Out & About with the Field Team

Posted June 27, 2025

Geologically Speaking

by Odell Ward, P.G.

Throughout my time with the district, I’ve encountered various terms used to refer to the Dockum. Let’s take a moment to clarify some of these.

Depending on the perspective, various terms are used to refer to the Dockum. Water users and drillers often call it the Dockum Aquifer, while geologists and hydrogeologists refer to it as the Dockum Group. Some individuals even designate the lower formation as the Santa Rosa Aquifer. The Dockum Group represents a collection of interconnected material layers deposited during the Triassic Period, approximately 252 to 201 million years ago (Texas Water Development Board, Report 359, 2003). During this era, all of Earth’s land masses merged into a single supercontinent known as Pangaea. This colossal landmass was subsequently divided into two regions: Laurasia, located north of the equator, and Gondwana, found to the south. The climate at this time was predominantly hot and arid, resembling a desert in the interior, while subtropical conditions prevailed along the coast.

The sediments of the Dockum Group developed from the accumulation of materials in various environments, including fluvial (rivers and streams), deltaic (river delta), and lacustrine (lake) settings, all within a closed continental basin.

Key Findings

  • Paleocurrent Analysis: This analysis indicates that the sediments were primarily of fluvial origin.
  • Depositional Environments:
  • Siltstones and mudstones were laid down on:
  • Floodplains
  • Interfluves
  • Isolated pond areas

The sediments in the basin were sourced from the erosion of Paleozoic sedimentary rocks that were exposed around the edges of the basin.

The Dockum Group consists of four interconnected formations. While all members of the Dockum are water-bearing, the quality of this water typically falls below EPA primary drinking water standards, ranging from poor to brackish or slightly saline.

Santa Rosa Formation

  • The Santa Rosa Formation is the oldest and lowest layer of the Dockum Group.
  • It features alluvial sandstones that range from red to brownish hues, along with conglomerates, which are a mix of coarse and fine-grained materials.

Tecovas Formation

  • The Tecovas Formation sits above the Santa Rosa Formation.
  • It is characterized by sandy mudstones interspersed with fine to medium-grained sandstones.

Together, these two formations constitute the lower section of the Dockum Group.

Upper Dockum Formations

  • The Trujillo Formation and the Cooper Canyon Formation make up the upper portion of the Dockum Group.
  • The Trujillo Formation comprises multi-colored fine to coarse-grained sandstones and sandy conglomerates.
  • The Cooper Canyon Formation is the uppermost layer of the Dockum Group, predominantly consisting of mud and siltstones, with sandstone lenses. It is noted for typically having the highest quality within the group.

Regardless of the terminology you choose, it’s crucial to safeguard and conserve this vital natural resource, just like the Ogallala Formation.

Texas Water Development Board, Report 380

Palo Duro Canyon State Park. Formation Outcrops