Lateral Tectonic Compression Research Articles

Impact of stylolitization on diagenesis of a Lower Cretaceous carbonate reservoir from a giant oilfield, Abu Dhabi, United Arab Emirates

Bed-parallel stylolites are a widespread diagenetic feature in Lower Cretaceous limestone reservoirs, Abu Dhabi, United Arab Emirates (UAE). Diagenetic calcite, dolomite, kaolin and small amounts of pyrite, fluorite, anhydrite and sphalerite occur along and in the vicinity of the stylolites. Petrographic observations, negative δ18OVPDB, fluid inclusion microthermometry, and enrichment in 87Sr suggest that these cements have precipitated from hot basinal brines, which migrated along the stylolites and genetically related microfractures (tension gashes). Fluid migration was presumably related to lateral tectonic compression events related to the foreland basin formation. The low solubility of Al3+ in formation waters suggests that kaolin precipitation was linked to derivation of organic acids during organic matter maturation, probably in siliciclastic source rocks. The mass released from stylolitization was presumably re-precipitated as macro- and microcrystalline calcite cement in the host limestones. The flanks of the oilfield (water zone) display more frequent presence and higher amplitude of stylolites, lower porosity and permeability, higher homogenization temperatures and more radiogenic composition of carbonates compared to the crest (oil zone). This indicates that oil emplacement retards diagenesis. This study demonstrates that stylolitization plays a crucial role in fluid flow and diagenesis of carbonate reservoirs during basin evolution.

Origin of over-pressure in clastic rocks in Yuanba area, northeast Sichuan Basin, China

Abnormally high pressures are widely developed in clastic formations with burial depths greater than 3500 m in Yuanba area, where the top depth of over-pressure zone encompasses different formations in different locations. With increasing depths, well logs response in mudstone to over-pressure zone show that no significant anomalies occur in sonic and density logs, suggesting that the over-pressured mudstones are normally compacted. Multiple evidences from well-log responses to over-pressure, low porosity, normal geothermal gradient, low sedimentary rate and velocity-density crossplots show that over-pressures in mudrocks were created by gas generation and not related to the disequilibrium compaction. Based on diagenetic evolution analysis, stratigraphic framework and gas accumulation model, pressure transference and oil-to-gas cracking are expected to be the main causes of over-pressure in sandstone. Because of the favorable sealing condition and gas-bearing properties, lateral tectonic compression has also been an important factor for over-pressure development and preservation since early Cretaceous.

A new helical trace fossil from the Lower Devonian of Spitsbergen (Svalbard) and its palaeoenvironmental significance

In Devonian molasse deposits (Old Red Sandstone) of Spitsbergen, vertical and regularly helical, tubular trace fossils are found predominantely in pedogenically overprinted calcareous clay to siltstones. Host rocks are interpreted as alluvial plain deposits with lacustrine intercalations and marine ingressions. Skolithos helicoidalis isp. nov. generally occurs as tubes with a round to oval cross section, continuously coiling downward from distinct bedding planes to a depth of 50 to 70 cm. Morphological variations with regard to cross-sectional shapes, amplitude of coiling and whorl distance are referred to primary trace heterogeneity, sediment composition, and secondary influences such as post-sedimentary deformation, diagenesis and lateral tectonic compression. Commonly occurring at the upper extent of fluvial, lacustrine and marginal marine beds, Skolithos helicoidalis isp. nov. probably marks periods of slow or no sedimentation, independent of facies. Its frequent association with pedogenic structures such as carbonate glaebules and slickensides suggests a plant origin (rhizolith). Alternatively, the extraordinary morphological regularity and wide distribution across facies could also point toward an animal, possibly of euryecological arthropod origin.

Overpressure associated with a convergent plate margin: East Coast Basin, New Zealand

The East Coast Basin of New Zealand lies in the frontal arc of the Hikurangi subduction zone. Overpressure forms a major hazard for exploration in the basin. Fluid pressures of 12.8 MPa at 600 m depth have been encountered, equivalent to 90% of lithostatic pressure, and mud weights of up to 19.5 ppg are required to control formation fluids at subsurface depths of only 1400 m. The distribution and magnitude of overpressure shows no relation to present-day depth. Lithostratigraphy controls the distribution of overpressure and widespread low-permeability bathyal mudstones form seals to overpressure across the basin. The transition from normal pressure to overpressure is not associated with any single stratigraphic unit. The Neogene evolution of the plate boundary has given rise to rapid Miocene sedimentation (400 m Ma −1 ) in areas of the basin and has caused disequilibrium compaction. Late Neogene compression has subsequently uplifted the overpressured, undercompacted sediments by up to 3000 m at rates of 1000 m Ma −1 . Lateral tectonic compression associated with the plate boundary has also caused undrained shear of thick mudstones, leading to extremely high overpressures in deformed sediments independent of depth.

