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USGS1940

Stratigraphic measurements in parallel folds

Folded rocks having bedding surfaces which are approximately parallel are said to lie in parallel folds. Utilizing the principle of evolutes and involutes, the author offers a more precise definition of parallel folds and points out inconsistencies in other concepts. With the idea of classifying parallel folds and possibly of deducing the mechanics of their formation, methods are presented for obtaining the differential equations of the families of involutes which, in certain cross sections, represent the traces of stratigraphic surfaces; and for obtaining the equations of the corresponding evolutes. Geometric methods are also given. These equations and geometric constructions may be deduced either from assumed structural postulates or from actual field data. Another part of the paper deals with the application of mean trigonometric functions to the measurement of thickness of strata, depth and distance to a stratum, and other stratigraphic dimensions in sections oblique to the strike of the rocks. This topic is considered under two headings: (1) where such measurements can be made from data collected at several stations along a line of traverse, and (2) where they must be made from a series of structural observations, considered in pairs. In the first case, no assumption is made regarding the curvature of the strata, but instead the mean values of the required functions are derived by mechanical integration. In the second case, the usual assumption of circular curvature is made, and the necessary functions are obtained by the use of definite integrals. Tables of the logarithms of these mean functions, with an increment of 5 degrees for the argument, are also presented. © 1940 Geological Society of America.

Geological Society of America Bulletin↗
USGS1940

Glacial chronology of the Southern Rocky Mountains

In order to extend the chronology and validate the five substages of the Wisconsin glaciation recognized in the Cache la Poudre Valley in the Colorado Front Range, a reconnaissance was made of the Southern Rocky Mountains, from southern Wyoming to Santa Fe, New Mexico. The chronology determined in the Cache la Poudre Valley was used to date the culture layer of the Lindenmeier (Folsom) Site in northern Colorado. The earliest or Twin Lakes substage is named for the "early moraine" mapped by Capps at Twin Lakes in the Upper Arkansas Valley. This substage is believed to have been contemporaneous with the Durango glaciation of the San Juan Mountains. Throughout the Southern Rocky Mountains the second, third, and fourth, or Home, Corral Creek, and Long Draw substages, can be readily correlated from valley to valley. The fifth or youngest substage, represented in some cirques by protalus ramparts, is named the Sprague substage. The validity of five distinct substages of Wisconsin ice advance, separated from one another by interstadial ice retreat or complete disappearance, is based on: (1) the character and topographic position of the moraines; (2) the relationship between successive terraces (valley trains) and the terminal moraines, as determined in the Cache la Poudre Valley; (3) the differences in weathering of the tills composing the moraines; and (4) the reported finding of an old soil zone between tills assigned to the Twin Lakes and Home substages. © 1940 Geological Society of America.

Geological Society of America Bulletin↗
USGS1941

Geology of the Moreno Valley, New Mexico

The Moreno Valley, located along the complex eastern boundary between the Sangre de Cristo Mountains and the Great Plains, is structurally a broad, northwardplunging syncline, disrupted by smaller folds and faults. This major synclinal structure is the result of the Laramide disturbance. Deformation, however, has continued possibly into the Quaternary. Intrusions, probably contemporaneous with those of the Spanish Peaks, have deformed the strata in the northern portion of the Moreno Valley. Scully Mountain is a northwestward-plunging anticline produced by a porphyry intrusion. A Mid-Tertiary surface of low relief is now represented by broad mountain flats at elevations near 10,000 feet. The Moreno Valley was initiated as a topographic feature about the close of the Mid-Tertiary. Its subsequent history is in many respects similar to that of the Rio Grande Depression. During the Quaternary a broad valley stage and three lower pediments have been developed. The lowest pediments are cut on deformed valley fill, believed to be Pliocene and here named the Eagle Nest formation. The early drainage of the valley was south and east into the Canadian River. Stream piracy by the Cimarron River diverted drainage to the east through the present Cimarron Canyon into the Canadian River. Drainage south of the point of capture has been reversed for about 14 miles, the present stream draining the southern portion of the Moreno Valley flowing northward to the Cimarron River. © 1941 Geological Society of America.

