The Lytle Formation is a geological formation of Early Cretaceous age found in the western United States, primarily within the Rocky Mountains region, including parts of Colorado, Wyoming, South Dakota, and New Mexico. It is typically recognized as the basal unit of the Lower Cretaceous sequence in these areas, often lying unconformably above Jurassic or older Paleozoic strata.
Stratigraphy and Lithology
The Lytle Formation consists predominantly of massive to cross-bedded, coarse-grained sandstone, often conglomeratic, with interbeds of finer-grained sandstone, siltstone, and mudstone. The sandstones are commonly quartzose to subarkosic and can exhibit reddish, grayish, or variegated colors. The presence of chert and other durable clasts in the conglomerates is common. Its thickness can vary significantly, ranging from a few tens of feet to over 300 feet (90 meters) in some localities.
Depositional Environment
The Lytle Formation is interpreted as having been deposited in a continental, fluvial (riverine) environment, characterized by braided river systems. These deposits represent the initial phase of Cretaceous sedimentation following a period of erosion and regional uplift (the "post-Morrison unconformity"). The clastic material was sourced from elevated landmasses to the west and south, gradually infilling topographic lows.
Age
Based on stratigraphic position, palynological data (spores and pollen), and correlation with other regional units, the Lytle Formation is assigned an Early Cretaceous age, generally spanning from the Valanginian to the Barremian or early Aptian stages. It predates the widespread marine transgression that deposited units like the overlying Skull Creek Shale.
Regional Context
The Lytle Formation is often considered the lowest unit of the Dakota Group in some regions, while in others, it underlies the Dakota Group and is sometimes correlated with other basal Cretaceous fluvial units such as the Lakota Formation (in the Black Hills region) or parts of the Burro Canyon Formation. Its type locality is near Lytle, Colorado, from exposures studied by T.W. Stanton in 1905.
Economic and Hydrogeological Significance
While generally not a significant source of hydrocarbons, the Lytle Formation can serve as an important aquifer in some areas, providing groundwater resources due to its porous sandstone lithology. Fossils are relatively rare within the Lytle Formation, primarily consisting of fossilized wood fragments, plant remains, and palynomorphs.