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2020 - Geology Tour: Glaciel Deposits Near Sperry Wash

Geology Tour:

Glacial Deposits Near Sperry Wash

by Bill Neill

Outside the Shoshone Museum is a geological display with a stratigraphic column showing rock formations of the Amargosa basin.  In the lower part of the stratigraphic column, one sedimentary unit labelled the Kingston Peak Formation is described as “mudstone with pebbles and boulders – ancient glacial deposit . . About 740-635 m.y.” 

The Kingston Peak Formation is sandwiched in the column between two carbonate units:  the Noonday Dolomite above, and the Beck Springs Dolomite below.  Because dolomite (magnesium calcium carbonate – MgCa2CO3) is derived from limestone (calcium carbonate - CaCO3) which normally is deposited in warm shallow tropical marine water, this arrangement raises the question – why was glacial sediment deposited in a tropical setting?

Before considering this question further, we’ll examine the Kingston Peak Formation at its most accessible outcrop, located south of Amargosa Canyon along the Sperry Wash Route, which is a legal OHV trail  through the Kingston Peak Wilderness.  Until 1974, trucks travelled the route carrying talc ore from the Western Talc Mine in the Alexander Hills to Union Pacific tracks at Dunn Siding, near Afton Canyon.

Outcrop of Kingston Peak Formation north of Dumont Dunes, next to Amargosa River flood channel.  Note vehicle on left for scale.

About 2/3 mile beyond the Sperry Wash gate, next to the Amargosa River flood channel, the outcrop of glacial deposit is not especially photogenic,but close inspection shows distinctive features:  a variety of mostly angular rock clasts embedded in a mudstone matrix, with the thin layering of mudstone either depressed by or draped over the clasts.  Geologists interpret these clasts as carried to an offshore marine basin by melting icebergs and dropped into mud that slowly accumulated from the settling of suspended clay and silt particles in quiet water. 

As shown on the Shoshone Museum display, the age of the Kingston Peak Formation is thought to be between 635 million and 740 million years – a time when land plants had not yet evolved, and the most advanced marine animals were soft-bodied like jellyfish.  Glacial deposits of similar age and character have been found elsewhere in the world – in Australia, Norway, India, Namibia – that apparently were deposited at tropical latitudes, as inferred from paleomagnetism and the association with carbonates.

A ”Snowball Earth” or “Slushball Earth” model has been developed to explain these features.  According to this theory, the Earth was heavily glaciated for 100 million years or more, mostly covered by thick ice extending nearly to the equator, at a time when most continental masses were grouped near the equator.  Before and after this prolonged cold period, temperate conditions prevailed and carbonate rocks were deposited below and above the glacial unit.

According to Wikipedia, the period of global cooling “from about 850-630 mya, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere.”  Although photosynthesis by marine algae started about 2 billion years ago, the atmosphere’s oxygen remained low during the first billion years because as oxygen was produced, it was removed by oxidation of dissolved iron in the ocean and iron minerals in exposed rock on the continents.

Eventually, dissolved iron in seawater was removed to form “banded iron formation”, now exposed in places like Michigan and Australia; and by covering much of the continents, the “Snowball Earth” glaciation slowed rock weathering and allowed the atmospheric oxygen content to rise nearly to present levels.  The increased oxygen level in the atmosphere, in turn, allowed larger, more complex animal life to evolve, resulting in the “Cambrian explosion” about 541 million years ago.

This is a complex story to derive from rust-colored rocks exposed along the Sperry Wash route south of Amargosa Canyon, but it’s humbling to recognize that the rock history contributes to explaining how complex life evolved, including us.   ~ Bill