Dry tributary under Congressman Bridge (Natural Bridges National Monument, Utah, USA) 1

            Dry tributary under Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin Natural Bridge) ), Natural Bridges National Monument, Utah, USA. (looking upstream to the ~NNE)

Natural Bridges National Monument is located in San Juan County in southeastern Utah.  It has the highest concentration of natural bridges on Earth.  A natural bridge is a rare weathering-erosional feature that consists of a rock arch over a river or stream (sometimes the stream is dry).  The park has three intact natural bridges in close proximity and erosional remnants of former natural bridges.  Sites for potential natural bridges in the future are also present.

Almost 100% of the surface bedrock in the park is Cedar Mesa Sandstone, which is about 1200 feet thick in this area.  All of the rocks surrounding the park’s three natural bridges are part of this stratigraphic unit.  The Cedar Mesa Sandstone (Wolfcampian Series, lower Lower Permian) consists of light-colored quartzose sandstones.  Almost all of the sand grains are composed of quartz (SiO2), with a small percentage composed of K-feldspar (KAlSi3O8) or marine fossil fragments.

The Cedar Mesa Sandstone has abundant, large-scale cross-bedding, formed by unidirectional wind currents (eolian).  The types of cross-bedding here are mostly tangential cross-bedding and trough cross-bedding, plus some tabular cross-bedding.  Soft-sediment deformation structures are occasionally present.

This cross-bedded quartzose sandstone lithofacies represents deposition in an ancient, large sand dune field (such extensive desert sand dune fields have been called ergs).  Sand dune deposits consist of wind-blown sand grains deposited in a variety of specific ways.  Some are deposited by saltation (wind-blown jumping) on the windward side of the dune.  Some are deposited as grainfalls from the air on the leeward side of the dune.  Some are deposited in small sediment slides (sand avalanches) on the leeward side of the dune.  Slumping of sand masses forms soft-sediment deformation.
Some interbeds of horizontally-bedded mudshales, siltstones, and fine-grained sandstones occur in Cedar Mesa Sandstone.  These represent interdune deposits - many of them have fossil root structures.

Regional studies have shown that the Cedar Mesa Sandstone dune field, during the Permian, was located near the Uncompahgre Uplift, an ancient mountain range.  Those mountains were, at times, bordered by the ocean.  The portions of the ocean proximal to the Uncompahgre Uplift received siliciclastic sediments derived from erosion of the mountains.  At times of lower sea level, nearshore, shallow marine sand deposits became exposed and were subject to wind transport.  Cross-bedding geometries in the Cedar Mesa Sandstone indicate that winds were predominantly blowing from the northwest (modern coordinates).

The normally-dry creek bed shown above occurs directly underneath Congressman Bridge.  The creek is mostly floored by hard sandstone, was formerly part of Tuwa Canyon, which now intersects Armstrong Canyon upstream from here.  Flowing water occurs here during storms and runs underneath Congressman Bridge.  The dark-stained sandstone shown below shows water flow paths under the natural bridge.

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Taken on: December 19, 2012

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Sets appears in: • Utah Rocks, Minerals, Fossils • Natural Bridges National Monument (Utah, USA)

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