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Landscape response in the California Coast Ranges to the stormy conditions of the Pleistocene to Holocene climatic transition part of Vignettes:Vignette Collection
Introduction: temporal scales, spatial scales, climate, and tectonics A long-standing problem in geomorphology is identifying the relative roles of climate and tectonism on landscape evolution. Here we examine the effect of the stormy climate of the time period between approximately 15,000 and 10,000 years ago ("15 to 10 ka"), known as the Pleistocene to Holocene climatic transition, on streams and hillslopes in Pancho Rico Valley. Pancho Rico Valley is an east-west trending stream valley within the area of modestly high topography known as the Gabilan Mesa (Figure 1). The Gabilan Mesa is a distinctive physiographic feature that is within the central Coast Ranges of California, and is adjacent to the San Andreas fault zone (Figure 1). A useful framework for understanding the relative influences of climate and tectonics on landscape evolution is to consider the temporal and spatial scales over which they influence geomorphic processes (Harvey, 2006). In many localities, the predominant influence of tectonics on landscapes is the creation of fundamental topographic relief over relatively long timescales (100s of thousands to millions of years; Harvey, 2006). In the central California Coast Ranges, tectonism over the last 400,000 years ("400 ky"), has resulted in down to the southwest tilting, with little folding or faulting, of the 110 km long and 25 km wide structural block that forms the plateau-like Gabilan Mesa (Page et al, 1998). On a regional spatial scale (>1000 km2), the effect of tectonism over the last 400 ky is the formation of the distinctive topography of the Gabilan Mesa, which consists of relatively uniform, low altitude, accordant ridges that slope exclusively toward southwest, and west- as well as southwest-trending stream valleys (Dohrenwend, 1978; Page et al, 1998; Figure 1). The influence of tectonics within most of the west- and southwest-trending stream valleys of the Gabilan Mesa (Figures 1) is subtle or undetectable (Dohrenwend, 1978). In contrast, the effect of climate on the geomorphology of Pancho Rico Valley since approximately 15 ka is profound. Climate influences landscape development in places like Pancho Rico Valley because: (1) climatic factors affect hillslope processes (eg. Reneau et al, 1990); and (2) climate directly controls stream discharge and strongly influences the magnitudes of sediment loads delivered to stream networks (Bull, 1979). The ratio of stream discharge to sediment load is a factor that influences landform development because, for example, within a stream channel having a given slope, a decrease in discharge or an increase in sediment load will cause sediment deposition, which ultimately leads to the development of landforms such as stream terraces or alluvial-fan lobes (Bull, 1979; Figure 2). The impact of the stormy climate of the Pleistocene to Holocene transition The potent storms of the Pleistocene to Holocene climatic transition caused many debris flows in Pancho Rico Valley. Garcia and Mahan (2009) showed that the debris flows stripped Pancho Rico Valley hillslopes of sediment, and scoured the bottoms of the uppermost parts of stream drainages (referred to herein as "low-order drainages"). The debris flows produced relatively broad scour scars (Stock and Dietrich, 2003) at the bottoms of most low-order drainages in Pancho Rico Valley (Figure 3; a discussion of the morphometric signature of debris-flow erosion, which Stock and Dietrich (2006) identified, is presented in Figure 4 and its caption). The sediment scoured by debris flows from the hillslopes and bottoms of low-order drainages exceeded the carrying capacity of Pancho Rico Creek, and the Pancho Rico Creek channel was buried by debris flow deposits that are up to 20 m thick (Garcia and Mahan, 2009). The responses of Pancho Rico Valley hillslopes and Pancho Rico Creek to the Pleistocene to Holocene climatic transition illustrate that influence of climate on landscape evolution and process can be both profound and complex. Firstly, the most conspicuous, local effect of the stormy Pleistocene to Holocene transition climate on the geomorphology of Pancho Rico Creek was that the convexity of interfluves were accentuated, and a low-relief valley floor was constructed (Figures 2 and 3). From a sedimentological perspective, the local effect of the Pleistocene to Holocene transition was significant sediment transport from Pancho Rico Creek hillslopes and low-order drainages to the bottom of Pancho Rico Valley (Figure 4). However, because the carrying capacity of Pancho Rico Creek was exceeded, sediment stripped from Pancho Rico Creek hillslopes and low order drainage bottoms remained within Pancho Rico Valley, and did not enter the larger Salinas River system during the Pleistocene to Holocene climatic transition (Garcia and Mahan, 2009). Incision by Pancho Rico Creek into the debris-flow deposits occurred some time after the Pleistocene to Holocene transition, likely as a consequence of stream capture (Figure 1) and the associated increase in stream discharge, but most of the sediment stripped from hillslopes remains stored in the prominent, constructional alluvial landform that dominates lower Pancho Rico Creek (Figure 2; Garcia and Mahan, 2009). Surprisingly, there are no landforms in the Salinas Valley that record the significant hillslope erosion that occurred in Pancho Rico Creek during the Pleistocene to Holocene transition climatic event (Garcia and Mahan, 2009). Lastly, although the Gabilan Mesa is adjacent to the San Andreas Fault zone, climate-driven processes more strongly influence landform evolution than tectonically-driven processes over thousand to 10 ky timescales in the west- and southwest-trending valleys of the Gabilan Mesa.
