Factors Controlling Stream Network Response to a Tectonically-Induced Incision Wave, Eastern Part of the Alpujarran Corridor, Betic Cordillera, Southern Spain (Almería)

Tony F. Garcia
Geological Sciences
University of California, Santa Barbara

Introduction

My dissertation is a field test of bedrock incision models that predict upstream propagation of base-level lowering (cf., Penck, 1953) is manifest in stream networks as an "incision wave" (Anderson, 1994; Seidl et al, 1994; Rosenbloom and Anderson, 1994; Safran, 1998). The term "incision wave" refers to a temporal and spatial pattern of stream channel incision first recognized in computer simulations of landscape evolution that employ the "stream power law" or use a "stream power rule" to govern bedrock-stream incision rates (eg., Rosenbloom and Anderson, 1994; Tucker and Slingerland, 1994; Seidl et al, 1994; Safran, 1998). Seidl and Dietrich (1992; p. 101) stated "The use of computer models to examine landscape evolution . . . has contributed to lifting geomorphic studies to a more quantitative level . . . ", but noted on p. 102 that "Much work is still needed in identifying properties of real landscapes that can be tested by theoretical predictions."

This statement strongly influenced the direction of geomorphic investigation, or perhaps was only prophetic. Subsequent studies documented incision-wave controlled landscape evolution (Anderson, 1994; Seidl et al, 1994; Seidl et al, 1997; Safran, 1998), or invoked incision-wave controlled landform or landscape evolution to account for observed features (Rosenbloom and Anderson, 1994; Heimsath et al, 1999). These studies were mostly based on computer modeling results, and on properties of real landscapes that could be easily used to calibrate numerical models, or properties that could be modeled and thereby quantitatively test empirically-derived hypotheses. The properties include elevation of topography or stream-channel beds above sea level, hillslope and channel gradient, vertical incision magnitude, soil thickness, and depth of rock exhumation (eg., Anderson, 1994; Montgomery, 1994; Seidl et al, 1994; Seidl et al, 1997; Heimsath et al, 1999).

Great progress was made in these studies, but large spatial and temporal scale field studies of stream incision histories are needed to further understand bedrock-stream incision (Sklar and Dietrich, 1998). Furthermore, I propose that a field test of the stream-power-law generated incision wave theory, based strictly on fluvial sediments and landforms, is necessary for a rigorous evaluation of the incision wave hypothesis. The well preserved fluvial stratigraphy of Rio Andarax's drainage basin in southern Spain provides an exceptional opportunity to deduce the history of a mountain range scale stream network whose pattern of incision in response to base-level lowering was controlled by an incision wave (García et al, 1999a; García et al, 1999b). It is an ideal setting for my dissertation because it about as large an area as can be mapped in detail the time allotted for a doctoral degree, and for which a detailed landscape evolution history based on morphostratigraphy of numerically dated landforms can be deduced. Testing the incision wave hypothesis with fluvial landform morphostratigraphy provides new insight into factors that control the rate at which an incision wave translates through a real fluvial system. Another criteria for a rigorous test of incision wave theory met by Rio Andarax is that it is a study site different from those where the incision-wave hypothesis was developed. The climatic and geologic setting of the Rio Andarax drainage basin is very different from localities used to calibrate and/or evaluate computer simulations that generated incision waves (Santa Cruz Mountains, California, studied by Anderson [1994], and Rosenbloom and Anderson [1994]; Kauai Island, Hawaii, studied by Seidl et al [1994] and Seidl et al [1997]; Bolivian Andes of South America, studied by Safran [1998]). It is a semi-arid climate, and mean annual precipitation in the Rio Andarax drainage basin is about 30 cm/yr (DCES), which is less than in the areas where previous incision waves were studied. Precipitation falls mostly in late Fall and Spring (DCES).

The content of my doctoral dissertation

In this section I will summarize each chapter of my dissertation and describe each chapter's context within the dissertation. Chapter two is the cornerstone of the dissertation. The chapter includes description and interpretation of fluvial landform morphostratigraphy, calcrete crust macro and micromorphology, and soil characteristics of fluvial sediments in the Rio Andarax drainage basin. These observations and interpretations are analyzed by evaluation in light of, and comparison to, well established models of fluvial system response to climatic perturbations (eg., Harvey et al, 1990), alluvial fan morphogenesis (eg., Harvey et al, 1999), calcretization (Alonso Zarza et al, 1998; Nash and Smith, 1998), soil profile development (Harvey et al, 1995), and tectonism (eg., Goy and Zazo, 1986; García et al, in review) in southern Spain. A product of this analyses is compelling evidence that an incision wave was the fundamental control on the temporal and spatial pattern of Rio Andarax's stream network evolution, and on the character of fluvial sediments in its drainage basin.

Chapter three is an evaluation factors other than the incision wave that affected bedrock-stream incision in the Rio Andarax drainage basin. Rio Andarax is the axial stream of 32 km long and 4 - 6 km wide structural basin formed in crystalline basement rock. Basin-fill sediments are mostly unconsolidated, and there are two structural bedrock highs that are transverse to the basin's E-W structural grain (CSdG map). In some cases, rock-type and along-axial-stream variation in catchment area can affect patterns of bedrock stream incision and fluvial sedimentation (eg., Gardner, 1983; Wohl et al, 1994; Personius, 1995; Safran, 1998). There are knickpoints in the longitudinal profiles of Rio Andarax and its major tributaries where they flow across contacts between contrasting rock types. However, in this chapter I show that rock type does not significantly affect stream incision or fluvial sedimentation. Similarly, there are no detectable changes in stream incision magnitude or fluvial sedimentation patterns near confluences of large tributaries where drainage area increases dramatically (for example, at the Rio Nacimiento-Rio Andarax confluence drainage area increases by 111%.) These results are important to my dissertation because they further support the hypothesis that the fundamental, first-order control on evolution of the Rio Andarax stream network was passage of the incision wave.

Chapter four places my dissertation, more specifically, the work outlined in chapter 2, in a regional context. The results of Chapter 2 have significant implications for regional sedimentologic and geomorphologic studies. The numeric age of travertine deposits in the Alhama de Almería area reported in García et al (1999a) and García et al (in review; see Appendix 1) is consistent with the age of the 8th depositional sequence of Pascual Molina (1997), which was previously inferred to be of upper Pliocene/lower Pleistocene age on the basis of sedimentologic and stratigraphic correlation. It is also important because it provides a numeric age of a paleofluvial level near the Rio Andarax-Rambla de Tabernas confluence. Although no numeric dates for paleofluvial levels have been determined in the Rambla de Tabernas drainage basin, it is the site of several decades of groundbreaking research (eg., Harvey, 1990, and references therein; Nash and Smith, 1998; Harvey et al, 1999b; Mather and Stokes, 1999), and is presently a site where well funded research is being conducted (Mather, 2000, written communication). The numeric age dtermination of a significant fluvial aggradation provides strong support of existing models of climatically-drive cycles of fluvial aggradation and degradation in southern Spain (eg., Harvey, 1990; Harvey et al, 1999a). The incision-wave story in the Andarax network compliments previuosly documented examples of drainage capture resulting from large-scale headward erosion (Harvey and Wells, 1987; Viseras and Fernandez, 1992; Mather, 1993; Mather, 2000). Finally, documentation of incision-wave controlled stream network evolution provides a larger-scale example of how headward erosion leads to changes in sedimentation patterns and alluvial fan formation in southern Spain. These processes were studied in drainage basins ranging in size from 4.25 - 123 km2 (Mather et al, 2000; Stokes and Mather, 2000).

For more information on this project please contact Tony Garcia: afg@geol.ucsb.edu

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