Geologist Alan Chamberlain said that in Wyoming, overthrusted
Paleozoic rocks are a couple of thousand feet thick, but in Nevada
they are 45,000 feet thick. Cretaceous source rocks in the Wyoming
overthrust are about 1 percent total organic content and a couple
of hundred feet thick, but in Nevada the
Mississippian source rocks average 5 to 6 percent total organic
content, and are thousands of feet thick.
"When the USGS published articles on Nevada they interpreted
the Mississippian rocks out here as swish turbidities, which are
not very good source rocks," Chamberlain said. "However,
I have found that the Mississippian source rocks are actually latchstrine
lake beds -- some of the richest source rocks in the world."
Nevada was part of a passive margin sequence with shallow water
carbonate build up until the Late Devonian when it was interrupted
by the Antler Orogeny, which effected the entire Cordillera from
Alaska to South America, according to Chamberlain.
"When I sat the Mukluk well in the Arctic Ocean in the 1980s,
the Sadlerochit Formation was part of the rocks associated with
the Antler Orogeny and the same Mississippian rocks I am dealing
with in Nevada," he said.
"The Antler Orogeny created a huge mountain range in central
Nevada and a seaway east of Nevada, similar to the Cretaceous seaway
that created all the source rocks for Wyoming and Montana,"
he continued. "The richest source rocks in that Antler basin
were the floodplain deposits in eastern Nevada where the lake beds
were. To the east it drops off the shoreline and becomes marine
rocks, and to the west the mountains begin and the rocks are more
sandstones with no shales."
When the Antler Orogeny subsided there was a transgression of Pennsylvanian
seas and normal deposition until the Late Cretaceous, when the Sevier
Orogeny, another major compressional event, took place, he said.
The Sevier Orogeny created the Wyoming overthrust belt, the Canadian
foothills as well as the structures around Monterey, Mexico.
"The same event created the great fault folds in Nevada,"
he said. "Following the Sevier Orogeny a blanket of volcanic
rock covered Nevada that sealed in the oil seeps associated with
the Cretaceous thrust belt."
Chamberlain said he is not entirely sure why volcanic rock covers
this part of the thrust belt and not others.
"It is likely there was not as much internal drainage in areas
not covered by volcanics," he said. "The Sierra Nevada
Mountains created an enormous area for internal drainage in what
is now Nevada, and the only outlet to the ocean was the Colorado
"If we had a major river cutting through central Nevada and
the Sierras, then the volcanics would likely have been stripped
like they are in other places on the overthrust belt."