B. C. Worssam
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The early Miocene (22 Ma) Tecuya volcanic rocks of the San Emigdio Mountains were erupted during a regional episode of crustal extension in coastal California. These rocks are correlative with similar rocks in outcrops and the subsurface over at least 800 kmz in the southern San Joaquin basin. Initial dacitic eruptions produced laterally continuous subaerial to submarine pyroclastic flows. These facies rapidly buried alluvial fan deposits of the lower Tecuya Formation and marine sandstone of the Temblor Formation farther west. Following dacitic volcanism, massive basalt was deposited subaerially along the southeastern basin margin. Farther west, the basalt triples in thickness across synvolcanic, high-angle faults and consists of hyaloclastite and submarine basalt flows. Facies associations, thickness and tectural trends, and paleodispersal directions of the volcanic rocks were controlled by synvolcanic extensional faulting throughout the southern San Joaquin basin. The regional extent of these volcanic rocks provides an important chronostratigraphic marker and potential stratigraphic trap in a variety of depositional settings, from nonmarine to outer shelf and slope. Preliminary rare-earth element data indicate that both dacites and basalts exhibit LREE enrichment patterns. The {epsilon}{sub Nd}(O) values cluster around +3. Although these data are typical of island arc volcanic rocks, geological data clearlymore » indicate that the Tecuya volcanic rocks were erupted in an extensional tectonic setting near the continental margin. Some other model, perhaps involving crustal contamination of magmas produced along the subducted East Pacific Rise, followed by syntectonic eruption along normal faults, is needed to explain this enigmatic combination of geochemical and geological data.« less
Subaerial
Outcrop
San Joaquin
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The present architecture of the southern Apennines results from the Neogene continental subduction of the Apulia margin (part of North Africa margin). An allochthonous complex is formed by the low-angle, large-scale, east-vergent tectonic stack of Triassic to upper Miocene (Messinian) units. The rocks represent both a shelf carbonate domain (Apennine platform) and a deep marine domain (Lagonegro-Molise basin). They are emplaced as a nappe during uppermost Miocene (Messinian) to lowermost Pliocene. They are transported to the northeast onto the flexed inner part of the Apulia platform (Mesozoic to late Miocene shelf carbonate series). During the middle-upper Pliocene, the inner Apulia carbonate margin (intermediate platform) was thrust against the outer Apulia domain (overthrust belt). Pliocene-Quaternary syntectonic clastic series were deposited in the flexural basin developed to the east of the Apenninic, allochthon outer ramp and also in piggyback basins carried on top of the allochthon. Oil occurrences are confined to the top of the platform carbonates. Discoveries have been made in (1) compressional structures of the overthrust belt (outer margin of the inner Apulian Platform), (2) late-stage compressive folds in the outer slope of the foredeep, and (3) slightly inverted structures of the foreland margin. Gas accumulations at the top ofmore » platform limestones occur only in antithetic fault block traps or in slightly inverted structures of the foreland. The most important and frequent biogenic accumulations are in clastic rocks of the foredeep, beneath and to the east of the frontal ramp of the allochthon.« less
Allochthon
Neogene
Passive margin
Molasse
Continental Margin
Thrust fault
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Surface geology interpreted from 1:24,000 scale Landsat Thematic Mapper images, in combination with topographic, borehole, seismic reflection, and field data were used to analyze the structure of the Thermopolis anticline, a Laramide Foreland structure in the southern Big Horn basin, Wyoming. Their results demonstrate that the anticline is asymmetric and verges to the southwest, away from the basin axis. Subsurface fold geometry can be predicted in cross section using a concentric fold model constrained by surface geology and balanced by bed-length measurements. The steep southwest limb is cut by multiple northeast-dipping thrust faults that thin the stratigraphic sequence approximately 25%. In the footwall, beds are upturned against the fault zone providing structural closure and a possible exploration target. Northeast-trending compartmental faults cut both the hanging wall and footwall, and segment the anticline into blocks that deformed independently from one another. Northeast-trending, low-amplitude folds are superimposed on the anticline. Flexural-slip folding, thrust faulting, and conjugate faulting were dominant mechanisms operating on the sedimentary sequence during deformation by compression. Seismic reflection data confirms predictions of subsurface geometry and indicate three stages of fault/fold development: (a) thrusting, initial brittle basement failure at low angles; (b) fold development, thrust fault propagation through the stratigraphicmore » sequence inducing folding in sedimentary strata and cataclasis in subjacent crystalline basement; and (c) back-limb thrusting, culmination of fold development by backlimb thrusting of sedimentary strata. Structural geometry is consistent with either: (1) multiple phases of Laramide compression, or (2) a single phase of Laramide compression with shear-zone deformation.« less
Anticline
Thrust fault
Monocline
Echelon formation
