Architectural features of the Kayenta formation (Lower Jurassic), Colorado Plateau, USA: relationship to salt tectonics in the Paradox Basin

Sedimentary Geology, 73 (1991) 77-99

77

Elsevier Science Publishers B.V., Amsterdam

Architectural features of the Kayenta Formation (Lower Jurassic), Colorado Plateau, USA: relationship to salt tectonics in the Paradox Basin Michael H. Bromley Department of Geology, University of Toronto, Toronto, Ont. M5S 3B1, Canada (Received May 27, 1991; revised version accepted June 14, 1991)

ABSTRACT Bromley, M.H., 1991. Architectural features of the Kayenta Formation (Lower Jurassic), Colorado Plateau, USA: relationship to salt tectonics in the Paradox Basin. Sediment. Geol., 73: 77-99. Fluvial sandstones of the Kayenta Formation were analyzed using architectural element analysis. Paleocurrent trends, the distribution of lacustrine facies and local silcrete development indicate that synsedimentary movement of evaporites in the underlying Paradox Basin created an unstable basin floor beneath the Kayenta fluvial system. This instability resulted in deflection of fluvial axes, local basin development and local areas of interrupted fluvial deposition with eolian dunes. Paleocurrent trends in the Kayenta system reflect periodic interruptions of southwesterly flow. Salt migrating laterally out of a rim syncline into an adjacent salt anticline resulted in a rim syncline of slight topographic relief. The resulting basin was probably rapidly filled, allowing the resumption of southwesterly flow. Differential movement of salt (incipient solution collapse features (?)) resulted in the formation of small centripetal basins in which playa mudstones formed. A laterally extensive resistant ledge underlies a horizontal surface, suggestive of deflation to the water table of an exposed section of valley fill. A channel scour in the top of one of these surfaces has margins much steeper ( > 60 °) than the angle of repose for unconsolidated sand. Early cementation of the exposed floodplain could account for this resistance.

Introduction

architectural element analysis (Miall, 1985, 1988a, b, c) to document the geometry of fluvial deposits.

Sediments of the Kayenta Formation (Lower Jurassic) of the Colorado Plateau (USA) were deposited in a fluvial system emanating to the southwest from the ancestral Rocky mountains ("Uncompahgria"). The fluvial sediments were deposited in an arid climate from large perennial trunk streams (Luttrell, 1987) and lesser ephemeral tributaries (Miall, 1988a). Sandstone dominates, with rare muddy facies in the region. The Kayenta fluvial interval is the middle formation of the Lower Jurassic Glen Canyon Group, which consists of the eolian Wingate, fluvial Kayenta and eolian Navajo Formations (Baker et al., 1927; Harshbarger et al., 1957). The study was carried out as part of the author's doctoral research using 0037-0738/91/$03.50

Location In the study area, large outcrops are located in the canyons of the Dolores and San Miguel rivers, along the axis of the Nucla Syncline, near the east flank of the Paradox Valley anticline, in southwestern Colorado (Fig. 1). Structural dip is from the horizontal to a maximum of 3 degrees. Kayenta Formation Kayenta lithologies are principally lavender, pink to maroon fine-grained sublitharenites to subarkoses (Folk, 1980) characterized by horizon-

© 1991 - Elsevier Science Publishers B.V. All rights reserved

M.H. BROMLEY

78

' ~Mesa'C/~L.rr 2n.~ COLORADO

Mesa Creek Entrada Formation J-2 Unconformity Glen Canyon Group Navajo Formation Kayenta Formation w/outcrop Wingate Formation 1 Kilometer ~ 1 Mile

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ARCHITECTURAL

FEATURES

OF THE KAYENTA

FORMATION~ COLORADO

PLATEAU.

USA

79

tal laminae to low-angle cross-stratification. Although intraformational mudstone-chip conglomerates indicate the deposition of muds, continuous mudstone beds are rare in the main body of the formation. Interbedded eolian strata are finer-grained than the fluvially deposited units and weather to a distinct mottled orange (Luttrell, 1987). In some sections, the top of the Kayenta Formation is marked by a number of isolated mudstone units. Elsewhere, the formation is truncated by the J-2 unconformity (Pipiringos and O'Sullivan, 1978). The contact with the underlying Wingate Formation is gradational over approximately 10 m. The transitional series has characteristics of subaqueous deposition, including erosional scours, ripples and mud drapes. The bulk lithologic characteristics are typical of Wingate sand, a buff-toorange-weathering, fine-grained quartzarenite. This observation is consistent with the interpretation of Harshbarger et al. (1957) that during the transition to fluvial domination, older Mesozoic formations were reworked before any new material was deposited. The thickness of the Kayenta Formation is defined by measuring upward from a laterally extensive horizontal surface (Fig. 2) at the base of the Wingate-Kayenta transition. This surface is overlain by a thin ( < 0.2 m) mudstone which weathers recessively. Measuring from this datum, the formation is 55 to 60 m thick in the study area. All Kayenta outcrops in the study area can be subdivided into a lower and upper "system" by a horizontal, resistant ledge about 24 m above the base of the recessive mudstone.

