TECTONIC CONTROLS ON INCISED VALLEY SYSTEMS ALONG EXTENSIONAL-LACUSTRINE MARGINS: AN EXAMPLE FROM THE MIOCENE HUMBOLDT FORMATION NEAR KNOLL MOUNTAIN (HD RANGE), NORTHEAST NEVADA

Incised valley systems produced by relative lake-level changes should be common along extensional-lacustrine margins but are rarely recognized in the rock record. Stratigraphic and structural analysis and geologic mapping of the Miocene Humboldt Formation near Knoll Mountain, Nevada has revealed an incised valley system that provides valuable insight into the formation of such systems. The incised valley system formed along the northeastern portion of a north-trending half graben with a west-dipping master normal fault. The incised valley is situated directly along the axis of a syncline that in turn is parallel to the master normal fault system in this portion of the basin. During a relative lake-level lowstand, a fluvial system incised to a depth of at least 50 m into lacustrine deposits and then filled with eolian and fluvial strata as relative lake-level rose and transgressed over the incised valley. This large relative lake-level fall and rise is only present in the northern part of the basin suggesting the fundamental control on the lake-level change was tectonic in nature. Structural analysis of strata in the syncline suggests that folding is related to fault propagation and indicates that folding occurred before, during, and after valley incision and filling. The syncline is located along the northern end of the half graben where fault slip is distributed among several normal faults instead of a single fault. This change in style of faulting in the northern part of the half graben may have created localized differential subsidence and could be responsible for the relative lake-level change that produced the incised valley. Our analysis of data suggest that fault slip and consequent folding and basin subsidence caused the local relative lake-level change responsible for incision and filling of the fluvial valley as well as controlled the location of the fluvial system. The analysis implies that incised valleys may be expected in similar tectonic settings and their locations may be predictable by recognizing specific tectonic elements.