Worldwide distribution of submarine mud volcanoes and associated gas hydrates

Abstract The list of known and inferred submarine mud volcanoes is presented in this paper. They occur worldwide on shelves, continental and insular slopes and in the abyssal parts of inland seas. Submarine mud volcanoes are distributed on the Earth more extensively than their subaerial analogs. The estimated total number of known and inferred deep-water mud volcanoes is 103–105. There are two key reasons for the formation of submarine mud volcanoes—high sedimentation rate and lateral tectonic compression. Submarine mud volcanoes form by two basic mechanisms: (1) formation on the top of a seafloor-piercing shale diapir; (2) formation due to the rise of fluidized sediments along faults. Fluid migration is critical to the formation of a mud volcano. Gas hydrates are often associated with deep-water mud volcanoes and have many common features from one accumulation to another. Gas hydrates form by conventional low-temperature hydrothermal process around the central part of a mud volcano and by metasomatic processes at its periphery. A preliminary global estimate of methane accumulated in gas hydrates associated with mud volcanoes is about 1010–1012 m3 at standard temperature and pressure.

Hydrodynamics and overpressure mechanisms in the Sacramento Basin, California

Subsurface pore fluid pressures exceeding 70 percent of lithostatic pressure are observed in large areas of the Sacramento basin and adjacent Coast Ranges, California. Basinor crustal-scale processes are required to explain the regional distribution of overpressures. Several plausible mechanisms of overpressuring are (1) depositional compaction by the Miocene and younger strata recently deposited, (2) aquathermal expansion under high heat flow conditions, and (3) horizontal or lateral tectonic compression. The latter mechanism is suggested by observed deformation patterns in the Sacramento basin. Throughout the geologic history, east-west compressive stresses by eastward subduction of the Juan de Fuca plate, subsequent tectonic wedging, right-lateral tectonism (primarily in southern part of basin), and east-west extension in the Basin and Range together created a primarily compressive stress regime. Such a stress regime has likely contributed to the cause and maintenance of overpressures observed in the region. We developed two-dimensional numerical modeling experiments to test these hypotheses. Results of the experiments suggest that aquathermal expansion is negligible as an overpressure mechanism. Sedimentary compaction is a possible mechanism but only for locally high sedimentation rates or for unusually low permeability and/or unusually high compressibility under lower sedimentation rates. Finally, model simulation results suggest that tectonic compression is a plausible, ongoing mechanism creating and maintaining high fluid pressures in the Sacramento basin. We speculate that other processes such as chemical diagenesis may be contributing to overpressures, but probably only on a local scale and of secondary importance.

Bear Mountain Igneous Complex, Klamath Mountains, California: an Ultrabasic to Silicic Cale-Alkaline Suite

