Segments from an Essay on Topographic and
Geomorphic Development of Southeastern Idaho
by H. Thomas Ore, Department of Geology, Idaho State University, May 1995.

H. Thomas Ore spent more than thirty years studying the geology and geomorphology of southeastern Idaho. He remains a geomorphologist of a classic style, unique in his tendency to think in terms of large-scale geomorphic evolution, and his ability to integrate observations from across large areas and from multiple geologic disciplines. Prior to his retirement from Idaho State University, Tom summarized his thoughts concerning southeast Idaho drainage evolution. We present his essay here in a condensed form, but one that conveys his ideas accurately and preserves the spirit of his thinking.

INTRODUCTION

This is an essay intended as a synthesis of thoughts concerning evolution of the landscape southeast of the eastern Snake River Plain. Proposing this model for Neogene evolution of southeast Idaho is prompted by an emerging view of the roles of evolution of basin-and-range structure and topography, and the almost concurrent passage of the North American Plate over a mantle plume (the "Snake River Plain-Yellowstone hot spot"). The essay is intended as a forum for thought, rather than as a compendium of research results of individual workers. It is, of course, based on the latter.

The overall theme is one of basin filling and emptying, and controls on those processes by base level, at a variety of scales. In fact, the whole subject of southeast Idaho topography is the story of the precursors to and the results of superposition, related to sequential basin separation and integration.

Miocene Eastward Regional Drainage

In Middle Miocene time, during early basin-and-range faulting, regional drainage was eastward, still responding to effects of uplift to the west, related to earlier subduction. Eastward progression of basin-and-range faulting made new sources and sinks available in that direction. An extensive east-sloping surface, part cut, part fill, was thus prepared, with deposition starting perhaps 8 to 10 million years ago. Drainage was toward the Atlantic, and the continental divide was west of our area, specifically on the volcanic plateau in the Owyhee uplands of southwestern Idaho, just as it is now on the Yellowstone plateau.

The principal theme of the above is that sediments were transported eastward, northward and southward away from the location of the volcanic tumescence at the hot spot. East-flowing Mio-Pliocene drainage systems thus aggraded the surface eastward into western Wyoming. The great distance to permanent base level to the east afforded no opportunity for dissection in that direction.

Swan Peak Formation Lag Gravels

On pediments adjacent to the nearly buried basin-and-range mountains, resistant boulders of Ordovician Swan Peak Quartzite formed a lag concentrate on the bedrock surfaces. That white bouldery quartzite is still present as scattered boulders and patches of gravel in areas not affected by streams since they were abandoned by subsequent downcutting drainage.

DRAINAGE DERANGEMENTS AND REVERSALS

The drainage in basins adjacent to the subsiding hot spot track reversed and became northwestward, tributary to the main axial stream of the hot spot track; i.e., the modern Snake River. The ancestral Raft River, Rock Creek (Rockland Valley), Bannock Creek (Arbon Valley), and Portneuf River, west to east, all had their inceptions as consequent streams responding to the new, lower base level at the collapsing hot spot track. They all started eroding headward toward the south and east, one by one capturing tributaries to old south- and east-flowing drainages that had been responsible for emplacement of the basin fill. These new north- and northwest-flowing streams also in part responded to tilting of the surface toward the Plain (at least near the Plain margin) by that same thermal collapse.

Role of the Bonneville Basin

A complicating factor was provided by downward displacement on the Wasatch Fault, providing a base level for streams to be captured by the Bonneville Basin. The unusual pathway of the Bear River may have originated in this manner. If so, it would have happened when the Gem-Gentile Basin was full of sediment, and a southward flowing drainage into the dropping Bonneville Basin evolved. Any rivers at that position would have had great competence at least near their entrance to the Bonneville Basin. The mouth of the ancestral lower Bear River into the Bonneville Basin was superposed onto the quartzite at Oneida Narrows south of Grace, and by headward erosion captured westward-flowing drainage of the ancestral upper Bear River at Soda Point. Similar captures, although not quite so dramatic as erosion through the Oneida Narrows, occurred to the west.

