EARLY TO MIDDLE PENNSYLVANIAN CONODONTS OF THE CENTRAL COLORADO TROUGH AS AN AGE CONTROL FOR THE EARLY STAGES OF THE ANCESTRAL ROCKY MOUNTAIN OROGENY

Early to Middle Pennsylvanian strata in the Central Colorado Trough, near Salida, Colorado, formed in response to the Ancestral Rocky Mountains (ARM) orogeny, but little is known about the timing of changes in depositional events. The Kerber Formation unconformably overlies the Mississippian Leadville Limestone, and contains fining upward sequences of quartzose sandstones and silty shales. It grades upward into the Belden Formation, which is characterized by carbonate wackestone to packstone units, with algal wackestone tops, that are interbedded with calcareous, gray shale. The Belden grades upward into the arkosic sandstones and reddish shales of the Sharpsdale Formation. Conodonts from the Belden Formation provide a way to date the beginning of erosion of the granitic sources for arkoses of the Sharpsdale in the early stages of the ARM orogeny.

Conodonts were obtained from limestones of the Belden Formation at three sections north and east of Salida. Faunas are dominated by elements of Idiognathodus and Adetognathus, taxa representative of shallow water marine environments. The presence of Idiognathodus sinuosis and Idiognathoides sinuatus indicates that the limestones are no older than mid-Morrowan and no younger than mid-Atokan. The appearance of I. klapperi in the upper Belden suggests a late Morrowan to early Atokan age. Idiognathodus parvus and similar, small Pa elements with long carinas occur in the upper Belden, and its first occurrence has been used elsewhere to identify the base of the Atokan. However, I. parvus appears to be juvenile forms of late I. sinuosis or I. klapperi. The Glenn Eyrie Formation near Colorado Springs, contains Neognathodus atokensis, a diagnostic Atokan species, in association with Idiognathodus morphotypes like those from Belden. This suggests an early Atokan age for the appearance of arkosic sandstones of the Sharpsdale Formation derived from erosion of granitic rocks during the ARM orogeny.