CHAPTER 2
PLEISTOCENE FLUVIAL TERRACE CORRELATION ON A TECTONICALLY ACTIVE COAST, SAN JUAN CAPISTRANO, CALIFORNIA
Abstract
The unique presence of more than 200 terraces throughout the San Juan Creek drainage basin in southern California has allowed the unparalleled opportunity to map and correlate fluvial terrace flights adjacent to a coastal marine system on an actively uplifting coastline. Detailed sedimentological analysis indicates that all of the terraces are fill terraces ranging from 6 to 13 m in thickness. Overall, 124 terraces have been mapped, grouped into levels on the basis of elevation, and correlated throughout both tributary and main canyons. Correlation was accomplished by the construction of 27 cross-valley profiles which were digitized from a topographic base map. Once correlative terrace treads were designated, longitudinal valley profiles of former stream levels were constructed on 2 branches of the San Juan Creek drainage basin. The terrace treads have been correlated into 12 distinctive terrace flights to assess the geomorphic history of the former stream levels. These correlations form the framework for dating of the terrace flights throughout the San Juan Creek drainage system. This research is necessary for the eventual assessment of seismic slip rates and recurrence intervals using the Cristianitos and Mission Viejo fault zones and former longitudinal valley profiles of the San Juan Creek drainage basin. The assessment of seismic slip rates and recurrence intervals could ultimately provide an increased understanding of rates of crustal displacement and geomorphic evolution that will aid in ascertaining a more complete picture of seismic interactions and motions along an actively deforming transform plate boundary.
2.1. Introduction
Over 200 well-developed terraces exist along the main branches of the San Juan Creek drainage system, near San Juan Capistrano in southern California (Fig. 2.1).
This study represents an effort to correlate 124 of those terraces into time sequential flights along 2 of the main branches of the drainage system; Bell Canyon and San Juan Creek (southern and northern portions). The correlation of terraces is important when analyzing landscape evolution because various terrace flights indicate a progressive time relationship between tectonics, sea level change, and fluvial systems. The analysis of terraces, and resulting relationships between tectonics and the coastal fluvial systems, can indicate rates of tectonic uplift and rates of seismicity in the vicinity of fluvial or marine systems. Furthermore, correlated terrace flights are ideal for quantifying rates of tectonic uplift, including identifying uplift differential on opposite sides of faults (Merritts and Bull, 1989; Merritts and Vincent, 1989; Merritts et al., 1994; Taylor et al., 2006). Correlated terrace flights can even be used to establish recurrence intervals for individual faults (Hanson et al., 1994; Saucier, 1987; Kern and Rockwell, 1992; Eisenberg, 1992). The correlation of terraces is critical to understanding, predicting, and mitigating geologic hazards in southern California.
2.1.1. Climate, Vegetation, Hydrology, and Topography
The San Juan Creek watershed is located predominantly in the southern portion of Orange County, California, with the eastern and southern ends extending into Riverside and San Diego Counties, respectively. This watershed encompasses a drainage area of ~456 km2. The main stream channel of San Juan Creek originates in the Santa Ana Mountains in the Cleveland National Forest and flows ~43.5 km from the headwater at ~1,280 m above sea level (asl), to the Pacific Ocean at Doheny State Beach near Dana Point Harbor. The watershed tilts generally southwest with the upper third of the drainage basin being exceptionally rugged, having steep slopes and deep cutting canyons. The center third of the watershed is dominated by rolling hills, while the downstream portion of the drainage basin is in a highly developed floodplain. Santiago Peak lies within the drainage basin at an elevation of 1,715 m asl, which also represents the total relief of the basin. Major tributaries to San Juan Creek include Trabuco Arroyo, Oso Creek, Canada Gobernadora, and Bell Canyon. These canyons appear as intermittent streams sometimes bearing water nearly all year as a result of urban and agricultural development (U.S. Army Corps of Engineers, 2002).
Southern California has a mild Mediterranean climate characterized by warm, dry summers and cool wet winters. Little stream flow occurs except during and immediately following rains, and runoff increases rapidly in response to rainfall excess (Kochel et al., 1997). The main flood season is from November to April. Rainless periods of several months frequently occur during the summer, and occasional snow (an unimportant contributor to runoff) may fall only in the most upstream part of the watershed. Precipitation is ~45 cm in the mountains and 33 cm near the coast (U.S. Army Corps of Engineers, 2002). Superimposed on annual variations are irregular but episodic cycles of wet (3-6 years) and dry (10-30 years) periods related to the Southern Oscillation (El Nino) climate phenomenon (Kochel et al., 1997).
