The Rocks of the Alamo Impact Site

The Alamo Breccia is a sedimentary unit within the Guilmette Formation and is largely made of a sedimentary rock called limestone. These rocks were formed beneath a shallow sea. Limestone is made of a group of minerals called carbonates. The minerals that make up carbonate rocks are typically calcite, aragonite and dolomite. In the rock record, the mineral aragonite is often replaced by either the mineral calcite or the mineral dolomite, which are relatively stable at Earth's surface conditions.

In the Guilmette Formation limestone and dolostone are the main rocks and can be easily distinguished from one another in the field by applying dilute hydrochloric acid. If the rock fizzes, it is generally made up of calcite and can be considered limestone. If it does not fizz, it is likely dolostone.

Breccia A

The Alamo Breccia is a limestone unit that was pulverized by the Devonian bolide. This caused the rocks to fragment into irregular pieces which were later cemented back together. This type of sedimentary rock is called a breccia.

The last kind of rock found in the Guilmette Formation is sandstone. This particular sandstone is made of nearly 100% mature quartz grains and is thus referred to as a quartz arenite. Sandstone is often found as the dominant rock in the uppermost units of the Guilmette Formation.