The plural of stratum is strata.
Stratum is a term used to describe a layer or level of material or substance that is formed naturally or through human intervention. It can refer to geological layers of rock or soil, social or economic classes within a society, or other hierarchical structures.
Singular:
Plural:
The graph shows the occurances of the plural of stratum in written English since 1800 using Google's Ngram Viewer.
Sources:
https://www.collinsdictionary.com/dictionary/english/stratum