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Late Quaternary rapid talus dissection and debris flow deposition on an alluvial fan in Syria
Catena 55 (2004) 125 – 140 www.elsevier.com/locate/catena
Late Quaternary rapid talus dissection and debris flow deposition on an alluvial fan in Syria Takashi Oguchi a,*, Chiaki T. Oguchi b a
Center for Spatial Information Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan b Japan Society for the Promotion of Science (Japan International Research Center for Agricultural Sciences), 1-1, Ohwashi, Tsukuba 305-8686, Japan
Abstract Landform development in an arid alluvial fan/source basin system in northwest Syria was reconstructed based on field surveys and correlated with dated stratigraphy in the nearby Dederiyeh Cave. During the Last Glacial Maximum, talus deposits were stored in the V-shaped valley of the source area and associated fan gravels became firmly cemented by carbonate, reflecting a slow deposition rate. In the Late Glacial, the lower parts of talus slopes were quickly dissected due to increased storm activity, and boulder debris flows were deposited on the fan surface. Subsequently, water flow removed the sandy matrix of the debris flow deposits on the northern half of the fan surface and that of talus deposits along the trunk stream of the upstream area. This was followed by a period of stability without significant geomorphic change during the Holocene. The inferred rapid geomorphic response to climatic change in the form of talus dissection, debris flows, and fan deposition contrasts with the notion that geomorphic systems in arid regions respond slowly to change in external variables. D 2003 Elsevier B.V. All rights reserved. Keywords: Alluvial fan; Talus slope; Debris flow; Climatic change; Syria
1. Introduction The responses of alluvial fans and their source basins to Quaternary environmental change have received considerable attention (e.g., Lustig, 1965; Ryder, 1971; Dorn et al., 1987; Oguchi, 1994; Harvey et al., 1999). An important concept in this field is ‘‘relaxation time’’ (i.e., how fast geomorphic systems adjust to an extrinsic environmental change such * Corresponding author. E-mail address: [email protected] (T. Oguchi). 0341-8162/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0341-8162(03)00112-7
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Fig. 1. Map showing the location of the study area. Fault distribution is after Garfunkel et al. (1981).
as increase in rainfall) (e.g., Brunsden and Thornes, 1979; Chorley et al., 1984). It is generally believed that drainage basins in humid regions respond more quickly to changes in external conditions and reach an equilibrium state more rapidly than river basins in arid areas (e.g., Wolman and Gerson, 1978; Graf, 1988, p. 40; Bull, 1991, p. 215). However, Oguchi (1996) and Oguchi et al. (2001) demonstrate that humid steep catchments in Japan exhibit long relaxation times in response to Pleistocene – Holocene climatic change. In Japan, marked increases in storm magnitude and frequency occurred at the Pleistocene – Holocene transition, resulting in bedrock gullying on hillslopes. Owing to the high resistance of bedrock to erosion, gullying can only proceed relatively slowly. Therefore, most Japanese catchments have not yet fully adjusted to Pleistocene– Holocene climatic change, as hillslope gullying continues to supply sediments to cause alluvial fan deposition. Fig. 2. Map (a) and satellite image (b) of the study area. Contour interval on the map is 25 m. The image is provided by the Corona Satellite Photography Library, US Geological Survey. Q1 and Q2 = two levels of alluvial fan surfaces; m = gently sloping surface below Q1; n = gently sloping surface below Q2; x = Wadi Dederiyeh; y = Wadi Hassen.
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In some other regions of the world, stripping of hillslope colluvium and resultant fan deposition is the major geomorphic response to Pleistocene – Holocene climatic change (e.g., Brazier et al., 1988; Church and Slaymaker, 1989; Bull, 1991, pp. 164– 166; Thomas and Thorp, 1995, 1996). These processes may proceed faster than bedrock gullying, since colluvium is generally less resistant to erosion than bedrock. For instance, some basins in the arid American Southwest underlain by schist adjusted to the Pleistocene – Holocene climatic change in a few to several thousand years because of the rapid removal of colluvium (Bull, 1991). As a result, current fan deposition is inactive. On the other hand, some basins in the American Southwest underlain by quartz monzonite have not yet adjusted to the Pleistocene – Holocene climatic change, and thus the removal of colluvium and alluvial fan deposition is still active (Bull, 1991). Further research is needed to investigate the relaxation times of alluvial fan/source basin systems linked to climatic change across a wider range of world regions. For example, there have been very few geomorphological studies in Syria, despite widespread and detailed archaeological surveys linked to the pivotal importance of this country in prehistory. Archaeological expeditions to northwest Syria have recently provided the authors with an opportunity to investigate the development of an alluvial fan/source basin system near the Dederiyeh Cave, a Neanderthal site in the Afrin region. This paper describes the geomorphology and surficial geology of the fan/basin system, and discusses the geomorphic development of the system in relation to late Quaternary climatic change.
Fig. 3. Alluvial fan and lower source basin seen from downstream. Q1 and Q2 are two levels of alluvial fan surfaces.
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2. The study area The alluvial fan and the source drainage basin are located in the Afrin region, northwest Syria (Figs. 1 and 2), at the eastern edge of a graben that corresponds to the northern extension of the Dead Sea Transform (Garfunkel et al., 1981). The alluvial fan has an area of ca. 0.19 km2 and a steep gradient of ca. 10%. The fan surface is covered with boulders, gravel, and sand (Fig. 3). The source basin has an area of ca. 3.6 km2 and a relative height of ca. 300 m. The basin is underlain by Miocene limestone and is covered with little vegetation (Fig. 3). Water flow in the trunk stream on the alluvial fan and the source basin (Wadi Dederiyeh) is nonperennial because of semiarid climate (mean annual precipitation is around 400 mm). The lower 1.2 km of the trunk stream in the source basin cuts deeply into bedrock to form a V-shaped valley. The valley-side slopes consist of two components: talus slopes in the lower to middle part, and steep bedrock cliffs in the upper part (Fig. 3). The V-shaped valley is surrounded by a plateau-like gentle landscape with karst features (Fig. 3). Bedrock is exposed along the lowermost and uppermost reaches of the V-shaped valley, but most of the valley bottom is covered with sand, gravel, and boulders. Bedrock exposure in the talus slopes is also limited. The trunk stream immediately above the fan apex bends sharply, causing a 300-m lateral shift in stream course (Figs. 2 and 3). The adjacent valleys also bend in the
Fig. 4. Topography and sediment of Dederiyeh Cave. (a) Cave in the bedrock cliff and talus slope, (b) inner chamber and chimney, and (c) section of sedimentary fill.
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same direction, in the direction of the left-lateral strike slip of the fault along the graben edge. Regional tectonic stress probably accounts for the fault movement because the slip direction corresponds with that for the southern Dead Sea Transform (Fig. 1). The lack of clear vertical deformation of recent geomorphic surfaces by the fault, however, indicates that the fault movement did not affect the base level of erosion. Some caves occur along the bedrock cliffs at the upper V-shaped valley, reflecting limestone solution by karst processes. One of the caves at the southern edge of the valley, called the Dederiyeh Cave (Fig. 2), has a hole in the roof (chimney) that is open to the sky in the inner chamber (Fig. 4a and b). Archaeological excavation of the cave floor since the late 1980s by Japanese, Syrian, and European scientists had led to the discovery of two well-preserved Neanderthal infant burial sites and a detailed cave sediment stratigraphy (Akazawa et al., 1995a,b, 1999; Oguchi and Fujimoto, 2002).
3. Landforms and surficial geology The landforms and surficial deposits of the alluvial fan and hillslopes in the source area were surveyed in the field to reconstruct their geomorphic development. Evidence for late Quaternary erosion and deposition in the Dederiyeh Cave (Oguchi and Fujimoto, 2002) was also examined in relation to landform development outside the cave. 3.1. Alluvial fan The alluvial fan consists of two surfaces of different heights and composed of different materials (Figs. 3, 5, and 6). The fan surface on the left bank of the trunk stream is higher than the fan surface on the right bank by ca. 2.5 m. This higher surface is referred to as Q1. It consists of deposits with angular boulders supported by a sandy matrix (Figs. 5 and 6). The boulders typically have a diameter of 20 –40 cm but the maximum is nearly 1 m. The thickness of the deposits is ca. 3 m. The boulders can be found over the Q1 surface, showing that they are not talus deposits supplied from bedrock cliffs behind the fan. The deposits are nonsorted and nonbedded, indicating that they resulted from debris flow. The boulder deposits are underlain by well-sorted fluvial deposits (Q0a; see Fig. 6) with a thickness of ca. 80 cm. The average long-axis length of 50 gravels sampled from the fluvial deposits is 8 cm. The maximum length is 25 cm, but more than 80% of gravel have lengths between 4 and 12 cm. The deposits are further underlain by wellsorted fluvial deposits cemented by carbonate (Q0b; see Fig. 6). Observation at the artificial outcrop near the fan toe has shown that the thickness of the cemented gravel is more than 5 m. Although it was impossible to take out gravels to measure their exact sizes because the deposits were firmly cemented, visual inspection of the outcrop indicates that the average gravel diameter is ca. 10 cm. The upper ca. 2 m of the cemented deposits (Q0b-1) are white in colour. The average penetration hardness of the cemented matrix, measured by the Yamanaka-type tester, is ca. 300 kg W/cm2. The
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Fig. 5. Two geomorphic surfaces and cemented gravel exposed along the trunk stream on the alluvial fan, seen from upstream.
lower cemented deposits (Q0b-2) have a redder colour and the average penetration hardness of the matrix is ca. 70 kg W/cm2. These values differ markedly from the matrix hardness values of the upper noncemented Q0a and debris flow deposits (Q1), which are typically ca. 1 kg W/cm2. It is unknown whether Q0a below the debris flow deposits already existed when cementation took place. Gile et al. (1966) and Mayer et al. (1988) suggest that the upper boundary of carbonate accumulation in desert soil generally
Fig. 6. Cross-section showing topography and inferred stratigraphy of the mid alluvial fan, seen from downstream.
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occurs within about 5– 60 cm of the soil surface, which is slightly less than the observed thickness of the noncemented fluvial deposits reported here. Q0a may postdate Q0b because their boundary looks sharp. The lower fan surface on the right bank is referred to as Q2. Large boulders are scattered over the Q2 surface (Fig. 5), showing that they are not talus deposits supplied from bedrock cliffs behind the fan. The lack of thick matrix also indicates that their current sedimentological structure did not result from deposition by the river. The stratigraphy below the boulders is similar to that below Q1, consisting of an upper noncemented gravel and a lower cemented gravel without boulders. The trunk stream on the alluvial fan cuts into the cemented gravel and, thus, the stream is lower than Q2 by a few meters. Some boulders are scattered in the incised channel (Fig. 5), and they do not show the lobe structure typical of debris flow deposits. Recent fluvial deposits are also limited in the channel (Fig. 5), indicating that the main geomorphological process along the channel has been linear erosion by water. Sloping surfaces gentler than the alluvial fan occur between the fan and the floodplain of the Afrin River (Figs. 2 and 3). Large boulders are rare on the surfaces, showing that they resulted from the deposition of fine sediment. No distinct incised channels can be seen on the surfaces. Distribution of landforms and the aspects of the surfaces observed in the field indicate that sediment forming the gentle surface below Q1 (m in Fig. 2) was supplied from the southern adjacent river (Wadi Hassen; y in Fig. 2), while sediment below Q2 (n in Fig. 2) was supplied from the alluvial fan and the V-shaped valley of the study area. The following mechanisms for the development of the alluvial fan surfaces have been inferred from the observations made above. The lower cemented fan deposits consist of well-sorted small gravel, indicating that fan deposition resulted from fluvial processes. The cementation of the deposits indicates that fluvial deposition was followed by a period of stability without marked deposition or erosion but with groundwater throughput. Then debris flows deposited coarse materials on the surface of the alluvial fan, including angular boulders derived from the source area. The boulders in the surface deposits of Q1 and Q2 have equivalent dimensions and shapes (Fig. 5), suggesting that the debris flows deposited boulders and sands on the whole alluvial fan with a thickness of ca. 3 m, but subsequent erosion by water flow through the northern half of the fan led to the selective removal of the sandy matrix, leaving a concentration of boulders on the surface as lag deposits. This resulted in a decrease in surface height of the northern fan surface by a few meters, leading to the formation of Q2. The removed fine materials were deposited on the gently sloping surface below Q2. Small-scale linear erosion by water along the trunk stream then incised the cemented gravel, also leaving boulders as lag deposits. This channel incision seems to have proceeded only gradually, since the cemented gravel has a very high hardness. 3.2. Hillslopes in the source area As noted above, the topography of the source area can be divided into two major components: (1) the V-shaped valley with talus slopes and bedrock cliffs, and (2) gentle plateau-like surfaces surrounding the V-shaped valley (Fig. 3). Although the gentle plateau surfaces occupy a much larger area than the V-shaped valleys (Fig. 2),
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the low topographic gradient and the lack of large gravels in soils on the plateau indicate that erosion and transport of course material have been very limited. Therefore, in this study, attention is directed toward the topography and deposits of the V-shaped valley as a main source of gravel transported to the alluvial fan. Eight topographic cross-sections across the valley were surveyed using a slope profiler with a span width of 2 m (Suzuki and Nakanishi, 1990). The surveyed sections and field observations have shown that the talus slopes in the V-shaped valley can be divided into three segments (Figs. 7 and 8). Segment A is the upper talus slope, and consists of angular boulders and gravel with sandy matrix. The maximum diameter of the boulders is ca. 2 m. The typical angle of Segment A is 20 –30j. Segment B is the lower talus slope, and also consists of angular boulders, gravel, and sands, but the density of boulders is lower than that of Segment A (Fig. 8). The typical angle of Segment B is similar to that of Segment A. Small cliffs, a few meters in height, occur at the boundary between the two segments (Fig. 7), indicating that Segment B formed due to the dissection of Segment A. It is unrealistic that Segment A formed by rock fall from bedrock cliffs after the formation of Segment B. If this had happened, the boundary of the two segments would have been more continuous, not bordered by cliffs. Moreover, Segment A would be always steeper
Fig. 7. Examples of the surveyed cross-sections of the V-shaped valley. Inset map shows the locations of the sections. (A) – (C) show slope segments.
