Grayburg formation, Texas

Sedimentary Geology, 56 (1988) 357-381 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

357

Interfingering of evaporites and red beds: An example from the Queen / Grayburg formation, Texas * H.S. N A N C E Bureau of Economic Geology, Unioersity of Texas, Austin, TX 78713 (U.S.A.) (Received April 2, 1986; revised and accepted December 4, 1986)

Abstract Nance, H.S., 1988. Interfingering of evaporites and red beds: An example from the Queen/Grayburg formation, Texas. In: G. Kocurek (Editor), Late Paleozoic and Mesozoic Eolian Deposits of the Western Interior of the United States. Sediment. Geol., 56: 357-381. Interbedded red sandstone, siltstone, mudstone, anhydrite and halite comprise the Queen/Grayburg formation (Guadalupian) of the Palo Duro Basin. These deposits accumulated in a 28,000 km2 epicontinental basin that was characterized by regional aridity and episodic, nearly basinwide influx of marine-derived hypersaline waters. In each of several recurrent progradational cycles a variety of environments developed in this basin including eolian dunes, interdune areas, sand sheets, saline mud flats, and broad, shallow expanses of marine-derived brine (salinas). Siliciclasticsources were in the west and northwest whereas brines were derived from the south and southwest. Eolian dune deposits are represented by medium- to high-angie (15-35 o) cross-stratified, fine-grained sandstone. Interdune areas accumulated flat- to low-angle bedded, wind-tippled strata, which are moderately to well sorted, fine-grained sand with frosted grains and illuviation structures. Sand sheets are characterized by deposits of horizontally bedded, rippled to massive, well-sorted, very fine-grained sandstone with illuviation structures. Sheetwash deposition on sand sheets and mud flats produced sandstones and siltstones with graded bedding, ripples with mud drapes, and intraclasts, and also flaser-bedded siltstones and mudstones deformed internally probably by haloturbation. Saline mud flats produced admixtures of mud and coarsely crystalline halite (chaotic halite-mudstone) and were located on the margins of regionally extensive hypersaline environments (salinas) in which bedded anhydrite and halite were precipitated subaqueously. Queen/Grayburg facies are cyclic and occur in characteristic vertical sequences that comprise, from base to top, bedded anhydrite, bedded halite, chaotic halite-mudstone, fine-grained elastics with displacive halite, siltstone and mudstone with deformed ripples, and eolian elastics. The sharp contacts between these shallowing-upward sequences suggest that transgressions were rapid.

Introduction T h e Palo D u r o Basin of the Texas P a n h a n d l e (Fig. 1) is o n e of several i n t r a c r a t o n i c s u b - b a s i n s that developed d u r i n g the late Paleozoic i n response to c o n t i n e n t a l collision associated with the

Q u a c h i t a O r o g e n y i n the s o u t h e r n U n i t e d States ( K l u t h a n d Coney, 1981). T h e b a s i n is b o u n d e d b y the A m a r i l l o U p l i f t a n d the Bravo D o m e i n the n o r t h a n d b y the M a t a d o r A r c h i n the south (Nicholson, 1960) a n d occupies a p p r o x i m a t e l y 28.000 kill 2. T h e p e a k of d e f o r m a t i o n of the Palo

* Publication authorized by the Director, Bureau of Economic Geology.

D u r o Basin occurred d u r i n g P e n n s y l v a n i a n time w h e n m o u n t a i n s c o r r e s p o n d i n g to the e a s t - w e s t t r e n d i n g A m a r i l l o - W i c h i t a U p l i f t developed. Exh u m e d b a s e m e n t rock shed siliciclastic debris to

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the north and south (Dutton, 1980). South of this uplift, marine conditions prevailed in the southern Palo Duro Basin where open-marine carbonates were deposited. By Leonardian time, eroded basement uplifts were buried and the Palo Duro Basin had become generally restricted from open-marine conditions; thereafter, cyclic deposition of Leonardian. Guadalupian and Ochoan evaporites and red beds prevailed. These deposits are similar in age and character to those discussed by Silver and Todd (1969) and Hills (1970) for areas down depositional slope generally south and southwest of the Palo Duro Basin. Leonardian and Guadalupian cyclic deposition recorded in the strata that pre-date and post-date the Queen/Grayburg formation in the Palo Duro Basin is discussed by Presley and McGillis (1982) and McGillis and Presley (1981). The Queen and Grayburg formations are part of the Permian-age Artesia Group (Tait et al., 1962), which occurs in the Texas/New Mexico Permian Basin. In type localities in New Mexico the Queen Formation is composed of mudstone with minor evaporite, whereas the Grayburg Formation is mainly dolomite. In the Panhandle of Texas, however, these formations are collectively sandstone, siltstone and mudstone with four thin evaporite beds. No distinct formational break between the two formations is present in the Palo

Duro Basin (Budnik and Smith, 1982). For this reason the Queen and Grayburg formations are treated in this report as a single genetic unit, the Queen/Grayburg formation. In the Palo Duro Basin the Queen/Grayburg formation is a clastic-dominant member of the evaporitedominated Artesia Group (Fig. 2). The remainder of the Artesia Group and overlying Salado Formation in the Palo Duro Basin is composed mostly of cyclic halite/mudrock (siltstone and mudstone) sequences. Stratigraphic nomenclature used for Guadalupian strata in the Palo Duro Basin follows that suggested by Tait et al. (1962), who published cross-sections composed of geophysical log correlations carried into southern Palo Duro Basin from outcropping type sections in the southwest. Formal nomenclature has not been established for the Palo Duro Basin where relatively more restricted-marine conditions than existed to the southwest prevailed through much of the Permian. Resulting lithologic differences between the Palo Duro Basin subsurface rocks and type sections render precise correlation of strata problematic. However, stratigraphic nomenclature used by previous workers in Palo Duro Basin studies (Hills, 1970; McGillis and Presley, 1981) will be observed in this report. This study is based on the integration of core analysis and facies interpretation with geophysical well-log mapping. Core from seven exploratory wells was recovered, visually examined, and graphically described. Textural analyses were performed at the Mineral Studies Laboratory of the Texas Bureau of Economic Geology. Scanning electron photomicrographs were obtained using a Joel T300 Scanning Electron Microscope. Regional correlations and mapping were based on geophysical well logs and sample logs from approximately 150 locations (Fig. 3).

