaceous polar shift was still Present for this continent in Late Cretaceoui time. This suggests that: i) South America and Africa were joined by their Atlantic margins up to the Early Cretaceous; and ii) the separation of these continenrs and the origin of the South Atlantic took place in late Early Cretaceous time (after the Serra Geral and Kaoko basaltic magmatism). However, the palaeomagnetic data do not rule out the possibility that the initial rifting in the South American- African block has occurred in Early Cretaceous time.

Geology and Petroleum Potential of Exmouth Plateau Area Off Western Australia

The Exmouth Plateau lies oceanward of Australia's Northwest Shelf petroleum province, in water depths of 800 to 2,000 m, and is flanked by the abyssal plains of the Wharton basin. The plateau is dominated by the northeast-trending Exmouth Plateau arch and Kangaroo syncline. The southwestern and northwestern margins are fault-bounded escarpments, but the northern margin consists of spurs and subplateaus separated by deep troughs. Magnetic basement is as deep as 10 km below sea level, but shallow crystalline basement and igneous bodies are present around the seaward margins. A crustal thickness of about 20 km, similar to that under the Northwest Shelf, is deduced from gravity data. The outer limit of continental crust is roughly coincident with the 4,000-m isobath. Interpretation of seismic profiles indicates that up to 5,000 m of Paleozoic strata and up to 5,000 m of younger strata overlie basement. The sediments have been gently folded, and northeast-trending block faults affect the pre-Late Jurassic sequence. The sediments beneath the Exmouth Plateau are considered to have been deposited in part of the Carnarvon basin. This part formed a north-facing embayment in Gondwanaland which received detrital sediments from the south until Early Cretaceous time. At least, 3,000 m of mainly paralic and shallow-marine detrital sediments were deposited from Permian to Middle Jurassic times. Rifting probably commenced in the Late Permian. It recurred in the Late Triassic, except in the northeast of the area and on the adjacent Rankin platform where it culminated in the early Late Jurassic. Prevailing hypotheses suggest that during the early Late Jurassic seafloor spreading around northeast-trending ridges formed the plateau's northwestern and northern margins, by rifting and transform faulting, respecti ely. About 1,000 m of shallow-marine and deltaic detrital sediment, derived from the south and east, covered the block-faulted surface in Late Jurassic and Early Cretaceous times. The plateau's southwestern margin formed by faulting, probably transform faulting, in the Early Cretaceous. About 200 m of marine terrigenous sediment was deposited in the middle Cretaceous, and 500 to 1,000 m of carbonate sediment in the Late Cretaceous and Cenozoic. In the Miocene, the rate of subsidence exceeded the rate of sedimentation and, thereafter, the Exmouth Plateau arch and Kangaroo syncline took their present form. Potential petroleum source rocks (especially Paleozoic to Cretaceous shale and siltstone) and reservoir rocks (especially Triassic and Neocomian sandstone) appear to be present. Numerous Triassic-Jurassic fault blocks, and a large Neocomian delta in the south, are obvious exploration targets.