Dunes on the northern Samoa Penninsula
Please note: this is a work in progress, currently being revised. Please send suggestions to me at jrpatton@northcoast.com. Thank You.
I. Introduction
The project area lies within the coastal dunes on the north spit of Humboldt Bay. This part of coastal California lies on the convergent zone of the North American plate, at the southern end of the Cascadia subduction zone, (Burke and Carver, 1992). The collision of the oceanic and the continental plates have caused the coastal landmass to rise rapidly, forming steep mountains (PWA, 1991). For 140 million years, the ocean bottom sediments have been scraped off by the North American plate to create a complex of highly fractured and deformed , easily erodable rocks which now compose the Franciscan Assemblage. This makes up the basement of the northern California coast region (Burke and Carver, 1992).
Along with intensive land use (logging and road building) over the last century, and with intense winter rain storms, this highly erodable bedrock has eroded rather fast. River sediment yields from the Eel and Mad Rivers exceed any other unglaciated watersheds in North America (brown and Ritter, 1971). This sediment provides the source of sand for the beaches and the coastal dune fields.
Plate tectonics have had a significant impact upon the geologic history of the area. The current tectonic setting and geomorphic processes continue to dominate the coastal strip.
II. Csz, Quaternary tectonics and Geologic Setting
The modern Cascadia subduction zone begins northwest of the northern end of Vancouver Island and extends to near Cape Mendocino, California (fig 1). Plate convergence has controlled the geologic development of this region since the Mesozoic. Global plate motion models and the magnetic qualities of the Gorda plate, combined, suggest the rate of convergence between the North American plate and the Juan de Fuca / Gorda plates to be between 2 and 4 cm per year. The northern movement of the Mendocino triple junction , and thus, the movement of the subductioning oceanic slab, is about 4 to 5 cm per year. (Engelbretsen, et al., 1985).
The Gorda plate originates at the Gorda rise, located 200-300 km west of the Csz. The Juan de Fuca and Gorda plates are relatively young (when compared to the Pacific plate). The Gorda plate age ranges from 5 ma at the north to 9 ma near the Mtj (Burke and Carver, 1992). The deformation front is located between 35 and 60 km offshore of northern California (Clarke, 1992; McPherson, 1989; Kelsey and Carver, 1988). East of the deformation front, the North American plate is deformed into a 75-100 km wide fold and thrust belt (Carver, 1987a; Clarke, 1992).
The fold and thrust belt includes fault caused anticlines, synclines and systems of north-northwest trending east-northeast dipping reverse faults. Most of this belt is located offshore, but it extends onshore from north of Cape Mendocino to north of Patrick's Point. The development of this fold and thrust belt has dominated the geologic and geomorphic development of this coastal region (Burke and Carver, 1992).
The fold and thrust belt contains two thrust systems. They extend offshore, parallel to the Csz. In northern California, where they come onshore, they are known as the Mad River fault zone (MRfz) and the Little Salmon fault zone (LSfz). These two systems are separated by the Freshwater syncline, a 20-30 km wide structure in the fold and thrust belt (Burke and Carver, 1992).
The MRfz comprises the eastern edge of the fold and thrust belt. The MRfz is 25km wide and intersects the coast from Big Lagoon to Arcata. The LSfz crosses the southern end of Humboldt Bay and continues east-southeast to the town of Bridgeville. Near the coast, the folds and thrusts of these fault systems deform late Pleistocene and Holocene sediments (Burke and Carver, 1992).
Humboldt Bay contains three sections. These sections are filled river valleys that are related to the alternating anticlines and synclines. Arcata Bay corresponds with the shallow Freshwater syncline, which can be observed in the offshore stratigraphy(FOP #10). Entrance Bay correlates with the Eel River syncline. South Bay corresponds with the South Bay syncline. Between the Freshwater and the Elk River synclines, is the Eureka anticline, which is uplifting about 0.3mm/yr(FOP #10). Between Entrance Bay and South Bay, are the Humboldt Hill anticline and the Little Salmon fault. Humboldt Bay is suggested to have formed about 3,000 ya (Leroy, 1997, personal communication).