Queen / Grayburg facies The Queen/Grayburg formation in the central and southern Palo Duro Basin is composed of 49% siltstone, 37% well-sorted, fine- to very finegrained sandstone, 8% mudstone (> 30% clay), and 6% bedded anhydrite and halite distributed

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among four evaporite beds. In the northwestern Palo Duro Basin all evaporites pinch out and the Q u e e n / G r a y b u r g Formation is a red siliciclastic unit comprising 81% well-sorted, fine- to very fine-grained sandstone, 13% siltstone, and 6% mudstone. Poikilotopic halite cement is ubiquitous, but is not included in the percents given above. Regionally extensive post-Permian halite dissolution has affected Permian strata in the northern and eastern parts of the study area (Gustavson et

al., 1980), but has not penetrated there to the Q u e e n / G r a y b u r g . Original lithologic relationships apparently have been preserved in the rocks used in this study. The rocks of the Q u e e n / G r a y b u r g formation are divisible into three general facies: (1) evaporite-bearing rocks, including anhydrite, halite and halite-dastic admixtures; (2)fine-grained clay-bearing siliciclastics including flaser-bedded siltstones and silt-rich red mudstones with de-

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Fig. 3. Map of study area showing data point and lines of cross-section. Section A-A' (Fig. 2) and fence diagram (Fig. 12) indicated by dashed pattern. formed ripples and rippled muddy fine- to very fine-grained sandstone with mudstone intraclasts and clay drapes; and (3) relatively clean sandstones ( < 5% clay) including cross-stratified to massive, very well sorted, red fine-grained sandstones and horizontally bedded, massive to subtly rippled, well-sorted, very fine-grained fight red sandstones to coarse-grained siltstones with illuviation structures. Environmental interpretations of these rocks are based on sediment composition,

texture, sedimentary structures and stratification types, stratigraphic relationships among component facies, and on similarities to rocks described and interpreted from recent and ancient analogs by other workers.

Evaporite/ halite-clastic facies A transitional continuum exists in Q u e e n / Grayburg lithologic cycles between purely chemical rocks and those that are dominantly siliciclas-

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tic and contain only minor amounts of halite. Because this continuum was also reflected in lateral facies distributions in the depositional environment, as will be shown, it is appropriate to consider Queen/Grayburg evaporite-bearing rocks collectively. Queen/Grayburg clastic-free evaporite facies include bedded anhydrite and halite. These rocks and halite-clastic admixtures occur in characteristically cyclic vertical sequences. A complete evaporite cycle comprises, from bottom to top, laminated anhydrite, anhydritic mosaic halite, mosaic halite, muddy mosaic halite, chaotic halite-mudstone, and mudstone with subordinate cubic halite. Siltstone and sandstone with minor skeletal displacive halite is found, in places, overlying the above-described sequence.

Salina subfacies Anhydrite of the Queen/Grayburg formation is characteristically thinly laminated (Fig. 4A) and, in some intervals, includes abundant halite pseudomorphs after bottom-nucleated selenite (Fig. 4B). Some occurrences are poorly laminated and dome-like (Fig. 4C), similar to selenite domes described from South Australia by Warren (1982) and Warren and Kendall (1985). Most original gypsum altered to anhydrite by dehydration during exposure to halite-saturated brine. Varying proportions of individual selenite crystals were replaced by halite or filled by halite after dissolution of gypsum in brine. In places, ripples and ripple cross-lamination traces can be seen in the anhydrite, suggesting an original transported detrital gypsum component. Probably abrasion of selenite provided gypsum detritus which, in turn, acted as seeds for subsequent nucleation of additional selenite. The composition and textures in these rocks indicate that in the sulfate depositional environment, substrates consisted of gypsum sand and less abraded, in situ selenite crystals and fragments. For additional details of the environmental conditions and processes by which modern subaqueous gypsum precipitates, see Schreiber (1978). Halite in Queen/Grayburg evaporite sequences is characteristically unlaminated and usually comprises a mosaic of centimeter- to decimeter-scale equant to vertically elongate crystals that contain

