Lower Pennsylvanian stratigraphy of the Central Colorado Trough

The Central Colorado Trough formed as a result of the uplift of the Ancestral Rocky Mountains. With uplift, sediments were eroded from the adjacent highlands and deposited in the trough. The Lower Pennsylvanian stratigraphy of the Central Colorado Trough was determined based on outcrops in the southern Mosquito Range and Arkansas River Valley east of Salida, Colorado.

The Kerber Formation is the oldest Pennsylvanian strata in the Central Colorado Trough and was derived from the erosion of older Paleozoic sedimentary rocks and some crystalline basement rocks. It was deposited in alluvial fan and braided streams environments near the edges of the trough. Along the axial portion of the Trough, it was deposited in braid-delta and shallow marine environments. As marine incursion occurred from the northwest, a retrogradational sequence developed. Based on conodont faunas, the upper Kerber Formation is late Morrowan to Atokan in age. Because the strata of the Central Colorado Trough are intimately related to the initial uplift of the Ancestral Rocky Mountains, the formation of the Ancestral Rocky Mountains began no later than the late Morrowan.

The Kerber Formation correlates with the Belden Formation of the northern portion of the Trough based on the first appearance of the conodonts Idiognathoides sinuatus, Idiognathodus sinuosus, and Idiognathodus parvus. At Willard Springs they occur within the Belden, whereas they occur within the Kerber at Wells Gulch, 8 km to the south. The dark marine sediments of the Belden Formation extend further southward than previously documented, and approximately 127 m of section occurs at Willard Springs, northeast of Salida. At Willard Springs, the Belden Formation lies conformably above the Kerber Formation and conformably below the Sharpsdale Formation. This is the first time that the Belden has been documented m contact with the Sharpsdale Formation. At Wells Gulch and Box Canyon to the south, the Belden Formation grades into shallow-water carbonates and shales interbedded with Kerberand Sharpsdale-type elastics.

The Sharpsdale Formation was derived from the erosion of Precambrian crystalline basement rocks of the Uncompahgre Highland as the climate of the Pennsylvanian grew more arid. It was deposited in alluvial fan and braided stream environments near the edges of the trough. Along the axial portion of the trough, it was deposited in braid-delta and rare shallow marine environments. A northward prograding sequence developed as sediment influx exceeded local sea-level rise.