The Geology of Carrollton, Texas

Overview of Carrollton’s Geologic History

Carrollton, located in north Texas just north of Dallas, has a fascinating geologic history spanning over 300 million years. The region was once located near the ancient shoreline of a shallow sea that covered much of central North America during the Paleozoic Era.

Over millions of years, this sea advances and retreated across the region, depositing sedimentary layers of limestone, shale, and sandstone.

During the Mesozoic and into the Cenozoic Era, the area was uplifted and eroded, creating the landscape we know today. The geology directly impacts soils, water, mineral deposits, and landforms in the Carrollton area.

Understanding the geologic history helps explain the region’s terrain, natural resources, and potential natural hazards.

Paleozoic Era (541-252 million years ago) #h2

The story of Carrollton’s geology begins over 300 million years ago in the Paleozoic Era, when most of central North America was covered by a shallow sea.

During the Carboniferous Period about 360-300 million years ago, limestones, shales, and sandstones were deposited across what is now north Texas:

• Limestone formed from calcite skeletons and shells of marine organisms
• Shale formed from muddy sediments
• Sandstone from sandy sediments

These marine sedimentary rocks still underlie the Carrollton area today at depths over 5,000 feet and are important oil and gas sources for Texas.

During the Permian Period ending 252 million years ago, an inland sea covered the region. As this sea advanced and retreated over millions of years, it left behind layers of sedimentary deposits including limestones, shales, evaporites like rock salt, and sandy red beds that also may contain fossils.

Mesozoic Era (252-66 million years ago)
#h2

During the Mesozoic Era, the Texas region was undergoing uplift, erosion, and deposition. The Cretaceous Period, the last period of the Mesozoic, spanned from 145-66 million years ago.

Shallow seas still covered the region early in the Cretaceous, depositing carbonate rocks and clays.

Later in the Cretaceous after the seas retreated, river systems flowed from highlands building up East Texas and Louisiana, depositing sands.

These Cretaceous sands form the Trinity and Woodbine Aquifers which today provide drinking water to Carrollton and the Dallas-Fort Worth area.

The close of the Mesozoic Era 66 million years ago marked the extinction of the dinosaurs. By this time North Texas was likely above sea level with a relatively flat landscape.

Cenozoic Era (past 66 million years)

The most recent era beginning 66 million years ago, the Cenozoic, saw significant changes in Texas’ climate and landscape.

As the Rocky Mountains and Rio Grande Rift were uplifted west of Texas starting about 35 million years ago in the Oligocene Epoch, regional drainages and erosion increased, cutting across eastern Texas.

Significant deposits of gravel, sand, silt, and clay built up by rivers and streams buried older bedrock layers across the region.

In the Quaternary Period of the past 2.6 million years which includes the current Holocene Epoch, the global climate fluctuated between cold glacial and warm interglacial periods.

While glaciers did not cover Texas, the cooler and wetter periods significantly impacted river systems, vegetation, and rainfall.

Major river systems like the Trinity River which now flows through Carrollton significantly changed course. Extensive deposits of gravel, sand, and silts were left by flooding rivers across the upper Trinity River watershed. These recent Quaternary deposits directly underlie the Carrollton area today.

Carrollton Regional Geology Features

Unique geologic features help define the Carrollton area’s landscape and environment today:

Trinity River Watershed

• Carrollton sits within the upper Trinity River watershed, with the Elm Fork of the Trinity defining the southern city edge
• Historical river deposits of sand, gravel and clay underlie the area
• Modern Trinity river flooding still impacts the city after heavy rain events

White Rock Escarpment

• Limestone bedrock layer creates 100-foot tall cliff band mark southeast Carrollton border
• Formed from ancient Cretaceous seas depositing Austin Chalk limestone
• Creates scenic vista and recreational opportunities like walks and climbs

Soils and Erosion

• Most soils derived from weathered river deposits or weathered Cretaceous bedrock
• Clay-rich Vertisol soils known for shrinking and swelling dominate
• Urban development increases erosion and water quality challenges

Local Mineral Resources

While Carrollton itself has no active mineral extraction, nearby mining activity utilizes regional geological resources:

• Sand and Gravel: Quaternary river deposits in Trinity River watersheds supply construction aggregate materials
• Limestone: Cretaceous Austin Chalk supplies cement and construction materials
• Clays: Weathered shales used for brick and tile manufacturing

Groundwater Resources

The local geology impacts regional groundwater resources Carrollton utilizes:

• Trinity Aquifer: Sands deposited by ancient rivers supply some city water
• Woodbine Aquifer: Cretaceous sandstones provide drinking water for city
• Lake Lewisville: Constructed reservoir stores water upstream on Elm Fork Trinity River

Earthquake Risks

While Texas has a low earthquake risk compared to more tectonically active regions west of the Rocky Mountains, seismic hazards still exist:

• DFW area has experienced minor earthquakes over past decade related to wastewater injection
• Highest risk zones statewide are West Texas Permian Basin and East Texas
• No active faults known in Dallas County per Texas Bureau of Economic Geology
• Building code requires engineered buildings to withstand minor ground shaking

Table summarizing the key geologic history events, era, period, epoch, and estimated years that shaped Carrollton’s landscape:

Era	Period	Epoch	Years Ago	Event
Cenozoic	Quaternary	Holocene	Current interglacial period last 11,700 years	Recent Trinity River deposits underlie city; modern river valley and soils formed
Cenozoic	Quaternary	Pleistocene	2.6 million – 11,700 years ago	Glacial-interglacial cycles; extensive Trinity River deposits
Cenozoic	Neogene	Pliocene	5-2.6 million years ago	Rocky Mountain uplift increased Texas erosion
Cenozoic	Paleogene	Oligocene	33-23 million years ago	Rio Grande Rift uplifted; drainage patterns crossed Texas
Mesozoic	Cretaceous	–	145-66 million years ago	Shallow seas deposited Austin Chalk Limestone; sands formed Trinity-Woodbine Aquifers
Paleozoic	Permian	–	300-252 million years ago	Inland seas deposited limestones, shales, evaporites like rock salt
Paleozoic	Carboniferous	–	360-300 million years ago	North Texas covered by shallow seas depositing limestones, shales, sandstones

The rich geologic history spanning over 300 million years has ultimately given shape to the North Texas landscape we know today.

Carrollton’s terrain, soils, water resources and mineral deposits reflect continual changes in paleo environments through geologic time periods as seas advances and retreated across the region and ancient river systems flowed.

Understanding this key history provides insight on local geology-related opportunities and challenges facing the city today and in the future.