numerous fluid inclusions. Most halite of this study lacks unequivocal environment-specific primary structures and could have been dissolved and reprecipitated subsequent to original deposition. However, some halite crystals have small (multi-millimeter-scale) cloudy, chevron-shaped internal structures with coigning-upward aspect. Examples of Queen/Grayburg halite chevrons are small, obscure, and have proved difficult to photograph satisfactorily; photogenic examples of larger, similar features can be found in Wardlaw and Schwerdtner (1966), Holdoway (1978) and Fisher (1985). Chevron-structures are composed of minute fluid inclusions that reside along common crystal lattice planes. Chevrons are indicative of subaqueous halite precipitation; foundered halite "rafts" (floating aggregates of fine-grained crystals) that formed at the brine surface in response to evaporative halite-supersaturation become sites of continued halite nucleation as they lay beneath the brine surface (Lowenstein and Hardie, 1985). Similar chevron halite was produced experimentally from sodium chloride-supersaturated brine by Arthurton (1973) and Southgate (1982). Halite crystals grow competitively in a vertical direction as lateral growth is inhibited by competition with adjacent developing crystals; crystals with corners pointed up have more surface area upon which to accumulate additional halite than crystals oriented otherwise and thus grow more rapidly. For these reasons coigning-upward tends to be more common than other orientations of halite chevrons from the Queen/Grayburg formation. Coi~'ning-upward, vertically elongate mosaic halite has been described from the Devonian Prairie Evaporite Formation of Canada (Wardlaw and Schwerdtner, 1966), the Triassic Lower Keuper Saliferous Beds of England (Arthurton, 1973), the Permian Blaine Formation of Kansas (Holdoway, 1978), and from the lower Clear Fork Formation (Handford, 1981) and Blaine-equivalent San Andres Formation (Hovorka, 1985) of the Texas Panhandle. In core, anhydritic halite occurs between anhydrite and overlying halite. Anhydrite in the form of irregular, laterally continuous (at scale of 10 cm core), sub-horizontally oriented laminations (partings) or very irregular wispy masses are seen

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Fig. 4. Evaporite facies. (A) Laminated anhydrite. Typical basal unit of evaporite cycle. (B) Laminated anhydrite with halite pseudomorphs after selenite (p). Laminations probably originally detrital gypsum and sites of selenite nucleation. (C) Unlaminated anhydrite domes. (D) Muddy mosaic halite containing minor interstitial mudstone at crystal boundaries. (E) Chaotic halite-mudstone rock. (F) Mudstone with displacive cubic halite. (G) Fine-grained sandstone with displacive skeletal halite.

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Fig. 4 (continued). (H) Displacivehalite growth in muddy sandstone. Angular relation between halite-displaced clastic sediment and overlying ripple-and-drape sequence indicates that sediment surface exhibited minor relief resulting from halite growth prior to deposition of undeformed interval. Note intraclasts (i) of host sediment included in halite mass. White specks in upper half of core are reduction spots. (I) Compacted and chaotic mix of mudstone intraclasts (i), rippled siltstone fragments (r) and anhydrite residue (a) produced by dissolution of halite components from strata of anhydritic halite, halite-mudstone, and halite-bearing silty mudstones.

in the midst of relatively clear mosaic halite. The stratigraphic relationship of anhydritic halite to underlying anhydrite and overlying, relatively sulfate-poor, halite suggests a transitional phase of evaporite precipitation whereby brines became progressively halite-enriched during desiccation. More probably, anhydrite partings resulted from initial evaporite precipitation following recurrent influxes of marine-derived waters or from accumulation of relatively insoluble sulfate originally included in the overlying anhydritic halite that dissolved in freshened water. Selenite nucleation sometimes may have occurred on relatively insoluble residue composed of fine-grained anhydrite particles originally disseminated throughout a subsequently dissolved halite interval. Rainfall could

have dissolved halite during periods of subareal exposure coincident with desiccation of the salina. Alternately, halite could have been dissolved while overlain with halite-undersaturated brine attendant with the influx of meteoric or marinefreshened water.

Saline mud-flat subfacies In the most completely preserved Q u e e n / Grayburg cycle, mixtures of halite and red mudstone (the mudstone contains about 30% clay) overlie clean halite, the proportion of mudstone increasing upward. (It is important to recognize that halite at the top of evaporite cycles may have been recrystallized wholly, partially, or removed completely by halite-undersaturated waters associ-

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ated with progradation of overriding terrestrial environments.) Four general varieties of halite/ siliciclastic combinations are seen: ( 1 ) m u d d y halite (very minor intracrystalline mudstone is present) with small irregular masses of mudstone conforming to crystal boundaries within an interlocking mosaic of centimeter-scale halite crystals (Fig. 4D); (2) chaotic halite-mudstone composed of larger (multi-centimeter-scale) straight-edged mudstone bodies occurring within a mosaic of interlocking centimeter-scale equant to cubic halite crystals (Fig. 4E); (3) halite-bearing mudstone to siltstone including millimeter- to centimeter-scale displacive cubic halite (Fig. 4F); and (4) sandstone containing large (decimeter-scale to larger) isolated crystals of displacive skeletal halite (Fig. 4G), which is distinguished by alternating latticeparallel bands of clear halite and halite-cemented sediment derived in situ from the clastic medium in which the halite crystallized. Spatial relationships between halite and mudstone in these mixtures provide information about the processes of formation. Small masses of mudstone within halite crystals in muddy halite indicate that halite crystallized within a medium that included mud. Mud between crystals suggests that mud was expelled to crystal boundaries during halite crystallization; hence, halite developed displacively in the mud. In halite-mudstone, straight edges on mudstone bodies suggest that halite crystallized displacively as cubes in soft sediment. The presence of numerous displacive halite cubes in halite-bearing mudstone and siltstone suggests that those halite crystals formed in sediment probably less frequently or more briefly inundated with supersaturated brine being, perhaps, more remote from the salina than areas that produced chaotic halite-mudstones. It is possible that the relative amount of mudstone in halite-mudstone mixtures is related to proximity of land to the site of halite-mudstone deposition and that the vertically increasing mudstone content seen in evaporite cycles reflects the progressive encroachment onto the desiccating salina of prograding terrestrial environments. If this is true, then fine-grained siliciclastic sediment seen as wisps in muddy halite was delivered from land to the salina, most likely by the wind prior to

