Response of Alluvial Rivers to Slow Active Tectonic Movement

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1985)962.0.CO;2 1985;96, no. 4;504-515Geological Society of America Bulletin

SHUNJI OUCHI

Response of alluvial rivers to slow active tectonic movement

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Response of alluvial rivers to slow active tectonic movement

SHUNJI OUCH)! c/o Institute of Geosciences, Faculty of Science and Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112, Japan

ABSTRACT

Alluvial rivers respond to valley-slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river. On the basis of experi-mental results and field examples, hypothetical models of river re-sponse to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers.

An experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift. With subsidence, aggradation in the central reach was the main response. Transverse bars developed downstream of the subsidence axis. An experimental meandering channel re-sponded to slope steepening with a sinuosity increase. Bank erosion and point-bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis. Upstream of the uplift axis and down-stream of the subsidence axis, where the slope was flattened, water flooded over bar!.

Local convexity in longitudinal profiles of the middle Rio Grande, New Mexico, is considered to be formed by a domal uplift. Local aggradation and degradation could be explained by the effect of uplift. The San Joaquin River, California, which is now highly controlled, does not show clear adjustment to the rapid subsidence due to ground-water withdrawid. It shows, however, a channel-pattern adjustment to active tectonic subsidence that has been occurring for a long time. The San Antonio and Guadalupe Rivers in Texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes.

INTRODUCTION

River morphology and channel behavior have been given much at-tention by geomoiphologists, who attempt to explain river morphology; by geologists, who study sedimentary structures of river deposits; and by civil engineers, who try to control rivers. Although many studies have been done on the controlling factors and their effects, little attention has been paid to active tectonic movement as a factor influencing river morphology and channel behavior, except in a few works, such as Tator (1958), Welch (1973), Adams (1.980), Russ (1982), and Burnett and Schumm (1983). Tectonic movement contemporaneous with the formation of modern river morphology is here referred to as "active tectonic movement." This study examines the hypothesis that alluvial rivers respond and adjust to active tectonic movement and describes the process of adjustment.

The main reiison why tectonic movement has largely been ignored as a factor influencing river morphology and channel behavior is its slowness.

Rates of Quaternary surficial deformation without faults are considered to be < 10 mm/yr (Schumm, 1963; Kaizuka, 1967; Bandy and Marincovich, 1973). Rates of active tectonic movements during shorter time spans should have a wider range than rates of Quaternary tectonic movements, which are expressed as average values during long periods. Aseismic deformation detected by geodetic surveys, however, seems to have a range similar to that of the longer-term Quaternary tectonic movement (Holdahl and Morrison, 1974; Reilinger and Oliver, 1976; Brown, 1978). Rites as much as 10 mm/yr may be a reasonable estimation for active aseismic deformation. This rate seems low as compared with the changes of alluvial rivers. When a period of some decades or a hundred years is considered, however, surficial deformation of this rate may deform valley slopes enough to affect alluvial rivers. The deformation of valley slope, the slope of the surface on which the channel is formed, will inevitably change channel gradient, which is a dependent variable determined by water and sediment discharge and by sediment size. This change of channel gradient will upset the equilibrium between channel slope and hydraulic properties of the stream.

Volkov and others (1967) indicated that scouring of the rivsr bed occurred where rivers flow through uplifted areas in the European part of the Soviet Union, but that the opposite situation occurred in subsided areas. Welch (1973) suggested that a decrease in sinuosity by bank erosion on the inside of bends in the Red River, Manitoba, Canada, is related to decreasing valley slope due to isostatic rebound. Adams (1980) showed remarkable positive correlations between tilt rates measured along valleys and changes in the sinuosity of those reaches. Burnett and Schumm (1983) observed channel changes across active uplifts in the southeastern United States, and they indicated that streams of different sizes are in different stages of adjustment to the same uplift. Nansen (1980) stated that the meandering Beatton River in British Columbia, Canada, has not yet com-pleted adjustment to tilting, which may have occurred some thousands of years ago.

Changes in numerous factors affecting alluvial rivers can certainly obscure the effect of slow movement of the Earth's surface. Recent activi-ties of man that have had large direct and indirect impacts on alluvial rivers make it much more difficult to identify and determine the effect of active tectonic movement. In this study, effects of local valley-slope de-formation were studied to reduce the complication to a certain degree. Larger-scale factors, such as climatic fluctuations, possibly can be elimi-nated from the causis of local changes in river properties.

