Northern Franciscan

October 30, 2011, 8:42 pm

The Northern Franciscan is in a steep mountainous area of the northern California Coast Ranges, USA, with substantial oceanic influence on microclimate, including summer fog.  It is bounded by the South Fork Mountain Thrust fault and a branch of it on the east, by the Pacific Ocean on the west, and by the Grogan fault on the southwest. The climate is temperate and humid (MLRA 4b). 

caption Subsection 263Ab, Del Norte Coast Redwood State Park (James R. Nelson)

Formation

Most of the Franciscan is uplift as a result of tectonic plate collisions with colliding plate movements generally southeast and northeast (explaining the resultant perpendicular ridgelines that run northwest; some of the collisions of the Pacific and Continental Plates abutted more  from opposite west southwest and east-northeast plate movements, which explain the north-northwest ridgelines of some formations. Most of the uplift began ten to 20 million years before present, and continues to the present. Some intervening volcanic activity complicates the Franciscan occurrence, but most of the mountainous elements of this region are derived from ancient sedimentary seabeds..

Lithology and Stratigraphy

This subsection is dominated by Jurassic and Cretaceous Franciscan sedimentary, minor volcanic, and metamorphic rocks of the Eastern Belt.  They are intensely folded and faulted.  Plio-Pleistocene marine and nonmarine sediments are the predominant strata between the Lost Man fault and the ocean, south of the mouth of the Klamath River.  There are small areas of recent alluvium along the Klamath and Smith Rivers and Pleistocene gravels on mountain ridges between the Klamath River and the Lost Man fault.

Geomorphology

This is a subsection of mountains with rounded ridges, steep and moderately steep sides, and narrow canyons.  Most of the mountains are elongated in north-northwest to northwest directions and have subequal summits with increasing elevation toward the interior.   Plio-Pleistocene sediments southwest of the Lost Man fault have been elevated nearly 1000 feet above sea-level and highly dissected. The subsection elevation range is from sea-level up to 3092 feet on School House Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.  The density (area/area) of landslides is very high.

caption Subsection 263Ab, Coastal bluff, scrub (James R. Nelson)

Soils

The soils are mostly Typic Humitropepts and Mollic Hapludalfs.  Typic Tropohumults and Ultic Hapludalfs are the common soils on Plio-Pleistocene sediments.  Typic Tropofluvents and Cumulic Humaquepts occur in alluvium along the major streams.  Most of the soils are leached free of carbonates and the older ones are strongly acid.  The soil temperature regimes are predominantly isomesic.  Soil moisture regimes are mostly udic with some aquic.

Vegetation

The predominant natural plant community is Redwood series.  Douglas-fir - tanoak series occurs on upper slopes and ridges.  Sitka spruce occurs along the coast.  There are patches of oak woodland communities including: Black oak series, Oregon white oak series, and Pacific reedgrass series on south-facing slopes and ridges. Red alder series is common in riparian areas.

Characteristic vegetative series by lifeform include:

  • Grasslands: Introduced perennial grassland series, Pacific reedgrass series.
  • Saltmarsh vegetation: Pickleweed series, Saltgrass series.
  • Shrublands: Salal - California huckleberry series.
  • Forests and woodlands: Black oak series, California bay series, Douglas-fir - tanoak series, Grand fir series, Red alder series, Redwood series, Sitka spruce series, Tanoak series, Western hemlock series.

Climate

The mean annual precipitation is about 60 to 120 inches.  Most is rain at lower elevations and some is snow at higher elevations.  Mean annual temperature is about 45° to 53°F.  The mean freeze-free period is about 250 to 300 days.

Surface Water

Runoff is rapid and many of the smaller streams are dry by the end of the summer.   The Klamath and Smith Rivers, which drain from the Klamath Mountains, cross this subsection to reach the ocean.  Natural lakes are absent, other than minor and temporary ponding by landslides.

See Also

Glossary

Citation

(2011). Northern Franciscan. Retrieved from http://www.eoearth.org/view/article/51cbee887896bb431f69889d

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