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Geological studies around the Texel-IJsselmeer High have been carried out for the regional subsurface mapping project of the Geological Survey of The Netherlands. The Texel-IJsselmeer High, in the northern part of the Netherlands, is a northwest-southeast-trending structural unit, slightly tilted to the northeast. The geological evolution of the Texel-IJsselmeer High and the adjacent areas can be linked to an extensional tectonic regime during which several Jurassic basins in the Netherlands originated. During the Late Jurassic, the southern border of the Texel-IJsselmeer High was characterized by normal faulting. Main faults are dipping southwest and are generally part of a half-graben structure. Faulting is accompanied by subsidence of the hanging wall (Jurassic basin area), while the footwall (the Texel-IJsselmeer High) is isostatically uplifted and eroded. The proposed model is based on thinning of the lower crust beneath the basins during Jurassic extension by pure shear. This mechanism is coupled locally with shear zones (simple shear) as a result of lower crustal failure. The model is supported by observations on deep regional seismics at the southern margin of the basin area. During the Late Cretaceous/early Tertiary, transpressional intraplate stresses reactivated the structural weakness zones in the lower and upper crust in a reversedmore » way (inversion). During this tectonic inversion the northwest-southeast-trending Texel-IJsselmeer High acted as a buffer zone perpendicular to the direction of maximum principal stress. Paleogeographical studies and geohistory analysis support the proposed tectogenetic model of the Texel-IJsselmeer High.« less

Devonian and Mississippian sedimentary rocks of western Montana and east-central Idaho were deposited on a cratonic platform that faced a northern extension of the Antler foredeep. Subsidence analyses of this sequence and isopach maps illustrate regional patterns of subsidence related to convergence along the western North American plate margin. Tectonic stresses affected deposition on platform areas which were hundreds of kilometers inboard from the ancient continental margin. Wavelengths of paleostructural elements, tectonic inversion of these structures (i.e., transition of a paleohigh into a depocenter), and time scales involved in the inversion process cannot be attributed solely to flexure or to vertical displacements by in-plane stresses but suggest reactivation of Precambrian structural trends. Late Devonian (Frasnian) platform sedimentation began during a brief interval of increased subsidence across western Montana. This interval of increased platform subsidence is greater than a Late Devonian eustatic sea level rise (determined from subsidence analyses of Devonian strata from stable cratonic areas) and suggests some tectonic event must have influenced subsidence in Montana. Thin uppermost Devonian Strata contain numerous unconformities that may be related to flexure of the platform plus eustatic sea level fluctuations. Rapid subsidence across Montana during the Early Mississippian (Kinderhookian) resulted in a condensedmore » platform sequence, which is overlain by deep water shaly carbonates. Rapid subsidence continued into the Osagean then slowed, allowing progradation of carbonate platform facies across Montana. A regional karst surface on top of the Meramecian platform coincides with conglomerate deposition and increased subsidence rates in the foredeep; unconformity durations on the platform also increase to the east.« less

The Arctic sector of Yakut ASSR includes the ancient Siberian platform, the late Hercynian Taimyr fold belt, the Mesozoic Verkhoyansk-Chukotsk fold ranges, and shelf seas that represent the recent Laptev-East Siberian Seas paraliageosynclinal region. The Taimyr fold structures are cut off at the continental slope. Fold systems in the Verkhoyansk-Chukotsk region are divided by median masses, and are traced within the shelf seas. The Verkhoyansk-Kolyma system is divided by the Shelon, Kular, and Laptev buried massifs into 3 branches; one of these, in the northwest, abuts the Hercynides of Taimyr and the others extend northward toward the continental slope. The Novosibirsk-Chukotsk fold system, confined on the north by the Novosibirsk and De-Longa massifs, is traceable northwest to the continental slope; on the southeast it is subdivided into 2 branches. One branch terminates with Mesozoides of the Anyuy zone, and another extends toward Wrangel Island. Outliers of fold structures of Mesozoides are exposed on islands of the shelf seas. Parts of median massifs represent uplifts of the basement of the recent paraliageosynclinal region. Within the latter there are northern and southern trough systems. The northern eugeosynclinal(?) system includes foreshelf and marginal-shelf troughs and uplifts, separated by systems of deep faults which probably have large associated intrusions of mafic magmatic rocks. The southern system consists of the miogeosynclinal sublatitudinal troughs that are separated by the Lyakhov uplift. Upper Mesozoic and Cenozoic strata of the troughs may reach thicknesses of 10-12 km. The cut-off by shelf seas and the continental slope of the Mesozoides, Hercynides, and median massif structures; features associated with the building of the Arctic Ocean crust; the nearby presence of mid-ocean and other submarine ridges, which are nearly perpendicular to the continental margins and which commonly are extensions of the continental fold structures of different age; and the formation of a juvenile paraliageosynclinal region within marginal areas of the ocean are End_Page 2496------------------------------ indicative of the extreme youth of large parts of the Arctic Ocean and of the formation of the ocean by oceanization of continental crust as a result of large vertical subsidence movements. End_of_Article - Last_Page 2497------------

