Mineral Fork Tillite
Age: Late Proterozoic Cryogenian Period (720-635 million years ago)
Location: Northern Utah along the Wasatch Front.
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     During the Late Proterozoic, the deposition of large volumes of carbonate rocks around the world might have reduced the amount of atmospheric Carbon.  This may have been the trigger that resulted in the greatest known ice ages in the history of the Earth.  Geologists refer to this long, intense period of global glaciation as the “Cryogenian”.  It is believed that during the Cryogenian, the polar ice sheets extended to, or at least very near to, the equator.  Geologic evidence for such an event can be found on every continent.  Some of that evidence is right here in Utah and exists as a type of glacial debris called “tillite”. Tillite is a rock that was deposited as glacial “till”.

     Glacial till is broken rock material that is entrained (picked un, included, moved along) within the ice of a glacier and then deposited at the melting terminus (end) of the ice. It typically consists of poorly sorted rubble of boulders, pebbles, sand, silt, clay, etc., which is gathered by the erosive power of the moving ice. It’s deposited, at the terminus of the glacier, as an unsorted, chaotic pile of rock fragments that are rounded, angular, and everything in between.


     The Mineral Fork Tillite is a different sort of till. It was not deposited on land but rather, in a marine environment. Glaciers on the continent moved out onto the sea, melting and calving as they interact with the warmer ocean waters.   This ice was still carrying a significant amount of sediment that had been plucked up on land.  As it melted, particles of rock matter embedded in the ice drop to the sea floor. Many of the larger, elongated rock fragments become hydrodynamically aligned into a vertical orientation as they fell through the water and, upon impacting the silty sea floor sediments, embed themselves penetrating and deforming the bedding of the sediments. Such rocks are referred to as “dropstones” and the Mineral Fork Tillite is rich with them.


     During the Cryogenian, What was later to become Utah was situated on the northwest of the paleocontinent “Laurasia”.  Paleomagnetic data puts Utah pretty close to the Earth’s equator.  This means that there were active glaciers at sea-level, at the equator.  This is strong evidence that the planet was either completely, or nearly completely covered in a crust of ice with a liquid sub-crustal ocean.  The Earth may have looked very much like Jupiter’s moon Europa or perhaps Saturn's moon Enceladus.


     There is uncertainty about what caused the warming of the planet which brought the Cryogenian Period to an end.  There is some evidence that the evolution of animal life in the oceans might have caused an increase of atmospheric Carbon dioxide.  About 650 million years ago sea sponges began to appear around seafloor hydrothermal vents.  These were among the earliest forms of animal life on Earth.  It appears that fluctuations in atmospheric Carbon might have both caused the Cryogenian freeze as well as caused it to end.  These are important lessons for us today.


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Outcrop of Mineral Fork Tillite at the mouth of Rock Canyon in Provo, UT

Outcrop of Mineral Fork Tillite in Provo Rock Canyon clearly showing large boulders as well as smaller dropstones embedded in marine mudstone.

Detail of Mineral Fork Tillite showing how a small dropstone deformed the bedding of the mud when it impacted. Small Swiss Army knife for scale

Cobble of Mineral Fork Tillite showing smaller grain sizes. Size 13 boot for scale.

Thick outcrop of Mineral Fork Tillite looking to the southeast toward Brigham City, Utah

Modern sea sponges, of the phylum Porifera, are very similar to ancient sponges.

Jupiter's ice-covered moon Europa