What types of rocks make up the Sierra Madre Occidental?

The Sierra Madre Occidental is primarily composed of a complex geological structure rather than a simple layer of volcanic rock. Its composition includes:

• Cenozoic Volcanic Deposits: Large quantities of lava and ash were deposited across the region during the Cenozoic Era.1
• Mesozoic Sediments: The range features underlying strata from the Mesozoic Era that were deformed by folding and faulting.1
• Paleozoic Strata: Materials from the latter part of the Paleozoic Era are found overlying older materials from the early part of that era.1
• Crystalline Rocks: The range contains old crystalline rocks that serve as part of the basement structure.1
• Intrusions: There are numerous igneous intrusions of varying sizes throughout the mountain system.1

While it was historically assumed to consist mostly of nearly horizontal lavas mantling older sediments, modern understanding confirms a more complex history involving multiple periods of deformation.1