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Fig. 8. Valley-side slope about 500 m above the fan apex and segments of talus slopes. A – C show slope segments.
than adjacent Segment B, because to keep talus deposits only on upper slopes, original slope angles should be lower than the angle of repose. On the contrary, actual Segment B is often steeper than adjacent Segment A (for instance, see the profile of the V-shaped valley at the upper left of Fig. 8). The trunk stream cuts down into Segment B by several meters to form a steeper segment (Segment C) of the lowermost talus slopes (Figs. 7 and 8). The processes of development of slope segments were inferred as follows, from the above observations. Segment A is an accumulation of debris over the bedrock valley slope supplied by erosion of the scarp above at the edge of the plateau. Boulders sometimes fell from the bedrock cliff above during the formation of Segment A. The lower part of Segment A was subsequently eroded to form Segment B both by the stripping of talus deposits and stream incision, and eroded material was evacuated from the valley. Fluvial processes along the trunk stream again led to riverine incision into the valley bottom to form Segment C. More boulders can be found along Segment C than in Segment B (Fig. 8), while the stream boulders do not exhibit the lobe structure typical of debris flow deposits. This observation suggests that fine material has been transported from the toe of Segment B to form Segment C, while leaving boulders in the valley bottom as lag deposits. Erosion to form Segments B and C rarely reached the bedrock except the uppermost and lowermost reaches of the V-shaped valley (Fig. 7), meaning that major geomorphic processes in the V-shaped valley have been the deposition and erosion of fragmented material.
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3.3. Topography and deposits in the Dederiyeh Cave Oguchi and Fujimoto (2002) described the sediment and topography in the Dederiyeh Cave at the top of the southern V-shaped valley (Fig. 4a). The history of deposition and erosion in the cave in relation to environmental change was reconstructed based on the topographic characteristics of the cave floor (Fig. 4b), grain size distribution of sediment samples from 27 stratigraphic levels (Fig. 4c), and chemical/ mineralogical properties of the sediment samples inferred from X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. Akazawa et al. (1999) estimated the age of the cave deposits based on the types of buried artefacts and carbon dating. They also reconstructed palaeovegetation around the cave based on the composition of animal fossils in the cave deposits. The results of these studies are summarised as follows. The cave originally had only one entrance. Carbonate and bat guano deposition were the main sedimentological processes in the cave. However, karstic processes opened up a hole in the roof in the inner chamber around 60 ka. This change permitted the inflow of sediments from the land surface above, resulting in the accumulation of relatively coarse sediments on the cave floor by fluvial processes. Such accumulation gradually became inactive between 60 and 40 ka because sediment inflow diminished due to forestation of the catchment outside the cave, and thus reduced sediment supply. Sediment deposition and erosion on the cave floor became largely inactive in the Last Glacial Maximum or Isotope Stage 2 when the cave floor below the hole in the roof became cemented by carbonate. This relative stability reflects reduction in the magnitude and frequency of storm events. However, in the Late Glacial, the cave floor was subjected to rapid erosion. This event reflects the significant increase in rainfall in the northern Levant due to the northward shift of the major storm track (Henry, 1989). By the end of the Late Glacial, this period of intense erosion had diminished and both deposition and erosion in the cave have been largely inactive during the Holocene.
4. Discussion 4.1. Correlation between hillslope development and fan development The inferred development of the alluvial fan can be correlated with that of the Vshaped valley (Fig. 9), based on the fact that most sediments that accumulated on the fan were supplied from the V-shaped valley. The present channel on the alluvial fan has been incised into cemented gravel, and the distribution of recent fluvial sediment in the channel is limited. These observations suggest that sediment supply from the Vshaped valley has been limited and linear erosion by water has been a major geomorphic process in the alluvial fan. The previous phase in the development of both the alluvial fan and the V-shaped valley was characterised by the selective transport of fine materials by fluvial processes, forming Q2 in the fan and Segment C in the upstream area. Prior to this phase, erosion of lower talus slopes to form Segment B occurred in the V-shaped valley, and the deposition of debris flow material occurred
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Fig. 9. Three phases of geomorphic development in the study area.
in the alluvial fan to form Q1. To examine whether this talus erosion and the debris flow deposition can be correlated, the approximate volume of sediment deposited in the alluvial fan was estimated and compared with estimates of sediment volumes eroded from the V-shaped valley. As noted before, the thickness of the debris flow deposits on the Q1 fan surface is ca. 3 m, and it can be assumed that the deposits covered the entire alluvial fan when the debris flows occurred since Q2 resulted from erosion of Q1. The product of the thickness and the area of the alluvial fan, or the estimated volume of the debris flow deposits is 5.7 105 m2. The form of the talus slopes before the formation of Segments B and C was reconstructed on the cross-sections (Fig. 7). The area of erosion on the cross-sections was integrated using the distance between the sections and the trapezoidal formula. The calculated volume of erosion in the V-shaped valley when Segment B formed is 4.5 105 m2, which is close to the estimated volume of the debris flow deposits on the alluvial fan. This observation supports the correlation between the rapid dissection of talus slopes to form Segment B and debris flow deposition on the fan to form Q1, although the estimated erosion volume may include some errors because of the uncertainty in profile reconstruction and the limited number of the cross-sections. The preceding phase was characterised by the accumulation of talus deposits to form Segment A in the V-shaped valley as well as the deposition and cementation of finer fluvial gravel (Q0) on the alluvial fan. Although the talus deposits of Segment A often include boulders, they were not transported to the alluvial fan, pointing to a more limited power of flow than the subsequent phase with debris flow deposition. Fan deposition even terminated to allow cementation of fan deposits by carbonate deposition, indicating that most talus deposits were stored within the V-shaped valley. In summary, the phases of alluvial fan development and those of the development of the V-shaped valley can be correlated (Table 1). Consequently, a model is proposed to describe the three phases of geomorphic development in the alluvial fan and the source basin (Fig. 9).