Substantial evidence for sudden subsidence events is recorded in the late Holocene stratigraphy of these synclines. (Burke and Carver, 1992). In the axes of these synclines there are sequences of tidal muds overlying salt marsh peats. The buried salt marsh plants are in the growth position and are completely covered by the overlying intertidal muds. This suggests episodes of rapid subsidence, followed by gradual emergence (FOP #10). This rapid subsidence is interpreted to correlate to large magnitude earthquakes.
Fossil Spruce trees are also found in their growth position in the upper intertidal layer in the Mad River Slough at the north end of Humboldt Bay (Burke and Carver, 1992). The growth rings of these trees suggest vigorous growth followed by sudden death, indicating they were not killed by slow sea water rise or by salt water intrusion (FOP #10). After comparing the dates of seven of these trees, it is suggested they all died within 2 years of each other.
The tree ring patterns of spruce trees killed in Alaska after the 1964 subduction zone earthquake resemble the rings at Mad River slough. This suggests a similar course of events.
The evidence of the fossil marsh plants, fossil trees, and the submergence recorded in the stratigraphy along the coast of Humboldt Bay and the Eel River valley indicates coseismic subsidence during large earthquakes generated by the Csz (Burke and Carver, 1992). Radiocarbon dating for these events recorded in the syncline stratigraphy suggest at least 7 late Holocene events. More than 60 c14 samples have been used for this evidence. The dates suggested by this method indicate events 300, 800, 1100 (two events), 1700, 2600 and 3000 years BP.
Another event associated with large subduction zone earthquakes is that of a tsunami. Tsunamis are a series of waves generated by the vertical displacement of a column of water in the ocean floor (or other large body of water). With tsunami waves the entire column of water from the sea floor to the ocean surface is displaced, instead of only the upper few meters as in normal ocean waves or storm surges (Bolt, 1995). Local tsunamis can have a runup height of 7 to 20 meters (Carver, 1997, p.c.). The local tsunami from the 300 ya earthquake is evident in the local Native American oral history. The village of Orekwa, with an elevation of 18 m, was partially destroyed due to the tsunami(Carver, 1997, p.c.).
III. shoreline processes
Local tectonics has been shown to have a vast impact on the geologic processes along the coast of Humboldt County (Clarke and Carver, 1992). The uplifting and subsidence has indeed had such an impact on the evolution and operation of the coastal sand dunes. Geologically recent events have produced much evidence for rapid vertical movements within the dunes, the adjacent sea floor, and in bay and terrace sediments (Carver, 1991; Clarke and Carver, 1992).
However, the waves and swells generated by local winds and distant storms are the driving factors controlling beach processes. During the summer, low (3-5 ft.), long period waves (called swell) are generated from northwesterly winds. During the winter, the predominant waves are high (10-25 ft.) which accompany cyclonic systems and strong winds from the southwest and southeast (PWA, 1991).
Sediment in the ocean is transported by these waves and swells in a process called longshore transport (O'Neill, 1985). During the winter months, sand is transported north, from the mouth of the Eel River to Trinidad Head. During the summer months, sand is transported in a southerly direction. However, there is no consensus as to what is the net transfer of sand during the year (PWA, 1991).
The mouth of Humboldt Bay provides a barrier to this sediment transport. Before the jetties were constructed, the mouth of the bay moved annually. The spits adjacent to the jetties have had an increased accumulation of sand which has been widening the spit. This provides evidence of north-south longshore transport.
Sand is also transported up and down the beach face in an annual, cyclical manner (Carte, 1988; Gares, et al., 1979; Weigers, 1979). During the spring and summer swells, sand is moved up and onto the beach from its off-shore location. This creates a summer beach profile with a steep and distinct sand berm on the upper strand. Waves carry and drop their sand here (PWA, 1991).
When the high energy winter storms come, they wear away the summer berm deposits. The wave action is often vigorous enough to affect the first protective dune ridge. These waves carry the sand back into the deeper water where it is deposited onto an off-shore bar (Komar, 1979). The first line of breakers, up to 1,000 feet off shore, indicates the location of this bar.
