Abstract: Field Descriptions of Unconsolidated Clastic Sediments for Environmental Investigations

George A. Robertson

Complete field lithologic descriptions of field samples collected from unconsolidated clastic sediments during environmental investigations provide the primary source of data for hydrogeological characterization of the site and surrounding area.

Most environmental site investigations are performed in unindurated sediments and soil. Thus, the focus of this paper addresses lithologic descriptions of unconsolidated clastic sediments. Due to the multidisciplinary composition of environmental professionals, this study reiterates the basic field skills required to adequately investigate the geological setting and discusses their applications in combination with geotechnical engineering field descriptions.

Complete lithologic descriptions provide the data needed to interpret and understand lithofacies and predict changes in hydraulic parameters across an aquifer. Additionally, lithofacies data are needed to determine the shape and boundary characteristics of an aquifer. Once the depositional environment is understood, it is possible to begin to model aquifer characteristics and aquifer geometry. Lithofacies mapping provides a method to advance beyond treating aquifers as homogenous, isotonic bodies and produce more realistic environmental site assessments resulting in more effective remedial action plans.

The procedure for describing "soils" for engineering purposes is found under the fixed American Society for Testing and Materials (ASTM) designation D 2488-84. This system applies to soil mechanics and may be of use to characterize naturally occurring "soils" for construction purposes. The ASTM system is designed for engineering purposes only and should not be considered in lieu of a more rigorous interpretative lithologic description from which a depositional environment can be interpreted.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas