E L S E V I E R Sedimentary Geology 90 (1994) 179-211

SEDIMENTARY GEOLOGY

Architectural element analysis within the Kayenta Formation (Lower Jurassic) using ground-probing radar

and sedimentological profiling, southwestern Colorado

Mark Stephens *

Department of Geology, University of Toronto, 22 Russell St., Toronto, Ont. M5S 3B1, Canada

(Received February 23, 1993; revised version accepted September 6, 1993)

Abstract

A well exposed outcrop in the Kayenta Formation (Lower Jurassic) in southwestern Colorado was examined in order to delineate the stratigraphy in the subsurface and test the usefulness of ground-probing radar (GPR) in three-dimensional architectural studies.

Two fluvial styles are present within the Kayenta Formation. Sandbodies within the lower third of the outcrop are characterized by parallel laminations that can be followed in the cliff-face for well over 300 m. These sandbodies are sheet-like in appearance, and represent high-energy flood deposits that most likely resulted from episodic floods. The remainder of the outcrop is characterized by concave-up channel deposits with bank-attached and mid-channel macroforms. Their presence suggests a multiple channel river system.

The GPR data collected on the cliff-top, together with sedimentological data, provided a partial three-dimen- sional picture of the paleo-river system within the Kayenta Formation. The 3-D picture consists of stacked channel-bar lenses approximately 50 m in diameter.

The GPR technique offers a very effective means of delineating the subsurface stratigraphy. Its high resolution capabilities, easy mobility, and rapid rate of data collection make it a useful tool. Its shallow penetration depth and limitation to low-conductivity environments are its only drawbacks.

1. Introduction

Over the past two decades, ground-probing radar (GPR) techniques have been used to solve many problems where high resolution imaging to depths of 30 m was required. G P R has been used with great success in mineral and groundwater exploration, geotechnical and archaeological in-

* Present address: 152 Baywood Court, Thornhill, Ontario L3T 5W3, Canada.

Elsevier Science B.V. SSDI 0037-0738(93)E0117-X

vestigations, the delineation of rock fabrics, and mine development. Some specific application ex- amples include mapping bedrock depth (Annan and Vaughan, 1982), fracture detection in bedrock and mapping of soil strata (Davis and Annan, 1986), and the location of old mine stopes (An- nan, 1988).

Early at tempts at facies analysis relied on ver- tical profile analysis and comparisons with facies models to explain ancient deposits. This method has limited usefulness as it can not adequately explain three-dimensional variations in composi-

180 M. Stephens / Sedimentapy Geology 90 (l 994) 179--211

~. ,MRVEY SITE " " ......... ' ......... ~OLORADO 1

108*49' 30" 108'49' 00" 108"48' 30"

4 0 0 m Contour Interval - 100 ft. s

Fig. 1. Location of survey area along the Dolores River in the Red Canyon area.

M. Stephens / Sedimentary Geology 90 (1994) 179-211 181

tion and geometry. Miall (1985), building on ear- lier work by Allen (1983), proposed a new method of facies analysis, known as architectural-element analysis. It subdivides fluvial deposits into a suite of basic three-dimensional architectural ele- ments, including channels, gravel bars and bed-

forms, sandy bedforms, downstream-accreting de- posits, lateral accretion deposits, sediment gravity flow deposits, laminated sand sheets and over- bank fines. Analysis also includes the delineation and classification of bounding surfaces, which subdivide the rocks into a hierarchy of deposi-

~Um~ok~Oc~ ~udie8 Dvmled channel in this me in ctiff face

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• -Direction in which line survey was conducted Fig. 2. Site of survey area showing the location and orientation of the radar lines. Lines are numbered.

182 M. Stephens / Sedimenta~ Geology 90 (1994) 179-211

tional units. This method has led to a greater understanding of the composition and architec- ture of deposits of diverse origin (Miall, 1988a, b; Bromley, 1991; Cowan, 1991; Miall and Tyler, 1991).

This research makes use of GPR and architec- tural-element analysis in order to elucidate the three-dimensional geometry of elements within a fluvial sandstone unit in the Kayenta Formation (Lower Jurassic) along the Dolores River Canyon, southwestern Colorado. A detailed sedimentolog- ical lateral-profiling study on a well-exposed cliff-face involved the gathering of paleocurrent data as well as the positioning of all bounding surfaces. A GPR survey was conducted immedi- ately behind the cliff-face in order to compare bounding surfaces visible in the cliff-face with those detected by GPR. Data collection required eight weeks of fieldwork in July-August 1990 (Stephens, 1992).

This study has three primary objectives: to test the usefulness of ground-penetrating radar in three-dimensional architectural studies; to delin- eate the subsurface; and to construct three-di- mensional pictures of the sandbodies or elements within the Kayenta Formation.

2. Location

The study area is located in southwestern Col- orado, approximately 48 km northwest of Natu- rita, in an area known as Red Canyon (Fig. 1). The outcrop under study lies on the west side of the Dolores River and is paralleled by Highway 141. Most of the area is accessible by State High- way 141 and a system of dry-weather roads (dashed lines in Fig. 1).

stratigraphy within the Kayenta Formation. Lines 1 through 4 were located on a downhill slope perpendicular to lines 5 through 7.

4. Kayenta Formation

The Kayenta Formation (Lower Jurassic age) is part of the Glen Canyon Group which com- prises, in ascending order, the Wingate sand- stone, the Kayenta Formation, and the Navajo sandstone (Blakey et al., 1988). The Wingate and Navajo sandstones both consist of large sweeping tangential cross-beds, indicating an eolian origin. The Kayenta Formation, on the other hand, is a continental redbed assemblage which represents the deposits of a fluvial system. The formation occurs over wide areas in southeastern Utah, western Colorado, northeastern Arizona, and the southern part of Nevada (Luttrell, 1986). Harsh- barger et al. (1957) divided the Kayenta into two distinct lithologic facies; the sandy and the silty facies. The sand facies, which occurs in the study area, crops out in a series of benches and ledges between the Navajo and Wingate sandstones, which are eolian in nature. This facies consists of a pale red-purple to pale reddish-brown fine- grained quartz sandstone interbedded with grey- ish-red mudstone. It also contains considerable quantities of red siltstone, thin bedded shale, and conglomerate. Individual sandstone beds are lenticular and discontinuous and interfinger with shale. The upper part of the Kayenta Formation is softer an weathers back to form a broad bench. The lower part is more firmly cemented forming resistant, thick ledges that protect the underlying Wingate sandstone from erosion.

3. GPR survey

The test area consisted of eight radar survey lines (Fig. 2). Radar data were acquired both stratigraphically above (test line 0) and within (lines 1-7) the Kayenta Formation. Test line 0 was erected on an Entrada sandstone outcrop approximately 6 m thick in order to see if the GPR would be able to penetrate it and detect the

5. Principles of ground-probing radar

The GPR technique is quite simple and is similar in principle to reflection seismic and sonar techniques (Annan, 1988). The GPR system con- sists of four main elements. These are the trans- mitting, receiving, control, and display units. A block diagram of the GPR system is shown in Fig. 3. The transmitter generates a high frequency EM pulse (10 to 1000 MHz) to the transmitting

M. Stephens /Sedimentary Geology 90 (1994) 179-211 183

I DISPLAY I AND

RECORO I

CONTROL UNIT ( TIMING )

TRANSMITTER RECEIVER

ANTENNA ANTENNA I I Fig. 3. Schematic diagram of radar system hardware.

antenna and radiated into the ground. Once transmitted, the receiver monitors the return en- ergy versus delay time. The pulse radiates out- wards from the antenna and travels at the EM wave velocity of the medium through which it passes. Whenever it encounters a change in elec- trical properties (associated with changes in stratigraphy, water content or rock type), part of the radar signal is reflected. The receiving an- tenna detects the reflected signals and transmits them to the receiver where the signals are ampli- fied, digitized and stored on disk. Overall opera- tion and control of the pulseEKKO IV is pro- vided by any IBM PC computer. The data can be displayed on the computer while the survey is being conducted. The transmitter and receiver units are linked to the control unit by fibre optic cables. They can also be interchanged with vari- ous types of antennas in order to vary the operat- ing frequency of the radar system.

