FERRICRETE FORMATION IN THE NORTH-CAROLINA COASTAL-PLAIN

A conceptual model is proposed to explain localized ferricrete formati ons on valley-side slopes in the North Carolina Coastal Plain. Iron-en riched groundwater flows from flat upland interfluves toward valley-si de slopes. Iron precipitation and oxidation occur where groundwater is discharged below a fluctuating water table. Exposure of the iron-ceme nted zone by erosion or mass wasting results in hardening. Field evide nce from three sites is entirely consistent with the model. The ferric retes are highly localized, occurring only at eroding sites and extend ing less than 1 m into the valley side. They are found just above the average water table, and in all stages of formation, from loose iron-s tained sand to red sandstone-like ferricretes. Ferricrete formation in eastern North Carolina is ongoing, localized, and is occurring in a s ubtropical climate. Its formation is dependent on exposure by erosion or slope failure. These results are consistent with recent work sugges ting that ferricretes are unreliable for relative dating, stratigraphi c correlations, and as climate indicators. Conversely, the relationshi p between ferricrete formation and slope retreat in the study area rai ses the possibility that ferricretes may be useful in inferring the ra tes and nature of coastal plain slope evolution.