Exploring the roles of discharge and bedload composition in strath terrace formation through a naturally occurring experiment in the California Coast Ranges part of Vignettes:Vignette Collection
Introduction Pancho Rico Creek is a tributary of the Salinas River that flows across an area of modestly high topography known as the Gabilan Mesa. The Gabilan Mesa is in the central Coast Ranges of California, and is an exceptional locality to study stream terraces because variables that influence stream processes are well known and relatively simple (Dohrenwend, 1978). Stream capture (Figure 1) occurred when San Lorenzo Creek eroded laterally and breached its southwestern drainage divide (Garcia and Mahan, in review). The re-routing of water and sediment from San Lorenzo Creek into Pancho Rico Creek makes possible studying the effect of an abrupt change in the fundamental factors influencing stream-terrace formation in a natural setting. This is a significant "experiment" because, unlike experiments conducted in laboratory flumes or using numerical simulations, which can yield unrealistic results, the results of this experiment are produced by real geomorphic processes. Definition and significance of strath terraces Strath terraces are the type of stream terraces that form in tectonically active mountains, especially in drainage basins underlain by fine grained, marine sedimentary rock (Montgomery, 2004). Straths are planar, erosional surfaces that act as floodplains for bedrock streams, and they form adjacent to stream channels when rates of lateral stream erosion exceed rates of downward stream erosion (Montgomery, 2004; Figure 2). A strath terrace consists of a strath, as well as alluvium that overlies the strath, and strath terraces are created when a stream incises sufficiently below the strath so that it is no longer affected by fluvial processes (Figure 2). Strath terraces are significant because the straths at the bases of strath terraces represent a former stream channel altitude, and the numeric age of the overlying alluvium can be determined using techniques such as radiocarbon dating of organic matter (e.g. Wegmann and Pazzaglia, 2002), or optically stimulated luminescence dating (e.g. Garcia and Mahan, 2009). It follows that strath terraces can be used to calculate rates of downward stream erosion, which in some instances are equal to rates of rock uplift (e.g. Pazzaglia and Brandon, 2001). Accordingly, strath terraces have been intensely studied during the last two decades, and some controversies exist. Factors that influence strath terrace formation The seminal paper by Montgomery (2004) provides a new model explaining how and why strath terraces form. This model is based on the observation that straths form most commonly in drainage basins composed of fine grained, clay rich, marine sedimentary rock, because clay-rich sedimentary rocks are highly susceptible to mechanical weathering caused by shrinking and swelling associated with periodic wetting and drying. When clay-rich sedimentary rocks are exposed on a strath (Figure 1), they rapidly disintegrate, and the weathered debris is rapidly eroded. Moreover, the rock in the channel is not subject to wetting and drying cycles and associated weathering effects, so rock within the channel is relatively hard and resistant to stream erosion. The contrast in the competence of weathered floodplain rock and unweathered channel rock insures that lateral erosion occurs at a faster rate than downward erosion. Once a strath forms, it is preserved only if it is capped by a relatively resistant cover of alluvium, which in most instances is composed of the gravel that was being transported as bedload in the stream channel (Montgomery, 2004). This compelling model requires reevaluation of the factors previously accepted as influencing to strath terraces formation, such as bedload composition and stream discharge. Merritts et al (1994) explored the relationship of stream discharge and uplift rate on development of strath terraces. They showed that straths will form only where there is discharge in excess of what is needed for a stream to erode downward at the same rate of rock uplift, and only a stream's capacity to erode beyond what is needed to keep