Sedimentology (1981) 28, 753-780

Significance of interdune deposits and bounding surfaces in aeolian dune sands

G A R Y KO CU RE K*

Department of Geology, Weeks Hall, University of Wisconsin, Madison, Wisconsin 53705, U.S.A.

A B S T R A C T

Bounding surfaces and interdune deposits provide keys for detailed interpretations of the develop- ment, shape, type, wavelength and angle of climb of aeolian bedforms, as well as overall sand sea conditions. Current alternate interpretations of bounding surfaces require very different, but testable models for sand sea deposition.

Two perpendicular traverses of Jurassic Entrada Sandstone, Utah, reveal relations among cross- strata, first-order bounding surfaces, and horizontal strata. These field relations seem explicable only as the deposits of downwind-migrating, climbing, enclosed interdune basins (horizontal strata) and dune bodies consisting of superimposed smaller crescentic dunes (cross-stratified deposits). 4 1:7 km traverse parallel to the palaeowind direction provides a time-transgressive view showing continuous cosets of cross-strata, first-order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree. Changes occur in the angle of climb, cross-strata structure, and interdune deposits; these,reflect changes in depositional conditions through time. A 1.5 km traverse perpendicular to the palaeowind direction provides aview at an instant in geological time showing first- order bounding surfaces and interdune deposits forming flat, laterally discontinuous lenticular bodies. The distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins, such as those on Padre Island, Texas.

Hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg. The presence of three orders of bounding surfaces indicates dune bodies consisting of smaller, super- imposed dunes. The geometry of first-order bounding surfaces is a reflection of the shape of the inter- dune basins. Second-order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type. Third-order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes. The presence of only two orders of bounding surfaces indicates simple dunes.

Modern and Entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet, damp, and dry conditions. Interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions.

I N T R O D U C T I O N

Aeolian cross-stratified deposits are universally interdune deposits record the history of a n erg (sand punctuated by various bounding surfaces, the most sea), but until recently interpretive efforts were extensive of which are commonly overlain by hori- concentrated upon dune cross-stratification. Bound- zontal interdune deposits. Together with dune- ing surfaces and horizontal strata within aeolian deposited cross-stratification, bounding surfaces and cross-stratified units were widely noted, but the

* Present address : Department of Geological Sciences, bounding surfaces were not generally attributed to University of Texas, Austin, Texas 78712, U S A . systematic dune development, nor were the hori- 0037-0746/81/1200-0753 $02.00 zontal strata interpreted as interdune deposits. 0 1981 International Association of Sedimentologists The most extensive, seemingly planar bounding

48 S E D zn

754 G. Kocurek

3

- _ - 2 - ~ - ~-~ 3 3 3

~

HORIZONTAL STRATA - ______

Fig. 1. Hierarchy of bounding surfaces in cross stratified sandstone viewed parallel to dune transport direction. First-order bounding suifaces (overlain by horizontal strata) indicated by ( I ) , second-order bounding surfaces by (2), third-order bounding surfaces by (3), foresets by dotted lines. After Brookfield (1977).

surfaces are prominently displayed in many ancient aeolian deposits (e.g. Stokes, 1968; Sanderson, 1974; Otto & Picard, 1975). The origin of these surfaces is attributed by some geologists to deflation to the water table, as proposed by Stokes (1968), who envisioned these surfaces originating as extensive deflation plains formed periodically in a subsiding aeolian basin whenever the rising water table reached the ground surface. Horizontal strata overlying these surfaces were then logically attributed to deposition in playas formed at the surface water table, rather than to interdune deposition per se.

McKee & Moiola (1975), reiterated in McKee, Breed & Fryberger (1977), appear first to have questioned the Stokes interpretation. They note that water tables are subdued reflections of the topography and, hence, these planar surfaces do not suggest a water table developed beneath dune relief. This argument has since been somewhat deflated by Walker & Middleton (1977), who suggest that progressive removal of dry sand by deflation from dune highs could ultimately allow a horizontal surface to develop. Based on cores taken at White Sands Dune Field, New Mexico, which show up to three generations of dune-deposited cross-strata separated by horizontal strata, McKee & Moiola (1975) propose that cross-strata, planar bounding surfaces and overlying horizontal strata result from the downwind migration of successive dunes and interdune areas across t:ie dune field. A bounding surface that truncates dune cross-strata, therefore, marks the passage of the interdune area over the dune deposits left as net sedimentation. Horizontal strata might then accumulate on the bounding sur- face within the interdune basin. With continued

migration both dunes and interdune deposits form continuous, but diachronous layers.

The McKee & Moiola (1975) hypothesis was greatly expanded by Brookfield (1977) to include not only the most extensive bounding surfaces, but subordinate bounding surfaces as well. Brookfield recognizes a hieiarchy of three bounding surfaces based upon their extensiveness and truncating relationships (Fig. l), and attributes their origin to the migration of a hierarchy of superimposed bed- forms. Brookfield argues that the most extensive bounding surfaces, termed by her first-order bound- ing surfaces result from the migration of the largest aeolian bedform, the draa (large-scale sand mound with dunes superimposed; terminology of Wilson, 1972a, b, 1973). Less extensive bounding surfaces truncated by first-order ones are termed by Brook- field second-order bounding surfaces, and are attributed to the migration of dunes over the draa. Still less extensive are third-order bounding surfaces, which are truncated by second-order bounding surfaces. Brookfield attributes the origin of third- order bounding surfaces to reactivation surfaces on the lee faces of individcal dunes. The analogous argument for the origir-3 of subaqueous hounding surfaces has been developed by Allen (1968a, b, 1973) and applied to the interpretation of ancient deposits (Banks, 1973).

Brookfield’s hypothesis is fully consistent with the structure of recent ergs, in which draas are fairly regularly spaced sodies with broad intervening interdune areas. Wilson (1971, 1972a, b, 1973) espe- cially emphasizes that the occurrence of aeolian bedforms within an erg is not random, but rather forms a hierarchy consisting of ripples, dunes, and draas. Each bedform can occur singly or superim- posed upon each other.

Objection to Wilson’s term draa has been raised (McKee et al., 1977; McKee, personal communica- tion) arguing that draas are only larger more com- plicated dunes formed by the same processes that form dunes and thus having similar structures. The global classification of dunes, based upon the shape of the sand body and the position and number of slipfaces, by McKee et al. (1977), McKee (1979a), Breed & Grow (1979), is excellent and needed. Whether those bedforms consisting of superimposed bedforms should be considered as separate bedforms (Wilson’s draa) and genetically distinct from dunes remains an open question as processes and factors of bedform formation are as yet poorly understood. The draa term is used here to refer collectively to all dunes that are not simple, but rather are compound (two or more of the same type of dune; combined by

Interdune cleposits and bounding surfaces 755

A. WATER-TABLE -CONTROLLED HYPOTHESIS

VIEW PARALLEL TO WIND DIRECTION WIND-

T I - - VIEW PERPENDICULAR

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B. CLIMBING BEDFORM HYPOTHESIS

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T I -

12 T 1-

VIEW PERPENDICULAR TO WIND D I R E C T I O N

T I -

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Fig. 2. Opposing models for aeolian cross-stratified deposits in views parallel and perpendicular to wind direction. (A) First-order bounding surfaces originating as water-table-controlled deflation plains, after Stokes (1968). (B) First-order bounding surfaces originating by migration of interdune areas and dunes. Shaded zones mark playa deposits formed at the surface water table in (A), and interdune deposits in (B). TILT3 indicate successive time planes depositional surfaces. TX marks a surface of subsequent erosion capping the aeolian deposit. In (A), time planes also mark water- table surfaces. In @), water-table surfaces are independent of depositional surfaces.