Emigrant atGrap anticline lies on the east side of the Casper arch and trends north-northwest in east-central Natrona County, Wyoming, from the western end of Casper Mountain to its plunge south of Petrie Siding, a distance of about 20 mi. Outcrops of Cretaceous Frontier sandstones define or outline the structure, especially its northern and central parts. Surface expression of faulting is minor; some beds are repeated by minor thrusting, and some are offset by small mainly north-northeast strike-slip faults. Joint sets include an approximately horizontal set, a high-angle northeastern set, and a high-angle northwestern set. A fourth high-angle approximately east-west set is prominent only in one area where the anticlinal axis is sinistrally offset almost 2 mi. An unexposed strike-slip/wrench fault is probably present at depth and probably offsets one or more major subsurface thrusts. These include the Owl Creek thrust fault, which extends beneath Pine Mountain, and probably one other east-dipping thrust that has possible surface extension adjacent to the anticline. Emigrant Gap anticline appears to be rootless, having been transported westward as part of a thrust sheet during the Laramide orogeny, the principal compressive stress directed east-northeast to west-southwest. Blind strike-slip or wrench faults probably offset the thrustmore » sheet.« less

The coast of Mexico between Puerto Vallarta (lat. 21/sup 0/N) and the Bay of Tehuantepec (long. 94/sup 0/) rises steeply from the Middle America Trench to expose deeply eroded terranes of metamorphosed ophiolitic, basinal to terrigenous sedimentary, and arc volcanic rocks of Pennsylvanian to middle Cretaceous age, in part lying on older Paleozoic and Proterozoic rocks. Granitic intrusios are of Late Cretaceous to early Cenozoic age. The terranes are overlapped by volcanic rocks of middle Cenozoic age and locally, along the coast, by marine Miocene strata. It is particularly significant to paleogeographic reconstructions that there are no known marine coastal deposits of Late Cretaceous or early Cenozoic age. Eight tectono-stratigraphic units are currently recognized. The Colima terrane is a complete sequence of red colvaniclastic beds and limestones from Neocomian to Aptian (ammonites, rudistids). The Tumbiscatio terrane is comprised of lavas and radiolarian cherts, at least in part Triassic. The Huetamo terrane is formed of turbiditic, volcaniclastic, and calcareous sequences of Late Jurassic and Early Cretaceous age (ammonites), locally containing fragments of ophiolite. The fourth unit is comprised of ophiolite terranes. Guerrero terranes are gently metamorphosed lavas, tuffs, and sediments of Late Jurassic to Aptian-Albian age. The Mixteca terrane is comprisedmore » of terrigenous calcareous sequences of Pennsylvanian and Early Jurassic ages lying on early Paleozoic basement. The Oaxaca terrane is a Paleozoic sedimentary sequence overlying metamorphic precambrian basement, and the Xalapa terrane is formed of migmatitic, gneissic rocks of Jurassic(.) age. However, this preliminary breakdown does not convey the chaotic complexity of the region.« less

