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George Plafker

Publications and source records attributed to George Plafker.

At least 73 records · Page 4Linked to original sources
USGS1976

Textural and mineralogical study of sandstones from the onshore Gulf of Alaska Tertiary Province, southern Alaska

Petrographic examination of 74 outcrop samples of Paleocene through Pliocene age from the onshore Gulf of Alaska Tertiary Province indicates that sandstones of the province characteristically are texturally immature and mineralogically unstable. Diagenetic alteration of framework grains throughout the stratigraphic sequence has produced widespread zeolite cement or phyllosilicate grain coatings and pseudomatrix. Multiple deformation and deep burial of the older Tertiary sequence--the Orca Group, the shale of Haydon Peak, and the Kulthieth and Tokun Formations--caused extensive alteration and grain interpenetration, resulting in low porosity values. Less intense deformation and intermediate depth of burial of the younger Tertiary sequence--the Katalla, Poul Creek, Redwood, and Yakataga Formations--has resulted in a greater range in textural properties. Most sandstone samples in the younger Tertiary sequence are poorly sorted, tightly packed, and have strongly appressed framework grains, but some are less tightly packed and contain less matrix. Soft and mineralogically unstable framework grains have undergone considerable alteration, reducing pore space even in the youngest rocks. Measurements of porosity, permeability, grain density, and sonic velocity of outcrop samples of the younger Tertiary sequence indicate a modest up-section improvement in sandstone reservoir characteristics. Nonetheless porosity and permeability values typically are below 16 percent and 15 millidarcies respectively and grain densities are consistently high, about 2.7 gm/cc. Low permeability and porosity values, and high grain densities and sonic velocities appear to be typical of most outcrop areas throughout the onshore Gulf of Alaska Tertiary Province.

Open-File Report↗
USGS1976

Tectonic aspects of the Guatemala earthquake of 4 February 1976

The locations of surface ruptures and the main shock epicenter indicate that the disastrous Guatemala earthquake of 4 February 1976 was tectonic in origin and generated mainly by slip on the Motagua fault, which has an arcuate roughly east-west trend across central Guatemala. Fault breakage was observed for 230 km. Displacement is predominantly horizontal and sinistral with a maximum measured offset of 340 cm and an average of about 100 cm. Secondary fault breaks trending roughly north-northeast to south-southwest have been found in a zone about 20 km long and 8 km wide extending from the western suburbs of Guatemala City to near Mixco, and similar faults with more subtle surface expression probably occur elsewhere in the Guatemalan Highlands. Displacements on the secondary faults are predominantly extensional and dip-slip, with as much as 15 cm vertical offset on a single fracture. The primary fault that broke during the earthquake involved roughly 10 percent of the length of the great transform fault system that defines the boundary between the Caribbean and North American plates. The observed sinistral displacement is striking confirmation of deductions regarding the late Cenozoic relative motion between these two crustal plates that were based largely on indirect geologic and geophysical evidence. The earthquake-related secondary faulting, together with the complex pattern of geologically young normal faults that occur in the Guatemalan Highlands and elsewhere in western Central America, suggest that the eastern wedge-shaped part of the Caribbean plate, roughly between the Motagua fault system and the volcanic arc, is being pulled apart in tension and left behind as the main mass of the plate moves relatively eastward. Because of their proximity to areas of high population density, shallow-focus earthquakes that originate on the Motagua fault system, on the system of predominantly extensional faults within the western part of the Caribbean plate, and in association with volcanism may pose a more serious seismic hazard than the more numerous (but generally more distant) earthquakes that are generated in the eastward-dipping subduction zone beneath Middle America.

