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Ichnopedologic Fabric Analysis: Developing a New Method That Incorporates Ichnofabrics and Pedofabrics in Sedimentologic and Stratigraphic Analyses

Stephen T. Hasiotis1, Mary J. Kraus2, Previous HitJonTop J. Smith1, Brian F. Platt1, and John W. Counts1
1Department of Geology, University of Kansas, Lawrence, KS
2Department of Geology, University of Colorado, Boulder, CO

Ichnofabric analysis is inaccurate when applied to pedogenically modified continental and marine deposits. There is no realistic method to tease out biotic- from abiotic-produced fabrics because biota is one of the five soil-forming factors that result in pedogenesis of subaerially exposed deposits. A new system is necessary that incorporates bioturbation and the other four soil-forming factors to interpret better the degree of pedogenic fabric and the genesis and history of the strata.

One solution is to create a fabric scoring system with a focus on ichnopedologic units—fabrics resulting from bioturbation and pedoturbation (abiotic and biotic) and named for the dominant ichnotaxon. A scoring system is used from 0 to 10—modified from current ichnofabric analyses (ii) that use scoring based on the degree of bioturbation (1 to 5 [6]). Shorthand also is used to delineate the contribution of fabrics from burrows (b), rhizoliths (r), mottles (m), nodules (n), and soil structure (s); a score of 1 is added to denote the presence of each category. Continental and marine sediments with no bioturbation, pedogenic features, or altered sedimentary fabrics have a score of 0. Bioturbation intensity is scored on a range of ichnofabrics from 1 to 5, paralleling divisions currently used in ichnofabric analyses. As pedogenic processes modify deposits with longer duration of subaerial exposure and pedoturbation outpaces sedimentation, deposits will have an increasing number of pedogenic fabrics: a score range of 6 to 10. For example, ii= 5b+1r+1m=7 denotes complete bioturbation (5b) as well as the presence of rhizoliths (1r) and mottles (1m) in the strata. Numbers in a stratigraphic succession reflect the interplay between sedimentation and pedoturbation, syn- and postdepositional modification processes, as well as the duration of subaerial exposure and landscape stability.


AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas