A rock stratigraphic or lithostratigraphic unit is defined as what?

• A. The unit is lithologically homogeneous or contains a characteristic combination of rock types
• B. The unit is defined by fossil assemblages
• C. The unit is defined by grain size distribution alone
• D. The unit is defined by color alone

Answer: (A)

Focus on rock type. A lithostratigraphic unit is defined by its lithology—the actual rock types present and how they’re arranged—so it’s either uniform in composition or shows a distinctive, recognizable combination of rock types that can be traced laterally across a region. This is why mapping is based on the rocks themselves, not on fossils or texture alone.

Fossils define different kinds of units (biostratigraphic or fossil zones), while grain size distribution or color can vary within a unit and don’t by themselves establish it. A unit can include multiple rock types if their particular combination is distinctive, but it’s the lithologic makeup that sets the boundary and name.