Geological Survey Professional Paper 516—E A Geophysical Study in Grand Teton National Park and Vicinity, Teton County, Wyoming

The area discussed in this report is of unusually great interest because (1) Jackson Hole was downfaulted in late Cenozoic time, thereby preserving a part of the geologic record that has been lost elsewhere in Wyoming; (2) it includes mountain ranges of different ages and origins; (3) its crustal disturbances are undoubtedly related to volcanism in the adjoining Yellowstone National Park area; (4) the thick stratigraphic section includes all systems except the Silurian; (5) strata of all Cenozoic epochs are thick, fossiliferous, and well exposed; and (6) the completeness of the stratigraphic record enables us to demonstrate the chronologic development of most structures.

The relation of Jackson Hole to upfolded and faulted mountains and to downwarps is shown in figure 1; the distribution of major rock units and significant structural features is shown on plate 1. The recognized sedimentary and volcanic rock units and their relation to tectonic events are shown in table 1. Details of stratigraphy, structure, petrography, geologic history, and paleontology of Jackson Hole and adjacent areas are given in reports listed in "References Cited."

TABLE 1.—Summary description of sedimentary and volcanic rocks and of tectonic events in the Grand Teton National Park and vicinity

DOWNWARPS

JACKSON HOLE

Jackson Hole is a complexly folded and faulted downwarp and within it are several structurally deep areas. One is an elongate trough adjacent and parallel to the west margin of Jackson Hole. Within it, at shotpoint 4.1, the top of the Precambrian is about 3 km below sea level (fig. 10), whereas on top of Mount Moran to the west the same horizon is about 4 km above sea level. The present structural configuration of this trough was caused by dropping in late Cenozoic time of the western part of Jackson Hole along the Teton fault and by sagging of the floor in the northeastern and southeastern sections, perhaps under the weight of a thick section of Miocene and Pliocene rocks in addition to the older strata (table 1).

A second deep area, which is also broad, lies between the Whetstone anticline and the Buffalo Fork thrust fault. Here, the top of the Precambrian is estimated to be 3.4 km below sea level. A third deep area, perhaps the deepest of all, is east of the north end of the Bacon Ridge anticline where the Precambrian surface is estimated to be at least 4.5 km below sea level.

GREEN RIVER BASIN

The Green River topographic basin lies southeast of Jackson Hole and is separated from it, but the two structural basins are connected by a narrow gooseneck-shaped enormously deep syncline that has been overridden by both the Hoback and Gros Ventre Ranges (fig. 2). Precambrian rocks at the bottom of this syncline are estimated to be at a depth of more than 9 km (Love and Keefer, 1965).

FIGURE 2.—Tectonic map of Teton County, Wyo. (click on image for a PDF version)

MOUNTAINS

Bordering Jackson Hole are four types of mountains: (1) a horstlike upfaulted block (Teton Range), (2) intricately folded and faulted mountains of sedimentary rocks underlain by low-angle thrust faults rather than by a Precambrian core (Snake River and Hoback Ranges), (3) folded asymmetric anticlinal uplifts with Precambrian cores and thrust or reverse faults along the steep flanks (Gros Ventre and Washakie Ranges), and (4) mountains that are remnants of thick piles of nearly horizontal pyroclastic rocks (Absaroka Range and Yellowstone Volcanic Plateau).

TETON RANGE

The Teton Range is a north-trending uplifted block of Precambrian and Paleozoic rocks about 60 km long and 15-25 km wide. It is bounded on the east by the Teton normal fault (pl. 1; cross section, fig. 7) and on the west by another, virtually parallel normal fault (unnamed and incompletely known; not shown in fig. 2) along the Wyoming-Idaho State line. This fault is not exposed but is inferred because a sharp linear topographic break in alluvial deposits coincides with steepened west dips in Paleozoic rocks of the Teton Range at the east margin of Teton Basin (fig. 1). If this interpretation is correct, the Teton Range is a horst between the two downfaulted blocks, Teton Basin and Jackson Hole. The core of the mountain is largely granite and metamorphic rocks (Reed, p. E12 of this report) overlain by west-dipping Paleozoic rocks on the west side; these are faulted off on the east side.

The Teton Range was formed in two stages. During Late Cretaceous and early Tertiary times it was the northwestern part of a broad northwest-trending fold continuous with the Gros Ventre Range. Later in Cenozoic time this uplift was thrust southwest. Then the overriding block was broken by a series of faults, the largest of which is the Teton fault. The area of downfaulted blocks became the southern part of Jackson Hole, and thereby separated the uplift into two very different segments. The Teton Range continued to rise, whereas the Gros Ventre Range did not.

