21 The Upper Cretaceous Mancos Group on the Colorado Plateau

Tuesday, 20 May 2025
8:00 AM - 9:45 AM
Cascade E

TYPE SECTION OF THE MANCOS SHALE REVISITED, MESA VERDE NATIONAL PARK, COLORADO

Paper number: 21-1
Program: Theme Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

A 700-m-thick section of the Mancos Shale was measured, described, and collected at the north end of the Mesa Verde National Park (MVNP), 8 miles west of Mancos, CO. Leckie et al. (1997) recognized 8 members in the Mancos Shale, two of which were new. Much of the stratigraphy of the Mancos Shale can be correlated to formations and members of the thinner and generally more calcareous stratigraphy to the east along the Colorado Front Range and into Kansas, in large part due to the increasing distance from the tectonic source of sediments from the west. For example, the Graneros Shale, Bridge Creek Limestone, Fairport Shale, Blue Hill Shale, Juana Lopez, and Smoky Hill members of the Mancos Shale are recognized along the Colorado Front Range. These units record the Greenhorn and Niobrara sequences of transgression and regression in the Western Interior Seaway (WIS). The paleoceanography of the seaway reflects the incursion of warm water masses and their biota far to the north during transgressions, with generally cooler and fresher waters along the western side of the seaway.

A regional disconformity separating the Montezuma Valley Mbr. and the Smoky Hill Mbr. records the omission of the Ft. Hays Limestone at MVNP related to uplift across a broad forebulge during the early Coniacian. The eastern extent of the forebulge lies between Durango and Pagosa Springs where the Ft. Hays Limestone is present.

The Mancos Shale at MVNP is transitional with the generally thicker sequences to the west in Utah where the Mancos Shale and correlative sediments fill the foredeep created by thrusting and subduction of the Farallon Plate. In these muddier and sandier lithofacies, the Mancos

Shale is proposed to be elevated to Group status. The Mancos Group in Utah includes lithostratigraphic units correlative with the type area of southwestern Colorado, including the Blue Hill Shale Mbr. of the Tununk Shale, Juana Lopez Fm., and the Montezuma Valley and Smoky Hill members of the Blue Gate Shale. The Prairie Canyon Formation is correlative with much of the Cortez Mbr. at MVNP. The principal reference section of the Mancos Shale at MVNP is an important lithostratigraphic and biostratigraphic tie point between the thicker siliciclastic-dominated sequences of the western foredeep, and the thinner carbonate-rich sequences in the central and eastern parts of the WIS.

Geological Society of America Abstracts with Programs. Vol. 57, No. 5

doi: 10.1130/abs/2025RM-409965

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