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

ABSTRACT-Middle Pennsylvanian, Atokan and Desmoinesian, crinoids and echinoids are described from the Belden and Minturn Formations of the Central Colorado Trough. The echinoderms from the Minturn Formation were living in an active intermontane trough setting. They are found in two environments, on the flanks of phylloid algal reefs and in patches on abandoned delta lobes on the gently sloping substrate. Faunas are dominated by cromyocrinids and are similar to stable platform Terrigineous Facies Belt faunas of the Midcontinent.

New species described are Dicromyocrinus beldenensis, Neoprotencrinus rockensis, and Sciadiocrinus wipsorum. Goleocrinus, and the columnal genera Blothronagma, Cyclocaudex, and Platyplateium are reported for the first time from the Minturn Formation. The columnal taxa Blothronagma cinctutum and Floricyclus angustimargo are believed to be cromyocrinids. B. cinctutum may be synonomous with Synarmocrinus molasensis, and Floricyclus angustimargo may be synonymous with Ulocrinus rockymontanus.

INTRODUCTION

MOST PENNSYLVANIAN or Late Carboniferous crinoids have been described from shallow marine settings in basins developed on stable platforms with little or no nearby tectonic and coarse clastic depositional activity. This generalization is most evident in the vast literature describing the Pennsylvanian crinoids of the Midcontinent of the United States (i.e., Moore and Plummer, 1940; Strimple and Watkins, 1969; Pabian and Strimple, 1993; among many others).

Articulated cups and crowns of Pennsylvanian crinoids from intermontane basins of Colorado have been described in papers by Tischler (1963), Strimple and Miller (1971), Strimple and Moore (1973), and Strimple (1976). All of these papers concern specimens from thin limestones or limestone nodules in calcareous shales within thick, clastic dominated sequences. Their occurrence in narrow, restricted seaways within an active intermontane basin setting was not considered significant if even noted.

Crinoid specimens reported herein were discovered mostly by Karen Houck and Martin Lockley during field studies of the paleoecology of the Minturn Formation in the McCoy/Dotsero area (Fig. 1) of Colorado (Houck and Lockley, 1986). Additional specimens were found by the Karl Hirsch family, Wayne Itano, John Chronic, students on geology field trips in the McCoy/ Minturn area, and members of the Western Interior Paleontological Society (WIPS), who helped to recollect the area specifically for crinoids. These specimens from the Minturn Formation and one additional specimen from the Belden Formation, as well as new information on the stratigraphy and locality of the specimens described by Strimple and Moore (1973) are the basis of this paper.

STRUCTURAL AND STRATIGRAPHIC SETTING

The Pennsylvanian stratigraphy and structural development of Colorado has been documented in numerous articles and theses on specific areas and parts of the stratigraphic section by various individuals during the past 65 years. These articles and theses are cited in papers that provide additional documentation, or a synthesis, of the Pennsylvanian stratigraphic and structural development of Colorado (Tweto, 1949; Mallory, 1972; De Voto, 1972; Chronic, 1958; De Voto, 1980; among others). The tectonic activity is perhaps best summarized in the following quotations from De Voto (1980, p. 95-96) "The Pennsylvanian was a time of aggressive tectonism; block-fault mountain building and basin subsidence resulted in the development of mountain ranges with as much as 10,000 ft (3,050 m) of relief. Thick sequences (up to 7,000 to 12,000 ft, 2,135 to 3,660 m, thick) of nonmarine, coarse-grained, alluvial sediments were deposited adjacent to the uplifted mountainous areas, while marine sediments accumulated in the actively subsiding or downdropped intermontane basins. ."; (p. 97) "The Des Moinesian was the time of most aggressive Pennsylvanian tectonic activity in Colorado. The Des Moinesian tectonic activity was effective in continued downdropping of the basin areas, particularly the Paradox basin and the several elements of the Central Colorado trough, so that during the Des Moinesian up to 5,000 to 10,000 ft (1,525 to 3,050 m) of sediments were deposited in marine and transitional environments...."

The stratigraphic development within an active tectonic region changes rapidly both vertically and laterally resulting in different stratigraphic units of equivalent age within a single basin and each basin having a unique set of stratigraphic units. Because there are a number of names applied to lithostratigraphic units of Pennsylvanian age in Colorado, (and some apply to areas not of concern herein,) only the pertinent units are given in Figure 2. The Minturn Formation and its seven formally named limestone members were defined by Tweto (1949) from the Minturn area. The Minturn Formation has since been applied to lithologically similar and age-equivalent strata of the Madera Formation in the southern part of the Central Colorado Trough (De Voto, 1980), but the limestone members have only been recognized in the vicinity of Minturn. The named stratigraphic units do not reflect the full character of vertical and lateral relationships of the diverse individual beds within the units.