All of these processes, the drainage basin erosion and sediment yield, the sediment transport by the nearby rivers, longshore sand transport, and the cyclic development and destruction of the summer berms, work together to provide the sand for the aeolian environment. Once the sand is on the summer berm, it may dry out and then enter the aeolian/dune environment (Carter, 1988). Ranwell suggests that 10-20% of the sand that ends up in the aeolian environment comes from the foreshore, the zone between mean low water and the mean high tide line. The remaining 80% or so come from the backshore, which runs from the mean high tide line to the first dune ridge.
IV. Dune geomorphology
Sands blown off the beach and upper strand, and then to the adjacent dune systems, maintain the coastal sand dunes in the study area. This process has been described by many researchers researchers (Bird, 1985; Carter, 1977; Carter, 1988; Hansom, 1988; Davies, 1980; Davidson-Arnott and Law, 1990; McLachlan, 1990). These processes are rather complicated and require more research (Nordstrom, et al., 1990). We have no clear evidence to determine the net transfer of sediment into the coastal dune system (PWA, 1991).
The factors significant to the annual sand movement from the beach to the dune system include: surface moisture, wide expanses of open sand, the abundance of wood and other litter, wind speed and direction, and vegetation density in the upper strand and primary foredune ridge (PWA, 1991).
V. North Spit Dune Morphology and Processes
The coastal dunes within the study area contain several morphologic features. These begin with the shadow dunes that develop behind the driftwood in the upper strand. These develop into embryonic dunes as sand is deposited from the strand. The embryonic dunes owe their existence to the vegetation that often invades and owe their destruction to the winter waves. If these dunes last long enough, they build up and become part of the foredune (Pethick, 1984).
The primary foredune ridge delineates the active upper strand, where beach processes guide development, and the dune system, which subsists from the aeolian influences (Steers, 1969). This foredune ridge is often continuous and due to constant wave attack, runs parallel to the beach (Bird, 1969). Vegetation that traps sand blown off the beach, is the key factor influencing the presence, width and height (Cooper, 1967; Bird, 1969; Davies, 1980; Pethick, 1984).
European Beachgrass (Ammophila arenaria or marram) is highly effective in trapping the moving sand to build up and stabilize open sand areas. Hansom (1988) suggests this grass to be the most effective trapping vegetation found in coastal dunes. Marram grows vigorously in areas of active sand deposition.
Behind the primary foredune ridge lies a series of older foredune ridges known as the foredune complex. This complex is 30 to 300 feet wide and has a lower rate of sand accumulation and growth (Carter, 1988; Hansom, 1988). Following the foredune complex lie deflation hollows and / or deflation plains (Cooper, 1967).
As the primary foredune ridge breaks in continuity, high wind velocities are created and sediment transport increases (Hansom, 1988). As these primary foredune ridges progress inland, they are replaced by new, more continuous primary foredunes (Pethick, 1984). Behind the primary foredune ridge, ridges have formed parallel to the summer wind direction.
The foredunes grow in volume and height until the sand is removed off the summit as fast as it is deposited. There are several events that can cause the removal of sand from the foredune complex. These events include, "man-caused mechanical disturbance (such as vehicle paths), burning or overgrazing of the protective vegetative cover, soil nutrient depletion, periods of aridity, trampling, high intensity wind or wave events, or gradual-to-rapid changes in sea level that result in increased wave attack and the eventual destruction of the primary foredune ridge" (PWA, 199?). Coseismic uplift or subsidence can also provide the elements necessary to change the dune forming rate and movement (Clarke and Carver, 1992).
As the blowouts concentrate the wind, they remove the sand from the vegetative cover, thus more sand is released. A parabolic shaped sand mass begins a trip inland. The margins of this sand mass are more vegetated than the center of the sand mass. Thus, because the wind is focused in the center and there is less vegetation here, the center of the "parabolic dune" moves inland faster than the margins (Bird, 1969). Where there are several blowouts in close proximity to each other, the emanating parabolic dunes often merge into a sand plain or sand sheet. Due to the overlapping of these parabolic dunes, and their margins, these sand plains or sand sheets have a smaller proportion of marginal ridges (Davies, 1980).
Where the foredune sand is removed down to the water table or down to below the prevailing winds, "deflation hollows" or "deflation plains" form. The erosion slows dramatically or stops completely when this level is reached (Hesp and Thom, 1990; Wiedeman, 1984). These plains enlarge as sand is removed down to this level.
