Cherty Shales Research Articles

Origin and Age Determination of the Neotethys Meliata Basin Ophiolite Fragments in the Late Jurassic–Early Cretaceous Accretionary Wedge Mélange (Inner Western Carpathians, Slovakia)

This study reports the Neotethyan Meliata Basin ophiolite fragments in the Late Jurassic–Early Cretaceous accretionary wedge mélange in the southern part of the Inner Western Carpathians (IWC). Here we present new lithostratigraphical, petrographical, geochemical, and geochronological data obtained from the mélange blocks used to reconstruct the Meliaticum paleotectonic zones in a tentative evolutionary model of this accretionary wedge. The Dobšiná mélange block continental margin carbonatic and siliciclastic sediments have calc-alkaline basalt intercalations. The basalt Concordia age dated to 245.5 ± 3.3 Ma by U–Pb SIMS on zircon most likely indicates the pre-oceanic advanced early Middle Triassic continental rifting stage. The evolving marginal oceanic crust is composed of Middle to Upper Triassic cherty shales to radiolarites. The detrital zircon U–Pb SIMS Concordia ages of 247 ± 4 Ma and 243 ± 4 Ma from a cherty shale, and the xenocryst zircon population Concordia age of 266 ± 3 Ma from a 0.5 m thick “normal” mid-ocean ridge (N-MOR) basalt layer in this cherty shale reveal the connection of the oceanic basin to the adjacent rifting continental margin. The chertified reddish limestone transition to radiolarite indicates syn-rift basin deepening. Upwards, regular alternating N-MOR basalts and radiolarites are often disturbed by peperite breccia horizons. The Nd isotope values of these basalts (εNd240 = 7–8) are consistent with their chondrite normalized rare earth element (REE) patterns and indicate a depleted mantle source. The Triassic ophiolitic suite also comprises rare ocean island (OI) basalts (εNd240 = 5) and serpentinized subduction unrelated peridotites. The Middle to Late Jurassic shortening and southward intra-oceanic and continental margin subduction at approximately 170–150 Ma enhanced the formation of the trench-like Jurassic flysch succession which preceded the closure of the Meliata Basin. The flysch sediments form a mélange matrix of olistolithic unsubducted, obducted, and MP–HP/LT metamorphosed exhumed blocks of the Triassic to Lower Jurassic successions. Blocks of peridotites, rodingites, blueschists, greenschists, rare amphibolites, deep-water shaly sediments and shallow- to deep-water carbonates are typical members of the mélange. The Meliatic accretionary wedge mélange nappe outliers were incorporated in the IWC orogenic wedge in the late Early Cretaceous according to metamorphic rutile U–Pb SIMS ages of 100 ± 10 Ma determined from a Jaklovce metabasalt.

New discovery of late early ordovician graptolites and discussion on stratigraphic sequence of paleozoic sediments in the Dinh Ca - Trang Xa Section, Vo Nhai Disctrict, Thai Nguyen Province, Northeast Vietnam