SNAKE RIVER AND HOBACK RANGES

The Snake River and Hoback Ranges are composed entirely of Paleozoic and Mesozoic sedimentary rocks that are intricately folded, overturned in places, and faulted. No Precambrian rocks are exposed. The ranges are piles of overlapping thrust sheets emplaced along low-angle thrust faults, such as the Jackson, Darby, Absaroka, St. John, Game Creek, and Bear Creek (fig. 2). Dominant movement was to the northeast, and the amount of displacement probably was many kilometers. The thrust sheets were apparently emplaced during early Eocene time, but some thrusting or gravity faulting occurred much later and in the opposite direction. For example, southwest of the mapped area, overturned Cambrian limestones along the southwest margin of the Snake River Range moved southwestward onto middle Pliocene rocks.

The Snake River and Hoback Ranges are structurally separated, but they are genetically related; both are composed of low-angle thrust sheets of about the same age, and, on their northern margins, both are underlain and bounded by the Jackson thrust. The trace of the Jackson fault curls around the east side of the Hoback Range where Paleozoic and Mesozoic rocks have been shoved onto conglomerates of latest Paleocene age.

GROS VENTRE RANGE

The Gros Ventre Range is an asymmetric anticlinal uplift that has Precambrian rocks in the core. The northeast flank is a broad area of gently dipping Paleozoic and Mesozoic strata. The steep southwest flank overrides the Green River basin along the Cache thrust (fig. 2). Vertical displacement on top of the Precambrian from the crest of the mountains to the bottom of the underlying basin is estimated to be more than 12 km. Maximum horizontal displacement is about 13 km.

The range was a broad gentle uplift as far back in time as latest Cretaceous. Major thrusting after middle Eocene time peeled back the north edge of the previously emplaced Jackson thrust block. Thrust masses (Love, 1956b), which were preserved because they were dropped by movement along the Hoback normal fault, 8 km southwest of the Gros Ventre Range (fig. 2), probably were emplaced during early Pliocene time as a result of rise of the mountain arch and movement on the Cache thrust. The separation of this uplift from the Teton Range by normal faulting during late Cenozoic time has been noted above.

WASHAKIE RANGE

The Washakie Range, like the Gros Ventre Range, is an asymmetric anticline that has a Precambrian core, a gentle northeast flank, and a steep thrust-faulted southwest flank. In several places the fault along the southwest side, the Buffalo Fork thrust, steepens and becomes a high-angle reverse fault. The throw on the Precambrian rocks between the top of the uplift and the bottom of the overridden syncline to the southwest is 6 km or more. During latest Cretaceous time the range was a broad low fold that subsequently rose and probably was thrust faulted during the Paleocene. At that time it may have been the dominant mountain range in the area. Post-Oligocene normal faulting near the south boundary of Yellowstone National Park lowered this part of the range by at least 300 m (meters).

ABSAROKA RANGE

The Absaroka Range is different from all other mountains in the region because it is not a structural uplift but an erosional remnant of a formerly vast nearly horizontal pyroclastic deposit several hundred meters thick that buried the adjacent folded mountains during Eocene and Oligocene time. The source of this debris was the Yellowstone-Absaroka volcanic area northeast and east of Jackson Hole. During middle and late Cenozoic time this was a relatively stable area affected by only minor warping and faulting in most places. Thus, it contrasts markedly with the spectacularly unstable major part of the Teton-Jackson Hole area only 30 km to the southwest.

Late Cenozoic erosion reexcavated the adjacent basins, exhumed the older folded mountains, and left the Absaroka Range as a series of high sharp drainage divides and steep-sided remnants of the original plateau. The underlying Washakie Range has been partly exhumed, and one type of mountain range can be seen overlapping the other.

YELLOWSTONE VOLCANIC PLATEAU

The Yellowstone Volcanic Plateau is not strictly a mountain range but is a broad rolling upland of more than 5,000 km², interrupted by a few peaks and ridges of volcanic rocks and by sharp steep-sided canyons. The rocks are chiefly andesite and basalt of Eocene and Oligocene age in the eastern part, rhyolite of Pliocene and Pleistocene age in the middle part, and rhyolite and basalt of similar age in the western part. These rocks have been warped and faulted in some places, but they are commonly nearly horizontal.