Science↗
USGS1976

Glaciomarine deposits of Miocene through Holocene age in the Yakataga Formation along the Gulf of Alaska margin, Alaska

Perhaps the world's longest and most complete onshore sedimentary record of late Cenozoic glaciation is preserved in the Gulf of Alaska Tertiary province that extends 800 km along the coast of southern Alaska. The Yakataga Formation, with an aggregate outcrop thickness of about 5,000 m, is characterized by variable amounts of distinctive neritic marine tillite-like diamictite and laminated siltstone containing dropstones interpreted as ice-rafted glacial debris. The lithology, sedimentary structures and molluscan fauna of the formation suggest that active tidal glaciers or an ice shelf were present along the landward margin of the basin possibly beginning in early or early middle Miocene time. Dropstone distribution in outcrop sections indicates that glaciers reached tidewater intermittently during the Miocene and were almost continually present throughout the Pliocene and much of the Pleistocene. Paleomagnetic and nannoplankton dating of the upper 1,181 m of the Yakataga Formation at Middleton Island indicate that this part of the sequence probably was entirely deposited during the Matuyama reversed polarity epoch of the Pleistocene during which the sedimentation rate was of the order of 1 m/1,000 years. Lithologically similar deposits of poorly consolidated sandy mud and pebbly mud continue to accumulate locally near tidal glaciers in the same area, and older late Quaternary deposits are undoubtedly present offshore. The anomalous late Cenozoic glaciation recorded in the sediments along the Gulf of Alaska margin reflects a fortuitous combination of a subsiding shelf basin adjacent to an area of extremely high elevations and exceptionally heavy precipitation, an environment that persists to the present.

Alaska↗
USGS1975

Interim report on petroleum resource potential and geologic hazards in the outer continental shelf of the Gulf of Alaska Tertiary province

The potential for discovering large accumulations of petroleum on the Outer Continental Shelf of the Gulf of Alaska Tertiary Province cannot be evaluated with much confidence at present because of the inadequacies of the available offshore geological and geophysical data. The 22 deep test wells that have been drilled since 1954 have been unsuccessful because structure is complex and because suitable reservoir rocks have not been found in favorable structural positions. Although it is possible that the factors controlling accumulation of petroleum may improve offshore, regional stratigraphic and structural considerations together with the limited geophysical data suggest that this is not necessarily the case. Extrapolation of onshore geology together with the limited amount of marine geophysical data, indicate that the eastern Gulf of Alaska Outer Continental Shelf is geologically complex and consists of several areas with markedly differing structural styles and petroleum potential.

Alaska↗
USGS1975

A computer-assisted procedure for information processing of geologic field data

We have examined several computer systems to see how they could aid information processing of geologic field data. The processing procedure we have chosen combines a systematic note-taking technique, a computer text-editor for creating files of data, and automated printing of selected data or of complete file copies. Traditional text notes are the foundation of each station record, and no coding of data is required. The text-editor used is an interactive computer system that allows any individual with typing skills to build and use data files. On the basis of our experience, an interactive text-editor can readily fulfill the data-handling needs of most geologic field projects and offers important advantages over other systems that were examined.

Journal of Research of the U.S. Geological Survey↗
USGS1974

Alaska

No abstract available.

Alaska↗
USGS1974

The Border Ranges Fault in south-central Alaska

The Border Ranges fault, a major fault of southern Alaska, can be traced for more than 1,000 km arcuately eastward from Kodiak Island to the St. Elias Mountains. Throughout its extent, the fault juxtaposes upper Paleozoic and lower Mesozoic rocks on the north against upper Mesozoic and Tertiary rocks. This report describes the Border Ranges fault and its geologic setting along an approximately 245-km-long segment in the McCarthy and Valdez quadrangles. It also summarizes information relevant to other parts of the fault and discusses its significance and tectonic implications. In the McCarthy and Valdez quadrangles the fault strikes between N. 60° W. and west. Its dips change from vertical and steeply northward in its eastern part to between 20° and 60° north throughout most of the Valdez quadrangle and reflect the transition from a high-angle reverse fault to a northward-dipping thrust. The Border Ranges fault is interpreted to mark a plate boundary that developed near the close of the Mesozoic or in the early Tertiary.

