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47 records · Page 3Linked to original sources
USGS1975

Middle tertiary volcanic field in the southern Rocky Mountains

A widespread volcanic field covered most of the Southern Rocky Mountains in middle Tertiary time, 40 to 25 m.y. ago (approximately Oligocene time). This field covered an erosion surface that beveled structures formed during the Laramide orogeny in Late Cretaceous and early Tertiary time. The source vents from which the volcanic rocks were derived were largely restricted to the deformed area. Recognized volcanic centers lie mostly within a broad triangular area bounded on the east by the Rocky Mountain front, on the northwest by the northeast-trending Colorado mineral belt lineament, and on the southwest by the southern margin of the recurrently active Uncompahgre-San Luis uplift. Local volcanic centers existed also in the Never Summer Mountains and Rabbit Ears Range north of the mineral belt lineament. The resulting volcanic field thus consisted of a major southern segment covering all of south-central Colorado and adjacent New Mexico and a northern segment extending into the mountain areas of north-central Colorado. The two segments were linked along the trend of the Colorado mineral belt. Most of the volcanic field consisted of volcanic rock of intermediate composition derived from many widely scattered volcanoes. Extensive aprons of volcaniclastic rocks around these volcanoes coalesced to provide a virtually continuous cover of volcanic material. In places, the volcanic activity became more silicic with time, great ash-flow tuff sheets were erupted, and the source areas subsided to form calderas. The largest ash-flow field and most of the calderas formed in the San Juan Mountains. Other ash flows apparently were derived from areas in the Sawatch Range in central Colorado and the Never Summer Mountains in northern Colorado. Large near-surface batholiths were emplaced beneath the San Juan Mountains and beneath the region that extends northeastward from the Elk Mountains and Sawatch Range along the trend of the Colorado mineral belt as far as the Rocky Mountain front. These batholiths are manifested by two large gravity lows and by many exposed epizonal plutons that represent cupolas on the larger underlying bodies. The batholith in central Colorado probably consists of plutons of both Laramide (70 to 55 m.y.) and middle Tertiary (40 to 25 m.y.) ages.

Rocky Mountains↗
USGS1981

Estimation of depth to magnetic source using maximum entropy power spectra, with application to the Peru-Chile Trench

Estimations of the depth to magnetic sources using the power spectrum of magnetic anomalies generally require long magnetic profiles. The method developed here uses the maximum entropy power spectrum (MEPS) to calculate depth to source on short windows of magnetic data; resolution is thereby improved. The method operates by dividing a profile into overlapping windows, calculating a maximum entropy power spectrum for each window, linearizing the spectra, and calculating with least squares the various depth estimates. The assumptions of the method are that the source is two dimensional and that the intensity of magnetization includes random noise; knowledge of the direction of magnetization is not required. The method is applied to synthetic data and to observed marine anomalies over the Peru-Chile Trench. The analyses indicate a continuous magnetic basement extending from the eastern margin of the Nazca plate and into the subduction zone. The computed basement depths agree with acoustic basement seaward of the trench axis, but deepen as the plate approaches the inner trench wall. This apparent increase in the computed depths may result from the deterioration of magnetization in the upper part of the ocean crust, possibly caused by compressional disruption of the basaltic layer. Landward of the trench axis, the depth estimates indicate possible thrusting of the oceanic material into the lower slope of the continental margin.

Memoir of the Geological Society of America↗
USGS1983

Fission-track ages of apatite in the Wasatch Mountains, Utah: An uplift study

Apatite fission-track ages from basement rocks in the Wasatch Mountains between Ogden and Bountiful, Utah, range from 5 m.y. near the Wasatch fault along the west margin of the mountains to 94 m.y. on the crest of the range and show a correlation with altitude within individual fault blocks. Analysis of these ages yields an uplift rate of 0.4 mm/yr for the last 10 m.y. for the fault blocks having the most sustained rapid uplift. A variety of intermediate ages of apatite from the crest of the range and at lower altitudes is interpreted as representing fault blocks with rocks having mixed ages due to various degrees of tilting and varying but lesser amounts of uplift.

