OF-23-02 Geologic Map of the Ignacio Quadrangle, La Plata County, Colorado

The purpose of this publication is to describe the geology, mineral and groundwater resources, and geologic hazards of the Ignacio 7.5-minute quadrangle. This map is part of the STATEMAP geologic mapping program in La Plata County near Durango, including the Bondad Hill, Loma Linda, and Gem Village quadrangles, which provide 1:24,000 scale geologic coverage of areas previously unmapped at that resolution. CGS geologists Nathan T. Rogers and Mary L. Gillam completed the field work on this project during the mapping seasons of 2022-23. Digital ZIP download. OF-23-02D

From the abstract:

The Ignacio 7.5-minute quadrangle, located within the northern San Juan Basin of southwestern Colorado, exhibits diverse topographic features ranging from the low-lying Los Pinos River valley to the prominent Mesa Mountains. This study provides a comprehensive geological map and report of the Ignacio quadrangle, including analyses of stratigraphy, structure, geological hazards, and resource potential.

The quadrangle’s geological framework comprises surficial deposits of Pleistocene to Holocene age and underling bedrock formations of Paleocene and Eocene age. The Nacimiento and San Jose Formations preserve a record of complex fluvial depositional environments characterized by significant lateral facies variations. A challenging bedrock mapping effort attempted to better delineate the intricate stratigraphic relationships between these formations, which have proven difficult to decipher in this as in previous investigations. Further study is needed. A notable structural element within the quadrangle is the Ignacio-Bondad Anticline, a subtle subsurface feature that may have influenced the deposition and distribution of Paleocene strata.

Surficial mapping identified fluvial, alluvial, colluvial, and eolian deposits that were influenced by regional uplift and Pleistocene climate fluctuations during glacial-interglacial cycles. Older gravels on the Mesa Mountains provide valuable insights into the evolution of regional drainage patterns. Younger deposits reflect progressive landscape lowering and the formation of alluvial terraces along the Los Pinos River.

This study also undertakes an assessment of potential geological hazards, with particular attention to landslides occurring along the flanks of the Mesa Mountains. Groundwater resources are evaluated, with attention to selenium contamination in some areas. The quadrangle has economic significance due to its petroleum and coalbed methane resources.