pace with rock uplift is expended on lateral erosion. For example, if rock uplift rates are relatively high, and discharge is relatively low, straths may not form, or straths that form will be relatively narrow. If rock uplift rates and rock type are the same farther downstream in the same stream network, where catchment area and discharge are greater, straths are more likely to form, and will be wider than straths farther upstream (stream discharge is largely dependant on catchment area, which is the amount of terrain contributing water to a segment of a stream). One way to test the hypotheses of Merritts et al (1994) is to conduct an experiment in which the catchment area and associated stream discharge of a stream increase abruptly. The history of strath terrace formation along Pancho Rico Creek is the result of such an experiment because its drainage basin expanded approximately 50% by stream capture sometime after approximately 11,800 to 15,500 years ago (Garcia and Mahan, 2009). Moreover, the bedrock geology of the Gabilan Mesa makes possible adding another facet to the experiment, which is the assessment of the influences of bedload composition on strath terrace formation. The experiment When the catchment area of Pancho Rico Creek expanded through stream capture (Figure 1), the total catchment area of Pancho Rico Creek increased from 99.5 km2 to 156.6 km2 (Garcia, 2006). Additionally, prior to stream capture, the entire Pancho Rico Creek drainage basin was formed in fine-grained, marine sedimentary rock, and gravel bedload consisted exclusively of relatively weak, clay rich, fine grained marine sedimentary rocks. Since stream capture occurred, Pancho Rico Creek's catchment includes areas underlain by the Franciscan Complex, and gravel bedload has been dominated by relatively hard, crystalline rocks of the Franciscan Complex (chert, as well as igneous and metamorphic rocks; Garcia, 2006). The effect of steam capture profoundly influenced the development of strath terraces along Pancho Rico Creek, which is readily apparent because pre- and post-capture strath terrace sediments have distinctly different compositions. Pre-capture terrace sediment was deposited during the Pleistocene to Holocene climatic transition, when the Pancho Rico Creek channel and Pancho Rico Creek floodplains were buried by extensive debris flows (Garcia and Mahan, 2009). The surfaces of pre-capture terraces are the remnants of the sedimentary deposit that filled the Pancho Rico Creek stream valley during the Pleistocene to Holocene climatic transition, therefore, pre-capture terraces are paired terraces (Figure 3; Garcia, 2006; Garcia and Mahan, 2009). On the other hand, post-capture strath terraces consist of relatively thin (5 m or less) Pancho Rico Creek channel gravel and floodplain deposits, are at many different heights above the Pancho Rico Creek channel, and are unpaired terraces that constitute an uninterrupted record of alternating lateral erosion and downward erosion by Pancho Rico Creek since stream capture occurred (Garcia and Mahan, 2009). A complete record of alternating lateral and vertical erosion by Pancho Rico Creek, which consists of straths at many different levels above the Pancho Rico Creek channel, is preserved because the alluvial covers protecting the straths are composed of relatively hard, post-capture, Franciscan-Complex derived bedload (Garcia and Mahan, 2009). The relatively strong gravelly covers protect the straths from weathering and erosion, insuring their preservation in the alluvial stratigraphic record. In pre-capture time, straths cut by Pancho Rico Creek were preserved only when buried under substantially thick (up to 20 m) alluvial covers associated with a significant climatic event (the Pleistocene to Holocene climatic transition), because the pre-capture bedload covering the straths was relatively weak (Garcia and Mahan, 2009). Lastly, in both pre- and post-capture time, straths formed only where Pancho Rico Creek's catchment area exceeds/exceeded approximately 60 km2 (Figure 4), because that is the size of catchment area needed to provide Pancho Rico Creek with sufficient discharge to erode downward at the same rate as rock uplift and also erode laterally (Garcia, 2006).