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During the last 4 years, the French research group Genegass has completed geological and geophysical studies in the Red Sea and the Gulf of Suez. Neogene infilling of the basin is related with basement quality and volume of clastic supply, and inherited structural features are important to the development of Miocene rifting. The Neogene series may be divided into four major groups (Listed A-D), each limited by unconformities that seem to reflect the major stages of rifting. (A) The lowermost formations begin with a conglomerate and are followed by a variegated unit of sand and clay. In the Gulf of Suez, especially on the eastern bank, these formations are marine. Along the Red Sea, tilted blocks may be capped by stromatolites, and the valleys between them are the site of shale and evaporite sedimentation (lower Miocene). (B) The main extension phase results in an invasion of marine shales. The lower zones contain coarse clastics, and the high zones contain reefs and bioclastic limestone (late Burdigalian to early Serravallian). (C) The middle to late Miocene corresponds to a regional basinward tilting. Stromatolites coat the slopes, and conglomerate fans are found in the lower zones. Evaporite sedimentation dominates; anhydrite is found onmore » the borders, and in the basin, thick halite is overlain by a clastic series. Basement shoulders are uplifted. (D) During the Pliocene and Pleistocene, the central part of the graben showed an important subsidence, and salt tectonism was active with diapirs and collapses.« less
Conglomerate
Basement
Neogene
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Two major petroleum plays are found in the Ouachita uplift region. Folded and thrusted strata constitute the shallower, and possibly the more extensive, of the two. Developed reservoirs include Bigfork Chert (Ordovician), Arkansas Novaculite (Devonian-Mississippian), sandstones within Stanley (Mississippian) and Jackfork (Morrowan) Groups, and Wapanucka Limestone-Spiro Sandstone (Morrowan-Atokan). The deeper play is characterized by underlying basement fault blocks. Development in the Arkoma basin indicates that major reservoirs will include Arbuckle group carbonates. This play is limited to the south by increasing depth and by seismic resolution of the fault blocks. Other potential reservoirs in both plays include Crystal Mountain and Blakely Sandstones (Ordovician), and Blaylock Sandstone (Silurian), with progressively older strata involved in thrusting southward across the uplift. The eastward extent of the Arbuckle facies of lower Paleozoic strata remains a major question.
Devonian
Basement
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Paleogene strata are exposed nearly the entire length of the Alaska Peninsula. They include continental and marine volcaniclastic rocks and a thick volcanic sequence. The strata are divided into the Tolstoi, Stepovak, Meshik, and Belkofski (in part) Formations in the southern part of the peninsula, and into the nonmarine clastic West Foreland Formation and the Hemlock Conglomerate in the northern part. The Tolstoi Formation (Paleocene and Eocene), 670-1380 m thick, consists mainly of continental quartz- and chert-rich sandstone and conglomerate, siltstone, and coal. Volcanic clasts and tuffaceous detritus increase in abundance upward. Neritic strata are present as interbeds in the type area. The formation overlies, with a major unconformity, strata ranging in age from Late Jurassic to Late Cretaceous. Partly coeval strata at the north end of the peninsula (West Foreland Formation) are mainly volcanic sandstone and conglomerate. The Stepovak Formation, 1800-2000 m thick, represents two contrasting depositional environments - a lower dark siltstone and sandstone turbidite, about 975 m thick, and a shallow neritic sandstone and siltstone, rich in volcanic material, about 1000 m thick. Locally, the upper part is deltaic sandstone, siltstone, and coal. An abundant metafauna of Eocene and Oligocene age is found in the neritic deposits.more » A thick coeval volcanic unit, the Meshik Formation, is present in the central part of the peninsula. Andesitic to basaltic lava, breccia, tuff, and lahars, as much as 1500 m thick, have been K-Ar dated at 27-38 m.y. Similar rocks with interbedded sediment at the end of the peninsula are included with the Belkofski Formation.« less
Siltstone
Conglomerate
Paleogene
Breccia
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ODP Leg 110 achieved a first-ever penetration of the detachment surface (decollement) between two converging plates. The Barbados forearc is the location of an eight-site transect drilled across this accretionary wedge and on the incoming Atlantic plate. Their results show offscraped sediments form repeated thrust-faulted sequences at the accretionary toe, with little internal deformation other than scaly fabric generally confined to thrust-fault planes. The decollement is characterized by very scaly, early Miocene orange claystone. This decollment zone is about 40 m thick at Site 671. Hemipelagic to pelagic sediments slip relatively undeformed beneath the accretionary wedge. Pore-water chloride and methane geochemical anomalies define two fluid flow systems in this area: (1) methane-enriched fluids derived from deeper within the arc travel along the decollement and through subducted, sand-rich layers, and (2) locally derived methane-poor fluids move through thrust-fault surfaces within the accreted series. A reference site was cored for comparison of structural, chemical, and physical properties of sediments not yet involved in the convergence process. The occurrence of minor scaly fabric, dewatering and shear veins, and pore-water geochemical anomalies here are clear indications of the propagation of tectonic collision 6 km seaward of the deformation front. Later stages of accretion aremore » typified by thrust packages of highly deformed and folded sediments. The high degree of deformation on the lower arc is accompanied by little loss of pore space. Major, out-of-sequence thrust faults are sources of landward-dipping seismic reflectors along the lower arc and form preferred dewatering paths for the offscraped sediments.« less
