CHAPTER 3
ESTABLISHMENT OF AGES FOR FLUVIAL TERRACE LEVELS AT SAN JUAN CREEK, CALIFORNIA
Abstract
The ages of 13 near-coastal fluvial terrace levels occurring in the San Juan Creek drainage basin in southern California are evaluated using 3 methods, including: 1) cosmogenic radionuclide dating; 2) correlation with previously dated marine terraces; and 3) correlation with the glacio-eustatic sea level fluctuation curve. The cosmogenic radionuclide dating analysis showed limited results due primarily to the lack of adequate sampling material on and within the terraces. Correlation with previously dated marine terraces is achieved using the proximity of the fluvial and marine systems. This correlation reveals an extremely close agreement between adjusted fluvial terrace elevations and the elevations of previously dated marine terraces. The correlation of elevations between the 2 sets of terraces implies shared chronology of formation. The evaluation of fluvial terraces with regards to the glacio-eustatic sea level fluctuation curve shows an extremely good correlation with previously plotted marine terrace age, and a strong correlation with glacio-eustatic sea level highstands. The fluvial terrace levels are assigned ages ranging between ~85 and ~460 ka.
3.1. Introduction
To understand, predict, and mitigate the geologic hazards in southern California, an appreciation of the tectonic evolution, uplift rates, and seismicity within the region is critical. The study of terraces is useful when analyzing the tectonic evolution because terraces indicate a progressive time relationship between tectonic, geomorphic, and marine/fluvial systems. Terraces can be used in establishing distinct uplift rates on opposite sides of fault systems and can even be used to establish recurrence intervals on individual faults.
To establish the age relationship of fluvial terraces in San Juan Creek in southern California, 3 dating methods are evaluated to establish the chronology of these fluvial terraces. These include: cosmogenic radionuclide (CRN) surface exposure dating; the correlation of elevations between fluvial and marine terraces at the mouth of San Juan Creek; and the correlation of the fluvial terraces with the glacio-eustatic sea level fluctuation curve. The San Juan Creek drainage basin lies within the coastal zone of southern California near San Juan Capistrano, and has a significant number of fluvial terraces contained within its tributaries (Fig. 3.1) occurring in association with marine terraces at the mouth of the drainage system.
Fluvial terraces are step-like landforms found on the sloping hills above a stream. Fluvial terraces can be expressed as notches cut into hillside bedrock and are sometimes sediment-covered, or they may be steps formed of alluvium representing abandoned alluvial floodplains, but they are always a result of stream entrenchment and incision. In contrast, marine terraces are erosional wave-cut platforms that formed during sea level highstands and are preserved on tectonically active coastlines by a combination of uplift and sea level decline. Marine terraces generally consist of a gently sloping seaward platform abutted against a remnant seacliff and covered by colluvium (Fig. 3.2).
When occurring in separate locations, the timing of formation of upstream fluvial terraces and marine terraces may or may not be linked by climate, uplift, or base level change, but in the area of the mouth of a river debauching into the ocean, fluvial and marine terraces are inextricably linked in all these regards.
3.1.1. Climate, Vegetation, Hydrology, and Topography
The San Juan Creek watershed is in southern California, primarily contained within the southern portion of Orange County. The watershed tilts generally southwest. Its southern and eastern portions extend into San Diego and Riverside Counties, respectively. Major tributaries to San Juan Creek include Oso Creek, Canada Gobernadora, Trabuco Arroyo, and Bell Canyon. The abovementioned are intermittent streams at times bearing water virtually the entire year due to the agricultural and urban development of the surrounding area. Originating in the Santa Ana Mountains in the Cleveland National Forest, the main stream channel of San Juan Creek flows ~43.5 km from the headwater at ~1,280 m asl, and empties into the Pacific Ocean near Dana Point Harbor at Doheny State Beach. The downstream section of the drainage basin is a well developed floodplain, while the middle third of the watershed is characterized by rolling hills. The upper third is exceptionally rugged, with deep cutting canyons and steep slopes. The total relief of the area is 1,715 m asl. This is the elevation of Santiago Peak which is situated within the drainage basin. The overall drainage area of this watershed encompasses ~456 km2 (U.S. Army Corps of Engineers, 2002).