Alaska↗
USGS1973

Field reconnaissance of the effects of the earthquake of April 13, 1973, near Laguna de Arenal, Costa Rica

At about 3:34 a.m. on April 13, 1973, a moderate-sized, but widely-felt, earthquake caused extensive damage with loss of 23 lives in a rural area of about 150 km 2 centered just south of Laguna de Arenal in northwestern Costa Rica (fig. 1). This report summarizes the results of the writer's reconnaissance investigation of the area that was affected by the earthquake of April 13, 1973. A 4-day field study of the meizoseismal area was carried out during the period from April 28 through May 1 under the auspices of the U.S. Geological Survey. The primary objective of this study was to evaluate geologic factors that contributed to the damage and loss of life. The earthquake was also of special interest because of the possibility that it was accompanied by surface faulting comparable to that which occurred at Managua, Nicaragua, during the disastrous earthquake of December 23, 1972 (Brown, Ward, and Plafker, 1973). Such earthquake-related surface faulting can provide scientifically valuable information on active tectonic processes at shallow depths within the Middle America arc. Also, identification of active faults in this area is of considerable practical importance because of the planned construction of a major hydroelectrical facility within the meizoseismal area by the Instituto Costarricense de Electricidad (I.C.E.). The project would involve creation of a storage reservoir within the Laguna de Arenal basin and part of the Río Arenal valley with a 75 m-high earthfill dam across Río Arenal at a point about 10 km east of the outlet of Laguna de Arenal.

Laguna De Arenal↗
USGS1971

Geological aspects of the May 31, 1970, Perú earthquake

Geological effects of the destructive May 31, 1970, Perú earthquake (Ms = 7.7) extended over roughly 65,000 km2 of west-central Perú. Earthquake-triggered slope failures of all types that occurred throughout the mountainous parts of the region extensively damaged transportation routes and irrigation canals and temporarily dammed some rivers and lakes. The geologically most important and spectacular of these, a cataclysmic debris avalanche from between 5,500 and 6,400 m altitude on the north peak of Huascarán Mountain, buried the city of Yungay and part of Ranrahirca (elevation about 2,500 m) with a loss of more than about 18,000 lives. The debris avalanche involved 50-100 million m3 of rock, ice, snow, and soil that traveled 14.5 km from the source to Yungay at an average velocity of between 280 and 335 km/hr. Exceptionally rapid movement of the avalanche is indicated by eyewitness accounts, by topographic irregularities as high as 140 m that were overridden, and locally by boulders weighing several tons that were hurled as much as 1,000 m beyond its margins. A pulse of muddy water from the debris avalanche that swept down the Río Santa 160 km to the sea inundated farms and small settlements, buried highway and railroad routes, and destroyed the diversion dam and access bridge to a major hydroelectric plant.

Bulletin of the Seismological Society of America↗
USGS1970

Mechanism of the Chilean Earthquakes of May 21 and 22, 1960

The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

GSA Bulletin↗
USGS1969

Preliminary geologic interpretation of aeromagnetic data in the Yakutat district, Alaska

An aeromagnetic survey covering portions of the Yakutat Feveland region in south-central Alaska was made to delineate concentrations of magnetic minerals possibly associated with heavy mineral placers in beach sands and raised beach ridges. However, the larger magnetic anomalies do not correlate with the beach placers in the surveyed area. The dominant magnetic feature, a strong linear positive anomaly almost parallel to the coastline, is thought to reflect a bedrock feature, probably a mafic igneous body enclosed within nonmagnetic rocks of the Yakutat group.

Open-File Report↗
USGS1969

The Alaska earthquake, March 27, 1964: Effects on communities

This is the second in a series of six reports that the U.S. Geological Survey published on the results of a comprehensive geologic study that began, as a reconnaissance survey, within 24 hours after the March 27, 1964, Magnitude 9.2 Great Alaska Earthquake and extended, as detailed investigations, through several field seasons. The 1964 Great Alaska earthquake was the largest earthquake in the U.S. since 1700. Professional Paper 542, in 7 parts, describes the effects of the earthquake on Alaskan communities.