into a system of linear intrusions and isolated domes (Cater, 1970). Salt flowage affected the dispersion and architecture of the Permian to middle Jurassic section (Landis et al., 1961; Cater, 1970; Doelling, 1981, 1982; Baars and Doelling, 1987; Chenoweth, 1987; Doelling et al., 1988). Some effects of evaporite movement include thinning of formations (or absence due to diapiric caprock emergence) over the axes of the intrusions, intraformational unconformities, isolated collapse structures due to dissolution, and diversion of fluvial axes due to subtle changes in basin floor topography (Tyler and Ethridge, 1983a, b). In the light of the effects on formations both older and younger than the Kayenta, it was thought that some of the features observed in these rocks may have an explanation in salt tectonics. Cater (1970) showed that the movement of salt was not uniform along the length of the intrusions. At different times, parts of the Paradox Valley salt anticline grew while others remained static. Depending on the lateral extent of evaporite flowage into anticlines, one could speculate on the degree of adjacent synclinal subsidence and whether it resulted in valleys (along which rivers could flow) or local centripetal basins. This paper will examine three aspects of possible salt-tectonic influence on the Kayenta fluvial system. These include the diversion of rivers into synclines resulting from underlying salt depletion, the occurrence of a fluvial hiatal surface and consequent preservation of an associated eolian deposit, and the formation of playa mudstones in small centripetal basins as fluvial processes decfined prior to and during the incursion of the Navajo erg.

Salt tectonics

Architectural element analysis (AEA)

The northeastern part of the Kayenta Formation is part of a continental clastic wedge overlying the Pennsylvanian Paradox Basin, a basement pull-apart structure in which developed evaporites of substantial thickness (Stevenson and Baars, 1986). Loading by Permian to Jurassic continental clastic sediments created an unstable density inversion within the evaporites. This resulted in flow of evaporites up the face of basement fault blocks

AEA uses photomosaics as base maps for the collection and presentation of outcrop data (Figs. 2 and 3). Tracing and identifying the rank of bounding surfaces separates essential geometric elements from the overwhelming information present in large outcrops. Line drawings serve as a guide to subsequent observations at the outcrop, as large-scale relationships are often difficult to observe at close range. Paleoflow and dip direc-

80

M.H. BROMLEY

tions of bounding surfaces are represented by differently ornamented arrows relative to a reference north; in this case the top of the drawing. Information on different photomosaics enables comparison of features at all scales between outcrops.

Discussion of a lateral profile photomosaic Figure 3 is a panoramic view around the outside of a meander bend in the San Miguel canyon (Fig. 1), oriented east-west to the left and n o r t h south to the right. Although an incomplete section of the Kayenta Formation, its internal organization is typical of all of the other outcrops studied, and illustrates two features discussed in this paper. Mudstones of the Kayenta Formation form talus-

covered slopes, some of which are exposed at the top of Profile 1 (Fig. 2). The outcrop was divided into 10 internally consistent architectural elements separated by majororder bounding surfaces such as 4th, 5th and 6th orders (Miall, 1988a). Paleoflow data is presented as arrows (with arrow point on the reading) with north at the top of the page; arrows bear no relationship to dips visible on the photomosaic. Paleoflow for each element or element complex is summarized in a rose diagram giving the vector mean and variance. Architectural element designations follow those suggested by Miall (1985, 1988, a, b, c; Table 2) except that the presentation bears an implicit genetic interpretation of element associations. Elements interpreted as being a part of a larger

TABLE 1 Lithofacies codes for fluvial environments (modified from Miall, 1978) Code

Lithofacies

Sedimentary structures

Interpretation

Gm

massive or crudely bedded gravel

horizontal bedding imbrication

longitudinal bars, lag deposits, sieve deposits

Gt

gravel, stratified

trough crossbeds

minor channel fills

Gp

gravel, stratified

planar crossbeds

linguoid bars or deltaic growths from older bar remnants

St

sand, medium to very coarse, may be pebbly

solitary or grouped trough crossbeds

dunes (lower flow regime)

