drainage analysis in geologic interpretation a summation_howard 1967.pdf

T H E A M E R I C A N ASSOCIATION OF P E T R O L E U M G E O L O G I S T S B U L L E T I N V. 51 . N O . 11 ( N O V E M B E R , 19671. P. "246-2259. 4 F IGS, , ' TABLE

D R A I N A G E ANALYSIS IN GEOLOGIC I N T E R P R E T A T I O N A SUMMATION'

ARTHUR DAVID HOWARD^ Stanford, California 94,̂ 0^

.\BSTR.ArT Drainage analysis is useful in structural interpretation, particularly in areas of low relief.

Analysis includes consideration of drainage patterns, drainage texture, individual stream patterns, and drainage anomalies.

Drainage patterns generally are subdivided into basic and modified basic. To these might be added pattern varieties. A basic pattern is one whose gross characteristics readily distinguish it from other basic patterns. Modified basic patterns differ from the type patterns in some fairly obvious regional aspect as, for example, a tendency toward parallelism of the larger tributaries in a dendritic pattern. Thus many modified patterns are transitional in character between basic patterns, and the naming of such patterns may be a matter of judgment. Pattern varieties are characterized by internal details, commonly obscure. In a broad sense, the basic patterns, the modified basic patterns, and the pattern varieties are analogous to the genera, species, and varieties of the zoological classification.

A complex pattern consists of two contemporaneous patterns adjacent to each other; a compound pattern consists of two unlike superimposed patterns. The palimpsest pattern consists of two super imposed patterns, but one is a paleopattern.

Drainage texture depends on a variety of factors. In any one small area where all other factors are constant, drainage texture may firovide information on underlying materials and indirectly on structure.

Individual stream patterns may display characteristics similai- to lho.se of the gross drainage pattern and may be referred to by the same name. Thus individual patterns may be referred to by such terms as rectangular, angulate, or contorted. Other stream patterns include irregular, rectilinear, meandering, braided, misfit, and beaded.

Drainage anomalies are local deviations from drainage and stream patterns which elsewhere accord with the known regional geology and/or topography. The expectable pattern is regarded as the norm; the anomalies indicate departures from the regional geologic or topographic controls. Analysis of drainage anomalies has revealed structural data in some ilalland reciions WIKTI' other methods of investigation have been unsatisfactory.

INTRODUCTION

Drainage analysis is an important tool in pho-togeologic interpretation, particularly in area? of low relief. It may provide clues to inactive structural features exposed at the surface, to structural features currently rising, and, possibly, to buried structural features. The density of drainage may provide information on permeability and texture of materials, and may infer the identity of materials. The characteristics and significance of drainage patterns, drainage texture, individual stream patterns, and drainage anomalies are considered here.

Techniques involving grid sampling and the use of digital computers eventually may result in the applicati(m of numerical values to drainage patterns (Merriam and Sneath 1966). I t is too early, however, to speculate on the advantages and disadvantages of this procedure.

^ Manuscript received, June 25, 1966; accepted, February i, 1967.

"Geology Department, Stanford University, The writer is indebted to Chester R. Longwell and Stanley N, Davis for review of the manuscript, but only he is responsible for its content.

D R . \ I \ , \ G K l'..\TTERNS

A drainat^e pattrni i< ;hc design formed by the aggregate of drainaiieways in an area regardless of whether they ,.irc o, cupied by permanent streams. A strniDi pii/lcn: is the design formed by a single drainageway

Both basic and iiKiditied basic drainage patterns have been des.ribed (Zernitz, 1932). In addition to these there are drainage varieties. A basic pattern is oiii' whose gross characteristics readily distinguish it from other basic patterns. A modified basic pattern iliffer- from the type basic pattern in some regional aspect as, for example, the close spacing o: small piirallel tributaries in the pinnale-dendriti' i i . ilirii or the preferred orientation of long-er iriliularics in the directional-trellis pattern i Fis. .?. 1! :uiii G) . Drainage vari

eties differ from the liasii and modified basic patterns in internal details. \'arieties are legion and the application of individual names is impractical. In a broad sense, the basii patterns, the modified basic patterns, and the pattern varieties may be likened to the genern, sfii ries and varieties of the zoological rlassificat ion

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DRAINAGE ANALYSIS IN GEOLOGIC INTERPRETATION 2247

BASIC PATTERNS

Most of the basic patterns are controlled by regional structure. Zernitz (1932) classified as major (basic?) the following patterns: dendritic, parallel, trellis, rectangular, radial, and annular. Because these are discussed in most elementary geology texts, only a pictorial review (Fig. 1, A-F) and a brief summation of characteristics and geologic significance (Table I) are included. Two other patterns, multibasinal and contorted, are grouped with the basic patterns in this report (Fig. 1, G and H; Table I) . The original or earnest known references to most of the basic and modified basic patterns are recorded in the footnotes to Table I.

