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The Tuscan Formation

Tuscan Formation cliffs in Bidwell Park

The Tuscan formation consists of a series of layers deposited by streams and mudflows between 2 and 4 million years ago. The mudflows spread out over the area burying older rock, filling low areas and gradually building a flat subdued landscape. The Tuscan Formation is characterized by:

  • Four million year old volcanic ash horizon at the bottom of the formation
  • Lower portion consisting primarily of reddish brown sandstone
  • Upper portion consisting of interbedded conglomerates and volcanic breccias
  • The layers within the formation are nearly horizontal

Rock Types

Sandstone

The lower portion of the formation is a fine to medium grained sandstone with a volcanic ash horizon at the very bottom that is 4 million years old. The sand was deposited in fine layers; some of which contain mud cracks. Mudcracks tell us that the sediment underwent times of wetting and drying. This basal sandstone also contains fossils of pinecones and other woody debris, which suggests that the climate was similar to today.

Breccia and Conglomerate

The upper portion of the Tuscan Formation is dominated by alternating thick beds of breccia and conglomerates. These sedimentary rock beds form the cliffs that cap the rims of the canyons. Both the breccias and the conglomerates are sedimentary rocks that consist of a mix of large and small particles. The particles that make up the rocks are volcanic indicating the source are for the particles was a volcanic terrain.

Breccia in the Tuscan Formation
Breccia

The breccias have a unique texture in that the large particles are angular and are completely surrounded by finer particles. This indicates that once the transportation mechanism began to slow down the large particles did not have an opportunity to settle to the bottom. The conclusion is that the breccia deposits represents mudflows that moved into the Chico area as thick slow moving slurries. These mudflows covered the land, buried the vegetation and filled stream valleys. In time the surface of the mudflow developed a new growth of pine forest and grasslands and the area recovered. The fossilized woody material found in the volcanic mudflows indicates the climate was similar to today.

Conglomerate in the Tuscan Formation
Conglomerate

Streams also reestablished their channels on top of the mudflows. These streams eroded the tops of the mudflows and re-deposited the materials as stream gravels. The stream gravel deposits are now represented in the rock record as conglomerates. The large particles were rounded as they tumbled down the streambed and when the stream slowed the large particles quickly settled to the bottom. The conglomerates consist of rounded boulders all touching each other and the voids between the boulders are filled with finer materials. An additional indicator of stream flow direction is the presence of crossbedding within the stream deposits. Cross bedding indicates the streams flowed towards the west, which is similar to the flow direction of modern streams.

Often the streams would abandon their channels and the gravel deposits would be exposed to weathering. The top of the gravel deposit would develop soils and plants would start to grow. Some conglomerate beds have paleosoil horizons at the top of the beds and relic root structures can be found in the clay rich horizon.

This sequence of deposition from breccia to conglomerate repeats itself often in the upper Tuscan Formation. It represents the repetitive cyclic character of the deposition regime in the area from mudflow to eroded mudflow to stream deposits.

Locations

With the exception of Quaternary sediments, the Tuscan formation is one of the youngest formations in the northern California foothills. As a result, the Tuscan formation mud flows are exposed in wide areas of the lower foothills and overlay many different types of rocks. Of the sites visited in this field trip, the Tuscan formation occurs at Bidwell Park and Pentz (view map).