SOUTHWESTIAN: THE BEARPAW MOUNTAINS and the SWEET GRASS HILLS, MONTANA and the North American Cordilleran mountain building (orogenic) events

GeoNotes Cypress Hills and area

THE BEARPAW MOUNTAINS and the SWEET GRASS HILLS, MONTANA

and the North American Cordilleran mountain building (orogenic) events

Location map. Image: Google Earth

The Cypress Hills were not formed by folding, faulting, or volcanic activity characteristic of the Late Cretaceous-Tertiary (see Fig. 1 geological time scale) orogenic uplifts and magmatic intrusive events that occurred to the south, southwest, and west that formed the Rocky Mountains, Bearpaw Mountains, and the Sweet Grass Hills. The Cypress Hills are simply an erosional sedimentary plateau of an ice age remnant. The first comprehensive study and the puzzling origin of the Cypress Hills began in 1885 by the Geological Survey of Canada (GSC). It’s not clear how they formed, but according to some researchers they may have been carved out in the sutures where the regional continental ice sheets converged or where the Cordilleran and the continental ice sheets met sometime during the many glaciations of the past 2.6 million years.

Figure 1: A generalized geologic time scale. ‘Ma’ is an abbreviation for millions of years. Image from USGS.

The earth’s crust is broken up into crustal tectonic plates that ride on and move with the circulation of slow-moving convection currents within the upper mantle (asthenosphere) and the deeper mantle. This movement creates two scenarios: an abduction zone where the crustal plates move apart, e.g., the mid-Atlantic ridge system, the West Antarctic Rift System, the Red Sea, and the Reo Grande and East Africa rift valleys, and a subduction zone where the plates collide, e.g., the cordilleran mountain regions around the world.

The tectonic events just off North America’s west coast during the Late Cretaceous or 80 million years ago (abbreviated Ma) occurred when an east moving heavier Pacific oceanic crustal plate collided and slid under the lighter west moving North American continental crustal plate creating a subduction zone that resulted in a major orogenic or mountain building event (Fig. 2). This collision caused a crinkling effect in the earth’s crust creating the rise of the western cordilleran mountain chain. The Cordilleran Mountains collectively contain all the western North American mountain ranges including the Rockies, Transverse Ranges, Cascade Range, Bear Paw Mountains, the Sweet Grass Hills, and several other intrusive uplifts in north-central Montana. Around 55-50 Ma (Eocene) and 34-23 Ma (Oligocene) renewed orogenies and volcanism added to the topographical evolution in the west. Since 23 Ma the western cordilleran mountains have been relatively stable with some orogenic activity (Pasadena Orogeny) in the Transverse Ranges of Southern California. Additional activity includes periodic major earthquakes (San Francisco, 1906), and an occasional major volcanic eruption (Mount St Helens, 1980) from dormant island arc volcanoes in the Cascade Range.

Figure 2: Subduction zone created by two converging crustal plates. Creation of the western North American Cordilleran mountains. The volcanic arc is represented by the dormant volcanoes of the Cascade Mountain Range. Image from Geologycafe.com.

The collision created by the subsiding tectonic plate produces immense heat and pressure resulting in various degrees of metamorphism of igneous and sedimentary rocks within the cordilleran mountain ranges. Frictional melting beneath the subduction zone creates magma that fuels volcanoes. Fracturing, fissuring, folding, faulting, volcanism including lava flows, earthquakes, and uplifts are relatively common. Magmatic intrusions move under tremendous pressure along faults, fractures, and into any weak areas in the subsurface stratigraphy causing the formation of plutons. Plutons are subsurface intrusions sills, dykes, volcanic necks, batholiths, stocks, and laccoliths found in Figure 3. Unlike the dykes, batholiths, and stocks that cut across the subsurface stratigraphy, the sills and laccoliths conform with the subsurface stratigraphy. The heat from the plutons bake the sediments they come in contact with creating a contact metamorphic halo in the sedimentary strata surrounding the plutons.

After millions of years of erosion, the surrounding country rock exposes the more erosionally resistant subsurface plutons within the igneous complex to become hills, mountains, and other volcanically interesting topographical features.

Figure 3: Examples of plutons. Unlike dikes, stocks, and batholiths, the sills and laccolith form parallel to the bedding planes. Not shown are stocks that are smaller than batholiths. Image from chegg.com.

The Bearpaw Mountains and the Sweet Grass Hills are part of the 55-50 Ma magmatic intrusive and extrusive igneous complex and uplift that occurred after the major mountain building event that produced the main cordilleran mountain chain. The intrusive activity in the Sweet Grass Hills moved from west to east over a 2 Ma period intruding shales, calcareous siltstones, and sandstones of the Upper Cretaceous Colorado Group.

Today, the Bearpaw Mountains and the Sweet Grass Hills are eroded remnants of that 55-50 Ma magmatic intrusive event. Over time the easily eroded Colorado Group exposed the more resistant igneous bodies of the Bearpaw Mountains, and the laccoliths of the Sweet Grass Hills which, today, form prominent rises in the horizon.

The Bearpaw Mountains are an insular-montane island range that rises to an elevation of 2108 m and is located 21 km south of Havre, MT. The Sweet Grass Hills are just south of the Alberta/US border with the following elevations; Mt Brown or East Butte 1770 m, Gold Butte 1985 m, and West Butte 2128 m. The Bearpaw Mountains are visible from the higher vantage points on the south side of the East, Center, and West Blocks of the Cypress Hills, and the Sweet Grass Hills can be seen from elevated vantage points to the SW and W of the West Block.

The Bearpaw Mountains and Sweet Grass Hills were exposed to the last continental glacier, and the Sweet Grass Hills were nunataks in a 305 m thick ice sheet.

The entire north-central Montana igneous intrusive uplift complex including the Bearpaw Mountains and the Sweet Grass Hills played a major roll in providing the fluvially transported source materials for the stratigraphic sequence making up the Cypress Hills plateau.

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