Sandstone, quartzitic sandstone rhythmically interbedded with siltstone, calcareous siltstone and cherty shale are largely exposed in the Dinh Ca - Trang Xa area, Vo Nhai District, Thai Nguyen Province, NE Vietnam. In previous studies, they were described as Mo Dong or Than Sa Formations (Middle-Upper Cambrian). The graptolite faunas newly discovered from the Dinh Ca Market - Na Mo - Dong Bo cross section are composed of Expansograptus urbanus (Monsen), Expansograptus abnormis (Hsu), Tetragraptus quadribrachiatus (Hall), T. approximatus Nicholson, T. pacificus Ruedemann, T. decipiens T.S. Hall, Schizograptus sp., belonging to the quadribrachiatus Biozone, indicating Floian Stage or late Early Ordovician. This serves as the basis for age determination and for establishing new lithostratigraphic units termed as the Dinh Ca Formation (O1 dc) and the Ba Nhat Member. The stratigraphic sequence of Paleozoic sediments in the Dinh Ca - Trang Xa area is also discussed in this paper.References Bergstöm S. M., Löfgren A., and Maletz J., 2004. The GSSP of the Second (Upper) Stage of Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergӧtland, Southwestern Sweden. Episodes, 27(4), 265-272. Bergstöm S. M., Chen X., Gutierrez-Marco J.C., and Dronov A. V., 2008. The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages to δ13C chemostratigraphy. LETHAI, 42(1), 97-107. Dovjikov, A. E. (ed) et al., 1965. (1971 in Vietnamese). The Geology of the Northern part of Vietnam. Geological Survey of Vietnam, Ha Noi, 583p. Doan Ky Thuy (Ed.), 1976. Geologyof the Lạng Sơn sheet (F-48-XXIII), 1:200.000 scale. Housed in the Central of Geological Information and Archives (in Vietnamese), Ha Noi. Doan Ky Thuỵ (Ed.), 2000. Geology and mineral resources of the Lạng Sơn sheet (F-48-XXIII), 1:200.000 scale. Published by Department of Geology and Minerals of Viet Nam (in Vietnamese with English summary), Ha Noi. Le Duy Bach, 1985. Structure and formation stages of Viet Nam in the tectonic framework of SE Asia. Abstract of PhD Dissertation (in Russian). The National Library of Viet Nam, 31 Trang Thi Street, Ha Noi. Mansuy H., 1908. Contribution à la carte géologiques de l'Indochine. Paléontologie, 73pp., Ha Noi. Nguyen Van Liem, 1978. On the Carboniferous system in North Viet Nam (in Vietnamese). Journal of Biology-Geosciences, 16(3), 78-85. Patte E., 1927. Études géologiques dans l'Est du Tonkin. Bull. Serv. Géol. Indoch., vol. XVI, fasc. 1, 314pp, Ha Noi. Pham Dinh Long (Ed.), 1968. Geology of the Tuyen Quang sheet (F-48-XXII), 1:200.000 scale. Housed in the Central of Geological Information and Archives. Ha Noi (in Vietnamese). Pham Dinh Long, Hoang Xuan Tinh, Phan Viet Ky, 1981. Some main features of regionalgeology of Northeast Viet Nam. Mapping, 48, 4-19, Ha Noi. Pham Kim Ngan, 1975. Some biostratigraphical sections of the Lower Paleozoic of North Viet Nam. Collections of works onstratigraphic studies in Viet Nam, 40-54. Science and Technics Publishing House (in Vietnamese), Ha Noi. Pham Kim Ngan, 1982. Some research results of the Cambrian and Ordovician biostratigraphy in Viet Nam. Geology and Minerals, 1, 7-17. Published by Institute of Geology and Mineral resources (in Vietnamese with English summary), Ha Noi. Saurin E., 1956b. Lexique stratigraphique international. Vol. III, fase, 6a: Indochine. Centre National de la Rech. Scient., 140p. Paris. 1970: published by Science and Technics Publishing House, 222pp. (in Vietnamese), Ha Noi. Tong Duy Thanh, Vu Khuc (Eds.), 2011. Stratigraphic units of Viet Nam.Vietnam National University Publishing House, 526pp, Ha Noi. Tran Van Tri, Ta Hoang Tinh, Phan Son, Le Duc An, 1964. On the Lower Paleozoic sediments in the Than Sa area. Journal of Geology, series A, 37, 6-11 (in Vietnamese with Russian summary), Ha Noi. Tran Van Tri, Vu Khuc (Eds.), 2011. Geology and Earth resources of Viet Nam. Publishing House for Science and Technology, 645pp, Ha Noi. Xu Chen, Strig M. Bergström, Yuan-Dong Zhang and Jun-Xuan Fan, 2009. The base of the Middle Ordovician in China with special reference to succession at Hengtan near Jiangshan, Zhejiang Province, Sothern China. LETHAI, 42, 218-231.

Supergene processes and uranium ore formation in the Ronneburg ore field, Germany

The Ordovician-Lower Carboniferous sequence of slightly metamorphosed gray carbonate-terrigenous rocks contains the Silurian black cherty shales enriched in carbon (6–9%), pyrite (6–7%), and uranium (∼30 ppm). The uranium ore is localized at the pinch-out of areal and linear zones of the Early Permian supergene (exogenic) oxidation of rocks expressed in reddening (hematitization). U, As, Sb, Cu, Ni, Mo, and Ag have been removed from the oxidized black shales and concentrated in the cementation zone in form of pitchblende and sulfides in wall-rock disseminations and veinlets largely hosted in carbonate-bearing rocks. In the Late Permian, during deposition of the upper Rotliegende and Zechstein, the fractures in the basement were filled with carbonates and sulfates; uranium was partly redeposited along with enrichment in Pb and Zn. Mesozoic and Cenozoic supergene processes altered uranium ore insignificantly.