A series of ex[>eriments was performed to obtain ideas about how streams respond to surficial deformation, and alluvial rivers flowing through areas of active tectonic movement were examined to determine whether changes of plane forms and longitudinal profiles could be detected.

Geological Society of America Bulletin, v. 96, p. 504-515, 15 figs., 1 table, April 1985.

504


RESPONSE OF RIVERS TO TECTONIC MOVEMENT 505

EXPERIMENTAL STUDY OF THE EFFECT OF ACTIVE TECTONICS ON ALLUVIAL RIVERS

Equipment and Procedure

All of the experiments were performed in a wood-framed flume set at the Engineering Research Center, Colorado State University, Fort Collins, Colorado, that is 9.1 m (30 ft) long, 2.4 m (8 ft) wide, and 0.6 m (2 ft) high. The central 2.4 m (8 ft) of the flume has a flexible bottom, which is supported by a steel beam across the flume at the center, 4.65 m from the upstream end of the flume. Anticlinal uplift or synclinal subsidence was simulated by jacking up the steel beam and adding or extracting a certain number of shims between the steel beam and the concrete blocks on which the beam rests. The shims are aluminum plates 1.27 mm (0.05 in.) thick. The rate of uplift or subsidence was empirically set rapidly or slowly enough to allow observation of the response of experimental channels. The rate is extremely rapid compared with active tectonic movement. The experiment is not a scale model, however, but it should be considered as an idea-generating method.

Water was introduced into the inlet box at the end of the flume upstream from the water pipe (braided channels) or recirculated by a small pump (meandering channels). A point gage was used for measuring eleva-tions of the sand surface along cross sections. Cross-section numbers indi-cate distance from the upstream end of the flume in metres. Time used in the experiment is expressed as hours of water flow from a certain initial time, excluding the time required for measuring cross sections.

The effects of both uplift and subsidence on braided and meandering channels were examined. Different channel patterns were formed by changing initial slope and discharge, and by introducing suspended load.

Braided Channel Experiments

The initial channel, 8.9 cm (3.5 in.) wide and 3.8 cm (1.5 in.) deep, was molded on a 2% slope, which was formed of a mixture of moderately sorted medium sand and a small amount of kaolinite (9:1). Additional sand was fed into the head of the channel by a vibrating sand feeder.

After 20 hr of running with clear water (Q = 100 ml/sec), a braided pattern had developed. This was the pattern used as the initial braided channel of the experiment, and the measurement was started from this point (0 hr).

Uplift. Uplift was started at 6 hr, 1 shim at a time, and it was continued every 2 hr until 48 hr except at 14,20, 30,42, and 46 hr. At 48 hr, 4 shims were added, to make a total uplift of 2.54 cm (1 in. or 20 shims). The measurement of cross sections was made every 2 hr, except from 19 to 22 hr (1-hr interval) and after 46 hr.

Bench marks were set on the sand surface, and their elevations were measured before and after each small uplift. Movement of bench marks at 4.65 m indicated that the surface of sand was uplifted almost the same amount as the added shims. No lateral tilting occurred.

In spite of the uplift, no significant local convexity in the area of uplift appeared in longitudinal profiles of mean bed elevation. The braided stream responded to uplift (1.27 mm/2 hr over distance of ~1.2 m) with degradation rapid enough to offset the uplift. In the uplifted area, channel depth increased with fluctuations (Fig. 1) caused by changes in pattern (Fig. 2) and by small-scale complex response. Degradation started near 5.0 m after the first uplift, where the slope was steepened the most, and it migrated upstream. Sediment produced by the degradation caused slight aggradation downstream. As degradation continued, terraces were formed in the central to downstream reach of the uplifted area after ~18 hr. Terraces were distinguished from bars by their fixed position and increas-

2 0 AO Cross Sect ion No.

Figure 1. Mean depth changes of the experimental braided chan-nel during uplift. Datum lines indicate the depth at 0.0 hr. Cross-section measurements were made every 25 cm from 3.75 m to 6.0 m, and every 50 cm in the rest of the flume from 1.0 m to 7.5 m.

ing height with uplift. The thalweg was fixed, and downcutting was accel-erated in this reach. At the same time, a multiple thalweg channel with submerged bars, which indicates an aggradational trend, formed in the upstream reach of the uplifted area. The terraces were gradually eroded by thalweg shift, whereas the height of the terrace surface increased with uplift, and they were destroyed by 32 hr. As the terraces were eroded, degradation migrated into the upstream reach (Fig. 1). A strongly braided pattern, also a result of aggradation, developed downstream from the terraces due to excess sediment supply from the uplifted area. When the terraces disappeared, the braided pattern in the downstream reach devel-oped a fixed thalweg with alternate bars as a result of degradation caused by the decline of sediment supply (Fig. 2). After 48 hr, when 4 shims were added, a similar process beginning with terrace formation was resumed. The difference in deformation rate did not seem to affect the trend of adjustment.