MODIFIED BASIC PATTERNS

Modified basic patterns, although usually recognized as belonging to one of the basic types, differ in certain regional characteristics. For example, the degree of parallelism of the main streams in a region of dendritic drainage is generally a function of the regional slope. On different declivities, therefore, there may be all transitions from dendritic to parallel drainage. Transitional types also may result from changes with time. The change toward parallelism might result from progressive steepening of a slope. Trellis characteristics may appear in a dendritic pattern as streams are superposed from an overlying cover onto dipping rocks. Transitions among all the basic types seem possible. Some of the modified patterns are considered below.

Dendritic Pattern Modifications Subdendritic.—This pattern differs from the

type dendritic only in the lack of perfection. Deviations are presumably due to secondary regional controls, either structural or topographic. Thus, in part of the Amazon basin recently studied by the writer (Howard 196S), the dendritic pattern, inherited from an unconformable mantle, is being transformed to a trellis pattern by adjustment of tributaries to the strike of underlying formations. Along the lower Yellowstone River in eastern Montana, the dendritic drainage is slowly developing trellis characteristics under the influence of a prevailing system of poorly expressed joints (Fig. 2, A).

Pinnate.—This pattern is characterized by many closely spaced, more-or-less parallel tribu

taries entering the larger streams at an acute angle. The drainage, therefore, has a featherlike or frondlike appearance (Fig. 2, B). The pattern is best developed in fine-textured, easily eroded materials such as loess. The fine texture of the materials accounts for the close spacing of the small tributaries, and the steep valley sides are the cause of their parallelism. On some slopes, particularly solifluction slopes in the Arctic, the closely spaced parallel tributaries are long compared with those in Figure 2. They are barely incised into the gentle slopes and extend to the crests of the rounded divides. The pattern resembles feathery plumes.

Anastomotic.—This pattern, characterized by a network of interlocking channels, sloughs, bayous, and oxbow lakes, is found on floodplains and deltas and in tidal marshes (Fig. 2, C). Varieties of the pattern have been termed "reticular" by Par-vis (1950, p. 43-44) and "reticulate" by White-house (1944, p. 9).

Distributary.—This is the branching pattern found on alluvial fans and deltas (Fig. 2, D). It resembles the dendritic pattern except that the tributaries diverge from, rather than converge toward, the main stream.

Parallel Pattern Modifications Subparallel.—The subparallel pattern (Zernitz,

1932, p. 518) shows less parallelism than the basic pattern. If due to slope alone, the pattern resembles that formed by the branches of a poplar tree. Where due to mild structural control by deformed strata of relatively uniform resistance to erosion, there is sufficient parallelism among segments of the main streams and tributaries to suggest the bedrock control, but streams commonly diverge from the geologic grain. The elongate streams are not ordinarily as continuous along the strike as those of the trellis pattern. These differences from the trellis pattern also apply to the subparallel pattern of drumlin areas (Fig. 2, E).

Colinear.—This pattern (Zernitz, 1932, p. 519) is characterized by remarkably straight parallel streams or channels which alternately disappear and reappear (Fig. 2, F). The pattern is found in areas of linear loess and sand ridges.

Trellis Pattern Modifications Subtrellis.—The subtrellis pattern differs from

the type trellis only in the degree of continuity

2248 ARTHUR DAVID HOWARD

FIG. 1.—Basic drainage patterns. Each pattern occurs in a wide range of scales. Examples shown may be regarded as types. Dendritic pattern resembles spreading branches of oak or cheslnul tree with tributaries entering at wide angles. In trellis pattern, small tributaries to long parallel subsequent streams are about same length on both sides of subsequent streams.

DRAINA(}E ANALYSIS IN GEOLOGIC I N T E R P R E T A T I O N 2249

TABLE I. SIGMFICANXE OF BASIC AND MODIPIED BASIC DRAINAGE PATTERNS

Basic Significance Modified Basic Added Significance or Locale

Horizontal sediments or beveled, uniformly resistant, crystalline rocks. Gentle regional slo]>e at present or at time of drainage inception. Type pattern resembles spreading oak or chestnut tree.

Subdendritic

Pinnate^

Anastomotic^^

Distributary (Dichotomic )i^

Minor secondary control, generally structural.

Fine-textured, easily erodable materials.

Floodplains, deltas, and marshes.

Alluvial fans and deltas.

tidal

Parallel^ Generally indicates moderate to steep Subparallel^^ slopes but also found in areas of parallel, elongate landforms. All transitions possible between this pattern and Colineari^ type dendritic and trelHs.

Intermediate slopes or control by subparallel landforms.

Between linea: loess and sand ridges.

Trellis^ Dipping or folded sedimentary, volcanic, or low-grade metasedimentary rocks; areas of parallel fractures; exposed lake or sea floors ribbed bv beach ridges. All transitions to parallel pattern. Type pattern is regarded here as one in which small tributaries are essentially same size on opposite sides of long parallel subsequent streams.

Subtrellis

Directional Trellis

Recurved Trellis

Faul t Trellis"

Joint Trellis

Parallel elongate landforms.

Gentle homocUnes. Gentle slopes with beach ridges.

Plunging folds.

Branching, converging, diverging, roughly parallel faults.

Straight parallel faults and/or joints.

Rectangular^ Joints and/or faults a t right angles. Lacks orderly repetitive quality of trellis pat tern; streams and divides lack regional continuity.

