Lake Meredith's Geology: Red Beds, Alibates Flint and a Valley Sunk by Dissolving Salt

On the mostly flat High Plains of Texas, Lake Meredith National Recreation Area and neighboring Alibates Flint Quarries National Monument sit in an unexpected piece of canyon country. Red hillsides strewn with white boulders drop to the water, side canyons cut into red rock, and seams of colorful Alibates flint run through the cliffs — one of the most scenic corners of the Texas Panhandle.

The Canadian Breaks

The Canadian River valley — the Canadian Breaks — is a rolling country of low mesas, buttes and hills that crosses the Panhandle and splits the High Plains into the Central High Plains to the north and the Southern High Plains to the south.

Its rim is the Caprock Escarpment, where the flat plains give way to canyons. The escarpment is a long line of eroding cliffs, capped by a hard zone of caliche up to 30 feet (9 m) thick in the upper Ogallala Formation.

Buried mountains

The park stands on the Amarillo uplift, a buried mountain range that rose about 300 million years ago, between two deep basins. These old structures controlled where the later Permian and Triassic rocks were laid down.

The rocks, oldest to youngest

Permian red beds. When they formed, the Panhandle lay near the equator in the supercontinent Pangaea, at the edge of an arm of the sea reaching up from the south. Some layers settled on coastal plains, some in marine basins, some on tidal flats, with huge amounts of salt and other evaporite minerals — thousands of feet thick underground, thickest to the south. Small amounts of iron oxides and clays color them red to orange. They are soft, so true outcrops are rare; they weather into the rounded hills that make up most of the landscape. Here they were mapped as one unit covering the Whitehorse Sandstone, Cloud Chief Gypsum and Quartermaster Formation, which can be told apart only by their order — and the marker beds that would show it are missing at Lake Meredith.

Alibates Dolomite. One of the few layers in the red beds that is easy to pick out: upper and lower dolomite with red bed between, though erosion has stripped the upper dolomite in many places. The dolomite formed in very salty water, on mud flats; some of it preserves algal mats. Harder than the red beds, it caps most of the mesas and buttes around the lake.

Triassic rocks. The Tecovas and Trujillo have been matched to the Chinle Formation at Petrified Forest National Park in Arizona and across the Southwest.

Ogallala Formation. Much younger than the rocks beneath, it lies on an unconformity — a gap in the record — and is made of debris shed from mountains to the west. Its top is caliche, a calcium carbonate hardpan that forms in dry climates.

Alibates flint

Flint — also called chert — is silica in crystals too small to see. At Alibates it occurs as lenses and nodules in the dolomite, formed long after the dolomite itself, when silica replaced the rock; trace elements give it its range of colors. American Indians prized it for stone tools for thousands of years, digging more than 700 shallow pits for unbroken, unweathered stone. The dolomite also served as a surface for petroglyphs.

Alibates Dolomite, a distinctive layer in the Permian red beds, at Alibates Flint Quarries National Monument. Photo from the National Park Service

How a small river carved a big valley

The Canadian River runs 906 miles (1,458 km), from the Sangre de Cristo Mountains in southern Colorado to the Arkansas River in eastern Oklahoma. After the Ogallala was laid down, it cut through the caprock within the last few million years, carving a valley up to 40 miles (64 km) wide and 1,000 feet (300 m) deep.

That is far too big a valley for so modest a river — and the reason lies underground. The river follows, at least in part, a broad zone where the ground has sunk as buried salt dissolved. Groundwater dissolves salt layers, and the rock above collapses; in that zone the base of the Ogallala has dropped 300–600 feet (100–200 m) relative to the rest of the High Plains. Most of the dissolving probably happened in the wetter Pleistocene, but it continues today — the river runs salty, and the region has sinkholes.

Karst and rock shelters

Most of the park is evaporite karst: salt (halite) and gypsum in the red beds dissolve, and the layers above fall in.

• Collapse chimneys — round or oval columns filled with fallen debris — are a signature feature. Most are buried and seen only in road cuts or bluffs, but some stand out, because the breccia filling them resists erosion better than the red beds around it.
• Alcoves, or rock shelters, form where soft layers wear back under a harder one. They are not caves and weren't dissolved out, but federal law protects them as cave-like features under the Federal Cave Resources Protection Act of 1988.

Fossils

The red beds have yielded no fossils, but the park has plenty of others:

Park fossils are protected by the Paleontological Resources Preservation Act of 2009.

Hazards

• Falling rock and slumping. Rockfall, creep, slumps and topples happen on the red-bed slopes, especially below caps of Alibates Dolomite; the white boulders scattered down the red hillsides fell from the rims and can still move. Most slumps are on the north side of the lake, on south-facing slopes; at high water, waves are the main trigger.
• Flash floods in the many side streams.
• Earthquakes — a low hazard. The USGS 2014 Seismic Hazard Map gives a 2% chance in 50 years of natural shaking exceeding 10–14% of gravity in peak ground acceleration — roughly VI on the Modified Mercalli scale. The Panhandle can expect 4 to 10 damaging shakings in 10,000 years.

Managing the land

• Oil and gas. Permian and Pennsylvanian rocks hold oil and gas, and producing wells stand in and around the park, managed under an oil and gas management plan.
• Wind. The Panhandle wind never stops. In drought, when the lake is low, large areas of lakebed lie open to it; off-road vehicle areas and oil and gas sites are vulnerable too, because traffic and new roads strip the plants and vehicles stir up dust.
• Water. The Canadian River Municipal Water Authority runs the dam and supplies 11 municipalities; the lake level rises and falls with their demand, rainfall and releases from upstream reservoirs. The Park Service provides the public's recreation on the water and the land beyond it.
• Old gravel pits. Two abandoned mineral land sites, both gravel pits from building Sanford Dam, remain: one has been cleaned up and the other needs no work. Stay out and stay alive near such sites.

Maps and reports

The park's geology is mapped and described by the NPS Geologic Resources Inventory, which has produced maps and reports for more than 270 parks — scoping summaries, digital GIS maps, a report and a poster. A Soil Resources Inventory has also been completed; both are on the NPS DataStore.

Geologic map of Lake Meredith National Recreation Area. Image from the National Park Service

Sources

Based on "NPS Geodiversity Atlas—Lake Meredith National Recreation Area, Texas," National Park Service; a work of the United States government in the public domain. A garbled sentence on dust control is left out, and the source's "evaporate minerals" is corrected to evaporite.