Unconfined groundwater exists in generally narrow, shallow alluvial valley fill that has been deposited in the area of the San Juan Canyon and its tributaries. Alluvial fill from the coast to the head of the tributaries reaches as much as 61 m (U.S. Army Corps of Engineers, 2002). The Cristianitos fault traverses the San Juan Canyon at a narrows separating the groundwater alluvium into an upper and a lower area. The total calculated storage capacity of the San Juan Creek Groundwater Basin is estimated to be ~0.111 km3 (U.S. Army Corps of Engineers, 2002).
Chaparral dominates the upper San Juan Creek watershed within the Cleveland National Forest, and a mixture of sage scrub (chaparral, riparian vegetation and sage/grassland) dominates the lower elevations. Coastal sage shrub (also termed southern coastal shrub or soft chaparral) is, along with chaparral, the major shrubland type of this lower area. Poorly developed oak woodland appears to be relictual, chiefly occupying small valleys and potreros on moister hillsides (Lewis, 1942). These oak woodlands are surrounded, and in many places all but engulfed, by the chaparral and coastal sage. Chaparral and coastal sage/grassland communities are the characteristic and widespread associations of the area. The Trabuco Arroyo watershed downstream of the Cleveland National Forest has been mostly developed (U.S. Army Corps of Engineers, 2002).
2.1.2. Regional, Seismic, and Tectonic Settings
The physiographic provinces of lower southern California include the Transverse and Peninsular Ranges, and the Los Angeles Basin (Fig. 1.1). This study area is located in the Santa Ana Mountains along their western border. These mountains occur in the northeast segment of the Peninsular Ranges south of the Los Angeles Basin near the coast. The Peninsular Ranges, found south of the Los Angeles Basin, contain numerous granitic blocks that have been uplifted along major faults. Easterly bounded by the San Andreas fault, these Cretaceous granitic blocks extend from the Los Angeles Basin southward through Baja California. The Santa Ana Mountains comprise Cretaceous granitic rocks covered with a succession of Cretaceous to Recent siliclastic rocks (Fig. 1.2; Fife, 1974; Jenkins, 1966; Jennings, 1977; Morton, 2004). The oldest formations are exposed to the east of the study area and progressively become younger westward. The San Juan Creek drainage basin, represented by active drainage of the west-facing southern Santa Ana Mountains, flows from the northeast to the southwest, and dissects the Late Cretaceous to Tertiary formations. The study area is just north of Camp Pendleton (Fig. 2.1), and lies within the fluvial drainage basin of the San Juan Creek within the Santa Ana Mountains (Jenkins, 1966; Greene and Kennedy, 1986).
Southern California physiographic provinces are bounded and divided by several major fault systems, including the San Andreas, San Jacinto, and Elsinore fault zones (Fig. 1.1). The Elsinore fault traverses the Peninsular Range passing on the eastern side of the Santa Ana Mountains, just to the east of the study area. The San Jacinto fault and the Elsinore fault are right-lateral strike-slip faults occurring west of the San Andreas fault zone; although a significant amount of oblique movement has occurred along the Elsinore fault resulting in the uplift of the Santa Ana Mountains. The Newport-Inglewood fault system, also along the coastal margin of the northern Peninsular Range, extends northward into the Los Angeles Basin, and strikes southward through Inglewood, Newport, and just offshore of the study area southwest of the Santa Ana Mountains (Barrows, 1974; Bryant, 1988; Wills, Wong, and Hart, 1990). The study area is confined between the Elsinore fault and the offshore extension of the Newport-Inglewood fault.
The Cristianitos and Mission Viejo faults transverse the major branches of the San Juan Creek and are considered inactive (Shlemon, 1992). In addition to these 2 faults, recent studies have suggested that a blind thrust is present beneath the San Joaquin Hills (Fig. 1.1) on the northern boundary of the study area (Grant et al., 1997; Grant et al., 1999). The blind thrust system may be associated with a plunging anticline that could extend within the boundaries of the San Juan Creek Basin. Progressive uplift and westward tilting of the Santa Ana Mountains have resulted in entrenchment and headward erosion of the river and tributary system and has caused the establishment of numerous flights of fluvial terraces throughout the drainage basin. These terraces (Fig. 2.2) are generally assumed to be Pleistocene (Hanson et al., 1994; Peska, 1986).