arrival of the prograding landward salinamargin. By modem analogy, fallout from desert dust storms comprises much of the marine sediment in the Persian Gulf (Kukal and SaadaUah, 1973). Alternatively, it is possible that all the halite-mudstone mixtures formed when halite crystallized displacively in dastic sediment along the landward margin of the salina after the margin had prograded to the location of subsequent muddy halite formation, perhaps locally elevating the land surface in the process. Concentration of the most clastic-free halite at the bottom of the halite-mudstone sequence may simply reflect brine residence-time in the clastic sediment, which would have been greatest at the base of the interval. Figure 4H suggests that the surface was sometimes elevated by displacive halite crystallization. This figure shows halite which has grown displacively into elastic sediment deformed overlying clay layers in the process. The deformed layers are in angular contact with overlying undeformed rippies and drapes so this process must have produced an irregular geomorphic surface. Note also the relative abundance of intraclasts in the halite-mudstone; it appears that halite crystallization proceeded displacively within the clastic medium that encased the halite. Also, the intraclasts are widely spaced. Obviously, the proportion of clastic material to halite in this example is unrelated to distance from land but is related to the displacive halite-crystallization process. Chaotic halite-mudstone mixtures and siliciclastics with cubic halite were definitely produced by displacive growth of halite following inundation by brine into saline mud-flat sediment on the landward periphery of salinas. In some of the halite-mudstone, siliciclastic intraclasts retain original sedimentary structures (including graded grain-size distributions, ripples and mud drapes) produced in the terrestrial environment prior to displacive growth of halite. Based on field studies of Bristol Dry Lake of the Mojave Desert in southern California, Handford (1982) concluded that, in spite of size-scale differences between the systems, halite-clastic interactions and sedimentary products were similar around giant Permian salinas in the Texas Panhandle to those around the continental-sabkha in his study. Handford

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postdated that chaotic halite-clastic fabrics resuited from cyclic expansions of the salina coupled with intervening desiccation phases. Undersaturated salinity of the initial flood waters caused dissolution of some halite and inundated surrounding clastic sediment with brine. During desiccation, halite was precipitated subaqueously above the sediment surface and displacively within the sediment. Handford concluded that chaotic halite-clastic mixtures bear the overprint of several of these dissolution/precipitation cycles. In the Queen/Grayburg formation some former halitebearing strata have lost most of their halite to dissolution which probably occurred soon after deposition during subsequent flooding by haliteundersaturated waters. The result is a chaotic mix of relatively insoluble siliciclastic and anhydritic residue compacted by burial (Fig. 41). In the fourth evaporite/clastic facies, isolated skeletal halite crystals (Fig. 4G) grew displacively in sandy sediment when the sand was inundated with brine during transgressions, wind-tidal flooding, or rise in a hypersaline water table. These rocks are very similar to halite-bearing sediments studied from the Holocene near the Dead Sea (Gornitz and Schreiber, 1981), which were produced under evaporative conditions, including elevated temperatures and salinities, by either downward ionic diffusion of halite during brineflooding or upward diffusion driven by evaporative pumping (capillary action). The history of an individual crystal might reflect both processes acting at different times. Gornitz and Schreiber (1981) concluded that the clear halite of such crystals formed by slow precipitation, whereas sediment was included along crystal lattice-planes during periods of rapid halite precipitation. A Permian analog for similar skeletal halite is described from the Flower-pot Formation of Kansas (Holdoway, 1978).

Rippled siltstone, mudstone, and muddy sandstone facies Two types of facies occur in the Queen/ Grayburg, whose components obviously were transported by and deposited in water. The first comprises graded (fining-upward) bedding, dis-

tinct ripple cross-lamination, and clay drapes. In some examples mudstone intraclasts occur that appear to be entrained fragments of previously deposited clay drapes. These deposits are thin (a maximum < 20 cm thick) and uncommon in Queen/Grayburg core. Close examination of clay drapes in some of these deposits reveals small cubic casts and "ghosts" probably recording the filling by clay, silt, or sand of cavities formerly occupied by halite cubes (Fig. 5A), suggesting that depositional waters were saline in those cases. Halite was probably dissolved subsequently by halite-undersaturated flood waters. Thinness of these deposits and their scarcity suggest that processes involving flowing water on the Permian landscape were ephemeral and that conditions were not conducive to their preservation (i.e., minor desert wadi deposits produced by rainstorm flooding or salina-margin flood deposits caused by wind-tidal action were probably reworked by eolian processes). The second water-laid clastic facies (Fig. 5B and C) is very common and includes poorly sorted clayey siltstone to mudstone (classification of Folk, 1974) with deformed small ripples (about 1 cm wavelength). Ripples are light-colored, composed of coarse-grained silt, and occur in a darker red fine-grained silt and clay matrix. The appearance of this facies resembles lenticular and flaser bedding except that the traditional role of clay in lenticular or flaser bedding is assumed by mediumto fine-grained silt in Queen/Grayburg siltstone and mudstone. This facies commonly occurs on top of chaotic halite-mudstone in evaporite sequences throughout the Guadalupian/Ochoan section in the Palo Duro Basin and also occurs interstratified with sand-sheet deposits (discussed below) interpreted to be down-slope from eolian dunes in the Queen/Grayburg formation. In many places its stratigraphic position directly overlying evaporite deposits implies lateral proximity to a salina. This suggests that ripple deformation may be the result of precipitation and dissolution of interstitial halite (" haloturbation" of Smith, 1971) derived from nearby brine sources. Other examples of this facies show greater abundance of coarser grains and little or no deformation (Fig. 5C), suggesting less inundation by brine probably