being superimposed to form a coherent larger bed- form in the terminology of McKee, 1979a), complex (two or more different types of dunes combined by being superimposed to form a coherent, larger bed- form in the terminology of McKee, 1979a), or consist of multiple, separate slipfaces. Dma is especially use- ful to refer to these bedform types in ancient deposits where the distinction between different dune types is far from obvious. As developed later in the text, the distinction in ancient deposits between draas (as used here) and simple forms, regardless of scale, is an easier and useful distinction. Similarly, interdune basin or area is used here to refer to all areas between sand

mound topographical highs, regardless of whether these mounds are dunes or draas. If first-order bounding surfaces originate by the

migration of interdune areas and the preserved thickness of dune cross-stratification and interdune deposits between successive first-order surfaces is the net deposition left by the migrating draa or dune and interdune, then each such sequence must climb downwind over the deposits left by the preceding generation. Climbing is a geometric necessity of downwind migrating draas or dunes and interdune

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756 G. Kocurek

areas if net sedimentation is occurring. Only if there is no net aggradation would this not be true. Documentation, however, that these sequences are in fact climbing has been very elusive. Shotton (1937) clearly envisioned what is referred to here as climbing dunes in his explanation of upwind- dipping bounding surfaces in the cross-stratified Lower Bunter Sandstone (England). McKee & Moiola (1975, fig. 8, p. 65) argue for climbing bedforms from the observation that the dune wind- ward surface is truncated to form a plane continuous with the interdune surface (Fig. 6a, b, p. 64). Wilson (1972a) and Brookfield (1977) assume climbing bedforms in their arguments without documenting their existence. Rubin & Hunter (1982) give a thorough theoretical argument to attest to the necessity and implications of large-scale climbing aeolian bedforms, as well as documenting climbing in modern, small-scale aeolian dunes (Oregon coast).

Criticial to the understanding of first-order bounding surfaces, as well as to a detailed inter- pretation of any ancient aeolian deposit, are the horizontal deposits that commonly overlie first- order bounding surfaces. Do the geometry and nature of these deposits in fact indicate deposition on extensive plains at the water table, or rather are they indicative of interdune deposition not neces- sarily associated with the water table? Unfortu- nately, interdune areas have received far less atten- tion than dunes, although interdune areas may cover a greater surface area than dunes. Descriptions of modern or ancient interdune deposits are given by McKee & Tibbitts (1964), McKee (1966a), Glennie (1970, 1972), Warren (1971, 1972), McKee & Moiola (1975), Gradziiiski, Gagol & Slaczka (1979), and Sharp (1979). Descriptions of sheet sand deposits by Fryberger, Ahlbrandt & Andrews (1979) apply equally well to some types of interdune deposits. These descriptions, however, do not by any means exhaust the variety of such deposits that exist.

Significantly, the two contrasting interpretations of first-order bounding surfaces and horizontal strata generate very different and testable models for aeolian deposits (Fig. 2). In the Stokes (1968) hypothesis where first-order bounding surfaces are deflation planes and horizontal strata are playa deposits (Fig. 2A), surfaces should be extensive, reasonably flat, and identical in cross-sections both parallel and perpendicular to the palaeowind direction. In this model horizontal strata should

show some evidence for deposition in water or at least on a damp surface as they were formed in conjunction with the water table. Where sand deflated from these surfaces is deposited it is not specified in the Stokes hypothesis, creating some- thing of a ‘sand disposal problem’. As illustrated by Stokes (1968, fig. 1, p. 512), the hypothesis is not realistic in its portrayal of most ergs because dune size and spacing appear random and no interdune areas occur. As illustrated by Stokes, the hypothesis does not have to contend with climbing bedforms for his dunes do not migrate. In his sketches of successive stages of deposition, individual dunes do not change position; new dunes only form on the tops of old, filling all open areas.

Conversely, if first-order surfaces are produced by interdune migration and horizontal strata are inter- dune deposits (Fig. 2B), then surfaces and horizontal strata should be continuous and climb downwind in cross-sections parallel to the palaeowind direction, but should be Rat in cross-sections perpendicular to the palaeowind direction. Horizontal strata should show the types and distributions of sedimentary structures characteristic of modern interdune de- posits.

This paper addresses three questions. First, what are the geometries of aeolian bounding surfaces and horizontal strata? Secondly, what sedimentary structures characterize the horizontal strata depo- sited upon first-order bounding surfaces and modern interdune areas? Lastly, what types of information do bounding surfaces and interdune deposits provide for the reconstruction of ancient aeolian deposits ?

Used here as the primary case-study is the aeolian Entrada Formation (Jurassic), which was studied over its northern extent in north-eastern Utah and north-central Colorado (Fig. 3). The regional depositional setting of Entrada and adjacent for- mations is presented in Kocurek (1981). In addition to standard stratigraphic sections, much of the information presented here is based upon two traverses (indicated on Fig. 3)-one parallel (Fig. 4A) and one perpendicular to the palaeowind direction (Fig. 4B). Recent aeolian deposits were studied primarily on Padre Island (south Texas), with less detailed study at Little Sahara Dune Field (Juab County, Utah), Great Salt Lake Desert (near Knolls, Utah) and at White Sands (New Mexico).

Interdiine deposits and boiinding surfaces 757

C

Fig. 3. (A) Index map with late Callovian palaeogeography superimposed. Modified from Hileman (1973), Imlay (1952, 1957, 1980), Johnston (1975), Peterson (1972) and Rautman (1975, 1978). (B) Study area indicated in (A), with late Callovian palaeogeography as determined from this study. Entrada Sandstone and equivalent age rock outcrops shaded. Numbers refer to section locations. Straight arrows and wavy arrows (labelled F) indicate mean cross-strata dip direction for aeolian and fluvial cross-sets, respectively, at section locations. (C) Area of traverses (Fig. 4), indicated in (B), with cross-set orientations.

GEOMETRY AND CHARACTERISTICS OF AEOLIAN BOUNDING SURFACES

AND HORIZONTAL STRATA

General sequence

Brookfield's (1977) three orders of bounding sur- faces are readily distinguishable in the Entrada Sandstone where the general vertical sequence consists of an orderly repetition of thick dune cross- stratification and thin horizontal strata. First-order bounding surfaces truncate cosets of cross-strata and are overlain by horizontal strata. Cross-strati- fication between first-order bounding surfaces consists of cosets of mutually truncating sets of cross-strata separated by second-order bounding surfaces. Third-order bounding surfaces occur separating subsets within a single set of cross-strata (Fig. 5) .

Variations in this general sequence occur in parts of the Entrada. Three orders of bounding surfaces are best developed near, but not immediately adjacent to, the palaeocoastline (sections 8, 9, 10, 1 1 , 15, area of traverses, Fig. 3B). Farther inland

(sections 12, 13, 14, 22, Fig. 3B) three orders of bounding surfaces occur, but horizontal strata are thinner or even absent above first-order bounding surfaces. Along the palaeocoastline (sections 4, 5, 6, 7, 18, 20, Fig. 3B) only two orders of bounding surfaces occur and aeolian cross-strata are inter- bedded with marine deposits. Similarly, near the Entrada's eastern edge (sections 17, 21, Fig. 3B) only two orders of bounding surfaces generally occur within aeolian cross-strata interbedded with fluvial deposits consisting of water ripples and small-scale, coarse-grained sets of cross-strata.