Marsh Creek was a temporary capture by the Bonneville basin base level. The eventual result, however, was that drainage to the ocean had an advantage over drainage to the more whimsical base level provided by a closed Bonneville basin, that was rapidly filling with sediment from all sides, and at later times filling with water as well, also raising its base level. Marsh Creek was thus captured by north-flowing drainage to the Snake River Plain.

Superposed Gaps, Basin Integration

Some bedrock slivers, uplifted along synthetic faults with offset less than those of the major range-bounding faults, had been deeply buried by the rising level of the Miocene Salt Lake Group basin fill. Higher, more uplifted bedrock that wasn't buried deeply by basin fill, was encountered early in the regional downcutting by the northwestward-flowing drainage network. The westward draining gap between Lava Hot Springs and McCammon was occupied earlier than other narrower gaps, explaining the wide, slope- and floodplain-dominated terrain there.

As the continental divide migrated to the east, westward drainage became superposed on the basin fill and on bedrock divides between basins, connecting those basins and allowing more upstream, eastern ones to become regraded to lower, more westerly ones. Major drainages through the Portneuf and Bannock ranges were established in this way. The cutting of the northwestward flowing main drainage of the system, through Portneuf Narrows between Inkom and Pocatello, has become a dominant westward egress to the Pacific.

A particularly elegant example of superposition and incipient basin capture, is the relationship between Hawkins Basin and Creek and the Garden Creek drainage. In the eastward flow of Garden Creek, draining the basin west of Scout and Old Tom Mountain, the superposed creek had encountered quartzite of the Scout Mountain Member of the Pocatello Formation, and became trapped, forming the narrow Garden Creek Gap. As the newly north-draining Marsh Creek continued to cut downward, both Garden Creek and Hawkins Creek followed along; the former in a steep canyon, the latter in a broader valley. The northern headwaters of Hawkins Creek today are eating headward into a narrow ridge of volcanigenic basin fill, all that remains separating Hawkins Basin from the Garden Creek drainage. Because of the temporary base level provided by the quartzites at Garden Creek Gap, the divide between the headwaters of Hawkins Creek and Garden Creek will continue to migrate northward. Eventually no water will flow through Garden Creek Gap. It will become a wind gap, common in the Valley and Ridge of the Appalachians.

OVERVIEW

In terms of the broad view of southeast Idaho topographic evolution, a Neogene relative chronology emerges, suggested by basin fill stratigraphy, locations of superposed gaps between basins, and migration of the hot spot and the associated continental divide. The first part of the chronology is relict eastward drainage toward the continental interior left from uplift to the west from the Sevier orogenic event. The onset of eastward younging basin-and-range grabens provided sinks for preservation of siliciclastic and volcanigenic sediment, the latter from silicic volcanic centers along the eastward-migrating hot spot. Basins filled syntectonically with sediment, some more than others, depending on activity along individual fault segments.

Eventually the basin system was filled with sediment, and an equilibrium drainage system delivered whatever sediment was derived from the highlands to the west to the drainage systems leading to the Gulf of Mexico. As eastward- migrating uplift occurred, as the continental divide followed the hot spot and basin-and-range extension, drainage shifted to the west.

As the Snake River Plain dropped, streams tributary to it at its margins were encouraged to eat headward into previously filled basins. Low-gradient westward-draining systems on the Pacific side of the eastward-migrating continental divide were locally captured by headward eroding steep gradient streams with local base levels provided by the Snake River. At times, the pattern was sidetracked, as by the immense Bonneville Basin locally capturing streams along its margins. Nevertheless, the general pattern was of a shift of drainage to the Pacific.

THIS EXCERPT IS FROM:
Link, P.K., Kaufman, D.S., and Thackray, G.D., 1999, Field Guide to Pleistocene Lakes Thatcher and Bonneville and the Bonneville Flood, southeastern Idaho, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History, p. 251-266.