Survey Notes

GeoSights: Stretched-Pebble Conglomerate in the Raft River Mountains, Box Elder County

by Jackson Smith and Rachel Willmore

Stretched-pebble conglomerate of the Elba quartzite.

Tucked in the northwestern corner of Utah in the Raft River Mountains, a stretched-pebble conglomerate found within the late Precambrian-age Elba Quartzite preserves an epic period of tectonic strain. Tectonic strain is the physical change a rock undergoes in response to tectonic forces. Stretched-pebble conglomerates are created when high heat and pressure transform originally spherical cobbles and pebbles into elongated, cigar-like, or disk-like shapes.

The key to understanding the creation of the Elba Quartzite stretched-pebble conglomerate is in the formation of the distinctive Raft River Mountains. This mountain range is one of only two ranges in Utah that trends east to west, and is part of the larger Raft River-Albion-Grouse Creek metamorphic core complex (see Survey Notes v. 44, no. 1). A metamorphic core complex forms when Earth’s crust rapidly extends and stretches enough that shallow layers of rock fracture and slide apart along large, low-angle normal faults, partially uncovering deeper rocks. Once unburdened by the weight of the upper layers, the underlying crust rebounds upwards into a mountainous dome. This tectonic process allows us to see rocks that have endured a history of enormous deformation from pressure and strain.

The Great Unconformity, a 2-billion-year time gap, separates the stretched-pebble conglomerate of the Elba Quartzite and the older schist of the Green Creek Complex.

In the Raft River Mountains, the middle-Precambrian-age Green Creek Complex* forms the foundation (“basement”) of the continental crust and is broadly divided into three main groups: the informal “older schist,” a suite of metamorphosed igneous rocks, and a banded granite. This basement complex comprises the oldest rocks in Utah—the “older schist” is roughly 2.6 billion years old (see Survey Notes, v. 54, no. 3).

The Elba Quartzite is much younger, around 700 million years old, and rests directly on top of the older schist. The contact between these two rock formations represents a massive 2-billion-year-long time gap in the geologic record! The missing rock record, known as the Great Unconformity, represents an episode of prolonged global erosion with pauses in deposition that were likely caused by the assembly and subsequent breakup of a supercontinent called Rodinia (see Survey Notes, v. 53, no. 3).

Around 700 million years ago as Rodinia was splitting apart, the crust in what would become northwestern Utah began to subside. Fed by intense erosion from rivers, this new basin filled with rounded quartz gravel and sand. With burial, this jumbled mixture of rocks was cemented together into the parent rock of the Elba Quartzite’s stretched-pebble conglomerate. As the basin sank further and sea level rose, the environment transitioned into a shallow marine shelf, further burying the gravel beneath layers of quartz sand.

For roughly 450 million years from the time that Rodinia split apart until the early Triassic Period, Utah was covered by oceans that deposited thick sequences of marine sediments over the former supercontinent’s coastal sand and pebble conglomerate. Then, western North America had a major shift in tectonics. Instead of warm shallow seas, volcanoes arose and mountain ranges formed. During the Sevier and Laramide mountain building events (between ~160 and 50 million years ago), the western edge of North America was compressed—rock layers were thrust to the east, stacked onto each other, and folded, creating mountain ranges at least 30 miles (50 kilometers) thick. The Elba Quartzite’s coastal sandstone and conglomerate were buried further—to as deep as 22 miles (35 kilometers), where intense heat and pressure metamorphosed the sedimentary rocks into quartzite.

Formation of a Metamorphic Core Complex

This episode of compression and mountain building eventually gave way to extension around 17 million years ago, and this area of overthickened crust began to relax and pull apart, essentially collapsing under its own weight. As this happened, the deeply buried rocks were subjected to brutal heat and strain from nearby plumes of rising magma that helped soften the rocks so that they deformed plastically. As the area extended along low-angle faults, an unroofing (removal) of the upper crust allowed the hot, softened basement rocks to rapidly rebound upwards, causing the Elba Quartzite’s quartz pebbles to stretch like taffy and shear at a microscopic level. The crustal rebound formed the mountainous dome (metamorphic core complex) that we now call the Raft River Mountains. In geological terms, this event was remarkably fast, taking roughly only six million years. At its most active, the Raft River detachment fault slipped at about 7 millimeters per year, compared to around 1–2 millimeters per year across the Wasatch fault’s five central segments.

Now that the supercontinent Rodinia’s coastline pebbles and sands have been exposed as metamorphosed rock, the fierce physical trauma on the Elba Quartzite is very apparent. The reason the quartz pebbles stretched instead of breaking apart is because the surrounding matrix (the finer-grained material surrounding the pebbles) contains the mineral muscovite. While quartz is naturally a very hard and stubborn mineral, muscovite is made of thin, flaky sheets that easily slide past each other, much like a deck of cards. By providing a slippery matrix, the muscovite acted almost like a lubricant, allowing the quartzite and pebble conglomerate to flow and stretch. Therefore, the Elba Quartzite was able to absorb the brunt of the tectonic strain affecting the area, accommodating massive deformation while the multi-billion-year-old schist beneath it was virtually unaffected.

The older schist beneath these stretched pebbles is intriguing. The physical juxtaposition of these two formations separated by eons is a stunning reminder of the immensity of geologic time. The highly deformed rocks spanning billions of years of earth’s history exposed in the Raft River Mountains are an incredible monument to the awesome forces that have shaped our very old world, and what is, for the time being, the world beneath our feet.

* Note: The word complex is used here to describe a chaotic group of different rocks that are intricately mixed together. In a metamorphic core complex, the word refers to the structural system.