Geometry of first-order bounding surfaces and horizontal strata

Description The overall geometry of first-order surfaces and of overlying horizontal strata is shown on Fig. 4(A, B). In the cross-section parallel to palaeowind direction (Fig. 4A), nearly all first-order bounding surfaces, their overlying horizontal strata, and major cosets of cross-stratification can be traced along the total length of the traverse. Marked changes in these do

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Fig. 5. Cosets of cross-strata between horizontal strata, drawn from outcrop, and typical of Entrada outcrops perpen- dicular to the palaeowind direction. First-order bounding surfaces indicated by ( I ) , second-order bounding surfaces by (2), third-order bounding surfaces by (3), generalized foresets by dotted lines. (Outcrop in E-W traverse, Fig. 3C). Outcrops trending parallel to the palaeowind direction show bounding surfaces as idealized in Fig. 1 .

occur, however, along the traverse. Some horizontal strata change thickness or terminate. For example, horizontal strata at the 29m level at section 1 terminate abruptly downwind. The underlying first-order surface, however, remains prominent and can be traced to neaI section 2 where horizontal strata recur and thicken downwind to near section 4, where they thin to near zero. The first-order surface persists, however, and horizontal deposits recur between sections 4 and 5. Within cosets of cross- strata between first-order bounding surfaces, some individual sets of cross-strata persist across the traverse, but changes occur in preserved thickness (e.g. cosets between 4 and 9.5 m, section 1). Other cosets change character as individual sets break down into multiple sets (e.g. 30-33.5 m, section 1) or, conversely, multiple sets merge into a single, larger set (e.g. 16-18 m, section 5).

Significantly, most cosets of cross-strata, hori- zontal deposits, and first-order bounding surfaces clearly rise, although irregularly, in the downwind direction with respect to the basal formational boundary. In contrast, a conspicuous 1-4 mm thick shale drape, which was almost certainly deposited as one event, remains consistently at 26.5 m in the cross-section. The two lower sequences of dune cross-stratification, first-order bounding surfaces and horizontal strata can be traced upwind to their points of origin, where they merge with the basal Entrada transitional zone. Relations in the central part of the outcrop (16.5-25 m, section 1 ) are less clear. Thin sets of cross-strata or cosets of thin cross-strata begin, grow and terminate across the traverse. Indeed, these cross-strata appear dispersed in the overall horizontal bedding. Most strata in this central zone show a very gentle rise in the section,

although others are essentially level or even dip very gently downwind.

In marked contIast to the outcrop viewed in cross-section parallel to the palaeowind (Fig. 4A), the cross-section perpendicular to the palaeowind direction (Fig. 4B) shows first-order bounding sur- faces and overlying horizontal deposits to form lenticular bodies, with horizontal deposits thinning laterally to zero from a central maxima and first- order bounding surfaces ending laterally. Signifi- cantly, horizontal strata and first-order bounding surfaces appear more regular and show no strati- graphic rise, as they do in the cross-section parallel to palaeowind.

Interpretation The geometry of dune cross-strata, horizontal deposits, and first-order bounding surfaces shown in Fig. 4(A, B) seem explicable only as the result of downwind migrating and climbing dune bodies and interdune areas. This geometry is consistent with the climbing bedform model (Fig. 2B).

Accumulations of superimposed sets of cross- strata between first-order bounding surfaces are best explained as remnant deposits left by passing draas. As illustrated in Fig. 2B, cross-strata between two first-order bounding surfaces must represent a single dune body. If this body represents a simple dune without multiple slipfaces or superimposed dunes (e.g. a transverse ridge), the resultant deposit would consist of a single set of cross-strata. If, however, the dune body were not simple, but rather consisted of superimposed dunes (draas, as used here; see comment on pp. 754-759, the resultant deposit would consist of cosets of superimposed sets of cross-strata. The Entrada cross-strata were clearly

Interditne &posits and bounding sitrjuces 76 I

Fig. 6. Exposed second-order bounding surface in the Entrada Sandstone showing well-developed wind ripple marks oriented with their long axes parallel to the dune migration direction. Tilt of the surface is tectonic. (Area of E-W traverse, Fig. 3C.)

deposited by the latter type of dune body. This distinction does not define the type of dunes in- volved, but is valuable in defining the overall structure of the dune body. Horizontal strata are interpreted as interdune deposits, and first-order bounding surfaces as having formed by the migration of interdune areas.

Specific features illustrated in Fig. 4(A, B) seem clear with this interpretation. Changes in thickness and character of dune cross-strata and interdune deposits in outcrops parallel to the palaeowind direction (Fig. 4A) are expected, considering that these features result from the time-transgressive migration of draas and interdune areas. Wilson (1972a) estimates maximum migration rates of draas at a few centimetres per year. Clearly, then, thou- sands of years were required for a draa to migrate the 1.65 km length of the traverse (Fig. 4A); average conditions of sedimentation and draa structure are bound to change over such a period. For example, the disappearance and re-emergence of interdune deposits above the first-order bounding surface at 29 m in section 1, Fig. 4(A), may result from varia- tions in the rate of advance of the draa across the

interdune area, variations in sedimentation rates within the interdune, or scour of interdune deposits. The first-order surface persists as it was formed by the migration of the interdune area over truncated draa deposits, but was not dependent on any of the above variables. Variations in the angle of climb of draa deposits and first-order bounding surfaces are, strictly speaking, the result of net sedimentation rate and draa migration rate (Rubin & Hunter, 1982). Sedimentation and migration rates, however, are affected by rates of sand supply, sand drift patterns over the erg, and basin subsidence. The merging of the lower draa and interdune deposits and first-order bounding surfaces with the basal Entrada (Fig. 6A) represents the points of origin or at least the points where net sedimentation and climbing began for these draas and interdune areas on the prograding Entrada erg. In the cross-section perpendicular to the palaeowind direction (Fig. 4B), the more regular nature of first-order surfaces and interdune deposits are also predictable from theory (Fig. 2B), as this view almost shows individual interdune basin deposits at moments in time.

The central zone in the outcrop parallel to the

762 G. Kocurek

Fig. 7. Modern wind ripples on second-order bounding surface. Dunes shown are part of a larger transverse dune ridge- the interdune area is far behind the camera, ridge advancing slipface not shown. Wind rippled surface in the fore- ground overlies more dune deposits and is being buried by the advancing next dune superimposed on the overall dune ridge. If preservation of this dune ridge occurred, the foreground surface would constitute a second-order surface showing wind ripple marks, as in Fig. 6 (Padre Island).

palaeowinds, noted earlier in Fig. 4(A), may repre- sent local development of sheet sands where only small dunes occurred. More probable, however, is that this zone represents a single, very thick inter- dune deposit formed in an unusually large interdune area. Thin sets of cross-strata occurring in this zone, owing to their small size, discontinuous nature, and apparent occurrence within overall horizontal deposits, may represent small dunes occurring within the interdune area. Apparent irregularities in the angle of climb there may be a reflection of buried dune relief within the interdune area. The shale drape at 263 m (Fkg. 4A), noted earlier, records a rare event, being one of two such drapes found in the Entrada in the study area. It is similar to shales found in the laterally adjacent Preuss Formation marine deposits (Fig. 3A). The shale drape and the other water-lain deposits forming the upper part of this interval, suggest that part of the unusual thick- ness of this interdune deposit is the result of storm- tidal flooding of the interdune area.

As is evident from Fig. 2(B), an estimate of the

angle of climb can be made where the generalized depositional surface can be determined. The shale drape almost certainly forms a time plane and marks a flat depositional surface parallel to the top of the underlying Carmel Fcrmation (used as datum), which is generally planar in outcrop and is an inferred flat depositional surface upon which the Entrada erg encroached. Assuming, therefore, a flat general depositional surface-not an unreasonable assump- tion for a draa erg-the maximum angle of climb shown in Fig. 4(A) is 1.5". The minimum angle is measured in hundredLhs of a degree, the mode being several tenths of a degree. Clearly, climbing would not be obvious in a single outcrop!

