OTR Field Trip – August 25, 2012

Title: Medicine Bow Mountains

Leader: Art Snoke

Description:  Published in the August 2012 Outcrop

More than two and a half billion years of geologic history are recorded in the rocks of the Medicine Bow Mountains. From an ancient continent and stromatolites to ocean basins and volcanic island chains, the Medicine Bow Mountains are a geologist’s dream to explore. This trip will specifically visit the Snowy Range, which consists of the magnificent quartzite peaks visible from the scenic byway. The Laramide uplift exposed this brilliant white rock with the highest peak, Medicine Bow Peak at an elevation of 12,013 feet.

Beyond the Laramide orogeny, the Medicine Bows have added structural complexity due to the Cheyenne Belt, a suture zone that runs through the mountains southwestnortheast. The metamorphosed rocks to the south of the belt were deposited in an ocean basin 2.0 to 1.8 billion years ago and then were intruded by granite plutons and amfic complexes until 1.4 billion years ago. To the north, a vastly different set of rocks are preserved with an Archean core overlain by metasedimentary and volcanic rocks 2.5 to 1.7 billion years old. These Archean and younger rocks are over 42,000 feet thick in some areas.

Paleozoic time was dominated by regressions and transgressions of numerous seas over this area of Wyoming. The region was structurally quiescent until the Pennsylvanian when uplift slightly to the west of the current Medicine Bows created the Ancestral Rocky Mountains. Erosion reduced this mountain to chain to low relief highlands into the Permian as oceans returned to the area.

The boundary between the Paleozoic Era and Mesozoic Era cannot be defined in the rock succession due to lack of fossils in the record. Things changed however in the Mesozoic Era with deposition of thousands of feet of sediment consisting of sands, silts, volcanic and even coal. Most of this deposition occurred during the Cretaceous due to the vast sea covering the land. By the end of the Cretaceous, the compressional forces of the Laramide orogeny likely produced islands in this seaway allowing erosive forces to remove the Paleozoic and Mesozoic sections exposing the Precambrian basement in the central part of the Medicine Bows. This uplift continued until the end of the Paleocene.

The current scenery of the Medicine Bow Mountains is dominated by the carving of glaciers that receded approximately 15,000 years ago. Shallow glacial basins are now filled with water throughout the alpine environment of the Snowy Range, adding to the magnificence of the  white quartzite cliffs.

Since the glaciers retreated, the appearance of the mountains have changed little except for small traces of mineral exploration that has occurred over the past 100 years. The initial gold rush brought hundreds to the mountains exploring stream bed and digging for the metal; few were successful. The early 1900s brought slightly more successful prospectors for platinum and palladium followed by copper and in the 1970s, uranium. Fluctuating prices caused prospectors to come and go with the last rush occurring in 1977 for placer diamonds, which were few and far between. Traces of mines and tailings can be seen in the area.