That sounds like a root problem. Sweet gum is not fond of your area (thus the iron chlorosis). In a clay soil when it rains a lot the roots can become waterlogged and start to die from lack of nitrogen. Something has damaged the roots whether it is to much water, weed and feed fertilizer, trenching, etc. If the initial damage is not too severe and the plant has time it may recover. Otherwise it is likely to die back some or all the way.
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