FeatvTes of bounding surfaces

First-order bounding surfaces Although generally planar in a broad view, first- order surfaces are commonly very irregular on a smaller scale, with relief ranging from a few mi lk metres to a metre. The microtopography consists of

Interdune deposits and bounding surfaces 763

irregular, steep projections rising up to a few centi- metres into the overlying interdune deposits. Some of this relief is attributed to wind sculpturing of a damp surface, in which cohesion due to capillary tension allows steep, irregular forms to stand (Cooper, 1958, p. 149; Sharp, 1966; Carter, 1978). Such wind-sculptured surfaces are common on damp interdune floors on Padre Island; many are eroded foresets from the previous generation of dunes. More pronounced relief on first-order bounding surfaces is attributed here to scouring by water or air. Shallow, local channels overlain by water ripples or small subaqueous-type cross-sets mark some first-order bounding surfaces in the Entrada. Small scours on first-order bounding surfaces that are mantled by coarser lag grains or wind ripples were probably formed by wind deflation. Other first-order bounding surface irregularities may be the result of obstacles, such as plants, on the interdune basin floor. Irregularities on first-order bounding surfaces must form after draa migration, but before the surface was covered by interdune deposits, hence suggesting their exposure as free surfaces for long periods of time.

Second-order bounding surfaces Second-order bounding surfaces in the Entrada are far more regular than first-order bounding surfaces, with relief largely confined to wind ripples. Where erosion has exposed a second-order bounding sur- face in the Entrada, well-developed wind ripples are commonly visible (Fig. 6). Similar development of wind ripples is readily seen on surfaces between advancing slipfaces on modern dunes (Fig. 7). Significantly, after heavy rains on Padre Island, some such surfaces several metres high on well- wetted dunes were seen to hold water for over an hour, and to remain damp long afterwards. Con- ceivably, both water-lain and adhesion structures could develop on a second-order surface.

Second-order bounding surfaces, because of their occurrence as surfaces separating individual sets of cross-strata, as shown in Figs 1, 4 and 5, must mark the migration of distinct, foreset-producing separate dunes or slipfaces. Their occurrence within se- quences of cosets between first-order bounding surfaces (interpreted as draa deposits) indicate their formation by the migration of superimposed dunes over the larger draa body, as stated by Brookfield (1977). Wind ripple marks and other surface features on second-order bounding surfaces form during

surface exposure before burial by the next advancing dune or slipface. Nearly always, this period of time would be far shorter than that experienced by first- order bounding surfaces, largely explaining why surface relief is far less pronounced on second-order surfaces than on first-order bounding surfaces.

Third-order bounding surfaces Third-order bounding surfaces within the Entrada and recent cross-sets are generally smooth surfaces truncating previous foresets but parallel to overlying foresets. The orientation of these surfaces (Figs 1 and 5 ) suggests that they are reactivation surfaces marking stages in the advance of a single dune under a particular set of conditions, echoing an interpretation by McKee (1966a). Similarly, Brook- field (1977) suggests that minor fluctuations in velocity or direction of winds and bedform-airflow interactions produce third-order surfaces. Hunter (1977a, fig. 9, p. 383) argues that the limited lateral extent of such surfaces indicates their origin by shifts in the wind direction. An abundance of third- order surfaces in a cross-set would, therefore, indicate variable, shifting winds.

SEDIMENTARY S T R U C T U R E S OF I N T E R D U N E DEPOSITS

Introduction

Clearly not all horizontal strata occurring within aeolian cross-stratified units are interdune in origin. As noted earlier, some horizontal strata in the Entrada, especially near the margins of the forma- tion, are marine or fluvial in origin. Those strata cited here as interdune deposits occur well within the Entrada erg, where they invariably overlie first- order bounding surfaces and display a geometry consistent with recent interdune areas.

Sedimentary structures in Entrada interdune deposits show a wide variety, and appear to have formed not only by direct interdune sedimentation, but also by modification of primary structures by the loading accompanying migration of dunes over interdune deposits. Interdune deposits have been grouped into three catagories-dry, damp and wet- based on the overall interpreted surface condition under which they formed. Table 1 summariies structures found in both ancient and modern inter- dune deposits, including minor structures not discussed in the text.

764

v)

t

W

v)

a

LL \

W

3 t- V

a I- v)

t

3

a a I- z W I - 0 W

C. Kocurek

D R Y I D A M P I W E T w - W I N D RIPPLES+*-

a- EOLIAN DUNE C R O S S - STRATA+*- 3 - L A G G R A I N SURFACES+*-

a- D E F L A T I O N S C O U R S + * - B I O T U R B A T I O N S T R U C T U R E S +

P L A N T R O O T S T R U C T U R E S +

- S A N D D R I F T BEHIND OBSTACLES++-

-ADHESION LAMINAEt%

-MlCRoToPoGRAF’H’f+*-

-RAIN-IMPACT R I P P L E S + - - B R E C C I A T E D LAMINAE+*- -ADHESION RIPPLESt*-

-ADHESION WARTS+*-

E V A P O R I T E S T R U C T U R E S +

, A L G A L S T R U C T U R E S + * - F E N E S T R A L POROSITY+- - C O N T O R T E D S T R U C T U R E S * - -RILL MARKS+ -

- W A V Y LAMINAE+*

- CHANNELS+*

MODERN INTERDUNE D E P O S I T S + - S M A L L DELTAS+*-

E N T R A O A INTERDUNE DEPOSITS* - W A T E R RIPPLES+*- -SUBAOUEOUS CROSS-STRATA+*-

--WRINKLE MARKS+-

Table 1. Summary of sedimentary structures and other features characteristic of interdune deposits and the range of depositional conditions under which they form

Dry interdune deposits

Wind rippIe structures Wind ripple deposits (DR in Fig. 4) are common features in Entrada interdune deposits, as well as in the recent interdune areas studied. Within the Entrada, wind ripples occur mostly on the edges of lenticular interdune bodies in those sections nearer the palaeocoastline (sections 8, 9, 10, area of traverses, Fig. 3B), but constitute entire interdune deposits farther from the palaeocoastline (sections 11, 12, 13, 14, 15, Fig. 3B).

Morphologically, wind ripple deposits consist of two types. One type is thinly laminated (1-50 mm), rather continuous, commonly inversely graded, and shows few, if any, preserved foresets (Fig. 8). These are interpreted as downwind-climbing translatent strata formed by wind ripples receiving enough sediment, under rather uniform conditions, so that laminae are generated with translation of the ripples (Hunter, 1977a, b). This ripple deposit seems identi- cal to the ‘type a’ deposit described by Fryberger et al. (1979), which they attribute to relatively continuous deposition, as during a single sand storm.

A second, morphologically distinct type of wind ripple deposit consists of ripple forms up to 1 cm thick occurring isolated or in short trains, generally well armoured by coarser grains, and not forming the thin, continuous laminae characteristic of the

first type (Fig. 9). This non-climbing type of wind ripple deposit is interpreted as forming under less uniform and less sand-saturated conditions than the first type; the translating ripple did not deposit a continuous lamination, but rather has been ‘frozen’ in place. Stabilization may be a result of armouring by coarser grains, which allows the ripple to grow in height (see Bagnold, 1941, p. 151; Sharp, 1963), but makes it less easily translated so that it may be buried by subsequent deposition. Stabilization may also result from dew settling on the ripple forms (McKee, personal communication). This type appears to be the same as the ‘type b’ deposit of Fryberger et al. (1979), which they attribute to alternating deposition and erosion.