Views of terraces are shown in Figure 1.4. The fluvial terraces extend to the mouth of San Juan Creek where they merge with marine terraces along the southern California coast. Of particular note is the Canada Gobernadora Canyon which extends parallel to Bell Canyon, both of which trend due north across the study area. Although others have noticed the “straight” nature of Canada Gobernadora Canyon (Taylor et al., 2006), the lineation which occurs in both Canada Gobernadora and Bell Canyons extends well beyond the confines of the canyon drainage patterns (Fig. 2.1 and 2.3).
These features, herein referred to as the Bell Canyon Lineament and the Gobernadora Lineament, strike perpendicularly across the southern San Juan Creek at the area of a large bend in the river system. The lineaments are subparallel to, and intersected by, both the Mission Viejo and Cristianitos fault zones. The Bell Canyon lineament is the largest, extending from the western base of Saddleback Peak in the Santa Ana Mountains to offshore Pacific Ocean sediments of the San Mateo Creek (Fig. 2.1), a distance of ~40 km. The Gobernadora Lineament is much shorter and lies west of, and nearly parallel to, the northern portion of the Bell Canyon Lineament. The Bell Canyon Lineament separates the high mountains of the Santa Ana Coast Range (including Santiago Peak) from the lower foothills adjacent to the Los Angeles Basin (Fig. 2.3). This topographic divide also extends offshore near the mouth of the San Mateo Creek.
2.2. Methodology
The study area has been divided into 3 segments each containing terraces distinctly separated in location from the other segments (Fig. 2.2). The northernmost terraces are located in Bell Canyon, a tributary stream to the main drainage. Here, over 50 terraces occur on both sides of the canyon. The next segment is designated northern San Juan Creek (NSJ) and is located above the confluence of Bell Canyon and San Juan Creek. Nearly 20 terraces occur on the eastern side of the main creek drainage in this area. The last segment is designated southern San Juan Creek (SSJ) and has over 40 terraces occurring on the northern side of the drainage.
2.2.1. Field Methods
Base maps were prepared at a scale of 1:6,000 by downloading USGS 10 m contour maps and compiling them in the GIS ArcMap program. Terrace risers, and eroded terrace material unassociated with identifiable tread geomorphic surfaces have been excluded from the geomorphic terrace map, but terrace risers are still visible in their topographic expression between various terrace treads (Fig. 2.2). Map data was collected from literature and compiled on the base maps (Transverse Mercator projection NAD27) including terraces, faults, and bedrock geology. Mapping relevant to the terrace study area includes maps by Morton (1974), Morton and Greensfelder (1976), Morton and Miller (1981), and Morton (2004) encompassing engineering geology, environmental geology, mines and mineral deposits, and the 30′ x 60′ Santa Ana Quadrangle SCAMP digital geologic map. Greenwood (1982) provides a mineral land classification map, and Blanc and Cleveland (1968) provide slope stability data for the region. Miller provides engineering aspect data (1976), while Collins and Dunne (1990) provide data on fluvial geomorphology.
An air photographic analysis of the drainage basin was conducted using aerial photographs from the Whittier College aerial photograph collection. The historic map data throughout the drainage basin was updated and compiled onto the base map (Whittier College, 2004). Geomorphic mapping of terrace surfaces was confirmed by field mapping of terraces of the lower San Juan Creek and Bell Canyon. All terraces were mapped except those occurring on the active floodplain and the lowest terrace flight (Fig. 2.2). Each mapped terrace tread was designated with a specific alphanumeric label for reference during terrace correlation.
The sedimentary sections were constructed using a Jacob’s Staff, a 15 m tape measure, a Brunton compass, and a GPS hand-held receiver. The columns were constructed along exposed terrace hillsides or along bulldozed road cuts by measuring vertical components of exposed bedrock and terrace material. All 3 sedimentary logs include the basal contact with Santiago Formation bedrock. The log locations are shown in Figure 2.2.
2.2.2. Data Analysis Methods
Analytical methodology for processing the cross-sectional data was facilitated with GIS ArcMap 9.1 (2005) by ESRI, Microsoft Office Excel (2003), and Adobe Illustrator 9.1.
Terrace flights are labeled on the map as Quaternary fluvial terraces (T1 as the oldest, and T12 as the youngest), to distinguish them from other studies (Taylor et al., 2006; Barrie et al. 1992) that use a variety of labels. Terrace correlation and level assessment were first conducted in the field by visual alignment. The visual assessment of terrace flight levels was hampered, however, by erosional drainages creating broad gaps between terraces, and the difficulty of designating terrace flights on opposite sides of the canyon and tributary systems.