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Fig. 5. Rippled siltstone, mudstone and muddy sandstone facies. (A) Poorly sorted muddy rippled-laminated fine to very fine sandstone with included clay drape intraclasts (i) probably deposited by sheet-wash on a sandy mud flat or sand sheet. Parts of cubic molds (m, formerly halite cubes) are filled with mud. In other places light-colored ghosts (g) in silty sand indicate former presence of halite cubes. (B) Poorly sorted slightly sandy clayey rippled siltstone probably deposited during sheet-flooding. Coarser gained bright areas are deformed ripples (dr) included in a finer silt matrix. Ripple deformation may have resulted from dissolution and reprecipitation of interstitial halite. Desiccation crack in mud-flat sediment (mr) is filled with very well=sorted fine sand (ss) and probably records passage of wind-blown sand over desiccated mud flat. (C) Similar to B but with greater abundance of coarse-grained flasers and less deformation.

b e c a u s e t h e d e p o s i t i o n a l e n v i r o n m e n t was m o r e u p s l o p e f r o m the s a l i n a o r o t h e r sources of h a l i t e s u c h as s a l i n e m u d flats c o n t a i n i n g b r i n e or crystaUized halite. O r i g i n a l l y u n d i s t u r b e d s i l t s t o n e / m u d s t o n e facies p r o b a b l y r e s u l t e d f r o m d e p o s i t i o n o r s e d i m e n t r e w o r k i n g b y s h e e t w a s h i n areas

368 near sea level between the desert and salina; periodic wind-tidal flooding of salina margins by brine or by meteoric water from land could account for sedimentary structures that resemble the flaser bedding described by Handford (1982) from channelized sheetflood deposits around the margins of Holocene Bristol Dry Lake, California. Similar rock is described from the Permian Lyons Formation in Colorado in sequences interpreted to record alternating sabkha-dune deposition (Adams and Patton, 1979). Recurrent Queen/Grayburg flooding introduced halite-undersaturated waters into the sediment that dissolved (then reprecipitated during desiccation) interstitial halite to produce the deformed fabric. Clean sandstone Jacies Sandstones from the Queen/Grayburg formation are horizontally bedded, cross-stratified or massive. They are composed of light red, moderately to very well sorted, subrounded (Fig. 6A), frosted, fine- to very fine-grained sand. Small amounts of very well rounded, frosted, mediumto coarse-grained sand is present in some intervals. Compositionaily, sandstones range from feldspathic sublitharenite to subarkose (classification of Folk, 1974). Included feldspar grains are relatively pristine in that they show no signs of extensive weathering. Some sandstone contains displacively crystallized skeletal halite (Fig. 4G). All Queen/Grayburg sandstones are cemented by poikilotopic halite. Most of the red siliciclastic rock of the Queen/ Grayburg display compositional, textural, and structural characteristics consistent with deposition in or near arid eolian environments. Grains in most Queen/Grayburg sandstones are exceptionally well sorted (0.25-0.50 phi units). Ripples and ripple cross-lamination, where present, are not well defined; Bigarella (1972) and Hunter (1977) pointed out that ripple cross-laminae are commonly not readily visible in eolian sediment. The single most indicative characteristic of eolian action, however, is the presence of wind-ripple lamination with inversely graded laminae produced by climbing bed forms under conditions of net deposition. Such structures are considered

unique to eolian deposits (Hunter, 1977; Kocurek and Dott, 1981). Examples of wind-ripple lamination are also present in the Queen/Grayburg formation (Fig. 6B). Larger-scale sedimentary structures consistent with eolian deposition sediment, including highangle planar dune cross-lamination and tangential intersection of cross-laminae with underlying bedding (McKee and Tibbitts, 1964; Bigarella, 1972; Walker and Harms, 1972; Sanderson, 1974; Hunter, 1980; Szigeti and Fox, 1981; Fryberger et al., 1983; Ross, 1983), occur within fine-grained sandstone intervals from the Queen/Grayburg formation. Features characteristic of sand-sheet and interdune deposits, including horizontal bedding, bimodal texture, and probable deflation lag surfaces (Bagnold, 1941; Bigarella, 1972; Walker and Harms, 1972; Steidtmann, 1974; Hunter, 1977; Fryberger et al., 1979; Walker and Middleton, 1979; Ahlbrandt and Fryberger, 1981; Andrews, 1981; Fryberger et al., 1983; Fryberger et al., 1984), are also present in Queen/Grayburg sandstones. Inspection of the medium and coarse sand grains with SEM reveals percussion indentations (Fig. 6C) and, at higher magnification, the nature of grain surface frosting as bumpy deposits of precipitated silica on upturned cleavage plates (Fig. 6D); these photomicrographs compare favorably to those of Krinsley and Doornkamp (1973) taken of eolian sand grains. The presence of uncorroded detrital plagioclase feldspar in Queen/Grayburg sandstone suggests a dry climate where chemical weathering was not a major factor. Eolian dune subfacies Cross-stratified bedding of the Queen/Grayburg formation is composed of low- to high-angle (5-32 ° ) sand-flow and wind-ripple deposits that occur in sets underlain by wind-rippled interdunal deposits (Fig. 6B and E) and marked at the top by bounding surfaces. Cross-lamination is visible mainly because pale green reduction mottling is present that tends to be located within coarser grained parts of the rock. However, cross-stratified sets defined by mottling may actually include several discrete sand flows. The excellent sorting of Queen/Grayburg eolian sand renders more

369

Fig. 6. Eolian dune/interdune facies. (A) Thin-section photomicrograph (plane light) of interdunal(?) bimodal very well sorted very fine sandstone with medium sand grains. (B) Very well-sorted wind-rippled fine sandstone deposited on interdune surface. Cross-lamination is tangential to lag surface ( I ) marked by thin layer of medium sand visible near bottom of photo.