As the sand actively moves inward from the deflation plains, several diagnostic geomorphic features develop. "Shadow dunes" form leeward of physical obstructions (Pethick, 1984). "Sand hummocks" form where parts of the moving sand field are stabilized by protective vegetation as surrounding sand is eroded away (Hesp and Thom, 1990; Wiedeman, 1984). "Transverse dunes" are the features easiest to identify, on these moving sand surfaces. These dunes form a ridge crest perpendicular to the summer winds (Cooper, 1967). The transverse dunes are from two to thirty feet high and up to 300 feet, or more, in length (Wiedeman, 1984). The dunes are spaced 100 to 250 feet apart.
The transverse dunes move at varying rates, often too fast for vegetation to establish itself. At the leading edge of the moving sand surface is a tall slip face that is prograding into the forest. The slip face is oriented parallel to the coastline. In the research area, a fifty foot tall slip face is invading forested, northwest trending ridge-and-swale topography of a paleo-parabolic dune system (PWA).
According to Cooper (1967), three things are necessary for the development of parabolic dunes. First a large volume of sand is required. Second, developed vegetation that allows for occasional blowouts. Lastly, "unidirectional" winds that are capable of the sediment transport are needed.
Vegetation along the lateral edges of the parabolic dune "anchor" the dune by slowing the sediment transport. Thus, the center of the dune progrades at a faster rate forming a u-shaped leeward front (King, 1959). The dune moves forward in response to the sediment supply of the transverse dunes and drops it off at the slip face (PWA). This is burying the spruce forest on the eastern side of the study area.
The rate of has been determined (PWA) for 19 parabolic dunes on the north spit using aerial photos from 1939 and 1988 and from field mapping in 1991. In the 50 year period, the dunes moved an average of 4.7 feet per year. The four year period yielded a 5.7 feet per year average.
VI. Dune history
One observes at least two age classes of dunes in the study area. The first is the active, or recently active, sand dune. The second is the dune that has been forested. The older, forested dunes are being over-run by the younger active parabolic dunes. A third, intermediate age dune form that occurs in isolated locations is currently being studied by Thom Leroy at Humboldt State University.
"These late Holocene tectonic events may be extremely important in the development and evolution of the coastal sand dunes. If the dune systems and beaches underwent the same episodic, vertical changes in elevation with each uplift or subsidence, the location of the shoreline and the expanse of beach available for sand transport would vary dramatically. Such uplifts twice exposed a wide expanse of sea floor and resulted in accelerated dune activity and deposition at Clam Beach in the last 1100 years (Clarke and Carver, 1992)."
The following is a timeline of events suggested by the Pacific Watershed Associates report:
"1. The coastal dune system was stable and largely vegetated most of the distance to the coast just prior to 300 years ago (wrong, it was much longer ago that the forest went to the beach 2000 yrs?);
2. Rapid tectonic subsidence of the area from roughly Manila north to Mad River County Park occurred approximately 300 years ago, in 1700, with the axis of greatest (p.28) subsidence, matching the area of greatest coastline incursion, in the north;
3. The foredune was destroyed by both winter storm waves and summer swells, and larger deposits of coarse, rounded beach cobbles and pebbles were laid down in swales and deflated areas of previously stable, forested dune system. The inland-most extent of coastal incursion is marked by the beach deposits, and exposures of paleosoils give evidence of the formerly stable dune topography which was still preserved to the east of the new coastline;
4. To regain a stable configuration, the coastline responded with gradual seismic rebound over the next few decades, coastal progradation and deposition of sand deposits in the lowered areas. Coastal erosion, followed by this influx of fresh sand, triggered a new episode of dune formation and inland migration, which continues to express itself today;
5. As new parabolic dunes and sand sheets moved inland, formerly stabilized valleys were filled with fresh sand and lower ridges of the former dune topography were eroded, exposing what are now seen as remnant paleosoils. The highest of the old, forested dune ridges remain today as remnants of the former topography, protruding out into the newer, younger fields of active sand;
6. Gradually, the coastline and foredune system has reestablished a relatively stable configuration, subject only to normal storm erosion and intervening periods of rebuilding (PWA)."