6. Radio wave propagation and electrical proper- ties

The propagation velocity of EM waves in soils varies between the speed of light (0.3 m/ns ) and 1/10 of the speed of light (Annan and Vaughan,

1982). The propagation of the radar signal de- pends on the dielectric and conductive properties of the ground. The dielectric constant (K) is a term which describes the ease with which a radar signal can pass through a medium. Both factors are primarily controlled by the water content of the soil (Davis and Annan, 1986). The higher the water content, the lower the EM wave velocity. In dry soils, propagation velocities are typically around 0.1 m/ns . In wet soils, velocities are normally in the range of 0.05-0.07 m / n s (Annan and Vaughan, 1982).

In materials with low electrical loss, the veloc- ity and attenuation are related to the dielectric constant K, and conductivity by the expressions:

c 0.3 V g l / 2 K1/2 m / n s

and:

(1.69 x 103)o . a = K1/2 d B / m

where o. is the conductivity, K is the dielectric constant of the material, and c is the speed of light

Unfortunately, GPR is not universally applica- ble. Wide variations in soil type present wide variations in attenuation characteristics (Annan and Vaughan, 1982). Coarse-grained sands and gravels are ideal materials for the GPR, whereas fine-grained soils such as silts and clays are less responsive (with respect to penetration depth) to the radar technique. Table 1 lists the dielectric constants (K), velocity (V), electrical conductivity (o.), and attenuation (a) of a number of geologi- cal materials at a frequency of 100 MHz.

7. Method

Photomosaics of the outcrop were produced from photographs taken from the opposite side of the canyon. Overlays were used to record bound- ing surfaces, bedding shapes, and paleocurrent directions (Figs. 4b, 5b and 6).

Paleocurrent directions were determined by the measurement of cross-bedding dip directions and the trend of parting lineations. The plotting

184 M. Stephens / Sedimentary Geology 90 (1994) 179-211

Table 1 Dielectric constants, electrical conductivity, velocity and at- tenuation for various geological materials at a frequency of 100 MHz (from Davis and Annan , 1989)

Material K o- V a ( m S / m ) ( m / n s ) ( d B / m )

Air 1 0 0.30 0 Distilled water 80 0.01 0.033 0.002 Fresh water 80 0.5 0.033 0.1 Sea water 80 30,000 0.01 1000 Dry sand 3 -5 0.01 0.15 0.01 Saturated sand 20-30 0.1-1.0 0.06 0.03-0.3 Limestone 4 -8 0.5-2 0.12 0.4-1 Shales 5-15 1-100 0.09 1-100 Silts 5 -30 1-100 0.07 1-100 Clays 5-40 2-1000 0.06 1-3000 Granite 4 -6 0.01-1 0.13 0.01-1 Dry salt 5 -6 0.01-1 0.13 0.01-1 Ice 3 -4 0.01 0.16 0.01

K = dielectric constant; ~r = electrical conductivity; V = velocity; a = attenuation.

of major bounding surfaces was done with the aid of binoculars and by walking the outcrop.

Sedimentological studies revealed the exis- tence of a channel (element 26a) trending approx- imately 300 °, lying about 6 to 10 m below the cliff-face (see Fig. 2). This information was used to set up the configuration of the radar survey lines. In order to get the best three-dimensional picture of the channel-fill, lines 1 through 4 were oriented perpendicular to the channel (trending 210°). Lines 5, 6, and 7 were oriented perpendicu- lar (parallel to channel) to the four lines in order to tie in lines 1 through 4.

A 1000 V pulse with a frequency of 50 MHz was transmitted for 30 ns at each station. The receiver recorded throughout a 512 ns time win- dow during which 128 pulses were stacked. The antennas, which were separated by 2 m, were moved 1 m along the survey line after each read- ing was recorded. The transmitter and receiver were always placed 1 m behind and in front of each station, respectively. Table 2 illustrates the radar survey parameters.

In order to determine the velocity of the radar wave, a common mid-point (CMP) sounding was conducted on line 1. The transmitting and receiv- ing antennas were placed 0.5 m on either side of

station 1 + 12 m (The common mid-point of the sounding). After a sounding, the antennas were moved in 0.5 m increments from 0.5 m to ] 9.5 m on either side of station 1 + 12 m for successive readings.

8. Data processing

The radar data were processed at MultiVIEW Geoservices Inc.'s data processing facility in seis- mic-like variable area trace format. The data were corrected for changes in surface topogra- phy.

The CMP sounding revealed a radar wave velocity of 1.1 x 108 m / n s . This velocity was used to determine a depth scale on the radar data sections. Prior to the radar survey, electrical property measurements were conducted on ran- domly selected sandstone samples from the sur- vey site by J.D. Redman at the Waterloo Centre for Groundwater Research at the University of Waterloo, Waterloo, Ontario. The results indi- cated that the site would be an ideal place for the GPR survey as the electrical conductivity in the area was low (1-5 mS/m) .

A low-pass filter was used to process the data. A 7-point running time average was applied along each trace to remove the high-frequency noise. A 2-trace horizontal averaging process was also em- ployed to reinforce horizontally continuous re- flections. A time varying gain function (Auto- matic Gain Control) was also applied to the radar data.

9. The profiles

The outcrop in this study forms a curved face approximately 550 m in length. It was split into two "straight line" profiles. Profiles 1 (Fig. 4b) and 2 (Fig. 5b) trend 190 ° and 237 °, respectively. The photomosaics of both profiles are shown in Figs. 4a and 5a, respectively. Regional recon- structions conducted by Middleton and Blakey (1983) indicate a generally southwestward pale- oflow. Both profiles are oriented approximately parallel to the paleoflow.

M. Stephens / Sedimentary Geology 90 (1994) 179-211

(a) POSITION (m)

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M. Stephens /Sedimentary Geology 90 (1994) 179-211 191

Profiles 1 and 2 display the various architec- tural elements within the Kayenta Formation along with their internal stratification, major bounding surfaces and bedding contacts, pale- oflow directions and order of deposition. Bound- ing surfaces in profiles 1 and 2 are indicated by varying line thicknesses. Miall (1988a) discusses the subject of bounding surfaces in detail. Major surfaces are indicated by capital letters. Surfaces of lesser importance which show evidence of ero- sion are indicated by capital letters with prime symbols. Each element is represented by a num- ber followed by a two-lettered code. The number and letters represent the order of deposition and the classification of the individual element, re- spectively (see Table 3 for explanation of element codes). Paleocurrent north on both profiles was rotated 80 and 33 ° clockwise, respectively, from the vertical to allow for easier visual interpreta- tion. Paleoflow directions that are parallel to the face of the outcrop appear as horizontal arrows pointing to the left on each profile. Orientation measurements for parting lineation and cross- bedding are illustrated by "open-headed" and "closed-headed" arrows, respectively. The dip of second- to fifth-order bounding surfaces is illus- trated by "closed hollow-headed" arrows. Litho- facies are classified using the coding system of Miall (1978) which is shown in Table 4. A sum- mary of lithofacies data is shown in Table 5. Fig. 6 illustrates a summary of profiles 1 and 2. Sur- faces of interest are high-lighted in this figure. They represent major erosional events and changes in fluvial style.

Table 2 Radar survey parameters

Nominal frequency 50 MHz Antenna separation 2 m Spacing between traces 1 m Pulser voltage 1000 V Range window 512 ns Number of stacks 128 Points per trace 640

10. Element descriptions

The sandstone bodies (elements) in profiles 1 and 2 will be discussed in vertical, depositional order. Note that profile 2 is the continuation of profile 1, the bend in the cliff being their points of intersection. Five major separate fluvial pack- ages occur within the cliff-face. They are sepa- rated by four major surfaces: J, Qa, T and AA.

