The Kaibab formation, formed during the Permian, consists of a sandy limestone. How would it be depicted on a lithologic log?

• A. Sandy limestone
• B. Limestone
• C. Sandstone
• D. Shale

Answer: (D)

Lithologic logs present each rock unit by its dominant lithology, with modifiers used to capture significant accessory components. A rock described as sandy limestone has substantial sand-sized grains within a limestone matrix, so the correct log depiction should reflect both carbonate composition and the clastic content. That means labeling it as sandy limestone, not pure limestone, sandstone, or shale. A pure limestone would imply little to no sand; sandstone is dominated by sand grains with little carbonate cement; shale is clay-rich and fissile. Therefore, for a Permian formation that is a sandy limestone, the most accurate depiction is sandy limestone.