The degradation occurring in the uplifted area did not exactly corre-spond to the uplift. There were fluctuations and pauses in the degradation while the uplift continued. These fluctuations and pauses seemed closely related to pattern changes.

Subsidence. The channel existing at 68 hr in the uplift experiment was used as the initial stage (0 hr) for the subsidence experiment. Subsi-


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hours

Thalweg

O Bar O

Terrace

Figure 2. Pattern change of the experimental braided channel during uplift.

dence was started at 2 hr by extracting 5 shims (6.35 mm) at a time, and 4 episodes (5 shims each) of subsidence were performed every hour for 5 hr (total 20 shims or 1 in.). Cross sections were usually measured every 30 min.

The braided stream responded to subsidence by deposition, but it was not enough to offs5t the subsidence. The rate of subsidence (6.35 mm/hr) apparently was too high for the stream to adjust completely. This resulted in a local concavi ty in channel profiles (Fig. 3). Deposition as the main response to subsidence occurred quickly in the upper central part of the subsided area, and. it migrated upstream. The aggradation, however, did not reach upstream to 3.75 m.

Transverse bars migrated slowly into the downstream reach of the subsidence, where the flow, slowed by slope flattening, flooded the entire channel width. In this "flooded" reach, all bed features were submerged under water, and the reach looked like a pool with no distinguishable thalweg (Fig. 4). The transverse bars did not reach the downstream end of the subsidence, and the deficiency of deposition resulted in gradient con-cavity by subsidence alone. In contrast, slope convexity at the upstream end of the subsidence was removed by degradation (Fig. 3). Degradation started in the uppeimost reach of the subsided area, where a slope discon-tinuity was formed with subsidence, and it migrated upstream into the reach, where no subsidence occurred. This degradation, which occurred mainly as bar destruction, provided sediment to the subsided area. As a result of this increase in sediment discharge and slope steepening by subsi-dence, a strongly braided pattern developed in the upstream part of the subsidence (Fig. 4). No significant movement of sediment through the flooded reach was observed. Thalweg degradation with alternate bar de-velopment occurred downstream from the subsided area.

Meandering Channel Experiment

A trapezoidal straight channel with a 4-cm bottom width was molded on a slope of 0.8%. The sand was composed of moderately sorted me-dium sand and a small amount of kaolinite (~ 10% to 20%). A meandering

thalweg pattern developed - 1 5 0 hr after water (100 ml/sec) was intro-duced from the inlei: with 30 deflection. This pattern was used as the initial stage (0 hr in Fig. 5). Clay was mixed with the water (-1,000- 2,000 ppm) that was circulated by a pump. Some clay deposited and formed a thin layer on the wet perimeter of the channel. This clay cover stabilized the pattern. No sand was supplied from the sand feeder.

Uplift. Uplift was started with 2 shims added at 8 hr and at 12 hr, and then 1 shim was added every 4 hr for a total of 8 shims (1.02 cm). The measurement of thalweg elevation and position and of some cross sections was made every 4 hr until 40 hr. After 60 hr, water was kept running until 73 hr.

The main response of the meandering channel to uplift was the increase in thalweg sinuosity in the downstream part of the uplift, where slope was steepened. The first notable change after the uplift started was removal of the clay cover in the downstream parts of meandering bends in the downstream side of the uplift. As the slope was steepened, flow eroded the outer bank on the lower half of a bend. Sediment produced here was deposited on the edge of the facing or the next point bar. The growth of point bars induced further bank erosion, and this process resulted in an increase in thalweg sinuosity (Fig. 5).

Thalweg elevations in the uplifted area increased with uplift as the change of valley slope was compensated for by increased sinuosity, and the convexity due to the uplift can be observed in projected thalweg profiles (Fig. 6). In other words, no significant degradation nor aggradation oc-curred. The projected thalweg profile is a plot of thalweg elevation versus distance from the upstream end of the flume.

