Rhizoliths in ancient eolianites

Abstract

Rhizoliths are abundant within Permo-Pennsylvanian eolian rocks of southeastern Utah. They are also present at the tops of several eolian sand bodies within the Pennsylvanian Fountain Formation of southeastern Wyoming and at a single stratigraphic level within the Jurassic Navajo Sandstone of southeastern Utah. Ranging in diameter from less than 1 mm to 15 cm, these cylindrical structures are composed of siliciclastic-free micrite and calcite spar. Cylinders ramify throughout the uppermost 2–3 m of thin, flat-topped sand bodies. Large cylinders are compound rhizoliths and display alveolar fabric, a feature resulting from rootlet calcification that is characteristic of modern and ancient calcretes. Development of rhizoliths required flat, geomorphically stable substrates for plant growth, highly permeable surficial sediments, and sources of calcium ions. The restriction of rhizoliths to planar deflation surfaces suggests that plants were unable to colonize or stabilize eolian bedforms. Paleozoic rhizoliths appear to be restricted to coastal areas with onshore winds. Associated evaporites suggest an arid to semi-arid paleoclimate. Rhizoliths formed when carbonate allochems derived from an upwind epeiric sea were dissolved and calcite reprecipitated around plant roots. Quaternary rhizoliths are best developed in similar sedimentologic and geographic settings. The abundance of rhizoliths and carbonate allochems in Quaternary and late Paleozoic low-latitude coastal dunes suggests that they are genetically related to glacio-eustatic sea-level fluctuations.