Abstract
The Lower Ordovician Marathon Formation, exposed in the Trans-Pecos part of the Ouachita-Marathon fold belt, consists of cyclic alternations of pelagic shale and gravity-displaced beds of siltstone, sandstone, calcarenite, calcisiltite, and conglomerate. Gravity displacement is inferred from the numerous examples of the Bouma turbidite cycle, from displaced faunas, slump structures, and boulder beds present in the formation. The Marathon Formation is a slope facies whose sediments, derived from mixed terrigenous and shallow-water sedimentary, igneous, and metamorphic sources, were deposited in relatively deep (30-300 m) water by turbidity currents, debris flows, slumps, and submarine landslides. The existence of this slope facies during Early Ordovician time implies tha a trough (Marathon geosyncline) with a well-developed shelf-slope-basin geometry already was functioning as a depositional basin. Three intraformational exotic boulder beds are evidence of locally intense tectonic movements along the northwest or cratonal side of the geosyncline. These movements produced large-scale (up to 21 cu km), isochronous downslope displacements (olistostromes) of shelf and upper-slope sediments. Volcanism occurred prior to, and probably during, this tectonic activity. Volcanic boulders are present in the boulder beds, and volcanic fragments are common in sandstone and calcarenite beds throughout the formation. The submarine landslides and the volcanic fragments show that cratonal areas marginal to the Marathon geosyncline were tectonically active even during very early stages of the geosynclinal cycle.
Published Version
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