Angulateis Joints and/or faults a t other than right angles* A compound rec-tangular-angulate pat tern is common.

Radials Volcanoes, domes, and erosion residuals. A complex of radial patterns in a volcanic field might be called multi-radial.

Centripetal IS Craters, calderas, and other depressions. A complex of centripetal patterns in area of multiple depressions might be called multi-centripetal.

Annular^ Structural domes and basins, tremes, and possibly stocks.

dia- Longer tributaries to annular subsequent streams generally indicate direction of dip and permit distinction between dome and basin.

Multibasinal ' Hummocky surficial deposits; differ- Glacially Disturbed entiaDy scoured or deflated bedrock; areas of recent volcanism, limestone Karst solution, and permafrost. This descriptive term is suggested for all Thermokarst^^ multiple-depression patterns whose exact origins are unknown. Elongate Bay^s

Glacial erosion and/or deposition.

Limestone.

Permafrost.

Coastal plains and deltas.

Con tor ted 8 Contorted, coarsely layered meta-morphic rocks. Dikes, veins, and mig-matized bands provide the resistant layers in some areas. Pat tern differs from recurved trellis (Fig. 2, H) in lack of regional orderliness, discontinuity of ridges and valleys, and generally smaller scale.

The longer tributaries to curved subsequent streams generally indicate dip of metamorphic layers and permit distinction between plunging anticlines and syncHnes.

1 Described by Dut ton (1882, p. 6, 62, 63) and applied as a drainage term at least as early as 1898 (Russell, p. 204). Classified as a basic pattern by Zernitz (1932, p. 499).

^Zernitz (1932, p . 510). 3 Willis (1895, p. 186). * First used in modern sense by Zernitz (1932, p . 503), but the pat tern was recognized much earlier (Daubree, 1879, p . 357-375; Kemj),

1894, p . 438-440; Hobbs, 1904, pi. 47). 6 Radial drainage is described and illustrated in Jaggar (1901, p . 174, pi. XVII I ) and is referred to by Dake and Brown (1925, p . 134). 6 Jaggar (1901, p. 277) refers to anniJar draniage, but Zernitz (1932, p. 507) may have been the first to apply the name to the drainage

pattern. ^ The descriptive term "multibasinal" is used here as a substitute for genetic terms such as "kettlehole" and "sinkhole" which have

been applied to patterns characterized by numerous depressions. The term "poly basin" (Parvis, 1950, p . 57) would have been appropriate had it not been restricted to the area of the Ogallala Formation in the Great Plains and specifically related to the presence of an impervious substratum.

8 Von Engeln (1942, p . 113, 336). s Zernitz (1932, p . 512). ^'^ Described as a pat tern by Zernitz (1932, p. 514). The descriptive adjective "anastomosing," however, had been used long prior to 1932.

Johnson (1932, p . 497) restricted the term "braided" to the interlacings of an individual stream. 1̂ Parvis (1950, p . 41) attributed the term "dichotomic" to Finch and Trewartha (1942). The writer was unable to locate the term in the

1942 reference or in the first edition of their Elements of Geography, but may have overlooked it. Distributaries are mentioned on pages 307, 342, and 355 of the 1st ed., 1936, and on pages 290, 326, and 340 of the 2d ed., 1942.

2̂ Zernitz (1932, p . 5181. 13 Zernitz (1932, p . 519). 1* Dake and Brown (1925, p . 191). 15 Zernitz (1932, p . 517). 16 Davis (1889. p. 249), 1" Muller (1943), p . SO. 18 Parvis (1950), p. 43.


FIG. 2.—^Modified basic patterns. Each pattern occurs in u v\ifle ran^e <if scales.

and parallelism of the dominant drainage. The Directional trellis.—This term is suggested for distinction between subtrellis and subparallel is a modification of the irellis pattern in which the commonly a matter of judgment. tributaries to the long subsequent streams are

DRAINAGE ANALYSIS I N GEOLOGIC I N T E R I ' K l / r A ' n O N 22>\

consistently longer on one side of the valley than on the other (Fig. 2, G ) . The pattern most commonly is found in areas of gently dipping homo-clinal beds, but also occurs on gentle slopes with parallel beach ridges.

Recurved trellis.—This is a modification of the trellis pattern in which the pattern as a whole forms sweeping curves around the noses of plunging folds (Fig. 2, H ) . I t is more orderly and systematic, and generally larger in scale, than the contorted pattern in metamorphic terrain. Comparison of the lengths of small tributaries on opposite sides of the curved subsequent <lreams, particularly at the noses of the folds, commonly permits distinction between plunging anticlines and synclines; the direction of flow of the longer tributaries generally indicates the direction of dip.

Fault trellis.—This pattern has been attributed by Dake and Brown (I92S, p. 191) to "alternating grabens and horsts or a succession of parallel rifts." It is described as less closely spaced than the trellis pattern on tilted or folded strata, with a tendency toward dendritic drainage between the faults. Right-angle turns are also less common. In the San Mateo quadrangle, just south of San Francisco, California, the fault-controlled streams, although grossly parallel, locally diverge, converge, and branch, and the broader inier-stream segments show dendritic, radial, or other drainage patterns (Fig. 3, A) .

