The area is considered to be a part of California Geomorphic Province called the Coast Ranges. The Humboldt Bay Area goes from the mouth of the Mattole River north to Gyon Point, and east to a line between Orick and Scotia Bluffs. This definition of area is unique to this report and has not been taken from other published works. Fossil hunters are primarily interested in the occurrence of sedimentary rocks and thus, little attention is paid to the vast tectonic, igneous and metamorphic history of the area except as it relates to the fossils. This report is preliminary and based on the work of others, many of whom are cited below and many more who have contributed. All are gratefully acknowledged. Additional sources are being sought, field trips planned, drawings and photos in preparation.
Due to tectonic forces the oldest rocks are not necessarily at the bottom.
Paleozoic
No Paleozoic sedimentary rocks are reported from this area. The region formed as island arcs, guyots, seamounts, seafloor and sediments were slowly pushed against the North American continent as the latter slowly moved westward since the opening of the Atlantic Ocean in the Triassic (about 180 million years ago). The oldest rocks tend to be in the eastern part of the area as they were accreted onto the continent before the rocks nearer to the present coast line.
Mesozoic
The oldest rocks in the Humboldt Bay area are the Franciscan Complex. The Franciscan is composed of metamorphic rocks and ranges in age from Middle Jurassic to Cretaceous, into the Tertiary, finally ending about 24 million years ago in the Miocene. There are three units in this complex, the Coastal belt, the Central belt and the Coastal belt. Rocks of the Coastal belt are predominantly graywacke (sandstone) and siltstone. Some are intensely deformed in shear zones along the numerous faults in the area (Woodward-Clyde, 1980).
Temperatures were higher then than now; whatever killed the dinosaurs was not climate change as all paleoclimate indicators stay relatively steady over the 65 million year ago boundary between the Mesozoic and the Cenozoic (Mackenzie, 1998: 357).
Franciscan rocks near Petrolia have been producing oil since it was discovered in surface seeps in 1865 (Stanley, 1995).
Cenozoic
Temperatures remained high until about 36 million years ago when they began a relatively abrupt decline to a point higher than midway between the extreme highs of the Mesozoic and the extreme lows of Quaternary glaciations (Mackenzie, 1998: 357).
About 41 million years ago, the area began to be slowly uplifted as the accretionary wedge formed. In the eastern parts of the area, the uppermost Franciscan includes the Yager Terrane which consists of submarine fan channels laid down at about 28 to 30 degrees North latitude. Its rocks include argillite, sandstone and conglomerate forming thin-beds of turbeditic mudstone interbedded with sandstone and contain spores, pollen and dinoflagellates in carbonate concretions and carbonate layers in the mudstone (McLaughlin et al., 2002). This shows that the Yager was near the continental margin, perhaps near a delta. Rocks from the Yager Formation are much harder than those of the older Franciscan and the younger formations which overlie it. Areas of outcrop of the Yager have a higher surface relief because of differential erosion. There are up to about 5,000 feet of rocks in the Yager which may date from the Paleocene, Eocene and or Oligocene in age (Stanley, 2000).
In the coastal area, generally south of the Mattole, the uppermost Franciscan is called the King Range Terrane (Stanley, 1995).
An unconformity separates the uppermost Franciscan Complex from overlying rocks. The Oligocene is represented in the area at Granite Butte on Fickle Hill Ridge and on a Highway 299 roadcut into the north edge of the trachyite intrusion (McLaughlin et al., 2000).
Neogene to the Present
Neogene (Miocene and Pliocene) strata in this area can be up to about 11 thousand feet thick and include the Bear River, Pullen, Eel River and part of the Rio Dell formations. These generally indicate deposition of sediment in a generally southeastward trending embayment. Uplift continued and great quantities of sediment were washed down into the bay and the offshore area (Stanley, 1995).
In some parts of the Humboldt Bay area, the Franciscan Complex is unconformably overlain by Miocene-age Bear River beds; in other parts, the Pliocene Pullen Formation lies atop the unconformity (Stanley, 1995).
The Bear River Beds contain volcanic ash of late Miocene or early Pliocene and may correlate to ash layers found in the bottom beds of the Wildcat Group exposed along the Van Duzen River. The sediments near the Bear River have mollusks that show the environment was a shelf or an inner slope and may indicate an early Pliocene regression (McLaughlin et al., 2000).
