The Geology of Jackson, Mississippi

An Overview of the City’s Natural Landscape

Jackson, the capital and most populous city in Mississippi, sits upon a landscape shaped by millions of years of geological processes.

The city’s location within the Gulf Coastal Plain physiographic region has influenced its topography, soils, and natural resources. Understanding Jackson’s geology provides insights into the region’s natural hazards, water resources, and building materials.

Jackson’s Physiographic Setting

Jackson is situated within the Gulf Coastal Plain, a low, flat region extending from Texas to Florida. The coastal plain consists of sedimentary rock layers gently dipping towards the Gulf of Mexico.

These sedimentary rocks were deposited by ancient seas that repeatedly advanced and retreated across the region.

The coastal plain has a series of terraces stepping up in elevation away from the coast. Jackson lies on the Jackson Prairie terrace, standing about 300 feet above sea level. This terrace has rolling topography along with some steeper hills and ravines.

Key Geologic Time Periods

Jackson’s landscape took shape over millions of years through the following key geologic time periods:

• Cretaceous Period (145-66 million years ago): Shallow seas deposited chalk, limestone, and marl across the region. These carbonate rocks produce the belt of the Black Prairie that runs just east of Jackson.
• Paleogene Period (66-23 million years ago): A rising Gulf of Mexico buried the area under sand, silt, and clay. Jackson’s terraces formed during this time.
• Quaternary Period (2.6 million years ago-present): Glaciation and changes in sea level led to deposition of loess soil and alluvial sediments.

Surface Geology

Jackson sits upon sedimentary bedrock formations covered in places by loose surface deposits:

• Citronelle Formation: This Pliocene-Pleistocene gravelly sand caps the hills north of Jackson. It provides sorted sands used for construction.
• Loess: This windblown silt deposited across uplands during the Pleistocene. Loess is easily eroded but fertile when mixed with organic matter.
• Alluvium: Gravel, sand, silt, and clay deposited by streams. Alluvium underlies the floodplains of rivers and creeks.

Structural Geology

Several major faults run through the Jackson area, including the Jackson Dome Fault Zone. This ancient zone of normal block faulting has shaped the modern landscape:

• Downthrown blocks created valleys subsequently filled by erosion and deposition.
• Upthrown blocks formed hills and ridges aligned northeast-southwest.
• Fault movement tilted rock layers now visible along stream bluffs.

Topography and Drainage

• Uplands: Loess hills and ridges with relief up to 300 feet above valleys. Examples include Georgetown and Livingston ridges.
• Lowlands: Broad floodplains and terraces bounded by streams and rivers. The city of Jackson lies within this area.
• Drainage: Creeks and rivers draining southward to the Gulf. Major waterways include Pearl River, Big Black River, and Town Creek.

Geologic Hazards

• Seismicity: Moderate earthquake risk from New Madrid Seismic Zone to the north. Zone of weakness related to ancient rifting.
• Landslides: Loess soils on steep valley walls and bluffs can fail after heavy rain events. Undercutting by streams exacerbates risk.
• Flooding: Low-lying parts of Jackson frequently inundate due to heavy rain, swollen rivers, and poor drainage.
• Sinkholes: Dissolution of subsurface carbonate rocks can create sinkholes, especially in easternmost areas.

Groundwater Resources

The Mississippi Embayment aquifer system provides water across central Mississippi:

• Shallow aquifers in Quaternary alluvium and loess – used for domestic supplies.
• Sparta Sand – important municipal source for Jackson city supply.
• Cockfield and Cook Mountain Formations – other units tapped for regional supply.

Mineral Resources

• Sand and gravel – alluvium and Citronelle used for construction aggregates.
• Limestone – Black Prairie belt east of Jackson. Used for cement and road materials.
• Clays – variety of clay deposits suitable for brick and tile manufacturing.
• Oil and gas – small fields produce from deeper formations like the Selma Chalk.

A Closer Look at Jackson’s Geology

Jackson’s landscape has been shaped by a long history of sedimentary rock formation, uplift, faulting, erosion, and Quaternary deposition. Here is a more in-depth look at key aspects of the city’s geology.

Bedrock Formations

Jackson sits atop the following sedimentary bedrock units, part of the larger Gulf Coastal Plain section:

Jackson Group

• Yazoo Clay – Paleocene-aged clay, mudstone, and siltstone. Often reddish due to iron oxide pigments.
• Forest Hill Formation – Paleocene sands, silts, and clays. Contains the Cocoa Sand, a regionally important aquifer.

Claiborne Group

• Tallahatta Formation – Eocene claystone, siltstone, and sandstone. Sandstone beds form ridges and bluffs.
• Winona Formation – Eocene sand, silt, and lignitic clay. Contains the Sparta Sand, a major aquifer.
• Zilpha Formation – Middle Eocene clay and limestone. Contains marine fossils like Turritella shells.

Wilcox Group

• Nanafalia Formation – Paleocene clay, sand, and gravel beds.
• Tuscahoma Formation – Paleocene sand, clayey sand, and sandy clay.

Structural Features

Jackson lies within the broad Mississippi Embayment syncline, where Gulf Coastal Plain strata dip gently southward into the embayment. Superimposed on this are a series of faults related to the Jackson Dome:

• Jackson Fault – Normal fault striking NE-SW with downthrown side to the SE.
• Flora Fault – Parallel normal fault to the southwest. Has displaced Eocene strata.
• Madison Fault – Begins near Madison and extends SW into Jackson area.

Loess and Terraces

As sea levels rose and fell during the Quaternary Period, a series of terraces formed in the Jackson area:

• Montgomery Terrace – Highest and oldest terrace, stands over 400 feet above modern floodplains.
• Bentley Terrace – Intermediate terrace around 320 feet elevation.
• Jackson Prairie Terrace – Lowest terrace, 210 to 300 feet elevation. Site of modern Jackson.

Thick loess deposits blanket the terraces, thinning to the east. Loess thickness:

• Montgomery Terrace – Up to 60 feet
• Bentley Terrace – 20 to 40 feet
• Jackson Prairie Terrace – 6 to 20 feet

Major Rivers

Jackson lies within the drainage basins of three major rivers:

Pearl River

• Rises in central Mississippi and flows southwest past Jackson.
• Occupies a downthrown valley aligned with the Jackson Dome Fault Zone.
• Jackson sits east of the Pearl River on the Jackson Prairie Terrace.

Big Black River

• Rises north of Jackson and flows southward through the city.
• Drainage basin covers ~11,000 km2 above Jackson.
• Forms a large floodplain through the western side of the city.

Yazoo River

• Formed by Tallahatchie and Yalobusha Rivers north of Jackson.
• Flows southward where it joins the Mississippi below Vicksburg.
• Drainage basin of ~34,000 km2 contributes water through western side of Jackson.

Impacts on Jackson and Surrounding Areas

Jackson’s geology has had major impacts on development and natural resources in the city and surrounding region. Some key influences include:

Settlement Patterns

• Early settlement concentrated on loess hills and terrace margins above floodplains.
• Terrace soils provided agricultural lands to support growth.
• Pearl River was a transportation route linking inland areas with the Gulf.

Water Resources

• Local aquifers provide abundant groundwater, but parts of the city deal with subsidence and reduced well output.
• Rivers and reservoirs supply surface water, but flooding and contamination are challenges.

Aggregate Resources

• Sand and gravel mines tap alluvium and Citronelle deposits across Rankin and Hinds counties.
• Ready availability of aggregates supports local construction.

Agriculture

• Loess soil supports crops like soybeans, cotton, corn, and sweet potatoes.
• Dairy farms occur on upland prairies.