The Riley Formation is a geological formation primarily recognized in the Llano Uplift region of central Texas, United States. It represents a significant component of the Cambrian System stratigraphy of the area, though its formal stratigraphic nomenclature has undergone revisions over time.
Geological Setting and Stratigraphy
The Riley Formation dates to the Middle to Late Cambrian epoch. Stratigraphically, it typically overlies the Hickory Sandstone and underlies the Lion Mountain Sandstone Member. Historically, the Riley Formation was sometimes considered a central member within the broader Wilberns Formation, which encompasses the entire Upper Cambrian sequence in the Llano Uplift. However, many contemporary geological studies and workers elevate the Riley Formation to its own formation status, comprising several distinct members.When recognized as a formation, the Riley Formation itself is commonly subdivided into three members, from oldest (bottom) to youngest (top):
- Welge Sandstone Member: A thin, basal unit consisting primarily of fine-grained, glauconitic sandstone.
- Morgan Creek Limestone Member: Overlying the Welge, this member is composed predominantly of fossiliferous, nodular limestone interbedded with shale.
- Cap Mountain Limestone Member: The uppermost member, characterized by massive to thick-bedded limestone, often dolomitic, with less shale than the Morgan Creek member.
Lithology and Paleontology
The dominant lithologies of the Riley Formation are limestone and shale, with subordinate sandstone beds. These units represent deposition in a shallow marine environment, progressively moving from nearshore sands (Welge) to deeper shelf limestones and shales (Morgan Creek, Cap Mountain).The formation is notably rich in fossils, particularly trilobites, brachiopods, and other marine invertebrates characteristic of the Cambrian period. These fossils are crucial for biostratigraphic correlation and understanding Cambrian life in North America.