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Table 1 Correlation among phases of geomorphic development in the alluvial fan, the source area, and the Dederiyeh Cave Phase
Alluvial fan
Source area
Dedeiryeh Cave
Age
b
Channel incision by water
No significant change
No significant change
Postglacial
a
Partial fluvial erosion to form Q2
Partial fluvial erosion to form Segment C
2
Debris flow deposition to form Q1
Rapid talus dissection to form Segment B
Rapid erosion
Late Glacial
1
Sediment cementation by carbonate
Talus deposition to form Segment A
Sediment cementation by carbonate
Last Glacial Maximum
3
Fluvial deposition
Fluvial deposition
4.2. Correlation between geomorphic processes inside and outside the cave The three phases of geomorphic development have a similarity to the erosion/ deposition history of the Dederiyeh Cave. The first phase (Fig. 9) can be correlated to the depositional phase in the cave during the Last Glacial Maximum or to the earlier period because both of them are characterised by the fluvial deposition of relatively fine material and subsequent cementation by carbonate. The following rapid erosion of the cave floor in the Late Glacial can be correlated with the dissection of talus slopes to form Segment B and debris flow deposition on the alluvial fan because all the processes can be triggered by enhanced storm activity. This correlation between sediment processes inside and outside of the cave is supported by the fact that Segment A of talus slopes below the cave is covered with sediment evacuated from the cave (Fig. 4a) because this observation shows that the erosion of the cave floor occurred after the formation of Segment A. The subsequent postglacial stable period in the cave can be correlated with the phase of reduced geomorphological processes outside the cave, associated with selective fluvial transport of the fine material along the trunk stream and small-scale incision along the current channel on the alluvial fan. If the above correlation is accepted, the ages of the three geomorphic phases in the alluvial fan and the source basin can be estimated: talus accumulation to form Segment A and the cementation of fan deposits (Q0) occurred during the Last Glacial Maximum or earlier; talus dissection and debris flow to form Segment B and Q1 occurred in the Late Glacial; and the selective fluvial transport of fine material to form Segment C and Q2 occurred in the postglacial period (Table 1). The direct validation of this age estimation is difficult because 14C datable organic material was found neither in the fan nor in the talus deposits. An indirect support is the type of artefacts found on geomorphic surfaces. The lack of Levallois/Mousterian lithic artefacts and the occurrence of flaked pottery on the alluvial fan and talus slopes indicate that all the geomorphic surfaces formed after 30– 40 ka, which is consistent with the above age
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estimation. In addition, previous studies in the northern Levant including northwest Syria pointed to an arid climate around the Last Glacial Maximum (e.g., Van Zeist and Bottema, 1982; Henry, 1989; Baruch, 1994), which may explain the reduced fluvial processes and enhanced gravel cementation during that period. 4.3. Characteristics of geomorphic responses to rapid environmental change The existence of Phase 2 (Fig. 9) points to the importance of rapid erosion and deposition caused by increased storm activity. The dissection of the lower talus slopes and subsequent debris flow seems to have occurred rapidly because the debris flow deposits on the alluvial fan consist of a single unit without unconformities. In other words, much of the debris stored in the source area as talus deposits was rapidly eroded. Although it is difficult to quantify the intensity of storms that triggered the rapid talus dissection and debris flows, there must have been an abrupt and marked increase in the storm power, as seen from the drastic change in the facies of the fan deposits. Phases 1 and 2 inferred by this study agree with a model of talus accumulation and dissection proposed in the Levant, in that Last Glacial talus accumulation switched to postglacial dissection (Gerson, 1982; Bowman et al., 1986; Bull, 1991). These previous studies, however, did not indicate that talus dissection proceeded over a short period of time. Vegetation does not appear to have played an important role in the geomorphic development of the hillslope/fan system. The analysis of animal fossils in the Dederiyeh Cave deposits indicates that there was forest outside the cave around 40 – 50 ka (Akazawa et al., 1999). Palynological surveys near the study area (Niklewski and Van Zeist, 1970; Van Zeist and Bottema, 1982) suggest that most of the forest disappeared during the Last Glacial Maximum due to increased aridity. These forest-free conditions were favourable for talus accumulation in the V-shaped valley. Therefore, the beginning of rapid talus dissection is not due to the retreat of forest but due to climatic change and the onset of greater storm intensity. The termination of rapid talus dissection and debris flow activity can be attributed to the further northward shift of the storm track around 10 ka (Henry, 1989). The inferred geomorphic responses outlined here are similar to an example from the American Southwest (Bull, 1991) in that the stripping of slope deposits led to fan deposition after the Pleistocene– Holocene transition. This study, however, points to a much faster geomorphic response to climatic change, involving rapid talus dissection, debris flows, and fan rebuilding. Abrupt increases in storm activity, steep talus slopes sensitive to erosion, and the hillslopes directly connected to the alluvial fan over very short distances together accounted for the rapid geomorphic response.
5. Concluding remarks This paper has presented an example of rapid geomorphic responses to late Quaternary climatic change in alluvial fan/source basin systems. Slow fan deposition
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and subsequent gravel cementation by carbonate under stable conditions switched to catastrophic debris flow deposition and the subsequent removal of fine material, which was followed by another stable period. These reflect changes in hillslope processes including talus deposition, dissection, and stabilisation. Although previous studies have suggested that drainage basins in arid regions tend to respond slowly to changes in external conditions, this study demonstrates that rapid geomorphological responses to extrinsic change and resultant drastic geomorphic change can occur within arid environments. It is now necessary to investigate how commonly these types of rapid geomorphic response occur across a range of arid environments. Detailed archaeological information in Syria can provide valuable ancillary information on the chronology of geomorphological events. It is hoped that further interdisciplinary archaeological and geomorphological research in Syria will yield important new insights into the nature of arid zone geomorphological processes and response times.
Acknowledgements We thank Helen Jarvie for reviewing an early draft of this paper and correcting the English. Adrian Harvey and Martin Thorp also gave us constructive comments to improve the draft. We also thank Takeru Akazawa, the leader of the archaeological mission to the Dederiyeh Cave, for giving us an opportunity to do research in Syria.