370

subtle the grading that usually characterizes sand flow cross-lamination. A similar observation was made regarding sorting and subtleness of crosslamination from the Unkpapa Sandstone of South Dakota (Szegeti and Fox, 1981). Similarly, the sharp lateral boundaries that characterize most eolian sand-flow deposits (Hunter, 1976) are not

visible in Queen/Grayburg cross-stratified sandstone, probably also because of excellent sorting. Continuous core was available through much of the Queen/Grayburg formation at Mansfield No. 1 making it possible to fit most pieces back into their original positions relative to each other and to measure ascending shifts in cross-lamination

371 -

"7--

~q rr~ U

,A

!1 J

Fig. 6 (continued). ((2) Scanning electron photomicrograph of medium- to coarse-grained sand extracted from interdune deposit. Note large percussion mark on grain at lower left (p). (D) Close-up of frosted medium sand grain surface showing bumpy precipitated silica (s) on subtle upturned cleavage plates. Large crystals are euhedral quartz overgrowths (q). (E) Part of dune sand-flow sequence showing thickness of intervals bounded by reduction lines (r); cross-lamina thickness is a function of original slip-face height.

dips; characteristically, dip azirauths rotated counter-clockwise through roughly ninety degrees terminating at an upper truncation surface, then returned to approximately the original bearing. Enough continuous core was available to detect several repetitions of this pattern although the unoriented nature of core rendered impossible the taking of paleocurrent measurements from these cross-sets. Azimuth shifts of 90 ° may reflect deposition on barchan or barchanoid transverse dunes in a unimodal wind regime (McKee, 1979). Q u e e n / G r a y b u r g sets of cross-strata range from 20 cm to 0.5 m between bounding surfaces and probably represent only a fraction of original dune height. However, recent investigations of sand-flow lamination thickness as a function of

slipface height in modern dunes suggest that thin sand-flow cross-laminae form on the slipfaces of small eolian dunes (Hunter, 1977; Kocurek and Dott, 1981). Examples of reduction-defined lamina are about 0.5-1 cm (Fig. 6E) and are, thus, quite thin in the Q u e e n / G r a y b u r g formation. Because the reduction-mottling may actually define packages of even thinner laminae, even smaller dunes may be implicated. Q u e e n / G r a y b u r g dune heights were probably less than two 2 m.

Dry interdune subfacies Dry interdune areas accumulated well-sorted, fine-grained sand and silt with iUuviation structures. Wind-ripple lamination is visible in some places at the base of eolian dune sequences (Fig.

372 6B). Some Q u e e n / G r a y b u r g rocks are interpreted here as interdune deposits because of their weak textural bimodality caused by the presence of sparse, very well rounded, frosted medium sand grains in conjunction with their proximity to eolian dune deposits in core. Textural bimodality is a common feature of m o d e m interdune deposits (Andrews, 1981; Ahlbrandt and Fryberger, 1981). Wind-ripple lamination is not abundant in many of these interpreted interdune deposits. Also, their development on sand sheets (discussed below) prior to passage of dunes cannot be ruled out. Very thin wisps of silt and clay that appear to have washed down (illuviated) through the sand are also present in interpreted interdune facies. Discussion of these features is deferred to the following section on Q u e e n / G r a y b u r g sand-sheet deposits where they are more abundant.

Sand-sheet subfacies Sand sheets accumulated well-sorted very finegrained sand and silt in areas where dune development was inhibited or where dunes may have been reworked by flood events (dune dissipation). Conditions that inhibit eolian dune development include high water tables, surface binding, flooding, a coarse-grain-size component, and vegetation (Kocurek and Nielson, 1986). Periodic flooding and halite-cementation of the sediment surface (perhaps resulting from high saline water tables or capillary action above water tables in the depositional environment) best explain Q u e e n / G r a y b u r g sand-sheet formation because no evidence of vegetation or coarse-grain texture is present. Also present in these deposits are features that resemble illuvial "dissipation structures" described by Ahlbrandt and Fryberger (1981) for

Fig. 7. Sand sheet facies. (A) Very well sorted horizontallyripple-laminated(?)very fine sandstone with possible illuviation structures (i) which appear to outline ripple-form and internal cross-lamination. (B) Horizontally laminated, very well sorted very fine sandstone with anhydrite cement on bedding planes(?).

373

wet interdune deposits in the Killpecker Dunes of Wyoming. Queen/Grayburg examples, however, are much thinner (millimeter-scale) and are more subtle (Fig. 7A) than those illustrated by Ahlbrandt and Fryberger. Such features probably result from illuviation into sand of clay colloids (McKee and Bigarella, 1979), which occurred dur-

m P~L~

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DEPTH

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API Units 45

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LOG 75

CLASTIC

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ing wet periods on the Queen/Grayburg landscape involving marine transgressions or flooding from wind-tides or rainstorms. The thinness of Queen/Grayburg examples probably reflects the scarcity of fine-grained silt and clay in the depositional environment where sand sheets developed. These illuviation structures appear to highlight

LITH

ENVI RONM ENTA L INTE~RPRETATIONS

SEDIMENTARY STRUCTURES

Sandstone I Mudstone Fn. Vfn, SOs. CIS. g

Structures not illustrated

N

m

~:~~'~:"~

~

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~

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~

~

850-

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i

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\

/

% 4

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Sedimentary structures - ~ Poorly defined ripples with illuvatlon structures ( ~

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----'~ Small deformed ripples

~

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cracks

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hevrons

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~

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..d,,on.