Joint trellis.—A second fracture trellis pattern, characterized by short, remarkably straight parallel streams, may be referred to as joint trellis, although the fractures may include faults. A good example is found in the Zion Park region of Utah (Fig. 3 , B ) .

Both of the fracture trellis patterns differ from the rectangular pattern in having one dominant set of parallel streams.

Rectangular Pattern Modifications

Angulate.—This pattern (Zernitz, 1932, p. 517)

is characterized by numerous acute-angle

bends and barbed tributaries. I t is generally

found in areas where an additional set (or sets)

of fractures is superimposed on a rectangular set.

There may be two superimposed rectangular sys

tems of different orientation. Figure 3C is a

generalized portrayal of the drainage of part of

the Yellowstone plateau. The drainage alignments

clearly indicate one rectangular system with elements oriented approximately north-south and east-west, and another system oriented northeast-southwest and northwest-southeast.

A remarkable cxanipU- of joint control is present in French Cuiana, where several sets of more or less equally s|)acid joints impart a geometric pattern to both the drainage and topography. The pattern has been referred to as "honeycomb" by Zonneveld el al \')^!. ]), I S3). Another geometric pattern, on a much jmaller scale, is found in permafrost areas where in.- wedges thaw around the margins of soil polygons. This pattern is best described as polyconal

Radial Pattern .Modifications

Centripetal.—This pattern (Davis, 1889, p. 249) is a modification of the radial pattern in which the streams flow inward toward a closed or nearly closed central dejircssion (Fig. 3, D ) . The pattern commonK' i> .issoi iated with caters, cal-deras, and a wide \ 'arirty nf depressions. In some areas, suth as the jian belt'' of the Union of South Africa (King, \'i?\. p. 91 ), there is a complex of centripetal pat I e m s The regional pattern might be icferir'il m ;L^ multicentripetal.

Multibasinal Pattern Modifications The multibasinal pattern occurs principally in

areas of glacial erosion and deposition, eolian erosion and deposition, solution, and permafrost. It also is found, however, in regions of recent volcanic activity and in landslide areas. There are many modifications of the pattern, even within individual regions. Thus in glaciated areas, the majority of the depressions may be small or large, closely spaced oi widely scattered, and the drainage may display \arii.-d amounts of integration. In sandy areas, the depressions may display great diversity in shajic MIKI size in accordance with the characteristics of ihe dunes within which they occur, and ina\' aUo display a certain amount of integrated drainage. The pattern may then closely resemble the drainage pattern in mo-rainal area.s.

In volcanic areas, the depressions may include craters and caldera>, lava-dammed valleys, interflow basins, or collapsed lava caves or tunnels. In many lava fields, depressions large enough to be shown on topographic maps are less profuse than in morainal or sand areas.

2252 A R T H U R DAVID HOWARD

FIG. ,•?.—Modified basic patterns (A-D); complex, compound, and |)alini|)sesl patternj (E-H). Each pattern oi-curs in a wide range of scales.


In landslide areas, depressions are found either behind rotated slump blocks, within chaotically jumbled landslide debris, or where drainage has been blocked. This multibasinal pattern is usually of small regional extent.

The multibasinal pattern i.s rarely diagnostic in itself of either process or material; patterns formed by different processes may be remarkably alike. A pitted outwash area in Minnesota illustrated by Cooper (r93S, Fig. 4, p. 10) is remarkably similar to the solution-pan landscape of parts of Florida. Multibasinal patterns in areas of moraine, sand dunes, limestone, recent lava flows, landslides, and permafrost may resemble each other at least superficially. Conclusions reached as to process or type of materials based on pattern alone could be in error. Nevertheless, several genetic terms have been suggested for varieties of the multibasinal pattern: glacially disturbed, deranged, kettle hole, swallow hole, karst, and others. If there is doubt as to genesis, the pattern is best referred to simply as multibasinal. If, on the other hand, the pattern includes features that leave no doubt as to process or material, there may be justification for using one of the established genetic terms. Thus, a multibasinal pattern with (f) depressions ranging from tiny steep-sided pits, many of which are circular, to large, deep, irregular valley-1'ke basins, (2) some depressions ahgned rectinearly, and (3) scattered disappearing and/or reappearing streams, may perhaps be referred to as a swallow-hole or karst pattern. Or, a multibasinal pattern associated with evidence of thawing permafrost, such as polygonal ground and beaded drainage, might be referred to as fhermokarst (Muller, 1943, p. SO). Parvis (1950, p. 43) suggested the name "elongate bay" for a multibasinal pattern in which the depressions are large, elliptical, and parallel. The pattern is found in some coastal-plain and delta areas and has been variously attributed to meteorite impact, solution, segmentation of lagoons at higher stands of the sea, and to thaw of formerly frozen ground. The value of the purely descriptive term "elongate bay" for this pattern is obvious.

