This mixed-origin rock forms when volcanic ejecta, such as ash and pumice fragments, settles into a basin alongside normal clastic sediments like sand or mud. To be classified under this name, the rock typically contains between 25 and 75 percent pyroclastic material. It represents a transitional phase between a pure volcanic tuff and a standard sedimentary rock.
Heard on the job
The core samples show a distinct layer of tuffite, indicating a volcanic eruption interrupted the limestone deposition.