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Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. Springer-Verlag, Berlin. Harvey, A.M., Wigand, P.E., Wells, S.G., 1999. Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA. Catena 36, 255 – 281. Henry, D.O., 1989. From Foraging to Agriculture: The Levant at the End of the Ice Age. University of Pennsylvania Press, Philadelphia. Lustig, K.L., 1965. Clastic sedimentation in Deep Spring Valley, California. U.S. Geol. Surv. Prof. Pap. 352, 131 – 192. Mayer, L., McFadden, L.D., Harden, J.W., 1988. Distribution of calcium carbonate in desert soils: a model. Geology 16, 303 – 306. Niklewski, J., Van Zeist, W., 1970. A late Quaternary pollen diagram from northwestern Syria. Acta Bot. Neerl. 19, 737 – 754. Oguchi, T., 1994. Late Quaternary geomorphic development of alluvial fan – source basin systems: the Yamagata region, Japan. Geogr. Rev. Jpn. 67B, 81 – 100. Oguchi, T., 1996. Relaxation time of geomorphic responses to Pleistocene – Holocene climatic change. Trans. Jpn. Geomorphol. Union 17, 309 – 321. Oguchi, T., Fujimoto, K., 2002. The sediment and paleoenvironment of the Dederiyeh Cave. In: Akazawa, T., Muhesen, S. (Eds.), Neanderthal Burials: Excavation of the Dederiyeh Cave, Afrin, Syria. International Research Center for Japanese Studies, Kyoto/L’Erma di Bretschneider, Roma, pp. 33 – 61. Oguchi, T., Saito, K., Kadomura, H., Grossman, M., 2001. Fluvial geomorphology and paleohydrology in Japan. Geomorphology 39, 3 – 19. Ryder, J.M., 1971. The stratigraphy and morphology of para-glacial alluvial fans in south-central British Columbia. Can. J. Earth Sci. 8, 279 – 298. Suzuki, T., Nakanishi, A., 1990. Rates of decline of fluvial terrace scarps in the Chichibu Basin, Japan. Trans. Jpn. Geomorphol. Union 11, 117 – 149. Thomas, M.F., Thorp, M.B., 1995. Geomorphic response to rapid climatic and hydrologic change during the Late Pleistocene and early Holocene in the humid and sub-humid tropics. Quat. Sci. Rev. 14, 193 – 207. Thomas, M.F., Thorp, M.B., 1996. The response of geomorphic systems to climatic and hydrologic changes during the Late Glacial and Holocene in the humid and sub-humid tropics. In: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Change: the Context of Palaeohydrology. Geological Society Special Publication, vol. 115. Geological Society, London, pp. 139 – 153. Van Zeist, W., Bottema, S., 1982. Vegetational history of the Eastern Mediterranean and the Near East during the last 20,000 years. In: Bintliff, J.L., Van Zeist, W. (Eds.), Palaeoclimates, Palaeoenvironments and Human Communities in the Eastern Mediterranean Region in Late Prehistory. BAR International Series, vol. 133. British Archaeological Reports, Oxford, pp. 277 – 323. Wolman, M.G., Gerson, R., 1978. Relative scales of time and effectiveness of climate in watershed geomorphology. Earth Surf. Process. 3, 189 – 208.
Late Quaternary rapid talus dissection and debris flow deposition on an alluvial fan in Syria Takashi Oguchi a,*, Chiaki T. Oguchi b a
Center for Spatial Information Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan b Japan Society for the Promotion of Science (Japan International Research Center for Agricultural Sciences), 1-1, Ohwashi, Tsukuba 305-8686, Japan
Abstract Landform development in an arid alluvial fan/source basin system in northwest Syria was reconstructed based on field surveys and correlated with dated stratigraphy in the nearby Dederiyeh Cave. During the Last Glacial Maximum, talus deposits were stored in the V-shaped valley of the source area and associated fan gravels became firmly cemented by carbonate, reflecting a slow deposition rate. In the Late Glacial, the lower parts of talus slopes were quickly dissected due to increased storm activity, and boulder debris flows were deposited on the fan surface. Subsequently, water flow removed the sandy matrix of the debris flow deposits on the northern half of the fan surface and that of talus deposits along the trunk stream of the upstream area. This was followed by a period of stability without significant geomorphic change during the Holocene. The inferred rapid geomorphic response to climatic change in the form of talus dissection, debris flows, and fan deposition contrasts with the notion that geomorphic systems in arid regions respond slowly to change in external variables. D 2003 Elsevier B.V. All rights reserved. Keywords: Alluvial fan; Talus slope; Debris flow; Climatic change; Syria
1. Introduction The responses of alluvial fans and their source basins to Quaternary environmental change have received considerable attention (e.g., Lustig, 1965; Ryder, 1971; Dorn et al., 1987; Oguchi, 1994; Harvey et al., 1999). An important concept in this field is ‘‘relaxation time’’ (i.e., how fast geomorphic systems adjust to an extrinsic environmental change such * Corresponding author. E-mail address: [email protected] (T. Oguchi). 0341-8162/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0341-8162(03)00112-7
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Fig. 1. Map showing the location of the study area. Fault distribution is after Garfunkel et al. (1981).
as increase in rainfall) (e.g., Brunsden and Thornes, 1979; Chorley et al., 1984). It is generally believed that drainage basins in humid regions respond more quickly to changes in external conditions and reach an equilibrium state more rapidly than river basins in arid areas (e.g., Wolman and Gerson, 1978; Graf, 1988, p. 40; Bull, 1991, p. 215). However, Oguchi (1996) and Oguchi et al. (2001) demonstrate that humid steep catchments in Japan exhibit long relaxation times in response to Pleistocene – Holocene climatic change. In Japan, marked increases in storm magnitude and frequency occurred at the Pleistocene – Holocene transition, resulting in bedrock gullying on hillslopes. Owing to the high resistance of bedrock to erosion, gullying can only proceed relatively slowly. Therefore, most Japanese catchments have not yet fully adjusted to Pleistocene– Holocene climatic change, as hillslope gullying continues to supply sediments to cause alluvial fan deposition. Fig. 2. Map (a) and satellite image (b) of the study area. Contour interval on the map is 25 m. The image is provided by the Corona Satellite Photography Library, US Geological Survey. Q1 and Q2 = two levels of alluvial fan surfaces; m = gently sloping surface below Q1; n = gently sloping surface below Q2; x = Wadi Dederiyeh; y = Wadi Hassen.
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In some other regions of the world, stripping of hillslope colluvium and resultant fan deposition is the major geomorphic response to Pleistocene – Holocene climatic change (e.g., Brazier et al., 1988; Church and Slaymaker, 1989; Bull, 1991, pp. 164– 166; Thomas and Thorp, 1995, 1996). These processes may proceed faster than bedrock gullying, since colluvium is generally less resistant to erosion than bedrock. For instance, some basins in the arid American Southwest underlain by schist adjusted to the Pleistocene – Holocene climatic change in a few to several thousand years because of the rapid removal of colluvium (Bull, 1991). As a result, current fan deposition is inactive. On the other hand, some basins in the American Southwest underlain by quartz monzonite have not yet adjusted to the Pleistocene – Holocene climatic change, and thus the removal of colluvium and alluvial fan deposition is still active (Bull, 1991). Further research is needed to investigate the relaxation times of alluvial fan/source basin systems linked to climatic change across a wider range of world regions. For example, there have been very few geomorphological studies in Syria, despite widespread and detailed archaeological surveys linked to the pivotal importance of this country in prehistory. Archaeological expeditions to northwest Syria have recently provided the authors with an opportunity to investigate the development of an alluvial fan/source basin system near the Dederiyeh Cave, a Neanderthal site in the Afrin region. This paper describes the geomorphology and surficial geology of the fan/basin system, and discusses the geomorphic development of the system in relation to late Quaternary climatic change.
Fig. 3. Alluvial fan and lower source basin seen from downstream. Q1 and Q2 are two levels of alluvial fan surfaces.