COMPLEX, COMPOUND, AND PALIMPSEST PATTERNS

Zernitz (1932, p. 521) proposed the term "complex" for an aggregate of dissimilar patterns reflecting different structural controls in adjoin

ing areas. Parvis (f9S0, p. 43) suggested the term "anomalous" for complex patterns found in areas of differing topography and materials. The terms "complex" and "anomalous" have thus been applied to situations that are in part similar and in part dissimilar. Inasmuch as the term "complex" has priority, it should be retained but perhaps with its scope enlarged to include all patterns representing an aggregate of adjoining dissimilar patterns due to structure, materials, and/ or differences in topography. In Figure 3, E, the contrasted patterns are due to differences in structural features. An example of drainage differences caused by differences in topography on identical materials is the multibasinal drainage of moraine versus the subparallel drainage of drumlin topography.

The term "compound" was applied by D. W. Johnson (personal commun., 1931) to drainage consisting of two or more contemporaneous patterns in the same area, as, for example, the combination of radial and annular patterns characteristic of many domes (Fig. 3, F). Dendritic and multibasinal patterns commonly are combined in areas where streams have cut youthful valleys into a relatively insoluble formation below a solution-pitted limestone formation. The depressions are restricted to the limestone-capped divides between the streams. A somewhat similar combination of patterns results from partial integration of drainage in morainal areas.

The writer encountered an interesting drainage pattern which he has called palimpsest (Howard, 1962, p. 2255). In the palimpsest pattern, an older, abandoned drainage or stream pattern forms the background for the present pattern. The example (Fig. 3, G) is in the western coastal plain of Taiwan. At the site of the anomaly, the present drainage pattern is radial. Faintly visible through the rice paddies is a meandering channel whose presence is indicated primarily by the somewhat smaller size of the paddies within its confines. The meandering channel crosses the present low topographic bulge toward its crest. Clearly, the topographic high was not present when the meandering stream crossed the area. The meandering stream apparently was deflected by the growing arch on which the present radial drainage came into existence. The situation suggests either active deformation within the coastal plain, not an unlikely possibility considering the

2254 A R T H U R DAVID HOWARD

instability of the island of Taiwan as a whole, or differential settling over a buried topographic high, or both. Any drainage pattern that includes traces of an older, unlike pattern may be referred to as palimpsest. Remnants of original stream courses are common in many areas of glacial and eolian activity, multiple piracy, and recent warping and faulting. Figure 3H illustrates in generalized fashion the relation of the Missouri River (or the Ohio River) to aliandoned preglacial valleys.

PATTERN VARIETIES

Pattern varieties differ from basic and modified basic patterns in internal details. They commonly provide useful geologic information.

Regional differences, such as contrasts in density of drainage, do not distinguish varieties. I t is expectable that a dendritic pattern in shale will be finer than that in sandstone, and that a Irellis pattern in slate will be finer than thai in in-terbedded sedimentary strata. Any drainage pattern may be fine, medium, or coarse textured.

Intrapattern differences in texture, however, do distinguish varieties. Thus, a dendritic pattern in an area in which thick, horizontal beds of sandstone and shale are exposed in the slopes may display a coarse texture in the sandstone and a finer texture in the shale. The pattern is 'le.xtur-ally zoned."

In another variety of the dendritic pattern, many streams consistently are closer to one side of their valleys than the other. In the Leavenworth Cjuadrangle (Kansas-Missouri), streams that flow generally east or west hug the steeper south (north-facing) slopes. The dendritic pattern suggests essentially horizontal sedimentary rocks or beveled, uniformly resistant crystalline rocks, but the valley a.symmetry suggests an additional influence such as a gentle southward dip, active tilting, or differences in degree of erosion of the valley slopes due to direction of exposure. That the asymmetry is not due to stream (leilecti(m resulting from terrestrial rotation is evident from the fact that the steep slope is on the left side of some streams and on the right side of others. •

Another variety of the dendritic pattern, characteristic of granitic areas, displays numerous sick-lelike curves. These apparently are the result of

' The term right and left apply when facing down-current.

deflection of .-streams around bodies of relatively unfractured or otherwise resistant rock.

Comparable varielie> ar t found in each of the other basic and modified basic patterns. A detailed treatment ol ihe>c i.̂ beyond the scope of this report. The irnportani point is ihal careful study of local departure^ from the regional pal-terns may reveal unsuspected information of considerable value. The anal>'sis of drainage varieties and of the related draiiuige anomalies discussed below presenis w. iiniiiuc challenge to the geologist .

DRAI.\A(,E TEXTURE

Drainage texture refer-, lo the relative spacing of drainage lines regardless of occupancy by perennial streams. The u-inis "fine," "medium," and "coarse" generall\ arc used in a relative sense to indie ati' ihe .-paung. A fine texture is one in which there i> a high degree of ramification of drainage lines resulting in a dense network involving myiiad small streams. Fine texture is typical ol i lay, -.hale, silt, and other relatively impervious materials. A coarse texture, in contrast, exhibits \ e r \ little ramification, and longer, more widel\- .^e])arated valleys prevail. Coarse texture i.-> typical of permeable materials such as sand, gravt'l. and rocks that weather into coarse fragment^. Medium texture is intermediate between the two extremes.

