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Isidore Zietz

Publications and source records attributed to Isidore Zietz.

At least 91 records · Page 5Linked to original sources
USGS1964

A magnetic anomaly of possible economic significance in southeastern Minnesota

An aeromagnetic survey in southeastern Minnesota by the U. S. Geological Survey in cooperation with the State of Minnesota has revealed a high-amplitude, linear, and narrow magnetic feature that suggests a possible source of Precambrian iron-formation of economic value. For the past few years the U. S. Geological Survey has been conducting detailed geophysical studies of the midcontinent gravity anomaly--a broad, high-amplitude feature that extends from Lake Superior through the States of Minnesota, Iowa, Nebraska, and part of Kansas. As part of this study an aeromagnetic survey of the southern part of the State was made in cooperation with the State of Minnesota during the summer of 1963, in which a linear high-amplitude anomaly of the order of 4,000 gammas was discovered. Because of the high amplitude, the linearity, and the narrowness of the magnetic feature, it is believed the source may be Precambrian iron-formation of possible economic value. The anomalous area is in Fillmore County, approximately between the towns of Lanesboro and Peterson in the extreme southeastern part of the State. (See figures 1 and 2.) At the site of the anomaly, Cambrian sedimentary rocks occur in the valley of the Root River, and Ordovician rocks (nearly flat lying) mantle the upland areas. The uplands are largely covered by glacial deposits, which are relatively thin (Paul K. Sims, written communication, 1964). Depths to the Precambrian are estimated to range from 500 feet to 1,000 feet below the surface. The aeromagnetic map shown in figure 2 was compiled from continuous magnetic profiles made along east-west flight lines 1,000 feet above ground, and spaced approximately 1 mile apart. Contour intervals of 20, 100, and 500 gammas were used depending on the intensity. The instrument for the survey was a flux-gate type magnetometer (AN/ASQ-3A) which measures total-field variations. The contour map displays variations in magnetic pattern which are typical of shallow Precambrian rocks. Anomalies of the order of 1,000 gammas are shown along the east and west edges of the map. The outstanding feature is the previously mentioned linear positive anomaly that trends northeast and reaches a peak of 3,960 gammas. The positive anomaly is contoured from data on four consecutive profiles, but only two show high amplitudes. The high-amplitude anomalies along traverses 1 and 2 are shown in figure 3. Depth calculations suggest that the source of the anomaly lies about 1,000 feet below the surface. Assuming a dikelike source and magnetization resulting entirely from induction in the earth's field, several calculations were made in an attempt to fit the magnetic profile taken along the line AA' (see figs. 2 and 4), considered to be a typical cross-section of the magnetic anomaly. Comparisons are shown between observed and computed profiles. The fixed parameters used were (a) distance from detector to source of 2,000 ft; width of dike of 5,000 ft; dip of dike of 75?, 90?, 105? , and 120? , as shown. The best fit occurs when the dike is vertical or dips 75? to the southwest. For these cases, the susceptibility, k, is computed to be 0.016 c.g.s, units, and is comparable to k = 0.02+ calculated by Bath (1962) for the relatively unmetamorphosed iron-formation of the Main Megabi district in Minnesota where the induced magnetization was most likely the dominant magnetization. If the dominant magnetization for the anomaly in Fillmore County were remanent rather than induced, the economic importance of the anomaly would be greatly reduced. This anomaly seems sufficiently promising to warrant further geologic and geophysical investigation. Detailed ground magnetic and electrical studies would be useful to delineate the feature. In the final analysis, however, the presence of iron-formation can be determined only by the drill.

Circular↗
USGS1963

Preliminary interpretation of an aeromagnetic survey in north-central Iowa

Publication of Lyons' gravity map of the United States in 1950 focused attention on a spectacular geophysical feature now generally referred to as the "midcontinent gravity high." This feature, the largest gravity maximum in North America, was discovered and later detailed by geophysicists at the University of Wisconsin (Woollard, 1943, 1951) and Thiel (1956). Their work reveals an essentially continuous large positive gravity anomaly extending southwestward for 800 miles from Lake Superior to the Salina Basin in Kansas. The midcontinent gravity high reaches its maximum intensity in central Iowa where the Bouguer anomaly ranges from -100 to 60 milligals. In September-October 1961, the U. S. Geological Survey made a 5,000 square mile aeromagnetic survey in north-central Iowa in cooperation with the Iowa Geological Survey. Objectives of the investigation were to prepare and interpret a detailed aeromagnetic contour map for part of the midcontinent gravity high in Iowa. Analysis of the magnetic data could be expected to yield estimates of local thickness of the Paleozoic-Mesozoic sedimentary section and provide a basis for informed speculation about the character, configuration, and distribution of the Precambrian rocks. Such information would have obvious scientific and possible economic value in an area covered by glacial till and for which little deep subsurface knowledge was available. Interpretation of the Indiana aeromagnetic survey (Henderson and Zietz, 1958) had previously demonstrated that aeromagnetic data from the central stable region of North America would provide a significant amount of geologic information. The 5,000 square mile area surveyed in 1961 has been combined with a previously unpublished 600 square mile aeromagnetic survey made in 1953, centered on Manson, Pocahontas Co., Iowa. The combined area is shown as the patterned area in figure 1. The unpatterned area outlined on this same figure is the additional 10,000 square mile cooperative aeromagnetic survey made in the summer of 1962 to complete coverage of the midcontinent gravity high in Iowa. The primary purpose of this report is to make the 1953 and 1961 aeromagnetic data available to the public in advance of more formal publication. Only salient magnetic features are discussed in this preliminary presentation and in a second report on results obtained during the 1962 flying. A more complete interpretation of the entire aeromagnetic survey will be prepared.

Iowa↗
USGS1960

An aeromagnetic reconnaissance of the Cook Inlet area, Alaska

Forty-two east-west aeromagnetic lines were flown across the Cook Inlet-Susitna Lowland between Chelatna Lake and Seldovia at a flight altitude of approximately 2,500 feet. The lines traverse all or part of five Mesozoic tectonic elements that dominate the structure of the Cook Inlet area. Each of these tectonic elements, the Alaska Range geosyncline, the Talkeetna geanticline, the Matanuska geosyncline, the Seldovia geanticline, and the Chugach Mountains geosyncline, has a characteristic magnetic pattern. The aeromagnetic data, compiled as total intensity aeromagnetic profiles, show several significant features which are consistent with the structural grain of the area. A two-dimensional anomaly was observed near the east edge of the area on all but the southernmost profiles, where it becomes obscure. Geologic evidence suggests that this feature, the Knik Arm anomaly, is produced by plutonic rocks that have been intruded along the Seldovia geanticline. Southeast of this anomaly the profiles are almost flat. This flatness indicates that magnetic rocks are deeply buried in this area, which is underlain by slate and graywacke deposited in the Chugach Mountains geosyncline.

Alaska↗