A Geophysical Study in Grand Teton National Park and Vicinity, Teton County, Wyoming: With Sections on Stratigraphy and Structure and Precambrian Rocks

An integrated geophysical study--comprising gravity, seismic refraction, and aeromagnetic surveys-was made of a 4,600km2 area in Grand Teton National Park and vicinity, 'Vyoming, for the purpose of obtaining a better understanding of the structural relationships in the region. The Teton range is largely comprised of Precambrian crystalline rocks and layered metasedimentary gneiss, but it also includes granitic gneiss, bornblende-plagioclase gneiss, granodiorite, and pegmatite and diabase dikes. Elsewhere, the sedimentary' section is thick. The presence of each system except Silurian provides a chronological history of most structures. Uplift of the Teton-Gros Ventre area began in the Late Cretaceous; most of the uplift occurred after middle Eocene time. Additional uplift of the Teton Range and downfaulting of Jackson Hole began in the late Pliocene and continues to the present. Bouguer anomalies range from -185 mgal over Precambrian rocks of the Teton Range to -240 mgal over low-density Tertiary and Cretaceous sedimentary rocks of Jackson Hole. The Teton fault (at the west edge of Jackson Hole), as shown by steep gravity gradients and seismic-refraction data, trends north-northeast away from the front of the Teton Range in the area of Jackson Lake. The Teton fault either is shallowly inclined in the Jenny Lake area, or it consists of a series of fault steps in the fault zone ; it is approximately vertical in the Arizona Creek area. Seismic-refraction data can be fitted well by a three-layer gravity model with velocities of 2.45 km per sec for the Tertiary and Cretaceous rocks above the Cloverly Formation, 3.9 km per sec for the lower Mesozoic rocks, and 6.1 km per sec for the Paleozoic (limestone and dolomite) and Precambrian rocks. Gravity models computed along two seismic profiles are in good agreement (u=± 2 mgal) if density contrasts with the assumed 2.67 g per cm Paleozoic and Precambrian rocks are assumed to be -0.35 and -0.10 g per cm for the 2.45 and 3.9 km per sec velocity layers, respectively. The Teton Range has a maximum vertical uplift of about 7 km, as inferred from the maximum depth to basement of about 5 km. Aeromagnetic data show a 400y positive anomaly in the Gros Ventre Range, which trends out of the surveyed area at the east edge. Exposed Precambrian rocks contain concentrations of magnetite and hematite. A prominent anomaly of about 100y is . associated with the Gros Ventre Range, and 100y anomalies are associated with the layered gneiss of the Teton Range. On this basis the unmapped Precambrian rocks of the Gross Ventre Range are interpreted as layered gneiss. The sources of the magnetic anomalies, as indicated by depth determination, are at the surface of the Precambrian rocks. A model fitted to a profile across the Gros Ventre Range gives a depth to the Precambrian surface and a susceptibility of 0.0004 emu ( electromagnetic units) for the source, which is consistent with modal analyses of the layered gneisses. A residual magnetic map shows that the granitic rocks and layered gneiss probably continue beneath the floor of Jackson Hole east of the Teton fault. The location of aeromagnetic anomalies is consistent with the interpretation that the Teton fault diverges from the front of the Teton Range. INTRODUCTION The area covered in this report consists of 4,&00 km (square kilometers), in Teton County, Wyo. (fig. 1), between lat 43°15' to 44°08' N. and long 110°23' to 111 °00' \V. , and includes Grand Teton National Park and parts of Teton National Forest. The Teton Range forms the west border of Jackson Hole, a downfa ulted basin containing a thick section of sedimentary rock. The Gros Ventre Range trends into the area from the southeast. The Mount Leidy and Pinyon Peak highlands form the east and northeast borders of Jackson Hole. Geologic work by Love ( 1956a) suggested a great thickness of Tertiary and Cretaceous sedimentary rocks in Jackson Hole. The Teton fault of post-middle Pliocene age generally has been considered to be a steeply dipping normal fault (Love, 1956b). Geophysical work was undertaken to obtain as much information as possible on the structural relationships and the subsurface geology of the region. Present investigation and acknowledgnwnts. -The text, except the sections by Love and Reed, was prepared by Behrendt. The 1964 seismic data were interpreted by Behrendt and Tibbetts. The original gravity survey was made in 1955 by Bonini and Lavin, of Princeton University; their ·work was pq,rtly supported by the National Science Foundation.