Sandstone and carbonate-rock aquifers

Science Center Objects

In scattered places in the United States, carbonate rocks are interbedded with almost equal amounts of water-yielding sandstone. In most places where these two rock types are interbedded, the carbonate rocks yield much more water than the sandstone.

Photograph of Edwards Plateau in Crockett County, Texas

The flat to gently rolling plains of the Edwards Plateau are locally interrupted by steep canyon walls. This view in western Crockett County, Texas, is toward the southwest and the Pecos River Valley. The road cut exposes the Fort Lancaster Formation.

In scattered places in the United States, carbonate rocks are interbedded with almost equal amounts of water-yielding sandstone. In most places where these two rock types are interbedded, the carbonate rocks yield much more water than the sandstone.

Most carbonate rocks originate as sedimentary deposits in marine environments. Compaction, cementation, and dolomitization processes might act on the deposits as they lithify and greatly change their porosity and permeability. However, the principal postdepositional change in carbonate rocks is the dissolution of part of the rock by circulating, slightly acidic groundwater. Solution openings in carbonate rocks range from small tubes and widened joints to caverns that may be tens of meters wide and hundreds to thousands of meters long. Where they are saturated, carbonate rocks with well-connected networks of solution openings yield large amounts of water to wells that penetrate the openings, although the undissolved rock between the large openings may be almost impermeable.

This map of sandstone and carbonate-rock aquifers in the United States shows the shallowest principal aquifer. In some places, other, sometimes more productive, aquifers underlie those mapped. Only small areas of some aquifers may be shown on the map because they are covered in many places by other aquifers closer to the land surface. In other places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped. Local aquifers are not shown because of the scale of the map. Some aquifers in sedimentary rocks are overlain by confining units, and the aquifers extend into the subsurface beyond the areas shown on the map.

Principal sandstone and carbonate-rock aquifers in the United States

Principal sandstone and carbonate-rock aquifers in the United States.

 

Sandstone and carbonate-rock aquifers include:

Edwards-Trinity aquifer system (Texas and Oklahoma)

Map showing the Edwards-Trinity aquifer system

The three major aquifers that constitute the aquifer system are the Edwards-Trinity, the Edwards, and the Trinity.

 

Valley and Ridge aquifers (eastern U.S.)

Map showing the Valley and Ridge aquifers of the eastern U.S.

The most important aquifers in the Valley and Ridge province are northeast- to east-trending carbonate rocks. Undifferentiated sedimentary-rock aquifers that consist mostly of sandstone and yield moderate volumes of water separate the bodies of carbonate rocks. Coal-bearing beds are prominanat in parts of Pennsylvania and in a local area of southeast Virginia.

 

Mississippian aquifers (central and eastern U.S.)

Map showing the Mississippian aquifers

Generally, thick-bedded limestones and sandstones constitute the Mississippian aquifers. In Kentucky and Tennessee, water in the limestone bedrock moves through bedding planes and fractures enlarged by slightly acidic water.

 

Paleozoic aquifers (northern Great Plains)

Map showing the Paleozoic aquifers

Paleozoic aquifers extend over large areas in the subsurface of Montana, North Dakota, South Dakota, and Wyoming. The aquifers commonly contain highly mineralized water where they are deeply buried.