10.1. Elements 1 to 8

Exposed portions of element 1 consist almost entirely of laminated sand (lithofacies Sh), at times exhibiting very low-angle dips (lithofacies SI). A parting lineation reading obtained from profile 2 suggested a paleoflow either toward the south or north (azimuth 170 ° or 350 ° , respec- tively). Element 1 is capped by two exposed thin sheets of interbedded mudstone and silty sand- stone. Mudstone layer L is approximately 16.8 m long and ranges from 0 to 43 cm in width.

Element 2 also consists of horizontal bedding (lithofacies Sh) and low-angle dipping surfaces (lithofacies S1). Measurements revealed an aver- age paleoflow toward the SSE (azimuth 161°). Two thin mud layers, G 1 and G2, occur at the top of this element in profile 2. Mudstone G1 is approximately 5.5 m in length and consists of chips of mudstone interbedded with thin layers of siltstone. Mudstone G 2 is similar in character to G 1. It is about 1 m wide and approximately 40 m in length.

Element 3a is mostly covered by talus. A pale- ocurrent reading (S1 184) indicated a flow parallel to the cliff-face. Bedding is relatively parallel. On the far right of this element (at position 318-340 m), there is evidence of minor channeling. A number of mudstone lenses is present. Mudstone B is approximately 2 m in length and ranges from 4 to 14 cm in thickness. It consists of mudstone chips (0.5-1.5 cm) in a matrix of silty sandstone. To its right, lies mudstone D. It is similar in character to mudstone B and is thought to repre- sent the continuation of mudstone B as it lies just under the upper bounding surface. Mudstone C consists of thin layers of interbedded mudstone and siltstone.

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M. Stephens / Sedimentary Geology 90 (1994) 179-211 193

Element 3b consists of lithofacies Sh, S1, Sp and St. Paleoflow data recorded within profile 1 were constant, and revealed an average paleoflow to the south (azimuth 185°). On the far right-hand side of element 3b (profile 1) lies a macroform, the lower part of which consists of large-scale, low-angle cross-bedding. Its geometry is similar to a cross-section of a downstream-accreting macro- form (element DA) described by Miall (1985; Fig. 4a). It consists mainly of lithofacies Sp and S1, with minor St. Six paleoflow measurements indi- cated an average paleoflow toward the southwest (azimuth 237°). The fourth-order surface repre- sents the upper bounding surface of this macro- form. Paleoflow readings gathered just above the macroform revealed a paleoflow toward the

southwest (azimuth 245°), varying greatly to those gathered in profile 1 (azimuth 185°). A possible interpretation for this difference could be that the sand sheet was confined within a wide river channel that flowed toward the southwest in pro- file 2, and swung around toward the south in profile 1. Mudstone A (position 157-200 m) caps element 3b. Just below this layer lies a smaller layer of mudstone about 4 cm thick and 14 m long.

Element 4 consists of horizontal bedding. It is composed mainly of Sh, Sp, and S1 lithofacies. Seven cross-bedding measurements gave an aver- age paleoflow toward the southwest (azimuth 223°). One parting lineation measurement taken from this element is similar in direction to the

Table 3 Architectural elements in fluvial deposits (modified from Miall, 1978, and Cowan, 1991)

Element Symbol Principal Geometry and relationships lithofacies assemblage

Channels CH Any combination Finger, lens or sheet; concave- up erosional base; scale and shape highly variable; internal concave-up secondary erosion surfaces common

SB lens, sheet, blanket, wedge occurs as channel fills, crevases splays, minor bars

DA lens resting on flat or channelled base, with convex-up second- order internal erosion surfaces and upper bounding surface

LA wedge, sheet, lobe; characterized by internal lateral accretion surfaces

Sandy bedforms St, Sp, Sh, SI, Sr, Se, Ss

Downstream accretion St, Sp, Sh, SI, deposits Sr, Se, Ss

Lateral accretion St, Sp, Sh, S1, deposits Sr, Se, Ss; less

commonly Gm, Gt, Gp

Laminated sand LS Sh, SI; minor sheet, blanket sheets St, Sp, Sr

Gravel bars and GB Gm, Gp, Gt bedforms

Hollow HO Si, SI

lens, blanket; usually tabular bodies; commonly interbedded with SB

Bowl shaped base; scale highly variable; inclined parallel strat- ification, or less than 10" angular discordance to the under- lying bounding surface

194 M. Stephens / Sedimentary Geology 90 (1994) 179-211

other readings. To the far left-hand side of ele- ment 4, horizontal bedding gives way to inclined surfaces possibly representing the edge of this flow.

Element 5 consists of lithofacies Sh, with mi- nor amounts of SI and St. The upper and lower bounding surfaces are horizontal. Paleoflow is toward the southwest (azimuth 219°).

Element 6 has a partial concave-up basal sur- face that becomes sub-horizontal (surface G). It consists of lithofacies Sh and S1. The beds within this element are horizontal and onlap against the concave-up surface on the left side of the ele- ment.

Element 7 has a concave-up erosional basal surface. The left-hand side and lower part of this element consists of horizontal bedding which has

been clearly truncated by an internal concave-up erosional surface marked H' (see Fig. 4b). This surface generally parallels the lower bounding surface marked H. A single reading on one inter- nal second-order surface indicated a dip toward the south (azimuth 177°), whereas a parting-linea- tion suggested a south-southwestern flow direc- tion (azimuth 212°). Two layers of mudstone, E and F, lie just above basal bounding surface H. Mudstone E is composed of a thin layer of mud- stone between 2.5 to 5 cm thick. It is connected via a dipping mudstone layer with mudstone F which consists of thin layers of mudstone 1-2.5 cm thick, laminated with sittstone.

Element 8 consists of horizontal bedding with minor facies Sp and St. Unlike the rest, the deposits are almost perfectly horizontal. Individ-

Table 4 Lithofacies classification (modified from MiaU, 1978, and Cowan, 1991)

Facies Lithofacies Sedimentary structures Interpretation codes

Gm massive or crudely horizontal bedding, longitudinal bars, bedded gravel imbrication lag deposits,

sieve deposits

Gp gravel, stratified planar crossbeds linguoid bars or deltaic growths from older bar remnants

Si/SI sand, very fine to sporadic parting tineation; upper to transitional very coarse inclined parallel stratification flow regime structure

(Si), or less than 10 ° angular discordance (S1) to the underlying bounding surface; in places broad trough shapes appear

St sand, medium to solitary (theta) or dunes (lower flow very coarse, may grouped (pi) trough regime) be pebbly crossbeds

Sp sand, medium to solitary (alpha) or linguoid, transverse very coarse, may grouped (omicron) bars, sand waves be pebbly planar crossbeds (lower flow regime)

Sh sand, very fine horizontal lamination, planar bed flow to very coarse, parting or streaming (lower and upper may be pebbly lineation flow regime)

Sl sand, fine low angle ( < 10 °) scour fills, crevasse crossbeds splays, antidunes

M. Stephens /Sedimentary Geology 90 (1994) 179-211 195

ual horizontal beds are approximately 0.5 cm thick. Average paleoflow toward the south (azimuth 173°).

10.2. Interpretation of elements 1 to 8

Elements 1 through 5 and element 8 represent laminated sand sheets. Their architecture is very similar to the laminated sand sheets described by Tunbridge (1981) and to the high-energy flood deposits described by McKee et al. (1967) at Bijou Creek, Colorado. Parallel laminations are the dominant sedimentary structure present in all the elements (present to a lesser degree in ele- ments 6 and 7). Beds exhibit both strong and faint internal laminations that are even and closely spaced. Primary current lineations are abundant on bedding surfaces. Many sequences within the cliff-face are continuous for well over 300 m. Element 8, in particular, can be traced for over 400 m. Some of the elements are capped abruptly by thin drapes of interbedded mudstone and silty sandstone. There is a lack of small-scale cross- laminated sequences above the parallel-laminated sands indicating a rapidly waning flow with no low-stage reworking of the sediments.