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2. The study area The alluvial fan and the source drainage basin are located in the Afrin region, northwest Syria (Figs. 1 and 2), at the eastern edge of a graben that corresponds to the northern extension of the Dead Sea Transform (Garfunkel et al., 1981). The alluvial fan has an area of ca. 0.19 km2 and a steep gradient of ca. 10%. The fan surface is covered with boulders, gravel, and sand (Fig. 3). The source basin has an area of ca. 3.6 km2 and a relative height of ca. 300 m. The basin is underlain by Miocene limestone and is covered with little vegetation (Fig. 3). Water flow in the trunk stream on the alluvial fan and the source basin (Wadi Dederiyeh) is nonperennial because of semiarid climate (mean annual precipitation is around 400 mm). The lower 1.2 km of the trunk stream in the source basin cuts deeply into bedrock to form a V-shaped valley. The valley-side slopes consist of two components: talus slopes in the lower to middle part, and steep bedrock cliffs in the upper part (Fig. 3). The V-shaped valley is surrounded by a plateau-like gentle landscape with karst features (Fig. 3). Bedrock is exposed along the lowermost and uppermost reaches of the V-shaped valley, but most of the valley bottom is covered with sand, gravel, and boulders. Bedrock exposure in the talus slopes is also limited. The trunk stream immediately above the fan apex bends sharply, causing a 300-m lateral shift in stream course (Figs. 2 and 3). The adjacent valleys also bend in the
Fig. 4. Topography and sediment of Dederiyeh Cave. (a) Cave in the bedrock cliff and talus slope, (b) inner chamber and chimney, and (c) section of sedimentary fill.
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same direction, in the direction of the left-lateral strike slip of the fault along the graben edge. Regional tectonic stress probably accounts for the fault movement because the slip direction corresponds with that for the southern Dead Sea Transform (Fig. 1). The lack of clear vertical deformation of recent geomorphic surfaces by the fault, however, indicates that the fault movement did not affect the base level of erosion. Some caves occur along the bedrock cliffs at the upper V-shaped valley, reflecting limestone solution by karst processes. One of the caves at the southern edge of the valley, called the Dederiyeh Cave (Fig. 2), has a hole in the roof (chimney) that is open to the sky in the inner chamber (Fig. 4a and b). Archaeological excavation of the cave floor since the late 1980s by Japanese, Syrian, and European scientists had led to the discovery of two well-preserved Neanderthal infant burial sites and a detailed cave sediment stratigraphy (Akazawa et al., 1995a,b, 1999; Oguchi and Fujimoto, 2002).
3. Landforms and surficial geology The landforms and surficial deposits of the alluvial fan and hillslopes in the source area were surveyed in the field to reconstruct their geomorphic development. Evidence for late Quaternary erosion and deposition in the Dederiyeh Cave (Oguchi and Fujimoto, 2002) was also examined in relation to landform development outside the cave. 3.1. Alluvial fan The alluvial fan consists of two surfaces of different heights and composed of different materials (Figs. 3, 5, and 6). The fan surface on the left bank of the trunk stream is higher than the fan surface on the right bank by ca. 2.5 m. This higher surface is referred to as Q1. It consists of deposits with angular boulders supported by a sandy matrix (Figs. 5 and 6). The boulders typically have a diameter of 20 –40 cm but the maximum is nearly 1 m. The thickness of the deposits is ca. 3 m. The boulders can be found over the Q1 surface, showing that they are not talus deposits supplied from bedrock cliffs behind the fan. The deposits are nonsorted and nonbedded, indicating that they resulted from debris flow. The boulder deposits are underlain by well-sorted fluvial deposits (Q0a; see Fig. 6) with a thickness of ca. 80 cm. The average long-axis length of 50 gravels sampled from the fluvial deposits is 8 cm. The maximum length is 25 cm, but more than 80% of gravel have lengths between 4 and 12 cm. The deposits are further underlain by wellsorted fluvial deposits cemented by carbonate (Q0b; see Fig. 6). Observation at the artificial outcrop near the fan toe has shown that the thickness of the cemented gravel is more than 5 m. Although it was impossible to take out gravels to measure their exact sizes because the deposits were firmly cemented, visual inspection of the outcrop indicates that the average gravel diameter is ca. 10 cm. The upper ca. 2 m of the cemented deposits (Q0b-1) are white in colour. The average penetration hardness of the cemented matrix, measured by the Yamanaka-type tester, is ca. 300 kg W/cm2. The
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Fig. 5. Two geomorphic surfaces and cemented gravel exposed along the trunk stream on the alluvial fan, seen from upstream.
lower cemented deposits (Q0b-2) have a redder colour and the average penetration hardness of the matrix is ca. 70 kg W/cm2. These values differ markedly from the matrix hardness values of the upper noncemented Q0a and debris flow deposits (Q1), which are typically ca. 1 kg W/cm2. It is unknown whether Q0a below the debris flow deposits already existed when cementation took place. Gile et al. (1966) and Mayer et al. (1988) suggest that the upper boundary of carbonate accumulation in desert soil generally
Fig. 6. Cross-section showing topography and inferred stratigraphy of the mid alluvial fan, seen from downstream.
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occurs within about 5– 60 cm of the soil surface, which is slightly less than the observed thickness of the noncemented fluvial deposits reported here. Q0a may postdate Q0b because their boundary looks sharp. The lower fan surface on the right bank is referred to as Q2. Large boulders are scattered over the Q2 surface (Fig. 5), showing that they are not talus deposits supplied from bedrock cliffs behind the fan. The lack of thick matrix also indicates that their current sedimentological structure did not result from deposition by the river. The stratigraphy below the boulders is similar to that below Q1, consisting of an upper noncemented gravel and a lower cemented gravel without boulders. The trunk stream on the alluvial fan cuts into the cemented gravel and, thus, the stream is lower than Q2 by a few meters. Some boulders are scattered in the incised channel (Fig. 5), and they do not show the lobe structure typical of debris flow deposits. Recent fluvial deposits are also limited in the channel (Fig. 5), indicating that the main geomorphological process along the channel has been linear erosion by water. Sloping surfaces gentler than the alluvial fan occur between the fan and the floodplain of the Afrin River (Figs. 2 and 3). Large boulders are rare on the surfaces, showing that they resulted from the deposition of fine sediment. No distinct incised channels can be seen on the surfaces. Distribution of landforms and the aspects of the surfaces observed in the field indicate that sediment forming the gentle surface below Q1 (m in Fig. 2) was supplied from the southern adjacent river (Wadi Hassen; y in Fig. 2), while sediment below Q2 (n in Fig. 2) was supplied from the alluvial fan and the V-shaped valley of the study area. The following mechanisms for the development of the alluvial fan surfaces have been inferred from the observations made above. The lower cemented fan deposits consist of well-sorted small gravel, indicating that fan deposition resulted from fluvial processes. The cementation of the deposits indicates that fluvial deposition was followed by a period of stability without marked deposition or erosion but with groundwater throughput. Then debris flows deposited coarse materials on the surface of the alluvial fan, including angular boulders derived from the source area. The boulders in the surface deposits of Q1 and Q2 have equivalent dimensions and shapes (Fig. 5), suggesting that the debris flows deposited boulders and sands on the whole alluvial fan with a thickness of ca. 3 m, but subsequent erosion by water flow through the northern half of the fan led to the selective removal of the sandy matrix, leaving a concentration of boulders on the surface as lag deposits. This resulted in a decrease in surface height of the northern fan surface by a few meters, leading to the formation of Q2. The removed fine materials were deposited on the gently sloping surface below Q2. Small-scale linear erosion by water along the trunk stream then incised the cemented gravel, also leaving boulders as lag deposits. This channel incision seems to have proceeded only gradually, since the cemented gravel has a very high hardness. 3.2. Hillslopes in the source area As noted above, the topography of the source area can be divided into two major components: (1) the V-shaped valley with talus slopes and bedrock cliffs, and (2) gentle plateau-like surfaces surrounding the V-shaped valley (Fig. 3). Although the gentle plateau surfaces occupy a much larger area than the V-shaped valleys (Fig. 2),
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the low topographic gradient and the lack of large gravels in soils on the plateau indicate that erosion and transport of course material have been very limited. Therefore, in this study, attention is directed toward the topography and deposits of the V-shaped valley as a main source of gravel transported to the alluvial fan. Eight topographic cross-sections across the valley were surveyed using a slope profiler with a span width of 2 m (Suzuki and Nakanishi, 1990). The surveyed sections and field observations have shown that the talus slopes in the V-shaped valley can be divided into three segments (Figs. 7 and 8). Segment A is the upper talus slope, and consists of angular boulders and gravel with sandy matrix. The maximum diameter of the boulders is ca. 2 m. The typical angle of Segment A is 20 –30j. Segment B is the lower talus slope, and also consists of angular boulders, gravel, and sands, but the density of boulders is lower than that of Segment A (Fig. 8). The typical angle of Segment B is similar to that of Segment A. Small cliffs, a few meters in height, occur at the boundary between the two segments (Fig. 7), indicating that Segment B formed due to the dissection of Segment A. It is unrealistic that Segment A formed by rock fall from bedrock cliffs after the formation of Segment B. If this had happened, the boundary of the two segments would have been more continuous, not bordered by cliffs. Moreover, Segment A would be always steeper
Fig. 7. Examples of the surveyed cross-sections of the V-shaped valley. Inset map shows the locations of the sections. (A) – (C) show slope segments.
