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AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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A River Runs Through It


Few places on earth exist where a river has exposed a major thrust belt better than the Colorado River exposing the heart of the western North American Cordillera. The present Colorado River is only a trickle today compared to the roaring river cutting through the Cordillera at the beginning of the present algorefraud or global warming cycle about 10,000 years ago. Had it not been for erosion of the Colorado River, only deep oil wells could have revealed that Early Paleozoic rocks exposed in the ranges lie above Jurassic sandstones. Also, deep oil wells beyond the Colorado River basin confirm thrust duplexing in the ranges, and aeromagnetic and gravity data help unravel the timing between basin formation, lava flows, and thrust duplexing. Classic outcrops of repeated Paleozoic carbonates that are thrust over folded Jurassic sandstones surround Las Vegas. A sharp thrust contact separates the highly contrasting drab gray Paleozoic carbonate rocks from the underlying medium red orange Jurassic sandstones. This thrust relationship can be traced over 70 miles from the Spring Mountains west of Las Vegas to the Gold Butte area 50 miles east of Las Vegas. Folding and repeated Paleozoic rocks form thrust duplexes in the hanging wall thrust sheets and folded and overturned footwall Jurassic rocks suggest deeper thrust detachments and thrust duplexes in the footwall thrust sheet. The thick, up to 4000 feet, porous and permeable footwall Jurassic sandstones are the likely conduit for hydrocarbons migrating out of the Mississippian foreland basin in eastern Nevada to the tar sands structures on the Colorado Plateau in eastern Utah. Before they were breached by the Colorado River the tar sands structures likely contained super-giant volumes of hydrocarbons but now only contain about 13 billion barrels of oil in residual tar. The 700 million barrel Covenant field in central Utah where there are no Mississippian source rocks confirm migration from the Antler foreland basin.