The first mountain-building episode
Near the close of the Mesozoic Era the earth was subjected to a series of intense crustal disturbances that geologists call the Laramide orogeny (orogeny means mountain-building). The origin and nature of the forces that bent and cracked the crust are unknown, but current theories being developed about sea-floor spreading and continental drift may shed light on this major upheaval that began about 75 million years ago. A significant effect of the Laramide orogeny was the uplift and contortion of many of the mountain ranges within what we today call the Rocky Mountains.
At the onset of the crustal disturbance, the gently rolling landscape of the Yellowstone region began to warp and flex into large upfolds (anticlines) and downfolds (synclines) ([fig. 13]). Gradually the mountain-building pressures increased, finally reaching such magnitude that the limbs of the folds could bend and stretch no further; thereupon, the rock layers broke and were shoved over one another along extensive reverse faults. The severely crumpled rocks within the Park area can now be seen only along the north edge and in the south-central part along the Snake River. In both places, the folds and faults are especially well displayed by the layered Paleozoic and Mesozoic sedimentary formations ([fig. 9]).
One of the most prominent Laramide structural features is a large anticline in the north-central and northeastern parts of the Park ([fig. 14], section B-B′); the road from Mammoth to the Northeast Entrance crosses much of this feature ([pl. 1]). Although originally forming a high mountain mass, the anticline has been eroded so extensively that it no longer appears mountainous ([fig. 18]). It displays a broad core of Precambrian gneisses and schists and is bounded along its southwest margin by a large reverse fault. Along the fault, the ancient gneisses and schists have been shoved over rocks as young as Late Cretaceous, a movement amounting to 10,000 feet or more. The Cretaceous rocks are those that are now exposed at Mount Everts ([fig. 10]).
COMMON KINDS OF GEOLOGIC STRUCTURES produced by deformation of the earth’s crust. An original horizontal rock layer may be upfolded into anticlines, downfolded into synclines, and broken by either reverse or normal faults. A fault is a fracture or a zone of fractures within the earth’s crust along which movement has taken place. A reverse fault is one generally produced by compression (squeezing together), and the hanging-wall block has moved up with respect to the footwall block. A normal fault is one generally produced by tension (pulling apart), and the hanging-wall block has moved down with respect to the footwall block. All these kinds of structures are present in Yellowstone National Park. (Fig. 13)
HORIZONTAL (undeformed) REVERSE FAULT (compressional) Hanging wall Footwall Forces ANTICLINE (upfold) Crest Limb NORMAL FAULT (tensional) Footwall Hanging wall Forces SYNCLINE (downfold) Limb Trough
During the Laramide orogeny, many folds and faults formed in the northwestern part of the Park, in the area now occupied by the Gallatin Range ([fig. 14], section A-A′). In south-central Yellowstone, the Paleozoic and Mesozoic sedimentary rocks were tightly folded into three anticlines separated from one another by synclines and faults ([fig. 14], section C-C′). Movement along one reverse fault in this area was locally more than 10,000 feet.
As the lands were uplifted and contorted, they came under vigorous attack by the ever-present agents of erosion. Tremendous quantities of rock were stripped from the highlands, and the debris was carried by streams into the adjacent lowland basins and deposited mostly as sand and gravel. As the highlands continued to rise, the basins continued to sink, and in a short period of time great thicknesses of basin-fill sediments accumulated locally. One such deposit, the Harebell Formation of latest Cretaceous age in south-central Yellowstone ([fig. 5]), is more than 8,000 feet thick.
Other similar anticlines, synclines, and reverse faults no doubt extend far into the interior of Yellowstone National Park, and perhaps entirely across it in places, but they lie buried beneath a thick capping of volcanic rocks. Nevertheless, it seems safe to conclude that none of the Park area escaped the effects of the great forces of the Laramide orogeny. These forces, regardless of how they originated deep within the earth, seem to have been compressional ([fig. 13]), pushing the upper layers of the earth’s crust from the east and northeast toward the west and southwest. This interpretation is based on the style of the structural features just described, which shows that the steep limbs of folds, as well as the direction of movements along reverse faults, point toward the west or southwest ([fig. 14]).
By early Eocene time, about 20 million years after they had begun, the deformational forces relaxed. But the effects of the giant earth movements were to last for a very long time. Crustal disturbances of such magnitude commonly produce conditions deep within the earth which, in places, gives rise to intense volcanic activity; one such place was Yellowstone.
CROSS SECTIONS SHOWING GEOLOGIC STRUCTURES in Yellowstone National Park. These illustrate the possible rock relationships that might be seen along the faces of vertical slices of the earth’s crust, if it could be cut and pulled apart (much like slicing a cake and looking at the different layers). The locations of the sections are shown on the geologic map, [plate 1]. Reverse faults and most folds originated during the Larimide orogeny, and normal faults originated chiefly during Pliocene and later times. The arrows indicate the relative movements of fault blocks. Geologic symbols: Qs, Quaternary superficial deposits; Qb, Quaternary basalt flows; Qy, Quaternary Yellowstone tuff; Tav, Tertiary Absaroka volcanic rocks; Mzr, Mesozoic sedimentary rocks; Pzr, Paleozoic sedimentary rocks; pCr, Precambrian metamorphic (“basement”) rocks. (Based partly on information supplied by E. T. Ruppel and J. D. Love.) (Fig. 14)