The mild Mediterranean climate of southern California is characterized by cool wet winters and warm, dry summers. Limited stream flow occurs other than during and immediately after rains, and rainfall excess rapidly increases runoff (Kochel et al., 1997). Irregular but episodic cycles of dry (10-30 years) periods and wet (3-6) year periods are superimposed on annual variations. These are attributed to the Southern Oscillation (El Niño) climate phenomenon (Kochel et al., 1997). The regular main flood season begins in November and lasts until April. Summer frequently brings rainless periods of several months, and snow occasionally may fall in the farthest upstream part of the watershed (any snow should be considered an insignificant contributor to runoff). Precipitation totals ~33 cm near the coast and ~45 cm in the mountains (U.S. Army Corps of Engineers, 2002).
Within the Cleveland National Forest, the upper San Juan Creek watershed is dominated by chaparral, while the lower elevations display an assortment of sage scrub (riparian vegetation, chaparral and sage/grassland). The major shrubland types of the lower area are chaparral and coastal sage shrub (also referred to as soft chaparral or southern coastal shrub). Oak woodland can only be found in poorly developed pockets of relictual growth, mainly in potreros on hillsides with more moisture and occupying small valleys which are surrounded by the coastal sage and chaparral (Lewis, 1942). Downstream of the Cleveland National Forest, the Trabuco Arroyo watershed has largely been developed (U.S. Army Corps of Engineers, 2002).
The typically narrow, shallow, alluvial valley fill in the area contains unconfined groundwater. This alluvial fill, ranging from ~61 to 0 m, extends throughout the San Juan Creek tributaries and can be found all the way to the coast (U.S. Army Corps of Engineers, 2002). The San Juan Canyon is traversed by the Cristianitos fault separating the groundwater alluvium into two areas. Storage capacity of the San Juan Creek Groundwater Basin is projected to be ~0.111 km3 (U.S. Army Corps of Engineers, 2002).
3.1.2. Geologic and Geomorphic Setting
As shown in Figure 1.1, southern California can be separated into the Transverse Ranges, the Los Angeles Basin, and the Peninsular Ranges. Several Cretaceous granitic blocks are contained within the Peninsular Ranges south of the Los Angeles Basin. These have been uplifted along major faults and extend southward to the tip of Baja California with the San Andreas fault making up their eastern boundary. The location of this study area lies within the Santa Ana Mountains, which occur in the northeast portion of the Peninsular Ranges near the coast. The study area is located on the western border of the Santa Ana Mountains, 32 km south of the Los Angeles Basin just north of Camp Pendleton, and consists of the fluvial drainage basin of the San Juan Creek. The San Juan Creek Basin is an active drainage of the west-facing southern Santa Ana Mountains. Westward tilting and progressive uplift have resulted in headward erosion and entrenchment of the tributary and river system, and have created fluvial terraces throughout the drainage basin. The river basin includes the tributaries of Oso Creek, Trabuco Arroyo, Canada Gobernadora, Bell Canyon and San Juan Creek (Fig. 3.1).
At the city of Dana Point, San Juan Creek empties into the Pacific Ocean. At this location the current fluvial floodplain (aggradational thalweg) is at the same level and chronology (inception) as the current interglacial sea level highstand (current marine wave-abraded platform). Although numerous fluvial terraces were once well preserved in this area, the construction of residential housing tracks, roads, and business properties for the cities of Dana Point and San Juan Capistrano have degraded terrace preservation. The fluvial terraces utilized in this study begin to occur ~9.6 km upstream on San Juan Creek (Fig. 3.3).
All of the fluvial terraces mapped in this area are fill terraces with thicknesses >6 m and in this regard represent terraces created by eustatic sea level fluctuation as opposed to strath and cut terraces which are primarily the result of the natural growth, incision, and extension of a river system (Merritts et al., 1994; Sections 2.3.1 and 4.1.3).
Striking north-south across the San Juan Creek Basin, are two fault zones. These include the eastern Mission Viejo fault zone, several branches of which crosscut San Juan Creek and Bell Canyon, and the western Cristianitos fault zone, which has several branches crosscutting San Juan Creek and Trabuco Arroyo. These faults trend through Late Cretaceous to Recent sediment and rocks that make up the coastal zone of southern California in the area of San Juan Capistrano. A sedimentary veneer of Miocene rocks overlays the Cretaceous Williams Formation. In the highlands of the Santa Ana Mountains numerous exposures of igneous Cretaceous plutons occur (Morton and Miller, 1981; Morton and Greensfelder, 1976).
