Ray trace travel time modeling of the seismic refraction record sections for a profile from near Soda Springs, Idaho, to near McKay, Idaho, was used to derive a crustal model across the eastern Snake River Plain (ESRP). The derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the ESRP. The interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the ESRP when compared with the upper crust beneath the adjoining Northern Rocky Mountain and Basin and Range provinces. The most prominent features of the crustal structure inferred by the ray trace modeling are as follows: (1) The northwest margin of the ESRP was modeled as a fault structure, downthrown on the SRP side (SE) with an offset of greater than 4 km. The southeast margin, conversely, appears to be downwarped with possible minor faulting. Paleozoic sedimentary rocks are present beneath the Cenozoic volcanics of the ESRP possibly to and beyond 40 km from the southeastern margin of the ESRP near Blackfoot, Idaho. (2) The modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the ESRP. (3) An intermediate 6.5 km/s layer occurring beneath the ESRP, interpreted from a refraction profile located along the axis of the ESRP, was found to be localized within the ESRP margins. This layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region. (4) A density model of the crust across the ESRP was prepared with the densities selected using the interpreted seismic velocities as a constraint. The gravity field calculated from this model resulted in a good match to observed gravity data over the eastern Snake River Plain.

Seismic refraction profiles recorded along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 Yellowstone‐Snake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q‐1) structure of the ESRP. Travel‐time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an upper‐crustal layer (compressional velocity ≅6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediate‐velocity (≅6.5 km/s) layer extends from ≅10 to ≅20 km depth. A thick (≅22 km) lower crust of compressional velocity 6.8 km/s, a total crustal thickness of ≅42 km, and a Pn velocity of ≅7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to low‐Q values in the volcanic rocks (Qp = 20 to 200) and throughout the crust (Qp = 160 to 300). Based on these characteristics of the crustal structure and volcanic‐age progression data, it is suggested that the ESRP has resulted from an intensive period of intrusion of mantle‐derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate layer. Crustal subsidence and periodic basaltic volcanism as represented by the ESRP complete the sequence of crustal evolution.

A 40 nanoTesla (nT) magnetic anomaly identified in an aeromagnetic survey over southern Illinois contours as a localized magnetic high on the west flank of a regional magnetic low. This magnetic anomaly is generally coincident with the Omaha Oil Field in northwest Gallatin County, Illinois. It was initially assumed that cultural sources of steel associated with this oil field were the primary source of the magnetic feature; however, similar oil fields overflown by the survey do not exhibit magnetic anomalies in the data set. The Luther Rister et ux ♯1 well, drilled near the apex of the Omaha structural dome, encountered two zones of ultramafic intrusive rock containing 9.0 percent by volume magnetite. These intrusives were identified to be alnöites which are a class of mantle‐derived ultramafic rock that can be associated with the incipient stages of crustal rifting. A ground magnetic survey verified the presence of the anomaly, and provided detailed data for 3-D modeling of the source. Petrophysical evaluations, magnetic susceptibility measurements and thin section modal analysis were made on drill cuttings from the ultramafic intrusives encountered in the Luther Rister ♯1 well. These measurements were made to constrain the 3-D magnetic modeling by the petrophysical characteristics of the source. After removal of the regional magnetic field, the resulting 140 nT residual magnetic anomaly was successfully modeled using two ultramafic sills with an igneous feeder plug. The two igneous sills adequately account for the structural closure exhibited in the Omaha Oil Field and raise the interesting possibility of other hydrocarbon trapping structures generated by intrusives emplaced into the sedimentary section.

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