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Fig. 8. Valley-side slope about 500 m above the fan apex and segments of talus slopes. A – C show slope segments.
than adjacent Segment B, because to keep talus deposits only on upper slopes, original slope angles should be lower than the angle of repose. On the contrary, actual Segment B is often steeper than adjacent Segment A (for instance, see the profile of the V-shaped valley at the upper left of Fig. 8). The trunk stream cuts down into Segment B by several meters to form a steeper segment (Segment C) of the lowermost talus slopes (Figs. 7 and 8). The processes of development of slope segments were inferred as follows, from the above observations. Segment A is an accumulation of debris over the bedrock valley slope supplied by erosion of the scarp above at the edge of the plateau. Boulders sometimes fell from the bedrock cliff above during the formation of Segment A. The lower part of Segment A was subsequently eroded to form Segment B both by the stripping of talus deposits and stream incision, and eroded material was evacuated from the valley. Fluvial processes along the trunk stream again led to riverine incision into the valley bottom to form Segment C. More boulders can be found along Segment C than in Segment B (Fig. 8), while the stream boulders do not exhibit the lobe structure typical of debris flow deposits. This observation suggests that fine material has been transported from the toe of Segment B to form Segment C, while leaving boulders in the valley bottom as lag deposits. Erosion to form Segments B and C rarely reached the bedrock except the uppermost and lowermost reaches of the V-shaped valley (Fig. 7), meaning that major geomorphic processes in the V-shaped valley have been the deposition and erosion of fragmented material.
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3.3. Topography and deposits in the Dederiyeh Cave Oguchi and Fujimoto (2002) described the sediment and topography in the Dederiyeh Cave at the top of the southern V-shaped valley (Fig. 4a). The history of deposition and erosion in the cave in relation to environmental change was reconstructed based on the topographic characteristics of the cave floor (Fig. 4b), grain size distribution of sediment samples from 27 stratigraphic levels (Fig. 4c), and chemical/ mineralogical properties of the sediment samples inferred from X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. Akazawa et al. (1999) estimated the age of the cave deposits based on the types of buried artefacts and carbon dating. They also reconstructed palaeovegetation around the cave based on the composition of animal fossils in the cave deposits. The results of these studies are summarised as follows. The cave originally had only one entrance. Carbonate and bat guano deposition were the main sedimentological processes in the cave. However, karstic processes opened up a hole in the roof in the inner chamber around 60 ka. This change permitted the inflow of sediments from the land surface above, resulting in the accumulation of relatively coarse sediments on the cave floor by fluvial processes. Such accumulation gradually became inactive between 60 and 40 ka because sediment inflow diminished due to forestation of the catchment outside the cave, and thus reduced sediment supply. Sediment deposition and erosion on the cave floor became largely inactive in the Last Glacial Maximum or Isotope Stage 2 when the cave floor below the hole in the roof became cemented by carbonate. This relative stability reflects reduction in the magnitude and frequency of storm events. However, in the Late Glacial, the cave floor was subjected to rapid erosion. This event reflects the significant increase in rainfall in the northern Levant due to the northward shift of the major storm track (Henry, 1989). By the end of the Late Glacial, this period of intense erosion had diminished and both deposition and erosion in the cave have been largely inactive during the Holocene.
4. Discussion 4.1. Correlation between hillslope development and fan development The inferred development of the alluvial fan can be correlated with that of the Vshaped valley (Fig. 9), based on the fact that most sediments that accumulated on the fan were supplied from the V-shaped valley. The present channel on the alluvial fan has been incised into cemented gravel, and the distribution of recent fluvial sediment in the channel is limited. These observations suggest that sediment supply from the Vshaped valley has been limited and linear erosion by water has been a major geomorphic process in the alluvial fan. The previous phase in the development of both the alluvial fan and the V-shaped valley was characterised by the selective transport of fine materials by fluvial processes, forming Q2 in the fan and Segment C in the upstream area. Prior to this phase, erosion of lower talus slopes to form Segment B occurred in the V-shaped valley, and the deposition of debris flow material occurred
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Fig. 9. Three phases of geomorphic development in the study area.
in the alluvial fan to form Q1. To examine whether this talus erosion and the debris flow deposition can be correlated, the approximate volume of sediment deposited in the alluvial fan was estimated and compared with estimates of sediment volumes eroded from the V-shaped valley. As noted before, the thickness of the debris flow deposits on the Q1 fan surface is ca. 3 m, and it can be assumed that the deposits covered the entire alluvial fan when the debris flows occurred since Q2 resulted from erosion of Q1. The product of the thickness and the area of the alluvial fan, or the estimated volume of the debris flow deposits is 5.7 105 m2. The form of the talus slopes before the formation of Segments B and C was reconstructed on the cross-sections (Fig. 7). The area of erosion on the cross-sections was integrated using the distance between the sections and the trapezoidal formula. The calculated volume of erosion in the V-shaped valley when Segment B formed is 4.5 105 m2, which is close to the estimated volume of the debris flow deposits on the alluvial fan. This observation supports the correlation between the rapid dissection of talus slopes to form Segment B and debris flow deposition on the fan to form Q1, although the estimated erosion volume may include some errors because of the uncertainty in profile reconstruction and the limited number of the cross-sections. The preceding phase was characterised by the accumulation of talus deposits to form Segment A in the V-shaped valley as well as the deposition and cementation of finer fluvial gravel (Q0) on the alluvial fan. Although the talus deposits of Segment A often include boulders, they were not transported to the alluvial fan, pointing to a more limited power of flow than the subsequent phase with debris flow deposition. Fan deposition even terminated to allow cementation of fan deposits by carbonate deposition, indicating that most talus deposits were stored within the V-shaped valley. In summary, the phases of alluvial fan development and those of the development of the V-shaped valley can be correlated (Table 1). Consequently, a model is proposed to describe the three phases of geomorphic development in the alluvial fan and the source basin (Fig. 9).