Two potential methods can be used to aid in the correlation of terrace flight levels for the construction of former longitudinal valley profiles. These methods include the collection of continuous geodetic survey data points along the terraces, or alternatively, the construction of cross-valley profiles. Both of these techniques have their advantages and disadvantages, and neither technique is ideally suited for all fluvial systems. For this current study, 27 cross-valley profiles were constructed to aid in terrace flight correlation and in the construction of former longitudinal valley profiles within the lower reach of San Juan Creek. Establishing the superior usefulness of the construction of cross-valley profiles methodology within the confines of San Juan Creek is critical to the process of correlating terrace flights throughout this drainage basin.
Merritts et al. (1994) use the continuous geodetic survey method on the Merced River, wherein multiple survey points are taken along terrace treads. These data are then used to indicate the correct alignment and inclination of reconstructed former longitudinal valley profiles. The Merced River system is over 80 km long with ~40 km of terrace surfaces. Individual terraces in their study ranged up to 1 km in length and are narrow compared to their overall length, so the Merritts et al. (1994) methodology works well to indicate trends in the elevation of former longitudinal valley profiles. Any choice of placement of the elevation survey points on a sloping tread will still create fairly uniform longitudinal profiles.
In contrast, the San Juan Creek/Bell Canyon fluvial system has a distance of only ~40 km with <25 km of the river system having terrace surfaces. The terraces are short (200 to 400 m long, and frequently as wide as they are long), and the river system itself is short. Terrace treads are sloping features, the valley-ward slope of the tread being controlled by the ratio of the rates of lateral migration to the rate of vertical incision (Merritts et al., 1994). If the terraces are short, wide, sloping features, then the subjective placement of elevation survey points becomes critical to the reconstruction of former longitudinal valley profiles. Variation in the placement of the survey points from terrace tread to terrace tread can introduce substantial vertical variation into the projection of former longitudinal valley profiles.
For example, walking down-valley on a short terrace tread taking several survey points will produce a longitudinal down-valley inclination that is completely dependent on the direction and location of the transverse across the terrace. This multiple-survey point methodology may allow elevation data disparities to be averaged out on terraces that are narrow and ~1 km long, but on short, wide sloping terraces it is of little use in discerning overall former longitudinal valley profile inclinations and correlations. The inherent error in choosing a direction to walk along the terrace will not be averaged out by the overall width/length relationship of short, wide terraces.
A case in point is the use of the Merritts et al. (1994) multiple-survey point methodology by Taylor et al. (2006) on San Juan Creek terraces in the lower portion (SSJ) of this study area. Using this methodology to correlate terrace levels at San Juan Creek probably introduced significant vertical error into their assessment of former longitudinal valley profiles. Many of the terraces in this region are short (200-400 m), and although a few are up to 600 m long, only one of the lower terraces is 1 km. These terraces are also quite wide (the 1 km terrace is 0.5 km wide), and they are all sloping toward the center of the valley. This section of San Juan Creek is only 10 km long and, although Taylor et al. (2006) uses terraces from other studies in his analysis, detailed mapping (Fig. 2.2) indicates that Taylor et al. (2006) could only have evaluated up to 38 terraces in this area above the 10 m level. (The actual number of terraces evaluated in the Taylor et al. (2006) study is unstated, and the report includes no map or indication of terrace locations and elevations.) The margin of vertical error introduced by Taylor et al. (2006) is critical because many terraces are close in elevation, being separated only by very slight risers. The Taylor et al. (2006) study apparently lumps these terraces together, effectively merging terrace flights into single levels. As a result of the vertical disparities within terrace levels, the Taylor et al. (2006) study must statistically average survey points to establish homogeneously straight former longitudinal valley profiles. Such averaging, however, blurs the distinction between terrace flights that are close in elevation, diminishing the ability of the study to use former longitudinal valley profiles to discern fault displacement.
Over short distances, a better method for plotting former longitudinal valley profiles is to correlate terraces using cross-valley profiles, and then simply connect correlated terrace tread centroid elevations to create former longitudinal valley profiles. The use of this methodology can be found in Bull (1991), which includes examples in Holocene terraces within channels in the Riverside Mountains (southeast corner of California), terraces in the Naomi basin (Negev Desert of Southern Israel) terraces in the North Fork of the San Gabriel River (Fig. 1.1), and terraces within Cajon Creek (Fig. 1.1) to name a few. These are short river systems 1.5, 1.4, 5, and 15 km in length respectively. This methodology has been found the most suitable and is used for this current study of the Bell Canyon/San Juan Creek fluvial terraces. The use of cross-valley profiles represents a more accurate means of establishing correlations between terraces that would otherwise be too close in elevation or too short in length to provide adequate elevation data for correlation using the Merritts et al. (1994) methodology.