Approximately 16 km to the northwest of San Juan Creek, the San Joaquin Hills form the southern boundary of the Los Angeles Basin fault system (Dolan et al., 1995). Ages for the first and second marine terraces in the suite, found on the western side of the San Joaquin Hills in Orange County between Newport Beach and Laguna Beach, were established by Barrie et al. (1992). These terraces were determined to be 80-85 ka, and 120-130 ka respectively, using amino acid racimization techniques on mollusks found within the terrace marine sediments. Barrie et al. (1992) use this data to calculate an uplift rate of 0.25 m/ka in agreement with dates established by Muhs et al. (2002),
The Barrie et al. (1992) data is then used by Grant et al. (1997) to study the Quaternary deformation and potential blind thrust fault on the northern edge of the study area in the San Joaquin Hills (Fig. 1.1). The marine terraces have been uplifted and deformed as the result of the formation of the northeast-vergent anticline that has formed these hills. It was proposed by Grant et al. (1999) that this anticlinal fold occured above an active, southwest-dipping blind thrust which has a slip-rate of 0.42-0.79 m/ka. This finding is based on uplift rates of 0.21-0.27 m/ka. Rivero et al. (2000) corroborates the reality of this blind thrust. In that study, it is proposed that, with an average southwest dip value of 23o, the San Joaquin Hills anticline is the northern onshore extension of the Oceanside thrust detachment. Grant et al. (1997; 1999) interprets the San Joaquin Hills anticline and blind thrust to be the product of partitioned strike-slip and compressive shortening across the southern Newport-Inglewood fault zones. These recent studies indicate the possibility of a blind thrust fault beneath the San Joaquin Hills on the northwestern boundary of this study area that may extend within the boundaries of the San Juan Creek basin (Grant et al., 1997).
3.1.3. Previous Work
Little previous work has been done on fluvial terraces in southern California. The closest fluvial terraces to the study area that have been surveyed are within Santa Ana Canyon, which is a major drainage for the San Gabriel Mountains. White (1980) establishes 3 terrace levels having the ages of 22.7 ka, 43.8 ka, and 63.3 ka by assuming a terrace soil chronosequence based on the rate of clay formation. South of this study area, Kochel et al. (1997) evaluated fluvial terraces along active fan channels in the San Felipe River in an attempt to document fluvial response to short-term wet cycles such as the 1978-83 wet interval. Within the San Juan Creek study area ~38 fluvial terraces have been surveyed, correlated, and dated by Taylor et al. (2006). These researchers established 6 fluvial terrace levels beginning at 10 m from the active stream. They provide a broad variety of possible dates for each terrace level based on a variety of dating techniques including luminescence dating, correlation with marine terraces, and use of the glacio-eustatic sea level fluctuation curve. The Taylor et al. (2006) study overlaps with the 124 fluvial terraces evaluated for this study, so their data will be utilized to assess, analyze, and correlate with data in this study.
3.2. Methodology
3.2.1. Fluvial Terrace Correlations with Local Marine Terraces
The correlation of previously dated marine terraces with fluvial terraces is possible because at the mouth of a river debauching into the ocean, fluvial and marine terraces merge and, therefore, approximate the same elevation and evolutionary chronology. Therefore, the subtraction of stream elevations from the longitudinal profile of a river debauching into the ocean will cause the profile to appear flat, and the profile will then represent mean sea level. To equilibrate fluvial terraces occuring along that stream to mean sea level, the elevation of the stream must be subtracted from the elevation of the fluvial terraces at the point where the terraces are projected perpendicularly onto the stream gradient. Presuming that the terraces occur in former longitudinal profiles that are parallel to the current stream profile (Merritts et al., 1994; see Chapter 2.2.2), this calculation equilibrates upstream former longitudinal profiles to downstream marine terraces and strandlines. The calculation has the effect of eliminating the curvature distortion of the stream profile and permits a more succinct evaluation of the terrace formative factors (Table 3.1) including age correlation. Figure 3.4 shows Bell Canyon and SSJ fluvial terraces equilibrated to sea level and also shows marine terrace elevations from the San Joaquin Hills 15 km northwest of the study area from Barrie et al. (1992) superimposed on the graph for visual correlation. Table 3.2 presents the results of the fluvial terrace correlations with the marine terraces that are shown on Figure 3.4.