VII. Disturbances
The activities of the people who inhabited the spit likely had little impact on most dune processes. Blowouts occurred in overused trails, but not to the extent of the future off-road vehicles. One of these such trails extends from the mouth of the Mad River and extends south (PWA).
Evidence of burning in prehistoric times is also represented in the gravel deposits. Many of the cobbles have been fire cracked.
There are three main factors , affecting the dune system, in the historic period. These include various constructs, planting efforts, and vehicular abuse. A fourth largely significant factor is the geographic location of the spit and its location's propensity to subside, uplift, and produce large local tsunamis. This factor is the only one that which is not created by people.
There are two constructs most significant. The first is the pair of jetties at the mouth of Humboldt Bay. These jetties affect the longshore transport of sediment. The second human effect is that of the water pipeline that traverses the project area. The route of this pipeline harbors a road. The pipeline has produced some shadow dunes within the study area.
Vegetation introduced in 1901 by the Vance Lumber Company, European Beachgrass (Ammophilia arenaria), quite effectively stabilized the active dune system. In the period from 1939 to 1989, there has been an increase of tenfold, the area covered by this grass. Due to its growth attributes, the beachgrass growing on the foredune ridge catches sand rather well. Thus, the foredune ridge grows at an accelerated rate. In the study area, this beachgrass covers seventy-eighty percent of the primary foredune ridge (PWA).
Off road vehicles have, by far, had the most significant impact upon the dune system. This type of erosion can cause irreversible conditions. The dune plants are fragile and are easily destroyed by the crushing power of the ORVs. These vehicles also force the sand to go from an elevated position (high on the dune) to a lower position. When the vehicles kill ample vegetation, dunes become unstable and soon begin to move (Godfrey, et al., 1978; Leatherman and Godfrey, 1979; Niedoroda, 1975).
Since 1988, the trails through the study area have been fenced and marked off. Some trails cut up to 16 feet deep into various dune ridges. Once the vegetation is removed, this vegetation requires ample time to recover. It is unlikely that some of these scars will revegetate (PWA).
VIII Humboldt Bay
Humboldt Bay is young geologically. The bay was formed at sometime between 3,000 and 8,000 years ago. This is due in part to the mean sea level. In terms of the location and shape of the bay we could look more to the geophysics associated with the Cascadia subduction zone.
In the geologic record of the Quaternary, we find evidence of large scale sea level fluctuations associated with the forming and melting of glaciers, ice sheets. During these periods of glaciation, the sea level fell when the water was trapped frozen on land. When the climate warmed, the water melted and returned to the oceans and the relative sea level rose. Around 18 kya, the Earth's climate was at an extreme low, and thus had a severely low relative sea level approximately 140 m lower than current levels (Bird, E. C. F., 1993).
The rise of the sea level occurred at roughly 1m per year from 18 ka till 6 ka. Since then, the relationship between land and sea has remained rather stable (Bird, E. C. F., 1993). Some localized discrepancies occur due to local emerging or subsiding coasts. The bay occupies one of these areas, the fold and thrust belt of the Cascadia subduction zone.
The northern part of Humboldt Bay lies in a syncline, where the relative land level is subsiding. The middle part of the bay lies on an anticline, where
Based on recent onshore geologic and tectonic studies, and offshore geophysical mapping and drilling data, the local coastline can be divided into a number of alternating zones of Holocene uplift and subsidence (fig. 3; Carver, 1991). These tectonic movements are generated along synclines and anticlines which respond to compression and thrusting along faults within the underlying bedrock. Such events are thought to be both episodic and instantaneous, probably resulting in both types of elevation changes during a single event at different locations along the north and south spits.
the relative land level is rising. The southern part lies on another syncline. Along the coast, synclines are associated with low lying wetlands. We also see marine terraces where there are anticlines.
Thus the site, residing in the northern part of the bay, is subsiding and the relative sea level is rising. This subsidence forms the low lying land area providing room for the bay.
The spit, having been formed as the result of the bay, cannot be older than the bay. The oldest dune complex, the paleo-parabolic dunes, are suggested to have formed about 3 ka.
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