Trace fossils and marine-nonmarine cyclicity in the Fountain Formation (Pennsylvanian: Morrowan/Atokan) near Manitou Springs, Colorado

Abstract

The Lower Pennsylvanian (Morrowan/Atokan) portion of the Fountain Formation in the Manitou Springs, Colorado, area commonly has been interpreted as a subaerial alluvial fan. However, approximately 12 marine-nonmarine cycles, each represented by a discrete progradational sequence, have been recognized within the lower third of the Fountain Formation in this area. The typical cycle is composed of six lithofacies: 1) transgressive-lag conglomerate; 2) offshore mudstone; 3) hummocky crossbedded sandstone; 4) planar crossbedded granular sandstone; 5) low-angle, crossbedded, coarse-grained sandstone; 6) lenticular conglomeratic sandstone. Only lithofacies 6 is nonmarine (alluvial) in origin. Two diverse trace-fossil assemblages (totalling 18 ichnogenera and 20 ichnospecies) within the Fountain Formation are restricted to the marine portions of the section, or to those portions within 1 m below marine-deposited strata. The assemblage includesArenicolites carbonarius,?A.ichnosp.,Aulichnites parkerensis,two types ofChondrites,?Conostichus broadheadi,?Crossopodiaichnosp.,Curvolithus manitouensis(ichnosp. nov.),Eioneichnosp.,Lockeiaichnosp.,Macaronichnus segregatis, Palaeophycus heberti, Palaeophycus striatus, Planolites beverleyensis, Psammichnites plummeri, Rhizocorallium irregulare,?Skolithosichnosp.,Taenidium serpentinum,?Teichichnusichnosp., ?Thalassinoidesichnosp., ?Zoophycosichnosp., and several unnamed traces.Trace fossils in the Fountain Formation can be used as indicators of sedimentary processes (e.g., rates of deposition, energy regime). Trace-fossil composition changes along a roughly south-to-north, nearshore-to-offshore gradient, generally reflecting increased influence and duration of normal-marine sedimentation. Ichnotaxa present and their distributions within marine strata in the lower part of the Fountain Formation suggest that they belong to theCurvolithusichnoassemblage, which has been shown to indicate high sedimentaiton rates in relatively nearshore shelf settings. TheCurvolithusichnoassemblage can be subdivided intoMacaronichnus-dominated andCurvolithus-dominated parts.Macaronichnus-dominated portions of the Fountain Formation indicate generally higher wave-energy sedimentation as compared withCurvolithus-dominated portions.