Differences in elevation between average San Juan Creek terrace elevations and those established by Barrie et al. (1992) range only from 1 to 3 m showing a excellent correlation. San Juan Creek fluvial terraces T2, T4, T8, T10, and T11c correlate with marine terraces 4b, 4a, 3, 2, and 1, respectively (Barrie et al., 1992).
3.2.2. Fluvial Terrace Correlations with the Glacio-eustatic Sea level Curve
Former longitudinal stream profiles constructed in Chapter 2 have been correlated with the glacio-eustatic sea level curve in order to determine terrace age chronology. Data from Barbados and New Guinea, combined with other sea level highstand and coral records, have been used to construct a glacio-eustatic sea level fluctuation curve back to ~250 ka (Bloom and Yonekura, 2000; Matthews, 2000). This curve has been supplemented with oxygen isotope data to extend the record of sea level fluctuation back ~1.5 Ma (Shackleton and Opdyke, 1973; Ruddiman, 2001; Lambeck et al., 2002).
When plotting marine terraces against the glacio-eustatic sea level fluctuation curve, the object is to correlate the marine terrace to a particular sea level highstand (or lowstand). The slope of the line (abscissa) drawn between the peak sea level highstand (or occasionally lowstand) and the marine terrace elevation represents the rate of tectonic uplift. If the uplift rate or the age of one or more terraces is known, the task of dating a specific strandline is reduced to correlating it to its appropriate interglacial or interstadial sea level highstand using an abscissa parallel to the known terrace (thereby presuming a constant uplift rate).
The former longitudinal stream profiles of San Juan Creek consist of fill terraces that formed from the eustatic sea level highstand aggradational episode (Chapter 2), and their ages are a direct result of the glacio-eustatic sea level fluctuation (Merritts et al., 1994). Therefore, the glacio-eustatic sea level fluctuation curve can be used to calculate their ages in the same manner as that used for marine terraces, based on marine and debauching fluvial terrace equivalency.
The correlation of former longitudinal stream profiles to the glacio-eustatic sea level fluctuation curve requires that the former longitudinal stream profiles be equilibrated to sea level elevations as described above (Fig. 3.4), and averaged to a single elevation. Ideally, once the stream gradient elevations have been subtracted from the former longitudinal stream profile terraces, the resulting gradient line would be perfectly horizontal. In reality, from the sea level equilibration shown in Figure 3.4, it can be seen that the fluvial terraces in the San Juan Creek/Bell Canyon complex have been dislocated near the northern portion of the study area, where they appear to slope downward. This dislocation may result from the thinning of the aggradational wedge upstream, but more likely it results from erosional and/or seismic interaction with the Mission Viejo fault which turns parallel to, and runs between, terraces in this area. In either case, such post-sea level terrace displacements will negatively influence the estimates of the original terrace level elevations. Therefore, terraces beyond 21 km have been eliminated from the average terrace level equilibration calculations. Table 3.3 shows the average San Juan Creek terrace level elevation calculated by summing all terraces in each level of the study area that occur within 21 km of the ocean and dividing by the number summed for that level.
If the uplift rate remains constant, the marine terrace age (or equilibrated former longitudinal stream profile age) is a result of the terrace elevation above modern sea level minus the elevation of the ancient sea level with relationship to modern sea level. The relationship between terrace height, terrace age, and uplift rate can be seen in the equation:
Age of Terrace = (Terrace Elevation – Ancient Sea Level Elevation) / (Uplift Rate)
This can also be written:
Terrace Elevation = (Uplift Rate x Age of Terrace) + Ancient Sea Level Elevation
The unknown values in this equation cannot be resolved knowing only the present height above sea level and the age of the terrace. To estimate the tectonic uplift rate, a reasonably constant uplift rate (within the last 500 ka) must be assumed, and an assessment of the initial sea level must be established (National Research Council, 2000). After plotting terraces with known ages on the glacio-eustatic sea level fluctuation curve, there formative height above or below modern sea level can be read from the curve, and with the assumption of a uniform uplift rate and the use of the formula stated above, the uplift rate can be calculated. This process can also be used for undated terraces projected with abscissas parallel to those from dated terraces.
Figure 3.5 shows San Juan Creek former longitudinal stream profiles (equilibrated to sea level as shown in Table 3.3) plotted on the observable/proxy glacio-eustatic sea level curve.
