Dictionary:

argillaceous

  (är'jə-lā'shəs)

Containing, made of, or resembling clay; clayey.

[From Latin argillāceus : argilla, argil; see argil + -āceus, -aceous.]

Clay-like in composition and texture, referring to rocks containing clay minerals and clay-sized particles, for example, shale.

Composed primarily of clay or shale; clayey.

The adjective has one meaning:

Meaning #1: resembling or containing clay
  Synonym: clayey
  Antonym: arenaceous (meaning #1)