Sp

sand, medium to very coarse, may be pebbly

solitary or grouped planar crossbeds

linguoid, transverse bars, sand waves (lower flow regime)

Sr

sand, very fine to coarse

ripple marks of all types

ripples (lower flow regime)

Sh

sand, very fine to very coarse, may be pebbly

horizontal lamination parting or streaming lineation

planar bed flow (lower and upper flow regime)

SI

sand, fine

low-angle ( < 10 °) crossbeds

scour fills, crevasse splays, antidunes

Sm ~

sand, fine to coarse, intraclasts

massive bedding flame structures

flood events debris flows

Spo a

sand, fine to coarse, may be muddy or pebbly

overturned parabolic crossbeds with minor massive intercalations

bed shear due to sediment saturation

" Additional lithofacies defined by author, 1991.

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in t h e D o l o r e s C a n y o n . A r r o w s i n d i c a t e f l o w d i r e c t i o n s , w i t h t h e r e a d i l g at t h e p o i n t . L a r g e a r r o w s o n b o t h p r o f i l e

i n d i c a t e r e s i s t a n t s u r f a c e d i s c u s s e d in text. S t o u t a r r o w in l o w e r r i g h t o f p r o f i l e i n d i c a t e s r-ecessive i r t e r v a l d i s c u s s e d in text.

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=86 161

margins. E l e m e n t

PALEOFLOW iNDICATOR BOUNDING SURFACE DIP DIRECTION SCOUR ORIENTATION Sp planar crossbeds Sm massive sand Sh horizontal laminations ~, SI low-angle crossbeds St trough crossbeds PALEOFLOW NORTH Sr rip les Sre eo~n ripples Spo deformed crossbeds Gm gravel, massive Gp gravel, planar crossbeds

/

by perspective ' ~ " ~ I-LS Sh Q ~ ) N = 6

IST & 2ND ORDER 3RD ORDER 4TH C 5TH C 6TH C

2 A - C H is part of a m u c h larger b r a i d e d

channel system that correlates with one a b o u t l km d o w n - c a n y o n . R e s i s t a n t surface i n d i c a t e d b y large arrows.

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ARCHITECTURAL FEATURES OF THE KAYENTA FORMATION. COLORADO PLATEAU. USA

complex of related elements bear the same number, but are separated chronologically by letters. A new two-letter architectural element code, ID, is introduced here (Table 2). Element 1-LS: laminated sand

Element 1-LS is poorly exposed in the profile and rests directly on the lowermost 6th-order surface which bounds the first Kayenta-type (Jk) lithology from that of the underlying K a y e n t a Wingate transition (Jkw) (Fig. 2). Lithofacies composition is predominantly facies Sh and S1 (Table 1) with minor Sp and St, allowing determination of paleoflow vectors. Internal bounding surfaces are horizontal to slightly dipping, often mirroring the geometry of shallow channels. Crossbedding is restricted to the lee of gentle bed topography. The lower bounding surface consists of shallow scours in the underlying Jwk unit. The first Jk element

above the transition has similar paleoflow and lithofacies content in all outcrops observed. Although the vector mean varies from outcrop to outcrop, overall the trend is west-southwest with low variance (Fig. 3). This element is interpreted as recording the onset of full-scale fluvial activity which introduced new, rather than reworked, material on top of the Wingate transition, from source areas uplifting to the east (Harshbarger et al., 1957; Luttrell, 1987). The predominance of high-regime sedimentary structures (Sh and S1), shallow nature of associated channels and low paleoflow variance suggest deposition from relatively unconfined sheet flow over a gently sloping braidplain or distal fan. Element 2A-CH." channel sandstones

This system is a multistory-multilateral complex of nested scour and channel fills (Friend,

TABLE 2 Lithofacies content of architectural elements (modified from Miall, 1985) Element

Symbol

Lithofacies assemblage

Geometry and relationships

Channels

CH

any combination

finger, lens or sheet; concave-up erosional base; scale and shape variable; nested or overlapping concave-up secondary erosion surfaces c o m m o n

Gravel bars and bedforms

GB

Gm, Gp, Gt

lens, blanket; tabular bodies commonly interbedded with SB

Sandy bedforms

SB

St, Sp, Sh, S1, Sr Se, Ss

lens, sheet, blanket, wedge; occurring as channel fills, crevasse splays, minor bars

Downstream

DA a

St, Sp, Sh, S1, Sr Se, Ss

lens resting on flat or channelled base with down-accretion stream-dipping set and coset bounding surfaces

Lateral accretion macroforms

LA

St, Sp, Sh, SI, Sr, Se, Ss, Gm, Gt, Gp

wedge, sheet, lobe; with internal bounding surfaces dipping cross-current

Sediment gravity

SG

Gm, Gms, Sm

lobes, sheets, interbedded with GB elements

Laminated sand sheets

LS

Sh, Sl, minor St, Sp, Sr

sheet, blanket

Overbank fines

OF

Fm, F1, P, Fr

thin to thick blankets commonly interbedded with SB

lnterfluvial dunes b

ID

Spe c, She, Sre

thin to thick tabular units above fluvial influence

Formerly element FM, or foreset macroform. h Defined by author, 1991. c Letter e denotes eolian process.