The channel did not fully adjust its thalweg slope by sinuosity in-crease even at the end of the experiment. Thalweg slope increased with projected thalweg slope (valley slope) steepening, although the rate of increase was lowered, by the sinuosity increase. Relatively stable banks probably prevented or at least delayed the complete adjustment of slope. In the upstream part of the uplift, where slope was flattened, flow velocity was reduced, and water flooded over the point bar. Clay was deposited, and the thalweg became indistinct.



cm

4 0 6 0 Cross Section No.

8 0

10

11 hours

I i hh \. I :. j

m

Figure 3. Mean bed profiles of the experimental braided channel during subsidence. Cross-section measurements were made every 25 cm from 3.75 m to 6.0 m, and every 50 cm in the rest of the flume.

Subsidence. The lower half of the meandering pattern was destroyed by the end of the uplift experiment mainly due to the introduction of clear water from 56 to 73 hr. After suspended load was reintroduced and banks were reshaped, a meandering pattern developed again. This pattern was used as the initial stage of the subsidence experiment (0 hr in Fig. 7). Subsidence was made by extracting 1 shim (1.27 mm) at a time every 4 hr from 8 to 36 hr (total 8 shims or 1.02 cm).

The main response of the meandering channel to subsidence was the increase in sinuosity in the upstream part of the subsided area. The sinuos-ity increase was similar to that which occurred in the downstream part of the uplift. After the first subsidence, the flow attacked the outer bank of the lower half of the bend in the upstream part of the subsidence, where slope was steepened. The clay cover was washed away, and the bank and the edge of a point bar were slightly eroded, and a small amount of sediment was deposited on the downstream part of the point bar and the upstream part of the next point bar. As the subsidence continued, this small deposi-tion developed into a new narrow bar attached to the old bar, and the outer bank was eroded (Fig. 7). This resulted in a slight sinuosity increase during the subsidence. The back-water effect of the central part of the subsided area may have reduced the rate and amount of sinuosity increase.

In the downstream part of the subsided area where slope was flat-tened, the flow was slowed, and it flooded over the point bar (Fig. 7). The point bar located approximately from 5.0 m to 5.75 m was almost com-pletely submerged by 36 hr. Clay was deposited over the channel width, and the thalweg became indistinct.

O B a r

Figure 4. Pattern change of the experimental braided channel during subsidence.

Thalweg elevations in the subsided area lowered with subsidence, because no significant degradation nor aggradation occurred. The convex-ity in projected thalweg profiles, which remained from the uplift experi-ment, disappeared with subsidence (Fig. 8). A slight concavity appeared in the subsided area at ~24 hr, and it persisted during the remainder of the experiment (Fig. 8).

Summary of Experiment

Response of the experimental channels to uplift or subsidence is summarized in Table 1.

The main feature of braided-stream response to uplift was incision and terrace formation in the uplifted area. Accompanied by terrace forma-tion and destruction, which occurred in the central to downstream part of the uplift, there was deposition in the upstream part of the uplift zone, and aggradation occurred in the reach downstream from the uplift. This ap-peared as the multiple thalweg channel with submerged bars (upstream) and the strongly braided stream (downstream). When the terraces were destroyed, these features disappeared, too. The aggradational condition in the downstream reach became degradational because of the decline in sediment transport from the uplifted area. After the uplift ended and the disturbance due to the uplift decreased, the channel pattern returned to one similar to the original pattern.

The main feature of the response of braided channel to subsidence was aggradation in the upper central reach of the subsided area. The degradation that occurred at the upstream end of the subsided area sup-plied some sediment to the central reach. This increase in sediment dis-charge and the steepened slope were manifested as a strongly braided pattern in the upstream part of the subsided area. Aggradation extended downstream into the downstream part of the subsided area (where slope


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m 0 5

0 . A*

1 - 0 i 1 - 5

2 0

2 5 M 3 - 0

3 - 5

0 v 1 l

4 - 5

5 0 f, % 5 5 i 6 0

6 5 J*' ' JA

7 - 0 \ v L

7 5 /


68 hrs.

8 m 50 cm

Figure 7. Pattern change of the experimental meandering channel during subsidence.

occurs as increased sinuosity, vertical adjustment may not occur. In the reaches where slope was flattened, water flooded over point bars, and the thalweg became indistinct. Clay deposition occurred in these reaches, but it did not affect bed elevation.

How much a channel can adjust its slope by a sinuosity change is not determined solely by the rate of slope steepening. Many other factors, such as bank stability and bed erodibility, may play important roles. In the experiment, relatively stable banks probably prevented the complete ad-justment of channel slope.