Construction of cross-valley profiles was accomplished by digitizing contour elevations and terrace locations along 27 transects on the north and east sides of San Juan Creek (9 and 5 transects respectively), and along the east side of Bell Canyon (14 transects; Fig. 2.2). Once the contour information and terrace locations were digitized from the base map, the data was entered into a spreadsheet and graphs were constructed using the scatter graph plot function. The graphs were compiled to superimpose, compare, and correlate multiple profiles. The presence of 3 delicately etched terraces on SSJ profile Line 1 allow this profile to be used as the standard for construction of the graphical correlation zones for the low elevation terraces. Most of the highest terraces in the study area occur in Bell Canyon and graphical correlation zones for these high terraces were designated using profiles Bell 8 (T1 and T3 levels), Bell 6 (T4 and T5 levels), and Bell 2 (T6 level) as standards. The graphical profile correlations are shown in Figures 2.4a, 2.4b, and 2.4c.
Once correlated, terraces on each transect were assigned a terrace level number. The terraces with their flight level numbers were transferred to the ArcGIS file, and a preliminary map was generated showing correlated terrace levels throughout the study area. Finally, remaining terraces not on cross sections were assigned a level number based on their proximity and relationship to terraces on cross sections, and a final map was generated (Fig. 2.5).
Once terrace correlation was completed and terrace levels were assigned to terraces, former longitudinal stream valley profiles were constructed for Bell Canyon and San Juan Creek. The former longitudinal valley profiles were constructed using a midvalley axial line drawn along the midpoint between the bedrock valley walls on the base map (Merritts et al., 1994). Then the approximate center of each terrace was projected perpendicularly to the modern stream, and the intersection of the terrace and the midvalley axial line was digitized (ArcGIS) to provide a horizontal component (distance to the river mouth at the Pacific Ocean) for the former longitudinal valley profiles. Each terrace surface was averaged into an elevation located at the centroid of the terrace and was entered as data to represent the vertical component of the former longitudinal valley profiles. The digitized vertical and horizontal components of the former longitudinal valley profiles were entered on an Excel spreadsheet using the scattergraph plot function (Fig. 2.6).
The Mission Viejo and Cristianitos faults were added to the graphs because of potential interaction with the former longitudinal valley profiles. Note that because of the alignment of the cross-valley profiles with the stream channel (midvalley axial line), the graphs do not represent a true profile, but rather represent a side view of the hillsides. Therefore, the true inclinations of the faults are not represented, but rather, the graph indicates the hillside location of each fault above the streambed as it weaves diagonally across the slope between or through terraces.
2.3. Results and Discussion
2.3.1. Sedimentary Logs
Figure 2.7 shows the graphic sedimentary logs which occur in terrace levels T1 (column No. 3), T8 (column No. 1), and T11bc (column No. 2).
All of the terrace deposits comprise alluvial material ranging from 6 to 13 m or more in thickness, and in this regard, all of the sedimentary logs are in fill terraces as opposed to cut terraces or strath terraces. All of the terraces observed in the study area during the fieldwork phase of this project were fill terraces.
Fluvial terraces in the lower San Juan Creek (SSJ) have been examined by Taylor et al. (2006) using drill holes and test pits. These authors report thicknesses ranging from 5 to 15 m for the terraces which overlap this study. They report compositions of up to 50% pebbles, cobbles, and boulders in a sandy matrix with infrequent layers of interbedded sands and silts. These descriptions again indicate fill terraces. The fact that all of the terraces examined in this study area are fill terraces is an important indicator as to the lower location of this stream reach within the San Juan Creek fluvial drainage system.
The nature of fill terraces found in near-coastal fluvial reaches occurring as a response to glacio-eustatic sea level changes have been evaluated on various river systems in both tectonically active and non-active regimes (Ramsay, 1931; Zeuner, 1945; Clayton, 1964; Merritts et al., 1994; Taylor et al., 2006, to name a few). River systems influenced by glacio-eustatic sea level change can exhibit 3 basic types of sediment in remnant terraces preserved by regional uplift. Thalassostatic sediment (estuarine sediment deposited in the thalweg by rising sea level; Fig. 2.8) will infill the lower estuarine reaches of coastal river systems in response to rising sea level resulting from glacio-eustatic oscillations and subsequent river mouth drowning (Merritts et al. 1994; Ramsay, 1931; Zeuner, 1945).
This gradient re-stabilization process most likely takes only a few thousand years.
