Rhizolite/Calcrete: Nonpedogenic Plant Root Diagenesis (Middle Pennsylvanian) from Kansas

Kimbell L. Knight

Rhizoliths, calcareous fabrics that accumulate in a host sediment as the direct or indirect consequence of plant roots, are widely noted as minor constituents in post-Silurian calcretes and soil horizons. However, rhizolites, calcretes whose petrologic character is dominated by rhizolithic fabrics, are apparently rare and have been confined to Quaternary examples. This report documents a widespread nodular to massive rhizolite horizon that developed in a siliciclastic sandstone host as part of a thick calcrete profile during a Desmoinesian minor lowstand of sea level coupled with increased aridity. Rhizoliths within the rhizolite include root casts, rhizosheaths, and most importantly rhizopatches. "Rhizopatch" is a term coined to designate nodular to massive rhizocretions containing displacive and replacive diagenetic carbonates with a plant root origin. This rhizolite is interpreted to have formed from the direct and indirect effects of plant roots in the zone of capillary rise just above the water table without a significant pedogenic influence. Misinterpretation of such "groundwater" calcretes as depositional, perhaps lacustrine, carbonates can and has resulted in stratigraphic inversion of the true ordering of events. It may be significant that all presently documented rhizolites are associated with periods of widespread continental glaciation (Carboniferous and Quaternary), and the role of glaciation and its effects on their origin--concomitant rapid fluctuations in sea level and climate--should not be overlooked.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.