THE WHITE RIVER SEQUENCE, THE FIRST WIDESPREAD POST-LARAMIDE DEPOSITS OF THE NORTHERN HIGH PLAINS

The White River sequence (Formation or Group) was the first widespread unit to be deposited in the northern Great Plains and Middle Rocky Mountains after the Laramide Orogeny. It was deposited on an erosional surface that had as much as 1200 m of relief in the crystalline-cored mountains of Wyoming but had approximately 50 m of relief in the Great Plains. On the plains this erosional surface is covered by a lag of silica-rich gravels composed of quartz, chert, and quartzite clasts that resulted from intense weathering during the Eocene. Erosion of the region was replaced by widespread deposition as large amounts of volcanic ash fell across the region from volcanic sources primarily in Nevada and Utah. The White River sequence is primarily an aeolian dust deposit that was locally reworked by streams and rivers in the lower half of the unit. Initially climates were moist and the volcanic glass in the dust was weathered into clay now represented by widespread claystone beds of the lower Chadron Formation. As climates dried through the late Eocene into the early Oligocene, the ash was preserved as mudstones of the upper Chadron and lower Brule formations. The upper Brule Formation is characterized by large massive sheets of siltstone preserving angular volcanic glass shards. These deposits were loessites that reflect deposition in dry, semiarid conditions at the end of White River deposition. The transition from claystone to mudstone to siltstone deposits reflecting increasing drying during White River deposition was diachronous, with siltstone loessites appearing earliest in deposits to the west and south, and latest in the northeast. There were two major river drainages during the deposition of the White River sequence. Rivers on the east side of the Laramie Mountains and Black Hills flowed to the southeast. Chadron rivers of this drainage flowed to the Gulf of Mexico, but later Brule rivers disappeared into the Great Plains as they lost their waters into the loessic deposits. North of the Granite Mountains and west of the Black Hills, White River rivers flowed to the northeast towards Hudson Bay. White River deposits in North Dakota have evidence of increasing wet conditions higher in the section, unlike their southern counterparts.