Pennsylvanian-Permian Hermosa Group in the Paradox Basin

Donald Rasmussen

The Hermosa Group is explicitly all of the strata between the regional Leonardian (Kungurian) angular unconformity, which lies at the base of the Organ Rock and/or White Rim formations (depending on location), and the regional Pennsylvanian Morrowan (Bashkirian) angular unconformity at the top of older rocks (Mississippian, Devonian, Cambrian or Precambrian, depending on location). The most important characters of the Hermosa Group are the angular unconformity surfaces at the top and base, and the cyclicity of the included strata. The Hermosa Group includes strata of the Cedar Mesa, Halgaito, Elephant Canyon, Honaker Trail, Paradox and Lower Hermosa formations, and those strata previously included in the Rico, Lower Cutler, Unidifferentiated Cutler, Pinkerton Trail and Molas formations. Of the 83 fourth-order cycles presently identified, 72 have evaporites, 41 have halite intervals, and 37 have potash intervals. There are 12 cycles in the Lower Hermosa Formation, 36 cycles in the Paradox Formation, 14 cycles in the Honaker Trail Formation, and 21 cycles in the Elephant Canyon Formation (which intertongues with the Cedar Mesa and Halgaito). If the time boundaries determined for the Atokan Series are correct, then there are 15 known cycles for the Morrowan, 14 for the Atokan, 22 for the Desmoinesian, 12 for the Missourian, 8 for the Virgilian, and 12 for the Wolfcampian. Continuous uninterrupted deposition of cycles in the Paradox Basin probably starts in the Lower Hermosa Formation (Morrowan), continues all though the Paradox Formation (Atokan and Desmoinesian), and possibly ends at the top of the Honaker Trail Formation (Missourian). It is suspected that some of the non-halite-bearing cycles could be fifth-order sequences. Additional new cycles or conformable erosional gaps may eventually be found in the Deep Fold and Fault Belt or northwestern part of the basin, especially for the Morrowan, Virgilian and Wolfcampian.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012