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At least 1,063 records · Page 59Linked to original sources
USGS1927

A comparison of the genera Metaplacenticeras Spath and Placenticeras Meek

In a recent examination of a considerable suite of specimens from the Western Interior of the United States belonging to the Cretaceous ammonite genus Placenticeras Meek the writer made comparisons of the species from the Interior with those from the Cretaceous of the Pacific coast ordinarily designated Placenticeras pacificum J. P. Smith, P. calijornicum Anderson, and P. sanctaemonicae Waring. The two groups of species differ widely in their characteristics, though each is a fairly homogeneous assemblage in itself. It was the writer's intention to propose a new name for the group of P. pacijicum - indeed, one was proposed and in type, when the publication by Spathof the name Metaplacenticeras met the need adequately, and that name is of course used here. Spath's name is based on the citation of Smith's original figure and a notation that Metaplacenticeras differs from Placenticeras in its falcoid ribbing and distinct suture line. Inasmuch as there has been some divergence of opinion as to the scope and probable derivation of the genus Placenticeras, based on the study by different writers of members of one or the other of the two groups, the writer has thought it worth while to record his observations and deductions and to give a fuller account of the new genus.

Professional Paper↗
USGS1927

The Montana earthquake of June 27, 1925

The earthquake of June 27, 1925, in Montana caused considerable damage within an area of 600 square miles or more, the center of which is in latitude 46° 5' N. and longitude 111° 20' W., a short distance southeast of Lombard. It was a seismic disturbance of the first order of magnitude, but, owing to the hour at which it occurred and to other fortunate circumstances, no lives were lost and no fires broke out. The shock was startling throughout an area extending 75 miles or more in all directions from the epicenter and was sensible to persons within an area of 310,000 square miles. Within the epicentral area brick buildings suffered severely, rocks fell from cliffs, cracks opened in the ground, and the inhabitants experienced the usual symptoms of illness and emotions of alarm. Isoseismals drawn according to the Rossi-Forel scale show a wide indentation at the south due to a rapid decline of intensity in the volcanic area of Snake River Plain and Yellowstone Park.

Montana↗
USGS1927

Geology and ore deposits of the Leadville mining district, Colorado

Adequate treatment of so large and so extensively developed a district as that of Leadville necessitates a voluminous report, in which the practical questions of prime interest to the commercial world can not be systematically answered until the data on which they depend are discussed. Many readers will no doubt wish to turn at once to the chapter on ore reserves, which will give them an appraisal of the district, without bothering with geologic detail. For those who wish a brief general account, a summary of the principal chapters of the report is presented below.

Colorado↗
USGS1992

Plutonism in the central part of the Sierra Nevada Batholith, California

The Sierra Nevada batholith comprises the plutonic rocks of Mesozoic age that underlie most of the Sierra Nevada, a magnificent mountain range that originated in the Cenozoic by the westward tilting of a huge block of the Earth's crust. Scattered intrusions west of the batholith in the western metamorphic belt of the Sierra Nevada and east of the Sierra Nevada in the Benton Range and the White and Inyo Mountains are satellitic to but not strictly parts of the Sierra Nevada batholith. Nevertheless, all the plutonic rocks are related in origin. The batholith lies along the west edge of the Paleozoic North American craton, and Paleozoic and early Mesozoic oceanic crust underlies its western margin. It was emplaced in strongly deformed but weakly metamorphosed strata ranging in age from Proterozoic to Cretaceous. Sedimentary rocks of Proterozoic and Paleozoic age crop out east of the batholith in the White and Inyo Mountains, and metamorphosed sedimentary and volcanic rocks of Paleozoic and Mesozoic age crop out west of the batholith in the western metamorphic belt. A few large and many small, generally elongate remnants of metamorphic rocks lie within the batholith. Sparse fossils from metasedimentary rocks and isotopic ages for metavolcanic rocks indicate that the metamorphic rocks in the remnants range in age from Early Cambrian to Early Cretaceous. Within the map area (the Mariposa 1 0 by 2 0 quadrangle), the bedding, cleavage, and axial surfaces of folds generally trend about N. 35 0 W., parallel to the long axis of the Sierra Nevada. The country rocks comprise strongly deformed but generally coherent sequences; however, some units in the western metamorphic belt may partly consist of melanges. Most sequences are in contact with other sequences, at least for short distances, but some sequences within the batholith are bounded on one or more sides by plutonic rocks. Proterozoic and Paleozoic sedimentary strata east of the Sierra Nevada and Paleozoic strata in remnants of country rocks within the eastern part of the batholith, although strongly deformed, are autochthonous or have been displaced only short distances, whereas some Mesozoic strata in the western metamorphic belt may be allochthonous. Probably the strata in the western metamorphic belt were deposited in marginal basins and island arcs, but the possibility that they were transported from distant places has not been disproved. All the country rocks have been strongly deformed, most of them more than once. Tectonic disturbances occurred during the Devonian and Mississippian (Antler? orogeny), the Permian and (or) Early Triassic (Sonoman? orogeny), the Late Jurassic (Nevadan orogeny), and at various other times during emplacement of the batholith and uplift that accompanied and followed its emplacement. The strata in the western metamorphic belt probably were deformed in an early Mesozoic subduction complex. The plutonic rocks range in composition from gabbro to leucogranite, but tonalite, granodiorite, and granite are the most common rock types. Most are medium to coarse grained, but some small rock masses are fine grained. Most have hypidiomorphic-granular textures and are equigranular, but some having compositions close to the boundary between granite and granodiorite contain large crystals of alkali feldspar. Serpentinized ultramafic rocks are present locally in the western metamorphic belt within and adjacent to the Melones fault zone. Except for serpentinized ultramafic rocks, trondhjemite, and most granites, all the plutonic rocks contain significant amounts of hornblende. Most of the granitoids are metaluminous or weakly peraluminous; strongly peraluminous granites are present only in the White Mountains. Most of the granitoids are assigned to units of lithodemic rank, and most of these units are assigned to intrusive suites. Plutons assigned to the same lithodeme are composed of rock of similar composition, fabric, and age and are presumed to h

Professional Paper↗
USGS1990

The Coalinga, California, earthquake of May 2, 1983

At 2342 G.m.t. May 2, 1983, a magnitude (ML) 6.7 earthquake occurred about 12 km northeast of the town of Coalinga, approximately halfway between Los Angeles and San Francisco. The shock was felt from Los Angeles to 200 km north of Sacramento and as far east as Las Vegas. Unlike other well-documented, major earthquakes in California in the 20th century, this event was not associated with any previously known or suspected active fault. Comprehensive geologic and geophysical investigations begun soon after the event have demonstrated the absence of a near-surface fault responsible for the earthquake. Instead, the earthquake was closely associated with a fault zone concealed beneath folds developed along the structural boundary between the Coast (Diablo) Ranges and the San Joaquin Valley.

California↗