Home Archived April 13, 2016

Investigations and Mapping of the Surficial Geology of Red Rock Lakes National Wildlife Refuge and Centennial Valley


The bedrock and parts of the surficial geology of the Centennial Valley and Range are relatively well mapped. The Centennial Valley is a Basin and Range half graben where the valley is actively subsiding and tilting into the fault zone at the front of the Centennial Range. Fault offset is about 10 m over the last 15,000 years. Such offset resulted in about 8 m downfaulting of the Centennial Valley, which is tectonically sustaining the basin and associated lakes and wetlands. Beneath the subsurface of the valley is a fill of young unconsolidated sediment more than a kilometer thick. In late Pleistocene time, a large lake about 20 m deep existed in the valley. Lake sediments accumulated in this lake and such sediments fine and become less permeable towards the basin center. As this lake shrank due to erosional lowering of its outlet, alluvial fans of gravel and sand built outward from the surrounding mountains to the margin of the shrinking lake. This near-surface surficial geology controls near-surface ground water flow. At shallow depths (<5 m), particularly in the post-lake alluvial fans, flow of ground water is concentrated along old gravelly channelways. At moderate depths (~10-20 m), ground water flow may follow gravelly alluvium beneath sediments of the late Pleistocene lake. Ground water may also discharge upward along faults and other breaches such as piping through the lake sediments.

Thermal anomalies shown by the thermal imagery will be examined and compared to possible geologic features controlling ground water discharge. Surficial geologic issues throughout the basin will be studied. Geologic topics important to understanding or predicting ground water flow are described in the following paragraphs, going counterclockwise from the south part of the lake basin.

  • The Humphrey Creek reentrant and associated abundant springs and peat will be studied. The postulate that gravels locally exposed by earthflow driven thrusting trace into the subsurface and carry water to this widespread spring area will be evaluated. Younger, fine-grained alluvial fan material mantling such gravels will be sampled for datable organic material, as well peat farther out in the basin to date the post-gravel history.
  • In the eastern part of the area, a sequence of gravel on lake sediment on gravel will be studied, including preparation of dark carbonaceous samples for dating the change from gravel deposition to inundation by the last large lake. The progradation of the Red Rock River gravel across this lake sediment will be mapped and dated if possible.
  • In the northern part of the area, the relation between lake levels and the sand source for the extensive dunes farther north will be studied, and times of activation of dunes dated based on buried humic soils. The dunes and adjacent gravelly alluvial fans represent an important source for ground water recharge.
  • In the western part of the area, an attempt will be made to better understand what controls base level and serves to hold water in Lower Red Rock Lake. Possible reasons include such processes as damming by alluvial fans, cross-basin structure, and eolian sand build-up of the low-gradient channel of Red Rock River. Does this constriction also affect ground water at depth and act as a ground water barrier causing upwelling of ground water east from this apparent constriction?
  • In the southwest part of the area, the extensive “fan form” of clayey sediment was built up from a sediment source rising to the Matsingale-Curry Creeks area. This clayey sediment has low permeability, contrasting with the gravelly, much more permeable other fan forms around the upper basin. The genesis and age of this “fan form” needs to be better understood to understand its effect on the hydrology and its role in constricting the outlet of Lower Red Rock Lake.
  • The central part of the basin (Upper and Lower Red Rock Lakes and River Marshes) will be studied, particular any sites with ground water anomalies. Lakes may be cored for paleoecologic studies, and to document and date the postulate that the present shallow lake has recently dried and then refilled. A postulated fault crossing the northern part of the central area will be evaluated. Such a fault is suggested by the following features, from southeast to northwest: (1) the linear north shore of Upper Red Rock Lake, (2) apparent offset of low shorelines 6 km further north west on the north side of the River Marshes, and (3) a large thermal spring in bedrock 6 km further northwest.

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