~

~, . . . . .

data.

Anhydrite

Fig. 8. Queen/Grayburg formation, Stone and Webster Mansfield No. 1, Oldham County, Texas. Environmental interpretations and facies relations discussed in text; key to symbols for Figs. 8-10.

374

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ENVIRONMENTAL INTERPRETATIONS

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Fig. 9. Queen/Grayburg formation. Gruy Federal Grabbe N o . 1, Swisher County, T e x a s . K e y to symbols in Fig. 8. Environmental interpretations and facies relations discussed in text.

ripples and internal ripple cross-laminae in places (Fig. 7A). Other evidence suggesting a wet phase for some of these sand sheets is drawn from their stratigraphic relationship with clearly subaqueous deposits, for example, the siltstones and mudstones with deformed ripples discussed above. Sand-sheet deposits are found interbedded with siltstones and mudstones in the upper half of the Queen/ Grayburg at both Mansfield No. 1 (Fig. 8) and Grabbe No. 1 (Fig. 9). No intervening eolian dune deposits are found with them and it is most reasonable to place these sand sheets between the dunes and the mud flats in the facies tract. In that position wind-tidal flooding and elevated saline

water tables, factors that help inhibit dune formation, would be more common. Additionally, in one particularly unusual sand-sheet interval contorted bedding is present with anhydrite cement deposited apparently along bedding planes (Fig. 7B). Small (1-2 mm) displacive halite crystals occur in part of the same sequence (not shown)• These chemical deposits could have precipitated during desiccation of a wetted sand sheet. The original sulfate could have been airborne gypsum dust which settled to the surface. Interpretation of this unusual rock remains problematic however; it is unique among all Permian core recovered from the Palo Duro Basin•

375

Integration of well logs and core: Implications for paleogeography and procession of facies Characteristic vertical sequences seen in core (Fig. 10) and areal distribution of component lithologies revealed by correlation and interpretation of geophysical well logs clearly define Queen/Grayburg deposition in the Palo Duro Basin as cyclic, progradational, and as having evolved in an arid environment. Clastic-free evaporite strata have a very low level of natural radioactivity and produce mappable gamma ray log patterns such as those shown in Fig. 8-11. The cross-section of the Queen/ Grayburg formation in Fig. 11 demonstrates that individual evaporite strata can be traced south across the Palo Duro Basin and the Matador Arch onto the Northern Shelf of the Midland Basin toward more open marine paleoenvironments where coeval carbonate-bearing sediments were deposited. These evaporite strata generally thicken, although very gradually, to the south and southwest. This thickening trend toward coeval marine-produced strata supports interpretation of a marine source for the precipitating brines. Silver and Todd (1969) and Ward et al. (1986) illustrate the Queen and Grayburg formations as deposited

SEDIMENTARY STRUCTURES

LITHOLOGY Gommo r y ~.,~ a

ENVIRONMENTAL INTERPRETATIONS

~ Mud flot

, ~//>>;,;7/i=~==

~
ft

.

~o r .....

::~;;iill;

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oio

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-----~..~

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Mud flof $olt/mud riot

Solln(l Mud flof Q& 5927

Fig. 10. Generalized stratigraphic interval: lower transgressive/progradational cycle, Queen/G-rayburg Formation, Gruy Federal Crabbe No. 1, Swisher County, Texas. Key to symbols in Fig. 8. Interval is capped by anhydrite deposited as gypsum during a subsequent depositional cycle.

in a broad restricted lagoonal environment north of the Goat Seep carbonate "reef" trend, which occupies the shelf north of the Delaware Basin. The geometrical and spatial configurations of these evaporites suggest that they are marine-derived and precipitated as broad uniform sheets following regional marine transgressions. Second, halite strata usually overlie anhydrite (Fig. 9). This pattern indicates that a progressively more saline water body developed which eventually achieved halite-supersaturation as desiccation and evaporite precipitation proceeded in the salina. Third, halite becomes increasingly mud-rich upward through a cyclic sequence, indicating the arrival of clastics into the desiccating salina (Fig. 10) or arrival of the prograding landward salinamargin at the site previously occupied by the salina. Continuing upward through a sequence mudstone, then sandstone occurs recording coarsening-upward progradational deposition. Fourth, sandstone maps show that deposits thicken toward the northwest and west, suggesting locations of sand sources in those directions. Queen/Grayburg core recovered from these sandstone intervals display textures and sedimentary structures diagnostic of deposition in and around eolian environments. Fifth, sandstones within individual progradational cycles tend to fine upward overall (Fig. 10). These finer-grained sandstones display textures and sedimentary structures consistent with deposition on sand sheets. This suggests that conditions were no longer optimal for dune development or preservation. Although it appears from other evidence that marine transgressions were relatively rapid, the actual rate of sea-level rise is unknown. Saline water tables may have risen on land as sea level rose in the basin. Elevated water tables could have maintained conditions generally too damp for dune development or provided halite cement that bound sand and prohibited entrainment by the wind. Sixth, clastic and halite-bearing clastic intervals within the Queen/Grayburg formation thicken at a higher rate down-slope than do the basal evaporite intervals (Figs. 11 and 12). Assuming consistent subsidence rates in the basin, the differences in thickening rates suggest that more time

376

8

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OLDHAM CO

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SWISHER CO

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is recorded in terrestrial and salina-margin processes than in the salina evaporites within cycles. Seventh, contacts between clastics and overlying evaporites are sharp and the clastics show little sign of having been reworked by marine processes since deposition in an eolian environment. This suggests that transgressions were rapid and that evaporite-precipitation commenced soon after a transgression. Eighth, the fence diagram (Fig. 12) constructed for the western half of the basin (based on core and log responses from the lowermost evaporite cycle of the Q u e e n / G r a y b u r g formation) il-

lustrates the facies relations of the six general rock-types that typify transgressive/progradational cycles of the Q u e e n / G r a y b u r g formation. Up-slope areas are dominantly sandstone while down-slope areas include several relatively clastic-free e v a p o r i t e beds. M u d r o c k s and halite/clastic mixtures compose transitional fades between sandstones and evaporites. Relations in eastern Palo Duro Basin are unclear because of lack of core and the effects of post-Permian halite dissolution. However, wall-log responses in the east suggest a finer-grained environment, perhaps of an alluvial nature. Further investigation is required in this part of the basin.

