Strange formation on Colorado Rockies sheds light on Earths past

first_imgThe Tava sandstone is unusual for another reason: It may be the only large-scale deposit in the world where sedimentary rocks such as sandstone have been injected into crystalline rocks such as granite, Siddoway says. Normally, molten material flows into cracks in sedimentary rocks and then solidifies, she explains.For well over a century, geologists have debated the age of the Tava sandstone, Siddoway notes. Although the deposit obviously must be younger than the surrounding granite, which has an estimated age between 1.03 billion and 1.09 billion years, some teams have suggested that the Tava sandstone might have formed as recently as 280 million years ago. Now, analyses by Siddoway and George Gehrels, a geologist at the University of Arizona (UA) in Tucson, shed new light on when the deposits may have formed.For their study, the researchers analyzed samples of Tava sandstone collected at six sites near Colorado Springs. First, they extracted between 100 and 125 tiny zircons—tiny bits of erosion- and chemical-resistant mineral—from each sample. Then they used uranium/lead dating to determine the age of each zircon (the time when its parent rock crystallized). Most of the zircons were between 1.33 billion and 970 million years old, and none were younger than 850 million years old, Siddoway says. Sometime after the zircons crystallized, possibly millions of years later, these bits of mineral eroded out of their parent rock and ultimately ended up being washed or blown into what is now central Colorado. So although the zircon ages provide clues to the age of the Tava sandstone, they aren’t definitive.To help narrow the range of possible ages for the sandstone, Siddoway and Gehrels compared the age distributions of the Tava zircons with the age distributions of zircons found in other sandstone formations in Colorado, Utah, northern Arizona, and southern California. Statistically, the Tava distribution most closely matched the patterns in sandstones that had been deposited between 680 million and 800 million years ago, the researchers will report in an upcoming issue of Lithosphere.The paper’s results are “strong, compelling evidence that this age range [for the sandstone’s deposition] is correct,” says Peter Reiners, a geophysicist at UA who was not involved in the new study.That was an interesting era in Earth’s history, Weil says. An ancient supercontinent called Rodinia was breaking up, he notes, and what is now western North America was being stretched apart—a process that likely cracked the Colorado granites apart, creating voids that were suddenly filled with immense amounts of waterlogged sand that had accumulated atop the granites or nearby. The new findings “will change [geologists’] perspective on the Rodinia breakup,” especially regarding when the event occurred and whether it happened in several phases, he notes. They also suggest that the Ute Pass fault formed during that era, hundreds of millions of years before the Rockies were even born, making it much older than researchers have previously suspected, he adds.If the fractures hosting the Tava sandstone were indeed formed during the breakup of Rodinia, the stretching and rifting of that supercontinent extended farther east than previously suggested, Reiners notes.It’s not yet clear where the reservoirs of sand that flowed into the granite fractures millions of years ago were situated, Siddoway says. She and her colleagues are now trying to figure that out. But the sands “almost certainly percolated downward from the surface into the older, underlying bedrock,” Reiners says. Scientists have suggested that similar formations in Sweden formed when the immense weight of glacial ice forced sand and other loose material into fissures in underlying rocks. “These rocks are unusual and not very common, but they’re not unheard of,” he says. Sign up for our daily newsletter Get more great content like this delivered right to you! Country Email Click to view the privacy policy. Required fields are indicated by an asterisk (*)center_img In the Front Range of the Colorado Rockies, smack in the middle of a cliff that overlooks U.S. Highway 24, resides a very unusual geological formation. This reddish gray, sharp-edged, and erosion-resistant swath of sandstone stands in stark contrast to the crumbling, heavily weathered granites that lie on either side. Now, scientists say they have narrowed