89

90

1983). Paleoflow is to the north-northwest (337 °) with a variance of 470, based on the strike of channel margins. A complex with similar features is exposed about one kilometre down-canyon in the same stratigraphic position. The nested channel and scour-fill geometry and moderate paleoflow variance is consistent with deposition from a freely shifting braid channel system (Coleman, 1969). If the complex exposed down-canyon is an extension of the same element, then the channel belt was at least 1200 m wide. The northwest paleoflow of element 2A-CH is interpreted as a record of the effects of episodic growth of the Paradox Valley salt anticline during Kayenta time. The paleoflow trend for the unit approximates the trend of the Paradox fold belt, as does the trend of other units from the same stratigraphic interval.

M.H. BROMLEY

Element 3B-DA: downstream accretion

This unit of planar crossbeds blankets all of the 3A-element deposits. Cross-laminae are deformed into recumbent parabolic folded foresets (Allen and Banks, 1972), but the lower bounding surface is nearly always planar, with no evidence of erosion. Crossbed sets are draped over and sweeping across shallow depressions in the lower surface. The deposits resemble the down-climbing sets described by Banks (1973). These units always occupy the upper part of a larger fluvial association, of buried channelled sands (Bromley, 1991). The upper surface of element 3B-DA, a resistant ledge, was used to subdivide the Kayenta Formation. Elements 3A and 3B represent the resumption of westerly flow after a period of backfilling of a salt-depletion valley by elements 2A and 2B.

Element 2B-LS Element 4-CH

This element is deposited over a broadly concave-up third-order surface at the top of the underlying channel belt. Relief of the channel margin is exaggerated by the camera position in the profile. The element is nearly 8 m thick at its thickest point. Similar to element 1-LS, lithofacies composition is predominantly Sh or S1, with only minor Sp and St in the vicinity of bed surface relief. Paleoflow has a northerly trend (2 °) similar to the underlying channel belt, suggesting that the two units are related systematically. The tabular geometry and horizontal laminae of these beds resemble the overbank deposits at Bijou Creek, Colorado (McKee et al., 1967) and other deposits attributed to flood events (Tunbridge, 1981; Stear, 1985). This element may represent abandonment of part of the associated channel system as the result of a large flood. Element 3A-LS: laminated sand

This element occupies a broad, shallow channel scour and is composed predominantly of Sh and SI. The lower part displays subtle nested channelfill geometry consisting of broad concave-up fills (similar to element 2A) which appear to record progressive shifting and abandonment of small channels.

This small element occupies a 5-m-deep scour in the top of the underlying 3-DA element. The element is composed of facies Sm (Table 1), chaotically disposed within the unit. The walls of the containing channel are oriented roughly n o r t h south, transverse to paleoflow in the underlying unit. The walls of the channel scour are substantially steeper than the angle of repose for unconsolidated sand. This indicates that the sand of the upper part of the underlying 3B-DA element resisted slumping during erosional event(s). Either the sand walls were unconsolidated, but damp, enabling them to stand at a steeper angle, or early cementation had occurred. Element 5-ID: interfluvial dunes

Element 5-ID is an example of a new type of architectural element defined in this paper (Table 2). ID (interfluvial dune) units are interpreted as eolian deposits coeval with or immediately postdating fluvial deposition, accumulating on bar tops and interchannel flats. Their preservation depends on having escaped the direct actions of the adjacent fluvial system, and they should occur above the highest fluvial beds in the related complex. Occupation of the uppermost position in a fluvial

ARCHITECTURAL

FEATURES

OF THE KAYENTA

FORMATION,

COLORADO

91

PLATEAU, USA

Y

i¸,

Fig. 4. Long-wavelength, low-amplitude rippled siltstone with lighter colored, coarser grains segregated on the crests, of possible eolian origin.

stratigraphic package should be an important clue as to their origin. Element 5-ID rests above 3-DA and 4-CH. In this outcrop, the element is composed of longwavelength, low-amplitude asymmetrical rippled siltstones of possible eolian origin (Hunter, 1977; Kocurek and Dott, 1981; Fig. 4). The rippled silts

are interbedded with crossbedded very fine sand and silt. Crossbedding dips in the interbedded material exceed 40 ° in places, and foresets are segregated into alternating coarse and fine laminae (Fig. 5). Only one reliable paleoflow reading was obtained from this exposure, but correlative units have anomalously eastward flow. The sum of these

Fig. 5. Festoon-crossbedded siltstone and very fine sandstone. Forset dip under compass is 38 °. This lithofacies forms an association with the ripples in Fig. 4.