New Mexico↗
USGS1942

Reconnaissance survey of the Roberts Mountains, Nevada

The Roberts Mountains region, central Nevada, provides an excellent section of Paleozoic rocks ranging from Upper Cambrian to Permian. Major low-angle thrusting is indicated by deformed Ordovician strata resting on Paleozoics of varying age. Overlying a thick breccia zone, the upper thrust plate consists of sandstones, andesitic flows and tuffs, black shales, and bedded cherts (Vinini formation). Ordovician age of the Vinini is established on the basis of graptolite faunules. A belt of Lower to Middle Ordovician graptolitic facies similar to the Vinini formation crosses the Great Basin west of Roberts Mountains. Deposits of roughly the same age in the Roberts Mountains meridian and eastward are dominantly limestone, carrying distinct faunas. Axial planes of overturned folds in the thrust plate dip west, a further indication that the upper thrust plate moved from west to east. Minimum horizontal displacement is 16 miles. The date of thrusting is uncertain, but presumably was later Cretaceous or early Tertiary. Following thrusting, an alaskite stock and rhyolite porphyry plugs were intruded; lava flows and tuffs covered the area in part. Thrust plate and cover of volcanics have been broken into normal fault blocks. The post-thrusting igneous rocks, like volcanic rocks of Utah and New Mexico, are characterized by high potash content.

Nevada↗
USGS1942

Correlation of the outcropping cretaceous formations of the Atlantic and Gulf Coastal Plain and trans-Pecos Texas

This is Number 9 of a series of correlation charts prepared by the Committee on Stratigraphy of the National Research Council, which will cover the post-Proterozoic sedimentary formations of North America. For general information about the project the reader is referred to the general introduction preceding this paper. The senior author, Lloyd W. Stephenson, is responsible for that part of the chart treating of the Upper Cretaceous series in the Atlantic and Gulf Coastal Plain (exceptions noted below) and the Lower Cretaceous series of the North Atlantic Coastal Plain; with respect to the Upper Cretaceous his presentation is based largely on his personal knowledge of the paleontology and stratigraphy of the area, gained from field and laboratory studies ranging from reconnaissance to detailed; all published information has been considered, and much of it utilized, but it is not practicable to include a complete bibliography. References are given to papers presenting views differing from those of the author, and to recent papers not yet listed in bibliographies. A complete bibliography of Texas geology through 1932 is given by Sellards (1933, p. 819-965). Watson H. Monroe has collaborated in the preparation of that part of the chart showing therelations of the Upper Cretaceous units in Alabama and Mississippi. The classification shown in the Upper Cretaceous part of the Delaware column is based on a recently published paper by Charles W. Carter (1937, p. 237-281). Philip B. King is responsible for the remainder of the chart covering the Lower Cretaceous (Comanche series) of central Texas (largely compiled from published sources) and all of the Cretaceous in trans-Pecos Texas (from personal knowledge and published sources). In March 1941 this part of the chart was checked by R. W. Imlay, and changes were made by King at his suggestion. References to the more important literature are given. In a correlation chart of this kind, in which age equivalencies are expressed by horizontal lining, it is obviously impossible to indicate relative thicknesses by vertical spacing. Hiatuses are indicated by vertical lining. The red-line overprint is intended to indicate certain important lithologic and faunal boundaries, and to assist the eye in recognizing the age relationships of the geologic units