ACTIVE TECTONICS AND CHANGES OF ALLUVIAL RIVERS

Middle Rio Grande, New Mexico

In the Rio Grande Valley, New Mexico, a rapid elliptical domal uplift has been detected from leveling data between Belen and Socorro (Reilinger and Oliver, 1976; Reilinger and others, 1980). The maximum uplift observed near the center is - 2 0 cm relative to the periphery, with the average rate ~5 mm/yr (Reilinger and others, 1980).

4 0 6 0 8 0 Cross Section No. m

Figure 8. Projected thalweg profiles of the experimental mean-dering channel during subsidence. Thalweg elevation was measured at every point required to describe a detailed thalweg profile.

The Rio Grande flows across the uplift approximately along its major axis. Late Pliocene alluvium of the ancestral Rio Grande is displaced 85 m vertically in the uplifted area (Bachman and Mehnert, 1978). Terrace remnants distributed along both sides of the Rio Grande in the area of uplift also show displacement by the uplift (Ouchi, 1983). Deformation of Tertiary deposits and Quaternary terraces indicates that the modern uplift is a part of long-term tectonic movement.

Thalweg profiles of the middle Rio Grande show a large convexity in the uplifted area, between Belen and Socorro (Fig. 9). Happ (1948) pointed out that this "hump" has existed at least since the 1918 mapping. He suggested that it has been caused by excessive local aggradation by two powerful tributaries, Rio Puerco and Rio Salado, which flow into the Rio Grande in this area. The fact that the center and the expanse of the convexity almost perfectly coincide with the uplift suggests, however, that it can be related to the uplift. A small bulge on the larger convexity of the 1936-1938 profile, which apparently was formed by the major flood of 1929 from Rio Puerco and Rio Salado, was removed by 1944 (Fig. 9). This indicates that the Rio Grande seems to be able to remove the sedi-ment contributed by these ephemeral tributaries before it makes a large convexity in the profile. Rio Puerco and Rio Salado are considered to have delivered less sediment prior to the 19th century (Bryan, 1928). The excess local sedimentation thus is not necessarily the main cause of the convexity in the Rio Grande thalweg profile.

Although the experimental results indicate that a braided river can compensate for slow uplift by degradation, the Rio Grande, which is braided in this reach, obviously could not maintain its profile. The sedi-ment dumped by Rio Puerco and Rio Salado into the Rio Grande at the reach where degradation is supposed to start may have prevented full slope


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adjustment, and hence the local convexity due to the uplift may have remained in the pr ofile.

Aggradation, which has been a problem in the middle Rio Grande, seems to have a close relationship with the uplift. Progressive aggradation has been the prevailing feature at least since 1918 in upstream and down-stream reaches of the uplift. In the central part of the uplift, no progressive aggradation is observed.

Alternate bar; with a braiding tendency upstream of the uplift and a strongly braided pattern in the downstream part of the uplift can be observed on the aerial photographs taken in 1948 by the U.S. Bureau of Reclamation.

San Joaquin River, California

Extremely rapid land subsidence due to ground-water withdrawal has occurred in some areas in the San Joaquin Valley, California, since the mid-1920s, especially since World War II (Poland and others, 1975). The San Joaquin River flows through a corner of one of the subsided areas. Topographic maps, (1:24,000) made in 1920-1921 (before the rapid sub-sidence due to ground-water withdrawal started) and in 1956-1962 do not show any significant effects of the subsidence on the river pattern. The river is highly controlled, and it probably could not adjust to the change in valley slope.

The San Joaquin Valley has been a region of nearly continuous deposition during :he late Tertiary and the Quaternary (Miller and others, 1971), and it has been a subsiding area for a long time. The lacustrine Corcoran clay layer, which is the principal confining bed in the San Joaquin Valley, shows postdepositional structural warping (Frink and Kues, 1954; Bull a nd Miller, 1975). Although the trend of warping of the Corcoran clay agrees well with the modern subsidence, ~ 152 m of down-warping cannot be explained by the ground-water withdrawal. If tectonic subsidence is still occurring, it is now overshadowed by subsidence due to ground-water withdrawal.

