Development of a Stage IV Calcium Carbonate Soil at Mormon Mesa, Nevada

Amy Brock
University of Nevada, Las Vegas, Department of Geoscience
Las Vegas, Nevada
[email protected]

Arid soils accumulate calcium carbonate through time and undergo specific morphologic changes (stages 1-6). Stages 5-6 morphologies are poorly understood but critical in understanding the length of time that a soil has developed and are used to obtain surface ages timing and lengths of unconformities in the rock record. The stage 6 soil that has developed on Mormon Mesa near Mesquite, Nevada is considered one of the oldest calcium carbonate soils in the world. Mormon Mesa is also a ~5 million year old remnant floodplain between the Muddy and Virgin Rivers (tributaries to the Colorado River). Preliminary fieldwork revealed unexpected (including some never seen before) features such as whole clast dissolution, reprecipitation of dissolved clasts into silica and silicate clays that have raised many interesting questions as to the genesis of the stages five and six designations. This study tests (1) whether or not pisolites are clasts (gravel, pebbles) that have undergone dissolution and reprecipitation and thus indicate gravel parent material textures (2) if other criteria should be used to determine stage six soil morphology in conjunction with pisolites and brecciation and (3) if Mormon Mesa exhibits stages 5 and 6 morphology across the entire landform and what any changes would tell about its paleohydrology and paleogeomorphology.

Applications of this research are applied to understanding other calcium carbonate soils throughout the world (genesis, age of unconformities in rock record, landscape ages) and regionally, has implications for the geomorphic history of the lower Colorado River over the past 5 million years.

AAPG Search and Discovery Article #90033©2004 AAPG Foundation Grants-in-Aid