Geological Society of America Bulletin↗
USGS1943

Geology of the Cimarron Range, New Mexico

In north-central New Mexico the rugged Cimarron Range marks the eastern margin of the Southern Rocky Mountains, abruptly rising more than 5000 feet above the adjacent Great Plains. Structurally the range is a northward-plunging anticline with a core of pre-Cambrian crystalline rocks. Faulting along the eastern and western margins of the anticline separates the mountains from the Great Plains and the Moreno Valley. North of Cimarron Canyon the anticline plunges beneath the Tertiary sediments of the Raton Coal Basin. To the south the structure is obscured by the basaltic flows of the Ocaté Mesa. Upturned Paleozoic to Tertiary sedimentary rocks crop out along the eastern and northern margin of the anticlinal uplift. Regional relations are obscured by extensive Tertiary intrusions in the northern portion of the area and by the lava flows to the south. In the valley of Rayado Creek a volcanic plug which fed some of the lava flows has been exposed by erosion. In this region there is evidence for a Mid-Tertiary surface of low relief in the central portion of the mountains, a later lava-capped surface believed to be the equivalent of the Broad Valley Stage, the surface of the Park Plateau, and the Ocate Mesa. Three well-developed gravel-capped Pleistocene pediment surfaces extend steplike from the mountain front into the Great Plains. Both structurally and topographically the Cimarron Range is similar to the Colorado Front Range. An attempt is made to reconstruct the geologic history of the Cimarron Range and to fit it into the broader story of the development of the Southern Rocky Mountains in New Mexico. © 1943 Geological Society of America.

New Mexico↗
USGS1980

Late Cenozoic volcanic rocks of the southern Sierra Nevada, California: I. Geology and petrology: Summary

The geology and petrology of the Cenozoic volcanic rocks of the region of California between 38° and 35°45'N latitude and 117°30' and 120°W longitude, including the rocks of a major potassic magmatic province on the west slope of the Sierra Nevada, have been described in Part I (Moore and Dodge, 1980) of this study. The geochemical features of these rocks may provide clues to their origin and aid in comparing and contrasting the Sierra Nevada potassic province with other localized potassic provinces elsewhere in the world. Basaltic rocks occur in numerous small lava-flow remnants, dikes, and plugs that intrude the predominantly granitic terrain of the western slope of the southern Sierra Nevada. Leucite-bearing rocks are present at several localities in this western region. More voluminous basalt, commonly associated with rhyolite, is present east of the range in the Basin and Range province. However, leucite-bearing rocks have been found-at only one locality in this region. Most of the late Cenozoic volcanic rocks in the southern Sierra Nevada eastward through Owens Valley and in the extreme western Basin-Range occur in five rather distinctive areas—the San Joaquin-Kings, Kern, Big Pine, and Coso volcanic fields, and the Mono-Long Valley volcanic center (Fig. 1).

Geological Society of America Bulletin, Part I↗
USGS1986

Role of replacement in the genesis of anorthosite in the Boehls Butte area, Idaho

In the Boehls Butte area, Idaho, three large and numerous small lenses of layered to massive anorthosite consisting of two, and locally three, types of plagioclase and minor hornblende and micas occur in aluminum silicate-rich garnet mica schist. In most of this anorthosite, andesine megacrysts with bytownite inclusions are embedded in a fine-grained groundmass of bytownite or anorthite. In places labradorite occurs instead of andesine. Some labradorite laths show Carlsbad twinning and rims of andesine around anorthite inclusions. Along the contacts, lenses of fine-grained bytownite anorthosite with some hornblende or garnet and quartz are common. These lenses could represent calcic parent rocks that were changed to two-plagioclase rocks by partial replacement of bytownite by andesine.

Idaho↗
USGS1987

Tourmaline (dravite) from the Boehls Butte anorthosite, Idaho, U.S.A.

Tourmaline (dravite) from a layered two-plagioclase anorthosite in the Boehls Butte quadrangle, Idaho, has the following structural formula: (Na 0.55 Ca 0.17 ) 0.72 (Mg 1.96 Fe 0.57 Mn 0.01 Ti 0.04 V 0.03 Al 0.47 ) 3.08 Al 6 (BO 3 ) 3.02 Si 5.98 O 18 (OH,F) 3.35 . The indices of refraction are ω = 1.647(1) and ε = 1.621(1), and the unit cell parameters are a = 15.9425(6) and c = 7.1883(3).