down when this anomaly and others like it in this region formed—a discovery that may give researchers new clues about the breakup of an ancient supercontinent hundreds of millions of years ago.Many outcrops of the “Tava sandstone”—derived from a Native American name for Pikes Peak, a local landmark—are found along the Ute Pass fault, which runs along the Front Range near Colorado Springs. First noted by geologists more than 130 years ago, these deposits have long been recognized as strange, says Christine Siddoway, a geologist at Colorado College, Colorado Springs. Many sandstone formations show layers of some type, signs they were laid down over time in distinct episodes by wind or flowing water. But the individual grains in the Tava sandstone, which typically are bits of quartz measuring from 125 to 250 micrometers across, are well mixed, and they’re peppered with larger bits of quartz up to 3 millimeters in diameter. Once free-flowing but now firmly cemented together with an iron-bearing mineral called hematite, the sand grains were apparently injected into cracks in ancient granite—some of them as much as 6 meters wide—under high pressure. The now-solid Tava deposits apparently flowed from vast reservoirs of once-waterlogged sand, some of them containing more than 1 million cubic meters of material.“This is a very unusual [sandstone],” says Arlo Weil, a structural geologist at Bryn Mawr College in Pennsylvania, who wasn’t involved in the new study. “It must have been formed by a very rapid, chaotic process.” Country * Afghanistan Aland Islands Albania Algeria Andorra Angola Anguilla Antarctica Antigua and Barbuda Argentina Armenia Aruba Australia Austria Azerbaijan Bahamas Bahrain Bangladesh Barbados Belarus Belgium Belize Benin Bermuda Bhutan Bolivia, Plurinational State of Bonaire, Sint Eustatius and Saba Bosnia and Herzegovina Botswana Bouvet Island Brazil British Indian Ocean Territory Brunei Darussalam Bulgaria Burkina Faso Burundi Cambodia Cameroon Canada Cape Verde Cayman Islands Central African Republic Chad Chile China Christmas Island Cocos (Keeling) Islands Colombia Comoros Congo Congo, the Democratic Republic of the Cook Islands Costa Rica Cote d’Ivoire Croatia Cuba Curaçao Cyprus Czech Republic Denmark Djibouti Dominica Dominican Republic Ecuador Egypt El Salvador Equatorial Guinea Eritrea Estonia Ethiopia Falkland Islands (Malvinas) Faroe Islands Fiji Finland France French Guiana French Polynesia French Southern Territories Gabon Gambia Georgia Germany Ghana Gibraltar Greece Greenland Grenada Guadeloupe Guatemala Guernsey Guinea Guinea-Bissau Guyana Haiti Heard Island and McDonald Islands Holy See (Vatican City State) Honduras Hungary Iceland India Indonesia Iran, Islamic Republic of Iraq Ireland Isle of Man Israel Italy Jamaica Japan Jersey Jordan Kazakhstan Kenya Kiribati Korea, Democratic People’s Republic of Korea, Republic of Kuwait Kyrgyzstan Lao People’s Democratic Republic Latvia Lebanon Lesotho Liberia Libyan Arab Jamahiriya Liechtenstein Lithuania Luxembourg Macao Macedonia, the former Yugoslav Republic of Madagascar Malawi Malaysia Maldives Mali Malta Martinique Mauritania Mauritius Mayotte Mexico Moldova, Republic of Monaco Mongolia Montenegro Montserrat Morocco Mozambique Myanmar Namibia Nauru Nepal Netherlands New Caledonia New Zealand Nicaragua Niger Nigeria Niue Norfolk Island Norway Oman Pakistan Palestine Panama Papua New Guinea Paraguay Peru Philippines Pitcairn Poland Portugal Qatar Reunion Romania Russian Federation Rwanda Saint Barthélemy Saint Helena, Ascension and Tristan da Cunha Saint Kitts and Nevis Saint Lucia Saint Martin (French part) Saint Pierre and Miquelon Saint Vincent and the Grenadines Samoa San Marino Sao Tome and Principe Saudi Arabia Senegal Serbia Seychelles Sierra Leone Singapore Sint Maarten (Dutch part) Slovakia Slovenia Solomon Islands Somalia South Africa South Georgia and the South Sandwich Islands South Sudan Spain Sri Lanka Sudan Suriname Svalbard and Jan Mayen Swaziland Sweden Switzerland Syrian Arab Republic Taiwan Tajikistan Tanzania, United Republic of Thailand Timor-Leste Togo Tokelau Tonga Trinidad and Tobago Tunisia Turkey Turkmenistan Turks and Caicos Islands Tuvalu Uganda Ukraine United Arab Emirates United Kingdom United States Uruguay Uzbekistan Vanuatu Venezuela, Bolivarian Republic of Vietnam Virgin Islands, British Wallis and Futuna Western Sahara Yemen Zambia Zimbabwelast_img

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