Utah↗
USGS1992

Chapter 4: Cretaceous thrusting and Neogene block rotation in the northern Portneuf Range region, southeastern Idaho

The Putnam thrust has long been recognized as an important Mesozoic structure in the northern Portneuf Range, southeastern Idaho. At most localities, the thrust places Ordovician rocks above Permian and Pennsylvanian rocks, although near its southeastern extent, it ramps laterally downsection to the southeast. At its southeasternmost exposures, Cambrian rocks are juxtaposed above Mississippian rocks. New work indicates that the hanging wall of the Putnam thrust contains three imbricate thrust slices or subplates, which are, from structurally lowest to highest (and generally from north to south), the Lone Pine subplate, the Narrows subplate, and the Bear Canyon-Toponce subplate. The steeply south-dipping, east-trending Narrows thrust overlies the Lone Pine subplate, underlies the Narrows subplate, and is a lateral ramp that merges eastward into the Putnam thrust. Where exposed, the Narrows thrust places Late Proterozoic quartzite of the Brigham Group over Ordovician and Cambrian rocks. The Bear Canyon thrust overlies the Narrows subplate and underlies the Bear Canyon-Toponce subplate, dips eastward along the west side of the Portneuf Range, and places lower Brigham Group quartzite above Cambrian limestone and Cambrian and Late Proterozoic upper Brigham Group quartzite and argillite. At its northern extent, the Bear Canyon thrust curves to the east, where it merges with the Putnam thrust. On the east side of the range, the intensely folded Toponce thrust places upper Brigham Group quartzite above Ordovician rocks; the Toponce is believed to be an eastward extension of the Bear Canyon thrust. East-dipping rocks within the Lone Pine subplate were not strongly deformed during Cretaceous thrusting, in contrast to rocks within the Narrows subplate, where east-vergent recumbent folds, cleavage directions that fan about northerly strikes, and tectonic thickening and thinning of beds indicate intense, thrust-parallel shear. The deformation and thrust geometry within the Narrows subplate suggest that the Narrows subplate actually consists of several horses within a foreland-dipping duplex. Late Miocene and younger basin deposits occur in north-trending valleys adjacent t o the northern Portneuf Range and, to the west, the Bannock and Pocatello ranges. At most places, the Neogene deposits dip to the east by as much as 35°, indicating that late Miocene and younger extension and down-to-the-east rotation occurred along mostly west-dipping listric faults that are inferred to merge on at least one regional detachment. Although range-bounding faults account for a large component of extension and rotation, an additional large component was contributed by numerous, relatively small-displacement normal faults within mountain ranges.

Idaho↗
USGS1954

Stratigraphy and history of the Moenkopi formation of Triassic age

The Moenkopi formation of Triassic age is composed of a series of deposits that form a wedge thinning eastward from a maximum of about 2000 feet in western Utah and southern Nevada to the vanishing point along an irregular margin in western Colorado, northeastern Arizona, and western New Mexico. Partly marine and partly continental in the thick western sections, it is entirely continental in the east. Invertebrate faunas indicate that deposition began either during or preceding the middle of the Early Triassic (Meekoceras zone) and continued into late Early Triassic (Tirolites zone) and probably into Medial Triassic time. Vertebrate faunas also indicate an Early Triassic and probably, in part, a Medial Triassic age . Studies of the deposits indicate three major transgressions and three regressions across southern Utah and northern Arizona. Analysis of sedimentary rock types and original structures in them suggests a complex mixture of environments involved in the development of the formation : stream beds, lagoons, playas, flood plains or tidal flats, shallow sea floors, and others. Some types are clear cut and readily demonstrated; others are open to question. Evidence from flora, fauna, and sediments indicates a semiarid to arid climate. Except for uplift in the Uncompahgre region of Colorado indicated by conglomeratic beds in the Moenkopi near by, the entire region probably remained very low and flat during Moenkopi deposition.

Colorado, Arizona, New Mexico↗
USGS1957

Chapter 22: Environments and facies of existing bays on the central Texas coast

The estuaries and neritic waters along the central Texas coast are characterized by broad ranges of environmental factors, providing conditions favorable to temperate organisms in winter and tropical organisms in summer. Dredging in the coastal waters revealed a number of distinct faunal facies, the distribution of which appears to be controlled primarily by salinity. Characteristic assemblages are found in the following areas: closed bays, polyhaline bays, a highly saline lagoon, passes, beaches on the open Gulf, and shallow waters of the open gulf. In the closed bays three subfacies are recognized-one in the bay heads, one on the reefs, and one in the interreef areas; in the open gulf the assemblages living close to shore are differentiated from those found 2-4 miles off shore. In most stations each facies is characterized by an abundance of certain living forms. Dead shells and other skeletal parts are much more widely distributed, but many such strays may be recognized as transported elements by their rarity or by their eroded surfaces. Each group of organisms has one or more centers of maximum development in numbers of species. The Foraminifera, for example, attain their greatest development in the polyhaline bays and near the heads of passes, the ostracodes near the bay heads, mollusks in the polyhaline bays and in the Gulf, and diatoms under normal marine conditions in the Gulf. The reef assemblage of the closed bays is poor in numbers of species, but the individuals are so numerous that they form extensive deposits.

