By JOHN C. REED, JR.
Precambrian rocks are extensively exposed in the core of the Teton Range and in several small areas in the Gros Ventre Range east and southeast of Jackson. The Precambrian rocks in Grand Teton National Park on the east flank of the Teton Range are being studied by the author. The geologic map (pl. 1) of this part of the Precambrian complex and the following descriptions are based largely on his unpublished data and on brief published descriptions by Horberg and Fryxell (1942), Bradley (1956), and Reed (1963). The Precambrian rocks in the Gros Ventre Range and on the western slope of the Teton Range have never been mapped or studied in detail, but presumably, they are similar to those in the eastern part of the Teton Range.
The oldest and most widely exposed Precambrian rocks in the Teton Range in Grand Teton National Park are layered metasedimentary gneiss of medium high metamorphic grade. These rocks, designated layered gneiss on plate 1, comprise a heterogeneous array of biotite gneiss, biotite-hornblende gneiss, quartz-plagioclase gneiss, hornblende gneiss, and amphibolite. Thin layers of mica schist, amphibole schist, amphibole-cordierite schist, and calc-silicate rocks are commonly interleaved with the gneiss, and locally, the sequence contains thin layers of iron formation and pods and irregular masses of metagabbro and ultramafic rocks.
Individual layers range in thickness from fractions of a centimeter to tens of meters, and some can be traced for several hundred meters. Most layers, however, are less continuous, and many display boudinage structure and sheared-out isoclinical noses. Later folds with diverse axial trends are superimposed on the isoclines, which indicates that the layered gneiss sequence has undergone at least two episodes of folding (Reed, 1963).
Light- to medium-gray fine- to medium-grained biotite gneiss comprises the bulk of the layered sequence in most areas. The biotite gneiss is generally even grained, strongly foliated, and conspicuously layered. Locally, it displays conspicuous augen of plagioclase, garnet, and magnetite. It is interlayered on all scales with dark-gray fine- to medium-grained biotite hornblende gneiss, light-gray to white quartz-plagioclase gneiss, and dark-green to black hornblende amphibolite.
Average modes of these rock types, which together comprise more than 95 percent of the layered gneiss, are given in table 2.
TABLE 2.Average modes, in volume percent, of principal rock types in the layered gneiss of the Teton Range
It is difficult to estimate the proportions of these rocks in the layered gneiss unit, but the number of specimens counted to determine the average modes (table 2) is a crude index of their volumetric proportions.
North of Leigh Canyon, granitic gneisses of two types are major components in the Precambrian complex in the Teton Range. Hornblende-quartz monzonite gneiss (pl. 1) forms a large elongate body that extends from near the mouth of Webb Canyon southward to the upper reaches of Moran Canyon. Similar gneiss crops out north and west of Lake Solitude and in several smaller pods and sill-like bodies between Leigh Canyon and Doane Peak. Biotite granodiorite gneiss forms several irregular discontinuous bodies on the east flank of the range north of the mouth of Leigh Canyon. Average modes of these rocks are given in table 3.
TABLE 3.Average modes, in volume percent, of granitic gneisses in the Teton Range1
The hornblende-quartz monzonite gneiss is a medium grained light-gray to pink rock. It is not layered, but displays conspicuous foliation and lineation defined by strongly alined dark clots of fine-grained hornblende and biotite. Contacts with the layered gneiss are sharp or transitional over a few tens of meters and are parallel to layering in the enclosing rocks. Foliation in the quartz monzonite gneiss is generally parallel to the contacts. Nowhere do contact relations suggest that the quartz monzonite is intrusive. In several areas the quartz monzonite contains continuous layers of dark hornblende amphibolite a few meters to several hundred meters thick. Some of these can be traced for as much as 5 km. They are parallel to foliation in the quartz monzonite gneiss and to layering in the enclosing layered gneisses. They are similar in composition to the amphibolite layers in the layered gneisses (table 2).
The biotite granodiorite gneiss is a medium- to dark gray strongly foliated rock. Commonly, it displays ovoid augen of gray potassium feldspar as much as 3 cm long. In many places it is rudely layered. Layering and foliation are parallel to layering in the enclosing rocks, and contacts with the layered gneiss are gradational over several hundred meters.
The patterns of mineral lineations and of the axes of minor folds in the granitic gneisses are similar to those in the layered gneisses, which shows that all these rocks were deformed during at least the last episode of folding and regional metamorphism (Reed, 1963).
Coarse-grained nonlayered weakly foliated hornblende-plagioclase gneiss comprises the bulk of the exposed Precambrian rocks in the Teton Range southeast of the Open Canyon fault (pls. 1, 3). Similar gneiss also occurs as thin layers and small pods in the layered gneisses at least as far north as Cascade Creek.
The rock consists of irregular equidimensional blotches of dark-green hornblende 1.2-5 cm across set in a matrix of milky-white altered plagioclase. Hornblende comprises 20-40 percent of the rock; plagioclase, 40-80 percent; quartz, 0-5 percent; and epidote, 0-10 percent. Magnetite and ilmenite are absent. The mineralogy and texture suggest that the rock may be a metamorphosed diorite.
The high peaks of the Teton Range between Buck Mountain and Cascade Creek are carved in an irregular mass of fine-grained light-colored granodiorite and coarse-grained muscovite and biotite pegmatite that extends northward into Leigh Canyon. Large bodies of similar rocks also occur north of Moran Canyon and near the head of Death Canyon. All the enclosing gneisses are laced with networks of discordant dikes of granodiorite and pegmatite ranging in thickness from several centimeters to more than 30 m. These dikes and small irregular intrusive bodies comprise as much as half of the country rock near the large intrusive masses; but they decrease in abundance away from these masses, and south of Granite Canyon and north of Webb Canyon only a few small dikes are present.
The intrusive bodies contain abundant tabular inclusions of the wallrocks. Contacts of the dikes and of the individual inclusions are knife sharp, but it is difficult to map the contacts of the larger granodiorite and pegmatite bodies because of their complex geometry and the transition between intrusive rocks containing abundant large inclusions and country rocks cut by networks of anastomosing dikes. The dikes cut cleanly across all structures in the enclosing rocks, and inclusions in the intrusive rocks have minor folds and mineral lineations that must have formed before the granodiorite and pegmatite were emplaced. These relations show that the granitic rocks invaded the surrounding gneisses after the last episode of folding and regional metamorphism.
The average modal composition of the granodiorite is given in table 4. No modal analyses of the pegmatite are available, but its bulk composition is probably similar to that of the granodiorite.
TABLE 4.Average mode, in volume percent, of granodiorite in the Teton Range1
Scattered dikes of dark-green to black diabase and basalt cut all the other Precambrian rocks in the Teton Range. The dikes trend N. 70°-85° W. and are nearly vertical. The largest of these dikes is 40-50 m thick, and has been traced for more than 11 km from near Leigh Lake to the head of Moran Canyon. The other dikes range in thickness from 1 to 20 m.
The dike rocks are composed principally of plagioclase and pyroxene. Some are holocrystalline and have ophitic or subophitic textures; others consist of phenocrysts set in a dark microcrystalline matrix. Some sensibly attract a hand magnet. No modal analyses are available, but chemical analyses suggest that the dike rocks are typical tholeitic basalts.
The dikes are clearly Precambrian, for they are unconformably overlain by Flathead Sandstone of Cambrian age on the summit of Mount Moran and on peaks on the ridge north of Moran Canyon. Post-Cambrian fault movement along some of the dikes in the south fork of Cascade Creek has sheared the dike rocks and displaced the overlying Paleozoic rocks.
Last Updated: 14-Jul-2009