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The Geology Of
Pagosa Springs

The Geology of Pagosa Springs

How The Hot Springs Are Formed

 


There are five geological requirements for a hot spring.

RECHARGE

Springs is a dominant discharge area for the headwaters of the San Juan Mountains. Flowing from the west, north, and east, streams and creeks feed into the San Juan River and recharge the aquifer.

There are two major rock formations in the Pagosa Springs area: Mancos Shale, which is the more recent, and lying beneath that the Dakota Sandstone. The Dakota Sandstone has been lifted up and tilted, actually being exposed to the surface west and southwest of Pagosa Springs.

Water movement within the tilted strata flows down-gradient to Pagosa Springs and recharges the aquifer. Being highly porous, the sandstone acts like a sponge and becomes an area of subsurface water storage.

AQUIFER

The aquifer that feeds the Pagosa Hot Springs lies in the Mancos Shale and Dakota Sandstone formations.  The Dakota Sandstone yields the hottest subsurface temperatures, averaging 160 degrees.

The hot water finds its way to the surface through extensive interconnecting vertical faults.

CAP ROCK

A layer of impermeable strata forms a natural lid that seals in heat and prevents evaporation. The cap rock for the Pagosa Hot Springs is the Mancos Shale.

HEAT SOURCE

On a world-wide scale, temperatures rise with increased depth as the earth releases its internal heat.  Localized increases of subsurface termeratures are associated with magma chambers that exist in volcanic fields.  These fields can be either active or dormant as it takes hundreds of thousands of years for this deep-seated magma to cool.

Faulting, which is associated with tectonic activity, can be a heat source also, due to the compressional or tensional forces that occur.

Locally, the heat source has not been definitely defined.  It will take additional research of the area to do so.

BEDROCK

The layers of sedimentary strata that lie between the heat source and the aquifer act as a heat conductor.

Pagosa Springs is located within the San Juan Basin and is bound to the north and east by the San Juan Mountains, which are sedimentary and volcanic in origin.

During the Jurassic and Cretaceous periods much of the western United States was covered by a shallow inland sea.  The rise and fall of the sea deposited sediments of alternating clay, sand, silt, and gravel.  The sediments were compacted and cemented to form the rock strata of the area.

After the deposition of marine sediments, volcanic activity deposited 15 layers of lava and ash over the San Juan Mountains for 20 million years.

Crustal movement has caused folding and faulting of the Earth’s crust.  Pagosa Springs lies on the eastern edge of the Archuleta Arch, which is a regional tectonic fold.  The uplift of the Arch has caused sedimentary strata to break and slip, creating faults and tilting the strata with a dip to the NNE.

Two smaller folds, the Sunetha and Stinking Springs Anticlines, locally altered the regional dip.  Smaller faults associated with the Nacimiento Fault are a source of migration for the hot mineral water from the aquifer to the surface.