Parallel laminations are often the main, or only, sedimentary structure found in sands de- posited by ephemeral stream floods and sheet floods (McKee et al., 1967). The Sh and SI facies present in all of the sandstones indicate upper flow regime plane bed and uppermost lower flow regime washed-out dune bed conditions which are common in rivers that undergo high seasonal discharge such as ephemeral streams (Miall, 1988a). Tunbridge (1981) states that dominantly parallel-laminated sands are rarely found in the deposits of perennial flowing rivers and that mod- ern perennial rivers do not appear to carry a sandy bedload exclusively as a plane bed. He suggests that dominantly parallel-laminated sands or sandstones may therefore be an indicator of deposition from ephemeral flows. Therefore, ele- ments 1 through 5 and element 8 most likely represent deposits formed by ephemeral pro- cesses.

The onlapping of near-horizontal beds against concave-up surfaces on the left side of element 6

suggests formation under high or intermediate flow-regime, plane-bed conditions. Element 6 may represent the deposits of a sand sheet confined within a channel, the concave-up surface repre- senting the bank of that channel.

Picard and High (1973) state that low-angle cross-stratification is restricted to and records the down-current migration of point and longitudinal mid-channel bars. In element 7, the low-angle cross-stratification most likely represents a mid- channel bar. Point bars are deposited on the inside of a curve in a stream where the water velocity is low, but evidence suggests that this is not the case here. Relatively deep water is indi- cated by the lack of ripple stratification upon this bar. The low-angle cross-stratification grades lat- erally into horizontal stratification. Long, parallel strata and the absence of current ripples in the general area suggest an upper-flow regime. The broad concave-up erosion surfaces, such as sur- face H' on the far left-hand side of element 7, may represent erosion surfaces created by suc- ceeding sheet flows. It is thought that succeeding sheet flows may have eroded previous sheets that were deposited on the non-horizontal basal sur- face (surface H) thus causing slight concave-up erosional surfaces. The fill of element 7 most likely represents a laminated sand sheet that filled in a channel scour.

10.3. Elements 9 to 15

Elements 9a and 9b (position 360-410 m) have concave-up basal erosion surfaces. Both consist of internal concave-up erosion surfaces and minor amounts of lag.

Element 9c lies directly above the sixth-order surface, J. No readings were obtained due to inaccessibility. Observations made with binocu- lars revealed a rather varied internal structure. Portions of the lower part of this element consists of minor lithofacies Sh. Above this lies lithofacies Sl and St.

Element 10a has a basal concave-up erosion surface. Many internal concave-up secondary ero- sion surfaces are present.

To the left of element 10a lie three smaller channels: 10b, 10c and 10d. The fill of element

196 M. Stephens / Sedimentary Geology 90 (1994) 179-211

Table 5 Summary of lithofacies data for profiles 1 and 2

Element Lithofacies N On Dn Interpretation

Profile 1 I-LS (?) Sh, SI 0 '~ '~ 2-LS (?) Sh 0 '~ '~ 3a-LS Sh, Sl 1 174 '~ 3b-LS Sh, SI, St 9 184 o 4-LS Sh, Sp, SI 8 223 '~ 5-LS Sh, SI, St 7 219 9 7-LS Sh, SI 2 212 177 8-LS Sh, Sp, St 12 173 9 9a-CH St 0 o ,~ 9b-CH St 0 ? 9 9c-SB SI, St, Sh 0 '~ 9 10a-LA St 0 '~ ? 10b-CH St 0 ? ? 10c-CH St 0 ? ? 10d-CH St 0 '~ 9 11-LA SI, St 1 252 ? 13-CH Sh, SI 0 ') ?

14-LA Sh, Sp 2 344 269 15-LA (?) SI 0 9 ? 16a-CH Sp, Gm 0 9 ? 16b-CH S1, St, Gm 0 9 ? 17-CH Sh, Gm 3 308 9 18-SB Sp, SI, St, Gp .6 220 ? 25a-CH SI, St 0 ? ? 25b-CH SI, St 0 9 9 26a-GB Gm, SI, Sh 6 238 312 26b-CH SI 2 262 9 27-CH S1, St 0 9 9 28-CH St, Sh, SI, Sp 9 242 9 30-LA S1, St, Sp, Sh 18 213 230 31-DA St, SI, Sp 51 203 214

Profile 2 I-LS Sh, SI 1 170 '~ 2-LS S1, Sh 9 161 9 3b-LS Sh, SI, Sp, St 11 245 ? -DA * Sp, SI, St 6 237 9

6-LS (?) Sh, SI 0 ? ? 12-CH SI, St 0 ? ? 19-HO (?) Si, SI 0 9 ? 20a-Slump (?) SI, Sh, St 0 ? ?

20b-LA St, SI, Gm 0 9 ? 21-CH St, Sl 1 159 ? 22-SB St, Sp, S1, Sh, Gm 1 214 ? 23-LA Sp, SI 4 275 ? 24-CH SI, St 0 ? ? 29-CH St, SI, Sh 3 301 292

Laminated sand sheet (?) Laminated sand sheet (?) Laminated sand sheet Laminated sand sheet Laminated sand sheet Laminated sand sheet Channel filled laminated sand sheet Laminated sand sheet Channel fill Channel fill Sandy bedforms Laterally accreted macroform Channel fill Channel fill Channel fill Laterally accreted macroform Channel consisting of planebedded

simple bar Side or lateral bar Side or lateral bar Channel fill Channel fill Channel fill Sandy bedforms Channel fill Channel fill Gravel macroform Channel fill Channel fill Channel fill Laterally accreted Downstream accreted macroform

Laminated sand sheet Laminated sand sheet Laminated sand sheet Downstream accreted macroform

within laminated sand sheet Channel filled laminated sand sheet (?) Channel fill Hollow (?) Slump structure on oversteepened

point bar (?) Side or lateral bar Channel fill Sandy bedform Side or lateral bar Channel fill Channel fill

M. Stephens /Sedimentary Geology 90 (1994) 179-211 197

10c consists predominantly of horizontal bedding. Bedding in the lower part of the channel dips gently and parallels the basal surface. Fill consist- ing of facies Sh reflects rapid aggradation, possi- bly representing flood events (Cant, 1978).

Element 11 represents a much wider channel system. Its basal surface, L, consists of a thin gravel lag (facies Gm). Paleoflow is toward the southwest (azimuth 252°). Its internal geometry consists of multiple concave-upward internal ero- sion surfaces.

Element 12 has a concave-up basal surface (surface M). Two cut-and-fill channels filled with coarser material are present on the far right side of this element. Small amounts of lag are also present just above the basal surface in profile 1.

Element 13 has a concave-up erosional sur- face. The beds within this element are parallel. Lithofacies Sh and SI are present.

Element 14 consists predominantly of horizon- tal parallel bedding (position 0-65 m). Individual beds are up to 0.5 cm thick and are laterally continuous for distances exceeding 60 m. The remainder of this element is comprised of facies Sp, St and Sh. One internal second-order bound- ing surface indicated a dip toward the west (azimuth 269°). The orientation of parting lin- eation suggested a paleoflow toward the north (azimuth 344°).

Element 15 rests on a convex-up erosional bounding surface (surface P). It is characterized by large-scale, gently dipping cross-bedding. Ele- ment 15 has the internal geometry of a laterally accreting macroform (see Miall, 1985, fig. 5C).

10.4. Interpretation of elements 9 to 15

Elements 9a, 9b, 10b, 10c and 10d represent channel-fill deposits. Element 9c was probably

formed by the migration of successive trains of bedforms. Periodic stage fluctuations must have occurred due to the presence of a minor plan bed (lithofacies Sh), and the presence of an erosional surface with some intraclast breccia. There is no evidence of lateral or downstream accretion. This sandbody is similar to element SB described by Miall (1985).

Elements 10a, 11 and 12 represent individual channel deposits that seem to have migrated pro- gressively toward the right with respect to the cliff-face. Elements 11 and 12 truncate the right sides of elements 10 and 11, respectively, the latter cutting right down to surface M. Elements 10, 11 and 12 could represent part of a large laterally accreted unit that was later eroded by succeeding elements above major surfaces T and AA.