Upstream from the wedge of aggradational fluvial thalweg sediment, strath terraces will form as the normal process of river system growth and incision continues to extend the river system inland, unimpeded by glacio-eustatic oscillations. This mountain-ward growth creates terraces (former longitudinal valley profiles) that are subparallel in nature, being spaced closer together in the downstream direction. Merritts et al. (1994) have shown, on the other hand, that the thalassostatic and fluvial thalweg aggradational sediments along the lower portion of the river system create terraces (former longitudinal valley profiles) that are parallel in nature. Therefore, the presence of thick, aggradational fill terraces throughout this study area indicates that this area lies within the fluvial system reach dominated by the fluvial thalweg aggradational wedge. The correlation of the terraces by this study into parallel former longitudinal valley profiles (rather than subparallel profiles) corroborates the interpretation of the sedimentary logs as occurring within the fluvial thalweg aggradational wedge reach of San Juan Creek.
2.3.2. Timing and Causation of Terrace Formation
Climate controls are important and strongly influence terrace formation. Not only will climatic controls initiate glacio-eustatic sea level fluctuation, but in southern California interglacial and glacial climatic fluctuations result in higher and lower precipitation respectively. Owen and Finkel (2003) suggest for example, that late glacial maximum mean precipitation rates in the San Gorgonio Mountains could have been 150% that of current mean precipitation rates. Interglacial climatic conditions in southern California therefore, represent a temporary environment of comparatively low rainfall, which is undoubtedly associated with higher sediment loads and stream aggradation. Conditions of high sediment load will be caused by reduced gradient, reduced velocity, and probably reduced discharge. Taylor et al. (2006) agrees stating that in the lower portion of the study area subsequent aggradation during the interglacial cycle creates a condition of higher sediment load. (They likely meant that higher sediment load creates the condition of stream aggradation because aggradation is a resulting factor rather than a causative factor.) These are precisely the conditions required to establish the aggradational wedge once thalassostatic deposition has ceased.
Western United States seems to have experienced a synchronicity of paleoclimatic conditions throughout glacial intervals, including increased precipitation resulting from southward migration of the mid-latitude westerly jet stream. The jet stream migration was controlled by fluctuations in the Northern Hemisphere ice sheets and oceans generating higher precipitation during glacial cycles (Owen et al., 2003). Moist climatic conditions during glacial periods are evident from the advance and retreats of glaciers within the last glaciation throughout western United States. These glacier movements are highly correlative suggesting that western United States glaciers were in phase with episodes of growth and collapse of the Laurentide ice sheet in association with North Atlantic Heinrich events. In general, over the last 60 ka, unified glacial advances throughout the Rocky Mountains and in the Cordilleran ice sheet have occurred at ~50-60, 47-54, 34-47, 21-23, 14-15, and 10-12 ka (Clark and Bartlein, 1995). Climate model experiments indicate a depression of from 4 to 10o C throughout western North America in association with abundant precipitation throughout this region (Thompson et al., 1993).
Therefore, it seems that southern California glacial cycles created conditions of relatively high rainfall, high discharge, and high stream power. These conditions interact to cause fluvial gradient readjustment to a lower base level. A new glacial regime causes sea level to drop drastically allowing stream incision through the newly stabilized thalweg material (stream power becomes greater than critical power leading to incision; Hancock and Anderson, 2002). Over-steepening of canyon walls and mass wasting then occurs, removing most of the former alluvial floodplain. The incision creates alluvial fill terraces; remnants of the floodplains abandoned high on the slope later to be tectonically uplifted, isolating them from the next phase of glacial/interglacial eustatic sea level change.
Taylor et al. (2006) suggests that interglacial cycles of low rainfall and stream discharge produce lateral planation (lateral cutting) which represents the ultimate cause of valley widening along the San Juan Creek, but this explanation is most probably not correct. Lateral planation is at a maximum only when a river system is stabilized during graded conditions, however, during low rainfall interglacial cycles, aggradation rather than planation will be the dominant process. For example, an examination of the current condition of San Juan Creek and Bell Canyon reveals braided streams throughout the entire length of the study area. The more probable cause of lateral cutting and valley widening has to do with the relationship between the wet glacial cycles, mass wasting, and the configuration of these canyons.
The well-preserved fluvial terraces in both the SSJ and NSJ study areas are on the inside curve (point bar) of broad bends in San Juan Creek. Likewise, the terraces in Bell Canyon are preserved on the inside of the broad curvature of the Canyon. Greatly increased discharge and stream power energy of Pleistocene glacial cycles will be concentrated on the outside edge of these curved river channels thereby causing both incision and lateral cutting as the outside bends in the channels are over-steepened and removed by mass wasting (Fig. 2.9). Figure 2.10 indicates that these areas have the steepest slopes along their cut banks, and in SSJ significant landsliding has occurred on this side of the canyon as well. It would therefore be the high energy of the glacial cycle combined with the curved configuration of the canyons that is responsible for the lateral cutting and widening of the valley, preserving former point bar deposits as terraces in the process.