Silurian and Devonian in Vietnam—Stratigraphy and facies

Silurian and Devonian deposits in Viet Nam are present in several zones and regions, including Quang Ninh, East Bac Bo, and West Bac Bo Zones of the Bac Bo Region, the Dien Bien-Nghe An and Binh Tri Thien Zones of the Viet-Lao Region, and the South Trung Bo, and Western Nam Bo Zones of the South Viet Nam Region (Fig. 1). The main lithological features and faunal composition of the Silurian and Devonian Units in all these zones are briefly described.The Silurian consists of deep-water deposits of the upper parts of the Co To and Tan Mai Formations in the Quang Ninh Zone, the upper parts of the Phu Ngu Formation in the East Bac Bo Zone and the upper parts of the Long Dai and Song Ca Formations in the Viet-Lao Region. Shallow water facies Silurian units containing benthic faunas are more widely distributed, including the upper part of the Sinh Vinh and Bo Hieng Formations in the West Bac Bo Zone, the Kien An Formation in the Quang Ninh Zone, and, in the Viet-Lao Region, the Dai Giang Formation and the upper part of the Tay Trang Formation.No Lower and Middle Devonian deposits indicate deep water facies, but they are characterized by different shallow water facies. Continental to near shore, deltaic facies characterize the Lower Devonian Song Cau Group in the East Bac Bo Zone, the Van Canh Formation in the Quang Ninh Zone, and the A Choc Formation in the Binh Tri Thien Zone. Similar facies also occur in the Givetian Do Son Formation of the Quang Ninh Zone, and the Tan Lap Formation in the East Bac Bo Zone, and consist of coarse terrigenous deposits—cross-bedded conglomerates, sandstone, etc. Most Devonian units are characterized by shallow marine shelf facies. Carbonate and terrigenous-carbonate facies dominate, and terrigenous facies occur in the Lower and Middle Devonian sections in some areas only.The deep-water-like facies is characteriztic for some Upper Devonian formations in the Bac Bo (Bang Ca and Toc Tat Formations) and Viet-Lao Regions (Thien Nhan and Xom Nha Formations). These formations contain cherty shale or siliceous limestone, and fossils consist of conodonts, but there are also brachiopods and other benthos. They were possibly deposited in a deep water environment on the slope of the continental shelf.Most Devonian units distributed in the North and the Central Viet Nam consist of self shallow water sediments, and apparently they were deposited in a passive marginal marine environment. The coarse clastic continental or subcontinental deposits are distributed only in some areas of the East Bac Bo and of the Quang Ninh zones of the Bac Bo Region, and in the south of the Binh Tri Thien Zone. This situation suggests the influence of the Caledonian movement at the end of the Silurian period that called the Guangxi movement in South China.

Geochronology Bearing on Age of Monterey Formation, California: ABSTRACT

The name Monterey Formation or Monterey Shale has been applied to lithologically similar pelitic marine rocks of Miocene age exposed in the coastal regions of California. These rocks, characterized by unusually high proportions of silica, have been variously termed diatomite, porcelanite, porcelaeous shale and mudstone, chert, and cherty shale. The type area for the Monterey Formation was designated near Monterey, California. Although the Monterey Formation in general contains numerous ash, tuff, and bentonite beds, only a few of these volcanic units have been dated. It has been necessary to make use of both the isotopically dated horizons and Kleinpell's Miocene benthonic foraminiferal stages to construct a time-stratigraphic framework for the Monterey Formation at different locations. Ages for the various stage and zone boundaries, based on published and unpublished data, are as follows: Repettian-Stratotype B. oblique (Delmontian), ca. 5 m.y.B.P.; Stratotype B. oblique-Mohnian, ca. 7 m.y.B.P.; Mohnian-Luisian, ca. 12 m.y.B.P.; Luisian-Relizian, 14.0 to 14.9 m.y.B.P.; Relizian-Saucesian, 15.7 m.y.B.P. At the type area, an ash from the Canyon del Rey Diatomite Member, the uppermost member of the Monterey Formation, was dated at 11.3 ± 0.9 m.y. (F-T on zircon). Coupled with a Luisian age for the oldest strata, the type Monterey Formation spans, at most, the time interval from 15 to 10 m.y.B.P. In contrast, the Monterey Shale in the Palos Verde Hills encompasses the time interval from 15 to 5 m.y.B.P. based on isotopic data from the Altamira Shale, Valmonte Diatomite, and Malaga Mudstone Members, which contain foraminiferal assemblages characteristic of the Relizian, Luisian, and Mohnian Stages, and the B. oblique zone (Delmontian). At other localities where biostratigraphic and isotopic data are available, the rocks termed Monterey are not exact time equivalents of the type foramation, an observation made earlier by both Kleinpell and Bramlette. End_of_Article - Last_Page 966------------

