New Mexico Geological Society Annual Spring Meeting — Abstracts

A soil toposequence is a sequence of soils located on different positions down a slope profile. A soil catena is a subset of a soil toposequence where individual soil properties are determined strongly by their position on the slope. The linkage between soils in a catena indicates a continuity of slope processes, such as soil creep or throughflow, which result in a continuum of soil variability from the top to the bottom of the slope. Catenary relations take time to develop so their presence indicates a peroid of slope stability. Identification of these relations can aid in determining the age of fault scarps and terrace risers and can also indicate the extent of throughflow and surface runoff off hillslopes.

Recent work on hillslope soils in the Sevilleta and near Escondida have identified three situations where catenary relations have not developed. Study of these toposequences allows us to identify situations in which other factors, apart from slope position, strongly influence soil forming processes.

On the Sevilieta, two sequences of Soils were described down opposing slopes. The SW facing slope is formed on Capirote quartz monzonite bedrock, whicn weathers to gruss. The gruss is easily transported off the slope and only a very thin soil forms in bedrock. There is litle variation in soil properties down the slope. The opposing NE facing slope, is formed on Ladron quartz monzonite which weathers to form a coarse colluvuim. Zones within this monzonite weather much faster than the surrounding bedrock, and these area appear to have formed topographic lows at some stage of slope development and they are now in the process of being filled in. Soil chemistry indicates that there is no significant throughflow on this slope.

The soils described on opposing slopes cut into upper Santa Fe Formation near Escondida, show a strong influence of the underlying sediments rather than a systematic variation in soil properties down slope. Catenary relations will not develop on these slopes until the underlying variation in sediments have been masked by the development of a thick colluvial layer.

The soils forming the toposequences in the Sevilleta have been forming for several thousands of years judging by the amount of calcium carbonate that has accumulated in these soils. This should be sufficient time for catenary relations to develop. In this case the bedrock lithology has an overriding influence on slope processes. The Escondida sequences on the other hand, have not been forming long enough for the establisment of a complete cover of colluvium down the entire slope and thus no catenary soil sequences have been developed.