2.3.3. Terrace Correlations
Terraces of various levels can be close in elevation in the study area (Fig. 2.6), so the discernment of an actual terrace tread (as opposed to a riser with no adjoining tread) was deemed critical for correlation of terrace levels. In addition, higher terrace material exists on the ridgeline, but the presence of a discernible fluvial terrace tread is not evident. Similarly, higher terrace material exists on the east side of San Juan Creek, but this material is associated with ancient alluvial fans rather than fluvial floodplain deposits.
Discernible terrace surfaces within the upper terrace flights are few in number due to the hillside erosion of these high locations. Only 1 tread surface exists at the highest T1 level, and it is present on the ridgeline between Bell Canyon and San Juan Creek (Fig. 2.5). Of the upper 4 terrace flights, 11 treads occur in Bell Canyon while only 4 occur in SSJ and none are present in NSJ (Table 2.1).
Of the five T3 terrace treads, 1 is on the west side of Bell canyon, while the remaining 4 are on the east side of Bell Canyon, and like the T1 and the T3 terrace levels, T5 treads do not occur in the SSJ or NSJ. The T6 level is the lowest level to which terrace treads on the west side of Bell Canyon descend.
None of the first 7 terrace levels occur in NSJ. The T8 terrace level is the first set of terraces to span all 3 of the distinct study areas and is the only terrace level to be prominently displayed in each area. Terraces above T8 occur more prominently in Bell Canyon, while terraces below T8 occur more prominently in SSJ. In this regard, the T8 terrace level represents the intermediary terrace level connecting the higher and lower elevation terraces throughout the San Juan Creek drainage basin.
The T9 and T10 terrace levels are mostly absent from Bell Canyon. One terrace tread having a unique but mappable surface includes both the T8 and T9 levels that have merged with their separating riser to form an indistinguishable surface herein classed as T8-9. This merger may be due to erosion of the tread, or possibly rapid stream down cutting during formation of the T8 and T9 terrace levels in this area. This unique terrace tread is shown on the terrace map (Fig. 2.5) but is not used in the construction of former longitudinal valley profiles (Fig. 2.6).
The T11 terrace level has the most terrace treads of any terrace flight in the study area and by far is the most complex, being represented by 40 terrace treads. Terraces within the T11 terrace level are represented by 3 distinct flight levels that frequently merge and sometimes have nearly indistinguishable or undeveloped risers between the terrace treads. The key to understanding the relationship between the terraces at the T11 level is at the upstream end of the SSJ segment where there are 3 delicately etched terraces in very close proximity to each other. These terraces are designated levels T11a, T11b, and T11c respectively. Although 3 terrace flights are clearly displayed at this location, at all other locations throughout the study area these 3 levels tend to merge in different combinations. At some locations one sublevel tread may exist as the single representative of the T11 level, while at other locations the “a,” “b” and “c” sublevels all merge together into a unified T11 terrace. In still other locations the “b” and “c” levels merge with no “a” levels present. For ease of representation, these T11b and the T11bc levels are combined on the terrace map (Fig. 2.5) though they remain distinct on the former longitudinal profiles (Fig. 2.6).
The large number of well-developed treads in the T11 level must be related to the youthfulness of the treads and the limited time for erosion to have occurred. In general, the delicate nature of treads, along with the propensity for T11 terrace flights to merge (failure to display riser development between levels) indicates that the T11 level may represent a time of continuous down cutting interrupted only by brief periods of gradient stability.
Terrace treads within the T12 level are the lowest hillside terraces throughout the study area. They occur primarily in the SSJ and Bell Canyon segments of the study area (Fig. 2.5 and 2.6). The age of these terrace treads are relatively young (OSL age reported by Taylor et al. (2006) ranges between 58 and 38 ka), and tend to be more closely related to the current floodplain than to the hillsides. These treads occur ~10 m above the current stream grade, so the mapping of the T12 level was considered beyond the scope of this tectonic landscape evolution study. One terrace tread has been mapped in Bell Canyon to show the relative relationship of the T12 level to other levels on Figures 2.6 and 2.11.
