When mapping thickness variations in a sedimentary sequence, what does an isopach map depict?

• A. The thickness variations of a bed
• B. The porosity distribution
• C. The bedrock age
• D. The surface elevation

Answer: (A)

Isopach maps depict the thickness of a particular sedimentary bed across a region. They use contour lines that connect points where the bed has the same vertical thickness, with thickness measured perpendicular to the bedding. These maps show where the bed was deposited thicker or thinner, helping interpret depositional settings and how erosion or changes in sediment supply affected the unit. Porosity distribution would require porosity data and is shown differently, not as thickness contours. Bedrock age and surface elevation relate to time or topography, not the thickness of a specific bed.