Although the San Joaquin River does not show clear adjustment to the rapid subsidence, due to ground-water withdrawal, the pattern change from upstream to downstream (Fig. 10) indicates a possible relationship between the river pattern and the long-term neotectonic movement identi-fied by the deformation of the Corcoran clay layer. The river is artificially fixed in some places, but it can be assumed that the river pattern shows

the natural pattern formed before the artificial control. The neotec-tonic subsidence had occurred for a long time prior to the artificial co ntrol in the same area as the subsidence due to ground-water withdrawal. The San Joaquin River flows in a very sinuous course toward the axis of the valley through the area where slope has been steepened by the neotectonic (in this case, active tectonic) subsidence. Slowly increasing slope in section C of Figure 10 could have caused the river to increase its sinuosity. From Mendota pool to Firebaugh (section D), the river has a less sinuous course, perhaps owing to slojje flattening by the active tectonic subsidence. Down-stream from Firebaugh, in sections E and F, where the flood plain widens, sloughs develop along the main channel. In section G, the river becomes straighter with smaller bends, and distributary sloughs increase in number and size. This gives the reach an anastomosing pattern. In section H, the main channel and other sloughs become very sinuous, and the pattern looks reticulate. There are numerous swamps and oxbow lakes on the flood plain, which is very wide.

The Merced River fan growing from the east side of the valley may have some effect on the river pattern in sections G and H; however, as in the case of the Tulare Lake interior drainage in the southern San Joaquin Valley, the large area of anastomosing or reticulate channel patterns in sections G and H is more likely to be a feature formed near the clown-stream end of the active tectonic subsidence. The Tulare Lake interior drainage, which was; explained by a damming effect of the alluvial fans of the Kings River and Los Gatos Creek developing from both sides of the valley, is now believed to be the result of the continued tectonic sub-sidence (Davis and Green, 1962).

Texas Coast

Long-term subsidence of the Texas coastal plain is well-known geo-logically as the "Gulf Coast Geosyncline" (Barton and others, 1933; Wa-ters and others, 1955; Bornhauser, 1958). Within this large tectonic structure, many smaller structures such as local folds, flexures, and faults

New Mexco

Figure 9. Local convexity appeared in longiltudinal profiles of the Rio Grande from Belen to Socorro, New Mexico (from Ouchi, 1983). Profiles from 1936 to 1972 are based on the data compiled by the U.S. Bu-reau of Reclamation (1967, 1972). The 1917-1918 profile was drawn from topographic maps surveyed b3' the State Engi-neer's Office in 1917-1918.

10 mi



Figure 10. Subsidence zone (due to ground-water withdrawal) and course of the San Joaquin River. Equal subsidence lines are from Po-land and others (1975). This subsidence zone generally agrees with the tectonic sub-sidence zone.

WftV Outline of va l ley

Contour l ine ( f t ) Line of equal subsidnce 1926-72 ( f t )

have developed (Bornhauser, 1958; Weaver, 1955; Waters and others, 1955; Colle and others, 1952; Shelton, 1968). One of the best-known examples is the Post-Vicksburg (Colle and others, 1952) or Vicksburg-Frio (Waters and others, 1955) flexure along the southern Texas coast (Fig. 11). Contemporaneous faults, which form while sediment is being deposited, are an important feature associated with the flexure (Born-hauser, 1958; Hardin and Hardin, 1961). A series of faults associated with the Post-Vicksburg flexure is the Sam Fordyce-Vanderbilt fault zone

(Honea, 1956). These faults, however, are not consistently contempora-neous with deposition. A fault may be contemporaneous in a certain horizon and postdepositional in a different horizon. Such a fault is formed by later movement along the same zone of weakness (Hardin and Hardin, 1961). Even long after the active deposition, the larger-scale tectonic movement of the Gulf Coast Geosyncline may reactivate the flexure zone. Weaver (1955) mentioned that a modern fault formed on the Post-Vicksburg flexure zone. Many active faults reported in the areas of rapid

Figure 11. Index map showing location of Guadalupe and San Antonio Rivers and Post-Vicksburg flexure in the Texas coast region.


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Post-Vicksburg Flexure

Figure 12. Longitudinal profiles (projected) of Guadalupe and San Antonio Rivers across Post-Vicksburg flexure zone (drawn from 1:24,000 topographic maps), p = sinuosity; Sp = projected channel slope (x I0"4); sc = channel slope (x 10"4).

subsidence due to ground-water withdrawal along the Texas coast are essentially the result of reactivation of old faults cutting through unconsol-idated sediments (Kreitler, 1976).

The Post-Vicksburg flexure or the Sam Fordyce-Vanderbilt fault zone may still be active and may have some effects on alluvial rivers flowing across it. Recent movement of the land surface in the Gulf Coast region detected from the National Geodetic Survey releveling data by Holdahl and Morrison (1974) generally supports this assumption.