Element 13 consists of parallel beds that may represent a plane-bedded simple bar within a channel. The description of this bar (?) is similar to the plane-bedded simple bars described by Allen (1983) which are illustrated in Fig. 15 (ele- ment 15 in profile 3) and Fig. 16 (elements 12 and 13 in profile 7). Element 13 is interpreted as channel-fill consisting of plane-bedded simple bars.

Elements 14 and 15 are also interpreted as having grown by lateral accretion. Lateral accre- tion is a characteristic of point- and alternate bars (Miall, 1988b).

10.5. Elements 16 to 18

Elements 16a, 16b, and 17 are all character- ized by a basal concave-up erosion surface, la- belled Qa, draped with intraclast breccia. A pale- ocurrent reading within element 16a revealed a paleoflow toward the southwest (azimuth 203°). It

Notes to Table 5: * This mesoform occurs within element 3b. N = number of readings; On = mean orientation of individual crossbeds and parting lineation; Dn = mean dip orientation of internal 2-5 order bounding surfaces A total of 179 readings were gathered from the cliff face. 167 of those represent individual cross-bed and parting lineation orientations. The remaining 12 represent the dip orientations of internal 2-5 order bounding surfaces. Information from elements 30 and 31 in profile 2 has been combined with that from elements 30 and 31 in profile 1.

198 M. Stephens / Sedimenta~ Geology 90 (1994) 179-211

consist mainly of Sp, St and SI facies. Element 16b consists of facies St and SI. Paleoflow direc- tion within element 17 is toward the northwest (azimuth 308°).

Element 18 has a varied composition including facies Sp, S1, St and minor amounts of Gp. Beds composed of stratified gravel (facies Gp) exist throughout this element. Six readings indicated a southwestern flow direction (azimuth 220°). Readings obtained from the stratified gravel re- vealed an average flow towards the southwest (azimuth 230°). The basal surface is characterized by minor cut-and-fill relief.

10.6. Interpretation of elements 16 to 18

Picard and High (1973) suggest that scours such as those found in elements 16a, 16b and 17, can result from the unequal distribution of water velocity across a channel cross-section. A helical flow near a bank is produced by pressure and velocity differences between the central part of the channel and the margins and can produce a trough-like scour at the channel edge. This seems to be the cause of five scoured features found at the edge of element 16a. These three elements are interpreted as the fill of a channel.

The stratified gravel present throughout ele- ment 18 is interpreted as linguoid bars. This element is similar to element SB (sandy bedform) in Miall's (1985) classification and is therefore interpreted as such.

10. 7. Elements 19 to 24

Just above elements 6 and 12 there exists a large sixth-order concave-up erosional surface (labelled T). It is approximately 260 m long and has a maximum scoured depth of 11.5 m. It is by far the largest and deepest concave-up surface within this outcrop.

Element 19 consists of interfingering beds (position 485-496 m). Over-lying beds cover un- derlying beds that seem to have been deposited from opposite sides of the element. This interfin- gering suggests the element was laterally filled from at least two sides.

Element 20a rests on a gently inclined basal surface (surface T). The element is approximately 6 to 7 m thick and thins out to zero towards the right. The left side is characterized by low-angle bedding and minor amounts of horizontal bed- ding. The right side is characterized by troughs and gently dipping beds. The central portion of this element consists of irregular, distorted bed- ding and faint unbroken strata that are bent into a series of folds.

Element 20b is characterized by low-angle cross-bedding. Small amounts of basal lag occur near the left side of this element.

Element 21 is composed of lithofacies St and SI. Internal concave-up secondary erosion sur- faces are common. Paleoflow is toward the south (azimuth 159°). A small mesoform consisting of two third-order erosion surfaces and trough- shaped bedding is present at position 382-429 m.

Element 22 has a gently dipping basal surface (surface W) and a complex internal structure. Sp sets occur throughout. Horizontal beds occur mostly along the base and are overlain by small amounts of gravely material. The remainder of the element consists of lithofacies S1 and St. Paleoflow is toward the southwest (azimuth 214°).

Element 23 rests on a sub-horizontal basal surface (surface X). It consists of lithofacies Sp and SI. Beds near the bottom are parallel, whereas beds in the upper two-thirds portion are charac- terized by large-scale, low-angle cross-bedding. Average paleoflow is toward the southwest (azimuth 275°).

Element 24 rests on a concave-up erosion sur- face (surface Y). Its base is characterized by patches of intraclast breccia. Internal concave-up secondary erosion surfaces are present.

10.8. Interpretation of elements 19 to 24

Cowan (1991) discovered large isolated scour features similar to element 19 within a large braided channel belt deposit of the Westwater Canyon Member (Morrison Formation) in the San Juan Basin, New Mexico. These unusual features, to which the term "hollow" was applied, have concave-up basal surfaces and are com- monly filled by lithofacies Si (inclined parallel

M. Stephens /Sedimentary Geology 90 (1994) 179-211 199

stratification) and SI (stratification that has a less than 10 ° angular discordance to the underlying bounding surface). Cowan (1991) indicates that the arrangement of the Si/SI lithofacies can vary quite dramatically from one hollow to the next. Some can be vertically filled, while others can be laterally filled, or a combination of both. Cowan found hollows as small as a few metres and as large as 80 m (see Cowan, 1991, fig. 10, p. 88). Mosley (1976), Mosley and Schumm (1976) and Best (1987, 1988) all documented deep scours (up to 6 times the mean channel depth) from both rivers and experiments. Their results suggested that deep scouring may play a significant role at channel junctions, particularly in the sandy braided stream environment where stream junc- tions are abundant.

Cowan (1991) interpreted the hollows as chan- nel confluence scours produced downstream of emergent channel sand bars (elements LA and DA). The three-dimensional aspect of element 19 is poorly known. Therefore, it is tentatively inter- preted as a "hollow" (element HO).

The description of element 20a is similar to slump structures that are normal to the slope (see Reineck and Singh, 1980, p. 90, fig. 144). In fluvial rocks slumps are restricted to bar deposits and indicate the downslope direction (Picard and High, 1973). Therefore, element 20a may repre- sent a bar deposit with slump features that flow into the cliff. It may have been one of a series of bars responsible for the underlying hollow (ele- ment 19).

Elements 20b and 23 are interpreted as later- ally accreted macroforms, and elements 21 and 24 as channel-fill deposits. Element 22 is similar to sandy bedforms described by Miall (1985) and is interpreted as such.

10.9. Elements 25 to 31

Elements 25-31 rest above an extensive sixth- order concave-up erosion surface, AA. It has a total length of approximately 443 m and a maxi- mum scoured depth of 13 m.

Elements 25a and 25b are both defined by basal concave-up erosion surfaces and clearly in- dicate channel migration towards the right.

Element 26a extends over 290 m. Its base consists of massive to crudely bedded gravel. Lithofacies Sh and SI are present. Grain size clearly diminishes upwards. A paleoflow reading obtained just above the gravel (position 145 m) suggested a southwestern flow (azimuth 238°). The dip orientation of five third-order surfaces (position 342-355 m) within the gravely material indicated an average flow toward the northwest (azimuth 312°).

Element 26b is characterized by concave-up erosion surfaces. Lithofacies Sl is present with minor Sh. Small amounts of basal lag are present. Paleoflow is toward the west-southwest (azimuth 262°).

Element 27 consists of a basal concave-up ero- sion surface and secondary concave-up surfaces. Minor lag deposits rest on its basal surface.

Element 28 has a concave-up basal surface that extends over 260 m. It consists mainly of lithofacies St, Sh, and SI, together with minor Sp. Average flow is toward the west-southwest (azimuth 242°). A small domed feature within this element (position 123 m) consists of parallel bed- ding near its base. The beds occupying the upper half of this feature parallel the curvature of its convex-up configuration. It may represent the tip of a bar travelling down the channel floor.

Element 29 consists mainly of lithofacies St and SI with minor Sh. Concave-up surfaces char- acterize the deposit's interior. Paleoflow is to- ward the northwest (azimuth 301°). Readings made on internal third-order surfaces indicate an average dip direction toward the northwest (azimuth 292°).