The Stratigraphy of the Ordovician rocks of Anglesey

AbstractArenig sandstones, grits and conglomerates rest unconformably on pre‐Cambrian rocks (the Mona Complex) and are of variable thickness and facies. Higher Arenig and Llanvirn shales, grits, conglomerates, shaly breccias and ironstones are related to contemporaneous fault movements, with abundant slump structures and rare turbidites. A pre‐Caradoc break in sedimentation was followed by deposition of shales, cherty shales, sandstones, breccias and conglomerates on Llanvirn, Arenig and pre‐Cambrian rocks. The Carmel Head Thrust acted as a major line controlling sedimentation, with upthrow to the north. Lower Silurian graptolitic shales at Parys Mountain rest on an acid volcanic group of Caradoc (?) age.

Fractured Shale and Basement Reservoir, Long Beach Unit, California

The Wilmington oil field in the south-central Los Angeles basin has produced oil from fractured basement rocks since 1945. Oil was discovered in a fractured upper or middle Miocene shale and basement reservoir by THUMS in the Long Beach Unit in March 1968. Oil production is from a southeasterly thickening prism of fractured, black, micaceous shale, siltstone, cherty shale, and marly limestone up to 1,385 ft (421 m) thick, and from fractured Franciscan schist. The black shale is correlated with the Palos Verdes peninsula Altamira Shale Member of early Mohnian and Luisian age. No definite unconformity has been recognized in the black shale member in contrast to the marked hiatus commonly seen between the upper and middle Miocene in the Los Angeles basin. The absence in Long Beach Unit wells of the thick schist breccia and associated volcanic rocks cored at Seal Beach, 3 mi (5 km) northeast, indicates a northeasterly source of sediment. Correlation with similar rocks across the Newport-Inglewood fault zone suggests right-lateral displacement of up to possibly 15 mi (24 km). The oil is believed to have originated in the shale and to have migrated into an interconnected network of vertical, high-angle, and horizontal fractures with migration occurring during several geologic episodes. The fracture system was produced by breakage of the brittle rocks along fold axes and adjacent to the larger faults.

Fractured Shale and Basement Reservoir, Long Beach Unit, California: ABSTRACT

The Wilmington oil field in the south-central Los Angeles basin has produced oil from fractured basement rocks since 1945. Oil was discovered in a fractured upper or middle Miocene shale and basement reservoir by Thums in the Long Beach unit in March 1968. Oil production is from a southeasterly-thickening prism of fractured black micaceous shale, siltstone, cherty shale, and marly limestone up to 1,385 ft (421 m) thick, and from fractured Franciscan schist. The black shale is correlated with the Palos Verdes Peninsula Altamira Shale of early Mohnian and Luisian age. No definite unconformity has been recognized in the black shale member in contrast to the strong hiatus commonly seen between the upper and middle Miocene in the Los Angeles basin. The absence in Long Beach unit wells of a thick schist breccia and associated volcanic rocks cored at Seal Beach 3 mi (4.8 km) northeast, indicates a northeasterly source of sedimentation. Correlation with similar rocks across the Newport-Inglewood fault zone suggests a possible 15 mi (24 km) of right-lateral displacement. The oil is thought to have originated in the shale and to have migrated into an interconnected network of vertical, high-angle and horizontal fractures with migration occurring during several geologic episodes. The fracture system was produced by breakage of the brittle rocks along fold axes and adjacent to the larger faults. End_of_Article - Last_Page 659------------

Pennsylvanian and Permian Basins in Northwestern Utah, Northeastern Nevada and South-Central Idaho: REPLY

