Two layers of rock contain trilobites that have the same body morphology; however, one rock layer is a shale and the other is an ironstone. Can these two layers be used for correlation?

Yes

Explanation:

If the trilobites are the same then two layers of rock contain trilobites that have the same body morphology; however, one rock layer is a shale and the other is an ironstone.

If a layer contain a distinct fossil then it can be use as marked bed. As we know that layers are formed due to weathering of mountains and erosion and these particle deposit on the sedimentary basin and after long time theses particles form layer of rocks.