SOIL CHRONOSEQUENCE INVESTIGATIONS OF KNICKZONE MIGRATIONS THAT RESULTED IN FLUVIAL TERRACES OF THE BELLE FOURCHE AND CHEYENNE RIVERS, SOUTH DAKOTA

Soil chronosequence investigations on flights of fluvial terraces of the Cheyenne and Belle Fourche Rivers support prior interpretations that describe knickzone migrations as time-transgressive processes responding to base level fall, related to tectonics or climate fluctuations. Soils were described to establish chronosequences on inset fluvial terraces and on single terraces, established by long-valley profiles, that young upstream in the wake of knickzone migration. Soil pits were excavated on five ages of fluvial terraces - from young to old: Marion Flat (MF), Farmingdale (F), Sturgis (S), Rapid (R), and Mountain Meadow (MM) along a 400 km stretch of the rivers extending from the town of Belle Fourche downstream to Lake Oahe.

At the upstream inset chronosequence, soil profiles vary from the youngest terrace at this site (F) A/Bw1/Bw2/Bktb/Bkb/Coxkb to the oldest terrace at this site (R) A/Bt1/Bt2/2Bkmjb/3Btkb/3Coxkb. For comparison, the downstream inset chronosequence, near Lake Oahe, has soil profiles that vary from the youngest terrace in this location (MF) Avj/Bwk/Coxk to the oldest terrace in this location (S) A/Bt1- Bt5/ Cox. Both show significant increases in soil development with terrace elevation.

A lateral chronosequence of three Farmingdale soils changes from the furthest upstream (youngest) site of Cu/Bwb1/Bwb2 to near Lake Oahe (oldest) of Cu/2Btk/2Btkb1/2Btkb2/2Btkb3/2Coxkq. A similar lateral chronosequence of the older Sturgis terrace shows changes from the furthest upstream (youngest) site of Ap/Bt/Btj/Bwk1b/Bwk2b/Coxkb to near Lake Oahe (oldest) of A/Bt1to Bt5/ Cox. Both show a decrease in soil development upstream, on a single surface. The variations in soil development within a terrace of a single ‘geomorphic age’ suggest that this ‘age’ may be time-transgressive over 10^4-10^5 years.

The A/C floodplain deposits and the oldest, poorly preserved, surface (MM) with a profile of A/Btj/Btjk/Btqk/2Bwk/3Bwqk/3Bwqkmj/3Bk/3Cqk1 were used as end members for the chronosequence age estimates. This study shows lateral chronosequence investigations are useful in understanding rates of knickzone migration. In addition, future comparisons of the downstream inset chronosequence with nearby glacial deposits for age control, may help explain causes of the onset of knickzone migration.