The Geography of the Alamo Impact

Late Devonian geography

The Alamo Impact Event took place on a carbonate shelf setting in southeastern Nevada that existed during the Devonian Period. This area of Nevada, near modern-day Lincoln and Nye Counties, was characterized by a passive margin, a calm setting absent of tectonic activity. This lack of tectonic activity allowed for the deposition of thick carbonate rocks within a typical shallow marine setting. During the Alamo Impact, the bolide struck the thick deposits of carbonate rocks, forming the characteristic Alamo Breccia we see today. The bolide is believed to have struck at the carbonate shelf-slope boundary, a natural transition in the marine realm characterized by a flat, shallower setting (carbonate shelf) passing into a steeply sloping, deeper setting (carbonate slope).

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Where Was the Alamo Impact in Lincoln and Nye Counties?

The largest extent of Alamo Breccia is exposed throughout Lincoln and Nye Counties of southeastern Nevada, with the majority of it exposed within Lincoln County. Within these counties lie several mountain ranges separated by flat valleys that formed during Basin and Range extensional faulting, roughly 17 million years ago. These mountain ranges expose much of the Devonian-aged rocks, including those associated with the Alamo Impact. Some noteworthy mountain ranges include: Timpahute Range, Hiko Range, Pahranagat Range, Worthington Mountains, Golden Gate Range, Seaman Range, South Pahroc Range, and North Pahroc Range.