The meandering Guadalupe and San Antonio Rivers flow across the Post-Vicksburg flexure zone (Fig. 11). Figure 12 shows longitudinal pro-files of both riven projected to straight lines along valleys. The profiles are divided into sections according to slope and pattern characteristics. The profiles indicate that the flexure has had some influence on the rivers. Both profiles have relatively steep (valley slope) sections (section B) 32-64 km (20-40 mi) upstream from the mouth, and the rivers have a highly sinu-ous course in this section. These steep slopes occur where the flexure is expected to deform the land surface. Morton and Donaldson (1978) rec-ognized the gradient change between this reach and the downstream reach, but they attributed the change to the difference between alluvial plain and delta plain. This interpretation, however, cannot explain the gradient in-crease from section C to section B of both rivers. The steep gradient of section B is very likely a direct result of the flexure movement. The rivers increase their sinuosity remarkably in this section (Fig. 12). This is what is expected in response to a local steepening of the valley slope. The channel-slope increase of steepened reaches is offset by the sinuosity increase to a certain degree. The Guadalupe River shows a reasonable change of chan-nel slope, when che general decrease in channel slope in a downstream direction is considered. In sections C and B of the San Antonio River, channel slope does not seem to have adjusted as well. Slope steepening in section B may be too large to be offset completely by sinuosity increase, and the sinuosity in this section (3.08) seems close to a maximum value. If

a meander cutoff occurs in this section, there possibly will be change in the entire channel pattern. Along the Guadalupe River, meander cutoffs and some degradation seem to have already occurred in the section of steep-ened slope. More sinuous abandoned channels are observed on the flood plain, which is >6 m (20 ft) above the channel bed. The response of the San Antonio River, which has more clay and less sand (Morton and Donaldson, 1978), may be slower than that of the Guadalupe River, which has more sand in both bed and banks. With the finer material, the San Antonio River may also be more able than the Guadalupe River to increase sinuosity.

CHANNEL PATTERN AND RESPONSE OF ALLUVIAL RIVERS TO ACTIVE TECTONIC DEFORMATION

Alluvial rivers respond to active tectonic movement in ways that are dependent on the types of deformation and rivers. The main observable effect of surficial deformation seems to appear first in the channel pattern, and degradation and aggradation as river adjustment to valley slope de-formation also affect the channel pattern.

Schumm (1981) developed a classification of alluvial-channel pat-terns, applying the concept of channel-pattern change with valley slope to the classification of alluvial-channel pattern with sediment load types. Although it is still hypothetical, this classification is pertinent to this study, because the valley slope is treated as an independent variable, and because it also appeared to be compatible with the observations reported here. Using this classification, hypothetical models of alluvial-river response to active tectonic movement (anticlinal uplift and synclinal subsidence) were developed for different types of alluvial rivers (Figs. 13,14,15). The term "reticulate" is used here for a widespread multiple channel network that has an angular cross-channel development. The reticulate pattern is sup-posed to form on flat and wide valley floors, and it is not necessarily a suspended-load channel, whereas the anastomosing pattern is the multiple-channel network representing the steepest suspended-load channel.

For braided rivers, which are the most obvious bedload channels, a valley-slope increase may change the channel pattern from meandering-thalweg-braided to bar-braided. Also, a valley-slope decrease may cause a change from a bar-braided or meandering-thalweg-braided pattern to an alternate-bar pattern.

In the case of anticlinal or domal uplift across a braided riv er, the bar-braided pattern will always be observed in the reach downstream from the uplift, where the slope is steepened and sediment discharge increases (Fig. 13a). Terraces are formed in the central part of the uplift where degradation occurs. The braiding tendency in the reach upstream from the uplift may be less than in the downstream reach, because the slope is flattened, and sediment discharge does not increase. Alternate bars with a braiding tendency in the upstream reach, terraces and a degradational trend in the central area, and a bar-braided pattern in the downstream reach from the uplift will be the dominant features.

The main feature of braided-river response to synclinal or basinal subsidence across the river is aggradation in the central to downstream area of the subsidence. A straight channel with transverse bars may de-velop in the downstream reach of the subsided area (Fig. 13b). In some cases, frequent overbank floods and channel avulsions may form multiple channels. At the upstream end of the subsidence, where a convex slope irregularity tends to be formed, degradation will occur. This degradation, which removes the convex slope irregularity, increases sediment load



Braided ( b e d - l o a d ) river Slope deformation River a d j u s t m e n t

Figure 13. Adjustment of a braided river to (a) anticlinal up-lift and (b) synclinal subsidence across it.

