In a regressive sediment sequence followed by delta development, which order from oldest to youngest is correct?

In a regressive sediment sequence followed by delta development, which order from oldest to youngest is correct?

• A. Shale, limestone and sandstone overlain by clay and marsh deposits
• B. Limestone, shale and sandstone overlain by sand, clay and marsh deposits
• C. Sandstone, limestone and shale overlain by marsh deposits
• D. Marsh deposits overlain by sandstone, shale, and limestone

Answer: (B)

In a regressive sequence that is followed by delta development, the environment shifts from deeper, quieter waters to progressively shallower, more energetic settings, with deltaic deposits building on top. Limestone forms in relatively deep, clear marine conditions, so it represents the older, earlier-deposited units. As sea level falls and shorelines prograde, the environment becomes shallower and more clastic, yielding offshore shale and then nearshore sandstone. When the delta develops, its architecture stacks on top of these earlier deposits, producing deltaic sands and muds, along with marsh deposits behind the delta front. Therefore, the oldest to youngest order is limestone first, then shale and sandstone, overlain by sand, clay and marsh deposits. The other sequences violate this deep-to-shallow progression or place deltaic or marsh deposits beneath earlier marine rocks.

In a regressive sequence that is followed by delta development, the environment shifts from deeper, quieter waters to progressively shallower, more energetic settings, with deltaic deposits building on top. Limestone forms in relatively deep, clear marine conditions, so it represents the older, earlier-deposited units. As sea level falls and shorelines prograde, the environment becomes shallower and more clastic, yielding offshore shale and then nearshore sandstone. When the delta develops, its architecture stacks on top of these earlier deposits, producing deltaic sands and muds, along with marsh deposits behind the delta front. Therefore, the oldest to youngest order is limestone first, then shale and sandstone, overlain by sand, clay and marsh deposits. The other sequences violate this deep-to-shallow progression or place deltaic or marsh deposits beneath earlier marine rocks.