92

accounts for the rose diagram in Fig. 3. The crossbedded facies weathers orange and outcrops in numerous exposures throughout the area. This unit can be traced intermittently for at least four kilometres to the northwest, transverse to its paleoflow. Wherever exposed, it rests on top of the resistant surface. The blanket-like geometry is so dissimilar to the lenticular, discontinuous fluvial beds of the Kayenta Formation that it would seem reasonable to invoke an eolian origin for this element. Element 6A-CH

Element 6A-CH is composed of complexly interfingered small scour-fills and horizontally laminated sand lenses, similar in character to, but more heterolithic than element 2A-CH. Scour fills are composed of granular intraformational conglomerate consisting of mud-chips, carbonate nodules with rare chert pebbles and silicified wood fragments. Size of scour fillings decreases upward in the element with a corresponding decrease in gravel content. The smaller scours may be bar-top chute channel deposits. The upper 2 m represent stagnant flow conditions, depicted by low-angle climbing ripples filling minor chute scours. This may indicate slow changes in stream gradient. Element 6B-DA

This element is similar to element 3B-DA, but is grouped with the underlying element on the basis of paleoflow similarity. Superposition above channeled sands suggests deposition from a midchannel sand flat like those described by Walker and Cant (1984). Element 7-LA: lateral accretion

This element displays the architectural features of lateral accretion, as in a point or lateral bar. Bounding surfaces dip at right angles to paleoflow indicators. Individual epsilon sets are composed of Sh and S1. The same element can be seen 800 m away up-canyon with little change in paleoflow and bounding surface dip direction. This element

M.H. BROMLEY

probably formed as a bank-attached alternate bar along a straight channel reach. Grain size distribution in this element is random, and lack the upward fining or stacking of bedforms which reflect an upward-diminishing flow regime. One epsilon set is composed of intraformational gravel, with fine sand above and below. Individual epsilon crossbeds thicken downbar. This lack of a pattern in the grain size distribution in the Kayenta Formation was also observed by Miall (1988a).

Paleocurrent trends

The dominant transport direction for Kayenta Formation sediments is towards the west-southwest (Poole, 1961; Luttrell, 1987; Miall, 1988a), reflected in the unweighted total data of all paleoflow indicators (Fig. 1). In this paper, two outcrop groups were studied, with paleocurrent analysis based on 931 readings (Fig. 1). The vector mean agrees with those of other workers. Low-rank structures such as parting lineation and channel margins (Allen, 1966) are strongly unimodal with low to moderate variance ( < 450), suggesting deposition from low-sinuosity streams (Fig. 3). Wide dispersion reflects to some degree the inclusion of high-rank sedimentary structures such as crossbedding and ripples in the total, and this dispersion is reflected in low-rank structures as well (Fig. 6). In general, the limits of dispersion about the regional mean are roughly northwest to southeast, which parallels the trend of the Paradox fold and fault belt. Some large features have anomalous paleoflow means (elements 2A-CH and 7-LA, Fig. 3). The LA-element at the top of Profile 1 (Fig. 2) has east-northeast paleoflow over a wide area. Such a large change of sediment movement in a direction opposite to the regional paleoslope suggests some structural control on the positioning of fluvial axes. In order to test for paleotopographic control on Kayenta drainage, the paleocurrent data were divided into five groups, each representing 10 m of stratigraphic thickness (Fig. 6). Parting lineation is by far the most common sedimentary structure in

ARCHITECTURAL

FEATURES OF THE KAYENTA FORMATION, COLORADO PLATEAU, USA

N=56 MEAN=246 30-40M

N=109 ~Atd--OA7

N=Sn ME/ 0-10

Fig. 6. Stratigraphic subdivision of total parting lineation from all outcrops. Note the high dispersion in the intervals from 20-30 and 4 0 - > 50 m.