The presence of wind ripple deposits is very indicative of a dry interdune floor. It was seen repeatedly on Padre Island interdune areas that wind ripples will not form on a damp surface. Instead, wind-blown sand is trapped on the damp surface to form adhesion structures (see also Hunter, 1980; Kocurek & Fielder, 1982). Only after a blanket of dry sand, at least a few millimetres thick, is deposited do wind ripples begin to form (Fig. 10). Those Entrada interdune deposits that consist of wind ripples directly overlying a first-order bounding surface are interpreted as forming on a totally dry surface showing no evidence of a water table, hence constituting additional evidence that formation of

Interdune deposits and bounding surfaces 765

Fig. 8. Entrada interdune wind ripple deposits of the first type described in the text. Each individual lamina is generated by the translation of a single wind ripple. Only a few actual ripple forms present and foresets are essentially absent. (Area of E W traverse, Fig. 3C.)

first-order bounding surfaces is not dependent on the water table.

Aeolian dune structures Aeolian dune deposits (DD in Fig. 4), which are distinguished from subaqueous dune deposits by their stratification types (see Hunter, 1977a, b), are common in interdune deposits within the Entrada. These ‘interdune dune’ deposits are typically small (a few centimetres to 1 m thick), isolated single sets of cross-strata, but they also occur as thin cosets of several small sets. The latter had to be traced laterally to see that they are totally contained within an interdune deposit.

Small dunes crossing interdune floors are a common feature on Padre Island and some other modern dune fields (see McKee & Tibbitts, 1964; Fryberger et al., 1979). Unlike wind ripples, however, aeolian dunes can be active on damp surfaces. Although receiving little new sediment as most saltating grains were trapped on to the damp surface to form adhesion structures, small dunes were seen actively migrating over damp interdune floors on Padre Island.

Damp interdune deposits

Adhesion structures Adhesion structures form by the adhering of wind- blown dry sand on to a damp surface; they consist of adhesion r@ples, adhesion faminations, and adhesion wurts (see Reineck, 1955; Hunter, 1973, 1980; Kocurek & Fielder, 1982). Although sharing the same basic origin, these different types of adhesion structures are morphologically distinct, and imply differing depositional conditions (Kocurek & Fielder, 1982).

Adhesion ripples and pseudo-cross-lamination formed, by the upwind climbing of adhesion ripples are very common on interdune surfaces on Padre Island. They were seen forming in abundance after a rain, when afternoon drying winds blew sand from dune crests on to the interdune floors. Some adhesion ripples, fed by grainfall over dune crests, were seen climbing up lee faces of dunes, where they became buried by grain flows (Fig. 11). Adhesion laminations are just as common as adhesion ripples on interdune areas on Padre Island but adhesion warts are gener- ally rare.

766 G. Kocurek

Fig. 9. Entrada interdune wind ripple deposits of the second type described in the text. Ripple forms (indicated by arrows) preserved. In this case, ripple forms are armoured by coarser grains. (Section 11, Fig. 3B.)

Given the abundance of adhesion structures on Padre Island interdunes, their general absence in Entrada interdune deposits is puzzling. Judging from other interdune structures, Entrada interdune areas were often wet or damp and seemingly favour- able for adhesion structure formation. Only a few possible adhesion ripple, lamination or wart structures were found.

Wet interdune deposits

Water ripple structures Water ripple structures (WR in Fig. 4) are common in Entrada interdune deposits, especially near the palaeocoastline (sections 8, 9, 10, 18, area of traverses, Fig. 3B). Both symmetric (Fig. 12) and asymmetric ripples (Fig. 13) occur, the latter being predominant and very commonly show climbing structures. The abundance of current ripples in interdune deposits was initially puzzling. Their presence indicates directed current flow and suggests wadi deposits (Glennie, 1970), yet the geometry of ripple-bearing interdune deposits does not seem

different from interdune deposits without current ripples, nor is any channel system evident. The origin of many of the water ripple structures in interdune deposits, however, was apparent during a visit to the Padre Island dune field in a particularly rainy period when, within flooded interdune basins, water ripple structures were being formed in great abundance. Symmetrical ripple marks were confined to the water’s edge, where small wind-generated waves lapped on to the interdune pond ‘shoreline’. Similarly, a variety of interference ripples were present along interdune pond edges where run-off and waves produced interfering sets. Current ripples, ranging from low-energy, straight-crested types to higher-energy, linguoid types, were very abundant but were confined to two distinct settings. First, sediment-charged run-off from dunes and interdune basin edges produced climbing current ripples. Secondly, cur- rent ripples occurred in local, very ephemeral channels, which formed in response to the filling and draining of interdune basins. Initially, the interdunes were fully enclosed basins separated

hterdune deposits and bounding surfaces 761

Fig. 10. Modern interdune floor. Dark surface (A) is damp and lacks wind-ripple deposits. Saltating grains striking the damp surface adhere and form adhesion laminations. Advancing from the left is a sheet of dry sand. Wind ripples do not form on the frontal edge of this dry sand sheet (B), and wind ripple formation begins only after a sufficient thickness of dry sand is deposited (C).

from each other by dunes. When heavy rains filled the basins to depths of several tens of centimetres, a break would sometimes develop across the dune barrier and a channel would be cut (Fig. 14). Channelling was exceedingly rapid in the loose dune sand. One channel nearly 1 m deep, over 3 m wide and floored by profuse ripples, was cut in a few minutes.

By this channelling process, a series of interdune basins would be connected. Once excess water in an interdune basin was released, channels sealed themselves. Most channel floors were covered with ripples, but larger channels showed a braided channel pattern with small, ripple-covered bars. Where the channel opened into the adjacent inter- dune area, small deltas would form (Fig. 15). Some tabular climbing-ripple structures seen in Entrada interdune deposits probably formed as small deltas (Fig. 16).

Many of the ripple structures in Entrada inter- dune deposits, therefore, could have resulted from heavy rains with no recourse to wadi or tidal flooding being necessary. This is not to deny that wadi or iidal

flooding did not occur in the Entrada erg, as such deposits have already been noted as occurring in erg-edge deposits and in the continuous shale drape of Fig. 4(A), respectively. The origin of some laterally extensive zones of ripple structures in Entrada interdune deposits is less clear, as these are unlikely to be the result of channelling or surface run- off. Analogous extensive surfaces with water ripple marks have been seen on modern interdune areas at Padre Island and coastal dune fields of Oregon (Hunter, personal communication). The origin of these extensive rippled surfaces may be by wind-drjven currents within a flooded interdune basin.

Wavy laminations Some Entrada interdune deposits show wavy laminations (WW in Fig. 4) suggestive of water-lain deposits, but commonly without well-developed ripple structures (Fig. 17). Most wavy laminae occur intercalated with well-developed water ripple struc- tures or grade laterally or downward to water ripple structures. Some wavy laminae are distinctly in-

768 G. Kocurek

Fig. 11. Adhesion ripples (A) climbing up the damp slipface of a dune following a rain. These adhesion ripples were receiving a generous supply of sand from grainfall (deposits above A on slipface) over the dune crest. Grain flow deposits (avalanche tongues) ha\? already buried a similar surface of adhesion ripples on the left side of the lee slopes (Padre Island).

Fig. 12. Symmetrical (oscillation) ripple structures in an Entrada interdune deposit. (Area of E-W traverse, Fig. 3c.I

Interdune deposits and bounding surfaces 769

Fig. 13. Climbing asymmetrical (current) ripple structures in an Entrada interdune deposit. (Section 22, Fig. 3B.)