Idaho↗
USGS2000

The Beaver River structure: A cross-strike discontinuity of possible crustal dimensions in the southern Mackenzie Fold Belt, Yukon and Northwest territories, Canada

A significant cross-strike structural discontinuity, the Beaver River Structure, has been recognized near the boundary between the north-central part of the Mackenzie Fold Belt and its southern part, the Liard Plateau. The Beaver River Structure is the northeast continuation of the Beaver Fault toward the confluence of the South Nahanni and Liard rivers and into the Interior Platform. It is oriented sub-parallel with the Liard Line farther south. Cumulative Laramide-aged dextral movement of 10 to 20 kilometres is inferred to have occurred along the Beaver River Structure. Small dextral offsets occur along the Beaver Fault, a possible surface manifestation of the Beaver River Structure and large, dominantly north-south oriented Laramide-aged folds and faults, such as the Kotaneelee and Liard synclines and the Nahanni Thrust Fault appear to have been locally rotated in a manner consistent with dextral strike-slip of at least 10 to 20 kilometres. The absence of Permian and Triassic strata in the Mackenzie Fold Belt north of the Beaver River Structure may indicate that uplift and erosion occurred north of the Beaver River Structure during the Early Cretaceous Columbian Orogeny. It is likely that the Beaver River Structure is an ancient, possibly Precambrian, structure that has been reactivated several times throughout Phanerozoic time.

British Columbia, Northwest Territories, Yukon↗
USGS2011

Reserve growth in oil pools of Alberta: Model and forecast

Reserve growth is recognized as a major component of additions to reserves in most oil provinces around the world, particularly in mature provinces. It takes place as a result of the discovery of new pools/reservoirs and extensions of known pools within existing fields, improved knowledge of reservoirs over time leading to a change in estimates of original oil-in-place, and improvement in recovery factor through the application of new technology, such as enhanced oil recovery methods, horizontal/multilateral drilling, and 4D seismic. A reserve growth study was conducted on oil pools in Alberta, Canada, with the following objectives: 1) evaluate historical oil reserve data in order to assess the potential for future reserve growth; 2) develop reserve growth models/ functions to help forecast hydrocarbon volumes; 3) study reserve growth sensitivity to various parameters (for example, pool size, porosity, and oil gravity); and 4) compare reserve growth in oil pools and fields in Alberta with those from other large petroleum provinces around the world. The reported known recoverable oil exclusive of Athabasca oil sands in Alberta increased from 4.5 billion barrels of oil (BBO) in 1960 to 17 BBO in 2005. Some of the pools that were included in the existing database were excluded from the present study for lack of adequate data. Therefore, the known recoverable oil increased from 4.2 to 13.9 BBO over the period from 1960 through 2005, with new discoveries contributing 3.7 BBO and reserve growth adding 6 BBO. This reserve growth took place mostly in pools with more than 125,000 barrels of known recoverable oil. Pools with light oil accounted for most of the total known oil volume, therefore reflecting the overall pool growth. Smaller pools, in contrast, shrank in their total recoverable volumes over the years. Pools with heavy oil (gravity less than 20o API) make up only a small share (3.8 percent) of the total recoverable oil; they showed a 23-fold growth compared to about 3.5-fold growth in pools with medium oil and 2.2-fold growth in pools with light oil over a fifty-year period. The analysis indicates that pools with high porosity reservoirs (greater than 30 percent porosity) grew more than pools with lower porosity reservoirs which could possibly be attributed to permeability differences between the two types. Reserve growth models for Alberta, Canada, show the growth at field level is almost twice as much as at pool level, possibly because the analysis has evaluated fields with two or more pools with different discovery years. Based on the models, the growth in oil volumes in Alberta pools over the next five-year period (2006-2010) is expected to be about 454 million barrels of oil. Over a twenty-five year period, the cumulative reserve growth in Alberta oil pools has been only 2-fold compared to a 4- to- 5-fold increase in other petroleum producing areas such as Saskatchewan, Volga-Ural, U.S. onshore fields, and U.S. Gulf of Mexico. However, the growth at the field level compares well with that of U.S. onshore fields. In other petroleum provinces, the reserves are reported at field levels rather than at pool levels, the latter basically being the equivalent of individual reservoirs. ?? 2010 by the Canadian Society of Petroleum Geologists.