The next group of formations fill the Eel River basin both on and offshore. The Pullen, Eel River, Rio Dell, Scotia Bluff Sandstones and Carlotta Formations are collectively known as the Wildcat Formation and are found along the Wild Cat Ridge south of the town of Ferndale, Centerville Beach, Rio Dell and at the Scotia Bluffs as well as near Bridgeville (McLaughlin et al., 2000).
The Pullen Formation is mostly diatomaceous siltstone and mudstone (Woodward-Clyde, 1980). Some iron rich limestone nodules and thin glauconitic sandstone beds were reported by Ogle (1953). The lowest layers appear to have been deposited in nearly 2,000 meters of water (McLaughlin et al., 2000). Outside of Bridgeville, CA, higher up the column, this formation has produced nearshore marine and whale fossils which Stanley (1995) suggests indicates regression of the sea. At Scotia, the bottom of the Pullen Formation unconformably overlies folded Yager terrane (McLaughlin et al., 2000).
Next up column is the Eel River Formation which has dark gray to black mudstones, siltstones and sandstones (Woodward-Clyde, 1980). Ogle (1953) describes the sandstones and some of the finer-grained sediments to be glauconitic. Dating of volcanic ash layers places the Pullen to Eel River age at about 3.4 million years ago (Clifton and Leithold, 1991), but the Eel River Formation is entirely Pliocene in age. It is overlain by the Rio Dell Formation.
The Rio Dell Formation is a massive marine siltstone with some claystone and very fine poorly sorted sandstone. Ingle (1976) studied microfossils at Centerville Beach and determined that water depths were about 1800 meters at the bottom of the formation, but shallowed to about 90 meters at the top.
Pliocene age sandstones in the Rio Dell and other formations in the area may reservoir natural gas. From 1937 to 1991 about 100 billion cubic feet of gas was taken from the Tompkins Hill Field (Stanley, 1995). Tompkins Hill, which produces from a depth of 2,000 to 5,000 feet, and Table Mountain are gas bearing anticline structures (PALCO-EIS, 1999).
Next up the column is the Scotia Bluffs Formation, sometimes called the Scotia Bluffs Sandstone. It lays on top of and interlayers with the Rio Dell Formation. Most of this unit is massive fine and medium grained sandstones, although some pebbly conglomerate and siltstone is also recorded. The Scotia Bluffs are the type section of this formation. Its megafossils are marine and suggest water depths of 30 meters or less (Faustman, 1964). Ash layers found in the interfingered Scotia Bluffs Sandstone and the Carlotta Formation date to about 1.3 to 1.5 million years ago (Clifton and Leithold, 1991).
The Carlotta Formation is on top of and interlayers with the Scotia Bluffs Sandstone. The Carlotta was laid down in predominantly continental environments. It has massive coarse conglomerates, poorly sorted sandstones, bedded and massive blue-gray siltstone, and blue-gray mudstone. Redwood logs to about 25 feet long have been found in some Carlotta Formation deposits. Along the Wildcat Ridge on the Petrolia-Capetown Road outside of Ferndale, the Carlotta is a massive sandstone with a thin pebble conglomerate. The coarse and massive conglomerate unit here is near the base of the formation.
Unconformably overlying the Carlotta are the yellow-orange gravels, sands, silts, and clays that comprise the Pleistocene Hookton Formation. It is often hard to tell the Carlotta from the Hookton, but Ogle (1953) cites the color as the primary differentiation. The Hookton contains an ash layer dated to about 450 thousand years ago (Clifton and Leithold, 1991). In other parts of the area, the Rohnerville Formation unconformably overlies the Carlotta.
Upper Pleistocene and Holocene Humboldt
High rates of tectonic uplift are shown by marine and fluvial terraces and deposits around the Cape Mendocino triple junction. During this time, the geological environment changed from subduction to a transform margin as the triple junction moved northward over time. In the late Wisconsinan (about 45,000 years ago) deposits indicate a large river system flowing parallel to the present coast prior to a rise in sea level associated with the melting of the last glaciers (McLaughlin et al., 2000).
Late Quaternary fluvial terraces can be seen along the Mad, Eel, Van Duzen and Mattole Rivers showing that uplift continues to the present time.