The Bear Mountain igneous complex, Klamath Mountains, California, can be divided into distinct lithologic suites (order according to apparent relative age): (1) satellitic masses of clinopyroxene-rich ultramafic and gabbroic rocks with subordinate dunite and hornblende-plagioclase pegmatoid; (2) two-pyroxene-biotite diorite and monzodiorite; (3) heterogeneous hornblende-rich rocks varying from gabbro to diorite; (4) leucocratic rocks, chiefly consisting of biotite tonalite and granodiorite; and (5) late dikes (mafic to felsic). Elongate masses of unit (1) flank a composite pluton consisting of units (2–4), while the late dikes (unit 5) intrude the adjacent country rocks. The rocks of the complex invaded an ophiolite allochthon during the Late Jurassic Nevadan orogeny, and well-defined contact aureoles surround the complex. Lower greenschist facies rocks, chiefly metabasalt, impure siliceous metasedimentary rocks, and serpentinized peridotite, have been dynamothermally metamorphosed to mineral assemblages indicative of hornblende-hornfels facies and locally pyroxene-hornfels facies. The emplacement of the igneous complex was chiefly by forcible shouldering aside, although local tectonic features such as faults in the ophiolite allochthon were instrumental in the emplacement history. The ultramafic and gabbroic rocks are interpreted as crystal cumulates of a fractionated basaltic magma. Mineral compositions and whole-rock chemical characteristics of the proposed cumulates suggest that the Mg/Fe ratio of the parental basaltic liquid was high. The activity of silica was low, while water vapor pressure apparently increased through time until it was moderately high during the late magmatic stage. These cumulates were subsequently remobilized during lateral tectonic compression and emplaced higher in the crust as hot, semisolid aggregates. A diverse array of data, including pyroxene compositions, major-, minor-, and rare-earth-element abundances and field relations, suggest that the two-pyroxene-biotite diorite/monzodiorite unit was consanguineous with the clinopyroxene-rich ultramafic and gabbroic rocks. The diorite/monzodiorite unit, therefore, is an intermediate differentiate of an early primitive basalt. Furthermore, major-, trace, and rare-earth-element data characteristic of the diorite/monzodionte unit indicate strong similarities to low-Si andesite and clearly suggest a calc-alkaline affinity. Age relations indicate that the hornblende-rich and leucocratic units are younger and represent the intrusion of other magmas into the same igneous locus. Petrographic and geochemical data from the hornblende-rich unit suggest recrystallization from hydrous magmas similar in composition to high-Al basalt and basaltic andesite. The leucocratic suite, consisting chiefly of calc-alkaline tonalitic rocks, is similar to other quartz-rich felsic rocks widespread throughout the Klamath Mountains-western Sierra Nevada. The available petrographic and geochemical data are consistent with formation of these rocks by either fractional crystallization of a wet basaltic magma or partial melting of amphibolite or eclogite. The Bear Mountain igneous complex is an example of a diverse but distinctive association of ultrabasic to silicic rocks which characterize numerous plutonic complexes in the Klamath Mountains-western Sierra Nevada. These intrusive complexes invade older ensimatic rocks and appear to define the roots of a complex, Middle to Late Jurassic calc-alkaline magmatic arc. The ultramafic and gabbroic rocks characteristic of this plutonic association are similar to Alaskan-type complexes but differ in detail. More significantly, these rocks are important clues to the composition of early magmas as well as the complex processes operative in reservoirs that form the core of calc-alkaline magmatic centers.

Mesozoic evolution, hinterland of the Sevier orogenic belt

West of the Mesozoic foreland Sevier orogenic belt in northern Utah and southern Idaho, a regional detachment of inferred mid-Jurassic age occurs in Upper Mississippian shale and can be traced westward to the margin of the coeval magmatic arc, represented by spaced calc-alkalic intrusions. The detachment may be the oldest and structurally highest intrusions. The detachment may be the oldest and structurally highest thrust of the Idaho-Wyoming thrust belt. If so, it bridges the apparent spatial and temporal gap between magmatism and thrusting, thus suggesting a direct genetic link. Thin-skinned deformation is interpreted here to be driven from the heated magmatic arc toward the colder craton by lateral tectonic compression created by lateral volume increase due to intrusion, elevated crustal geotherms, and possible thickening in the arc. The metamorphic complexes occur at the crustal interface between these two thermal-tectonic regimes and may themselves be allochthonous.

Tectonic Significance of Microstructures in Idaho-Wyoming Thrust Belt and Hinterland: ABSTRACT

Mechanics of foreland thrust belt development can be explained using paleostress orientations from dynamic analyses of microstructures in quartz, calcite, and dolomite. Deformation in the hinterland must also be considered since the two basic models--gravity spreading and lateral tectonic compression--predict substantially different stress fields in this region. Petrofabric studies in the Meade plate show that compression was dominantly layer-parallel, trending approximately east-west. On overturned fold limbs, compression at 50-80° to bedding suggests a locking angle which agrees well with existing theoretical and experimental analyses of kink-folding. Observed kink-fold geometries may be a necessary result of ramp configurations in the decollement thrust surface. These data are in accord with either of the two principal models. Dynamic analyses at scattered localities in the southern Idaho hinterland show primarily layer parallel or subparallel, east-west compression in all demonstrably allochthonous rocks at all structural levels. Fold vergence and local overturning indicate eastward translation along the undated, but probably Mesozoic, younger-over-older thrusts characteristic of this region. Near metamorphic core complexes, Tertiary thermal events may have affected preservation of older microstructures. Parautochthonous Precambrian metasediments between foreland and hinterland record compression at high angles to bedding. The age and origin of these microstructures are unknown at present. These studies indicate that maximum compression was nearly horizontal and oriented approximately east-west throughout southeastern Idaho during thrust belt activity. Therefore, the lateral tectonic compression model is favored. End_of_Article - Last_Page 669------------