Bulletin of Canadian Petroleum Geology↗
USGS2025

Peak streamflow trends in North Dakota and their relation to changes in climate, water years 1921–2020

Standardized guidelines for completing flood-flow frequency analyses are presented in a U.S. Geological Survey Techniques and Methods report known as Bulletin 17C, https://doi.org/10.3133/tm4B5 . In recent decades (since about 2000), a better understanding of long-term climatic persistence (periods of clustered floods or droughts, or wet or dry periods) and concerns about potential climate change and land-use change have caused a reexamination of the stationarity assumptions underlying methods in Bulletin 17C. Bulletin 17C does not offer guidance on incorporating nonstationarities and further identifies a need for flood-frequency studies that incorporate changing climate or basin characteristics. As part of that reexamination, a study of annual peak streamflow (peak flow) has begun in the Midwest. This chapter of the study summarizes how hydroclimatic variability affects peak flows in North Dakota. In this analysis of peak flow, daily streamflow, and climate metrics, four periods were selected: (1) a 100-year period, 1921–2020; (2) a 75-year period, 1946–2020; (3) a 50-year period, 1971–2020; and (4) a 30-year period, 1991–2020. Output from a monthly water-balance model was used for the climate data. Statistical analysis of peak flow consisted of evaluations of autocorrelation, trends, and change points and was augmented with analyses of seasonality and daily streamflow. The long-term pattern of decreasing peak flow in the west and increasing peak flow in the east is a pattern of opposing signals on either side of the 100th meridian. Analyses indicate that a key factor in changing hydroclimatology is the increase in fall precipitation. The trends in soil moisture closely match the trends in annual precipitation. Nonstationary flood-frequency analysis necessitates detailed exploratory data analysis and additional data and information about climate, land use, and other factors. This study provides extensive exploratory analysis for peak flow, daily streamflow, and climate data for North Dakota, setting the stage for informed nonstationary flood-frequency analysis.

North Dakota↗
USGS1986

Late Cretaceous stratigraphy, deformation and intrusion in the Madison Range of southwestern Montana ( USA).

Dating of orogenic rock units in the central part of the Madison Range shows that Laramide deformation was virtually completed by the end of the Cretaceous. Early Campanian K-Ar dates of about 79 m.y. were obtained from welded tuffs in the basal part of the Livingston Formation, a volcanic and volcaniclastic assemblage that is conformable with underlying Cretaceous clastic rocks and with the overlying Sphinx Conglomerate. The Sphinx and the Livingston were deformed by the Hilgard fault system which extends along the western side of the southern two-thirds of the range. This north-trending fault system represents the culmination of Laramide shortening within the range. Dating of hornblende indicates an approximate date of 68-69 m.y. B.P. for emplacement of the igneous suite. The dacite postdates movement along faults of the Hilgard fault system, and postdates the synorogenic Sphinx Conglomerate. -from Authors

Geological Society of America Bulletin↗
USGS1989

Mid-Cretaceous alluvial-plain incision related to eustasy, southeastern Colorado Plateau

Eustatic effects on the deposition of ancient coastal and marine rocks are well known, but eustasy also can affect depositional patterns and processes well inland from the sea and play an important role in the development of nonmarine unconformities. In the southeastern part of the Colorado Plateau, fluvial rocks of the lowermost Cenomanian (lowermost Upper Cretaceous) Encinal Canyon Member at the base of the Dakota Sandstone fill paleovalleys incised into underlying formations. In the latter part of the Early Cretaceous, an epicontinental sea lay about 240 km east of the southeastern Colorado Plateau and was base level for streams in the plateau region. Near the end of the Early Cretaceous, sea level fell, base level was lowered, and streams incised valleys into alluvial deposits of the Burro Canyon Formation and into older formations. The resulting incised paleodrainage surface was preserved as the sub-Dakota unconformity when the succeeding sea-level rise, in earliest Late Cretaceous time, caused Dakota streams to aggrade and backfill the paleovalleys with alluvial sediments of the Encinal Canyon Member. -from Author