The area is tectonically active and is cut by the Mad River Fault Zone, the Little Salmon Fault, the Russ and False Cape fault zones and perhaps others unnamed and unmapped until earthquakes reveals their locations.
South of the Humboldt Bay area, south of Cape Mendocino the San Andreas Fault permits the Pacific tectonic plate to move north relative to the North America plate. Near the Cape, in an area often referred to as the Cape Mendocino Triple Junction, the fault zone bends westward and becomes the Mendocino strike-slip fault which separates the Pacific plate from the Gorda plate which is being subducted beneath the North American plate. The compressive forces associated with subduction cause thrust-faulting and folding which forms the northwest trending ridge and valley systems so prevalent in the area.
Recent Geology
The processes in effect today are shaping the landscape of the future. It just all moves too slowly for us to see; except in rare catastrophic events like the 1902 and 1992 earthquakes which devastated parts of the lost coast and the Eel River Valley.
California Geological Survey has geotours, maps, the sadly extinct California Geology Magazine index and much more. The best issue for Humboldt also has Joshua Tree in it, so if you are travelling by that National Park, pick up a copy. No one up here seems to have one.
Aalto, K.R., 1989, Geology of Patrick's Point State Park, Humboldt County, California Geology, v. 42, p. 125-133.
Bailey, E.H. 1966. Geology of Northern California. Bulletin 190. California Division of Mines and Geology. San Francisco, California. 508 pages.
Clifton, H.E., and Leithold, E.L., 1991, Quaternary coastal and shallow-marine facies, northern California and the Pacific Northwest, in Morrison, R.B., ed., Quaternary Non-Glacial Geology: Conterminous United States: Decade of North American Geology Volume K-2, Geological Society of America.
Fossil localities are from the HSU Natural History Museum local fossil display case, publication and online sources and the author's personal experience.
Ingle, J.C. 1976. Late Neogene paleobathymetry and paleoenvironments of the Humboldt Basin, northern California. In: Fritsche, E.A., TerBest, Harry, Jr., and Wornardt, W.W., eds. The Neogene Symposium, Pacific Section, Society of Economic Paleontologists and Mineralogists Book Series. San Francisco, California. From: Woodward-Clyde, 1980.
Lehre, Andre. 2000. HSU Geology 700 Field Trip Road Guide. [http://www.humboldt.edu/~geodept/geology700/landslides_floods/ g700_field_handout.pdf -- this is not given as a link to prevent unneccessary downloading]
Mackenzie, Fred T. 1998. Our Changing Planet: An introduction to Earth System Science and Global Environmental Change. Second Edition. Prentice Hall. ISBN 0-13-271-321-7.
McLaughlin, R.J., S.D. Ellen, M.C. Blake, Jr., A.S. Jayko, W.P. Irwin, K.R. Aalto, G.A. Carver, and S.H. Clarke, Jr. 2000. Geology of the Cape Mendocino, Eureka, Garberville, and Southwestern part of the Hayfork 30 x 60 Minute Quadrangles and Adjacent Offshore Area, Northern California. Miscellaneous Field Studies MF-2336, Version 1.0. [http://wrgis.wr.usgs.gov/map-mf/mf2336/ceghmf.pdf -- this is not given as a link to prevent unneccessary downloading]
Stanley, Richard G., 1995, Northern Coastal province (007). In:Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds. 1995 National Assessment of United States Oil and Gas Resources-Results, methodology, and
supporting data: U.S. Geological Survey Digital Data Series DDS-30, 1 CD-ROM. [http://certmapper.cr.usgs.gov/data/ noga95/prov7/text/prov7.pdf -- this is not given as a link to prevent unneccessary downloading]
Woodward-Clyde Consultants, October 1980. Evaluation of the Potential for Resolving the Geologic and Seismic Issues at the Humboldt Bay Power Plan Unit No. 3, prepared for Pacific Gas and Electric Company. (See especially the technical appendices), October, 1980. In: Lehre, Andre. 2000. HSU Geology 700 Field Trip Road Guide. [http://www.humboldt.edu/~geodept/geology700/landslides_floods/ g700_field_handout.pdf -- this is not given as a link to prevent unneccessary downloading]