377 P

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Polo Duro Basin

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Matador Arch

Clean fine sandstone

[ ~

Halite/clastic mix

~

Very fine sandstone/siltstone

~

Halite

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Mudstone

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Anhydrite/gypsum

County names P - POTTER

A

-

0

S

- SWISHER

- OLDHAM

DS- DEAF SMITH R -RANDALL

ARMSTRONG H-HALE

( ~ Well

designation

Datum: base of Queen/Grayburg middle evaporite bed Data : Core~gamma -ray ; sonic~ and sample logs

L- LUBBOCK

PA- PARMER C -CASTRO

QA 5 9 1 8

Fig. 12. Generalized fence diagram depicting Queen/Grayburg evaporite depositional cycles and subsequent siliciclastic progradation in the central and western Palo Duro Basin. Based on lower evaporite cycle (see Figs. 9 and 10) in central part of basin (Swisher County) and correlative clastic intervals in western part of basin. See Fig. 3 for lines of section.

Collectively these individual aspects suggest a scenario of deposition for each cycle such as illustrated in Fig. 13. Following a regional marine transgression from the south and southwest anhydrite, then halite precipitates were deposited subaqueously in a broad shallow salina. Some silt and clay were probably transported to the salina interior from land by wind. Clean evaporites in the salina graded at the up-slope salina margins

into muddy halite, chaotic halite-mudstone, and sandstone with displacive skeletal halite. Landward salina margins migrated toward the salina center as desiccation and clastic progradation proceeded. Because of regional aridity, which characterized the region, eolian processes dominated terrestrial deposition. Sand was transported from sources in the northwest and west and deposited on dunes, in

378

I

(a)

~"_-..-_-..~

+ +~HHHI;;II=::===H?+

~'+ + + + + + ÷III;==;=I=III;H=II~ ~ + + ÷ + + + + +""===I=IIIHH=~;~ + + + ÷ + + + =,3~,,[;=I~I~=~IH~

-÷ +J ÷ +|

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AU

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.

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. ::.".:

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(f)

Fine sand

~

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~

Amarillo Uplift

Very fine sand

~H+t+H+HHalite

~

Matador Arch

Silt

~

Gypsum

Mud

~

Halite dissolution zone . . . .

?" uncertain Nature of facies Stream QA 5917

Fig. 13. Generalized paleogeographic reconstruction of a typical transgressive/progradational cycle during Queen/Grayburg deposition: (a) post-transgression regional sulfate precipitation with landward marginal production of halite-siliciclastic mixtures; (b) regional halite precipitation and initiation of siliciclastic progradation; (c) shrinkage of salina, continuation of progradation including establishment of eolian environments in the study area; (d) late desiccation stage of isolated brine pools, progressive dominance of eolian environments prograding from the northwest and west; (e) maximum eastern extent of fine sand deposited on dunes, interdunes and sand sheets; (f) recession of dune/interdune environments, dominance of marginally finer-grained sand sheet environment.

379 interdune areas, and on sand sheets. Sheetwash flooding by wind-tides and intermittent rains produced thin mud-flat deposits, mainly between salinas and eolian environments. Downward flow of ephemeral flood-waters produced illuviation structures in sand deposits and, in combination with evaporative pumping, deposited displacive halite crystals within the sediment.

Condusions (1) Rapid marine transgressions resulted in minimal reworking of preexisting sediments and produced sharp contacts between eolian-clastic sediments and overlying evaporite d e p o s i t s . (2) Cyclic evaporites were precipitated (gypsum followed by halite) in regionally extensive, subaqueous, marine-derived, hypersaline environments (salinas). (3) Evaporite deposition was followed by progradation of terrestrial environments that produced coarsening-upward sequences of siliciclastic strata. (4) During most of the time between transgressions Q u e e n / G r a y b u r g deposition was dominated by salina-margin and eolian processes. (5) Terrestrial siliciclastics were deposited in a desert setting that included small barchan or barchanoid transverse eolian dunes, dry interdunal areas, eolian-derived sand sheets, and m u d flats produced b y flood-induced sheetwash. (6) Periods of eolian dune deposition tended to be followed b y periods of sand-sheet deposition. (7) The sequence of depositional environments was, from land to basin, eolian d u n e / i n t e r d u n e , sand sheet, mud flat, saline m u d flat and salina. Sand sheets were essentially transitional between d u n e / i n t e r d u n e areas and coastal m u d flats. Saline m u d flats were transitional between m u d flats and salinas.

Acknowledgments This study was funded by the United States Department of Energy under contract n u m b e r DE-AC97-83WM46651. The quality of this study and manuscript benefited immeasurably from discussions with and reviews by G a r y Kocurek, Susan

Hovorka, Jay Raney and John Warren. Technical editing was performed b y Jules Dubar. SEM photographs were provided b y Patty Granger. Figures were drafted by N a n Minchow-Newman. Core photographs were processed by James Morgan.

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