Geological Society of America Bulletin↗
USGS1991

Paleogeographic implications of an erosional remnant of Paleogene rocks southwest of the Sur-Nacimiento Fault Zone, southern Coast Ranges, California

A small tract of heretofore-unrecognized Paleogene rocks lies about 30 km northeast of Santa Maria and 1 km southwest of the Sur-Nacimiento fault zone near upper Pine Creek. This poorly exposed assemblage of rocks is less than 50 m thick, lies unconformably on regionally distributed Upper Cretaceous submarine-fan deposits, and consists of three units: fossiliferous lower Eocene mudstone, Oligocene(?) conglomerate, and basaltic andesite that has a radiometric age of 26.6 ?? 0.5 Ma. Both the sedimentary and igneous constituents in the Paleogene sequence are unlike those of known sequences on either side of the Sur-Nacimiento fault zone. The Paleogene sedimentary rocks near upper Pine Creek presumably are remnants of formerly widespread early Eocene bathyal deposits and locally distributed Oligocene(?) fluvial deposits southwest of the fault zone. The 26.6 Ma basaltic andesite, however, may not have extended much beyond its present outcrops. An episode of Oligocene(?) displacement is required by the contrast in thicknesses, depositional patterns, and paleobathymetry of the juxtaposed rock sequences. -from Authors

Geological Society of America Bulletin↗
USGS1999

Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction

This study documents three localities in the Franciscan accretionary complex of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 Ma: Point Delgada-Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane). Vein assemblages of quartz, carbonate, sulfide minerals, and adularia were precipitated locally in highly fractured wall rock. Vitrinite reflectance (Rm) values and illite crystallinity decrease away from the zones of metalliferous veins, where peak wall-rock temperatures, as determined from Rm, were as high as 315??C. The ??18O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher ??18O fluids produced widespread quartz and calcite veins that are typical of the regional paleothermal regime. The widespread veins are by-products of heat conduction and diffuse fluid flow during zeolite and prehnite-pumpellyite-grade metamorphism, and we interpret their paleofluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks. Lower ??18O fluids, in contrast, evolved from relatively high temperature exchange between seawater (or meteoric water) and basaltic and/or sedimentary host rocks; focused flow of those fluids resulted in local deposition of the metalliferous veins. Heat sources for the three paleothermal anomalies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advected either from a site of ridge-trench interaction north of the Mendocino fracture zone or from a "slabless window" in the wake of the northward migrating Mendocino triple junction. A separate paradox involves the amount of Quaternary offset of Franciscan basement rocks near Shelter Cove by on-land faults that some regard as the main active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by A.D. 1906 surface rupture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-slip component of San Andreas offset at a rate of 3-4 cm/yr, then its activity began only 83-62 ka. This interpretation would also mean that a longer term trace of the San Andreas fault must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak heating of King Range strata north of the Mendocino triple junction. Conversely, shifting the fault to the east would be compatible with a slabless window heat source and long-distance northward translation of the King Range terrane after peak heating.

