Paleosols: Sequence Boundaries in Continental Rocks

HANNEMAN, DEBRA L., Whitehall Geogroup, Inc., Whitehall, MT, and CHARLES J. WIDEMAN, Montana College of Mineral Science and Technology, Butte, MT

Paleosols often mark unconformity surfaces on continental rock packages and thus may be used to delineate sequence boundaries. Paleosols are characterized by various diagnostic features such as calcic, argillic, and oxic horizons. Calcic-rich paleosols are of particular importance in seismic stratigraphy because of their morphology and mode of occurrence on and below major unconformity surfaces. Calcic layers are typically well indurated, range from 0.2 to 1 m in thickness, and contain thin zones of root mats. As determined by laboratory measurements of outcrop samples, densities of calcic paleosols range from 2.1 to 2.6 g/cm3 as opposed to densities of 1.4 to 1.7 g/cm3 for underlying, nonpedogenic rocks. The calcic paleosols are usually interbedded with nonpedogenic strata for up to 0 m below a major unconformity. Calcic paleosol zones can be projected from surface exposures into the subsurface for correlation with features on seismic reflection data. The use of paleosols as sequence boundary markers is analogous to the use of marine sequence markers and can provide a new and realistic basis for interpreting continental rock sequences.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)