Cenozoic Surfaces and Deposits in the Southern Rocky Mountains

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Abstract
The Cenozoic erosional history of the eastern part of the Southern Rocky Mountains is documented by ancient surfaces and deposits. During Laramide tectonism, streams eroded the sedimentary cover, exhuming pre-Mesozoic surfaces and beveling Precambrian rocks. By late Eocene time, erosion had produced the only widespread surface of low relief developed in the mountains. In early Oligocene time, broad shallow channels were cut into this surface, and these were partly filled with alluvial deposits and overlapped by Oligocene and Miocene volcanic rocks. Faulting in early Miocene through Pliocene time displaced mountain versus valley blocks as much as 12,000 m vertically. The late Eocene surface was fragmented, the mountains were deeply eroded, and adjacent grabens filled. Channels were again cut below the late Eocene surface and filled with equivalents of the Santa Fe Formation. Uplift accelerated in late Pliocene time, and concurrent erosion cut canyons 180 to 300 m deep at the mountain flanks. Tertiary surfaces can be differentiated by their extent and degree of development, height above streams, and character of covering deposit. All widespread surfaces on Precambrian rocks are exhumed older surfaces or were cut during Tertiary time. Because the late Eocene surface is fragmented, reconstruction requires consideration of Neogene block faulting. Broad gentle channels cut into the late Eocene surface in early Oligocene and in Miocene or Pliocene time contain stream gravels that permit reconstruction of the geomorphic history. Some of these gravels have been called till, but they are not glacial and extensive icecap glaciation is unknown in the Southern Rockies. Quaternary surfaces are narrow and confined to valleys, and are not over 140 m above streams. Surfaces can be dated best where overlying deposits furnish direct evidence of age.

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