The basal surface of element 30 (surface EE) is approximately 460 m long. Lithofacies SI, St, Sp, and Sh are present. Indicated paleoflow is variable, but on average is toward the southwest (azimuth 213°). There are at least two well-de- fined internal erosion surfaces, EE' and EE", that dip at a low angle and extend from the top to the base of the macroform. The geometry of the beds above and below these surfaces are similar in character. Low-angle cross-bedding is domi- nant in the right-half of the element. The left-half is dominated by horizontal bedding. A cut-and-fill structure, approximately 8 m in width, is evident

20() M. Stephens / Sedimentary Geology 90 (1994) 179-21 ]

_ _ - - - - ~ - - , , m ~ , . . , ~ , ~ , ~ .__ S U R F A C E i i i i i i i r lU ~ . _L_

5m Fig. 7. Interpretation of line 0. Depth of penetration is approximately 15 m (about 5 m deeper than other lines). The upper 6 m consist of overburden (an Entrada sandstone mound which is eolian in nature). Deeper penetration depth may therefore be due to dryer material and lower conductivity within overburden.

at position 115-123 m. Readings from a second- and third-order surface gave an average orienta- tion of 230 ° .

Element 31 rests on a sub-horizontal basal bounding surface, FF. It consists of lithofacies St, SI, and Sp. Paleoflow is toward the south-south-

(m)

0 80 e0 90 I I I I I I I I I I

11110 l m t 8 0 I I ! I I I I t

i m

T I m N ) Bmmk)n (m)

Fig. 8. GPR section of line 1. Vertical exaggeration t.4.

M. Stephens / Sedimentary Geology 90 (1994) 179-211 201

west (azimuth 203°). Beds within this element dip at a low angle, and are at times sub-horizontal. One reading from a second-order surface indi- cated a dip toward the southwest (azimuth 214°).

Element 31 is interpreted as having grown by downstream accretion due to the small difference (11 °) between the dip direction of a second-order surface and the average paleoflow.

10.10. Interpretation of elements 25 to 31

Elements 25a, 25b, 26b, 27, 28 and 29 all represent channel-fills. Element 26a is similar in character to the description of longitudinal bars given by Miall (1977). The dip direction recorded from five third-order surfaces (position 242-355 m) diverges by 74 ° from the paleoflow direction. The dip direction of the third-order surfaces most likely represents the asymmetric flow across a longitudinal bar within a southwesterly flowing channel. The deposits within this element are interpreted as a gravel macroform (element GB). The abundance of low-angle cross-bedding within element 30 suggest it formed by lateral accretion.

1 1 . R a d a r s u r v e y r e s u l t s

A total of 990 m of survey lines were profiled with the GPR. It had an effective penetration depth of approximately 10 m. Figs. 7-11 illustrate both a radar section and the interpretation of lines 0 to 7. A summary of the radar parameters is shown in Table 6. The results from test line 0 (Fig. 7) indicated that the GPR was able to penetrate the Entrada overburden. This line is the least complex and illustrates continuous, sub-parallel layers within the 6 m high Entrada hill.

The interpretations of radar lines 1-7 (Figs.

L I N E 1 Line 5 Line 6 Line 7

I I I

: ~ / ' SURFACE

,<-...- ..... o

Line 5 Une 6 Une 7 LINE 2 ......... :;:::':::~i-" I I I

SURFACE

I I I

- Thane linm repmee~ surfaces ¢xxrrelated with " ' -- .~C thoee in olher mdw ¢ectior~ 5m

- Ropreeentsamrfaouthatcould not be corrolated 5m Vootk:al exmggomtion = 1.4

Fig. 9. Interpretation of lines 1 and 2. Individual surfaces are i, ndicated by four different types of matted lines and capital letters.

2112 M. Stephens / Sedimentary Geology 90 (l 994) 179 2/1

LINE 3 Line 5 Line 6 Line 7

I I I

. SURF, CE

i i::ii::

Line 5 Line 6 Line 7 LINE 4 I I I

I .............................................. ::::::::~:::~! :i

- - } L_ - These lines r e p r e ~ mJrfaces correl~ed with 5m thow in ocher racbr sections

- Represents surfaces that could not be correlated 5rn Vettimd ~ n = 1.4

Fig. 10. Interpretation of lines 3 and 4. Individual surfaces are indicated by different types of matted lines and capital letters.

9-11) indicate a complex stratigraphy. Radar re- flections that could be correlated have been la- belled by capital letters and are illustrated by various matted lines and thicknesses for clarifica- tion.

Two surfaces (C and J) were chosen to pro- duce a 3-D image of the channel floor as they were extensive and appeared on almost all the lines. Fig. 12 illustrates the gradual shallowing of the channel floor near the cliff edge (forward left side of diagram). Lateral profiling on the cliff-face indicated a flow toward the southwest (azimuth 203°). Another channel (indicated by a black ar- row) moves away from the cliff edge to the left and may represent a major channel, whereas the southwestern flow may represent a tributary or minor channel flowing around a depositional fea- ture.

An effort was made to correlate bounding surfaces visible in the cliff-face with the strafigra-

phy detected by the GPR. Reflector G was the only reflector that was not truncated by the sur- face (see Fig. 11). Assuming it remains at a con- stant depth until it meets the cliff-face, an equiva- lent depth to the reflector was plotted on profile 2. It was found to be very similar to the depth of a second-order surface in the middle of element 30 (marked 2 in Fig. 5b). Unfortunately, reflector G has been truncated by overlying reflectors in the other sections making it difficult to delineate its shape.

12. Three-dimensional geometry of sandbodies within the Kayenta Formation

Three-dimensional diagrams of sandbodies within the Kayenta are illustrated in Figs. 13-17. Their position within the outcrop is shown in the upper left corner.

M. Stephens / Sedimentary Geology 90 (1994) 179-211 203

Fig. 13 illustrates the approximate topography of the river system. Information concerning the topography was obtained from the radar survey, and paleoflow directions were obtained from sed- imentological profiling. In this figure, a deep channel runs approximately parallel to the cliff- face with a smaller channel flowing out of the

cliff (azimuth 203°). The positions and line num- bers of the GPR lines are indicated by thin dashed lines and bold numbers, respectively. The ex- posed cross-sections in this figure represent lines 1 and 5. Solid thick lines denote subsurface fea- tures detected by the GPR. Dashed thick lines represent inferred features. A downstream-

L I N E 5

SURFACE

CLIFF FACE

Line I Line 2 Line 3 Line 4 I I I I

............ ~. ........ I I I I

L I N E 6

SURFACE

CLIFF FACE

Line I Line 2 Line 3 Line 4 I I I I

G' . ¥

I I I I

L I N E 7 SURFACE

CLIFF FACE

Line I Line 2 Line 3 Line 4 I I I I

W Z

I I I I

- These lines represent stxfaces correlated I V.E. = 1.4 wilh thoee in other radar sections 5m

........................... - R q x m m s sudaoes that could not be correlmed 5m

Fig. 11. Interpretation of lines 5, 6 and 7. Individual surfaces are indicated by four different types of matted lines and capital letters. Capital letters are located either above or at the side of the individual surfaces.

2( ~4 M. Stephens /Sedimenta~ (;eology 90 (1994) 179-211

~ _ LINE5 LINE6 LINE 7

lOrn LINE 4 ~ ' C C4 I - ~ - ~ ' / - - - -~ - - - ~ ~ _ _J,_ I

C 3 . . . . . . ~ ' " " " ~ . . . .

I : ~ __ OR/ENI"ATIONOFUNE$1,2,3,&4 I / ,,#' /

Fig. 12. Diagram illustrates the shape and depth of two surfaces, C and J, detected by the GPR. They indicate that the centre of a channel existed near the back of the survey area. The channel seems to shallow to the right, as indicated by surface C and underlying surface J. Not drawn to scale. Sedimentological studies indicate a paleoflow to the southwest (this is shown by a white arrow). Black arrow represents channel paleoflow. Subscripts denote line numbers; the asterisk represents the shallowing or the meeting of surface J with the groundsurface; this "high" could represent and exposed bar or sandflat.

PROFILE 2

Point bar or mid-channel bar Main channel (?)