Fig. 14. Small channel cut across dunes to connect two flooded interdune basins. Once the channel was cut, water level in the higher basin (lower left) fell rapidly as indicated by water level marks in the foreground (Padre Island).

49 SED 28

770 G. Kocurek

Fig. 15. Small delta formed at the end of a channel connecting two flooded interdune areas (Padre Island).

Fig. 16. Climbing tabular cross.sets in an Entrada interdune deposit showing well-formed topset and foreset deposits, indicating deposition during rising water level. These structures probably were formed by a prograding small delta, as in Fig. 15. (Section 8, Fig. 3B.)

Interdune deposits and bounding surfaces 771

Fig. 17. Wavy laminations, here largely in-phase, in an Entrada interdune deposit. (Area of E-W traverse, Fig. 3C.)

phase and, although resembling some algal mat deposits, are thought to represent vertically climbing ripple laminae-in-rhythm.

Other, more poorly formed wavy laminae are probably the result of modified ripples formed during falling water or the result of weak, inter- fering currents. Possible analogous features were seen on some Padre Island flooded interdune surfaces where large areas were covered with irregular, poorly defined water ripples, but with falling water their forms became modified. Others seemed to origi- nate in very shallow water by interfering, weak currents.

Contorted structures Some Entrada interdune deposits are characterized by various contorted structures (WC in Fig. 4) consisting of irregular swirls, broad to narrow over- steepened folds, near-vertical fluid escape pipes, and more rarely, ball and pillow structures (Figs 18 and 19). These structures show a pronounced regional occurrence near the palaeocoastline (sections 8, 9, 10, 18, area of traverses, Fig. 3B). Contorted struc- tures may occupy the total thickness of an interdune deposit or form discrete horizons intercalated with

uncontorted, interpreted wet interdune structures. Where they occupy the laterally central part of interdune lenticular deposits, they usually grade into brecciated structures (discussed below) toward the edges of the interdune deposit. Where they occur in a position between the centre and edge of an interdune deposit, they tend to grade to more central water-rippled or wavy-laminated deposits.

Contorted structures may originate in interdune deposits by several mechanisms. Some structures are clearly penecontemporaneous deformation struc- tures. Ball and pillow structures (Fig. 19) are similar to late-stage flood deposits described from fluvial deposits (McKee, 1966b, p. D98; McKee, Crosby & Berryhill, 1967, figs 7a, b and 9) and represent inter- dune floods. Other contorted structures, however, may be the result of loading by dune encroachment over water-saturated interdune deposits (Fig. 18). Deformational structures formed in this way have been noted previously (Brown, 1969; Jones, 1972). Load deformation, however, occurs mostly where a pronounced textural difference exists between the overlying aeolian dune sand and the underlying saturated sediment. In the Entrada, grain size contrast between dune and interdune deposits is

49-2

772 C. Kocurek

Fig. 18. Contorted bedding in an Entrada interdune deposit. Such deposits are commonly characterized by broad to narrow folds, irregular swirls, and fluid-escape pipes (arrow). (Area of traverse, Fig. 3C.)

Fig. 19. Ball and pillow structures in a zone of contorted bedding within an Entrada interdune deposit. (Area of N-S traverse, Fig. 3C.)

Interdune deposits and bounding surfaces 773

Fig. 20. Peel showing abundant fenestral porosity characterizing the sediment beneath algal mats on modern wind tidal flats. Fenestral porosity consists of millimetre-sized pores, apparently gas-generated by mat decay, and larger porous chunks of decaying algal material (arrows). Area bracketed shows a small (aeolian?) cross-set partly obliter- ated by fenestral pore development. Peel is about 30 cm high (Padre Island).

minor, being entirely within the sand range. This lack of a dramatic grain size contrast and the occur- rence of contorted structures in discrete zones argues strongly for a specific precursor fabric that was very susceptible to liquefaction and contortion upon loading. Indeed, all inter dune deposits sandwiched between dune-deposited cross-strata have necessarily been overridden by dunes, yet most Entrada inter- dune deposits are undeformed; deformed ones are restricted in their regional and stratigraphic occur- rence.

One plausible precursor fabric is the abundant fenestral fabric associated with algal mats. This inter- pretation stems from observations on Padre Island, where it was noted while walking that most inter- dune deposits, including subaqueous ones, are quite firm. Interdune and wind-tidal flat surfaces covered by algal mats, however, experienced much vigorous degassing and fluid escape when loaded. Cores taken through surfaces covered by algal mats show a significant amount of fenestral porosity (Fig. 20). Laterally from the central, lushest algal growth,

fenestral porosity decreases markedly to near ab- sence beneath surfaces covered with only brecciated mat crusts. This type of abundant fenestral porosity is almost never preserved in ancient sandstones but could serve as a precursor fabric that gives rise to contorted strata developed with unequal loading by dune encroachment from one side. Presumably, structureless interdune deposits might originate through total collapse and liquefaction upon loading of sands with much fenestral porosity.

An additional cause of deformatic- might be the dissolut,ion of interdune evaporites during early diagenesis. No direct evidence for evaporites, however, was seen in Entrada interdune deposits.

Brecciated laminae Brecciated laminae (WB in Fig. 4) are thin, wavy laminae showing a spotty or brecciated appearance (Fig. 21). Laminations may be slightly contorted into gentle folds, but deformation is rarely pronounced. As with contorted structures, multiple causes of brecciated laminae may occur. Some brecciated

114 G. Kocurek

Fig. 21. Brecciated laminae in an Entrada interdune deposit. (Area of E-W traverse, Fig. 3C.)

laminations may originate as surfaces of algal crust chips such as those occurring along the edges of lusher algal mats. This interpretation is enhanced by the occurrence of brecciated laminated structures lateral to the central contorted structures if those contorted structures did originate from a fenestral precursor fabric associated with algal mats. Load deformation of algal crust deposits would be less severe because these sediments have far less fenestral porosity than do those beneath more lush mats.

Brecciated laminae also may result from broken crusts of salts or salt-cemented sand, or from cohe- sive flakes of sand, as would result from a heavy dew or light rain (see McKee, 1945, 1979b).

Variations in interdune deposits

Vertical sequences Most Entrada interdune deposits show complex vertical sequences; some having a seemingly syste- matic vertical gradation, but others reflecting a series of unrelated events. Some simple drying-upward sequences occur, as in interpreted wet deposits grading upward to damp deposits, thence to dry deposits. Wetting-upward sequences are less com-

mon. A few interdune deposits, especially those consisting of contorted or of wind-rippled structures, remain unchanged in character throughout their vel tical thickness. A very common pattern, and one used as a separate class in Fig. 4 is the interlamina- tion of brecciated laminations and wind-ripple deposits (WB/DR in Fig. 4). Such sequences may represent wet or damp surfaces alternating with dried surfaces marked by wind-ripple deposits. A few interdune sequences show exceedingly complex histories, as in Fig. 22.