Geological Society of America Bulletin↗
USGS1999

Surface faulting and paleoseismic history of the 1932 Cedar Mountain earthquake area, west-central Nevada, and implications for modern tectonics of the Walker Lane

The 1932 Cedar Mountain earthquake (Ms 7.2) was one of the largest historical events in the Walker Lane region of western Nevada, and it produced a complicated strike-slip rupture pattern on multiple Quaternary faults distributed through three valleys. Primary, right-lateral surface ruptures occurred on north-striking faults in Monte Cristo Valley; small-scale lateral and normal offsets occurred in Stewart Valley; and secondary, normal faulting occurred on north-northeast-striking faults in the Gabbs Valley epicentral region. A reexamination of the surface ruptures provides new displacement and fault-zone data: maximum cumulative offset is estimated to be 2.7 m, and newly recognized faults extend the maximum width and end-to-end length of the rupture zone to 17 and 75 km, respectively. A detailed Quaternary allostratigraphic chronology based on regional alluvialgeomorphic relationships, tephrochronology, and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone. A late Wisconsinan alluvial-fan and piedmont unit containing a 32-36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements. Exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late Quaternary faulting along the Cedar Mountain fault zone. Three trenches display evidence for six faulting events, including that in 1932, during the past 32-36 ka. Radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best-fit ages of the pre-1932 events at 4, 5,12,15, and 18 ka, each with ??2 ka uncertainties. On the basis of an estimated cumulative net slip of 6-12 m for the six faulting events, minimum and maximum late Quaternary slip rates are 0.2 and 0.7 mm/yr, respectively, and the preferred rate is 0.4-0.5 mm/yr. The average recurrence (interseismic) interval is 3600 yr. The relatively uniform thickness of the ponded deposits suggests that similar-size, characteristic rupture events may characterize late Quaternary slip on the zone. A comparison of event timing with the average late Quaternary recurrence interval indicates that slip has been largely regular (periodic) rather than temporally clustered. To account for the spatial separation of the primary surface faulting in Monte Cristo Valley from the epicenter and for a factor-of-two-to-three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake, we hypothesize two alternative tectonic models containing undetected subevents. Either model would adequately account for the observed faulting on the basis of wrench-fault kinematics that may be associated with the Walker Lane. The 1932 Cedar Mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the Walker Lane region. In contrast to most other historical events in the Basin and Range province, the 1932 event did not occur along a major range-bounding fault, and no single, throughgoing basement structure can account for the observed rupture pattern. The 1932 faulting supports the concept that major earthquakes in the Basin and Range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard.

Geological Society of America Bulletin↗
USGS2011

Zircon U-Pb age of the Pescadero felsite: A late Cretaceous igneous event in the forearc, west-central California Coast Ranges

Downstream grain-size fining in stratigraphy is driven primarily by selective deposition of sediment, and the long-term efficiency of this process is determined by: (1) the magnitude and characteristics of the input sediment supply; (2) the spatial distribution of subsidence rate, which creates accommodation for sediment preservation; and (3) the dynamics of sediment transport and deposition. A key challenge is to determine how these first two factors control the caliber and spatial distribution of deposits over time scales of 10 4 –10 6 yr without incorporating sediment transport details that are largely unknowable for time-averaged stratigraphy in the geological past. We address this using grain-size data collected from fluvial conglomerates in the Eocene Pobla Basin, Spanish Pyrenees, a synorogenic basin where the timing of sediment deposition is well-constrained; the sediment budget is closed; and good exposure enables time lines within stratigraphy to be picked out unambiguously. For successive stratigraphic horizons, downstream trends in grain size and composition are derived for basin-filling sediment-routing systems with length scales of 6 and 40 km, respectively. Our data show that the rate of grain-size fining varies over time and with system length and can be linked to changes in source area. These results are contrasted with grain-size data from the Antist Group, a 60-km-long Oligocene system that mantles the Southern Pyrenees, where very slow rates of grain-size fining on the wedge top of this fold-and-thrust belt are observed. We apply a self-similarity–based selective deposition model to quantify the competing controls of tectonic subsidence and sediment supply on derived grain-size trends, and model results are compared with independent constraints on the Eocene–Oligocene evolution of the Pyrenees. Our results suggest that it is now possible to invert time-averaged grain-size trends in stratigraphy to gain quantitative information on the geological boundary conditions governing the evolution of sedimentary basins.

Geological Society of America Bulletin↗