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Dwight Lyman Schmidt

Publications and source records attributed to Dwight Lyman Schmidt.

9 recordsLinked to original sources
USGS1983

Middle Tertiary continental rift and evolution of the Red Sea in southwestern Saudi Arabia

Middle Tertiary rift volcanism in a continental-rift valley in the Arabian-Nubian Shield was the first surface expression of active mantle convection beneath an axis that was to become the Red Sea. Investigation of the coastal plain of southwestern Saudi Arabia suggests that the rift valley was filled with basaltic and felsic to rhyolitic volcanic rocks (Ad Darb and Damad formations), cherty tuffaceous siltstones (Baid formation), and subordinate Nubian-type quartz sandstone (Ayyanah sandstone) between about 30 and 20 Ma ago. These rocks are named herein the Jizan group. At the same time, alkali-olivine basalt was erupted on the stable Precambrian craton at locations 100 to 200 km east of the rift valley axis. First-stage spreading of the Red Sea began about 20 Ma ago when diabasic dikes and gabbro and granophyre plutons (Tihamat Asir complex) of oceanic-tholeiitic parentage were intruded into the continental-rift deposits. The new oceanic crust was injected into a region of thinned continental crust. Because of thinning, the continental crust was extended and became fractured into a series of fault blocks. Crustal extension in the attenuated zone was accompanied by mafic and silicic volcanism producing a continental rift-valley tectonic environment. Fault blocks of the rift valley were initially rotated toward the Red Sea during the first-stage spreading episode. Inland from the continental rift, the thick continental crust was distended on long parallel fractures that were intruded by gabbroic to quartz syenitic magma to form continental dikes. This period of continental-margin extension was short lived, lasting perhaps only 1 or 2 Ma. The first-stage sea-floor spreading of the Red Sea continued until about 15 or 14 Ma ago at a half-spreading rate of about 2.2 cm/yr. Throughout early Tertiary time, the Arabian Shield erosion surface remained near sea level. First-stage uplift of the Red Sea Escarpment began during middle Miocene time, as evidenced by the coarse polymictic boulder conglomerate of the Bathan formation. Second-stage scarp uplift and second-stage sea-floor spreading followed during Pliocene, Pleistocene, and Holocene time.

Open-File Report↗
USGS1978

The Antarctic and its geology

To the seamen of the 18th century who skirted its ice-choked seas in their wooden vessels, Antarctica was known as "Terra Australis Incognito" - the unknown land of the south.

General Information Product↗
USGS1962

Quaternary geology of the Bellevue area in Blaine and Camas Counties, Idaho

The Bellevue area covers about 350 square miles of a foothill belt between the Rocky Mountains to the north and the Snake River plains to the south. Complexly deformed impure quartzites and limestones of the Mississippian Milligen and Pennsylvanian-Permian Wood River formations were intruded by large bodies of quartz diorite and granodiorite along regional structures trending northwesterly; the intrusions are part of the Cretaceous Idaho batholith. Erosional remnants of the Challis volcanics, dominantly latitic to andesitic in composition and early(?) to middle Tertiary in age, rest unconformably on the older rocks. A sequence of Pliocene Rhyolitic ash flows and basaltic lava flows unconformably overlies the Challis and older rocks and is in turn unconformably overlain by olivine basalt of late Pliocene or early Quaternary age. The main valleys of the area, partly Erosional and partly structural in origin, are underlaind by late Quaternary olivine basalt flows (Snake River basalt) and intercalated lacustrine, fluvial, proglacial sediments. The Big Wood River, the master stream of the area, flows southward through a narrow steep-sided valley in the mountainous country north of the Bellevue area and debouches into a broad alluvial valley, the Wood River Valley, in the foothill belt. The valley has the shape of an isosceles triangle with a ten mile long, east-west base consisting of a ridge of Pliocene volcanics which separates the valley from the Snake River Plains to the south. The river now flows through a narrow gap in the southwest corner of the triangle. A similar, but wider, gap around the east end of the ridge was formerly occupied by the river. The river has been shifted back and forth between these two gaps at least four times during an interval in which six late Quaternary basalt flows erupted in the Bellevue area. Two of the flows caused direct diversion of the river and another was influential in bringing about a diversion on an aggradational fan upstream from the lava dam. Just prior to the Bull Lake stage the river, flowing out the east gap, was blocked but not diverted by the youngest basalt flow in the Bellevue area. During the proglacial aggradation, the river shifted widely on its fan and spilled alternatively out both the east and west gaps. After the Bull Lake stage, the west gap had an advantageous base level relative to the lava-blocked east gap, and the river cut down in the west gap. After the second, Pinedale, proglacial aggradation in the Wood River Valley, the west gap still maintained an advantageous base level, and the river again cut down in the west outlet valley where it remains today. Periglacial deposits completely dominate the sidestream valleys of the Bellevue area. They formed under a rigorous climate during the Pinedale stage, when slope erosion accelerated by frost activated processes caused aggradation of valley floors by local detritus. Even at present the larger sidestreams are so choked with detritus that the streams have not regained control of their valley floors. Recent basalt, comparable in age to the younger flows of the Craters of the Moon National Monument, spread from a rugged, cratered vent several miles south of the Bellevue area. Using degree of weathering, erosion, and soil development as a basis of comparison, this flow provides and end point for estimating the relative ages of the six late Quaternary flows in the Bellevue area.

Idaho↗
USGS1953

Reconnaissance geology of placer deposits containing radioactive minerals in the Bear Valley district, Valley County, Idaho

A reconnaissance of the Bear Valley district was undertaken to provide a geologic interpretation of placer deposits drilled by the U.S. Bureau of Mines. The placer minerals are monazite and a group of uranium bearing rare earth columbates and tantalates here referred to loosely as radioactive blacks. The monazite is an accessory mineral in the granitic country rock; the radioactive blacks occur in pegmatite dikes. The supply of these minerals to the placers was controlled (1) by the geography of their occurrence in the parent rock, and (2) by the distribution of alpine glaciers during two late Pleistocene glacial stages. By reason of a favorable combination of these factors, the richest placer deposits of the district are in Big Meadow, a valley fill formed as a result of the blocking of Bear Creek by a glacier from a tributary valley during the Illinoian (?) stage. The Big Meadow fill consists of intertonguing depositional units formed by Bear Creek and its tributaries, including both normal alluvium and glacial outwash, and ranging from rich to barren. The richest phase that has been blocked out by drilling was derived from the drainage basin of Casner Creek, an east tributary of Bear Creek. The geologic relations suggest that a neighboring stream, Howard Creek, should have supplied equally rich material, but the part of the valley fill formed by Howard Creek has not been tested. The Howard Creek deposits and shallow alluvium in the upper valleys of Casner and Howard Creeks may considerably increase the reserves of the district.

Open-File Report↗