Former longitudinal stream profiles were plotted using abscissas based on amino acid racemization strandline dates from Barrie et al. (1992). Elevation age estimates have been tabulated and correlated on Figure 3.5 for San Juan Creek former longitudinal stream profiles and Barrie et al. (1992) marine terraces. Ancient sea level for each former longitudinal stream profile projection was estimated from the glacio-eustatic sea level curve and can be seen tabulated in Table 3.3. Local sea level at about 80 ka in coastal California is estimated to have been approximately 5 m below present sea level (Barrie et al., 1992). Likewise, the sea level at the 125 ka interglacial is estimated to be 6 m above present sea level (National Research Council, 1990). Table 3.3 also shows the calculations and age projections of each of the San Juan Creek equilibrated terrace levels which are tabulated and graphically presented on Figure 3.5.
3.2.3. Sampling Procedures Numerical dating within the San Juan Creek fluvial terrace study area was hampered by the lack of appropriate sample material. The fluvial terraces at all levels were devoid of cobbles and boulders, having instead a well-developed silty soil horizon established on their surfaces. The 5 CRN samples collected for this survey were located near the edges of the terraces and showed possible indications of having been exhumed by erosion of the terrace treads (Fig. 3.6).
Sample 13 and 14 were the largest boulders sampled, and both were found on the tread toe/riser top slope break (terrace levels T11a and T9 respectively) which formed a fairly steep angle. Sample 17 (terrace level T4) also occurred on the tread toe/riser top slope break, but the angle of this slope break was much more subdued. Samples 7 and 19 (terrace levels T8 and T11c, respectively) were the smallest boulders sampled and were both found within 30 m of the tread/riser slope break. In the case of both of these samples, rills and drainage channels had developed within this 30 m zone indicating that the upper soil level was in an active state of erosion. It is anticipated that the chemical age of the samples underestimates the true age of the terrace surfaces.
Other factors that may have affected the reliability of the samples collected for this study include weathering (which may lower the geomorphic surface age estimate by removing 10Be in the outer layers of the rock), and inheritance, which may increase the geomorphic surface age estimate due to the sample having been previously exposed to cosmic rays before finally coming to rest on the terrace surface (Owen et al., 2002; Finkel et al., 2003). Typically, multiple samples would be taken from each landform to provide confidence in the reproducibility of the dating and to estimate CRN inheritance; however, due to the lack of adequate sample material, only 5 representative samples from different terrace levels were selected from this study area for analysis. Sample locations were recorded in the field using a hand-held GPS receiver (Fig. 3.3). The boulders sampled range from ~0.3 to 1 m in diameter.
3.2.4. Laboratory Analysis
Cosmogenic radionuclide surface dating samples were prepared in the geochronology labs at the University of Cincinnati, while accelerator mass spectrometer (AMS) measuring was performed at the center for AMS at Lawrence Livermore National Laboratory (LLNL) by measuring CRN 10Be in quartz (Kohl and Nishiizumi, 1992). At the University of Cincinnati, the rock samples were individually crushed and sieved, and 100 gm of the 250-500 micron fraction of quartz was prepared for each sample. The Be was isolated by leaching the sample fractions in a series of HCl and HF/HNO3 baths (for methodology, see Kohl and Nishiizumi, 1992). A Be carrier was added to the quartz samples, and the samples were then dissolved in 3:1 HF/HNO3. The Be was then isolated by ion exchange chromatography and precipitation, and finally was ignited in a furnace to produce BeO (Dortch et al., 2008). The final samples were mixed with Nb and placed into cathodes for targeting by the LLNL AMS to determine the 10Be /9Be ratios (Davis et al., 1990). An ICN 10Be standard prepared by K. Nishiizumi (half-life of 1.36 x 106 years) was used to determine the 10Be /9Be ratios (Nishiizumi et al., 2007). The equations and production rates in Lal (1991) as modified by Stone (2000) were the basis for age determinations. The final age of each sample was then determined by applying a correction for variations (latitude and altitude) in the geomagnetic field (Nishiizumi et al., 1989). A detailed description of the methodology used for CRN age determination can be found in Owen et al. (2001; 2002) and Balco et al. (2009). Final age results for CRN samples can be found in Table 3.4.