93

the Kayenta Formation, as well as a reliable indicator of paleoslope direction (Allen, 1966, 1967), and consequently these structures were used for analysis. In two stratigraphic intervals, dispersion is at a maximum; 20 to 30 m and 40 to 50 m. Readings were taken from macroforms that show low internal variance, and it appears that the orientation of macroforms was influenced by local paleoslope. Radial patterns might develop on alluvial fans, where channels or sheetflow radiate from the apex over time. This might explain the pattern here, as the study area is close to the postulated source for the sediments in the PaleoUncompahgre highlands. A problem with the fan model is the areal extent over which some of the macroforms display anomalous flow directions. For example, element 2A-CH (Fig. 3) is traceable for a large distance along its flow length, parallel to the Nucla Syncline axis. The element is a ribbon cut transversely through the lowermost LS-element found in all of the outcrops. Another example is the upper LA element in Profile 1, which can be traced transverse to its paleoflow for about 2 km. This system had to be well-established in order to maintain its east-northeast flow long enough to deposit a sand sheet of this magnitude. Anomalous currents are common throughout the salt anticline region, especially near the top of the formation (Luttrell, 1987; pers. commun., 1989). An examination of Poole's presentation of dispersal patterns (1962, p. C140, fig. 199.1-D) shows anomalous data in the vicinity of the present study. Intervals of highest dispersion probably record the sporadic subsidence of the Nucla salt syncline in the path of the southwest-flowing fluvial system. As the anticlines grew, they did so at the expense of salt in the immediately neighboring synclines, resulting in subsidence. If subsidence happens beneath a fluvial system, some diversion of its axis might be expected. A period of stream diversion was followed by a period of vertical aggradation upstream of the anticline, and finally, the resumption of southwesterly flow as the basin became filled (Fig. 7). Other possibilities for resumption of southwesterly flow could be the antecedent breaching of the anticline by west-flowing streams, or stream cap-

94

M.H. BROMLEY

(b)

(a)

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limitof backfill

(e)

(d)

(f)

" ~ pulseof anticlinegrowth subsidencedue to saltdepletion Fig. 7. Block diagram reconstructing stratigraphy in the profile in Fig. 3: (a) fluvial system at onset of Kayenta deposition; (b) system during deposition of elements 2A-CH and 2B-LS; (c) backfilling of basin formed during halokinetic pulse in (b), deposition of element 3A-LS and 3B-DA; (d) another pulse of anticlinal growth diverts major fluvial axis away--unit 4-CH is deposited (an arroyo?), perhaps coeval with eolian material in 5-ID; early cementation of floodplain; (e) elements 6A-CH and 6B-DA fill in behind anticline; (f) anticline rises, diverting river axis to the southeast, depositing element 7-LA.

ture from streams on the west flank of the anticline. Intraformational disconformities

Fluvial m a c r o f o r m s generally rest entirely above a resistant ledge with a planar upper surface, with minimal downcutting through it. Where erosion occurs, oversteepened margins developed (element 4-CH, Fig. 2), suggesting that the sand was at least partially consolidated and obviously able to stand at angles substantially greater than the angle of repose.

Petrographic analysis of the strata immediately beneath this surface shows occasional grain-tograin contact, with a b u n d a n t euhedral, syntaxial quartz overgrowths p r o t r u d i n g into adjacent pore space (Fig. 8). This textural characteristic appears to indicate that cementation took place before c o m p a c t i o n of the grains. In strata immediately below the resistant horizon, grain contacts in the plane of the section increase markedly with a corresponding reduction in quartz overgrowths (Fig. 9). This f o r m of o p e n - f r a m e w o r k with quartz overgrowth has been attributed to vadose zone cemen-

ARCHITECTURAl- FEATURES OF THE KAYENTA FORMATION, COLORADO PLATEAU, USA

95

Fig. 8. Photomicrograph of sample taken from the top set of crossbeds in element 3B-DA, Profile 2. Note the open framework and euhedral overgrowths on quartz grains. Bar scale = 0.25 mm.

tation in an arid climate (Waugh, 1970a, b; Scholle, 1979; Summerfield, 1983). The time needed to firmly cement sand under such conditions varies with temperature and solubility of silica, which is greatly affected by the presence of alkaline solutions (Mizutani, 1970). Regardless, a substantial period is needed to accomplish quartz cementation at earth-surface conditions. For sediments to remain undisturbed for any length of time, means

that reworking by active river systems had to be absent. Movements of the basin floor, related to salt depletion, may have diverted the axis of fluvial deposition elsewhere for an extended period. Left undisturbed by fluvial erosion, upward capillary movement of groundwater through evaporation may have resulted in the early cementation of floodplain sediments by silica. The resulting

Fig. 9. Photomicrograph of specimen taken from about 2 m below the specimen in Fig. 8. Note the near absence of overgrowth cementation and the concavo-convex grain contact in the field of view. Bar scale = 0.25 mm.