The use of these te.Mural terms without clarification is inadvisal)le. not only because they mean different things to different people, but because texture varies vviih -cale. Attempts have been made lo exj)re^^ textures cjuantitatively on the basis of the numbei i >l ream frequency) and total length (drainage den-iiy) of drainage lines per unit area (Honon. 'U^', Smith, 1950), However, quanti tat i \e ilelerniiiiations of texture involve laborious, time-consuming procedures, and the resulting degree- ef refinement are greater than necessary for many geologic problems, A satisfactory ])ro( edure :'()r leports is to prepare diagrams showing the drainage textures, at the scale of the maps or photos, that are regarded as fine, medium, and loai.-e atid perhaps as ultrafine and ultracoarse.

Drainage texture is influenced by (1) climatically controlled factors ^Ul:ll as amount and distribution of i^recipitation, \-egetafion, and permafrost; (2) rock characteristics, including te,x-


ture and size of fragments released by weathering; (3) infiltration capacity; (4) topography; and (S) stage and number of erosion cycles. In any one small area of study, the climatic factors, the topography, and the stage and number of erosion cycles may be reasonably constant, so that the variations in te.xture will reflect differences in rock characteristics and infiltration capacity.

In unconsolidated sediments, the drainage texture is related directly to grain size. On similar declivities, small rills can easily move particles of clay and silt and develop myriad small channels, whereas larger streams, that is, the accumulations from larger watersheds, are required to move sand and gravel. Hence the channels are more widely spaced. As Schumm reported (1956, p. 607), a certain minimum drainage area is required to maintain a stream channel in an area of uniform lithology and simple structure. He expressed this quantitatively as a constant of channel maintenance, which is actually an expression of texture. The reader is referred to Schumm's paper for further discussion of this relationship.

In areas of hard rock, the size of the fragments provided for transport is the decisive factor. The removal of large blocks ordinarily requires larger streams than does the removal of small fragments, if there are no strong contrasts in stream gradients. Texture of drainage in granite areas, for example, may range from fine in closely fractured zones to coarse where the fractures are more widely spaced. On very gentle slopes in humid climates, deep weathering may result in a fine-textured soil regardless of the rock type below. The fine-textured debris, however, generally influences the texture only of that part of the drainage system that has not eroded through the surficial mantle.

Infiltration capacity, the rate at which water soaks into the ground, depends to a large degree on permeability. Deposits of sand and gravel, as well as permeable rocks including those in which the permeability is the result of fractures, readily absorb precipitation. Therefore, they have few surface streams and display a coarse drainage texture. The pattern may be finer on steep slopes, however, where velocity of flow results in reduced infiltration and greater surface runoff. Clay, with a low infiltration capacity, has a large surface runoff and a dense network of surface drainage.

Vegetation, with its absorbent root mat and underlying soil, retards runoff and reduces development of rills. Thus, the texture of drainage in humid climates is generally coarser than in arid climates, and the texture is coarser on heavily vegetated slopes than on barren slopes.

Some gravel deposits display a medium or even fine texture of drainage. Such gravels may have a high content of "fines"—materially reducing the permeability—or may be exposed on steep slopes, such as terrace scarps or steep dip slopes where the velocity of flow is rapid enough to insure considerable runoff.

Drainage texture may vary within the confines of a single drainage pattern depending on the nature of the rocks exposed. Theoretically, the cross-country trend of the boundary between tex-tural zones should assist in correlation of rock units from one drainage basin to another.

STREAM PATTERNS

The names apphed to stream patterns are self-explanatory, and most of the patterns are so well known that further explanation is not required. However, a few comments seem pertinent.

Some individual stream patterns show the characteristics of the overall drainage pattern and are referred to by the same names (Johnson, 1932). Thus, a stream showing right-angle bends may be referred to as rectangular; one with acute angle bends, as angulate; and one with tight hairpin turns, as contorted. The geologic implications of these stream patterns are the same as for the corresponding drainage patterns.

Other distinctive stream patterns are: the irregular pattern characterized by a more or less random course and suggesting an absence of structural or topographic control; the rectilinear pattern, with abnormally long straight reaches, generally indicating fracture control; the meandering pattern, indicating competency on the part of the stream to transport available bed load (Leopold and Wolman, 1957, p. 39); and the braided pattern, indicating an inability to handle bed load.^ Alternate meandering and braided reaches, therefore, suggest local differences in the texture of the materials being supplied to the stream and may indicate alternate exposures of

* Detailed discussions of floodplain stream patterns appear in Melton (1936) and Russell (1939).


unlike materials. Misfit meandering streams, in which the dimensions of the meanders do not agree with those of meander scars or of flood-plain scrolls, suggest geologic or climatic change. The sickle pattern displays some arcuate curves and is most common in areas of plutonic rocks and migmatites. The barbed pattern indicates either piracy or the presence of joints, faults, or layers of weak rock trending obliquely across the path of the stream. The term "beaded" has been applied to streams in the subarctic along which small thaw sinks are present at irregular intervals. Successions of beaver dams give a superficially similar pattern, as do, on a larger scale, strings of glacial lakes.