ANTICLINES

Many large anticlines 15-30 km long and with 1,500-3,000 m amplitude, which range in age from pre-middle Paleocene to Pleistocene, are present in the Jackson Hole area (pl 3). Most of these folds have thrust or reverse faults on their southwest sides, and at least two were monoclines until Pliocene time. A few of the anticlines are described in order to show the variety present.

The Spread Creek anticline, which is 30 km long and has at least 2,000 m of closure, is bounded on the southwest by a thrust fault (crossed in one deep oil well). The anticline involves Paleozoic rocks (penetrated in two oil wells) and was formed before deposition of the Paleocene Pinyon Conglomerate. The Bacon Ridge anticline, which is more than 30 km long and has only a small amount of closure, plunges to the northwest and is bounded on the southwest by a small thrust fault. This anticline involves Paleozoic rocks, and is in part older than the Pinyon Conglomerate and in part younger. The Whetstone anticline is a broad open southeastward-plunging fold formed in post-Paleocene time.

The Red Hills anticline is a sharp fold involving Paleozoic rocks but is older than the Pinyon Conglomerate. The adjoining Ramshorn anticline to the northwest, however, was apparently a northeast-dipping monocline until post-middle Pliocene subsidence of the western part of Jackson Hole. At that time, westward tilting was sufficient to produce the west limb of the fold. The Munger anticline south of Jackson Hole was formed likewise; it was a southwest-dipping monocline until the Hoback normal fault became active in Pliocene time. Movement along this fault caused the rocks to the west to hinge down in such a way that the northeast flank of the anticline was created. This fold is one of several caused by tension rather than compression.

The Little Granite anticline, east of the Munger, was formed after Paleocene time, for it involves youngest Paleocene rocks, but before emplacement, along the Jackson thrust, of the Hoback Range which, moving northeastward, completely overrode the anticline in places (fig. 2). Subsequently, the north flank of this anticline was overridden by the Gros Ventre Range block, which moved southwestward along the Cache thrust.

THRUST FAULTS

Three types of thrust faults are present in the mapped area. (1) The low-angle type which involves both competent and incompetent sedimentary beds but no mountain cores of Precambrian rocks, as in the Snake River and Hoback Ranges, and which commonly have many kilometers of horizontal displacement. (2) The steeper type, which involves Precambrian cores of the mountain uplifts and which have only a few kilometers of horizontal displacement, such as the Cache and Buffalo Fork thrusts. These faults grade from thrust to reverse in many places. (3) Thrust faults which involve incompetent beds on anticlines and whose displacement is less than 2 km, such as the Spread Creek and Bacon Ridge thrusts. In places along the crest of the Bacon Ridge anticline, some of the bentonitic Cretaceous rocks are so incompetent that they were squeezed into recumbent folds before breaking.

NORMAL FAULTS

Three normal faults are especially important in the structural development of Jackson Hole. (1) The Teton normal fault, which is 80 km long and has a maximum vertical displacement of about 7 km with the east block down, is responsible in part for the uplift of the Teton Range as we know it today; it originated within the last 10 million years and is still active. (2) The Hoback normal fault, which is 40 km long and has at least 3 km displacement, with the west block down, probably began slightly later than the Teton fault and it also continues to be active. (3) The Warm Spring fault, cutting across the southern part of Jackson Hole, is about 19 km long and has at least 2 km of displacement, with the north block down; it is of Pliocene age. Subsidence of the north block during middle Pliocene time probably was responsible for impounding Teewinot lake in which 2 km of sediment accumulated. The fault has been inactive since late Pleistocene time.

CHRONOLOGY OF TECTONIC EVENTS AND THEIR RELATION TO VOLCANISM

The chronology of tectonic events in Grand Teton National Park and vicinity is summarized in table 1. The Teton Range apparently rose at the time the valley floor to the east subsided, but the volume of the lifted block is considerably less than that of the dropped block. This discrepancy still requires explanation.

Volcanism began in the Yellowstone-Absaroka area in early Eocene time and continued to the Pleistocene. Considerably more than 4 X 10⁴ km³ (cubic kilometers) of debris were blown out and spread to the east by wind and water. Jackson Hole did not subside during the first 20 million years of volcanism in the adjacent Yellowstone-Absaroka area despite the extrusion of an enormous volume of debris. However, from Miocene time to the Pleistocene, large-scale subsidence of Jackson Hole was coincident with extensive volcanic activity, not only in the sinking area (for the first time) but to a much greater extent in the Yellowstone-Absaroka area which did not subside. This relation suggests that Jackson Hole sank as subcrustal material moved laterally into the area of greatest volcanism.