Observations on Padre Island showed that pro- cesses operating in interdune areas could change rapidly and differ over a single interdune area. Indeed, the very nature of interdune basins favours the simultaneous occurrence of multiple processes within single interdune basins or in adjacent inter- dunes basins depending on the exact location within the interdune and the extent of filling of the basin. For example, on a windy, drying afternoon following a heavy rain, channels, algal mats, current and oscillation ripples, adhesion ripples and adhesion laminae, and wind ripples were all seen to be forming in interdune areas simultaneously. Interdunes are ephemeral basins not characterized by any one

Interduiir deposits and bounding surfaces 775

250 cm

200

150

100

5 0

0 ’ . I I I I

0 50 100 150 200 2 5 0 c m

Fig. 22. Sketch of a portion of an interdune deposit in the Entrada Formation showing a very complex history of deposition. The top of an underlying dune deposit is scoured. Lower portions of the scour are filled by water ripple interdune deposits (WR). These are overlain by wavy laminae (WW), which are capped by brecciated laminae (WB). Upward, a complex record of small aeolian dune (DD) and wind ripple deposits (DR) occurs, interrupted by numerous shallow scours and water-lain deposits. (Area of E-W traverse, Fig. 3C.)

sequence (model) of filling, but rather the structure is simply a function of what events occur during the lifespan of the interdune. On Padre Island, where interdune areas commonly begin wet and become progressively drier as filling proceeds, a n idealized vertical sequence is predictable (Fig. 23). Numerous cores in these interdune areas, however, showed that this drying-upward sequence is only rarely fully realized as many disruptive events occur. Idealized model sequences for interdune basin fillings are, therefore, far less reliable than sequences for some other types of sedimentation (e.g. Bouma sequence). Significantly, many of the sedimentary structures characterizing interdune deposits (e.g. climbing, water-ripple structures, channels, deltas, and ball and pillow structures) indicate flood conditionsand subse- quent rapid fall-out of sediment. As with many other ecvironments, some interdune deposits may be biased toward the rare storm event, rather than daily events.

Lateral variations

The types of interdune deposits shown in Fig. 4 are generalized, but despite these generali7ations

p/A T RAN SG RE S S I V E DUNE FORESETS

WIND RIPPLES AND/OR SMALL EOLIAN CROSS - STRATA

ADHESION LAMINAE

ADHESION RIPPLE PSEUDO-CROSS-STRATA A L G A L MAT STRUCTURES FENESTRAL POROSITY WATER RIPPLES

TRUNCATE D DUNE FORESETS

Fig. 23. Idealized drying-upward sequence of structures associated with progressive filling of an interdune basin deposit on Padre Island. Water ripple and algal structures may characterize lower interdune deposits if sufficient currents and long-standing water occur, respectively. Continued filling results in the formation of damp surfaces upon which adhesion ripples form. Dry interdune de- posits characterize the later stages of interdune deposition before the interdune sequence is terminated by the next advancing dune.

a significant contrast in cross-sections parallel t o and perpendicular to the palaeowind direction can be seen. In Fig. 4(B), wherein interdune deposits are shown across the interdune basins essentially along time lines, structures differ laterally in a consistent and meaningful manner. In these inter- dune deposits located near the palaeocoastline, the general trend is for wet deposits to occupy the interdune lens centre, and to grade laterally to damp deposits, and finally to dry deposits a t the edge of the lens. For instance, in the lower interdune deposits in Fig. 4(B), a central facies, which consists of alter- nating brecciated laminae and wind ripple deposits, consistently changes to exclusively wind ripple deposits on the edges of the lens, indicating periodic wetting and drying in the centre, but prevailing dry conditions on the flanks. Other such traverses, as across the interdune lens a t 19 m in section 15, show that water ripples and wavy laminations in the lens centre grade laterally to brecciated laminations, to intercalated brecciated laminations and wind ripple deposits, and finally to wind ripple deposits on the edges of the lens.

In contrast, interdune deposits viewed parallel to

776 G. Kocurek

palaeowind direction and across the time planes (Fig. 4A) either remain fairly constant in type or change in a less orderly manner than interdune deposits in views perpendicular to the palaeowind direction (Fig. 4B). These contrasting views of interdune lenses are fully consistent with some recent interdune basins, such as those studied on Padre Island, and supportive of their origin as deposits formed within migrating, climbing interdune basins. At Padre, interdune areas that are periodically flooded show a concen- tration of water-lain structures in their lower centres, damp structures toward their flanks, and predominantly dry structures on their higher edges. This scheme is very similar to that in the Entrada (Fig. 4B). Interdune deposits in a time-transgressive view (Fig. 4B) do not show a sequence across an inter- dune at a single time but show the general conditions through time as the interdune basins migrated down- wind. Thus, the two contrasting cross-sections to- gether give both views of Entrada interdune deposits at an instant in geological timeandacross time planes.

Interdime varieties

Certainly those structures occurring in the Entrada and other interdune deposits described here do not cover all of the variations occurring in interdune deposits. Interdune deposits can differ in sediment type alone. Triassic-Jurassic Nugget-Navajo inter- dune deposits tend to contain more mud than do those of the Entrada and some contain burrows (Dott, 1979), which are notably missing from the Entrada. Hanley & Steidtmann (1973) describe ostracodal-peloidal-fenestral limestone lenses in the Permian Casper Formation and attribute these to freshwater shallow ponds. Some of these lenses might be interdune deposits; also some limestone beds in the Pennsylvanian Weber (Fryberger, 1979) and Jurassic Navajo formations (Pipiringos & O’Sullivan, 1975) may be interdune deposits. Evap- orites characterize some interdune deposits form- ing interdune sabkhas. The Permian Rotliegends of north-west Europe, described by Glennie (1 970, 1972) and Nagtegaal (1973) is an ancient example.

The additional factors of fauna and flora are important in some eoIian settings, especially in interdune areas where water may be more abundant than on dunes (see Ahlbrandt, Andrews & Gwynne, 1978; Glennie & Evamy, 1968). It is not clear when animals and/or plants first became adapted to the arid, aeolian environment. With the exception of possible algal structures, no plant or animal remains or definite traces were seen in the Entrada Sandstone,

but these are well known from the aeolian Permian Coconino, Permian Lyons, Triassic-Jurassic Navajo- Nugget and Permian Cedar Mesa Sandstone (see Loope, 1980).

ERG RECONSTRUCTIONS BASED UPON BOUNDING SURFACES AND

INTERDUNE DEPOSITS

Bounding surfaces

The extent of the development of the hierarchy of bounding surfaces in an aeolian deposit is significant as it will reflect the extent of the development of the hierarchy of bedforms present in the erg. Given a series of downwind-migrating dune bodies separated by interdune areas, with first-order bounding surfaces forming with interdune migration (Fig. 2B), then the cross-strata between two successive first-order bounding surfaces record a single dune body. Where that body consists of a simple, unidirectional dune, such as a simple crescentic dune, the resultant de- posit will show, in general, only a single set of cross- strata between two first-order bounding surfaces. Third-order bounding surfaces could occur within the set of cross-strata. Part of the Navajo Sandstone in the Zions area shows this situation, and may reflect dunes similar to those of the coastal Namib erg of south-west Africa, which is characterized by simple crescentic dunes.

Where the dunes on an erg are compound or complex (draa as used here) such as compound crescentic dunes, the resultant deposit will show cosets of cross-strata, between first-order bounding surfaces with second-order bounding surfaces separating individual sets of cross-strata. Again, third-order bounding surfaces could occur within sets of cross-strata. Most of the Entrada Sandstone, where three orders of bounding surfaces are charac- teristic, appears to be such a draa deposit. Adjacent to the eastern terminus of the Entrada (sections 17, 21, Fig. 3B) and in a band along the palaeocoastline (sections 4, 5 , 6, 7, 18. Fig. 3B), only two orders of bounding surfaces occur. Here draa development is not indicated; simple dunes prevailed.