The writer has named a new pattern, spatulate, which could be included under beaded, but which he believes is distinctive enough to warrant a separate designation. In essence, it consists of alternate broad valley segments and narrow defiles. The pattern is displayed by some of the valleys, such as the Aragva, that drain south from the Caucasus in southern Russia. The Aragva and its sister streams pass intermittently through resistant and weak Cretaceous sedimentary rocks (Renngarten, 1937, p. 104). The streams are restricted to defiles where the more resistant carbonate rocks of the Upper Cretaceous are brought down to river level in the troughs of synclines, but they meander in broad open reaches in the weaker, sandy-argillaceous Lower Cretaceous sediments of the anticlinal cores. The defiles and open reaches range in length from O.S to 2 mi or more. The pattern is quite regular in these open folds, with the broad, elongate segments occurring at uniform intervals along the valley.

Other spatulate patterns may have no structural significance. The spatulate pattern displayed by the Missouri River in eastern Montana and western North Dakota is glacial in origin (Howard, 1958). The Missouri trench is locally 1 mi or less in width; in intervening areas its width may exceed 4 mi. The narrow segments represent ice-marginal paths cut across former divides, whereas the broad elongate segments represent parts of preglacial valleys. The pattern is irregular in that the broad segments inherit their trends from an ancestral drainage whose trends were opposed to the trend of the ice front. Thus the broad segments are considerably varied in

orientation and are irregularly distributed along the present valley.

DRAINAGE ANOMALIES

Anomalies in drainage patterns and in the patterns of individual streams have been the subject of discussion in recent years. They are of particular importance in the flatlands. The analysis of drainage may provide clues to structural features undetectable by other methods.

A drainage anomaly can be defined as a local deviation from the regional drainage and/or stream pattern which elsewhere accords with the known regional structure and/or topography. The expectable pattern is regarded as the norm (DeBlieux, 1949, p. 1253-1254), and the deviations are anomalies. An alternation of broad valley segments and narrow defiles along transverse streams in areas in which the structure is known to consist of folded weak and resistant rock is herein regarded as normal, as are sicklelike curves in granite areas. However, in many other geologic environments these phenomena are anomalous. Anomalies suggest structural or topographic deviations from the regional plan. Many composite patterns, for example, involve a small enclave of one pattern within another, rather than two adjacent patterns of equal magnitude. An illustration is the local occurrence of radial and annular drainage within a regional dendritic pattern (Fig. 4, A). Many pattern modifications and varieties also involve anomalies as, for example, local parallelism of streams in a dendritic pattern (Fig. 4, B). Many anomalies are localized along individual streams. Some of these are listed below.

Rectilinearity.—Long, rectilinear segments of streams, particularly if aligned across divides with rectilinear segments of other streams, constitute an anomaly if the regional pattern is other than rectangular, angulate, or fault-trellis. A fracture, or an easily erodable vein or dike is indicated. In Figure 4C the arrow indicates a rectilinear stream.

Abrupt and localized appearance of meanders.— DeBlieux (1949, p. 1259) has described an interesting stream anomaly at the Lafitte oil field in Jefferson Parish, about IS mi south of New Orleans (Fig. 4, D). The channel of an abandoned Mississippi River distributary is relatively straight and simple for several miles upstream

DRAINAGE ANALYSIS IN (iEOLOGIC INTERPRKTA'l'ION 2257

A. Dendritic with radia l -onnular enclave

D. Local meandering

Abandoned Mississippi distributary

G. Pinched valley

J. Variation in levee width

2 miles

B. Dendritic. Trellis influence

E Compressed meanders

H. Anomalous flore in valley

I' m Scherr

C Rectilmeoi

s-r̂ —v / mile

^^i_ ^ I ocal braiding

Abandoned* * > \ \ \ e.ees W I K \ \ \

I Anomolous pond.morsh, or alluviQi fiM

1 L Kiogo

L. Anomalous :;urves and turns -

/ Schematic

FIG. 4.—Examples of drainage anomalies. A. E, C, G--Amazon basin; E Kent County, Te-icas, after DeBlieux and Shepherd, IP.'il; D, F, J—Louisiana, after DeBlieu.x, 1Q45: K bouisiana. generalized after DeBlieux, 1940; I—East Africa, after Holmes, 106,=̂ ; H, L generalized exumpU-

and downstream from the Lafitte salt dome. At the dome, however, two meanderlike curves are present. This interruption of the normal pattern may be related to a subtle upstream reduction in stream gradient caused by the appearaiu:e of the dome along its path.

Compressed meanders.—DeBlieux and Shepherd (1951, p. 98) described a stream pattern in which several meanders of an otherwise normal and continuous series are squeezed, compressed, and incised (Fig. 4, E). The anomaly, along the Double Mountain Fork of the Brazos River in Kent County, Texas, is at the site of a subsequently demonstrated structural anomaly.

No explanation of ihc anomaly is offered. Mc-Kenzie Creek, a tributary from the south, displays an anomalous (ur\e apparently influenced by the dome.

