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Bulletin of the Geological Society of America Vol BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 65, PP. 175-190, 7 FIGS.. 2 PLS. FEBRUARY 1964 GEOLOGY OF ALLUVIAL FANS IN SEMIARID REGIONS BY ERICH BLISSENBACH ABSTRACT An alluvial fan is a body of detrital sediments built up at a mountain base by a mountain stream. Bold relief is essential, moderately arid to semiarid climate favorable for the development of fans. The depositing agents are sheet floods, stream floods, and streams. Compound alluvial fans result from lateral coalescence of single fans. Development of alluvial fans is affected by changes in the course of a cycle, varying base level, climatic changes, tectonic movements, and slumping of fan deposits. Telescoped or superimposed structure may be developed. Fan deposits are arkosic or graywacke. Sorting and roundness of particles range widely. The matrix is primary or secondary. In general alluvial-fan deposits are stratified. Channel cut-and-fill is pronounced. Individual strata in fans are up to 20 feet thick. Particles in stream deposits are imbricated. Talus-slope deposits at the apex of a fan and floodplain deposits at its base can be separated from those of an alluvial fan by particle sizes, angularity and orientation of fragments, sorting, and original dip of strata. Mudflow deposits in an alluvial fan indicate certain climatic conditions during its formation. Many ancient fan deposits may have escaped recognition because of the common misconception that fan deposits are necessarily unstratified, composed of angular fragments, poorly sorted, and without distinctive sedimentary structures. CONTENTS TEXT Page Ancient alluvial-fan deposits 187 Page Occurrence 187 Introduction 176 Relative abundance 187 Purpose of investigation 176 Criteria for recognition of ancient alluvial- Areas of field study 176 fan deposits 188 Acknowledgments 176 References cited 189 Geology of alluvial fans 176 Definitions 176 General statement 176 ILLUSTRATIONS Process of forming alluvial fans 177 Figure Page Late-stage development 179 1.—Compound alluvial fan formed by lateral Changes in normal course of a cycle 179 coalescence of single alluvial fans 177 Effects of varying base level 179 2.—Alluvial fan with telescope structure 180 Effects of climatic changes 180 3.—Superimposed alluvial fan 181 Effects of tectonic movements 180 4.—Pseudo-telescope structure (modified after Effects of slumping of unconsolidated fan Gilbert) 181 material 181 5.—Distribution of maximum particle sizes and Secondary alluvial fans 181 surface angles along a radial profile on an Geology of alluvial-fan deposits 181 alluvial fan of the Santa Catalina Moun- Fades 181 tains, Arizona 182 Particle sizes and particle-size distribution.. 182 6.—Distribution of roundness and sphericity Composition 182 of alluvial fan particles along the same Sorting 183 radial profile as Figure 5. Base of Santa Roundness 183 Catalina Mountains, Arizona 184 Sphericity 184 7.—Characteristic sedimentary structures in an Matrix 184 alluvial fan of the Santa Catalina Moun- Cement 185 tains, Arizona. Main direction of trans- Color 185 port from right to left 186 Porosity 185 Permeability 185 Plate Facing page Sedimentary structures 185 1.—Alluvial fans 186 Organic contents 187 2.—Alluvial-fan deposits 187 175 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/65/2/175/3441421/i0016-7606-65-2-175.pdf by guest on 26 September 2021 176 ERICH BLISSENBACH—ALLUVIAL FANS INTRODUCTION differs from an alluvial fan in that it is built up predominantly by gravitational sliding. Purpose of Investigation The apex of an alluvial fan develops at the This paper gives the results of a compre- point where the stream emerges from the moun- hensive study of alluvial fans. Though detailed tain. It is the point of highest elevation on the field work was carried out only in the south- alluvial fan. Fanhead applies to the area on the western United States, most of the data may alluvial fan close to the apex; midfan desig- be applied to alluvial fans in other areas where nates the area between the fanhead and the similar conditions prevail. Thus, through de- outer, lower margins of the fan. Base of an tailed study of recent alluvial-fan deposits, alluvial fan is the term applied to the outermost geologists may be aided in determining whether or lowest zone of the fan. ancient rocks were formed under similar con- If the fanhead area reaches far into a moun- tain canyon the term fan-bay (Davis, 1938, ditions. p. 1374) is applied to it. Fan-mesa (Eckis, 1928, p. 243-244) is the term applied to an Areas of Field Study alluvial fan remnant left standing in the process Alluvial fans studied in southern Arizona of degradation of a fan. include those at the southern base of the Santa Catalina Mountains, north and northeast