Middle Pennsylvanian, late Atokan-early Desmoinesian echinoderms from an intermontane basin, the Central Colorado Trough

Journal of Paleontology, Nov 1998 by Webster, G D, Houck, Karen J

Most of the Minturn crinoid species are endemic. Three species are identified as forms originally reported elsewhere. Aglaocrinus magnus and the columnal taxon (0) OBlothronagma cinctutum were initially reported from the Atokan Pumpkin Creek Limestone of southern Oklahoma (Strimple, 1949; Moore and Jeffords, 1968) and Metaffinocrinus perundatus from the Desmoinesian part of the Millsap Lake Formation of Texas (Moore and Plummer, 1940). Two other species are referred to species initially reported from other formations. Synarmocrinus molasensis is a late Atokan or early Desmoinesian form found in the Pinkerton Trail Limestone of southwestern Colorado (Strimple and Miller, 1971) and oPlatyplateium texanum is a Desmoinesian form reported from the Millsap Lake Formation of northeastern Texas (Moore and Jeffords, 1968).

The late Atokan and early Desmoinesian age (Ross, 1993, personal commun. to Houck) of the Minturn crinoid faunas is based on fusulinids found within the same limestones as the crinoids. The crinoids support this age because the species identified from the Minturn Formation that were initially reported from Texas and Oklahoma are of Atokan and Desmoinesian age. Also, the ages of all Minturn genera, as reported from other areas in the U. S., are within late Atokan or early Desmoinesian time (Webster, 1977, 1986, 1988, 1993).

PALEOECOLOGY

Paleoecologic interpretations of late Paleozoic crinoids are based largely on studies from the Midcontinent of the United States (Pabian and Strimple, 1970; Holterhoff, 1988; Pabian et al., 1989; among others). Those studies were made on faunas from a wide, shallow shelf setting, which are well removed from high-relief mountains undergoing active faulting. However, the setting of the Minturn Formation is grossly different from that of the Midcontinent.

The paleoecology of the Minturn crinoid faunas is related to the sedimentary cycles within the formation. In ascending stratigraphic order, the cycles typically contain 1) red arkosic conglomerate; 2) red, brown, or gray sandstone and shale; 3) gray limestone with marine fossils; and 4) sandstone and shale of variable color. The cycles are bounded by erosional surfaces that may truncate underlying strata (Chronic, 1964). Stevens (1965, 1969), Walker and Harms (1991), and Houck (1991, 1993, 1997) interpreted the conglomerates as braided fluvial deposits. The sandstones and shales are interpreted as marginal marine delta and washover fan deposits, and the limestones are interpreted as shallow marine deposits. The conglomerates are devoid of marine fossils. The sandstones and shales commonly contain marginal marine trace fossils (Lockley et al., 1987), but body fossils are rare; a few orbiculoid brachiopods, bellerophontid gastropods, and myalinid bivalves have been found (Euphemites fauna of Stevens, 1965). Most of the marine fossils known from the Minturn Formation in the McCoy area, including the echinoderms described herein, occur in the limestone beds, or in shales associated with them.