96

M.H. BROMLEY

quartzitic silcrete (as defined by Smale, 1973) would certainly resist subsequent erosion. If eroded, this resistant material would certainly stand at greater than the angle of repose. Similarly stepped, oversteepened channel margins were attributed to the presence of silicious duricrusts in the Carboniferous of Nova Scotia (Gibling and Rust, 1990). The very fine, earthy sandstones to coarse siltstones above the silcrete horizon are cemented by hematite (Element 5-ID, Fig. 3). This unit rests conformably on beds of apparent fluvial origin, on a surface believed to have been subjected to long periods of subaerial exposure. The planar upper surface of the resistant element may represent eolian deflation to the water table. The paleoflow direction of the above element is diametrically opposed to the underlying strata, and agrees with northwesterly wind directions prevalent during Glen Canyon Group sedimentation (Poole, 1962; Middleton and Blakey, 1983; Luttrell, 1987). The rippled silts resemble supercritical eolian ripples (Hunter, 1977) and the very steep foresets in crossbedded sand and silt both appear to indicate deposition by wind. The architectural position, sedimentary structures and paleoflow directions strongly support an eolian origin. Substantial intertonguing of the eolian Navajo Sandstone with the Kayenta Formation is documented in the western part of the basin (Mid-

dleton and Blakey, 1983; LuttreU, 1987), so there is no reason to assume the absence of eolian deposition in the study area. Subsequent fluvial incursions eroded these beds but rarely cut through the underlying resistant horizons (Fig. 3).

Playa deposits The upper Kayenta Formation is marked in many places by a discontinuous mudstone unit (parts of which cap Profile 1, Fig. 2). The units are composed of deep maroon mudstones and shale with interbeds of very fine sand. Some of the sand beds contain abundant rhizoconcretions (Fig. 10). Surfaces within the mudstone display contorted desiccation cracks infilled with very fine sandstone (Fig. 11). The sandstone infillings are probably of eolian origin, as the curled mudchips show no signs of disturbance. Mudstone lenses are sparsely distributed along two parallel northwesttrending belts, the easternmost belt approximately coincident with the Nucla Syncline axis (Fig. 1). The mudstone lenses suggest an intermittent period of playa conditions prior to the incursion of the Navajo erg. The linear trend of the eastern belt of mudstones infers a relationship to subsidence along the proto-Nucla Syncline. The western chain may have formed in a compressional flexure adjacent to the actual salt intrusion, which, judging from field relationships, was not more

Fig. 10. Sandstone bed in the upper Kayenta mudstone displaying abundant rhizoconcretions(arrowed).

A R C H I T E C T U R A L FEATURES OF THE KAYENTA FORMATION~ C O L O R A D O PLATEAU, USA

97

Fig. 11. Contorted mudcracks in upper surface of mudstone lens. Cracks superficially resemble roots, but digging reveals these structures to be planar features.

than 2 km away. The depressions may be due to the uneven movement of salt (Cater, 1970), resulting in a chain of shallow centripetal basins. The anomalously located mudstone outlier (Fig. 1) is located near a modern-day salt-dissolution collapse structure which may have started sagging as early as the Jurassic (Fig. 12). Conclusions

Jurassic fluvial sedimentation in the Kayenta Formation was influenced by the growth of salt-

tectonic structures. Through the conservation of mass, growth of the salt anticlines proceeded at the expense of salt beneath adjacent synclines. The resulting subsidence periodically deflected drainage into parallelism with the trend of the salt structures. This is reflected by northwest-or southeast-directed secondary trends in paleocurrents. A major bounding surface in the Kayenta Formation represents a period of fluvial non-deposition due to subsidence-driven deflection of fluvial axes elsewhere in the system. The strata immediately beneath the surfaces show evidence of

Fig. 12. Paleogeographic reconstruction of study area at the time of m u d s t o n e deposition. Playas nearest the salt intrusion m a y have formed in a flexure, although evidence for this in the field is difficult to assess.

98

M.H. BROMLEY

early

cementation

Kayenta period.

time,

and

eolian

indicating

During

deflation

exposure

for

this p e r i o d , a s u b s t a n t i a l

during a long eolian

b l a n k e t a c c u m u l a t e d o n t h e d e f l a t i o n surface. The formation of small basins from the lateral migration

o f salt r e s u l t e d

in t h e f o r m a t i o n

of

c h a i n s of s m a l l p l a y a lakes.

