Abstract

Metamorphic and sedimentary rocks of pre-Tertiary age are exposed over an area of about 200 square miles in the northern part of Yellowstone National Park, Wyo. and Mont., mostly in remote and seldom-visited parts of the park along its north boundary. The main outcrops are in the Gallatin Range in the northwest corner of the park, where the entire section of rocks from Precambrian to Late Cretaceous age is exposed; in the area around Mammoth Hot Springs and Mount Everts, where Jurassic and Cretaceous rocks are exposed; and in the north-central and northeastern parts of the park where rocks of Precambrian and Paleozoic age are exposed.

These outcrops consist of crystalline metamorphic rocks of Precambrian age and sedimentary rocks that represent every system except the Silurian. The Precambrian crystalline rocks are mainly granitic gneiss and quartz-biotite schist, but they also include amphibolite and pegmatite. These rocks are overlain by a mostly conformable sequence of sedimentary rocks about 8,000 feet thick, consisting of about 3,000 feet of marine rocks of Paleozoic age, a few hundred feet of Triassic and Jurassic non-marine and marine rocks, and about 4,000 feet, or half the exposed sequence, of marine and nonmarine Cretaceous rocks. The Paleozoic rocks—limestone, dolomite, and, in subordinate amount, clastic rocks—range in age from Middle Cambrian to Permian.

The Mesozoic sedimentary rocks consist of marine deposits of Triassic and Jurassic age—mainly shale, sandstone, and, in subordinate amount, limestone—and overlying nonmarine mainly clastic rocks of later Jurassic and Early Cretaceous age and a thick sequence of marine and nonmarine clastic rocks of later Cretaceous age. Triassic and Jurassic rocks together are only a little more than 5000 feet thick. The 4,000 feet of Cretaceous rock is a monotonous sequence of shale, mudstone, and sandstone, broken by a few beds of conglomerate and limestone, and, in the upper part of the section, by a few beds of coal, carbonaceous sandstone, and shale. The youngest exposed pre-Tertiary sedimentary rocks are sandstone and conglomerate of the Landslide Creek Formation, which consists mainly of volcanic fragments.

The sedimentary rocks in the northwestern part of the park have been intruded by igneous rocks that form stocks, laccoliths, sills, and a few dikes. The oldest of these intrusives is the Mount Holmes stock of fine-grained biotite quartz monzonite, a steep-sided stock that domed its roof of Middle Cambrian sedimentary rocks and intensely hornfelsed both the roof rocks and its own chilled margin. It apparently was emplaced in early Tertiary time.

The laccoliths, sills, and related dikes were emplaced after the Mount Holmes stock, but still in early Tertiary time, and these bodies of biotite rhyodacite porphyry and biotite quartz latite porphyry are the most widespread intrusive masses in the northern part of Yellowstone National Park. The Indian Creek laccolith, emplaced in Cambrian rocks near the south end of the Gallatin Range, is the largest of the group and is about 3 miles in diameter and more than 1,000 feet thick. The Snowshoe and Gray Peak laccoliths, emplaced in Jurassic and Cretaceous rocks in the Gallatin Range, are each about 1 mile in diameter and before erosion were probably each about 1,000 feet thick; they are surrounded by multiple sills that extend out for several miles from the laccolithic core to form an intrusive field 6-8 miles in diameter. The Snowshoe and Gray Peak laccoliths, thus, are the trunk, and the broad connected sills are the branches of a very large Christmastree laccolithic complex. The multiple thin sills and their interconnecting dikes on Electric Peak were emplaced in Cretaceous rocks. The laccoliths and sills were emplaced in shaly horizons. Their ultimate shape was possibly determined by depth—a comparatively simple laccolith if deep, multiple thin sills if shallow, and a combination of these, aChristmastree Laccolith, if intermediate in depth. termediate in depth.

The Electric Peak stock, of Eocene age, is an irregular complex, about 1 mile across, on the east side of Electric Peak; it intrudes Upper Cretaceous sedimentary rocks and comprises several varieties of fine- to medium-grained rocks ranging in composition from calcic granodiorite to near granite, although most of the rock is granodiorite. The stock has long been considered the neck through which the volcanic rocks of Sepulcher Mountain were erupted.

The faults and folds in the pre-Tertiary rocks of northern Yellowstone National Park appear outwardly to have little relation to structural features in surrounding areas, probably because the park is in an unusual setting where several structural provinces come together. The Precambrian crystalline rocks reflect extreme deformation in Precambrian time. The sedimentary rocks are in parallel beds, which imply structural quiet throughout Paleozoic and most of Mesozoic time and to the latest Cretaceous, when they were folded at the beginning of a period of deformation that probably is still going on. The folding formed the broad, poorly defined northwest-trending Gallatin anticline, which was intruded in its axial part by both the Mount Holmes stock and the Indian Creek laccolith. Doming over the laccolith apparently caused the massive upper Paleozoic rocks to slide northward on the gently dipping Crowfoot fault zone.

In late Paleocene and Eocene time, the rocks in the northern part of Yellowstone Park were broken by steep northwest-trending reverse faults, which include (1) the faults of the north-dipping Gardiner fault zone and the associated reverse faults northeast of the Gallatin Range and (2) the south-dipping Grayling Greek fault southwest of the Gallatin Range. The Gallatin block was depressed between these bounding faults. Displacement on the Gardiner fault zone exceeds 10,000 feet, and that on the Grayling Creek fault is uncertain, but it probably exceeds 1,000 feet. The Gardiner fault zone is the southwest boundary of the uplifted Beartooth block, which is the dominant structural feature in the north-central and northeastern parts of the park.

In later Tertiary time, pre-Tertiary and younger rocks in the north-central and northeastern parts of the park were broken by northwest-trending normal faults, which now define the Lamar Valley and the south front of the Buffalo Plateau. Still later, perhaps in middle or late Pliocene time, the reverse faults and northwest-trending normal faults were broken by north-trending near-vertical normal faults, some of which have displacements of thousands of feet. These faults bound the Gallatin Range and are represented in the north-central part of the park by the Buffalo Creek faults, and they probably control many of the thermal areas in the park. They may be reflected through younger volcanic rocks elsewhere in the park by a striking series of north-trending lineaments. The east and West Gallatin faults, major faults in the north-trending group, perhaps are continuous with somewhat similar faults south of Yellowstone National Park; north of the park they appear to merge into the northeast-trending Emigrant fault. The present Gallatin Range is an uplifted block much like the Teton Range and probably is genetically related to it. The structural features in the northern part of the park, however, do not appear to be related to most structural features in adjacent areas in Montana, and the structural pattern in and near the northern part of the park appears to reflect intersection and termination of the Snake River downwarp against the older, reverse-fault-bounded blocks of the Middle Rocky Mountains tectonic province.