GROWTH STRATA DEVELOPMENT IN QUATERNARY ALLUVIAL FAN DEPOSITS, LOST RIVER RANGE, IDAHO

Growth strata are allostratigraphic packages of coarse-grained deposits that accumulate in response to structural rotation of the depositional surface through time. Although common in thrust-cored anticline, reverse fault, and salt diaper settings, they have been little recognized along normal faults. Detailed surface mapping of several Pleistocene-Holocene alluvial fans along the west flank of the extensional Lost River Range, Idaho reveals alluvial fan gravel packages with bounding unconformities (diastems) comprising surfaces of differing degrees of dissection, slope, and age. Drainage basin size is the major control on alluvial fan type. Calibrated surface ages using carbonate clast coat thickness values show large-radius (> 5km) sheetflood fans (Upper Cedar Creek; Jones Creek) contain surfaces with minimum age values of 36,000 (±20,000) yrs. and 28,333 (±20,000) yrs. for strongly- and moderately-dissected Pleistocene surfaces, respectively, and 6,667 (±5,000) yrs. for a weakly-dissected Holocene surface. Surface slope values decrease with decreasing age (7.2-2.7⁰). These fans were incised to their toe during late Holocene time as they achieved grade to the axial Big Lost River drainage; incised canyons have subsequently begun backfilling with modern gravel. The tilted dissected fan surfaces can be traced beneath one another and bound packages of fan gravel. Small-radius (<2km) debris flow fans (Jones Creek North; Elkhorn Creek) possess a single weakly-dissected Holocene surface dated to approximately 8,333 (±3,333) yrs. Active depositional lobes on these aggrading fans comprise debris flow levees and lateral lobes along the incised gully and terminal lobes below the intersection point.

Development of alluvial fans along the Lost River normal fault is spatially and temporally disparate. Presently, modern debris flow fans are aggrading adjacent to Pleistocene-Holocene fans undergoing canyon dissection and backfilling. Complexly interacting controls on fan development and rotation include: 1) temporal changes in locus of maximum displacement along Lost River fault segments, 2) local development of extensional fault propagation folding, 3) basinward stepping of the extensional fault plane, 4) drainage capture by the Big Lost River, and 5) changes in Pleistocene to Holocene discharge.