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The ages of the continental, Upper Cretaceous, Fruitland Formation and Kirtland Shale based on a projection of ammonite zones from the Lewis Shale, San Juan Basin, New Mexico and Colorado

James E. Fassett
The ages of the continental, Upper Cretaceous, Fruitland Formation and Kirtland Shale based on a projection of ammonite zones from the Lewis Shale, San Juan Basin, New Mexico and Colorado (in The Cretaceous-Tertiary boundary in the San Juan and Raton basins, New Mexico and Colorado, James E. Fassett (editor) and J. Keith Rigby (editor))
Special Paper - Geological Society of America (1987) 209: 5-16

Abstract

The Kirtland Shale or Fruitland Formation directly underlies the Cretaceous-Tertiary boundary throughout most of the San Juan Basin of northwest New Mexico and southwest Colorado. These formations have been known to be Late Cretaceous in age since the early 1900s. Now, with the greatly renewed interest in rocks adjacent to mass extinction boundaries, it is important to place more precise ages on such rock units as the Fruitland and Kirtland. Deposition of the Fruitland and Kirtland was closely related to deposition of the underlying marine-regressive Pictured Cliffs Sandstone. Because the Pictured Cliffs was deposited as a strandline sandstone in a subsiding seaway, its stratigraphic expression, when related to a time horizon (the Huerfanito Bentonite Bed), is a series of rising-to-the-northeast, time transgressive, stair steps. Thus, time lines (or horizons) drawn parallel to the Huerfanito cut through the marine Lewis Shale, the strandline Pictured Cliffs Sandstone, and the continental Fruitland Formation and Kirtland Shale. Ammonites have been collected and identified from various stratigraphic levels within the Lewis Shale around the northwest, north, and east sides of the San Juan Basin. These fossils can be tied in to the established ammonite zonation of the Western Interior seaway. Because some of these ammonite zones have been radiometrically dated outside the San Juan Basin, it is possible to project these dated faunal zones from the Lewis Shale along time lines into the Fruitland Formation and Kirtland Shale and thereby estimate the age of those rocks. Based on these projections the part of the Fruitland and Kirtland laterally time-equivalent to the Lewis Shale is estimated to range from 73.2 + or - 0.7 Ma to 73.9 + or - 0.8 Ma. The average age for this interval based on these dates is 73.5 + or - 0.5 Ma; the maximum range of the interval at the 95 percent confidence level is 71.8 to 75.5 Ma. This age range puts these rocks in the Campanian Stage of the upper Cretaceous in the San Juan Basin.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 209
Title: The ages of the continental, Upper Cretaceous, Fruitland Formation and Kirtland Shale based on a projection of ammonite zones from the Lewis Shale, San Juan Basin, New Mexico and Colorado
Title: The Cretaceous-Tertiary boundary in the San Juan and Raton basins, New Mexico and Colorado
Author(s): Fassett, James E.
Author(s): Fassett, James E.editor
Author(s): Rigby, J. Keith, Jr.editor
Affiliation: U. S. Geol. Surv., Reston, VA, United States
Affiliation: U. S. Geol. Surv., Reston, VA, United States
Pages: 5-16
Published: 1987
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Geological Society of America, Rocky Mountain and Cordilleran sections, joint meeting ; The Cretaceous-Tertiary boundary in the San Juan and Raton basins, New Mexico and Colorado
Meeting location: Salt Lake City, UT, USA, United States
Meeting date: 198305May 1983
References: 17
Accession Number: 1987-041938
Categories: Stratigraphy
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sects., geol. sketch maps
N35°49'60" - N37°30'00", W108°30'00" - W106°49'60"
Secondary Affiliation: Univ. Notre Dame, Dep. Earth Sci., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 1987
Program Name: USGSOPNon-USGS publications with USGS authors
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