Final answer:
At the bottom of brackish estuaries, the sediment typically consists of alternating beds of sand and silty clay, with bioturbation indicators like root structures and shell fragments.
Step-by-step explanation:
The type of sediment found at the bottom of brackish estuaries typically consists of a mix of organic and inorganic material. This includes alternating beds of poorly sorted sand and silty clay, often with signs of bioturbation such as root structures and shell fragments. The dynamic environment of estuaries, with fluctuations in salinity and sediment deposition due to tides and river flow, creates conditions that deposit various types of sediments, ranging from fine muds to coarser sands mixed with organic detritus from estuarine organisms.