112. Jay Cooke State Park, MN

Jay Cooke State Park (Carlton, MN)

1.9-billion-year-old Thomson Formation featuring severely folded slate and metagraywacke beds cut by basalt dikes.

The bedrock of Jay Cooke State Park preserves a classic Precambrian structural record, displaying two major continental-scale geological events recorded directly in the bed and walls of the St. Louis River gorge.

The Thomson Formation (~1.9 Billion Years Ago)

The foundation of the park is the Thomson Formation, a thick sequence of Paleoproterozoic sedimentary rocks that underwent low- to medium-grade regional metamorphism.

• Original Deposition: Approximately 1.9 billion years ago, turbidite currents deposited alternating layers of mud, silt, and poorly sorted sand into an ancient marine basin.

• Protolith to Metamorphic Rock: Lithification and subsequent metamorphism transformed the fine mudstone layers into fine-grained, dark grey slate with strong slaty cleavage. The sand-rich layers became dense, quartz-rich metagraywacke.

• Differential Erosion: Along the riverbed, the softer, fissile slate layers weather rapidly under hydraulic action, while the massive, durable metagraywacke beds resist erosion. This uneven wear produces the distinctive "tilted tombstones" and vertical rock ribs sticking out at steep angles across the rapids.

Tectonic Deformation: Penokean Orogeny

Shortly after deposition, during the Penokean Orogeny (~1.85 billion years ago), a microcontinent collided with the ancestral North American craton (Laurentia).

• Severe Folding: Mountain-building compressional stresses from the south squeezed the flat horizontal beds, warping them into tight anticlines (upward arches) and synclines (downward troughs).

• Oriented Bedding: In many exposures along the park's trails, the beds are tilted near-vertically or completely overturned, allowing visitors to walk across millions of years of rhythmic deposition in just a few dozen paces.

Midcontinent Rift & Basalt Dikes (~1.1 Billion Years Ago)

Nearly 800 million years after the Thomson Formation was folded, the continent began pulling apart along the Midcontinent Rift System (the same event that formed the Lake Superior basin and the North Shore Volcanics).

Thomson Deposition

~1.9 Ga

Turbidity flows deposit marine muds and greywacke sands in a deep basin.

Penokean Orogeny

~1.85 Ga

Compressional tectonic forces fold the rocks and metamorphose muds to slate and sands to metagraywacke.

Keweenawan Rifting

~1.1 Ga

Basaltic magma intrudes through fractures in the Thomson Formation, forming dark basalt/diabase dikes.

Glacial Meltwater Incision

~10,000 BP

Outflow from Glacial Lake Duluth carves the modern St. Louis River gorge, exposing the bedrock sequence.

• Cross-Cutting Intrusions: As the crust stretched, deep fractures opened in the brittle slate and metagraywacke. Feeder magma for overlying lava flows filled these fissures, cooling beneath the surface into dark, fine- to medium-grained basalt and diabase dikes.

• Cross-Cutting Relationships: These vertical dikes cut sharply across the older folded slate layers, providing a text-book example of cross-cutting relationships where the dark intrusive basalt is distinctly younger than the host Thomson Formation.