The Pennsylvanian and Permian basins in northwestern Utah, northeastern Nevada, and south-central Idaho are downwarped segments of the Cordilleran geosyncline superposed on a complex structural pattern of Precambrian and early and middle Paleozoic age. The Oquirrh basin contains Pennsylvanian and Lower Permian sedimentary rocks as much as 26,000 feet thick; the area of maximum sedimentation was in northwestern Utah. Three principal parts are recognized: a lower part of clastic limestone, sandy limestone, and minor shale; a middle cyclically bedded limestone and sandstone; and an upper sandstone, chert, and shale unit. The lower part was deposited in an offshore shallow-water environment; the middle and upper parts in offshore moderately deep water; these grade laterally both eastward and westward into shallow-water nearshore facies. The Permian Phosphoria basin was partly coextensive with the Oquirrh basin, but the area of maximum sedimentation was in northeastern Nevada and southern Idaho, where locally 3,500 feet of shale, cherty shale, chert, dolomite, and limestone accumulated. This facies was deposited in an offshore deep-water environment, favoring formation of thick sponge-spicule chert and cherty shale units; these grade southward and eastward into carbonates and shales that were deposited in a shallow nearshore environment. In Cretaceous time the Paleozoic and lower Mesozoic rocks of northeastern Nevada and western Utah moved eastward on great thrust plates that extended from southern Utah into Idaho. Movement took place on the Willard-Charleston-Nebo thrust belt in the Wasatch Mountains. Westward continuations of these thrusts crop out in northwestern Utah and eastern Nevada. Imbricate thrusts and tear faults within the upper plate have resulted in complex distribution of upper Paleozoic rocks.

Is the Tay Nappe post-Arenig?

Evaluation of the structural data from the Highland Border is important in the discussion of the age of the Caledonian orogeny in Scotland. There is no published account setting out the detailed structural history of the Highland Border Series and making a comparison of the structures in the Highland Border ‘Arenig’ with those in the adjacent Dalradian. This note gives the results of a study in the N. Esk section which was chosen because it provides the most complete exposures of the Highland Border Series and adjacent Dalradian rocks. The section in the N. Esk was examined in detail, from the mouth of the Mooran Burn [NO. 583735] to the Highland Boundary Fault [NO. 594719]. The rocks exposed (Fig. 1) may be conveniently subdivided in two groups separated by the N. Esk Fault which is exposed in the right bank of the river [NO. 587732]. Upstream from the Fault, Dalradian pelitic rocks are succeeded by grits which have been correlated with the Ben Ledi Grits (Anderson 1942, p. 229). Downstream from the Fault lie two bands of grits and shales with limestone (assigned to the Margie Series = Leny Grit and Leny Limestone, Anderson 1947, p. 490), separated by a group of rocks consisting of greenstones and cherty shales. The greenstones and cherty shales of the N. Esk section, together with other rocks at the Highland Border in Kincardine, Callander and in Glen Sannox, have long been regarded as Arenig in age. As a result of recent re-examination of the ...

Pennsylvanian and Permian Basins in Northwestern Utah, Northeastern Nevada and South-Central Idaho

The Pennsylvanian and Permian basins in northwestern Utah, northeastern Nevada, and south-central Idaho are downwarped segments of the Cordilleran geosyncline superposed on a complex structural pattern of Precambrian and early and middle Paleozoic age. The Oquirrh basin contains Pennsylvanian and Lower Permian sedimentary rocks as much as 26,000 feet thick; the area of maximum sedimentation was in northwestern Utah. Three principal parts are recognized: a lower part of clastic limestone, sandy limestone, and minor shale; a middle cyclically bedded limestone and sandstone; and an upper sandstone, chert, and shale unit. The lower part was deposited in an offshore shallow-water environment; the middle and upper parts in offshore moderately deep water; these grade laterally both eastward and westward into shallow-water nearshore facies. The Permian Phosphoria basin was partly coextensive with the Oquirrh basin, but the area of maximum sedimentation was in northeastern Nevada and southern Idaho, where locally 3,500 feet of shale, cherty shale, chert, dolomite, and limestone accumulated. This facies was deposited in an offshore deep-water environment, favoring formation of thick sponge-spicule chert and cherty shale units; these grade southward and eastward into carbonates and shales that were deposited in a shallow nearshore environment. In Cretaceous time the Paleozoic and lower Mesozoic rocks of northeastern Nevada and western Utah moved eastward on great thrust plates that extended from southern Utah into Idaho. Movement took place on the Willard-Charleston-Nebo thrust belt in the Wasatch Mountains. Westward continuations of these thrusts crop out in northwestern Utah and eastern Nevada. Imbricate thrusts and tear faults within the upper plate have resulted in complex distribution of upper Paleozoic rocks.