3.3. Results and Discussion
3.3.1. CRN Dating Chronology
As shown in Table 3.3 and Figure 3.5, all of the CRN dates for samples analyzed in this study (with the exception of the date from terrace level T11c to be discussed later) are within 14 ka of the MIS stage 5a interstadial highstand, and in this regard their ages closely match the inception of the 85 ka stadial western United States wet cycle. An examination of the possible scenarios by which these boulders could have accrued cosmic ray exposure can indicate the history of the boulders. The CRN exposure ages of boulders from terraces in this study could result from inheritance alone, from cosmic ray bombardment while exposed on a terrace surface, from cosmic ray bombardment after being buried in a floodplain and later exhumed by a wet cycle near the edge of a terrace tread, or from some combination of all three (Table 3.5).
In the first scenario, if the age of each boulder sampled results from inheritance alone, then the boulder would have gained its primary exposure to cosmic ray bombardment while it remained in situ, and while it traveled through the fluvial system to the terrace. Once on the terrace it would have been buried near the surface, and exhumed only recently to be sampled. Given this scenario, the exposure age of the boulder could be much older or younger than the actual terrace surfaced age where the boulder was found, depending on the amount of in situ exposure time, and the travel time within the fluvial system. Thus, boulder samples with cosmic ray exposures resulting primarily from inheritance can yield a wide range of CRN ages. This wide variation of CRN ages would be augmented by the fact that the boulders sampled were from a wide variety of terrace levels with an accompanying wide variation in formative timing ranging between terrace levels T11c and T4 (Table 3.4).
In the second scenario, if the CRN ages resulted primarily from cosmic ray bombardment while the boulders were exposed on terrace surfaces, then the CRN ages recovered from the boulders would be the same age as the terrace on which they were found. Because the boulders are from a wide variety of terrace levels, again there should be a wide variety of ages between the boulders sampled for the study.
In the third scenario, boulders would have undergone immediate burial on the fluvial floodplain with limited inheritance exposure time. Then the boulders would have to be exhumed near the edge of the tread surface during a strong wet cycle. Most probably the boulders would have been exhumed during the same strong wet cycle, while boulders exhumed in previous strong wet cycles would have been washed away from the edge of the eroding terrace surface. The wet cycle would have to have been the most recent strong wet cycle, or these boulders too would have been tumbled away from the terrace surface. In this case the small degree of variation between boulder exposure ages may represent variations in inheritance travel time accrued as boulders arrive at various terraces before burial, may represent slight differences in the timing of erosional exhumation, or may represent degradation of the boulder surfaces by weathering. This scenario is in agreement with the clustering of CRN dates around the 85 ka stadial wet cycle.
Finally, a scenario combining any or all of these inheritance/cosmic ray exposure scenarios would yield a significant spread in the degree of variation between the ages of the boulders sampled because two out of three of the scenarios provide for a wide variation in CRN sample age.
The high degree of CRN age correlation between boulders from various terrace levels indicates that the samples were indeed exhumed at or around the commencement of the recent 85 ka stadial wet cycle of the most recent glacial event, and that there was minimal inheritance accrued in the CRN exposure aging process of the boulders. This is the same timeframe during which terrace T11c was formed. If a boulder was buried on the T11c floodplain at this time and later exhumed from the T11c terrace level, it would have an age younger than the 85 ka stadial. Hence sample 19 found on terrace level T11c has an age of 53 ka and must have been exhumed after terrace formation and erosion. Further, the exhumation process must not have represented a significantly strong wet cycle or many of the other boulders would have been tumbled away from their precipice locations. Indeed, all of these samples were found on the toe of primarily cobbleless terrace tread surfaces and showed every indication of being exhumed boulders.
Although these data do not provide confirmation of terrace surface ages, they do provide some indication of the geomorphic evolution of fluvial terraces throughout the Pleistocene in western United States. Apparently, while glacial wet cycles provide the impetus for floodplain incision, some wet cycles are also erosive enough to push back terrace risers throughout fluvial systems removing boulders that have accumulated at the toes of terrace treads at every level and replacing them with newly exposed boulders. In particular, the wet cycle that accompanied the 85 ka stadial must have been the most recent strong wet cycle in coastal southern California, and may have had a pronounced effect on erosion and incision within fluvial terrace systems throughout much of western United States.