Alaska↗
USGS1969

Effects of the earthquake of March 27, 1964, on various communities

The 1964 earthquake caused wide-spread damage to inhabited places throughout more than 60,000 square miles of south-central Alaska. This report describes damage to all communities in the area except Anchorage, Whittier, Homer, Valdez, Seward, the communities of the Kodiak group of islands, and communities in the Copper River Basin; these were discussed in previous chapters of the Geological Survey's series of reports on the earthquake. At the communities discussed herein, damage resulted primarily from sea waves of diverse origins, displacements of the land relative to sea level, and seismic shaking. Waves took all of the 31 lives lost at those communities; physical damage was primarily from the waves and vertical displacements of the land relative to sea level. Destructive waves of local origin struck during or immediately after the earthquake throughout much of Prince William Sound, the southern Kenai Peninsula, and the shores of Kenai Lake. In Prince William Sound, waves demolished all but one home at the native village of Chenega, destroyed homesites at Point Nowell and Anderson Bay, and caused varying amounts of damage to waterfront facilities at Sawmill Bay, Latouche, Port Oceanic, Port Nellie Juan, Perry Island, and western Port Valdez. The local waves, which ran up as high as 70 feet above tide level at Chenega and more than 170 feet in several uninhabited parts of the Sound, took nearly all of the lives lost by drowning at these communities. Destructive local waves that devastated shores of Anderson Bay and adjacent parts of western Port Valdez probably were generated primarily by massive submarine slides of glacial and fluvioglacial deposits ; the origin of the waves that caused damage at most of the other communities and at extensive uninhabited segments of shoreline is not known. At these places the most probable generative mechanisms are: unidentified submarine slides of unconsolidated deposits, and (or) the horizontal tectonic displacements, of 20 to more than 60 feet, that occurred in the Prince William Sound region during the earthquake. A train of long-period seismic sea waves that began about 20 minutes after the start of the earthquake inundated shores along the Gulf of Alaska coast to a maximum height of 35 feet above tide level. At the communities described, they virtually destroyed two logging camps at Whidbey Bay and Puget Bay on the south coast of the Kenai Peninsula, caused moderate damage to boat harbors and docks at Seldovia and Cordova, floated away some beach cabins in the Cordova area, and drowned two people, one at Point Whitshed near Cordora and one at the Cape Saint Elias Light Station. The seismic sea waves were generated by regional tectonic uplift of the sea floor on the Continental Shelf. Vertical tectonic displacements of the land relative to sea level that accompanied the earthquake affected virtually all the coastal communities. Tectonic subsidence of 5 to 6 feet, augmented locally by surficial subsidence of unconsolidated deposits required either the relocation or raising of structures at Portage, Girdwood, and Hope on Turnagain Arm. Shoreline submergence resulting from about 3½ feet of tectonic subsidence at Seldovia necessitated raising all waterfront facilities and the airstrip above the level of high tides. On the other hand, tectonic uplift of the land in the Prince Williams Sound region required deepening of the small-boat harbors at Cordora and Tatitlek, dredging of the waterways in the Cordova area, and lengthening of some docks or piers at Cordova, the Cape Hinchinbrook Light Station, and in Sawmill Bay. Significant structural damage from direct seismic shaking was largely confined to fluid containers and a pier facility near Kenai. Indirect damage from fissuring and differential settling of foundation mterials in the vicinity of the Cordova airfield mused damage to a building, underground utilities, an airfield fill, and the highway. Minor amounts of direct and indirect damage from seismic vibrations were sustained by most of the communities situated on unconsolidated deposits as far east as Yakutat, north to Fairbanks, and west to King Salmon. Except for a few cracked or toppled chimney, all the damage from shaking was confined to areas of thick, unconsolidated deposits. Foundation damage was almost entirely restricted to water-saturated unconsolidated deposits which, when liquefied by seismic shaking, could spread laterally toward free faces and (or) settle differentially through compaction.

Alaska↗