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Table 1 Correlation among phases of geomorphic development in the alluvial fan, the source area, and the Dederiyeh Cave Phase
Alluvial fan
Source area
Dedeiryeh Cave
Age
b
Channel incision by water
No significant change
No significant change
Postglacial
a
Partial fluvial erosion to form Q2
Partial fluvial erosion to form Segment C
2
Debris flow deposition to form Q1
Rapid talus dissection to form Segment B
Rapid erosion
Late Glacial
1
Sediment cementation by carbonate
Talus deposition to form Segment A
Sediment cementation by carbonate
Last Glacial Maximum
3
Fluvial deposition
Fluvial deposition
4.2. Correlation between geomorphic processes inside and outside the cave The three phases of geomorphic development have a similarity to the erosion/ deposition history of the Dederiyeh Cave. The first phase (Fig. 9) can be correlated to the depositional phase in the cave during the Last Glacial Maximum or to the earlier period because both of them are characterised by the fluvial deposition of relatively fine material and subsequent cementation by carbonate. The following rapid erosion of the cave floor in the Late Glacial can be correlated with the dissection of talus slopes to form Segment B and debris flow deposition on the alluvial fan because all the processes can be triggered by enhanced storm activity. This correlation between sediment processes inside and outside of the cave is supported by the fact that Segment A of talus slopes below the cave is covered with sediment evacuated from the cave (Fig. 4a) because this observation shows that the erosion of the cave floor occurred after the formation of Segment A. The subsequent postglacial stable period in the cave can be correlated with the phase of reduced geomorphological processes outside the cave, associated with selective fluvial transport of the fine material along the trunk stream and small-scale incision along the current channel on the alluvial fan. If the above correlation is accepted, the ages of the three geomorphic phases in the alluvial fan and the source basin can be estimated: talus accumulation to form Segment A and the cementation of fan deposits (Q0) occurred during the Last Glacial Maximum or earlier; talus dissection and debris flow to form Segment B and Q1 occurred in the Late Glacial; and the selective fluvial transport of fine material to form Segment C and Q2 occurred in the postglacial period (Table 1). The direct validation of this age estimation is difficult because 14C datable organic material was found neither in the fan nor in the talus deposits. An indirect support is the type of artefacts found on geomorphic surfaces. The lack of Levallois/Mousterian lithic artefacts and the occurrence of flaked pottery on the alluvial fan and talus slopes indicate that all the geomorphic surfaces formed after 30– 40 ka, which is consistent with the above age
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estimation. In addition, previous studies in the northern Levant including northwest Syria pointed to an arid climate around the Last Glacial Maximum (e.g., Van Zeist and Bottema, 1982; Henry, 1989; Baruch, 1994), which may explain the reduced fluvial processes and enhanced gravel cementation during that period. 4.3. Characteristics of geomorphic responses to rapid environmental change The existence of Phase 2 (Fig. 9) points to the importance of rapid erosion and deposition caused by increased storm activity. The dissection of the lower talus slopes and subsequent debris flow seems to have occurred rapidly because the debris flow deposits on the alluvial fan consist of a single unit without unconformities. In other words, much of the debris stored in the source area as talus deposits was rapidly eroded. Although it is difficult to quantify the intensity of storms that triggered the rapid talus dissection and debris flows, there must have been an abrupt and marked increase in the storm power, as seen from the drastic change in the facies of the fan deposits. Phases 1 and 2 inferred by this study agree with a model of talus accumulation and dissection proposed in the Levant, in that Last Glacial talus accumulation switched to postglacial dissection (Gerson, 1982; Bowman et al., 1986; Bull, 1991). These previous studies, however, did not indicate that talus dissection proceeded over a short period of time. Vegetation does not appear to have played an important role in the geomorphic development of the hillslope/fan system. The analysis of animal fossils in the Dederiyeh Cave deposits indicates that there was forest outside the cave around 40 – 50 ka (Akazawa et al., 1999). Palynological surveys near the study area (Niklewski and Van Zeist, 1970; Van Zeist and Bottema, 1982) suggest that most of the forest disappeared during the Last Glacial Maximum due to increased aridity. These forest-free conditions were favourable for talus accumulation in the V-shaped valley. Therefore, the beginning of rapid talus dissection is not due to the retreat of forest but due to climatic change and the onset of greater storm intensity. The termination of rapid talus dissection and debris flow activity can be attributed to the further northward shift of the storm track around 10 ka (Henry, 1989). The inferred geomorphic responses outlined here are similar to an example from the American Southwest (Bull, 1991) in that the stripping of slope deposits led to fan deposition after the Pleistocene– Holocene transition. This study, however, points to a much faster geomorphic response to climatic change, involving rapid talus dissection, debris flows, and fan rebuilding. Abrupt increases in storm activity, steep talus slopes sensitive to erosion, and the hillslopes directly connected to the alluvial fan over very short distances together accounted for the rapid geomorphic response.
5. Concluding remarks This paper has presented an example of rapid geomorphic responses to late Quaternary climatic change in alluvial fan/source basin systems. Slow fan deposition
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and subsequent gravel cementation by carbonate under stable conditions switched to catastrophic debris flow deposition and the subsequent removal of fine material, which was followed by another stable period. These reflect changes in hillslope processes including talus deposition, dissection, and stabilisation. Although previous studies have suggested that drainage basins in arid regions tend to respond slowly to changes in external conditions, this study demonstrates that rapid geomorphological responses to extrinsic change and resultant drastic geomorphic change can occur within arid environments. It is now necessary to investigate how commonly these types of rapid geomorphic response occur across a range of arid environments. Detailed archaeological information in Syria can provide valuable ancillary information on the chronology of geomorphological events. It is hoped that further interdisciplinary archaeological and geomorphological research in Syria will yield important new insights into the nature of arid zone geomorphological processes and response times.
Acknowledgements We thank Helen Jarvie for reviewing an early draft of this paper and correcting the English. Adrian Harvey and Martin Thorp also gave us constructive comments to improve the draft. We also thank Takeru Akazawa, the leader of the archaeological mission to the Dederiyeh Cave, for giving us an opportunity to do research in Syria.
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