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Re-evaluation of Correlations for Early Cretaceous Strata along the Margins of the Western Interior Basin: Lytle Formation and Equivalents


Early Cretaceous stratigraphic units of the Western Interior Basin (WIB) have long been considered to be temporally and physically correlative based on their unconformable stratigraphic position above the Late Jurassic Morrison Formation and similarities in lithologic character. The Lytle Sandstone is present in this position from northeastern New Mexico to southern Wyoming and is interpreted to have been deposited under fluvial and lacustrine conditions. It consists of lenticular fine- to medium-grained silty sandstone and varicolored mudstone. An analysis of published literature reveals significant problems with accepted stratigraphic interpretations, including:

1) Morrison/Lytle contact uncertainty: The Lytle Sandstone strongly resembles the underlying Morrison Formation, and placement of the contact is often uncertain. The contact has been defined by most authors as an erosional surface; however, incision can be minor to negligible and might represent lateral channel migration, rather than a true unconformity. At some localities, an erosional surface is missing altogether, suggesting a conformable stratigraphic relationship.

2) Highly disparate ages for the Lytle and for hiatal length of the basal unconformity: Dating of the Lytle is problematic; few, if any, dates from radiometric or biostratigraphic sources have been reported. Published dates range from middle Kimmeridgian to early Albian, a disparity of up to 50 my.

3) Questionable regional correlations: Using stratigraphic position and similarity in lithology, the Lytle has previously been assumed to correlate with Early Cretaceous units across much of the WIB. These include the Cedar Mountain Formation (Utah), lower Gannet Group (Idaho), Cloverly Formation (Wyoming), Kootenai Formation (Montana), Burro Canyon Formation (Colorado), Cheyenne Sandstone (Kansas), and Lakota Sandstone (South Dakota); however, these “correlative” units also show a wide range in age. Given they were deposited in the earliest stages of foreland basin development, under entirely continental conditions, and prior to incursion of the interior seaway, and that they were sourced from opposite sides of the WIB, they were most likely deposited in several separate basins with unique, unrelated histories.

4) Inconsistent use of the term “Lytle:” The hierarchal relationship of the Lytle to enveloping stratigraphic units changes from north, where it is of formational rank and part of the Dakota Group, to south, where it becomes a member of the Purgatoire Formation.

This study recommends a re-evaluation of the Lytle and other Early Cretaceous units of the WIB to refine lithostratigrapic and sequence stratigraphic relationships in order to better understand early development of the WIB, prior to establishment of base-level controls by an encroaching seaway. This is particularly important for the WIB, as it has become a global standard as a litho- and sequence stratigraphic analog for foreland basin depositional patterns. Therefore, misidentification and miscorrelation of stratigraphic units and surfaces can negatively impact application of the WIB model to hydrocarbon exploration, as well as strategies for environmental contamination remediation.