A lithostratigraphic unit is primarily defined by which criterion?

• A. Fossil assemblages
• B. Grain size distribution
• C. Magnetic properties
• D. The lithology and rock-type combinations

Answer: (D)

Lithology and rock-type combinations are what define a lithostratigraphic unit. In lithostratigraphy, units are distinguished by the observable rocks themselves—their mineral composition, textures, and how different rock types are arranged and stacked in the sequence. Boundaries between units are drawn where there is a noticeable change in lithology, not because of time or fossils. That’s why a formation or member is defined by the dominant rock types and their relationships.

Fossil assemblages belong to biostratigraphy and help interpret age and correlate beds based on fossils. Magnetic properties tie to magnetostratigraphy, used for dating and correlation through past geomagnetic reversals. Grain size distribution relates to depositional texture and environment but does not define lithostratigraphic units, which are defined by lithology and rock-type combinations.