Revised Stratigraphic Nomenclature of Pennsylvanian System, Paradox Basin

Abstract

Pennsylvanian rocks of the Paradox basin provide an excellent example of rock-stratigraphic units occurring in a clear-cut relation with time-stratigraphic units. These rocks have had many names applied to them; some are confusing and contradictory. A system of simple, formal rock-stratigraphic and time-stratigraphic terms is proposed that is usable even in difficult transition areas where evaporite facies grade into shelf carbonate and terrigenous clastic rocks. The Hermosa Group includes rocks that overlie the Molas Formation of Late Mississippian and Early Pennsylvanian age and underlie the Permian Cutler Group, except in the northwestern part of the basin where the Elephant Canyon Formation of Early Permian age disconformably overlies lithologically similar Pennsylvanian rocks. The Hermosa Group consists of three distinctive formations. The oldest is the Pinkerton Trail Formation consisting mainly of marine carbonate rocks and shale. The overlying unit is the Paradox Formation, a sequence of evaporites, carbonate rocks of restricted marine origin, and dark-colored shale. The Paradox Formation is restricted to strata containing evaporites; time-equivalent rocks containing no evaporites are excluded from the formation. Outside the area of ev porite deposition, the Hermosa Group is called the Hermosa Formation undifferentiated because the individual units become indistinguishable. The youngest formation of the group is the Honaker Trail Formation consisting of carbonate, shale, and sandstone. The Honaker Trail Formation disconformably underlies the Permian rocks in the greater part of the Paradox basin. The group contains rocks of Morrowan(?), Atokan, Desmoinesian, Missourian, and Virgilian Series, identified for the most part by abundant fusulinids. A formal time-stratigraphic nomenclature is proposed for further subdivision of the Paradox and Honaker Trail Formations. This system uses the pay zones of the petroleum geologists. These units are bounded at top and base by dark-colored shales of regional distribution which are approximate time-marker beds according to fusulinid determinations. These time-stratigraphic shale units can be traced through many thousands of square miles, even from the evaporites into non-evaporitic facies of the Paradox basin. The shale markers are 10-25 feet thick in most of the basin, but are thicker toward the east. A formal time-stratigraphic unit named the Four Corners Stage is proposed as a subdivision of the Desmoinesian Series. The Ismay, Desert Creek, Akah, and Barker Creek Substages are included in the new stage. It is suggested that these units (formerly the pay zones) be called substages when used as time-stratigraphic units in the Paradox basin. Alternate type sections are proposed for some Pennsylvanian formations: (1) an alternate (subsurface) type section for the Pinkerton Trail Formation, (2) a subsurface type section for the Paradox Formation, and (3) an alternate type section for the Honaker Trail Formation.