What type of sedimentary rock comprises the majority of the Franciscan Formation?

The Franciscan Formation is primarily composed of graywacke, a type of sedimentary rock characterized by a significant proportion of sand-sized particles mixed with clay or silt. This formation is notable for its complex geological history, having been formed through a combination of deep oceanic sedimentation and tectonic activity associated with the subduction of oceanic plates beneath the North American tectonic plate.

Graywacke forms in a range of environments, but within the context of the Franciscan Formation, it is indicative of rapid sediment accumulation, often in a submarine setting. This sedimentary rock type reflects the dynamic geological processes at play, including the input of materials from nearby land masses and the remobilization of sediments due to tectonic activity.

While the formation does contain other rock types such as sandstone and shale, graywacke makes up the bulk of its composition. The presence of claystone and limestone in other sedimentary contexts does not reflect the predominant features of the Franciscan Formation, which is more characterized by the high-energy processes associated with graywacke deposition. This makes graywacke the dominant lithology in this significant geological unit.