Abstract

A solitary, thin, widespread conglomerate is found in latest Cretaceous–Paleocene sections in each of these Laramide basins of the Southern Rockies—the Table Cliff (northwestern Kaiparowits) Plateau and Uinta Basin (Utah), the Piceance Creek and Denver Basins (Colorado), and the San Juan Basin (New Mexico). These units are unusual compared to typical syntectonic conglomerates in that they occur as a thin interval underlain by a disconformity, are far-traveled across most of the length of their basins, generally reflect a change in dispersal direction relative to underlying deposits and were deposited during a period of reduced subsidence. All units were deposited well after local initiation of the Laramide orogeny but within ∼8 m.y. of each other. However, each conglomerate unit was separately derived from individual source areas and is not part of a once-continuous gravel sheet. Similar widespread conglomerate units of this age are apparently absent in other Rocky Mountain Laramide 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-traveled conglomerates are grossly consistent with vertical motions and surface tilting along the transport path of the oceanic plateau. An exact match of conglomerate timing differs from previously proposed models, but uncertainties in timing of deposits, and the lack of constraints on the shape and structure of the subducted plateau preclude our ability to further refine the comparison. Elsewise local tectonic activity, climate change, and sea-level fluctuations fail to explain the observed spatiotemporal pattern. Hence, we suggest that these deposits record a transient mantle-derived tectonic effect traversing the Rocky Mountain region and demonstrate the utility of stratigraphy as a record of subtle tectonic events.

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