Texas↗
USGS1945

American old and middle tertiary larger foraminifera and corals

The scleractinian coral fauna of the Eocene Upper Scotland formation of Barbados comprises 27 species and varieties belonging to 23 genera and subgenera. There is a mingling of hermatypic and ahermatypic forms suggesting a tropical shallow-water, non-littoral environment at depths at or even beyond the lower limits of temperature for vigorous reef-coral growth. Most (16) of the species and 2 of the genera are new and generally show relations with the middle Eocene Claibornian coral fauna of somewhat similar environment in the United States. A few species are either the same as or have affinity with Claibornian forms; one or two are allied to western European or Mediterranean Tertiary species. Of particular interest is the occurrence of Endopachys maclurii. Two corals are described from the Miocene of Martinique. One is a new species of the odd subgenus of Sphenotrochus, Eusthenotrochus, heretofore known only from one recent species and another from the Eocene of the Paris Basin. The other is Dominicotrochus dominicensis (Vaughan), a turbinolian previously known from a single Dominican specimen of uncertain origin and horizon.

Memoir of the Geological Society of America↗
USGS2011

Chapter 39 The Edwardsburg Formation and related rocks, Windermere Supergroup, central Idaho, USA

In central Idaho, Neoproterozoic stratified rocks are engulfed by the Late Cretaceous Idaho batholith and by Eocene volcanic and plutonic rocks of the Challis event. Studied sections in the Gospel Peaks and Big Creek areas of west-central Idaho are in roof pendants of the Idaho batholith. A drill core section studied from near Challis, east-central Idaho, lies beneath the Challis Volcanic Group and is not exposed at the surface. Metamorphic and deformational overprinting, as well as widespread dismembering by the younger igneous rocks, conceals many primary details. Despite this, these rocks provide important links for regional correlations and have produced critical geochronological data for two Neoproterozoic glacial periods in the North American Cordillera. At the base of the section, the more than 700-m-thick Edwardsburg Formation (Fm.) contains interlayered diamictite and volcanic rocks. There are two diamictite-bearing members in the Edwardsburg Fm. that are closely related in time. Each of the diamictites is associated with intermediate composition tuff or flow rocks and the diamictites are separated by mafic volcanic rocks. SHRIMP U–Pb dating indicates that the lower diamictite is about 685±7 Ma, whereas the upper diamictite is 684±4 Ma. The diamictite units are part of a cycle of rocks from coarse clastic, to fine clastic, to carbonate rocks that, by correlation to better preserved sections, are thought to record an older Cryogenian glacial to interglacial period in the northern US Cordillera. The more than 75-m-thick diamictite of Daugherty Gulch is dated at 664±6 Ma. This unit is preserved only in drill core and the palaeoenvironmental interpretation and local stratigraphic relations are non-unique. Thus, the date for this diamictite may provide a date for a newly recognized glaciogenic horizon or may be a minimum age for the diamictite in the Edwardsburg Fm. The c. 1000-m-thick Moores Lake Fm. is an amphibolite facies diamictite in which glacial features have not been observed. However, it is part of a sedimentary cycle from unsorted siliclastic deposits to mud and carbonate deposits. Using lithostratigraphy and available geochronology, the Moores Lake Fm. is correlated with a younger succession of Cryogenian glaciogenic rocks in southeastern Idaho. Traditional correlations of Neoproterozoic rocks in the Cordillera recognize two levels of Cryogenian diamictites. The Edwardsburg and Moores Lake diamictites along the middle Cordillera fit well into the scenario of two glacial events. Because of the correlations, dates that provide ages for the diamictites in central Idaho (and corroborated in southeastern Idaho, Link & Fanning 2008) could constrain the age of correlated glaciogenic deposits elsewhere in the Cordillera. However, in the absence of dates for the glaciogenic diamictites in Canadian and southern US Cordilleran sections, the correlations are considered possible but uncertain.

Idaho↗