Here's why:

* Sedimentary structures form within sedimentary rocks, which are formed from the accumulation and cementation of sediments. They are features like ripple marks, cross-bedding, mud cracks, and fossils.

* Magma emplacement is the process of molten rock (magma) intruding into existing rock layers. This intrusion creates igneous rocks, not sedimentary rocks.

Igneous rocks form from the cooling and solidification of magma or lava. Features associated with magma emplacement, such as dikes, sills, batholiths, and volcanic necks, are igneous structures.

Let me know if you have any other questions about geology!