3.3.2. Fluvial Terrace Correlation with Marine Terraces
The evaluation of terraces conducted in and adjacent to the study area includes: the mapping and dating of marine terraces by Barrie et al. (1992); the mapping and dating of San Juan Creek fluvial terraces by Taylor et al. (2006); the mapping and evaluation of terrace levels shown in Chapter 2; and the dating analysis from this study. The various elevations and age estimates from these studies are correlated in Table 3.6 and Figure 3.7. From Figure 3.7 it can be seen that the San Juan Creek fluvial terrace levels (Chapter 2) are correlative in elevation (variations range only from 1 to 3 m) with the marine terraces established by Barrie et al. (1992), but the terrace level elevations from Taylor et al. (2006) do not correlate well with these 2 studies.
Elevation variations between the Taylor et al. (2006) and Barrie et al. (1992) studies range between 4 to 12 m.
Several general observations can be made from the correlations shown in Table 3.6 and Figure 3.7. First, Figure 3.7 indicates that the Taylor et al. (2006) authors correlate their Qtr6 terrace (10 m) with the Barrie et al. (1992) 80-85 ka Tm No. 1 terrace (18 m). The Taylor et al. (2006) authors state that they make this correlation because Tm No. 1 is the lowest terrace in the Barrie et al. (1992) suite, disregarding an elevation difference of 8 m. This correlation is probably incorrect. It is important to realize that the Barrie et al. (1992) marine terrace suite is undoubtedly an incomplete set of terrace flights leaving many highstand events unrecorded. The T12 terrace level of this current study (9 m) is correlative to the Taylor et al. (2006) Qtr6 terrace (10 m), but this study makes no correlation between the T12 level and marine terraces on the coast in recognition of the incomplete nature of marine terrace suites in southern Orange County.
This same inappropriate rationale can be seen in the Taylor et al. (2006) correlation of Qtr2 (76 m) to marine terrace Qtm3 (95 m) found west of San Juan Creek. These authors correlate Qtm3 with Tm No. 4a (80 m) of the Barrie et al. (1992) suite (Fig. 3.7). Taylor et al. (2006) equate Qtm3 and Tm No. 4a based on the assumption that Qtm3 must have a correlation within the Barrie et al. (1992) suite when, in fact, it need not. These two suites of marine terraces are in completely different locations, and may easily have recorded different sea level highstands based on different wind, wave, and current directions that prevailed at the time. Erosion and mapping inconsistencies may also have played a role in creating a variety of terrace locations. There is no requirement that all terrace flights within these 2 marine suites must be correlative. Making forced correlations that negate elevation equivalency is inappropriate because it implies extreme tectonic adjustment over very small, local areas.
These observations imply that the combination of the marine terraces found west of San Juan Creek (Taylor et al., 2006), and those surveyed by Barrie et al. (1992), may begin to approximate an accurate record of the total sea level highstands on the southern Orange County coastline. If none of the terraces from these 2 suites are overlapping (possibly Qtm1 and Tm No. 2 are equivalent), then 11 marine terrace levels are recorded over a time span of ~700 ka. The discrimination of 14 fluvial terrace flights younger than ~500 ka within San Juan Creek, all probably correlative to sea level highstands, corroborates the requirement of a significantly high number of marine terrace flights to provide an accurate record of sea level highstands on the southern Orange County coastline. Ultimately, the mapping of several suites of marine terraces in various related locations with specific attention paid to mutual uplift rates may make it possible to assemble a complete regional suite (southern Orange County) of marine terrace flights.
Finally, a fourth observation that follows from the third, is that at least 2 higher marine terrace flights (Tm No. 5 and Qtm5; Barrie et al., 1992 and Taylor et al., 2006, respectively) are preserved on the coastline when no remnant of equivalent fluvial terrace tread can be found inland. This suggests that the preservation of highstand events older than ~500 ka may be difficult to find along inland fluvial terraces that form on river systems in southern California.