of General Statement Tucson, Arizona, and fans of the Tucson Moun- tains, west and northwest of Tucson, Arizona. The occurrence of alluvial fans has been reported from all continents. However, due to Investigations in the Mammoth area, Ari- zona, were carried out on alluvial fans on both conspicuous development and easy accessi- bility, alluvial fans in California have been sides of an intermontane valley of the Black studied in more detail than any others (Law- Hills, Arizona, west of Mammoth. In northern Arizona alluvial fans were studied son, 1913, p. 332; Eckis, 1928, p. 232-246; Krumbein, 1937, p. 586-594; Buwalda, 1951, along the Aubrey Cliffs, northwest of Selig- man, Arizona, and along the Bright Angel p. 1491). Trail, north of the Grand Canyon Village, Investigators generally agree that an alluvial Arizona. fan resembles geometrically the segment of a Studies were also carried out on alluvial cone. From the apex of the fan the surface dips fans in southern California and southern New toward the base in which direction the angles Mexico. of dip gradually become flatter. Thus, a radial Certain alluvial fans were studied in every profile through the fan is concave upward; a detail, whereas only certain properties were profile at right angles convex. The steepest examined in other fans. angle of dip on the alluvial fan is encountered close to its apex. The angle of dip of the fan surface rarely A cknowledgments exceeds 10° (Dana, 1894, p. 194-195; Eckis, The aid and counsel of Professor Edwin D. 1928, p. 223; Scott, 1932, p. 269; Eardley, McK.ee of the University of Arizona, under 1938, p. 1408). Some authors report no angle whose guidance this work was undertaken, is greater than 5° or 6° (Lawson, 1915, p. 25; sincerely appreciated. Vaughan, 1922, p. 341). The maximum angles of slope of alluvial fans studied by the writer GEOLOGY OF ALLUVIAL FANS range from 5° observed on fans of the Aubrey Cliffs, Arizona, to 9° on fans of the Black Hills Definitions of Arizona. An alluvial fan is a body of detrital sediments Surface angles greater than 5° are charac- built up by a mountain stream at the base of a teristic of the upper half or more of small mountain front. It develops because all streams alluvial fans with a radial extent of a few hun- tend to form a graded course. A talus slope dred feet like those of the Black Hills of Arizona. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/65/2/175/3441421/i0016-7606-65-2-175.pdf by guest on 26 September 2021 GEOLOGY OF ALLUVIAL FANS 177 Large alluvial fans such as those of the Santa under such conditions will there be profound Catalina Mountains, Arizona, with a radial erosion and transportation together with a extent of about 4 miles, exhibit surface angles strong tendency for deposition as the mountain greater than 5° only within the upper one streams reach areas of low gradient. twentieth or less of their extent. Practically all reductions of rock slope and In geologic literature small alluvial fans are contributions to alluvial embankment are commonly called alluvial cones. As no definition specifies the difference between an alluvial fan and a cone, this classification is not followed. Instead the terms steep, gentle, and flat angles of dip are proposed to denote the degree of slope of alluvial fans or parts of them. Because angles of dip greater than 5° are rare these are called steep. Angles of dip be- tween 2° and 5° are termed gentle, and sloping angles below 2° flat. On the basis of these definitions the slope of the surface of a par- ticular alluvial fan can be described as follows: Alluvial fans of the Santa Catalina Mountains with a radial extent of about 4 miles have steep FIGURE 1.—COMPOUND ALLUVIAL FAN FORMED slopes in the uppermost 0.2 mile of the radial BY LATERAL COALESCENCE OF SINGLE extent of the fans. In the adjoining 0.7 mile ALLUVIAL FANS surface angles are gentle, while the remaining part, about 3.1 miles, has flat angles of dip. affected by brief and infrequent periods of The radius of an alluvial fan may be as great downpour (Lawson, 1915, p. 28). This type of as 40 miles under exceptional conditions (Gra- precipitation is characteristic of areas with arid bau, 1913, p. 584). The radii of alluvial fans and semiarid climate. Under humid conditions studied by the writer range from 4 miles at also alluvial fans are developed as seen along the Catalina Mountains, Arizona, to about the Alps and the Himalayas. Fans formed in 500 feet in the Black Hills, Arizona. humid environment commonly are flatter than By lateral coalescence of single alluvial fans, those of arid environment owing to the abun- a compound alluvial fan may result (Miller, dance of running water which favors the de- 1926, p. 164-166) (Fig. 1; PI. 1, fig. 1). The velopment of gentler gradients.
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