The character of the clastic depositional environments represented in the lower Mary Lee coal zone of the Pennsylvanian Pottsville Formation in the Warrior basin Alabama (tidally influenced mud flats and alluvial swamps) was controlled by the compaction of buried peat bodies. The lowest mineable coal in the Mary Lee coal zone, the Jagger, is overlain by laminated shale and sandstone exhibiting pronounced cycle bedding. This bedding records daily tidal cyclicity in the form of sand-mud couplets. These correspond to flood-current deposition of the coarser fraction followed by fallout of the finer grained fraction during ensuing slack-water periods. These couplets are cyclically bundled-sandier bundles corresponding to spring tides and muddier bundles to neap tides (lamination counts suggest a 24-30-day cycle). The clastic sequence above the overlying Blue Creek coal is characterized by a series of stacked alluvial swamp horizons. These can be identified by autochthonous fossil plants and pedological features indicative of gleyed paleosols. Catastrophic flooding buried and preserved these horizons. The rapid, early compaction of the buried Jagger and Blue Creek peat bodies created accommodation space that allowed both the preservation of tidalites in the Jagger coal to Blue Creek coal interval and the stacking of alluvial swamp paleosols abovemore » the Blue Creek seam. Carboniferous peats were comprised of highly compressible plant parts and hence, were sensitive to sediment loading. Once the peat bodies had compressed to a certain extent, stability of the overlying sediment surface created conditions amenable to resumption of peat accumulation.« less

Surface observations along the southern margin of the Wind River Range in Fremont County, Wyoming, indicate that early motion along the Wind River and Continental faults controlled depositional patterns and lithologic characteristics of the local syntectonic sediments, and that the latest motion on a segment of the Wind River fault between Oregon and Pacific buttes folded some of these same sediments into a monocline. The stratigraphic sequence exposed in the monocline consists (in ascending order) of a lower distal fan or alluvial plain unit (main body of the Wasatch Formation), a lake margin unit (Tipton Tongue of the Green River Formation), a fluvial and deltaic sandbody (Tipton Sandstone), and an alluvial fan unit (Cathedral Bluffs Tongue of the Wasatch Formation). Cu rent direction, clast composition, and clast-size data indicate that a granitic and mafic distal source to the east and a proximal granitic source to the north supplied sediment. Subsequent movement on the Wind River fault warped this sequence into a monocline 2 mi long. This structure dies out in both a northwest and southeast direction along the inferred trace of the Wind River fault and is overlain by undeformed middle Eocene sediments. Other syntectonic units (e.g., Fort Union, Ice Point, White River, Arikaree, and South Pass conglomerates) occur in patches along the Wind River and Continental faults in this area. Each deposit is of local extent, exhibits rapid thickness and petrofacies changes, and probably represents proximal alluvial fan deposition. These characteristics are typical of syntectonic sediments in transcurrent-faulted terrains, and we are investigating the possibility of such faulting in this area. Tectonic implications of these interpretations are: (1) early motion on the Wind River fault controlled the margin of Eocene Lake Gosiute and generated a distal sediment source to the east; (2) late early Eocene uplift of the north side of the Continental fault provided a proximal source for pegmatitic and granitic boulders to the north; (3) last motion on the Wind River fault was latest early Eocene or earliest middle Eocene between Oregon and Pacific buttes; (4) the Wind River fault consists of several segments which moved separately rather than as one, long continuous zone of concurrent faulting; (5) while the Wind River Range was being thrust to the southwest it may have been uncoupled from the basins to the south by a zone of transcurrent faulting; (6) Pliocene or younger recurre t motion along the Continental fault was opposite to that in the Eocene. End_of_Article - Last_Page 1357------------