DA - Downstream ~--'reting microform 237° <" Orienllltton of cliff-face

Fig. 13. This diagram was produced from information provided by both ground-probing radar (GPR) and lateral sedimentological profiling. Solid lines on both sides of the block diagram represent bounding surfaces detected by the GPR and are labelled appropriately. Dashed lines represent inferred bounding surfaces. Numbers 1-7 represent GPR survey lines; Black box in upper left comer illustrates the position of the block diagram in profile 2.

M. Stephens /Sedimentary Geology 90 (1994) 179-211 205

Table 6 Summary of radar survey data

Line Start (m) End (m) Number (m) (m) of traces

0 0 + 75 0 - 02 78 1 1 + 00 1 + 188 189 2 2 + 180 2 - 1 4 195 3 3 + 00 3 + 183 184 4 4 + 180 4 + 01 180 5 5 - 0 5 5 + 4 5 51 6 6 + 0 0 6 + 5 6 57 7 7 - 0 7 7 + 5 5 63

accreting macroform (element 31 in Fig. 5b) oc- curs near the comer of the diagram. The "pinch- ing out" of surfaces on lines 1 through 4 most likely represents an exposed sandflat and is shown as such in this diagram. This figure is similar to model 10 from Miall's (1985) fluvial models.

Fig. 14 represents the lower portion of profile 2. It is dominated by laminated sand sheets (ele- ment LS) indicating an extremely flashy dis- charge. Channels are poorly defined. Average

flow is toward the southwest. The left corner consists of gently inclined surfaces that are a part of a downstream-accreting macroform (element DA) and is similar to model 12 from Miall (1985).

Fig. 15 illustrates a river system approximately 300 m wide, flowing into the cliff-face (azimuth 238°). The base of this channel (element 26a) consists of massive to crudely bedded gravel (lith- ofacies Gm). The lag ranges from 0 to 10 m in height and represents part of a longitudinal bar. Flow directions within the bar deviate by 74 ° from the main flow and is explained by an asym- metric flow across the bar. The 10 m thickness of the bar suggests the river had a minimum depth of 10 m. Water flow over the bar is shallower and slower than that within the channel to its left. The channel is relatively deep (4.5 m) and is characterized by a fast moving flow due to an abundance of lithofacies Sh. This diagram is simi- lar in character to model 10 from Miall (1985).

In Fig. 16, elements 5 and 7 represent the deposits of a southwesterly flow. Element 7 rests on an undulating channel surface which may have been continuously carved out and filled by suc-

PROFILE

[ Mid-channel bar .... - - - - - - I , .ph.m.,' . l .o,. /

"/. _..z. ~ ~ ~-'-----./, ",, ,s

~ " \ ~ %----__ " - " - / - - - - - x \ Average flow d i l l o n Sh -- N

i

-Troughs ~ ~ 1Ora l IT]- Ekm~rd number DA - Dowrmream acoreting macroform lore

Fig. 14. 3-D depiction of e lement 3b. Sheet flood fluvial plain subject to highly flashy discharge. Model is similar to model 12 of Miall (1985). Position within outcrop shown in upper left corner.

206 M. Stephens / Sedimentary Geology 90 (1994) 179-211

f

PROFILE 1

~ 3 1 2 *

!

J A

\ \

\ \ \

\

\ \ \

) - flow direction (or pendpal ~ trar,~port) - probee~e o~Uno of longitudinal bar

D - longitudinal bar ~ diagonal flow R - eroded bar remnant

Fig. 15 .3 -D depiction of element 26a in profile 1. Model is similar to models 10 and 11 of Miall (1985).

PROFILE 1 Sheet flood confined within river channel

.. . . . ~ ~ ~ LS ->/\ R ~ , ~ , , ~ , ~ , ~ ~ ~ - ~

l a n d n e t e d m m d s h e e t _ ~ - - - - ~ ~ _ _ - - - -

i< ~ ~ 212 ~ ~ ~

:~ - Dunes

Fig. 16. 3-D depiction of elements 5 and 7. Element 7 flow occurred after element 5, but did not completely destroy the deposits of the preceding flow. Model is similar to model 12 of Miall (1985).

M. Stephens / Sedimentary Geology 90 (1994) 179-211 207

PROFILE 1 Paa of large mind fiat (?)

Major fast flowing channel

o" " L

..ndlnce ot , n h o ~ ~ c--~" ' ° m L 20m

Fig. 17. 3-D view of element 30 in profile 1. Similar to model 10 of Miall (1985). Minor features from model 11 are also present.

ceeding high-energy flows. The left edge of the channel can clearly be seen. Element 7 represents a rightward shift in the river with respect to element 5. Only part of the sheetflood deposits can be seen in element 5 as it has been truncated by element 7. The left side of element 5 consists of low-angle cross-bedding, which probably repre- sents accretion surfaces. This figure is similar in character to model 12 from Miall (1985) although some features of model 11 are present.

Fig. 17 represents a cross-section of deposits found in the top portion of profile 1. A south- westerly flowing channel is present in the left half of this figure. The right side of element 30 con- sists of a large downstream accreting macroform possibly representing a bar or part of an exposed large sandflat. This diagram is similar to model 10 from Miall (1985).

13. Major surfaces of interest

There are a number of important surfaces present in the outcrop studied. They include three

sixth-order bounding surfaces (surfaces J, T and AA), three fifth-order surfaces (K, L and M), and a major down-cutting surface, Qa. These surfaces are shown as darker lines in Fig. 6.

The surface of greatest interest is the sixth- order surface (J), lying about half-way down the outcrop (see Fig. 4b). It is rather flat and extends over most of profile 1. Deposits found below this surface consist of sheet flow deposits; those above consist mainly of channel deposits.

The second sixth-order surface (labelled T) starts from the right edge of profile 1 (position 334 m) and extends all the way through profile 2. This concave-up surface may represent a conflu- ence scour produced within a wider and shal- lower channel system. At position 417 m, the slope of surface T may represent the edge of the scour. It is possible that after the surface was initially scoured by a channel system, channel sand bars or bedforms began to emerge. This would have caused the water flow to diverge and converge upstream and downstream around these bars or bedforms. The convergence of this flow around one of these structures led to the forma-

20~ M. Stephens / Sedimentary Geology 90 (1994) 179-211

tion of a channel confluence scour (?) further deepening the channel system. The scour was then filled with sediments (element 19), which were in turn covered by element 20a. During this process, element 20a eroded most of the upper portion of element 19 producing the gently tilting surface seen today in the cliff-face (see surface U a between positions 430 m and 530 m). Element 19, which has tentatively been interpreted as a hollow, lends support to this interpretation.

Surface AA represents the third sixth-order surface present within the cliff-face. It too was probably scoured by a converging water flow. Since element 26a fills most of this concave-up surface, it is possible that the river that carved this surface had a similar flow direction (azimuth 238°). Assuming this, and knowing that surface AA extends 443 m along the cliff-face, the river must have been at least 329 m wide.

Surfaces K, L and M are interpreted as fifth- order surfaces that have truncated part of the sixth-order surface (J) and some of the underlying sheet flow deposits. These three surfaces are illustrated as lighter coloured lines (dashed in Fig. 6) where they have truncated surface J and separate two different depositional styles.

Surface Qa is thought to have been scoured by a river or channel rather than by stream flow convergence. This conclusion was reached as the three overlying elements 16a, 16b and 17 do not seem to represent Cowan's (1991) hollows. (They are not characterized by lithofacies Si and SI.) Element 16a conforms to Miall's (1985) descrip- tion of element CH. There is no evidence to suggest that elements 16b and 17 represent hol- lows.