Acknowledgements The

author

Alexander,

acknowledges

A.D.

Miall and

F.G. an

Ethridge,

anonymous

D. re-

v i e w e r for c r i t i c a l l y r e a d i n g the m a n u s c r i p t ; t h e i r comments

w e r e useful. D. N u m m e d a l

instigated

s o m e v i g o r o u s d i s c u s s i o n at the o u t c r o p c o n c e r n i n g t h e i d e a o f salt t e c t o n i c s . A b l e field a s s i s t a n c e was p r o v i d e d b y S. T r i b e . F u n d i n g for this p a p e r a n d the l a r g e r e f f o r t of w h i c h it is a p a r t has c o m e from both Natural Sciences and Engineering Research Council of Canada and American Chemical S o c i e t y g r a n t s u n d e r t h e s u p e r v i s i o n o f A . D . Miall.

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ARCHITECTURAL FEATURES OF THE KAYENTA FORMATION. COLORADO PLATEAU, USA stones: lessons from outcrop studies. Bull. Am. Assoc. Pet. Geol., 72: 682-697. Middleton, L.T. and Blakey, R.C., 1983. Processes and controls on the intertonguing of the Kayenta and Navajo Formations, Northern Arizona: eolian-fluvial interactions. In: M.E. Brookfield and T.S. Ahlbrandt (Editors), Eolian Sediments and Processes. Developments in Sedimentology, 38, Elsevier, Amsterdam, pp. 613-634. Mizutani, S., 1970. Silica minerals in the early stage of diagenesis. Sedimentology, 15: 419-436. Pipiringos, G.N. and O'Sullivan, R.B., 1978. Principal unconformities in Triassic and Jurassic rocks, Western Interior United S t a t e s - - a preliminary survey. U.S. Geol. Surv. Prof. Pap., 1035-A, 29 pp. Poole, F.G., 1961. Stream directions in Triassic rocks of the Colorado Plateau. U.S. Geol. Surv. Prof. Pap., 424-C: 139141. Poole, F.G., 1962. Wind directions in late Paleozoic to middle Mesozoic time on the Colorado Plateau. U.S. Geol. Surv. Prof. Pap., 450-D: 147-150. Scholle, P.A., 1979. Constituents, textures, cements and porosities of sandstones and associated rocks. Mem. Am. Assoc. Pet. Geol., 28, 201 pp. Smale, D., 1973. Silcretes and associated silica diagenesis in southern Africa and Australia. J. Sediment. Petrol., 43: 1077-1089. Stear, W.M., 1985. Comparison of the bedform distribution and dynamics of modern and ancient sandy ephemeral flood deposits in the southwestern Karoo region, South Africa. Sediment. Geol., 45: 209-230.

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Stevenson, G.M. and Baars, D.L., 1986. The Paradox: a pullapart basin of Pennsylvanian age. In: J.A. Peterson (Editor), Paleotectonics and Sedimentation in the Rocky Mountain Region, United States. Mem. Am. Assoc. Pet. Geol., 41: 513 540. Summerfield, M.A., 1983. Petrography and diagenesis of silcretes from the Kalahari Basin and Cape Coastal Zone, southern Africa. J. Sediment. Petrol., 53: 895-909. Tunbridge, I.P., 1981. Sandy high-energy flood sedimentation - - s o m e criteria for recognition, with an example from the Devonian of S.W. England. Sediment. Geol., 28: 79-95. Tyler, N. and Ethridge, F.G., 1983a. Fluvial architecture of Jurassic uranium-bearing sandstones, Colorado Plateau, western United States. In: J.D. Collinson and J. Lewin (Editors), Modern and Ancient Fluvial Systems. Spec. Publ. Int. Assoc. Sedimentol., 6: 533-547. Tyler, N. and Ethridge, F.G., 1983b. Depositional setting of the Salt Wash Member of the Morrison Formation, Southwest Colorado. J. Sediment. Petrol., 53: 67-82. Walker, R.G. and Cant, D.J., 1984. Sandy fluvial systems. In: R.G. Walker (Editor), Facies Models. Geosci. Can. Reprint Ser., 1: 71-89. Waugh, B., 1970a. Formation of quartz overgrowths in the Penrith Sandstone (lower Permian) of northwest England as revealed by scanning electron microscopy. Sedimentology, 14: 309-320. Waugh, B., 1970b. Petrology, provenance and silica diagenesis of the Penrith sandstone (lower Permian) of northwest England. J. Sediment. Petrol., 40: 1226-1240.