Oquirrh and Phosphoria Basins in Northwestern Utah, Northeastern Nevada, and Southern Idaho: ABSTRACT

The Oquirrh and Phosphoria basins in northwestern Utah, northeastern Nevada, and southern Idaho are downwarped segments of the Cordilleran geosyncline superposed on a complex structural pattern of Precambrian and early and middle Paleozoic age. End_Page 1878------------------------------ The Oquirrh basin contains Pennsylvanian and Early Permian sedimentary rocks as much as 26,000 feet thick; the area of maximum sedimentation was in west-central Utah. Three principal units are recognized: a lower unit of cyclically bedded bioclastic limestone and sandy limestone, a middle unit of interbedded limestone and quartzite, and an upper unit of quartzite. The lower and middle units were mostly deposited in an offshore shallow water environment; the upper unit in offshore moderately deep water; these grade laterally both eastward and westward into shallow nearshore facies. The Middle Permian Phosphoria basin was partly coextensive with the Oquirrh basin, but the area of maximum sedimentation was in northeastern Nevada and southern Idaho, where locally 3,500 feet of shale, cherty shale, chert, dolomite, and limestone accumulated. This facies was deposited in an offshore deep-water environment, favoring formation of thick sponge-spicule chert and cherty shale units; these grade southward and eastward into carbonates and shales that were deposited in shallow nearshore environment. In Cretaceous time the Paleozoic and early Mesozoic rocks of northeastern Nevada and western Utah moved eastward on great thrust plates that extended from southern Utah into Idaho. Movement took place on the Willard-Charleston-Nebo thrust belt in the Wasatch Mountains. Westward continuations of these thrusts crop out in northwestern Utah and eastern Nevada. Imbricate thrusts and tear faults within the upper plate have resulted in complex distribution of late Paleozoic facies. End_of_Article - Last_Page 1879------------

Nuka Formation of Late Mississippian to Late Permian Age, New Formation in Northern Alaska

An unusual succession of fine clastic, feldspathic coarse clastic and cherty strata, and correlative coarse clastic rocks in a 170-mile-long band along the northern front of the Brooks Range in northern Alaska is named the Nuka Formation. The succession is more than 6,500 feet thick. It is composed of 13 members that are characterized by light-colored arkosic to calcarenitic layers, by dark-colored layers of mud shale to calcilutite, or by varicolored chert and cherty shale. Rhythmic repetition of the three main rock types may represent depositional cyclicity. The succession is overlain by Jurassic (?) graywacke-type strata; the base, though not exposed, may be gradational with the Lisburne Group (Mississippian). Fossils in the upper half are Permian in age and those in t e lower third are Late Mississippian; no diagnostic Pennsylvanian fossils have been recognized. Away from the type area, the Nuka consists predominantly of arkosic and calcarenitic rocks that contain fossils of Permian age. The feldspathic rocks indicate that a granitic provenance in northern Alaska spread over a considerable area during Permian time and was at least periodically exposed locally for the estimated 80 million years of Late Mississippian to Permian time. Granitic detritus and rocks deposited under varied and active conditions, rather than in a stable shelf environment, differentiate the Nuka Formation from other upper Paleozoic units in the region. These contrasting facies are now juxtaposed, probably as a result of large-scale structural dislocations. The Nuka may extend into the subsurface toward the north and have characteristics favorable to the accumulation of petroleum.

The Tesnus Formation of Trans-pecos Texas

ABSTRACT Field and laboratory investigations indicate a twofold division of the Tesnus formation. The lower unit is marine and consists chiefly of siliceous shales and massive low-rank gray-wackes; the clay-mineral fractions of these rocks are composed mainly of illite and a mixed-layer chlorite-vermiculite. The upper unit is predominantly nonmarine and consists of fine-grained clastic sediments, mainly bentonitic platy shales and cherty shales in which illite and montmorillonite are the clay fraction.

Pennsylvanian Stratigraphy of Elko and Northern Diamond Ranges, Northeastern Nevada