Forearc
Accretionary wedge
Décollement
Thrust fault
Echelon formation
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The Lospe Formation contains at least five mappable tuff units (17-18 Ma) which were erupted during initial stages of Neogene Santa Maria basin subsidence. Individual tuff units are lenticular, as much as 15-20 m thick, and 1-3 km wide; they were deposited predominantly in a lacustrine setting. Subaqueous deposition is indicated by facies of the interbedded nonvolcanic Lospe Formation. The lowermost Lospe Tuff unit, however, which overlies Jurassic basement, is interpreted as a subaerial deposit. Each subaqueous tuff unit contains two or more eruption units. Each eruption unit consists of three zones which are, from base to top: (1) massive vitric tuff comprising about 50% of the eruption unit, (2) thin- to medium-bedded vitric tuff with pumice concentrations at the tops of beds and mud drapes between beds, and (3) a thin-bedded interval of massive to planar laminated tuffaceous siltstone-mudstone. The predominance of delicate cuspate vitric shards and pumice, and the near absence of nonvolcanic detritus indicates that little or no reworking of the ash occurred prior to deposition. The Lospe tuffs are predominantly distal pyroclastic flow (zones 1 and 2) and pyroclastic turbidite (zones 2 and 3) deposits, derived from subaerial magmatic eruptions. A possible source for the Lospemore » tuffs is located at Tranquillon Mountain, 30 km to the south in the westernmost Transverse Range, where 17-18 Ma proximal pyroclastic deposits of welded lithic tuff breccia and thick pumiceous fallout tuffs are present. The similar ages, stratigraphic positions, and petrology, as well as the lateral facies relations suggests a correlation between the Tranquillon volcanic center and the Lospe tuff units. The authors are currently testing this hypothesis on the basis of geochemical and isotopic analyses.« less
Subaerial
Pumice
Siltstone
Neogene
Breccia
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Looking at tectonics from a global perspective may open new horizons in the structural studies of thrust belts as well as other tectonic settings. Inferences from plate motions over the past 40 m.y., based on structural data, indicate the presence of a coherent flow of plate motions. Moreover, a [open quotes]westward[close quotes] drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot-spot reference frame. Independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative [open quotes]eastward or northeastward[close quotes] mantle flow also support this global polarity, both in the general directions and in the [open quotes]westward[close quotes] component. Because of this, thrust belts related to east-northeast-dipping subduction show conspicuous structural and morphologic relief, involve deep crustal rocks, and are associated with shallow foredeeps. However, thrust belts related to west-dipping subduction show relatively low structural and morphological relief; they involve only shallow upper crustal rocks and are associated with deep foredeeps as well as back-arc extensions. In the case of west-dipping subduction, the area of the foredeep (filled or unfilled) is bigger than the area of the associated elevated orogene, and it is smallermore » in the opposite subduction. For this reason, in west-dipping subductions in the foredeep maintains longer conditions of deep sea facies with respect to the opposite end member. Foredeep depth in west-dipping subductions is controlled mainly by the rollback of the subduction hinge pushed by the relative [open quotes]eastward[close quotes] mantle flow, and foredeep depth in east- or northeast-dipping subductions is generated mainly by the load of the thrust sheets and by the rollback of the subduction hinge due to the advancing upper plate, contrasting the upward push of the mantle.« less
Asthenosphere
Thrust fault
Eclogitization
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Recognition of laterally extensive, delta-margin sheet sandstone facies in the Lower Pennysylvanian Breathitt and Lee Formations of Kentucky has enabled us to identify laterally persistent coal seams directly overlying sheet sandstones. The Mississippian-Pennsylvanian systemic contact is a pronounced unconformity, and relief on the erosional surface exceeds 30 m. Discontinuous coal and coal-bearing rock units directly overlie the unconformity because precursor peats and associated deltaic sediments were deposited in topographic lows, which prevented blanketlike sedimentation. Southwesterly progradational deltaic sediments progressively filled in topographic lows, and eventually the lower delta plain became nearly flat and horizontal. The interplay between subsidence, detrital influx, clay dewatering, and wave loading influenced the development of sandy substrate locations on the lower delta-plain surface at which peats could accumulate. The sand platforms were required for widespread accumulation of peats because the platforms provided surfaces of considerable areal extent on which vegetation could establish. Depositional sites on the upper delta and lower fluvial plain were more localized; therefore, peat accumulations in those areas were areally restricted. Surfaces that experienced optimum conditions for peat and sheet sand deposit accumulation are preserved as laterally persistent complexes of delta-margin sheet sandstone-coal facies, elongated parallel to the northwesterly depositional strike and dippingmore » gently southwest. Coal exploration programs focusing on the Lower Pennsylvanian rocks of south-central Kentucky should concentrate on delineating similar delta-margin sheet sandstone-coal facies complexes.« less
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