THE MISSISSIPPIAN BORDEN FORMATION, VISEAN ARIDITY IN SOUTHERN LAURENTIA, AND EOLIAN SEDIMENTATION IN MARINE ENVIRONMENTS

The widespread occurrence of marine carbonates, evaporites, and terrestrial Aridisols across southern Laurentia are indicative of aridity during the early Visean (Mississippian Sub-period, Osagean Stage). The Osagean Borden Formation (informally, the Borden delta or Borden siltstone) is enigmatic within what is otherwise an arid carbonate-chert-evaporite depositional realm. Unlike coeval chemical strata indicating arid deposition, the Borden is a widespread fine-grained siliciclastic unit that extends from the west central Appalachian basin into the southern part of the Illinois basin.

The Borden is dominated by quartz (~ 80%), feldspar (~10%), clay minerals (~9%), plus heavy minerals and undetermined material (~1%). Particles sizes are mostly silt, followed by fine sand and clay. A fluvio-deltaic depositional model for Borden sedimentation appears untenable, as extreme Osagean aridity would have severely limited fluvial sediment supply. Also, the paucity of medium to coarse sand in the Borden appears inconsistent with fluvio-deltaic sedimentation. Alternatively, we propose an eolian sediment supply for the Borden. The formation preserves rounded zircons with morphologies characteristic of eolian transportation. U-Pb ages from these zircons indicates both Laurentian and Gondwanan affinities. Uniquely Gondwanan ages comprise about 7% of the dated population, but the proportion of material with a Gondwanan source could be substantially higher given considerable age-overlap in Laurentian and Gondwanan sources.

The paleogeography, paleoclimate, and probable paleo-wind directions are all consistent with an eolian sediment source in peri-Gondwanan and Laurentian terrains. Furthermore, the grain size and composition of Borden sediments are comparable to the grain size and composition of many Quaternary loess deposits as reported by Basucca, et al (2004). Thus, eolian deposition may have been the predominant source of Borden sediments, and is compatible with most submarine depositional features.