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G. D. Robinson

Publications and source records attributed to G. D. Robinson.

At least 19 recordsLinked to original sources
USGS1971

Interpretation of an aeromagnetic strip across the northwestern United States

This report discusses the results of aeromagnetic survey bounded by latitudes 45°30′ N. and 47°00′ N. and extending from the Rocky Mountains, to approximately 120 mi offshore in the Pacific Ocean. East of the Rocky Mountains, a larger area has been surveyed in the Great Plains bounded approximately by latitudes 44°50′ N. and 48°10′ N. and by longitudes 104°W. and 110°W. Throughout the area of the survey, the magnetic map is marked by conspicuous northeast and northwest anomaly trends, lineaments, and breaks in the anomaly pattern. Their regional distribution, over-all magnetic character, and geologic evidence suggest that they are major structural features in the basement rocks. The close correspondence of structural and geologic features in younger rocks with these basement magnetic and structural trends suggest that basement trends controlled or at least greatly influenced intrusion, deposition, and structural history of younger rocks. In some cases, evidence suggests that basement structures have been reactivated during later tectonic activity. Perhaps even more striking than the northeast- and northwest-trending features are large east-west magnetic discontinuities which, in some cases, extend completely across the strip to the edge of the shelf, and which, in some cases, can be correlated with large-scale discontinuities dating back to the Precambrian.

Idaho, Montana, North Dakota, Oregon, South Dakota↗
USGS1963

Geology of the Three Forks quadrangle, Montana, with sections on petrography of igneous rocks

The Three Forks quadrangle at the head of the Missouri River, has a long stratigraphic succession. The oldest rocks are Precambrian gneiss, derived from arkosic sandstone, and amphibolite, probably altered from mafic sills and lava flows. The unmetamorphosed sedimentary rocks range in age from late Precambrian to Recent, have an aggregate thickness of about 11,000 feet, and underlie about 85 percent of the quadrangle. The only systems not represented are Ordovician, Silurian, and Triassic.

Montana↗
USGS1961

Origin and development of the Three Forks Basin, Montana

The Three Forks Basin sprawls where the intricately deformed sedimentary and volcanic rocks of the Disturbed Belt along the Rocky Mountain front are faulted against the Precambrian metamorphic rocks that make the core of the Tobacco Root, Madison, Gallatin, and Beartooth ranges. Its eastern edge is linear, controlled by steep faults at the west front of the Bridger Range. All other boundaries are sinuous and show little sign of structural control. Tertiary deposits in the basin , rich in contemporaneous rhyolitic and latitic ash, are about equally of lake, bolson, and stream origin . The western part of the basin is dominated by moderately folded Eocene and lower Oligocene rocks, more than 2000 feet thick. They dip eastward beneath apparently unfolded upper Miocene and Pliocene rocks, more than 1300 feet thick, that also dip gently eastward to the basin edge. Thin but extensive Quaternary deposits lying unconformably on the Tertiary and pre-Tertiary rocks are mainly of rounded terrace and flood-plain gravel, angular fan gravel, and wind-blown silt. The basin began as part of an east-flowing stream system that developed in Late Cretaceous and Paleocene time, concurrently with Laramide folding and thrusting; the faulted contact between metamorphic and sedimentary rocks was especially erodible and became a main drainage way. Recurrent uplift to the west throughout the Tertiary provided gradient and load to the streams; additional load was provided by showers of ash from unknown vents. Relative uplifts of the Bridger Range in Eocene and early Oligocene time, and again in late Miocene and Pliocene time, impeded flow from the basin and led to deposits in channels, flood plains, and lakes. During most of Oligocene and Miocene time, however, the basin was being eroded. By the end of the Tertiary the basin was deeply filled and became part of a regional surface of low relief. Regional northwestward tilting stimulated headward erosion of the Missouri River which then captured the formerly east-draining or closed basin . The Tertiary deposits have been deeply eroded, and the rugged pre- basin surface partly exhumed.

Montana↗