The Todilto Limestone of Middle Jurassic age in the Ambrosia Lake uranium mining district of McKinley County, New Mexico, is the host formation for numerous small to medium-size uranium deposits in joints, shear zones, and fractures within small to large-scale intraformational folds. These folds probably were formed as a result of differential sediment loading when eolain sand dunes of the overlying Summerville Formation of Middle Jurassic age migrated over soft, chemically precipitated, lime muds of the Todilto shortly after their deposition in a regressive, mixed fresh and saline lacustrine or marginal-marine environment of deposition. Encroachment of Summerville eolian dunes was apparently restricted to relatively narrow beltlike zones trending radially across the Todilto coastline toward the receding Todilto body of water. Intraformational folding is believed to be confined to the pathways of individual eolian dunes or clusters of dunes within the dune belts. During the process of sediment loading by the migrating dunes, layers of Todilto lime mud were differentially compacted, contorted, and dewatered, producing both small, and large-scale plastic deformation structures including convolute laminations, mounds, rolls, folds, and small anticlines and synclines. During the processes of compaction and dewatering, the mud, in localized areas, reached a point of saturation at which sediment plasticity was lost, causing shearing, fracturing, and jointing of the contorted limestone beds. These areas or zones within the limestone became the preferred sites of uranium mineralization because of the induced transmissivity created by sediment rupture during prolonged sediment loading. Along the Todilto coastline adjacent to the eolian dune belts, both interdune and coastal sabkha environments dominated the Summerville on the margins of the Todilto body of water. Sediment in these areas consists mainly of claystone, siltstone, sandy siltstone, and very fine grained sandstone which was apparently derived from the winnowing of the finer grained fraction of sediment from adjacent eolian dune fields during eolian activity. Most of the sabkha sediments were probably carried in airborne suspension to the low-lying, ground-water saturated, coastal areas where they were deposited as relatively uniform blanketlike layers. Deposition of sabkha deposits was apparently slow and uniform over most of the Todilto coastal and interdune areas, and did not cause the formation of othe than small-scale deformation features in underlying Todilto rocks. Large-scale deformational features as well as uranium deposits are notably absent in the Todilto where it is overlain by finer textured sabkha deposits in the Summerville. End_of_Article - Last_Page 560------------

The evolution of Australia's rifted northwestern margin has been investigated by interpreting its stratigraphic history within the framework of a regional plate tectonic/paleogeographic model. The Permian through Cretaceous stratigraphy of the five major basins, the Bonaparte, Browse, Canning, Carnarvon, and Perth, was summarized using a standardized nomenclature and time scale. These data were used to construct 12 subsidence profiles and to provide the lithologic and paleoecologic foundation for interpreting regional paleogeography. Subsidence analysis was performed to study varying responses to tectonism, eustatic and climatic change, as well as variations in sediment input/loading. Two types of events (unconformities) became evident through this analysis. The first type tend to be diachronous across the margin, becoming younger toward the Southwest. These events were primarily driven by regional tectonic forces, that were usually extensional in nature. The second type were more regionally synchronous and appear to have been related to third order changes in base level and/or variations in regional stresses. The timing and magnitude of events, as indicated by subsidence analysis and sub-basin-level structural analysis, were used to construct the paleogeographic model. The model is illustrated in a series of 10 regional paleogeographic maps depicting changing environments and zones of structural deformation frommore » the Permian through the Cretaceous. as eastern Gondwana was disassembled. This work points out the significant effects of far-field tectonic forces on basin evolution and serves as a prerequisite for comprehensive investigation of petroleum systems in this prolific area.« less

Along the lower Colville River near Ocean Point, Alaska, Late Cretaceous to early Tertiary sediments (Colville Group and the Sagavanirktok Formation) record depositional environments from delta plain to prograding delta to shallow marine shelf. The unit hosts the northernmost known dinosaur remains and is less than 100 m thick with numerous tephra deposits in its lower sections. Furthermore, it is characterized by cyclic, relatively fine-grained sediments indicating mostly depositional and few erosional events. The major depositional elements of the delta plain are 3.5 to 5.45-m thick tabular fining-upward cycles that are cut by sand-filled fluvial channels (up to 10 m thick). The cyclic sediments contain abundant roots and grade upward from small-scale cross-beds to laminated and then structureless silt and clay terminating in organic-rich layers. The channel-fill sequences fine upward and change vertically from large to small-scale cross-beds. Over-bank flooding as well as lateral migration of small meandering fluvial channels formed the cyclically interbedded deposits, meandering rivers deposited the thick cross-bedded sands, and soil development or marsh deposits formed the organic-rich horizons that cap each cycle. Plant debris, nonmarine invertebrates, and vertebrate fossils are locally concentrated in the delta plain sediments. Subsidence related to compaction of the deltaic sediments alongmore » with possible delta lobe switching resulted in repeated progradation of the delta front over the delta plain. Delta front sediments are 3 to 10-m thick tabular deposits of large and small-scale cross-bedded sands and silt bounded by organic-rich beds. Also, there are abundant roots, rare channels and invertebrate fossils that suggest a transitional environment from sand-flats to estuarine or bay.« less