The shape of first- and second-order bounding surfaces also can be used to deduce interdune geometry and dune types. As shown in Fig. 4(B), Entrada first-order bounding surfaces and overlying interdune deposits define fully enclosed interdune basins, such as those in the modern Mar7Ciq erg,

Interdune deposits and bounding surfaces 777

Libya. As shown in Fig. 5, second-order bounding surfaces in the Entrada are trough-shaped, a geo- metry most likely formed by crescentic or barch- anoid dunes. Entrada draa erg structure, therefore, probably consisted of compound crescentic dunes, such as those in the modern eastern Rub’al Khali, Saudi Arabia, separated by fully enclosed interdune areas.

As evident from the model for unidirectional dunes (Fig. 2B), the intersection of first-order surfaces with any time line or generalized depositional surface in a cross-section parallel to dune migration direction will yield an estimate of dune wavelength on the erg surface. In the cross-section parallel to the palaeo- wind direction for the Entrada (Fig. 4A), the distance between points of intersection of the two lower first- order bounding surfaces with the base can be measured directly, yielding a wavelength of 1600 m. More speculatively, the projection of subsequent first-order surfaces along their general upwind dip direction to points of intersection with the assumed near-horizontal depositional surface in Fig. 4(A) yields wavelengths from 900 to 3000m. These wavelengths fall within the range for recent draas observed by Wilson (1972a) to range between 500 and 5000 m. Simple dune wavelengths reported by him are shorter, between 10 and 500 m.

Development and geometry of bounding surfaces in major dune types besides the unidirectional crescentic dunes described here cannot be deter- mined at present. Migration paths and internal structuie of linear and star dunes are largely un- known. Indeed, even in unidirectional, crescentic dune ergs, the shape of the interdune area can vary greatly, hence resulting in varied geometries of the first-order bounding surfaces.

The rock record, however, is probably biased toward preservation of draas. Simple, isolated barchan dunes occur along erg margins where sand availability is low. These are followed inward by longitudinal forms in areas of greater but still discontinuous sand cover, to various complex draas characterizing the erg centre with thick, continuous sand cover (Wilson, 1972a). Very few examples of longitudinal or isolated barchan dunes exist in the geological record. Draa deposits are surely the most preservable as they form on the thickest accumula- tion of sediment and commonly occur in the centres of subsiding aeolian basins. Wilson (1972a) states that a draa erg will develop given sufficient time for nucleation (estimated at 10,000 y) and sufficient loose sand cocer (estimated at 3 m). Several other

mechanisms, however, may inhibit draa develop- ment. For example, an extensive protective armour- ing pebble surface or a sufficiently high water table would limit the thickness of sediment available for transport and inhibit draa growth. Similarly, periodic disruptive events such as extensive flooding or fluvial activity could affect the time needed for draa nucleation.

Interdune deposits

As noted above, the thickness of Entrada interdune deposits ranges from a few centimetres to a few metres. The structures present are a function of dune- field location, rate and type of sedimentation, climste and other factors. Interdune deposits are, therefore, accurate indicators of the overall deposi- tional environment. Clearly, they are far more sensitive indicators than dune structures, which are less variable with changing conditions. In addition, interdune deposits are more likely to be preserved than deposits of dunes, which are topographic highs. Entrada interdune deposits indicate that the Entrada erg near the palaeocoastline (sections 8, 9, 10, 18, area of traverses, Fig. 3B) was frequently flooded. as indicated by the abundance of wet, interdune de- posits. Flooding may also have been rapid at times, probably by storms, as indicated earlier. Farther inland (sections 11, 12, 13, 14, 15, Fig. 3B) interdune areas were dry, as indicated by the geneial absence of wet or damp interdune deposits and the abundance of dry interdune deposits. This conclusion is not evident from the dune deposits.

Where the interdune deposits consist of wind- blown sand, the thickness of the deposit is likely to be a sensitive indicator of sand diift saturation level (the ratio of the actual transport to the transporting capacity of a given wind velocity). Thin to absent interdune deposits result from low sand drift saturation levels or actual deflation. For example, some interdune areas, such as those of parts of the Algodones Chain, southeastern California (Norris & Norris, 1961), and the Little Sahara Desert, Utah, are barren and are covered only by advancing dunes. Dune bodies tend to conserve their shape during migration and act as sand traps (Wilson, 1972a; Howard et a/., 1978). As a result, where sand is not abundantly available. interdune areas are likely to be barren of actual interdune deposits- interdune areas essentially receive only the residual scdimcnt, as noted by Wilson (1973).

778 G. Kocurek

C O N C L U S I O N S

Based upon comparison of the ancient Entrada Sandstone and modern ergs, the following conclu- sions are forthcoming: (1) Alternating horizontal strata and cross-stratified deposits in the Entrada are not random in their structure, but reflect systematic deposition by successive, downwind- migrating and climbing interdune areas and dune bodies consisting of superimposed bedforms (draas). A general model for the migration of dunes and inteidune areas can be generated (Fig. 2B), but as this model is based on the slow migration of large- scale bodies, it is especially subject to complications such as changes in the depositional factors that are bound to occur through time.

(2) Bounding surfaces record the growth and geometry of the hierarchy of superimposed bed fox ms. Based upon this study, the geometry of first-order bounding surfaces is not compatible with an origin as water table surfaces (Fig. 2A). Instead, the surfaces mark the migration of interdune areas. Second-order bounding surfaces arise from the migration of superimposed dunes over larger draas. Third-order bounding surfaces represent reactiva- tion faces on individual dunes.

(3) Horizontal strata overlying first-order bound- ing surfaces are interpreted as forming within migrating interdune basins. The geometry and the lateral variations of these deposits, viewed in cross- sections parallel and perpendicular to the palaeo- wind direction, indicate their origin as interdune deposits. Sedimentary structures and their distribu- tion in Entrada interdune deposits are very similar to those seen in recent interdune areas, and conform to the climbing bedform model (Fig. 2B). (4) Interdune deposits are the most variable

aspect of the erg and, therefore, are the key indi- cators of specific depositional conditions; dune structures are less sensitive to varying environmental factors.

( 5 ) New, detailed aspects for the interpretation of aeolian deposits arise from insights into bounding surfaces and interdune deposits. Some possible interpretations that become feasible are the extent of the development of hierarchial bedforms on the erg, bedform shape, type, angle of climb and wave- length, sand drift saturation level, and overall depositional conditions.

(6) Clearly, in some aeolian settings, the water table does control the level to which deflation can occur and, hence, influence the net rate of sedimenta-

tion and the amount of loose sediment available for transport. The water-table-controlled hypothesis does, however, require very specific conditions. It is not compatible with deposits studied here, is not realistic in its portrayal of erg structure, and has some inherent mechanical problems, such as ‘sand disposal’ and inability to account for migrating, climbing bedforms. It does not emerge as the general model for aeolian deposits. Bounding surfaces necessarily arise by the migration of bedforms, and they will develop regardless of the water table.

ACKNOWLEDGMENTS

This study constitutes part of my doctoral thesis at the University of W:.consin under the direction of R.H. Dott, Jr, whose guidance and reading of this manuscript are greatly appreciated. The manuscript was also critically reviewed by E.D. McKee (U.S.G.S., Denver), R.E. Hunter and D. Rubin (both of the U.S.G.S., Menlo Park), L. Pray and- C.W. Byers (both of the University of Wisconsin), and H.S. Chafetz (University of Houston). All of the above made very valuable suggestions toward improving this paper. I am indebted to R.E. Hunter and D. Rubin for discussions in the field and for introducing me to aspects of climbing bedforms. Traverses of the Entrada Sandstone were made with the able assistance of C. Ptacek and W. DeMiss (both of the University of Wisconsin). Figures were drafted by my wife, Dianna. This study was gener- ously supported financially by Atlantic-Richfield Co.

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(Manuscript received 30 M a y 1980; revision received 13 November 1980)