(FR) Sedimentary and Igneous Franciscan Assemblage Suite -- California


(FR) Sedimentary and Igneous Franciscan Assemblage Suite -- California


This 14 sample collection primarily consists of clastic sedimentary rocks with metamorphic and igneous components, representing a Franciscan Assemblage. This assemblage is indicative of a subduction zone along the California coast.


Western Minerals, Inc. - Collection was supervised by Dr. Frederick C. Marshall, Assistant Professor of Geology at Principia College.

Spatial Coverage

These specimens were collected across Marin County and the San Francisco Peninsula.

Temporal Coverage

The specimens are Mesozoic in age.


Start here:
Ernst, W.G., 2015, Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex: American Mineralogist, v. 100, p. 6-13. doi: 10.2138/am-2015-4850

  • This source supplies useful information on the low temperature, high pressure environment in which these rocks formed. 
Additional resources:
Barnes, J., Eldam, R., Lee, C.T., Errico, J., Loewy, S., and Cisneros, M., 2013, Petrogenesis of serpentinites from the Franciscan Complex, western California, USA: Lithos, v. 178, p. 143-157. doi:10.1016/j.lithos.2012.12.018
  • Describes the petrogenesis of serpentine as seen in one of the items in the collection (Fr-4).

Cooper, F., Platt, J., and Anczkiewicz, R., 2011, Constraints on early Franciscan subduction rates from 2-D thermal modeling: Earth and Planetary Science Letters, v. 312, p. 69-79. doi:10.1016/j.epsl.2011.09.051

  • This paper suggests that the rate of subduction in this area may have been five times lower than previous studies have found.

Ghatak, A., Basu, A., and Wakabayashi, J., 2013, Implications of Franciscan Complex graywacke geochemistry for sediment transport, provenance determination, burial-exposure duration, and fluid exchange with cosubducted metabasites: Tectonics, v. 32, p. 1480-1492. doi: 10.1002/tect.20078

  • This article states that the europium anomalies of the greywacke samples suggest provenance from the Sierra Nevada batholith.

Langenheim, V., Jachens, R., Wentworth, C., and McLaughlin, R., 2013, Previously unrecognized regional structure of the Coastal Belt of the Franciscan Complex, northern California, revealed by magnetic data: Geosphere, v. 9, p. 1514-1529. doi: 10.1130/GES00942.1

  • Using magnetic data, this article provides an overview of the regional structure of the Franciscan Complex.

Collection Items

OCR PDF of supplemental information of the "Sedimentary and Igneous Franciscan Assemblage - California" collection.

Black Silty Shale
Clay sized feldspars and quartz present, but majority of crystal grains too small to identify in thin section. Surface contains some calcite that formed after lithification. Fresh surfaces do not effervesce with acid meaning reaction is caused by…

Equal mix of angular and rounded sand-sized grains of quartz, with clay sized feldspars. Surface contains significant coverage of calcite that formed after lithification. Fresh surfaces do not effervesce with acid meaning that reaction is based on…

Sand sized quartz grains and smaller feldspars make up the matrix. Grey-brown sand weathers to a rusty golden color.

(Thin Section FOV: 1.28mm)

Radiolarian Chert and Shale
More orange in color that FR-3 (Red Chert). Radiolaria fossils are visible in thin section amongst red-orange martix.

(Thin Section FOV: 1.28)

Arkosic Graywacke
Coarse sand grains made of quartz, surrounded by smaller feldspar grains. Greenish blue surface color, minimal surface weathering.

(Thin section FOV: 1.28)

Calera type Limestone
Fresh surfaces contain grey-white, anhedral calcite and dolomite crystals. Weathered surfaces are dark brown, with a fine powder observable after handling in hand sample. Bubbles from acid reaction turn dark brown on weathered surfaces. These…

Calera-type Limestone with Chert
Groundmass dominated by grey calcite and dolomite. Calcite and dolomite crystals are subhedral in thin section and hand sample.. (FOV: 1.28)

Angular sand size grains, dominated by quartz, have a brown color with a greenish tint. Calcite found on weathered bedding planes after lithification. Slight folding of the bedding planes in one direction. (FOV: 1.28)

Holocrystalline, medium grained, anhedral, ophitic, dibastic crystals. Low silica percentage (46.3%) for a Diabase, bordering on ultramafic compostion. Calcite is present on weathered surfaces, but not on fresh surfaces. (FOV: 3.2mm)

Graywacke Siltstone
Angular, silty size gains, composed of equal parts quatz, feldspars and rock fragments. Prominent orange and brown surface oxydation on all samples. Some samples contain evidence of bedding planes. Calicte is found along bedding planes, as well as…

Fresh surfaces are dark blue in color, while weathered surfaces range in shades of green. Asbestos veins intersect throughout the rock, making up ~5% of the crystal mass. (FOV: 1.28)

Red Chert
Red jasper matrix with interescting veins of while quartz. Prominent physcial weathering, brittle areas fall apart to the touch. Several prominent fractures run through the rock, contributing to the weakened structure. Most fresh surfaces are reddish…

Arkosic Graywacke
Grain size is fine sand to silty sand, mostly angular with some rounded crystals. Various shades of surficial weathering with distinct pockets of orange-brown oxydation. Fresh surfaces brown-green in color. Collected from same quarry as sample FR-1.…

Arkosic Graywacke
Grain sizes sand to silty sand, rounded and angular with very minor examples of weathering and thin quartz vein present. Collected from the same quarry as FR-2. (FOV: 1.28mm)
View all 15 items

Collection Tree

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