Abrupt and localized braiding.—DeBlieux (1949, p. 1'2S9) reported the abrupt and local appearance of braiding at Scully salt dome in abandoned distributaries of Bayou Lafourche about 30 mi southwe.'̂ l of New Orleans Fig. 4. F). Braiding generally indicates inal)ilit\- of a stream to transport its bed load (Leopold and Wolman, 1957, p. 50). Inability may resull from local acquisition of a coarser load than the .stream is competent to handle, loss of volume ilue to locally increased


underflow, loss of velocity caused by flattening of the gradient (perhaps by a rising structure), or some other geologic or hydrologic factor. De-Blieux attributed the braiding to flattening of the gradient. The presence of similar anomalies in neighboring streams may permit regional delineation of the area or zone of anomalous behavior and allow a more informed consideration of cause. Correlation of meandering and braided reaches in adjacent streams conceivably might permit the delineation of formational boundaries. The same may be indicated by more subtle variations in stream patterns (Tator, 1954, p. 414), such as zonal variations in drainage density within the drainage pattern.

Anomalous pinching or faring of valleys or channels.—Local widening or narrowing of valleys or channels, not a repetitive feature of the regional drainage pattern, may indicate local structure. A shallow upwarp, for example, might bring slightly weaker or more resistant materials to stream level, thereby influencing the rate of valley widening; or upwarping might result in incision of the stream, the valley being broader upstream and downstream (Fig. 4, G and H).

Anomalous ponds, marshes, or alluvial fills.— The presence of an isolated pond, marsh, or alluvial fill along the path of a mature stream where landslides or other surficial causes can be excluded, may indicate damming by subsidence or by uplift directly downstream. Some streams have been able to maintain their courses across rising obstructions; other streams have been diverted. Excellent examples of anomalous ponding are provided by Lakes Victoria and Kioga in East Africa (Fig. 4, I ) . The lake basins originally drained westward by way of the streams labeled A and B in the figure. Relative subsidence of the central area contemporaneous with creation of the western and eastern rift valleys resulted in drowning of the lake basins and reversal of the direction of flow of the outlet streams, many of whose tributaries are barbed and locally drowned. Blocking of the western outlets diverted the waters of newly created Lake Victoria northward to Lake Kioga and thence northwestward around the northern end of the western rift valley. Although these drainage modifications are on a grand scale, similar phenomena may occur at all scales.

Anomalous breadth of levees.—Russell (1939, p. 1212) noted that leaves of abandoned channels

along the Mississippi River are narrower in some places than others. He suggested that subsidence of the levees at these places permitted encroachment by the neighboring swamp or marsh resulting in the reduced levee width.

It is recognized generally that subsidence in the Mississippi delta is differential, being retarded over the sites of buried structural features. Thus, levees are generally broader where they cross such structural features than they are up- or downstream. This is true of the levees of the abandoned Bayou Lafourche (Fig. 4, J) where it crosses the Valentine dome about 30 mi southwest of New Orleans in Lafourche Parish (De-Blieux, 1949, p. 12S3). DeBheux recognized that levee broadening may be caused by factors other than subsidence, such as crevassing, bifurcation, and coalescence, but believed that these causes are readily recognizable.

Flying levees.—In many parts of the Mississippi delta, former channels have subsided below marsh level and only small fragments are preserved, perhaps because they are on buried structural features (Fig. 4, K). Because these levee remnants are completely isolated, the expression "flying levee" is herein proposed. De-Blieux (1949, p. 1253) cited the levee remnants at Four Isle dome, about 70 mi southwest of New Orleans in Terrebonne Parish, as an example. Here, the flying levees are more than 3 mi downstream from the present terminus of Bayou Grand Caillou.

Anomalous curves and turns.—^An anomalous curve or turn is one that is abnormal within the drainage pattern in which it occurs. The varieties are legion, being most common in the flatlands (Fig. 4, L). For example, a domal upwarp across the path of a stream may gently "shoulder" the stream aside, forcing it to follow a curved, commonly semicircular path around the structural feature. Barbed junctions similar to those resulting from piracy may be formed where tributaries to one stream are blocked by an upwarp and are deflected sharply into neighboring drainage. If a domal upwarp takes place between parallel streams, both streams may be deflected, resulting in a peculiar "blowlegged" pattern. A stream crossing an active strike-slip fault may be offset laterally and display sharp right-angle turns where it enters and leaves the rift. Faults may lead to anomalous lengthening and flattening of a curve.

DRAINAGE ANALYSIS I N GEOLOGIC I N T E R P R E T A T I O N 2259

SUMMARY

Drainage analysis may provide information on structural features and type of materials. The analysis should consider not only basic patterns, but also modified basic patterns, pattern varieties, drainage texture, stream patterns, and anomalies.

The drainage patterns, individually and in combination, provide a certain amount of information which, in the flatlands at least, may not be obtainable by ordinary field methods. The pahmp-sest pattern is of special interest inasmuch as it may indicate current tectonic activity.

Drainage texture within any one small area in which climate, topography, and erosional history are reasonably constant commonly may be indicative of the permeability of materials or of the size of particles provided by weathering.

Individual stream patterns may provide information on structural features, rock type, hydraulic conditions, or geomorphic changes.

Drainage anomalies may provide information on local structural features, active deformation, differential subsidence, or changes in the hydrolo-gic regimen.

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