The 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
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
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
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.
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