2.3.4. Inter-study Terrace Level Correlations
Establishing the equivalency of terrace levels with the Taylor et al. (2006) article is difficult because of the lack of geomorphic information provided in their paper. Taylor et al. (2006) do not mention the presence of risers, which suggests that these admittedly fine features were not taken into consideration by them in the process of delineating terrace treads. Neglecting the location of terrace risers has apparently resulted in the blending together of terrace treads and levels in the Taylor et al. (2006) study. Additional difficulties in establishing correlations with this study results from the fact that the Taylor et al. (2006) report does not include; a terrace map, cross-valley profiles, a statement of the number of terraces mapped, GPS waypoint locations, number of GPS waypoints per terrace, GPS waypoint and/or bearing for their “mid-valley axis” location, or an appreciable discussion of data collection methodology. Taylor et al. (2006) do include, however, a description of terrace levels by elevation above the current floodplain which is the only information provided to indicate where their terrace treads are located. Figure 2.11 and Table 2.1 show the possible overlap of terrace levels established by this study and that of Taylor et al. (2006).
The single T2 terrace tread in SSJ is described as the Qtr1 level in the Taylor et al. (2006). This location consists of 1 possible tread that is cut by 2 strands of the Mission Viejo fault intersecting at this tread location. The T4 and T7 levels in this study may be equivalent to the Taylor et al. Qtr2 and Qtr3 terrace levels, respectively. The T8 level in the upstream region of the Mission Viejo fault seems to be equivalent with Taylor et al.’s (2006) Qtr4 terrace, but near the downstream Cristianitos fault, the Qtr4 terrace of Taylor et al. (2006) changes elevations to align with the T9 level of this study, perhaps again representing a lumping and miscorrelation of terrace treads. Terrace sublevel T11a seems to be the most equivalent with the Qtr5 terrace level of Taylor et al. (2006), but these authors provide no indication of having separated the various terrace sublevels present in the T11 level, indicating the probable merger of these terrace treads in their study. The T12 terrace level is reported as Qtr6 in Taylor et al. (2006).
2.3.5. Former Longitudinal Profiles
Figure 2.6 shows the correlated former longitudinal valley profiles throughout the SSJ/Bell Canyon and San Juan Creek (SSJ/NSJ) segments of the study area. The SSJ/Bell Canyon profile shows the isolation of T1, T3, and T5 which are exclusively confined to Bell Canyon. The T7 and T8 terrace levels are the longest terrace profiles, extending all the way through the study area from upper Bell Canyon to lower SSJ, possibly indicating an extensive time period of aggradational terrace formation followed by valley down cutting.
The San Juan Creek (SSJ/NSJ) profile (Fig. 2.6) shows that only 2 isolated high terrace treads (T2 and T5 levels) exist in San Juan Creek proper. The T8 and the T9 levels are the most continuous terrace levels throughout San Juan Creek Canyon, while the T11 levels are also prominently displayed there with the T11a predominating.
2.4. Conclusions
Overall, 124 terrace treads have been correlated into 12 discrete terrace levels throughout the extent of both the San Juan Creek main canyon and its tributary, Bell Canyon, demonstrating the remarkable continuity of terrace formation throughout the lower San Juan Creek drainage basin. The pronounced continuity of the terrace correlations indicates that formative influences reflect major regional activities such as climatic regimes and tectonic uplift, rather than minor localized depositional events resulting from sentiment loads, gradients, or stream velocities exceeding localized threshold factors that inhibit or permit deposition. Moreover, the presence of thick alluvial fill terraces throughout this portion of San Juan Creek and Bell Canyon indicates that deposition in this reach of the drainage basin has been dominated by glacio-eustatic sea level oscillation. These fill terraces were probably deposited during sea level rise associated with interglacial cycles, while valley widening (lateral cutting) may not have occurred until the next glacial cycle began, causing both incision and undercutting/mass wasting of the outside curves along this river system. The thalweg will consist of both estuarine sediment (thalassostatic sediment; downstream and stratigraphically lower) and a fluvial thalweg aggradational wedge (upstream and stratigraphically higher) all resulting from synchronously depositing environments imposed by glacio-eustatic sea level fluctuation.
Above the T8 level, terrace treads occur more commonly in Bell Canyon, while below T8 terrace treads occur more prominently in San Juan Creek. This could indicate that fluvial activity (deposition and down cutting/valley widening) was more pronounced on tributaries rather than on San Juan Creek proper during the formative stages of drainage system development. After the time of the development of the T8 terrace level, headward erosion and valley extension may have allowed San Juan Creek proper to capture a larger drainage area allowing for greater development of terraces within the southern San Juan Creek, with a corresponding decrease in valley widening and terrace development in other tributaries of the drainage system.