RECONSTRUCTING TRANSIENT LATEST CRETACEOUS-PALEOGENE MANTLE-DERIVED UPLIFT IN THE SOUTHERN ROCKY MOUNTAIN BASINS

Mantle processes are the primary mechanisms for generating long-wavelength, low-amplitude, tilting in plate interiors. Since tractive transport in rivers is directly tied to slope, fluvial stratigraphy provides a means of reconstructing the past history of large-scale continental tilting in aggradational settings. As an example we focus on interpreting the origin of a single, thin, widespread conglomerate found in Latest Cretaceous–Paleogene sections in each of these Laramide basins of the Southern Rockies — in Utah the northwestern Kaiparowits Plateau (Canaan Peak Fm.) and Uinta Basin (Dark Canyon sequence); in Colorado the Piceance Basin (Ohio Creek Mbr. of the Mesa Verde Group) and Denver basin (Arapahoe Conglomerate), and, in New Mexico, the San Juan Basin (Ojo Alamo Sandstone). In each of these basins, the widespread conglomerate unit is found across most of the length of the basin, is underlain by a disconformity, and was deposited following a period of reduced subsidence. All units were deposited well after local initiation of the Laramide Orogeny, but within c. 8 m.y. of each other. However, each conglomerate unit was separately derived from individual source areas and not part of a once continuous gravel sheet. Similar widespread conglomerate units of this age are apparently absent in other Rocky Mountain basins.

Previous studies postulate the shallow subduction of an oceanic plateau – the conjugate Shatsky Rise – beneath the U.S. Cordillera during the time interval of conglomerate deposition. The distribution, timing and dispersion directions of the far-travelled conglomerates are overall consistent with hypothesized vertical motions and surface tilting along the transport path of the oceanic plateau. Local tectonic activity, climate change, and sea-level fluctuations are insufficient to explain the observed spatiotemporal pattern. Hence, we suggest that these deposits record a transient mantle-derived tectonic effect traversing the Rocky Mountain region.