14. Fluvial style

There is a number of points to consider or summarize in order to determine the fluvial styles present within the Kayenta Formation in this area. They are as follows:

(1) A number of fining-upward cycles occur within the outcrop at various scales. Godin (1991) defined three separate scales defined by the ranks of the top and bottom bounding surfaces of the

cycle, namely the macrofrom-scale cycle, the channel-scale cycle and the sheet-scale cycle. Macroform-scale cycles are generally 1-6 m in thickness, and bounded above and below by third- or higher-order surfaces, one of which must be of third- or fourth-order. Channel-scale cycles are deposits infilling large-scale scours and possibly individual channels. They are 1-10 m in thick- ness, and are bounded above and below by fifth- or sixth-order surfaces. Sheet-scale cycles (4-16 m in thickness) are the products of autocyclic channel avulsion processes and are bounded both above and below by laterally extensive (> 100 m to 1 km or more), planar fifth- or sixth-order surfaces.

Element 21 (position 380-428 m) contains a number of cycles that fall within the macroform- scale category. Within this element, two to three cycles of cosets bounded by second-order surfaces are present (Fig. 5b). Due to inaccessibility, it was difficult to see if there were any fining-upward cycles.

Elements 10b and 10c are the only macroforms within the channel-scale cycle that show an up- ward decrease in average grain size. Apart from these two elements, channel-scale fining-upward cycles do not seem to be present in this outcrop.

Sheet-scale fining-upward cycles are visible within this outcrop. The most visible of these is element 26a. It consists of massive to crudely bedded gravel at its base which is covered by finer-grained sandstone. A number of channel systems and other sheet-scale units such as ele- ments 11, 12, 14, 16a, 16b, 20b and 27 contain patches of intraclast breccia at their bases. Some laminated sandsheets in the lower-third of the outcrop (elements 1 through 7, excluding element 6) also fine upward. They are capped partially or extensively by mudstone drapes or thin lenses of mudstone and silty sandstone.

(2) Some elements are capped by thin mud- stone drapes or thin lenses of mudstone and silty sandstone. There is a lack of small-scale cross- laminated sequences suggesting a rapidly waning flow with no low-stage reworking of the sedi- ments.

(3) Parallel laminations are the dominant sedi- mentary structure present in the sandstones in

M. Stephens / Sedimentary Geology 90 (1994) 179-211 209

the lower third of the outcrop. Primary current lineations are abundant on bedding surfaces and exhibit a NE-SW orientation which is consistent with the dip of the paleoslope.

(4) Beds can be followed in the cliff-face for well over 300 m producing a "layer-cake" se- quence. These sandstones are sheet-like in ap- pearance and consist of horizontal or sub-hori- zontal bounding surfaces. Channel forms are faint to absent. The sheet-like deposits are similar to the type A facies deposits of Tunbridge (1981).

(5) Lithofacies Sh and S1 are common throughout the outcrop (more dominant in the lower third). These lithofacies typically indicate upper flow regime plane bed and uppermost lower flow regime washed-out dune bed conditions, re- spectively, which are common in ephemeral rivers and rivers that are characterized by high seasonal discharge.

(6) Fine-grained flood plain and levee deposits are rare or not present.

(7) Channel deposits show an approximately horizontal top, and the lower surface is erosional and trough-shaped. Meander widening scour sur- faces are not present.

(8) There is an abundance of lateral and downstream (or oblique) accreting macroforms in the upper two-thirds of the outcrop.

(9) A total of 179 readings were gathered from the cliff-face. 167 of those represent paleoflow directions, while 12 represent the dip of second- to fifth-order surfaces. Average flow directions within individual elements ranged between 164 ° and 308 ° azimuth (Table 5). Dip directions of upper bounding surfaces show a similar range, from 177 ° to 312 ° azimuth. This is interpreted as indicating a generally low-sinuosity fluvial envi- ronment.

(10) Bristow (1987) found that the depth of a channel in the modern Brahmaputra River was typically between one and two times the height of a macroform. The largest macroform within the cliff-face, element 26a, is approximately 7.5 m high. This means that the Kayenta rivers may have been as deep as 15 m.

The above points suggest the occurrence of two fluvial styles within the Kayenta Formation. The lower third of the Kayenta is characterized

by laminated sand sheets (high-energy flood de- posits) which were most likely the product of ephemeral flows (Figs. 14 and 16) and is similar in character to Miall's (1985) facies models 11 and 12. A dramatical change in fluvial style oc- curs at the sixth-order bounding surface (J). From this surface upwards, concave-up channels are abundant and consist of bank-attached macro- forms (element LA) and mid-channel macroforms (element DA) suggesting a multiple channel model (Figs. 13, 15 and 17). The fluvial style in this part of the outcrop is similar to facies model 10 described by Miall (1985). Paleocurrent evi- dence within elements are rather uniform, sup- porting a low- to moderate-sinuosity model for the Kayenta rivers.

15. Usefulness of ground-probing radar

One of the objectives of the study was to test the usefulness of the GPR in the delineation of subsurface features for future aid in mapping the geometry of sandbodies (architectural-element analysis).

It should be noted that GPR reflections are not the direct result of the visually derived bounding surfaces. They are the result of varia- tions in soil electrical properties in the rocks caused by changes in water content. These changes, which are related to lithofacies and bounding surfaces, give rise to reflections that are not necessarily identical to the bounding surfaces present in the cliff-face.

A number of prominent surfaces detected by the GPR allowed for the construction of a three- dimensional block diagram of the study area (Figs. 12 and 13).

The GPR can transmit and receive signals thousands of times per second and its light weight made it easy to transport in rough terrain. As a result, radar survey data were collected rapidly. In total, 997 traces were recorded at 1 m intervals within 4 h.

The GPR has two drawbacks. It cannot be used in all environments and it does not have a very deep penetration depth. The conductivity of the medium under study has to be low in order

21 (I M. Stephens / Sedimenta~ Geology qO (1994) 179-2l I

for the G P R to obtain a deep penetrat ion depth. The study area had a conductivity value in the range of 1-5 m S / m allowing a penetrat ion depth of about 10 m. The GPR, therefore, is applicable in low conductivity environments. Its penetrat ion depth limits it to shallow subsurface mapping.

16. Conclusions

(7) The lower third of the Kayenta Formation is characterized by laminated sand sheets which were most likely the product of flash floods. The upper portion of the cliff-face is characterized by river channels consisting of lateral- and down- stream-accreting macroforms. Their presence suggests that a multiple channel river system was responsible for the deposition of sediments in the upper portion of the Kayenta Formation.

Field work in southwestern Colorado illus- trated the effectiveness of high-resolution G P R in the delineation of subsurface features and its usefulness in enhancing architectural element analysis in the third dimension. Several results can be drawn from this study.

(1) The field example demonstrated that G P R is a practical and economic method for obtaining high-resolution soundings as it can gather infor- mation rapidly and can be moved easily over rough terrain.

(2) GPR reflections are a result of water con- tent fluctuations in the rocks, which are related to lithofacies and bounding surfaces, but are not necessarily identical to them.

(3) Radar signals penetrated approximately 6 m of Entrada sandstone overburden along line 0 and detected a number of layers within the Kayenta Formation.

(4) The G P R was useful in detecting and map- ping subsurface features. A number of dominant features representing paleochannel bases were detected revealing the subsurface topography in the survey area.

(5) G P R is limited to low conductivity envi- ronments. Fine-grained soils such as silts and clays, which have high conductivities, cause high signal attenuation and reduce the penetrat ion depth to a few metres. Seismics are currently the only techniques that can provide high-resolution sounding in environments where radar instru- mentation fails.

(6) A combination of radar soundings and sed- imentological lateral profiling was instrumental in the construction of detailed and precise three-di- mensional architectural diagrams of individual el- ements in the subsurface.

Acknowledgements

I am indebted to my supervisor Andrew D. Miall who suggested this project. The radar sur- vey was conducted by Tom Cuthbertson and Pe- ter Giamou of Mult iVIEW Geoservices Inc. and myself. I thank Mike Bromley for his assistance in the field. His mountaineering skills were impor- tant in collecting sedimentological data from in- accessible parts of the cliff-face. Photographs used to construct profiles 1 and 2 were taken by Mike Bromley. J.D. Redman conducted the electrical property measurements on the sandstone rock samples prior to arrival in the field.

The Natural Sciences and Engineering Re- search Council (NSERC) supported this project through research grants to Andrew D. Miall, who read early drafts of this paper.

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