Mappable formations of late Mississippian, Pennsylvanian, and early Wolfcampian age have been designated in northeast Nevada. Since the Fortieth Parallel Survey (1876-1878), these have been referred to the Lower Coal Measures Weber quartzite or conglomerate, and Coal Measures The Tonka formation (2,000-3,000 feet thick) consists of chert-pebble conglomerate and quartzite with fossiliferous limestone and clay-shale interbeds. It conformably overlies and grades eastward into the White Pine? shale except in Carlin Canyon where it appears to rest unconformably on allochthonous cherty shales. Folding and thrusting during the Antler orogeny in central and western Nevada had raised lands from which gravels were shed into the west margin of the miogeosyncline. Though tentatively correlated with the Diamond Peak formation of central Nevada, the Tonka may not have been laterally connected with it. Deposition was essentially continuous during late Mississippian and early Pennsylvanian, and the systemic boundary can not be precisely defined within these formations.The Moleen formation (1,100-1,400 feet thick) conformably overlies the Tonka in the north and the Diamond Peak formation farther south. It is characterized by widespread cherty limestone and indicates abrupt change to more uniform stable conditions. Algae and abundance of fragmental fossils, detrital quartz and chert, diastems, and rare oolite indicate shallow water and frequent agitation by currents and waves. The widespread diagnostic brachiopod Rhipidomella nevadensis Meek occurs in the lower member, considered Springeran in age. The middle member comprises the Millerella-Paramillerella faunizone of Morrowan age, and the upper member of Atokan or Derryan age has a widespread, synchronous Chaetetes zone associated with the lowest abundant fusulinids, principally Profusulinella. The Tomera formation (0-2,000 feet thick) conformably overlies the Moleen except where later Pennsylvanian erosion removed it completely. It has much chert-pebble conglomerate interbedded with quartz-sandy limestones and coquinites, presumably indicating slight revival of crustal instability farther west. The lower two-thirds (Atokan) contains Fusulinella and the upper part (early Desmoinesian), where preserved, has rare Wedekindellina and Fusulina. The upper contact is unconformable where observed. The long established Ely limestone of east-central Nevada is equivalent to, and is an easterly, less clastic facies of, both the Moleen and Tomera formations. The Ely is elevated to group rank in the Elko-Eureka region with these two formational subdivisions. The latter were deposited nearer lands than the more homogeneous type Ely limestone. The Strathearn formation (0-1,500 feet thick) unconformably bevels the Tomera and older formations, and conformably underlies Wolfcampian siltstones near Elko. It consists of limestone, siltstone, and fine conglomerate with Missourian-Virgilian Triticites (lower part) and primitive Wolfcampian Schwagerina (upper part). Equivalent rocks ocur east (upper Oquirrh?) and west of Elko (Antler Peak), but a short distance south and southeast no Upper Pennsylvanian is known, and presumably was not deposited. Most of the strata show evidence of cyclic deposition, although cyclothems were not recognized. End_Page 2211------------------------------ The rocks are lithologically like many shallow-water cratonal deposits, but are thicker and occur in a linear stratigraphic belt. Principal hiatus in eastern Nevada occurred in late Pennsylvanian. About the end of Desmoinesian, increasing crustal instability differentiated the miogeosyncline into broad, low, emergent areas and adjacent negative basins. Deposition was more continuous in basins, as near Las Vegas (Bird Springs formation) and Elko (Strathearn formation), but transgression of intervening emergent east-central Nevada was not completed until Wolfcampian time. Thus in the latter region Wolfcampian strata disconformably overlap the Ely limestone. Definition of the base of the Wolfcampian is generally difficult without fusulinid information, but is especially so where the very similar Upper Pennsylvanian strata are present.

Geology and Exploration for Oil in the Huasna District, San Luis Obispo County: ABSTRACT

The Huasna district, an area of approximately 100 square miles, lies at the southern end of the Santa Lucia Range in the California Coast Range province about 12 miles northeast of the town of Santa Maria. Geologically it consists of a synclinal basin filled with Miocene sediments and interbedded volcanics. The basin is roughly limited on the east and west by two nearly parallel faults. On or near these boundaries Miocene strata are found in fault or depositional contact with Franciscan rocks on the west and Cretaceous sediments on the east. Anticlines have formed on the flanks of the major syncline. Middle and upper Miocene (Monterey) shales and sands crop out along the eastern flank of the syncline. Basinward they are overlain by 2,000 feet of Santa Margarita sands and shales. Wells drilled within the deeper portion of the basin penetrate 4,000-6,500 feet of brown cherty and siliceous shales and dark brown calcareous shales representing all the foraminiferal stages from upper Mohnian to Relizian. Shows of tarry oil occur throughout this interval both in outcrop and in wells, probably accounting for many of the shows of oil recorded by early tests in the district. At and throughout the basal portion of this predominantly shale section volcanic interbeds of tuff, agglomerate and ashy sands of middle and lower Miocene (Saucesian ?) age are encountered. Incomplete records on earlier wells and lack of deep stratigraphic penetration in more than one well near the center of the basin lead to some conjecture as to the nature and distribution of lower Miocene sediments. Thirty-five wells have been drilled in the Huasna district since 1898. Two or three wells have produced a small amount of oil. The Barneberg No. 2, drilled on the Tar Springs anticline, has produced 11,400 barrels of 16 degrees A.P.I. oil, which appears to have come from near the Santa Margarita-Monterey gradational contact and from within the underlying locally fractured shales. Recent wells drilled nearby failed to develop production. The possibilities of stratigraphic traps at depth within the basin and of better structural position on some of the asymmetric anticlines are indicated as being the objectives of future exploration in the district. End_of_Article - Last_Page 2630------------

