The Cook Mountain Formation is a geologic formation exposed in the Gulf Coastal Plain of the United States, principally in Texas and Louisiana. It is assigned to the early Eocene epoch of the Paleogene period (approximately 56–47 million years ago). The unit was named after Cook Mountain, a locality in Texas, where characteristic exposures of the formation were first described.
Lithology
The formation consists mainly of marine clastic sediments. Typical lithologies include fine‑ to medium‑grained sandstones, siltstones, and mudstones, often displaying glauconitic or ferruginous cement. In certain sections, thin beds of limestone and calcareous marl are present, reflecting intermittent shallow‑marine carbonate deposition.
Stratigraphic relationships
Regionally, the Cook Mountain Formation conformably overlies older Cretaceous units (such as the Woodbine Group) and is generally overlain by younger early‑Eocene or late‑Eocene deposits, including formations of the Wilcox Group. Exact thickness and the nature of bounding contacts vary across the formation’s extent.
Geographic extent
Outcrops and subsurface occurrences of the Cook Mountain Formation are documented across central and east‑central Texas, extending eastward into southwestern Louisiana. The formation is part of the larger Gulf Coastal Plain sedimentary sequence that records the early stages of Atlantic‑western interior sea transgression during the early Eocene.
Paleontology
Fossil assemblages recovered from the Cook Mountain Formation are indicative of a shallow marine environment. Representative fossils include foraminifera, nannoplankton, bivalves, gastropods, and occasional vertebrate remains such as fish teeth and shark spines. These faunal elements have been used for biostratigraphic correlation with other early‑Eocene marine successions across the Gulf Coast region.
History of investigation
The unit was formally defined in the early 20th century by geologists mapping the Gulf Coastal Plain, who recognized its distinctive lithologic character and fossil content. Subsequent studies have refined its age through biostratigraphic and, in limited cases, radiometric dating of interbedded volcanic ash layers, confirming an early Eocene assignment.
Significance
The Cook Mountain Formation provides important insight into early Paleogene marine conditions along the southern United States margin, documenting sedimentary responses to rising sea level and tectonic subsidence following the Cretaceous‑Paleogene boundary. Its fossil record contributes to understanding the early evolution of Eocene marine ecosystems in the Gulf of Mexico realm.