3.3.3. San Juan Creek Correlations with Glacio-eustatic Sea Level Changes
Figure 3.5 indicates that all of the fluvial terrace levels within the study area correlate with glacio-eustatic highstands. This correlation is consistent with the postulation that this region of the San Juan Creek drainage basin has been dominated by glacio-eustatic sea level fluctuation (Chapter 2 this study; Merritts et al. 1994; Ramsay, 1931; Zeuner, 1945; Clayton, 1964) resulting in a parallel suite of fluvial fill terraces. Age results from this study indicate that in addition to the 5 marine terrace levels younger than 500 ka that were mapped by Barrie et al. (1992), at least 5 more marine terrace levels could be present on the southern California coastline, including highstands at 105, 195, 240, 310, and 460 ka. Also indicated in this figure is the recognition that some highstands may result in the formation of more than 1 fluvial terrace level. While 8 fluvial terraces are associated with single highstands, MIS 5c has 2 fluvial terrace levels associated with it (T11b and T11a), and the MIS 9 highstand has 2 fluvial terrace levels associated with it (T3 and T4). The uplift rate calculations for this study are based on the amino acid racemization dates derived by Barrie et al. (1992). After constructing the glacio-eustatic sea level curve and correlating the abscissas, the ancient sea level has been estimated for each of the fluvial terraces in the study (Table 3.3). Calculation of the uplift rates indicate that individual uplift rate estimates range from 0.27 to 0.32 m/ka yielding an average uplift rate of 0.29 m/ka (Fig. 3.8).
This uplift rate is consistent with 0.25 m/ka established by Barrie et al. (1992), and is also consistent with other uplift rates calculated throughout southern California (Fig. 3.9).
The average 0.29 m/ka uplift rate is inconsistent, however, with the 0.4 m/ka uplift rate calculated by Taylor et al. (2006). The uplift rate of Taylor et al. (2006) is higher than uplift rates obtained throughout most of southern California.
Uplift rates along the southern California coast between San Diego and Orange County fall into 2 distinct groups (Fig. 3.9). The southern group of uplift rates range from approximately 0.10-0.15 m/ka, which varies sharply with the northern group of uplift rates ranging from approximately 0.25-0.30 m/ka. The dividing line between these 2 groups is quite sharp and seems to occur just a few kilometers south of the study area. This boundary coincides with both the Cristianitos and Mission Viejo faults and the Bell Canyon Lineament (Chapter 2), raising the possibility of a major structural boundary transecting San Juan Creek at its deviating bend, and controlling the uplift and seismicity of coastal southern California.
3.4. Conclusions
The dating of fluvial terraces that occur on a near-coastal river system in an actively tectonic uplifting environment can be accomplished by correlation of fluvial terraces to local dated marine terraces. This is possible because the formative relationship between marine and fluvial terraces at the mouth of a debauching river is the same during a sea level highstand event. A river thalweg actually consists of thalassostatic sediment (filling the canyon to sea level) and a wedge of sediment (filling the canyon above sea level) representing aggradation resulting from gradient readjustment. Therefore, upstream fluvial terraces are related to downstream fluvial and marine terraces by reason of the aggradational wedge which forms above, and upstream to, the thalassostatic sediment infilling the canyon. Once the elevation differences between fluvial terraces and marine terraces have been taken into account using a sea level equivalency calculation, fluvial terraces can be correlated with dated marine terraces near the mouth of the river.
This study also suggests that fluvial terraces in a near coastal environment influenced by eustatic sea level fluctuation can be dated using the glacio-eustatic sea level fluctuation curve appropriate to that local area. The correlation of a suite of 13 San Juan Creek fluvial terrace levels to sea level highstands ranging back to 460 ka validates the basic premise assumed but unstated by Taylor et al. (2006), and discussed in Chapter 2 concerning the dominating influence of eustatic sea level fluctuation on the formation of these fluvial terraces (also see Merritts et al., 1994). This correlation also establishes the basic premise of a base level controlled aggradational wedge linking fluvial and marine systems during highstand events (Chapter 2).
The San Juan Creek fluvial terrace levels correlate with 10 sea level highstands (Figure 3.5) ranging in age from 85 to 460 ka, showing that in addition to the 5 marine terraces mapped by Barrie et al. (1992), at least 5 more marine terraces could be present on this stretch of southern California coastline. Using amino acid racemization dates established by Barrie et al. (1992), this study calculates an uplift rate of 0.29 m/ka for the San Juan Creek study area which is very similar to uplift rates established by other researchers from Newport Beach to Dana Point (with the exception of Taylor et al., 2006). This is the southernmost uplift rate above approximately 0.25 m/ka before the abrupt change at San Onofre to 0.09 m/ka suggesting that the Cristianitos fault, the Mission Viejo fault, and the Bell Canyon Lineament may all be part of a structural divide separating distinct tectonic regimes with significantly different rates of motion. This divide could represent a boundary between separate and distinct seismotectonic zones within southern California, or it could itself be a boundary that is seismically and tectonically active.