Profile

Pattern

b a r - b r a ided or

meanderi ng - talweg braided

b a r - braided

b. Subsidence Profile

Pattern

al ternate bars

downstream. On the upstream side of the subsidence, therefore, a bar-braided pattern will be dominant (Fig. 13b).

In the case of anticlinal or domal uplift across a meandering river, which is the most common mixed- or suspended-load channel river, sinuosity increase will be observed on the downstream side of the uplift as the valley floor is steepened (Figs. 14a and 15a). On the upstream side of the uplift, channel straightening can be expected, but the damming effects of the uplift may be more apparent. As a result, there will be inundation of flood-plain and channel avulsions, and a swampy condition with deposition of fine material will occur. The reticulate (or in some cases, anastomosing) channel pattern will probably develop (Figs. 14a and 15a). After the pattern threshold is exceeded by meander cutofls on the steepened slope, full-scale degradation will start and migrate upstream. The sinuous- or island-braided pattern will develop on the downstream

side of the uplift for a mixed-load river (Fig. 14a), and the anastomosing pattern will develop for a suspended-load river (Fig. 15a). The convexity formed by the uplift will be reduced as the degradation proceeds, and the swampy reach will be drained. For suspended-load rivers, which are more stable and can accommodate higher sinuosity than can mixed-load rivers, the whole process will proceed more slowly.

Sinuosity increase also occurs on the upstream side of subsidence across a meandering river (Figs. 14b and 15b). In the downstream part of the subsidence, a condition similar to that occurring on the upstream side of uplift is expected to occur; however, because slope adjustment by ag-gradation is a slow process even after the pattern threshold is exceeded on the oversteepened upstream side of the subsidence, the swampy condition in the lower downstream part of the subsidence has a better chance to develop and remain than on the upstream side of uplift. The multiple-


Figure 14. Adjust-ment of a mixed-load meandering river 1o (a) anticlinal uplift and (b) synclinal subsidence across it. Time sequence is expressed in the order from top to bottom.

Suspended-load meander ing river a. Upli f t b. Subsidence f

Slope deformation ana adjustment Channel pattern Slope deformation and adjustment Channel pat tern

S x j O S I S reticulate: w

reticulate

cutoff - cutoff

anastomosing * anastomosing

Figure 15. Adjust-ment of a suspended-load meandering channel to (a) anticlinal uplift and (b) synclinal subsidence across it. Time sequence is expressed in the order from top to bottom.



channel reticulate pattern most likely forms near the downstream end of subsidence for suspended-load meandering rivers. As on the downstream side of uplift, the sinuous-braided pattern for a mixed-load river (Fig. 14b) and the anastomosing pattern for a suspended-load river (Fig. 15b) will develop with meander-bend cutoffs on the upstream side of subsidence. As the slope restoration by aggradation in the subsided area will take a longer time than that by degradation in the case of uplift, widespread reticulate channels or lakes are likely to be a common feature in the area of subsidence.

Changes in channel pattern caused by surficial deformation will ap-pear as changes in sedimentary fades. Detailed studies will make it possi-ble to detect facies changes of sedimentary layers from known tectonic movements and to detect slow contemporaneous paleotectonic movement from changes in sedimentary facies, as Slack (1981) attempted. In the case of braided-river deposits, a strongly braided pattern will form horizontally bedded longitudinal-bar facies downstream of anticlinal uplift and up-stream of synclinal subsidence. Downstream of subsidence, transverse-bar facies, the dominant feature of which is planar cross-stratification, will form. Meandering river deposits may show a cyclic vertical facies change in response to slow contemporaneous tectonic movement. Downstream of uplift and upstream of subsidence, point-bar growth will intensify deposi-tion of point-bar sand. The point-bar facies will be interrupted by sinuous (or island)-braided or anastomosing channel facies, when the channel pattern change occurs on the oversteepened slope. The point-bar deposi-tion will resume after the slope is restored. Downstream of subsidence or upstream of uplift, a widespread reticulate channel pattern will enhance flood-plain deposition.

ACKNOWLEDGMENTS

I wish to thank S. A. Schumm for his guidance during the study and for reading the manuscript.

This study was supported by a grant from the National Science Foundation (Project No. EAR-7727573). R E F E R E N C E S C I T E D

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Response of Alluvial Rivers to Slow Active Tectonic Movement