The Cretaceous of Colombia: ABSTRACT

During the Cretaceous varying amounts of marine and non-marine sediments were deposited in the North Andean geosyncline which passed through Venezuela, Colombia, and Ecuador. During the maximum period of flooding, all except extreme Western Colombia, part of Eastern Colombia, and a few islands appear to have been covered by the seaway. The Cretaceous sediments usually begin with a sandstone or limestone, which is followed by a thick sequence of black shales with occasional limestone members. Following the black shales, there is a more or less cherty shale or limestone, which is followed by either sands and shales or shales and limestones. The thickness of the sediments varies from around 2,000 feet to more than 40,000 feet. Marine faunas are often abundant and show marked relationships to both the Gulf Coast and the European Cretaceous faunas. From the faunas collected at various localities it appears that most, if not all of the standard Cretaceous section is represented in Colombia. End_of_Article - Last_Page 2089------------

Notes on the Stratigraphy of the Santa Maria District: ABSTRACT

Seven oil fields in the Santa Maria District produce oil from four different formations. The most prolific of these is the Monterey shale (middle and upper Miocene). Oil in the Knoxville sandstone, discovered in 1943, created a mild drilling boom, but was limited in areal extent. Lospe production is the latest economic interest in the district. Variation in the age of the Monterey cherty shale in the Santa Maria Valley and other fields makes correlations difficult. Ditch sampling of development as well as wildcat wells has been a valuable practice in stratigraphic control and aid in electrolog correlations. Microscopic study of washed samples shows definite lithologic changes that carry over wide areas. End_of_Article - Last_Page 2087------------

Stratigraphy and Paleontology of Santa Maria District, California

Nine producing or potentially productive oil fields or areas are located in the Santa Maria district, which lies in the Santa Maria basin in coastal California. Formations in outcrop and subsurface sections range in age from Jurassic(?) to Recent. They have a maximum outcrop thickness of about 13,000 feet. The Monterey shale (middle to upper Miocene) and the Sisquoc formation (upper Miocene (?) to middle Pliocene) are the formations of greatest economic interest, the principal oil zones of the largest fields being in fractured chert and cherty shale of the Monterey. The basin facies of the Sisquoc formation represents a Monterey-like facies of later date than the Monterey proper. The stratigraphic relations of the Sisquoc formation and of overlying Pliocene formations sho a close correlation with structural features and structural history. Deformation took place at the eastern margin of the basin and on anticlines in the basin during late Miocene and Pliocene time. Most of the present anticlines were evidently then growing ridges on the floor of the sea and the present synclines were basins. The principal deformation, affecting the entire region, took place after deposition of the non-marine Paso Robles formation (upper Pliocene to lower Pleistocene (?) ).

Miocene Stratigraphy and Paleontology of Palos Verdes Hills, California

The Miocene of the Palos Verdes Hills embraces a thickness of about 2,500 feet of rocks here designated as Monterey shale, and the base is not exposed in the region where the oldest beds crop out. Five main lithologic units are recognized, consisting, in ascending order, principally of silty shale, porcelaneous and cherty shale, phosphatic shale, diatomite, and radiolarian mudstone. Three local members are named and mapped, the lowest one of which includes three of the main lithologic units and also two named tuff beds. Foraminifera are found in all the main units and are assigned to nine faunal zones indicating a time range extending from early Middle Miocene to the top of the Upper Miocene, as these terms are generally used in Coast Range chronology. These beds overlap orthward onto a schist basement of Franciscan (?) rocks, and this northward overlap apparently continues to the Torrance and Playa del Rey oil fields in the southern part of the adjoining Los Angeles Basin. Mollusks from detrital rocks at the base of the overlapping beds on the north slope of the hills represent a fauna that in many respects is a new one for the Coast Ranges, as it includes a number of tropical genera not heretofore recorded there.