A primer on the geologic history of western Colorado | PostIndependent.com

A primer on the geologic history of western Colorado

John Colson
Post Independent Staff
Glenwood Springs, Colorado CO
Illustration courtesy Colorado Geological SurveyThis illustration shows a not-to-scale, north-south cross-section of the Colorado River Valley at Parachute. Oil and gas deposits are being developed from the Mesaverde Group layers. The wavy black line represents a layer of coal.

RIFLE,Colorado – Where the Colorado Rockies now stand was, 100 million years ago, a shallow sea that stretched from the Arctic to the Gulf of Mexico.

Today, that ancient seaway has yielded to the awe inspiring, mountainous vistas of Western Colorado. It also left behind vast deposits of organic material that were compressed, heated and transformed into the coal, oil shale, natural gas and oil that attract the modern energy industry.

Peter Barkmann, and environmental geologist and hydro-geologist for the Colorado Geological Survey, offered a primer on the deep geologic history of western Colorado in a presentation last week to the Northwest Colorado Oil and Gas Forum, which meets quarterly in Rifle.

Barkmann described the formation of the Mesaverde and other energy-rich rock layers formed from coastal plains sediments deposited 75 million years ago.

The draining of the seaway and the uplift of the Rocky Mountains resulted in much of Colorado’s coveted energy resources, said Barkmann.

By 50 million years ago, the Rockies uplift had begun. The ancestral Rockies had risen and worn away a couple of hundred million years earlier.

As the mountains rose, along with the surrounding terrain, the rising peaks and plateaus pushed the seaway eastward.

In what is now northwest Colorado and southwest Wyoming, Barkmann said, several large inland lakes remained, fed by freshwater rivers and streams.

The departing sea took its time, fluctuating back and forth across Colorado. It spread vast deposits of sand, mud and organic material from the plant and animal life that had once teemed along the coasts and in the depths of the seaway.

Over time, the regional uplift continued and the seaway retreated eastward, helped along by the movement of tectonic plates, shoved by volcanic eruptions and carved by erosion.

The organic deposits of the seaway, laid down over eons, were covered by accumulating layers of rock and sediment. Buried deep underground, subjected to extreme pressure and heat, the organic materials gradually decomposed and permeated the surrounding rock, forming deposits of coal, oil, gas and oil shale.

Barkman’s presentation graphically showed how the coal- and oil-bearing formations sank over time in relation to the uplift. Erosion continued to play a powerful role in shaping the landscape.

A “large delta system encroached into the seaway across northwest Colorado,” Barkmann said.

The Piceance Basin (pronounced PEE-aunce) was in its early stages of formation, and over the past 10 million years, geologic forces raised up the Roan Plateau, and Battlement Mesa and Grand Mesa.

Meanwhile, the Colorado River carved downward through the rising earth.

The Piceance Basin now appears as a sharp depression between the Cathedral Bluffs on the west, near Rangely, and the Grand Hogback on the east, bearing northwest from New Castle.

At either end, the upward-bending formations rise to the surface, while they “sag” in the middle, Barkmann said.

“This is a really deep basin,” he added, explaining that hydrocarbon deposits reach down to sea level and below.

The basin is permeated by a large number of folds and faults, he said, which complicates the search for gas, oil and coal deposits.

Barkmann also touched on the issue of groundwater contamination, and whether residents should be concerned about the possibility that gas drilling activities could irretrievably pollute the well water and groundwater aquifers needed for household use and farming.

Pointing out that nearly all water wells are no more than 300 feet deep. Gas drilling rigs, however, are reaching for hydrocarbon deposits at depths of 8,000 to 10,000 feet below the surface.

“Intervening strata are generally low in permeability,” he said, meaning the likelihood of contamination spreading upward through thousands of feet of rock is fairly low.

“That’s not to say that you can’t have fractures and faults that can form a pathway,” he noted.

Barkmann said he believes hydraulic fracturing of gas wells, in and of itself, could not result in cracks reaching far enough upward to pollute groundwater.

“It’s very unlikely that an artificially induced fracture could travel that distance,” he said, but it’s not impossible. “I wouldn’t say it won’t happen.”


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